Device and method

Abstract

Calibrating a gamma radiation detection and measurement device comprising a scintillator module, optical transducer module and a comparator for comparing the output of the optical transducer module wi

Description

The present invention relates to a gamma radiation detection and measurement device, and associated methods, especially for use in measuring the levels of gamma radiation downhole in a subsurface formation. The invention also relates to the testing and calibration of a gammaradiation detection and measurement device. A gamma radiation detection and measurement device is an instrument for measuring the intensity (and / or spectrum) of gamma radiation. One application of such a device is downhole in mining (e.g. geothermal exploration, oil or gas extraction, etc), as layers of strata with different rock compositions will emit different levels of gamma radiation. For example, a gamma radiation detection and measurement device can be used to aid with the assessment of hydrocarbon potential in a formation. The main components of a typical gamma radiation detection and measurement device used in downhole applications are sensitive to mechanical shock and vibrations which can result in costly tool failures. A scintillation crystal is typically supplied in a hermetically sealed housing due to the hydroscopic nature thereof. The crystals may be protected using a combination of mechanical springs and compliant materials surrounding the crystal in order to reduce shock levels and prevent fracture. PMTs are typically manufactured as glass vacuum tubes with sensitive internal components, sometimes packaged using compliant materials in an attempt to mitigate damage caused by shock and vibration. The effectiveness of these measures is somewhat limited for a variety of reasons and there is very little maintenance or inspection possible to determine if the internal packaging has degraded. Manufacturers of downhole gamma radiation detection and measurement devices that use these modules therefore need to incorporate further shock and vibration absorbing methods to reduce the likelihood of downhole failures. Whilst several of the sources of downhole vibration are largely periodic, e.g. rotary drilling, drilling motor and / or pump induced noise, bearing noise, roller cone bit noise, bit precession etc; and could potentially be reduced using a tuned active absorber, the potential range of frequencies is too large to make this a practical proposition. Many of the damaging shocks or transient periods of severe vibration are stochastic in nature and due to changes in drilling parameter dynamics potentially caused by variations in rock formation, sub-optimal bottom hole assembly design or poor procedures e.g. bit bounce, stick-slip, whirling etc, the inventors of the present invention consider the most practical method of protecting the delicate components is through passive isolation. A gamma ray, also known as gamma radiation, is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the short wavelength electromagnetic waves, having a frequency of above 30 exahertz (3xl019 Hz). Gamma rays are similar to x-rays but come from different parts of the atom. In particular, gamma rays originate inside the nucleus of an atom. In contrast, X-rays are emitted from processes outside the nucleus. X-Rays are also lower in energy than gamma rays and, therefore, less penetrating than gamma rays. Because the energy of each photon of gamma radiation is proportional to its frequency, gamma rays have sufficient energy that they are typically observed by counting individual photons. The detection of a number of individual photons requires very sensitive and precise components (e.g. photomultiplier tube) and electronics, and can be easily distorted in many ways. As a result, detectors used for this purpose must be of high accuracy, in order to resolve any significant counts above the background radiation. For this reason, the preferred choice of detector is one using a scintillation counter. The scintillator is used to convert the interacting gamma photons to lower-energy photons. Preferably the lower-energy photons are in the ultraviolet, visible, and near-infrared part of the Electromagnetic (EM) spectrum - i.e. EM radiation having a wavelength of between 100 pm - 10 nm. A Photomultiplier Tube (PMT) is then used to convert the resulting visible light to an electrical signal. Once an electrical signal has been obtained, the signal can be processed e.g. using an electronic pre-amplifier and any suitable processing electronics. When the scintillator converts interacting gamma photons to a lower-energy photon output, the number of resulting lower-energy photons is proportional to the gamma radiation received by the scintillator. The scintillator is typically optically coupled to a photomultiplier tube (PMT). A PMT is a detector of light in the ultraviolet, visible, and near-infrared ranges of the electromagnetic spectrum. They are members of the class of vacuum tubes, more specifically vacuum phototubes. These detectors convert the photons of light output by the scintillator into an electrical pulse and multiply the electrical pulse by as much as 160 dB, enabling, for example, individual photons to be detected when the incident flux of light is low. In particular, the light output from the scintillator interacts with a photocathode within the PMT to emit a small amount of electrons. With most PMTs, the pulse output magnitude is extremely small and very fast (nS). The electrons are then multiplied across a number of dynodes, biased with successively higher voltage at each dynode stage. This results in the PMT producing an electrical pulse with amplitude proportional to the interacting gamma photon energy. The output electrical pulse from the PMT is then passed through a DC 'blocking' capacitor to remove DC bias. The DC capacitor is coupled to a preamplifier for amplification, in a positive high-voltage (HV) biased system. For a negative HV biased system, the blocking capacitor may optionally be eliminated and the output signal from the PMT can be directly coupled to the electronics preamplification, due to the lack of a large DC voltage. For a reliable and meaningful measurement of downhole gamma radiation emissions, in use, a number of the PMT characteristics need to be taken into account. For example, the PMT's gain is a function of voltage between the dynodes of the PMT, temperature, and PMT 'ageing'. As a