Imaging of alpha emitters

Abstract

Optical system 12, ultraviolet bandpass filter 16 and ultraviolet-sensitive camera 14 are coupled in line of sight. Filter 16 passes alpha-induced radioluminescence frequencies, and comprises either two reflective filters (58, 59, Fig. 8) arranged mutually skew, or absorptive filter (62 Fig. 7) arranged between two reflective filters (58, 59, Fig. 7). A background image, obtained using a background bandpass filter (32, Fig. 2) which passes ultraviolet wavelengths not corresponding to alpha-induced radioluminescence, may be subtracted from the ultraviolet image. The ultraviolet image may be overlaid on a visible image obtained using a visible-spectrum filter (34, Fig. 2). First and second ultraviolet bandpass filters (16b, Fig. 13; 36, Fig. 2) may respectively pass the 337 nm and 357 nm fluorescence emissions of nitrogen. Filter 16 may pass the 276 nm fluorescence emission of air. Filter changing system 30 may bring one of filters 16, 32, 34, 36 into line of sight. Camera 14 may be a deep-cooled or intensified CCD.

Description

FIELD OF THE INVENTION

[0001] The present invention relates to alpha particle sensors. In particular to an alpha particle sensing apparatus for long range detection of alpha particle via alphainduced radio-luminescence. BACKGROUND OF THE INVENTION

[0002] Alpha particles, typically emitted by heavy nuclides and their decay products, are a significant concern in the commercial nuclear industry due to their potential harm to human health through ingestion, inhalation, or injection. Alpha particles are composed of two protons and two neutrons. Alpha particles can only travel a few centimetres in air, which makes them hard to detect. Despite their limited travel distance, alpha particles can cause considerable damage to DNA if they enter the body. Current detection methodologies necessitate close proximity (e.g., usually millimetres) to these particles, resulting in a time-intensive, laborious process that increases the risk of exposure and contamination for personnel. Furthermore, slight increments in contamination levels necessitate the use of more stringent personal protective equipment (PPE), while the decontamination or replacement of contaminated hand-held alpha detectors incurs significant costs. There is a need for devices capable of detecting alpha particles from distances of several meters to enhance safety and efficiency in applications such as glovebox characterization for decommissioning, lab or wide-area contamination assessment, real-time alpha spillage monitoring, and open environment dispersion monitoring. SUMMARY OF THE INVENTION

[0003] A first aspect of the invention provides an alpha particle sensing apparatus for obtaining images of alpha-induced radio-luminescence, the apparatus comprising: an optical system for directing ultraviolet, UV, light; a UV sensitive camera coupled to the optical system in an optical line-of-sight path; a UV bandpass filter for passing UV light at radio-luminescence wavelengths induced from alpha particles for reception by the UV sensitive camera, wherein the UV bandpass filter is coupled to the UV sensitive camera in an optical line-of-sight path; and, a processing system configured to receive a UV image from the UV sensitive camera based on UV light passed through the UV bandpass filter, wherein the UV bandpass filter comprises at least one of: an angled reflective filter arrangement comprising at least a first and second reflective filters arranged in series, wherein the first reflective filter is skew with respect to the second reflective filter; or a reflective-absorptive-reflective filter arrangement comprising two reflective filters, and an absorptive filter, wherein the absorptive filter is arranged between each of the two reflective filters.

[0004] Since the UV bandpass filter may comprise multiple filters in at least one of an angled reflective filter arrangement and / or a reflective-absorptive-reflective filter arrangement, the UV bandpass filter may be referred to as a UV bandpass filter arrangement herein.

[0005] The alpha particle sensing apparatus may be used to determine the location of an alpha particle source.

[0006] Advantageously, the UV bandpass filter provides improved blocking rate outside of the target band of the UV bandpass filter which enables the alpha particle sensing apparatus to be suitable for obtaining images of alpha-induced radioluminescence: (i) at greater distances than known systems, (ii) with lower alpha source strength than known systems; and / or (iii) in faster time than known systems.

[0007] Optionally, the optical system comprises UV fused silica lenses. Directing light, such as UV light, may include gathering and focussing light, such as UV light. The UV bandpass filter may be described as a narrowband filter due to a short bandwidth of 5 to 40 nm.

[0008] Optionally regarding an angled reflective filter arrangement, the first reflective filter may have a first optical axis and the second reflective filter may have a second optical axis, wherein the angle between the first and second optical axes are non-zero, i.e., the optical axes are non-parallel. The optical axis may be defined as an angle with respect to the optical system.

[0009] Optionally regarding a reflective-absorptive-reflective filter arrangement, all filters (e.g., two reflective filters, and an absorptive filter) are arranged in series and the optical axis of each are parallel with respect to each other, e.g., all filters comprise a coaxial axis.

[0010] Optionally, the alpha particle sensing apparatus further comprises a background image detection means configured to generate a background image in UV electromagnetic spectrum. Optionally, the processing system further configured to subtract the background image from the UV image to generate an UV-background image. If the background image detection means is distinct from the UV sensitive camera then, optionally, the background image has a known offset relative to the processed image, and the UV-background image is generated based on the known offset. Advantageously, this improves the alpha particle sensing apparatus' sensitivity with respect to background sources.

[0011] Optionally, the background image detection means is a background bandpass filter for passing UV wavelengths which are not wavelengths corresponding to alpha-induced radio-luminescence, the background bandpass filter comprising at least one of: an angled reflective filter arrangement comprising at least a first and second reflective filters arranged in series, wherein the first reflective filter is skew with respect to the second reflective filter; or a reflective-absorptive-reflective filter arrangement comprising two reflective filters, and an absorptive filter, wherein the absorptive filter is arranged between each of the two reflective filters. The absorptive filter may be a UG11 absorptive filter (Schott™ part number). That is, the absorptive filter may be UV transmitting between 275-375nm, which has minimal effect on the detected signal.

[0012] Since the background bandpass filter may comprise multiple filters, the background bandpass filter may be referred to as a background bandpass filter arrangement herein.

[0013] Optionally, the background bandpass filter passes light corresponding to the wavelengths between the 337nm and 357nm fluorescence emission of nitrogen for reception by the UV sensitive camera. This is particularly advantageous when operating the alpha particle sensing apparatus in the absence of (direct) sunlight (e.g., an indoor lighting condition, or at night-time).

[0014] Optionally, the alpha particle sensing apparatus further comprises: a visible-spectrum image detection means configured to generate a visible-spectrum image in the visible electromagnetic spectrum; and the processing system further configured to determine an estimated location of alpha particles based on: the UV image or the UV-background image; and, the visible-spectrum image. If the visible-spectrum image detection means is distinct from the UV sensitive camera then, optionally, the visible-spectrum image has a known offset relative to the processed image, and the estimated location is generated based on the known offset. Advantageously, this improves the alpha particle sensing apparatus' ability to determine the location of an alpha particle source.

[0015] Optionally, the visible-spectrum image detection means is a visible-spectrum filter for passing visible light wavelengths via the optical system. Visible light wavelengths may be lower than 475nm, ideally 450nm. Advantageously, this may reduce chromatic aberration. Alternatively, the visible image could be taken using other visible wavelengths.

[0016] Since the visible-spectrum filter may comprise multiple filters, the visible-spectrum filter may be referred to as a visible-spectrum filter arrangement herein. Optionally, the visible-spectrum filter is a visible-spectrum passband filter.

[0017] Optionally, the processing system further configured to overlay the UV image or the UV-background image with the visible-spectrum image to generate an overlayed image which identifies an estimated location of alpha particles within the visible-spectrum image. If the visible-spectrum image detection means is distinct from the UV sensitive camera then, optionally, the visible-spectrum image has a known offset relative to the processed image, and the visible-spectrum image is based on the known offset.

[0018] Optionally, the apparatus further comprising a filter changing system configured to change between a two or more states. Optionally, each state corresponds to a respective bandpass filter being in the optical line-of-sight path (and the other bandpass filters are not in the optical line of sight path). Optionally, each respective bandpass filter is selected from one of: the UV bandpass filter; the first UV bandpass filter; the second UV bandpass filter; the background bandpass filter; and the visible-spectrum filter. Advantageously, the filter changing system results in a simpler, reduced weight alpha particle sensing apparatus.