result, gain can be increased by increasing voltage. Further, gain can be reduced by increasing the temperature of the PMT. In addition, it is difficult to manufacture two PMTs with identical gain characteristics. As a result, there are typically significant variations of gain characteristics across a number of PMTs of the same model from the same manufacturer. A second important characteristic of a PMT is its noise edge. The noise edge of a PMT is the background noise generated by the PMT. This background noise can be considered as an effective noise floor of the device. If this noise floor is higher than the amplitude of the signal resulting from gamma radiation, then it is no longer possible to distinguish the signal from noise. The noise edge increases in amplitude as a function of temperature. Again, the characteristics of each PMT may be somewhat different between individual PMTs and types. Also, as the gain is increased the noise floor is also increased. Thus, the noise edge defines the minimum amplitude of a pulse threshold generated in response to gamma radiation that can be reliably detected at the output of the PMT. The electronics required to bias the PMT and process the output typically comprises a low-power High Voltage Power Supply, charge preamplifier, and either a pulse discriminator or threshold comparator (referred to herein as a comparator). The high voltage acts to bias a resistor ladder network of the PMT. The resistor ladder network is arranged across the dynodes of the PMT. After amplification of the pulse by the charge amplifier, a pulse threshold voltage is typically applied to the comparator to discriminate between a pulse from the PMT that has been triggered by light output by the scintillator in response to gamma radiation (a gamma pulse) and lower energy pulses from the PMT caused by noise (noise pulses). Correct calibration maximises the percentage of gamma pulses that pass through to the pulse counter and minimises the percentage of noise pulses that reach the pulse counter. Typically, in a 'gross count' scintillation system there is no defined upper energy threshold. As the gain characteristics of both the PMT and the scintillator can vary between batches and model types, calibration of the PMT is required to obtain an accurate output from the PMT. In order to calibrate the PMT, the characteristics of each assembled scintillator and PMT need to be determined independently. One method to achieve the characterisation, is to undertake a plateau test, as exemplified in Fig. 5. A plateau test usually takes the form of adjusting the gain of the PMT, by adjusting the voltage applied and recording the pulse count output at a predefined pulse threshold. At present, following the ambient and elevated temperature plateau tests, a suitable fixed operating voltage and pulse threshold voltages are estimated based on the tests and a predetermined preferred operating temperature for the gamma radiation detection and measurement device. At present, these thresholds must be determined for each configuration and set manually until adjusted at some time in the future - i.e. a fixed gain is assumed and subsequent gain changes are not compensated for. It is also known to estimate a plurality of suitable fixed operating voltages and pulse threshold voltages for different temperatures between the ambient and elevated temperatures for use to obtain a stable pulse count output over the entire operating temperature range of the gamma radiation detection and measurement device. These systems essentially adjust the detection threshold by a predetermined amount as temperature increases and assume that the gain of the PMT remains constant over time and is essentially linear. However, present inventors recognise that the gain of a PMT deteriorates as it ages and deteriorates as temperature increases. In an attempt to overcome these issues, some gamma radiation detection and measurement devices utilise gain stabilisation using various methods, for example utilising a constant (known) light source as an input to determine the PMT gain, typically adjusting the HV value to compensate. However, amongst other issues, this technique does not compensate for crystal light loss versus temperature, and there are problems maintaining the stability of the light output by the light source at different temperatures. Another common technique is to expose the scintillator to a small radiation source for a predetermined time period at predetermined fixed distance, often Am-241. This method allows for constant gain stabilisation for all elements; however, among other issues, it requires the use of a potentially harmful radiation source, and the stochastic nature of radiation emitted by it may interfere with the normal detection of ambient gamma radiation. In addition, as noted above, the characteristics of any PMT will change over time and with exposure to temperature and shock / vibration. As a result, the entire gamma radiation detection and measurement device will need periodic 're-calibration'. This requires sending it away to a designated calibration laboratory, which requires advance planning to fit in with tight operational schedules and increased downtime as the device needs to be retrieved from down hole. A further issue is that, because the device needs to be transported back from calibration and lowered back downhole, it can be disturbed between calibration and positioning downhole. This disturbance can result in the characteristics of the PMT changing between calibration and use - i.e. as present there is a risk that the PMT is incorrectly calibrated when it is being used post calibration. The present invention is directed to solving the calibration problems set out above a method and system for automatic plateau testing and optimisation of the voltage settings of a gamma radiation detection and measurement device. Preferably the present invention facilitates the calibration of the gamma radiation detection and measurement device in situ. Preferably within the limitations of a downhole environment (preferably a drilling environment), allowing the test to be performed on much a shorter timescale than traditional plateau testing. As such, calibration can be performed more efficiently and with more control of the testing parameters. While other attempts have been made to address this problem, none have been particularly successful. For example, the system disclosed in WO 2020 / 226766 discloses a system configured to detect a 'knee' of the plateau curve. This knee is typically located at the start of the plateau mode of a PMT. However, this knee is difficult to accurately detect due to, for example, the magnitude of the pulse counts around the knee. Further, using the knee for calibration is problematic due to the possibility of a change of gain producing a drop in counts (effectively 'slipping off’ the knee and off the plateau). As such the level of data that needs to be collected, and the frequency that it needs to be collected, to accurately locate this knee and maintain proper calibration means that this system is insufficiently fast to be used without either calibrating for a small range of voltages or increasing the steps between calibration voltages both of which negatively impact the accuracy of the system. As such there is a continuing need to provide a system that addresses the problem set out above and which are of practical use in real-life mining operations. According to a first aspect of the present invention, a computer implemented method of calibrating a gamma radiation detection and measurement device is provided. The device comprises a scintillator module, an optical transducer module coupled to the scintillator module, and a comparator for comparing the output of the optical transducer module with an operating pulse energy threshold for detecting gamma radiation. The device has an operating voltage for powering the optical transducer module, and is switchable between a first mode of operation for detecting gamma radiation and a second mode of operation for calibrating the operating voltage for powering the optical transducer module. The method comprises setting a calibration voltage for powering the optical transducer module of the device to a predetermined first voltage value, wherein the first voltage value is below the operating voltage; increasing the calibration voltage by a predetermined magnitude and measuring a pulse count rate detected by the device over a predetermined period of time, wherein the step of increasing the calibration voltage is performed until the measured pulse count rate is greater than a predetermined target pulse count-rate value; and when the measured pulse count rate reaches the predetermined target pulse count-rate value, determining an adjusted operating voltage based on the value of the calibration voltage. The method may further comprise a step of changing the operating voltage used in the first mode based on the adjusted operating voltage. The method may further comprise a step of switching the device from the first mode of operation for to the second mode of operation, optionally the switching to the second mode comprising the step of setting the calibration voltage for powering the optical transducer module to the predetermined first voltage value. The method may further comprise a step of switching the device from the second mode of operation to the first mode of operation, optionally the switching to the first mode comprising the step of changing the operating voltage used in the first mode. According to a second aspect of the present invention, a further computer implemented method of calibrating a gamma radiation detection and measurement device is provided. The device comprises a scintillator module, an optical transducer module coupled to the scintillator module, and a comparator for comparing the output of the optical transducer module with an operating pulse energy threshold for detecting gamma radiation. The device is switchable between a first mode of operation for detecting gamma radiation and a second mode of operation for calibrating the operating pulse energy threshold. The method comprises: setting a calibration pulse energy threshold of the device to a predetermined first value, wherein the first value is above the operating pulse energy threshold; decreasing the calibration pulse energy threshold and measuring a further pulse count rate detected by the device, wherein the step of decreasing is performed until the measured further pulse count rate is greater than a further predetermined target pulse count-rate value; and when the measured further pulse count rate reaches the further predetermined target pulse count-rate value, determining an adjusted operating pulse energy threshold based on the value of the calibration pulse energy threshold. The method may further comprise a step of changing the operating pulse energy threshold used in the first mode based on the adjusted operating pulse energy threshold. The method may further comprise a step of switching the device from the first mode of operation to the second mode of operation, optionally the switching to the second mode comprising the step of setting the calibration pulse energy threshold of the device to the predetermined first value. The method may further comprise a step of switching the device from second mode of operation to the first mode of operation, optionally the switching to the first mode comprising the step of changing the operating pulse energy threshold used in the first mode. According to a third aspect of the present invention, there is provided a method comprising the steps of the first and the second aspects. Preferably the predetermined period of time is less than a minute. More preferably the predetermined period of time is less than 10 seconds. Usually the device is located in a borehole. The method may comprise: receiving a signal from a sensor proximate to the device indicating that axial movement of a drill string drilling has ceased or been paused; and on receipt of the signal, switching to the second mode of operation. Usually the step of determining the adjusted operating voltage comprises ignoring pulse readings that fall above a first error threshold and below a second error threshold. Normally determining the adjusted operating voltage comprises finding the centre of the plateau mode of operation of the PMT. Preferably this comprises determining a portion of a best fit curve of the pulse readings with a minimum gradient. Preferably, the best fit curve is a fit to the pulse readings plotted as a function of the voltage, normally the voltage being the calibration voltage. More preferably this comprises obtaining a derivative of the best fit curve. Optionally, the derivative is a first derivative or second derivative. The method may comprise comparing the adjusted operating voltage to previously obtained adjusted operating values. The method may comprise