[0019] Optionally, the processing system is configured to receive from the UV sensitive camera a series of images, each of the images corresponding to a respective state of the filter changing system. Optionally, in a first state of the filter changing system the UV sensitive camera captures the UV image. Optionally, in the first state the UV bandpass filter (or first UV bandpass filter) is in the optical line of sight path. Optionally, in a second state of the filter changing system the UV sensitive camera captures the second UV image. Optionally, in the second state the second UV bandpass filter is in the optical line of sight path. Optionally, in a third state of the filter changing system the UV sensitive camera captures the background image. Optionally, in the third state the background bandpass filter is in the optical line of sight path. Optionally, in a fourth state of the filter changing system the UV sensitive camera captures the visible-spectrum image. Optionally, in the fourth state the visible spectrum bandpass filter is in the optical line of sight path.

[0020] Optionally, the UV bandpass filter is the reflective-absorptive-reflective filter arrangement. Optionally, the UV bandpass filter passes light corresponding to the 337nm or 357nm fluorescence emission of nitrogen for reception by the UV sensitive camera. This is particularly advantageous when operating the alpha particle sensing apparatus in the absence of (direct) sunlight (e.g., an indoor lighting condition).

[0021] Optionally, the UV bandpass filter is a first UV bandpass filter and passes light corresponding to the 337nm fluorescence emission of nitrogen for reception by the UV sensitive camera, and the apparatus further comprises a second UV bandpass filter comprising a second reflective-absorptive-reflective filter arrangement comprising two reflective filters, and an absorptive filter. Optionally, the absorptive filter is arranged between each of the two reflective filters. Optionally, the second UV bandpass filter passes light corresponding to the 357nm fluorescence emission of nitrogen for reception by the UV sensitive camera. This is particularly advantageous when operating the alpha particle sensing apparatus in the absence of (direct) sunlight (e.g., an indoor lighting condition).

[0022] Optionally, the UV image is based on: a first UV image based on UV light passed through the first UV bandpass filter and received by the UV sensitive camera; and, a second UV image based on UV light passed through the second UV bandpass filter and received by the UV sensitive camera. This is particularly advantageous when operating the alpha particle sensing apparatus in the absence of (direct) sunlight (e.g., an indoor lighting condition, or at night).

[0023] Optionally, the UV bandpass filter is the angled reflective filter arrangement. Optionally, the UV bandpass filter passes light corresponding to the emission of air (e.g., nitrogen, and nitrogen molecules) at or under 276nm (276nm, 272nm, 268nm, 264nm, 259nm, 256nm, 248nm)fluorescence for reception by the UV sensitive camera. This is particularly advantageous when operating the alpha particle sensing apparatus in the presence of (direct) sunlight (e.g., in an outside lighting condition).

[0024] Optionally, the UV sensitive camera is a deep cooled CCD camera. Advantageously, this provides improved sensitivity of the alpha particle sensing apparatus.

[0025] Optionally, the UV sensitive camera is an Intensified CCD camera.

[0026] Optionally, the processing system is further configured to applying a median filter to at least one of: the UV image, the first UV image, the second UV image, the background image, and the visible-spectrum image, from the UV sensitive camera. Advantageously, applying a median filter removes false positive effects which may occur due to cosmic rays.

[0027] A further aspect of the invention provides a method of obtaining an image of alpha-induced radio-luminescence using the alpha particle sensing apparatus of any preceding claim, the method comprising: arranging the UV bandpass filter for passing UV light at radio-luminescence wavelengths induced from alpha particles for reception by the UV sensitive camera; and, capturing a UV image with the UV sensitive camera based on the light passed through the UV bandpass filter.

[0028] Optionally, the method comprises: applying a median filter to the UV image; and, generating an image of alpha-induced radio-luminescence based on the median filtered UV image.

[0029] Optionally, the method comprises: arranging a background bandpass filter for passing UV light which are not wavelengths corresponding to alpha-induced radio-luminescence for reception by the UV sensitive camera; capturing a background image with the UV sensitive camera based on the light passed through the background bandpass filter; and, generating an / the image of alpha-induced radio-luminescence based on: the UV image or the median filtered UV image; and, the background image.

[0030] Optionally, the method comprises: applying a median filter to the background image; and, generating the image of alpha-induced radio-luminescence based on the median filtered background image.

[0031] Optionally, the method comprises: arranging a visible-spectrum filter for passing light in the visible electromagnetic spectrum for reception by the UV sensitive camera; capturing a visible-spectrum image with the UV sensitive camera based on the light passed through the visible-spectrum filter; and, generating an / the image of alpha-induced radio-luminescence based on the UV image and the visible-spectrum image.

[0032] Optionally, the method comprises: generating a UV-background image by subtracting the background image or the median filtered background image from the UV image or the median filtered UV image; and, generating the image of alphainduced radio-luminescence by overlaying the visible-spectrum image and the UV-background image. This is particularly advantageous when operating the alpha particle sensing apparatus in the presence of (direct) sunlight (e.g., in an outside lighting condition) or when detecting one emission spectra in the absence of (direct) sunlight (e.g., an indoor lighting condition, or at night-time).

[0033] Optionally, the UV bandpass filter is a first UV bandpass filter and passes light corresponding to the 337nm fluorescence emission of nitrogen, the UV image is a first UV image, and the method further optionally comprises: arranging a second UV bandpass filter which passes light corresponding to the 357nm fluorescence emission of nitrogen for reception by the UV sensitive camera; capturing a second UV image with the UV sensitive camera based on the light passed through the second UV bandpass filter; and generating the image of alphainduced radio-luminescence based on the first UV image and the second UV image.

[0034] Optionally, the method comprises: applying a median filter to the second UV image; and, generating the image of alpha-induced radio-luminescence based on the median filtered second UV image.

[0035] Optionally, the method comprises: generating a nitrogen UV-background image by: adding the first UV image or the median filtered first image, to, the second UV image or the median filtered second UV image; and subtracting the background image or the median filtered background image; and generating the image of alpha-induced radio-luminescence by overlaying the visible-spectrum image and the nitrogen UV-background image. This is particularly advantageous when operating the alpha particle sensing apparatus in the absence of (direct) sunlight (e.g., an indoor lighting condition, or at night-time).

[0036] A further aspect of the invention provides a high blocking optical bandpass filter comprising two reflective bandpass filters, and an absorptive bandpass filter, wherein the absorptive filter is arranged between each of the two reflective filters, wherein the bandpass bandwidth of the absorptive filter is greater than the bandpass bandwidth of the reflective bandpass filters.

[0037] Optionally, the UV bandpass filter of any aspect may further comprise a reflective-absorptive filter arrangement comprising two reflective filters, and an absorptive filter. Optionally, a first reflective filter is arranged between the absorptive filter and a second reflective filter. Optionally, the reflective-absorptive filter is arranged such that the second reflective filter is the closest filter of the reflective-absorptive filter arrangement to the UV sensitive camera. Alternatively, the reflective-absorptive filter is arranged such that the absorptive filter is the closest filter of the reflective-absorptive filter arrangement to the UV sensitive camera. That is, in certain conditions (e.g., in the absence of direct sunlight (e.g., an indoor lighting condition)) configurations like 'reflect-reflect-absorb' or 'absorb-reflect-reflect' might also be effective.

[0038] Aspects of the invention may provide the following advantages:

[0039] Enhanced Safety: The long-range detection ability of the invention may reduce the need for direct contact with radioactive materials. This may significantly lower the risk of exposure to harmful alpha radiation, enhancing the safety of users and potentially reducing the costs associated with radiation protection measures

[0040] Portable and easy to use: The invention may weigh less than 10 kg and may fit in a suitcase. The power requirements of the alpha particle sensing apparatus may allow its use with a portable power supply that enables 5-6 hours of running time. Setup may be quick and easy, and the alpha particle sensing apparatus may be operated by one person in a few minutes.

[0041] Flexible and Adaptable: The invention may be capable of capturing megapixel resolution images, enabling the precise location of alpha sources. Alternatively, pixels may be combined to achieve an even lower detection limit, albeit at the expense of reduced resolution.

[0042] Time-saving: The invention may provide a short setup, detection, and processing time, allowing for the detection of alpha emitters over large areas in a few minutes. This leads to a faster and more efficient clean-up process.