computing a statistics of a predetermined number of adjusted operating voltages and using the statistics to change the settings of the device. The statistic may be any standard statistics used to obtain a representative value of a data set. For example, the statistics may comprise at least one of a straight average, a moving average and a weighted average. The average may be defined, as an arithmetic mean, a geometric mean, a median, or a mode. Preferably switching to first mode comprises gradually changing the settings, such as the operating voltage, whilst obtaining a plurality of adjusted operating voltage values. As such, the switching to first mode may comprise gradually changing the settings over a further period of time, said further period of time spanning the time taken to obtain a plurality of adjusted operating voltage values. According to a fourth aspect of the present invention, a computer program is provided. The computer program comprises instructions which, when executed by a controller coupled to a gamma radiation detection and measurement device comprising an optical transducer module coupled to a scintillator module, cause the controller to perform a method as described above. According to a fifth aspect of the present invention a controller is provided. The controller is configured to control a gamma radiation detection and measurement device comprising a scintillator module, an optical transducer module coupled to the scintillator module, wherein the optical transducer module is powered by a voltage supply, and a comparator for comparing the output of the optical transducer module with a pulse energy threshold for detecting gamma radiation; wherein the controller is configured to: provide an operating pulse energy threshold to the comparator in a first mode of operation for detecting gamma radiation; and adjust the operating pulse energy threshold provided to the comparator to determine the operating pulse energy threshold in a second mode of operation for calibrating the gamma radiation detection and measurement device. Preferably the controller is further configured to: control the voltage supply to provide an operating voltage of a first magnitude to the optical transducer module in a first mode of operation for detecting gamma radiation; and control the voltage supply to adjust the magnitude of the voltage provided to the optical transducer module to determine the magnitude of the operating voltage in a second mode of operation for calibrating the gamma radiation detection and measurement device . Optionally the controller is configured to switch from the first mode of operation to the second mode of operation on receipt of a signal from a sensor. According to a sixth aspect of the present invention, a gamma radiation detection and measurement system is provided. The gamma radiation detection and measurement system comprises the controller and the gamma radiation detection and measurement device of any of the other aspects of the present invention. According to a seventh aspect of the present invention, there is provided a gamma radiation detection and measurement device which comprises: an elongate housing; a scintillator module mounted in said housing and arranged to respond to the presence of gamma radiation in the local environment in which the device is located and said gamma radiation passing through a wall of the housing and into the scintillator module, said module having an optical output for transmitting an optical signal; and an optical transducer module mounted in the housing and having an input arranged to receive the optical output from the scintillator module, said optical transducer module being able to convert the instantaneous optical output into an electrical output corresponding to the energy and / or flux of the incoming gamma radiation as detected in the scintillator module; at least one dampening ring mounted on an outer periphery of at least one module and contactable with an inside of the housing; wherein in a given radial location of the at least one dampening ring, the ring has at least two radial protuberances contactable with the inside of the housing. The two protuberances may be referred to as a 'pair of wings' or a 'wing pair'. Each wing of the pair may be symmetrical compared to the other, over an axis of symmetry between the two. The wings are normally resilient. There may be a plurality of wings pairs spaced radially around the ring, such as more than three pairs, or more than five pairs. There may be up to twelve pairs or up to ten pairs. The outer diameter of the rings, measured at the extremity of the wings, may be larger than the inner diameter of the housing. In this way, the wings are under compression. The optical transducer module is herein also referred to as a transducer module. The optical transducer module may comprise a photomultiplier tube (PMT). A space may be defined inside the housing between the scintillator module and the transducer module and may be filled with an optical fluid to improve the transmission of the optical signal to the input of the transducer module and / or damping movement of either or both of said modules within said housing. The optical fluid may be oil. As well as the normal central aperture of a ring, in which the module extends, the ring may also define at least one, or normally, a plurality of further apertures. The pairs of wings may be discontinuous around the perimeter of the ring. Thus, when there are at least two pairs of wings, the space(s) defined between one pair and the next pair is / are said further aperture(s). Said space(s) / further aperture(s) is / are also normally defined, in part, by the inside of the housing. The rings and further aperture(s) may be arranged such that in the event of a shock to the device, and movement of the module(s), the optical fluid flowing through the aperture(s) in the ring absorb such a shock. The wings of the ring may also flex in response to a shock, then return the module to its generally central position. The fluid flowing around the modules in response to a shock, also normally provides some shock absorption, most likely the majority of the lateral damping force. The wings may be at an angle of 10 - 120 degrees relative to an axis through the main plane of the ring. This angle may be varied to account for applications requiring more or less resilience. For example, a narrow angle, such as 10-40 degrees, may be used for firmer resilience, a midrange angle, such as 40 - 80 degrees, for a medium amount of resilience, whilst a wider angle may be used, such as 80 - 120 degrees, where less resilience is required. The shock absorption may also be varied by thickness of the wings. For example, the thickness of the wings (defined as the depth in the direction of reflection) is typically in the range of 1-3.0mm optionally 1.2-2.0 mm, and may be tapered. The shock absorption may be varied by varying the material properties, such as by using elastomers with different durometer values, typically in the range of 60-90 on the Shore A Hardness Scale. An example material is fluorocarbon based fluoro-elastomer with a value of 85 on the Shore A Hardness Scale (Viton™). Each module may have at least one ring. Preferably at least one ring is provided around the scintillator module. According to an eight aspect of the present invention, there is provided a gamma-ray detection and measurement device which comprises: an elongate housing; a scintillator module mounted in said housing and arranged to respond to the presence of gamma radiation in the local environment in which the device is located and said gamma radiation passing through a wall of the housing and into the scintillator module, said module having an optical output for transmitting an optical signal; and an optical transducer module mounted in the housing and having an input arranged to receive the optical output from the scintillator module, said optical transducer module being able to convert the instantaneous optical output into an electrical output corresponding to the energy and / or flux of the incoming gamma radiation as detected in the scintillator module; wherein the optical transducer module is mounted in a container, with a support sleeve therebetween. Optionally, a split ring with an inverted conical seat retains the sleeve. A spring may be provided to urge the split ring towards the support sleeve, especially a wave spring. Preferably the conical shape of the seat, the split in the split ring and spring combine to urge the split ring radially outwards, as well as urge the split ring towards the support sleeve. The support sleeve may comprise apertures, especially axially extending apertures. There may be 3 to 16 of such apertures. In effect, the support sleeve can function as a bow-spring centralizer. The scintillator module and the optical transducer module may be rigidly coupled together, optionally with a space defined therebetween, and said modules may be movable as a unit within the housing. Resilient means may be arranged within the housing to resist reciprocating movement of said unit. The resilient means may comprise spring arrangement(s) arranged towards each end of the housing, and each applying inward biasing, directly or indirectly, to a respective one of the modules of said unit. Such end springs may be any kind of mechanical spring. The end springs may be wave springs, coil springs, elastomeric tubes or cones. Each of the modules may therefore be slidably mounted within the housing in the manner of a piston. The optical transducer module normally has an input window to receive an optical signal from the scintillator module. The scintillator module normally has a window, such as a sapphire window for an optical output signal. The container may comprise a rubber bump stop between each end of the combined unit, and other components in the housing, such as the spring arrangement. The device may comprise a fluid expansion piston mounted for sealed movement within said housing in order to provide compensation for thermal expansion of the optical fluid in service. This may include a flow restrictor consisting of a small hole connecting a main damping chamber to an equalizing cylinder chamber to allow the transfer of a small volume of optical fluid to equalize the pressure as the optical fluid expands with temperature, but essentially choke any significant transfer of fluid when the assembly is subject to significant axial displacement velocities. Alternatively, metal or elastomeric bellows / bladder may be provided instead of, or in addition to, the fluid expansion piston to provide compensation for the thermal expansion of the optical fluid. An elastomeric disc valve can be provided which can close the flow path as soon as any pressure difference exists, thus preventing the possibility of the piston being displaced and as a consequence, force all the oil through the damping ring flow channels. Alternatively, a reed valve may be used. A spring biasing the compensating piston may also be added to further reduce the likelihood of cavitation occurring due to the piston stiction. The scintillator module preferably comprises a vacuum tube optionally made from glass or ceramic with electrodes (and crystal) normally provided inside the tube. Whilst ceramic scintillators are more robust internally, preferred embodiments here are made from glass. The crystal is normally hermetically sealed. The crystal typically comprises sodium iodide. The housing may be of circular cross-section, and each module may be of circular cross-section and of smaller diameter than the diameter of the housing, to define annular chambers between the inner wall of the housing and the outer surface of each module, and which may be filled with said optical fluid. The invention also provides a drilling tool comprising the device, and drill string and optionally a drill bit. All optional features relate to all aspects of the invention unless relating specifically to features only present in one or more aspects of the invention. Different aspects of the invention may be combined. Embodiments of the present invention will now be described by way of example only, with reference to the accompanying drawings, in which Fig. 1 is a side-sectional view of a gamma radiation detection and measurement device in accordance with an aspect of the present invention; Fig. 2a is a front view of a ring which forms part of the Fig. 1 device; Fig. 2b is a side-sectional view from line A-A of the Fig. 2a ring; Fig. 2c is a side view of the Fig. 2a ring; Fig. 2d is an enlarged view of Detail A of the Fig. 2a ring; Fig. 2e is a perspective view of the Fig. 2a ring; Fig. 3a is a known detail of a gamma radiation detection and measurement device not in accordance with the present invention; Fig. 3b is first embodiment of the ring, showing contact point detail, which forms part of the Fig. 1 device in accordance with one aspect of the present invention; Fig. 