[0043] High accuracy and sensitivity: By overlaying the alpha fluorescence signal with images in the visible spectrum (e.g., RGB images) from the camera, the invention may achieve millimetre accuracy in localizing alpha sources, such as detecting a 2mm diameter smoke detector ionisation source from several meters away. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Embodiments of the invention will now be described with reference to the accompanying drawings, in which:

[0045] Figure 1 illustrates an isometric view of an alpha particle sensing apparatus for obtaining images of alpha-induced radio-luminescence.

[0046] Figure 2 illustrates an isometric exploded view of a filter changing system of the alpha particle sensing apparatus of Figure 1.

[0047] Figure 3 illustrates an example flow diagram showing multiple images being combined to produce an image of alpha-induced radio-luminescence.

[0048] Figure 4a shows a UV-background image.

[0049] Figure 4b shows a visible-spectrum image.

[0050] Figure 4c shows an overlayed image generated by overlying the UV-background image of Figure 4a with the visible-spectrum image of Figure 4b.

[0051] Figure 5a shows a UV image received by the processing system with artifacts due to cosmic rays.

[0052] Figure 5b shows a processed UV image.

[0053] Figure 6 illustrates a side view of an example of a known reflective-reflective filter arrangement.

[0054] Figure 7 illustrates a side view of an example of a reflective-absorptive-reflective filter arrangement.

[0055] Figure 8 illustrates a side view of an example of an angled reflective filter arrangement.

[0056] Figure 9 graphically illustrates both an emission spectrum graph of the relative intensity of alpha-induced radio-luminescence (around 276nm) of air on the y-axis with wavelength (A) on the x-axis, and a system graph of quantum efficiency (QE) of cameras and transmittance of filters as a percentage on the y-axis with wavelength (A) on the x-axis.

[0057] Figure 10 illustrates a side view of an alpha particle sensing apparatus suitable for obtaining images of alpha-induced radio-luminescence in the presence of sunlight (e.g., direct sunlight). That is, in an outside lighting condition.

[0058] Figure 11 illustrates an example flow diagram showing multiple images generated by the alpha particle sensing apparatus of Figure 10 being combined to produce an image of alpha-induced radio-luminescence.

[0059] Figure 12 graphically illustrates both an emission spectrum graph of the relative intensity of alpha-induced radio-luminescence of Nitrogen (N2 molecule) on the y-axis with wavelength (A) on the x-axis, and the system graph of quantum efficiency (QE) of cameras and transmittance of filters as a percentage on the y-axis with wavelength (A) on the x-axis.

[0060] Figure 13 illustrates a side view of an alpha particle sensing apparatus suitable for obtaining images of alpha-induced radio-luminescence in the absence of sunlight (e.g., direct sunlight). That is, an indoor lighting condition, or at nighttime.

[0061] Figure 14 illustrates an example flow diagram showing multiple images generated by the alpha particle sensing apparatus of Figure 13 being combined to produce an image of alpha-induced radio-luminescence.

[0062] Figure 15 illustrates a flow chat of a method of obtaining an image of alphainduced radio-luminescence using an alpha particle sensing apparatus. DETAILED DESCRIPTION OF EMBODIMENT(S)

[0063] Alpha-induced radio-luminescence is a phenomenon where alpha particles excite nitrogen molecules in the air to emit photons in the ultraviolet region. These emitted photons can be harnessed to determine the position of alpha emitters at extended distances. The underlying phenomenon may be known, however, the description herein provides a sensing apparatus to harness alpha-induced radioluminescence. When sensing alpha-induced radio-luminescence there is a trade off between alpha source strength, maximum detection distance, capture time from a UV sensitive camera, and sensitivity of a UV sensitive camera.

[0064] By way of a non-limiting description of the invention, the alpha particle sensing apparatus has an optical system (e.g., a lens system), an optical bandpass filter, and a UV sensitive camera. The optical bandpass filter provides a very high blocking ability by an arrangement of known filters designed to block ambient light that is billions of times brighter than the alpha particle signal. The optical bandpass filter may be: (i) a 'sandwich' configuration utilising two narrow bandwidth reflective filters and one broad bandwidth absorptive filter positioned in a reflective-absorptive-reflective order, or (ii) an arrangement of two or more skew reflective filters. Advantageously, the blocking ability of the optical bandpass filter allows the alpha particle sensing apparatus to have improved sensitivity, allowing the alpha particle sensing apparatus to detect weaker alpha particle sources, detect alpha particle sources from a further distance, and / or detect alpha particle sources in reduced time. By way of an example, the alpha particle sensing apparatus disclosed herein is capable of detecting a 3 MBq point Am-241 alpha source from a distance of 30 meters within a span of five minutes, or detecting a 29 kBq Am-241 smoke detector ionisation source at 3 meters in 15 minutes. The resulting high-resolution image enabled location of alpha sources with millimetre accuracy.

[0065] Figure 1 shows an alpha particle sensing apparatus 10 for obtaining images of alpha-induced radio-luminescence. Alpha-induced radio-luminescence of nitrogen (N2 molecule) and air occurs in the ultraviolet (UV) range of the electromagnetic (EM) spectrum. The alpha particle sensing apparatus 10 comprises an optical system 12 (specifically, a lens system 12), a UV sensitive camera 14, a UV bandpass filter 16 (which may comprise multiple or a combination of filters), and a processing system 17. The processing system 17 may be integrated into the UV sensitive camera 14 or may be external to the UV sensitive camera 14 and in (wireless or wired) communication with the UV sensitive camera 14.

[0066] The lens system 12 is arranged to direct (e.g., includes gathering and focussing) ultraviolet (UV) light, and is therefore largely transparent to UV light. As shown in Figure 1, the lens system 12 is optical and comprises a primary UV lens 18, a secondary lens 20, and a lens focusing arrangement 22. The primary UV lens 18 and secondary UV lens 20 are arranged to substantially share an optical axis 25. The primary UV lens 18 may be crafted from UV fused silica, may have a diameter between 120mm and 300mm (e.g., 200mm, 150mm), and may have elongated focal length between 200mm and 600mm (e.g., 400mm, 300mm). The primary UV lens 18 advantageously gathers and focuses UV light onto the camera sensor of the UV sensitive camera 14. The secondary UV lens 20 is arranged to further focus the light from the primary UV lens 18 onto the camera sensor of the UV sensitive camera 14. The secondary UV lens 20 may be crafted from UV fused silica, may have a diameter between 20mm and 200mm (e.g., 50.8mm, 50mm), and may have elongated focal length between 30mm and 120mm (e.g., 60mm, 100mm). Advantageously, the second UV lens 20 is smaller in diameter than the primary UV lens, this reduces weight and cost. However, alternatively, the second UV lens 20 may be equal to or larger (in diameter) than the primary UV lens 18.

[0067] UV fused silica provides good transmission properties from at least 195nm to 400nm wavelengths, that is, in the ultraviolet and blue / violet visible region. Alternatively, other lens materials which provide good transmission of UV light may be used, such as, Calcium Fluoride (CaF2).

[0068] The optional lens focusing arrangement 22 comprises a stepper motor 24 connected to a threaded rod 26 at one end, and a frame 28 at the other end. The lens focusing arrangement 22 is arranged to adjust the position of the primary UV lens 18, such that the distance between the UV lenses 18, 20, in the optical axis is adjusted. The stepper motor 24 is configured to rotate the threaded rod 26. The primary UV lens 18 is coupled to the threaded rod 26 such that by turning the threaded rod, the stepper motor 24 can precisely move the primary UV lens 18, allowing the alpha particle sensing apparatus 10 to focus on signals from various distances.

[0069] The UV sensitive camera 14 is coupled to the lens system 12 in an optical line-of-sight path 25, such that UV light may be directed by the lens system 12 on to the sensor of the UV sensitive camera 14. The UV sensitive camera 14 may be a deep-cooled CCD camera (which is a highly sensitive CCD camera featuring deep cooling), or an Intensified CCD (ICCD) camera. The cooling of the deep-cooled CCD camera enhances the sensitivity of the UV sensitive camera 14 (e.g., by reducing noise), which is especially beneficial for detecting faint alpha signals amidst dominant ambient light. ICCD cameras which, when equipped with the appropriate photo cathode, lack responsiveness to infrared light which may also minimize ambient light noise. Alternatively, other suitable UV sensitive camera's may be used, e.g., an Electron multiplying CCD (EMCCD), etc..

[0070] The UV bandpass filter 16 is suitable for passing UV light at radioluminescence wavelengths induced by alpha particles for reception by the UV sensitive camera. The UV bandpass filter 16 is coupled to the UV sensitive camera in an optical line-of-sight path 25. The UV bandpass filter 16 is positioned between the lens system 12 and the UV sensitive camera 14. Alternatively, the UV bandpass filter 16 may be positioned before the lens system 12. Advantageously, the UV bandpass filter 16 provides improved blocking rate outside of the pass band and may be a narrowband filter. The UV bandpass filter may be housed in an optional filter changing system 30 which will be described with reference to Figure 2.

[0071] Figure 2 shows an isometric exploded view of the filter changing system 30. The filter changing system 30 may comprise a plurality of states, each state corresponding to arranging one of a plurality of filters positioned in the optical line-of-sight path 25. The filter changing system 30 may be formed as a filter wheel. The filter changing system 30 provides a mechanism for rapid and efficient filter interchange. Advantageously, this permits a collection of images to be taken by the UV sensitive camera 14 across various wavelengths, thereby expanding the detection range and enhancing the versatility of the alpha particle sensing apparatus 10. The filter wheel 30 is a rotatable mechanism housing two or more filters 16, 32, 34, 36, each with specific optical properties, designed to capture or block certain wavelengths of light. The filter wheel is coupled to a stepper motor 38 and an optional gearbox (e.g., a 25:1 gearbox) which is configured to rotate the filter wheel to align one of the filters 16, 32, 34, 36 in the optical line-of-sight path 25, such that the UV sensitive camera 14 may capture an image via the filter in the optical line-of-sight path 25. That is: in a first state, the UV bandpass filter 16 is in the optical line-of-sight path 25. In a second state, the filter 32 is in the optical line-of-sight path 25. In a third state, the filter 34 is in the optical line-of-sight path 25. In a fourth state, the filter 36 is in the optical line-of-sight path 25. The optional gearbox may enhance the torque of the stepper motor 38 and allowing it to move the filters inside the filter wheel with heightened accuracy. The optional gearbox may also ensure precise positioning of the filters 16, 32, 34, 36 in front of the camera sensor of the UV sensitive camera 14. The UV bandpass filter 16 is comprised of multiple optical filters as shown in Figure 2. In addition, each of the filters 16, 32, 34, 36 may be comprised of multiple optical filters.

[0072] Figure 2 shows three filters 32, 34, 36, in addition to the UV bandpass filter 16. The filter changing system 30 allows the UV sensitive camera to capture images corresponding to each filter 16, 32, 34, 36. These images are received by the processing system 17 and may be combined to produce an image of alpha-induced radio-luminescence.

[0073] Figure 3 shows an example flow diagram 40 showing how multiple images 42, 44, 46 may be combined by the processing system 17 to produce an image of alpha-induced radio-luminescence, such as an overlayed image 48.

[0074] A UV image 42 is received from the UV sensitive camera 14. The UV image 42 is generated from light of UV wavelengths corresponding to alpha-induced radioluminescence passing through the UV bandpass filter 16 to be received by the UV sensitive camera 14.

[0075] A background image 44 may be received from the UV sensitive camera 14. The background image 44 captures wavelengths in the UV electromagnetic spectrum. A background image detection means is configured to generate the background image 44. The background image 44 is an image of UV light wavelengths which are not wavelengths corresponding to alpha-induced radioluminescence. Thus, the background image 44 provides a baseline of ambient UV light. Advantageously, the use of a background image 44 improves the sensitivity of the alpha particle sensing apparatus 10.

[0076] The background image 44 may capture wavelengths which are in a band which is close to (but not overlapping any known alpha-induced radio-luminescence wavelengths) the band of wavelengths captured by the UV image 42. Advantageously, this improves the sensitivity to the alpha-induced radioluminescence wavelengths and improves the image of alpha-induced radioluminescence.

[0077] The background image detection means may be a background bandpass filter 32 arranged in the optical line-of-sight path 25 (e.g., by the filter changing system 30) with the UV sensitive camera 14. The background bandpass filter 32 may be suitable for passing UV wavelengths which are not wavelengths corresponding to alpha-induced radio-luminescence.

[0078] A UV-background image 50 is generated by subtracting 52 the background image from the UV image. Advantageously, the UV-background image 50 provides a clearer image of the alpha-induced radio-luminescence wavelengths by eliminating any undesired environmental noise patterns.

[0079] A visible-spectrum image 46 may be received from the UV sensitive camera 14. The visible-spectrum image 46 captures wavelengths in the visible electromagnetic spectrum. A visible-spectrum image detection means is configured to generate the visible-spectrum image 46. The visible-spectrum image 46 is an image of visible-spectrum light wavelengths. Thus, the visible-spectrum image 46 provides an image of the environment. Advantageously, the use of a visible-spectrum image 46 provides a location of the alpha particle source when combined with the UV image 42 or the UV-background image 50.

[0080] The visible-spectrum image detection means may be a visible-spectrum filter, such as filter 34 of Figure 2, arranged in the optical line-of-sight path 25 (e.g., by the filter changing system 30) with the UV sensitive camera 14. The visible-spectrum filter may be a visible-spectrum bandpass filter 34 and may be suitable for passing visible-spectrum wavelengths. The visible-spectrum filter 34 may pass wavelengths which are in the blue / violet visible range, e.g., lower than 500nm, such as a filter centred at 450nm with a 40 nm bandwidth. The visible-spectrum filter 34 may be comprised of a singular reflective filter centred at 450 nm with a 40 nm bandwidth and an OD4 ND filter arranged in series with each other (with their optical axis arranged in parallel). Alternatively, any suitable visible-spectrum filter 34 may be used which is suitable for capturing a visible-spectrum image.

[0081] Advantageously, the lens system 12 and / or the UV sensitive camera 14 operate more efficiently and reduced chromatic aberration with visible-spectrum light blue / violet wavelength range when compared to higher wavelength visible light, such as light in the red wavelength range.

[0082] The overlayed image 48 may be generated by overlaying the UV image 42 or the UV-background image 50 with the visible-spectrum image 46. The overlayed image 48 identifies an estimated location of alpha particle source within the visible-spectrum image based on the of alpha-induced radio-luminescence of an alpha particle source.

[0083] Figure 4a shows an example of a UV-background image 50a (alternatively it may show a UV image 42). Figure 4b shows an example of a visible-spectrum image 46a captured using a visible-spectrum bandpass filter centred at 450nm with a 40 nm bandwidth. Figure 4c shows an example of an overlayed image 48a generated by overlaying the UV-background image 50a with the visible-spectrum image 46a. Thus, the location 54 of the alpha particle source may be easily identified within the overlayed image 48a.

[0084] The accuracy of the alpha particle sensing apparatus 10 may be improved by using image processing techniques. A potential problem with strongly filtered images in the UV electromagnetic spectrum (such as the UV image 42 and / or the background image 44) captured with the UV sensitive camera 14 is shown in Figure 5a. Figure 5a shows an example UV image 42a received by the processing system 17. The example UV image 42a has some noisy artifacts, which have been found to be caused by cosmic ray detections by the UV sensitive camera 14. Figure 5b shows an example of a processed UV image 42b. To improve the quality of the example UV image 42a the processing system 17 applies a median filter to the example UV image 42a to generate the processed UV image 42b. The median filter is an image processing technique which removes noisy artifacts caused by cosmic rays. The median filter may be applied to each received UV image 42, and / or background image 44 before further processing, such as the generation of the UV-background image 50, and / or the overlayed image 48. Advantageously, applying a median filter to a UV image 42a or a background image 44 optimally reduces environmental noise and improves the signal-to-noise ratio, facilitating more precise and reliable detection of alpha emitters at long distances.