3c is a second embodiment of the ring, showing contact point detail, which forms part of the Fig. 1 device in accordance with one aspect of the present invention; Fig. 3d is a third embodiment of the ring, showing contact point detail, which forms part of the Fig. 1 device in accordance with one aspect of the present invention; Fig. 4a is a support sleeve which forms part of the Fig. 1 device; Fig. 4b is a first perspective view of a split ring which forms part of the Fig. 1 device; Fig. 4c is a second perspective view of the Fig. 4b split ring; Fig. 5 is an example plot of the output of a plateau test for a gamma radiation detection and measurement device; Fig. 6 is a schematic representation of electronic components of a gamma radiation detection and measurement device in accordance with the present invention; Fig. 7 is a plot of pulse counts per second against kiloelectronvolts (keV); Figs 8a - 8d show example plots of the output of a fast plateau test for a gamma radiation detection and measurement device; and Figa 9a - 9d show example plots of the output of the plateau test for a gamma radiation detection and measurement device according to the present invention, compared with the state of the art. A gamma radiation detection and measurement device 1 is shown in Fig. 1 comprising a housing 10 which houses two fundamental components - scintillator module 16 which detects gamma rays, and a photomultiplier tube (PMT) module 14 which converts an optical input from the scintillator module 16 to an electrical signal. The two modules are connected together by a threaded connection 15. To hold the modules 14,16 in place, a plurality of rings 20 are provided around the scintillator module 16 and PMT module 14, which contact on their outer edges an inside 10b of the housing 10. The ring 1 is shown in more detail in Figs 2a to 2e. Spaced radially around the ring are pairs of protuberances / wings 22 arranged in a "gull-wing" shape, providing two contact points at each given radial location. The wings are resilient. Accordingly, when the tool experiences a lateral shock during use, the gull wings can flex to absorb the shock, then return the modules to their generally central position. Such an arrangement provides an advantage over pre-existing arrangements as it allows a lower lateral spring rate to be implemented whilst still providing a lateral bump stop to prevent metal to metal contact of the modules with the inside of the housing, and provides more scope for lateral movement leading to a higher degree of shock absorption. Between the pairs of wings 22, spaces 23 are defined. When the tool experiences shock, the oil in the spaces around the modules, flow through these spaces and dampens the shock, especially axial shock, experienced by the modules 14,16. Moreover, in addition to lateral shock absorption, when flow impinges on a ring due to axial shock, the wings deflect radially (in a similar manner to a lip seal being energised by differential pressure). This provides a transient increase the lateral spring rate which helps stabilize the modules when subject to high shocks. Returning to Fig. 1, end springs 11, 13 are provided towards each end of the of the pair of modules, which allows them to move together axially within the housing, and function to mitigate the effects of shock on the device 1. A fluid expansion piston 30 is provided at one end to allow for thermal expansion of the fluid. A spring 31 is provided to bias the piston 30 towards the scintillator module 16. The characteristics of the lateral shock absorption can be varied using oil viscosity and the effective spring rate of the shock ring. This is optimized with the axial shock absorption which is varied by altering the size of the spaces 23 through the shock ring, the oil viscosity and varying the rate of the axial wave springs 11,13. The inventors have noted that despite being precise in many ways, there are often large variations in the outer diameter and circularity of a PMT module. Moreover, in downhole conditions, thermal expansion occurs. As shown in Fig. 1, the PMT module 14 is mounted in a container 24. To accommodate such manufacturing tolerances, and thermal expansion, a support sleeve 25 is provided between the PMT module 14 and the container 24. The sleeve 25 maintains the PMT module 14 at a fixed axial distance from the scintillator module 16 (variations in spacing could spoil the results). The PMT module 14 is restrained at the opposite end to the scintillator module 16, by a split ring 27 with an inverted conical seat 28. In use, the conical seat 28 bears against the rear of the PMT module 14, which possesses an unspecified profile (due to the manufacturing process). The split ring 27 is axially biased by means of a wave spring 29 which keeps a window of the PMT module 14 in contact with an internal flange of the support sleeve 25 to maintain its proximity to the scintillator module 16. The function of the split S in the split ring 27 is to transfer a proportion of the axial spring force into a lateral supporting force by means of the conical seat and it being able of expanding circumferentially. The distribution of the forces is determined by the angle of the seat 28. This results in a mounting that can accommodate a wide range of dimensional variations under a wide range of operating temperatures. The support sleeve 25 and split ring 27 are shown in more detail in Figs. 4a and 4b respectively. The support sleeve 25 is made from a temperature resistant composite polymer such as carbon reinforced polyether ether ketone. The split ring 27 is made from a temperature resistant fluoropolymer such as tetrafluoroethylene. The support sleeve 25 comprises slots, which allow for sections of the sleeve to deflect radially outwards independently to accommodate non-circularity and other dimensional variations as well as thermal expansion of the PMT module, without introducing stress onto the scintillator module 16. With refence to Fig. 6, a system 2000 is presented in accordance with an aspect of the present invention. The system 2000 comprises a PMT 2010 coupled to a scintillator module 2020. Any suitable scintillator can be used, however an Nai crystal is particularly suitable. The PMT 2010 is powered by a high voltage power supply 2030. The high voltage power supply 2030 is configured to provide a voltage in the range of 1000 - 2000 volts. The output from the PMT 2010 is coupled to a pre-amplifier 2050 for amplification. Preferably