[0085] Optionally, the processing system 17 applies: (i) the median filter to an image (e.g., the UV image 42, background image 44, etc.); (ii) a clipping process to clip pixel values above a threshold (e.g., the threshold may be predetermined, or based on the 99th percentile of the pixel values of the image) to a maximum value (e.g., the maximum value may be predetermined, the maximum value may be the threshold [that is, all values above the threshold value should be truncated to the threshold value], or the maximum value may be based on the 99th percentile of the pixel values of the image); (iii) a normalization process to normalize the image (e.g., the UV image 42, background image 44, etc.). Optionally, the above process (i.e., (i) to (iii)) is applied to the UV image 42 (which may include multiple UV images) and the background image 44. Optionally, the processing system 17 further subtracts the processed background image from processed the UV image to generate the UV-background image 50.

[0086] Figure 6 shows an example of a known reflective-reflective filter arrangement 56 consisting of two reflective bandpass filters 58, 59 arranged in series (i.e., positioned sequentially in an optical line-of-sight path) and with their optical axes in parallel or are coaxial. Reflective bandpass filters can be manufactured with a narrow bandwidth and a high blocking rate, i.e., each bandpass filter 58, 59 may be designed to block light outside of their pass band with a rate of 10T Therefore, reflective bandpass filters 58, 59 are well suited to primarily obstruct ambient light.

[0087] Reflective filters are one of two types of optical filters, the other being absorptive filters. Absorptive filters are not generally used for precision applications (such as radio-luminescence detectors) due to their generally larger bandwidths, shallow slopes, and material limitations (such as low blocking ability).

[0088] However, the inventors found that when combined in the reflective-reflective filter arrangement 56, unwanted light (i.e., light of wavelengths not in the pass band of the bandpass filter) that passes through the first bandpass filter 58 may be reflected by the second bandpass filter 59. This causes light to "bounce" or oscillate between the first and second bandpass filters 58, 59. That is, the reflective-reflective filter arrangement 56 cannot achieve a blocking rate which is cumulative of the first and second reflective-reflective filter arrangement 56. For example, the overall blocking rate of the reflective-reflective filter arrangement 56 only reaches around 0.5 * 10-4, if each bandpass filter 58, 59 has a blocking rate of io-4. This blocking efficiency is not enough to adequately detect alpha particle sources by receiving light generated by alpha-induced radio-luminescence in most practical conditions.

[0089] Figures 7 and 8 show examples of high-blocking filter arrangements 60, 64 which may achieve a blocking rate of 10-10 which may effectively eliminate ambient light to isolate a faint alpha-induced radio-luminescence signal for detection by the alpha particle sensing apparatus 10. The filter arrangements 60, 64 each comprise two or more bandpass filters with overlapping pass bands and so each filter arrangement 60, 64 may simply be described as an improved optical bandpass filter.

[0090] Figure 7 shows an example of a reflective-absorptive-reflective filter arrangement 60. The reflective-absorptive-reflective filter arrangement 60 comprises two reflective bandpass filters 58, 59, and an absorptive bandpass filter 62. The absorptive bandpass filter 62 is arranged between each of the two reflective bandpass filters 58, 59. All bandpass filters 58, 59, 62 are arranged in series (i.e., positioned sequentially in an optical line-of-sight path) and the optical axis of each are parallel with respect to each other or are coaxial. The absorptive filter 62 is strategically placed between the two reflective bandpass filters 58, 59 to provide an added barrier against unwanted light (i.e., light of a wavelength outside of the pass band of the filters 58, 59, 62). This strategic placement ensures that unwanted photons which pass through the first bandpass filter 58, which could potentially be reflected by the second bandpass filter 59, may be absorbed by the absorptive filter 62. As such, the reflective-absorptive-reflective filter arrangement 60 may achieve a blocking rate of 10-10.

[0091] Figure 8 shows an example of an angled reflective filter arrangement 64 comprising at least a first and second reflective bandpass filters 58, 59 arranged in series (i.e., positioned sequentially in an optical line-of-sight path), wherein the first reflective filter 58 is skew with respect to the second reflective bandpass filter 59. That is, as shown in Figure 8 the first reflective bandpass filter 58 has a first optical axis represented by arrow 68 and the second reflective filter has a second optical axis represented by arrow 69, wherein the angles % between the first and second optical axes (i.e., arrows 68, 69) are different, i.e., non-parallel, or the filters 58, 59 are tilted with respect to each other. By angling the filters 58, 59, light that might reflect between them is directed off-path. After a few reflections, this stray light may be absorbed by a lens tube surrounding the filters 58, 59. This angled approach effectively minimizes the light-bunching effect observed in the example of Figure 6. A suitable angle % between the first and second optical axes (i.e., arrows 68, 69) may be 10 degrees, however, other suitable angles may be 5 degrees, 8 degrees, 12 degrees, 15 degrees, or 20 degrees. It is possible to further improve the blocking rate of an angled reflective filter arrangement by arranging multiple reflective bandpass filters in series (i.e., positioned sequentially in an optical line-of-sight path) each skew with respect to a neighbouring reflective bandpass filter.

[0092] Figure 9 shows two graphs: an emission spectrum graph 70 of the relative intensity of alpha-induced radio-luminescence (at approximately 276nm or under approximately 280nm) of air on the y-axis with wavelength (2) on the x-axis; and a system graph 72 of quantum efficiency (QE) of cameras, and transmittance of filters, as a percentage on the y-axis with wavelength (A) on the x-axis.

[0093] The system graph 72 shows: • at line 73, the QE of a CsTe Photoncathode ICCD camera; • at line 74, the QE of a deep-cooled CCD camera; • at line 75, the transmittance of a reflective 276nm bandpass filter with a 40nm bandwidth from 240nm to 280nm; • at line 76, the transmittance of a reflective 337nm bandpass filter with a lOnm bandwidth from 332nm to 342nm; • at line 77, the transmittance of a reflective 343nm bandpass filter with a 5nm bandwidth from 340.5nm to 345.5nm; • at line 78, the transmittance of a reflective 355nm bandpass filter with a lOnm bandwidth from 350nm to 360nm; and, • at line 79, the transmittance of a reflective 450nm bandpass filter with a 40nm bandwidth from 430nm to 470nm.

[0094] As shown in Figure 9, an alpha particle source can excite air molecules (which may include nitrogen monoxide) to emit photons with energies corresponding to the emission spectrum graph 70. Therefore, the alpha particle sensing apparatus 10 may be configured to detect these photons using appropriate filters. For example, the UV bandpass filter (e.g., shown by line 75 of system graph 72) passes light corresponding to fluorescence emissions of air (such as the 276nm fluorescence emission) for reception by the UV sensitive camera 14, such as a deep-cooled CCD camera (e.g., shown by line 74 of system graph 72) or an ICCD camera (e.g., shown by line 73 of system graph 72).

[0095] Figure 10 shows an example of an alpha particle sensing apparatus 10a suitable for obtaining images of alpha-induced radio-luminescence in a sunlit environment. The alpha particle sensing apparatus 10a shows many of the features of the alpha particle sensing apparatus 10 of Figure 1, in addition to certain optional features. The same reference numerals are used to denote the same / corresponding features in relation to Figure 1 and will not be described in detail again below.

[0096] When contending with direct sunlight, the intense UV background light may bleed through filters with strong blocking ability and obscure any alpha-induced radio-luminescence. However, since sunlight below 280nm wavelength is absorbed atmospherically, a UV bandpass filter 16a with a pass band below 280nm is suitable for counteracting the sunlight background while being sensitive to alpha-induced radio-luminescence.

[0097] A suitable UV bandpass filter 16a for operating in a sunlight condition (i.e., alpha particle sensing with sunlight present in the environment) is the angled reflective filter arrangement 64 of Figure 8. Alternatively, if known, an appropriate absorptive filter may be used in a reflective-absorptive-reflective filter arrangement 60 of Figure 7 may be used.

[0098] The UV bandpass filter 16a in the angled reflective filter arrangement 64 ensures that light potentially reflecting between the filters is side-tracked, and after minimal reflections, may be absorbed by the lens tube 81. Advantageously, this curtails the light-bunching effect of conventional arrangements.