this coupling is via a capacitor 2040 to remove the DC component from the output of the PMT. The output from the pre-amplifier 2050 is passed to a pulse conditioner 2060 to further amplify and shape the pulse generated by the PMT 2010. The output of the pulse conditioner 2060 is passed to first input of a comparator (or a pulse discriminator) 2070. The second input of the of the comparator 2070 is coupled to a first potentiometer 2080, which provides a pulse threshold voltage to the comparator 2070. A second potentiometer 2090 provides a reference voltage to the high voltage supply 2030 to control the voltage output by the high voltage supply 2030, which in turn controls the gain of the PMT 2010. Preferably the first and second potentiometers are digital potentiometers. The first potentiometer 2080 and the second potentiometer 2090 are also coupled to a reference voltage 2100 voltage set-point. The output of the comparator 2070 is operatively connected to a controller 2200. The controller 2200 can be any suitable microcontroller. The controller 2200 is configured to control the first potentiometer 2080. In particular, the micro controller 2200 is configured to control the first potentiometer 2080 to adjust the pulse threshold based on the output of the comparator 2070 as discussed below in more detail. In addition, the micro controller 2200 is configured to control the second potentiometer 2080. In particular, the micro controller 2200 is configured to control the second potentiometer 2080 to control / adjust the gain of the PMT 2010 based on the output of the calibration process described herein. In use, the controller has two modes of operation. In the first 'standard' mode, the controller 2200 is configured to provide readings from the system to a suitable user display. The controller 2200 also has a calibration mode. In calibration mode, the pulse threshold is initially set at a value that is higher than that used in the standard operating mode. The controller 2200 the iteratively and incrementally reduces the pulse threshold while monitoring the output from the comparator 2070. With reference to Fig. 7, when the pulse threshold voltage is reduced below the noise edge 300, the count rate, or number of pulses counted per second (CPS) will increase sharply. As a result, it is possible for the controller to determine the noise edge. In particular, if the controller detects that the number of pulses per second is greater than a predetermined CPS threshold, this is indicative that the pulse threshold voltage is at the noise edge. The use of a controller with one or more potentiometers may increase the initial cost of a system in accordance with the present invention by comparison to prior art systems. However, this cost increase is outweighed by a cost saving as it provides a means for the system to conduct 'automatic' plateau generation and correction for the PMT 2010. Further, the accuracy of the system is increased as calibration will account for scintillator and electronic variations, as well as the PMT's ageing. The configuration of a system in accordance with the present invention allows control of both the pulse threshold and voltage provided to the PMT. As a system in accordance with an aspect of the present invention is adapted to provide control over the pulse threshold and gain through control of the voltage provided to the PMT, it is possible to perform dynamic plateau tests to determine the optimum voltage settings for the PMT and scintillator combination at a given operational temperature. In order to increase the accuracy of the dynamic plateau test, it should be preferably performed when the gamma radiation background is constant in order to optimally characterise the system. Therefore, if a dynamic plateau is to be conducted in a drilling environment, the gamma radiation background needs to be sufficiently stable throughout the period of the dynamic plateau testing. Typically, a dynamic plateau test takes 15 - 30 minutes to perform, depending on background activity, plateau length and number of voltage 'steps' between the data points. For example, the dynamic plateau test can be performed using decreasing steps of around 40V steps and a 60 second period between steps to obtain and average a count rate. This results in a period of around 23 minutes for the ambient temperature plateau and around 18 minutes for the plateau at temperature (due to the reduced plateau length). During the drilling process, there are occasional 'static' periods - i.e. periods where the device is down-hole, but no drilling activity is taking place. These periods occur generally during a 'connection', which is the time required to disconnect the pumps, and add a length (or lengths) of pipe, reconnect and re-start the pumps. A static period may only last a minute or two, but during the static period, the device is in practice held in a fixed position. In addition, as there is no drilling activity, there are also no substantial mud-flow during a static period. Thus, the static period provides the opportunity for undertaking a plateau test. This has not been considered as a feasible approach in the past due to problems with accounting for variable and possibly very low background activity of adjacent rock formations, which can affect the stability of the plateau and measurement accuracy. Further, given that a static period will only last a minute or two, and that measurement noise will increase as a function of the reduction in the averaging period, there appeared to be insufficient time to obtain reliable readings. A plateau test usually takes the form of adjusting the gain of the PMT, by adjusting the voltage applied and recording the pulse count output at a pre-defined pulse threshold. This resultant plot of CPS versus voltage has three significant modes of operation as the voltage, and hence gain, increases: A first mode (1) - During the first mode, the PMT is under amplified. In particular, many pulses produced by the PMT typically fall below the pulse threshold voltage and are not counted. As the gain of the PMT is increased, a greater proportion of pulses achieve the required peak value to cross the pulse threshold voltage. As such approaching the knee of the plot the pulse count increases as the voltage increases the PMT gain. Continually increasing the gain of the PMT eventually causes the PMT to leave the first mode and enter the plateau mode of operation. A plateau mode (2) - During the 