[0099] As described with reference to Figure 8 the UV bandpass filter 16a comprises multiple reflective filters arranged in series, wherein each reflective filter is skew with respect to its neighbouring reflective filters (e.g., so neither two are parallel to each other). Specifically, the UV bandpass filter 16a comprises four reflective filters 82a, 82b, 82c, 82d. The reflective filters 82a an 82b have a 10° angle between them. The reflective filters 82b an 82c have a 15° angle between them. The reflective filters 82c an 82d have a 10° angle between them. The reflective bandpass filters of the 82a, 82b, 82c, 82d of the UV bandpass filter 16a may each be the same filter, e.g., a 276nm bandpass filter with a short bandwidth (so no exact bandwidth, although the bandwidth may be around 40 nm from a datasheet transmission curve). Optionally, additional lenses 80a, 80b may be present in the alpha particle sensing apparatus 10a before and after the UV bandpass filter 16a. Advantageously, the additional lenses 80a, 80b allow a greater distance between the secondary lens 20 and the camera 14 to compensate for the thickness of the UV bandpass filter 16a.

[0100] The UV sensitive camera 14 may be any suitable camera, e.g., an ICCD camera (e.g., equipped with a CsTe photon cathode sensitive in the 200nm-300nm wavelength range), or an EMCCD camera. However, a deep-cooled CCD camera is less susceptible to noise and provides good sensitivity. Therefore, in a sunlight condition, the UV sensitive camera 14 may be a deep-cooled CCD camera. The UV sensitive camera may use commercial-off-the-shelf (COTS) solar blind technology. For example, using a CCD camera COTS solar blind technology, the alpha particle sensing apparatus 10a may detect a 370 MBq Po-210 source at 30 meters in 8 minutes. The alpha particle sensing apparatus 10a may be capable of capturing mega-pixel resolution images, enabling the precise location of alpha sources. Alternatively, pixels may be combined to achieve an even lower detection limit, albeit at the expense of reduced resolution.

[0101] Figure 11 shows an example flow diagram 84 showing how multiple images 42a, 44a, 46a may be combined by the processing system 17 to produce an image of alpha-induced radio-luminescence, such as an overlayed image 48a. The flow diagram 84 shows many of the features of the flow diagram 40 of Figure 3, in addition to certain optional features. The same reference numerals are used to denote the same / corresponding features in relation to Figure 3 and will not be described in detail again below.

[0102] A UV image 42a is received from the UV sensitive camera 14. The UV image 42a is generated from light of UV wavelengths corresponding to alpha-induced radio-luminescence passing through the UV bandpass filter 16a to be received by the UV sensitive camera 14, such as a deep-cooled CCD camera. Processing system 17 receives the UV image 42a and applies a median filter 86 to the UV image 42a to generate a processed UV image.

[0103] A background image 44a may be received from the UV sensitive camera 14. Although, the atmosphere may provide suitable blocking of the majority of background sunlight photons, as such the capturing of a background image 44a and subsequent background subtraction for 276 nm system may not be necessary. The background image 44a is generated from light of UV wavelengths which are not wavelengths corresponding to alpha-induced radio-luminescence passing through a suitable background bandpass filter (which may be different to the background). The background bandpass filter may be formed from a reflective-absorptive-reflective filter arrangement 60 of Figure 7. That is, the background bandpass filter may comprise a UG11 absorptive filter (Schott™ part number) arranged between two reflective 343nm bandpass filters. The two reflective 343nm bandpass filters may have a 5nm bandwidth respectively. A UG11 absorptive filter may have a bandpass ranging from 275nm to 375nm. Alternatively, the background bandpass filter may be any suitable filter for filtering UV wavelengths which are not wavelengths corresponding to wavelengths of alpha-induced radio-luminescence. Processing system 17 receives the background image 44a and applies a median filter 86 to the background image 44a to generate a processed background image.

[0104] A UV-background image 50a is generated by subtracting 52 the processed background image from the processed UV image. Advantageously, the UV-background image 50a provides an improved image of the alpha-induced radioluminescence wavelengths from air.

[0105] The visible-spectrum image 46a may be received from the UV sensitive camera 14. The visible-spectrum image 46a is generated from light of visible-spectrum wavelengths passing through the visible-spectrum bandpass filter 34 to be received by the UV sensitive camera 14, such as a deep-cooled CCD camera. The visible-spectrum bandpass filter 34 may be formed from a single reflective 450nm bandpass filter with a 40nm bandwidth and a OD4 neutral density (ND) filter. Advantageously, the 0D4 ND reduces overall intensity of light by 10-4 of its original intensity and may be used to prevent the UV camera from easily saturating an image. Alternatively, the visible-spectrum filter may be any suitable filter for capturing visible-spectrum images. Alternatively, no visible-spectrum bandpass filter may be used if the camera is already sensitive to visible-spectrum light.

[0106] The overlayed image 48a is generated by overlaying the UV-background image 50a with the visible-spectrum image 46a.

[0107] Figure 12 shows two graphs: an emission spectrum graph 90 of the relative intensity of alpha-induced radio-luminescence of nitrogen (N2 molecule) on the y-axis with wavelength (2) on the x-axis; and the system graph 72 of quantum efficiency (QE) of cameras and transmittance of filters as a percentage on the y-axis with wavelength (2) on the x-axis.

[0108] As shown in Figure 12, an alpha particle source can excite nitrogen (N2) molecules in the air to emit photons with energies corresponding to the emission spectrum graph 90. Therefore, the alpha particle sensing apparatus 10 may be configured to detect these photons using appropriate filters. For example, the UV bandpass filter (e.g., shown by lines 76 and 78 of system graph 72) passes light corresponding to fluorescence emissions of nitrogen (such as the 337nm and 355nm fluorescence emission) for reception by the UV sensitive camera 14, such as a deep-cooled CCD camera (e.g., shown by line 74 of system graph 72) or an ICCD camera (e.g., shown by line 73 of system graph 72).

[0109] Figure 13 shows an example of an alpha particle sensing apparatus 10b suitable for obtaining images of alpha-induced radio-luminescence in an environment in the absence of sunlight (e.g., suitable for indoor and night-time imaging). The alpha particle sensing apparatus 10b shows many of the features of the alpha particle sensing apparatus 10 of Figure 1, in addition to certain optional features. The same reference numerals are used to denote the same / corresponding features in relation to Figure 1 and will not be described in detail again below.

[0110] In the absence of sunlight, it is possible to use the filter arrangements of Figures 7 and / or 8 to detect the alpha-induced radio-luminescence from nitrogen and / or air in the presence of an alpha particle source. UV bandpass filters with a pass band corresponding to the 337nm or 357nm fluorescence emission of nitrogen are suitable (i.e., corresponding to the strongest signals from alpha-induced fluorescence of nitrogen). It is noted that light associated with the alpha fluorescence emission of air under 280nm is approximately 1% of the light associated with the total alpha radio-luminescence emission of air. Thus, in an environment in the absence of sunlight it is practical to configure the alpha particle sensing apparatus to detect the alpha-induced radio-luminescence from nitrogen.

[0111] A suitable UV bandpass filter 16b for operating in an environment in the absence of sunlight (i.e., alpha particle sensing in the absence of sunlight present in the environment) is the reflective-absorptive-reflective filter arrangement 60 of Figure 7. Alternatively or in addition, an angled reflective filter arrangement 64 of Figure 8 may be used.

[0112] The UV bandpass filter 16b in the reflective-absorptive-reflective filter arrangement 60 advantageously curtails the light-bunching effect of conventional arrangements, e.g., as shown in Figure 6. The UV bandpass filter 16b passes light corresponding to the 337nm or 357nm fluorescence emission of nitrogen for reception by the UV sensitive camera 14.

[0113] As described with reference to Figure 7 the UV bandpass filter 16b comprises multiple reflective filters arranged in series, and may comprise a UG11 absorptive filter arranged between two reflective 337nm or 355nm bandpass filters. The two reflective 337nm or 355nm bandpass filters may comprise a lOnm bandwidth respectively. Alternatively, the UV bandpass filter 16b may be any suitable filter for filtering UV wavelengths which are wavelengths corresponding to alpha-induced radio-luminescence.

[0114] The UV sensitive camera 14 may be any suitable camera. However, a deecooled CCD camera is particularly suitable for use in an environment in the absence of sunlight. The cooling enhances the sensitivity of the UV sensitive camera 14.

[0115] Figure 14 shows an example flow diagram 92 showing how multiple images 42c, 42d, 44a, 46a may be combined by the processing system 17 to produce an image of alpha-induced radio-luminescence, such as an overlayed image 48b. The flow diagram 92 shows many of the features of the flow diagram 40 of Figure 3 and / or the flow diagram 84 of Figure 11, in addition to certain optional features. The same reference numerals are used to denote the same / corresponding features in relation to Figure 3 and / or Figure 11 and will not be described in detail again below.

[0116] Two UV images 42c, 42d are received from the UV sensitive camera 14. The UV images 42c, 42d are generated from light of UV wavelengths corresponding to alpha-induced radio-luminescence passing through a first UV bandpass filter 16b and a second UV bandpass filter 36 respectively, to be received by the UV sensitive camera 14, such as a deep-cooled CCD camera, at first and second times. Processing system 17 receives the UV images 42c, 42d and applies a median filter 86 to each of the UV images 42c, 42d to generate respective first and second processed UV images. The first and second processed UV images are added 94 to each other (or otherwise amalgamated) to generate a combined UV image 42e. The first UV bandpass filter 16b may be formed from a reflective-absorptive-reflective filter arrangement 60 of Figure 7. That is, the first UV bandpass filter 16b may comprise a UG11 absorptive filter arranged between two reflective 337nm bandpass filters (with a lOnm bandwidth respectively). The second UV bandpass filter 36 may be formed from a reflective-absorptive-reflective filter arrangement 60 of Figure 7. That is, the second UV bandpass filter 36 may comprise a UG11 absorptive filter arranged between two reflective 355nm bandpass filters (with a lOnm bandwidth respectively).

[0117] A UV-background image 50b is generated by subtracting 52 the processed background image from the processed combined UV image 42e. Advantageously, the UV-background image 50b provides an improved image of the alpha-induced radio-luminescence wavelengths from nitrogen.

[0118] The overlayed image 48b is generated by overlaying the UV-background image 50b with the visible-spectrum image 46a.

[0119] Figure 15 shows a flow diagram of an example method 100 of obtaining an image of alpha-induced radio-luminescence using the alpha particle sensing apparatus 10. The method may be computer implemented, or may be stored as instructions on a non-volatile memory. If the UV sensitive camera 14 is a deep-cooled CCD camera, then the first step 102 is to cool the UV sensitive camera to 100-110 degrees Celsius less than the ambient air temperature (i.e., deep cooled). For example, if the air temperature is 20 degrees Celsius, the sensor of the UV sensitive camera may be -80 degree Celsius. The method 100 may comprise the following steps:

[0120] Step 104: determine if the alpha particle sensing apparatus 10 is to be operated in an environment with sunlight. This may be predetermined or may be determined with a sensor.

[0121] Step 106: arrange a UV bandpass filter 16a for passing UV light at radioluminescence wavelengths induced from alpha particles for reception by the UV sensitive camera (e.g., via a filter changing system 30). Capture a UV image with the UV sensitive camera 14 based on the light passed through the UV bandpass filter 16a. Optionally, apply a median filter to the UV image and / or other processing to generate a processed UV image.

[0122] Optional step 108: arrange a background bandpass filter 32 for passing UV light which are not wavelengths corresponding to alpha-induced radioluminescence for reception by the UV sensitive camera 14 (e.g., via a filter changing system 30). Capture a background image with the UV sensitive camera 14 based on the light passed through the background bandpass filter 32. Optionally, apply a median filter to the background image and / or other processing to generate a processed background image.

[0123] Step 110: arrange a visible-spectrum bandpass filter 34 for passing UV light which are in the visible electromagnetic spectrum for reception by the UV sensitive camera 14 (e.g., via a filter changing system 30). Capture a visible-spectrum image with the UV sensitive camera 14 based on the light passed through the visible-spectrum bandpass filter.

[0124] Optional step 112: generate an air UV-background image by subtracting the processed background image from the processed UV image. The wavelength of the captured air UV-background image may correspond to wavelengths under 280nm (e.g., at approximately 276nm). Generate an image of alpha-induced radioluminescence by overlaying the visible-spectrum image and the air UV-background image.

[0125] Step 114: arrange a first UV bandpass filter 16b for passing UV light at radio-luminescence wavelengths corresponding to the 337nm fluorescence emission of nitrogen induced from alpha particles for reception by the UV sensitive camera 14 (e.g., via a filter changing system 30). Capture a first UV image with the UV sensitive camera 14 based on the light passed through the first UV bandpass filter 16b. Optionally, apply a median filter to the first UV image and / or other processing to generate a first processed UV image.

[0126] Step 116: arrange a second UV bandpass filter 36 for passing UV light at radio-luminescence wavelengths corresponding to the 357nm fluorescence emission of nitrogen induced from alpha particles for reception by the UV sensitive camera 14 (e.g., via a filter changing system 30). Capture a second UV image with the UV sensitive camera 14 based on the light passed through the second UV bandpass filter 36. Optionally, apply a median filter to the second UV image and / or other processing to generate a second processed UV image. Alternatively, the second UV bandpass filter 36 may arranged to pass UV light at radio-luminescence wavelengths corresponding to 315nm, 353nm, 375nm, or a wider bandpass filter which include all the emissions between 300nm-400nm.

[0127] Step 118: arrange a background bandpass filter 32 for passing UV light which are not wavelengths corresponding to alpha-induced radio-luminescence for reception by the UV sensitive camera 14 (e.g., via a filter changing system 30). Capture a background image with the UV sensitive camera 14 based on the light passed through the background bandpass filter 32. Optionally, apply a median filter to the background image and / or other processing to generate a processed background image.

[0128] Step 120: arrange a visible-spectrum bandpass filter 34 for passing UV light which are in the visible electromagnetic spectrum for reception by the UV sensitive camera 14 (e.g., via a filter changing system 30). Capture a visible-spectrum image with the UV sensitive camera 14 based on the light passed through the visible-spectrum bandpass filter.

[0129] Step 122: generate a nitrogen UV-background image by: (i) adding the processed first image to the processed second UV image to; and (ii) subtracting the processed background image. Generate an image of alpha-induced radioluminescence of nitrogen by overlaying the visible-spectrum image and the nitrogen UV-background image.

[0130] Although specific arrangements have been described above with the Figures, the alpha particle sensing apparatus in embodiments may comprise a lens system 12 suitable for use with both lens arrangements of Figures 7 and / or 8. That is, to obtain images (in a sunlight condition and in the absence of a sunlight condition.) a different state of the filter changing system 30 may be selected. Alternatively, a lens changing system may be implemented to change between suitable lenses for each image capture, or it may be done manually by an operator. Alternatively, two distinct alpha particle sensing apparatus' may be used, one configured for obtaining images of alpha-induced radio-luminescence in a sunlight condition and one configured for obtaining images of alpha-induced radioluminescence in the absence of a sunlight condition.

[0131] The optical system 12 may be a lens system as described above and may be any suitable lens system. However, the optical system 12 may be an arrangement of one or more mirrors (i.e., a reflector system) and / or lenses. Specifically, mirrors are often used in UV systems and may improve UV light throughput. The optical system 12 may comprise a reflecting telescope (e.g., Cassegrain or similar). Additionally or alternatively, the optical system 12 may comprise diffractive optics to achieve suitable directing of UV light.

[0132] The processing system 17 may be any suitable processing system and may include one or more of the following: computer, a data processor, a controller, a microcontroller, a CPU, etc.

[0133] The background image detection means may comprise a first alternative camera system. The processing system 17 may also receive a known offset corresponding to the offset between the point of views of the UV sensitive camera 14 and the first alternative camera system. The UV-background image 50 may be generated based on the known offset.

[0134] The visible-spectrum image detection means may comprise a second alternative camera system. The processing system 17 may also receive a known offset corresponding to the offset between the point of views of the UV sensitive camera 14 and the second alternative camera system. The overlayed image 48 may be generated based on the known offset.

[0135] The optical axis may, when used in reference to a lens, be a line along which there is some degree of rotational symmetry in an optical system such as a camera lens. The optical axis may, when used in reference to a filter, be any line perpendicular to the surface of the filter, e.g., the optical axis may be the optical line of sight, or parallel to the optical line of sight, between the lens system 22 and the UV sensitive camera 14.

[0136] In an example, an ICCD camera may comprise an CsTe Photocathode, although the image intensifier may comprise alternative materials. Beneficially, CsTe Photocathode technology can be described as 'solar blind'. Different materials have different sensitivity to different wavelength.

[0137] The UG11 absorptive filter may be a u340 absorptive filter or a ZWB1 absorptive filter, or more generally any suitable absorptive filter which may has suitable passband wavelengths and achieves a blocking rate of approximately io-4 (or greater) outside of its passband.

[0138] A deep-cooled UV sensitive camera may be defined as a UV sensitive camera cooled to 100-110 degrees Celsius less than the ambient air temperature. For example, if the air temperature is 20 degrees Celsius, the sensor of the UV sensitive camera may be -80 degree Celsius

[0139] Where the word 'or' appears this is to be construed to mean 'and / or' such that items referred to are not necessarily mutually exclusive and may be used in any appropriate combination.

[0140] Although the invention has been described above with reference to one or more preferred embodiments, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims.

Claims

1. An alpha particle sensing apparatus for obtaining images of alpha-induced radioluminescence, the apparatus comprising:an optical system for directing ultraviolet, UV, light;a UV sensitive camera coupled to the optical system in an optical line-of-sight path;a UV bandpass filter for passing UV light at radio-luminescence wavelengths induced from alpha particles for reception by the UV sensitive camera, wherein the UV bandpass filter is coupled to the UV sensitive camera in an optical line-of-sight path; and,a processing system configured to receive a UV image from the UV sensitive camera based on UV light passed through the UV bandpass filter,wherein the UV bandpass filter comprises at least one of:an angled reflective filter arrangement comprising at least a first and second reflective filters arranged in series, wherein the first reflective filter is skew with respect to the second reflective filter; ora reflective-absorptive-reflective filter arrangement comprising two reflective filters, and an absorptive filter, wherein the absorptive filter is arranged between each of the two reflective filters.

2. The alpha particle sensing apparatus of claim 1, further comprising:a background image detection means configured to generate a background image in UV electromagnetic spectrum; andthe processing system further configured to subtract the background image from the UV image to generate an UV-background image.

3. The alpha particle sensing apparatus of claim 2, wherein the background image detection means is a background bandpass filter for passing UV wavelengths which are not wavelengths corresponding to alpha-induced radio-luminescence, the background bandpass filter comprising at least one of:an angled reflective filter arrangement comprising at least a first and second reflective filters arranged in series, wherein the first reflective filter is skew with respect to the second reflective filter; ora reflective-absorptive-reflective filter arrangement comprising two reflective filters, and an absorptive filter, wherein the absorptive filter is arranged between each of the two reflective filters.

4. The alpha particle sensing apparatus of claim 3, wherein the background bandpass filter passes light corresponding to the wavelengths between the 337nm and 357nm fluorescence emission of nitrogen for reception by the UV sensitive camera.

5. The alpha particle sensing apparatus of any preceding claim, further comprising: a visible-spectrum image detection means configured to generate a visible-spectrum image in the visible electromagnetic spectrum; andthe processing system further configured to determine an estimated location of alpha particles based on: the UV image or the UV-background image; and, the visible-spectrum image.

6. The alpha particle sensing apparatus of claim 5, wherein the visible-spectrum image detection means is a visible-spectrum filter for passing visible light wavelengths via the optical system.

7. The alpha particle sensing apparatus of any of claims 5 or 6, wherein the processing system further configured to overlay the UV image or the UV-background image with the visible-spectrum image to generate an overlayed image which identifies an estimated location of alpha particles within the visible-spectrum image.

8. The alpha particle sensing apparatus of any preceding claim, wherein the apparatus further comprising a filter changing system configured to change between a two or more states, wherein each state corresponds to a respective bandpass filter being in the optical line-of-sight path, wherein each respective bandpass filter is selected from one of: the UV bandpass filter; the background bandpass filter; and the visible-spectrum filter.

9. The alpha particle sensing apparatus of claim 8, wherein the processing system is configured to receive from the UV sensitive camera a series of images, each of the images corresponding to a respective state of the filter changing system.

10. The alpha particle sensing apparatus of any preceding claim, wherein the UV bandpass filter is the reflective-absorptive-reflective filter arrangement, wherein the UV bandpass filter passes light corresponding to the 337nm or 357nm fluorescence emission of nitrogen for reception by the UV sensitive camera.

11. The alpha particle sensing apparatus of claim 10, wherein the UV bandpass filter is a first UV bandpass filter and passes light corresponding to the 337nm fluorescence emission of nitrogen for reception by the UV sensitive camera, and the apparatus further comprises a second UV bandpass filter comprising a second reflective-absorptive-reflective filter arrangement comprising two reflective filters, and an absorptive filter, wherein the absorptive filter is arranged between each of the two reflective filters, wherein the second UV bandpass filter passes light corresponding to the 357nm fluorescence emission of nitrogen for reception by the UV sensitive camera.

12. The alpha particle sensing apparatus of claim 11, wherein the UV image is based on: a first UV image based on UV light passed through the first UV bandpass filter and received by the UV sensitive camera; and, a second UV image based on UV light passed through the second UV bandpass filter and received by the UV sensitive camera.

13. The alpha particle sensing apparatus of any of claims 1 to 9, wherein the UV bandpass filter is the angled reflective filter arrangement, wherein the UV bandpass filter passes light corresponding to the 276nm fluorescence emission of air for reception by the UV sensitive camera.

14. The alpha particle sensing apparatus of any preceding claim, wherein the UV sensitive camera is a deep cooled CCD camera.

15. The alpha particle sensing apparatus of any of claims 1 to 13, wherein the UV sensitive camera is an Intensified CCD camera.

16. The alpha particle sensing apparatus of any preceding claims, wherein the processing system is further configured to applying a median filter to at least one of: the UV image, the first UV image, the second UV image, the background image, and the visible-spectrum image, from the UV sensitive camera.

17. A Method of obtaining an image of alpha-induced radio-luminescence using the alpha particle sensing apparatus of any preceding claim, the method comprising:arranging the UV bandpass filter for passing UV light at radio-luminescence wavelengths induced from alpha particles for reception by the UV sensitive camera; and,capturing a UV image with the UV sensitive camera based on the light passed through the UV bandpass filter.

18. The method of claim 17, further comprising:applying a median filter to the UV image; and,generating an image of alpha-induced radio-luminescence based on the median filtered UV image.

19. The method of any of claims 17 or 18, further comprising:arranging a background bandpass filter for passing UV light which are not wavelengths corresponding to alpha-induced radio-luminescence for reception by the UV sensitive camera;capturing a background image with the UV sensitive camera based on the light passed through the background bandpass filter; and,generating an / the image of alpha-induced radio-luminescence based on: the UV image or the median filtered UV image; and, the background image.

20. The method of claim 19, further comprising:applying a median filter to the background image; and,generating the image of alpha-induced radio-luminescence based on the median filtered background image.

21. The method of any of claims 17 to 20, further comprising:arranging a visible-spectrum filter for passing light in the visible electromagnetic spectrum for reception by the UV sensitive camera;capturing a visible-spectrum image with the UV sensitive camera based on the light passed through the visible-spectrum filter; and,generating an / the image of alpha-induced radio-luminescence based on the UV image and the visible-spectrum image.

22. The method of claim 21, further comprising:generating a UV-background image by subtracting the background image or the median filtered background image from the UV image or the median filtered UV image; and,generating the image of alpha-induced radio-luminescence by overlaying the visible-spectrum image and the UV-background image.

23. The method of any of claims 17 to 21, wherein the UV bandpass filter is a first UV bandpass filter and passes light corresponding to the 337nm fluorescence emission of nitrogen, the UV image is a first UV image, and the method further comprising:arranging a second UV bandpass filter which passes light corresponding to the 357nm fluorescence emission of nitrogen for reception by the UV sensitive camera;capturing a second UV image with the UV sensitive camera based on the light passed through the second UV bandpass filter; andgenerating the image of alpha-induced radio-luminescence based on the first UV image and the second UV image.

24. The method of claim 23, further comprising:applying a median filter to the second UV image; and, generating the image of alpha-induced radio-luminescence based on the median filtered second UV image.

25. The method of any of claims 23 or 24, further comprising:generating a nitrogen UV-background image by:adding the first UV image or the median filtered first image, to, the second UV image or the median filtered second UV image; andsubtracting the background image or the median filtered background image; andgenerating the image of alpha-induced radio-luminescence by overlaying the visible-spectrum image and the nitrogen UV-background image.

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