'plateau' mode, the total CPS remains somewhat constant over a wide range of voltages. In this phase, gamma pulses typically have sufficient energy to cross the pulse threshold voltage, while noise pulses typically fall below the pulse threshold voltage. As such, all but the lowest energy gamma pulses are counted. As noise pulses typically have lower amplitude than gamma pulses (in this mode), continually increasing the gain can cause very little change to total CPS detected while the PMT is in plateau mode. However, at a certain point, increasing the voltage of the PMT will eventually causes the PMT to leave the plateau mode and enter the third mode of operation. A third mode (3) - During the third mode, the PMT becomes over amplified. In this mode of operation, the CPS rises markedly after leaving the plateau mode as voltage is further increased because noise pulse will have sufficient amplitude to cross the pulse threshold voltage. To put it differently, if the PMT is constantly operating in this mode, the noise edge is amplified sufficiently to cross the pulse threshold voltage and can proceed to produce a huge number of CPS potentially swamping the background gamma count. A plateau test is typically undertaken at an ambient temperature and repeated at an elevated temperature. The length of the plateau mode indicates a range of voltage values at which a stable gamma count rate can be produced, indicating the voltage 'setting' at which the PMT can be used. A complication of the plateau test is that the length of the plateau mode is typically significantly reduced at elevated temperature. This occurs (primarily) because of the reduced PMT gain at the elevated temperature requiring a higher voltage for an equivalent gain to that obtained at ambient temperature. Intrinsically, PMT noise increases with temperature. In addition, noise is also increased by the elevated voltage required to compensate for the negative effects of elevated temperature on gain. Thus, the elevated temperature results in a longer first mode, a later start to the plateau mode as voltage increases, and an earlier start to the third mode, as a lower gain will cause the noise edge to cross the voltage threshold due to the increased voltage required to obtain the gain and the additional noise from the elevated temperature. Referring again to Fig. 5, the solid line 51 shows the plateau plot from an ambient temperature plateau test and the dashed line 52 shows the plateau plot from a plateau test that was performed at an elevated temperature - in this case 150C°. The data shown in Fig. 8a were obtained using 22 voltage steps and 5 second sample periods. This results in a reduction by a factor of 12 by comparison to the use of a 1-minute sample period. This in turn causes an effective noise increase of around x 3.5 times. As a result, the data obtained is much noisier compared with the standard plateau test results shown in Fig. 5, making the determination of optimal voltage appear impossible. However, a usable calibration can be obtained as follows. Data acquisition is initiated by a 'flag' from a downhole control module or an internal sensor measurement (e.g. a pressure sensor or an accelerometer). The flag is any signal suitable for indicating that mud-flow has stopped. This in turn indicates that the drill-string likely to be stationary. After receipt of the flag, the systems initiates a short delay to ensure that the flag was not triggered by a short flow / pressure cycle, rather than a connection. After the delay, data acquisition is initiated by the controller. In particular, the controller sets a data acquisition time sufficiently short that the plateau test can be completed during a connection. During the data acquisition time, voltage is successively increased in steps from a pre-defined starting point. The steps are of configurable magnitude. Once raw data has been acquired, a first and a second CPS threshold are applied to the raw data to eliminate outliers. In particular, if a data point is above the first count threshold it is ignored. Similarly, if a data point is below the second CPS threshold it is also ignored. These CPS thresholds may be set as a percentage of the total average sample CPS. For example, if a percentage Y is chosen, the first count threshold may be 100 + Y percent of the total average sample CPS. Similarly, if the second CPS threshold may be 100 - Y percent of the total average sample CPS. For example, if the percentage is 10%, any readings above 110% and below 90% of the total average sample CPS will be ignored. This is shown in Fig. 8b. The data obtained (i.e. the CPS obtained and the settings used to obtain them) can then be used to calibrate the system. Preferably the centre of the plateau is determined and used for calibration because it provides a greater degree of insensitivity to gain change in absence of a recent downhole plateau, due to the gain / HV distance from the knee or noise floor. Preferably this comprises fitting a curve to the data. For example, Fig. 8c shows a 3rd order polynomial fitted to the remaining data points. Fig. 8d shows the later stages of the data processing. In this stage, a derivative of the best fit curve is obtained to extract information about the curve. In particular, a first derivative of the best fit curve is performed to determine the minimum gradient of the curve (i.e. the flattest portion of the best fit curve indicative of the plateau). The lowest point on the first derivative of the best fit curve may then be determined mathematically. Once the location of the minimum gradient of the curve has been determined, the system derives the voltage corresponding to that point on the best fit curve. The derived voltage value may then be used as the optimal operating voltage setting for the PMT. Alternatively, second differentiation of the best fit curve may be performed to approximate a straight line through a count change of zero (i.e. a zero slope of the first derivative). In this case, a linear trendline is fitted to the second derivative. The system then derives the voltage value at the zero slope. The derived voltage value may then be used as the optimal operating voltage setting for the PMT, or as a weighted input to multiple plateau tests that may be conducted throughout the drilling process. Figs 9a - 9d show how the output of the plateau test compares with an output of the state of the art plateau test. Figs 9a shows a 'well behaved' standard plateau acquired using a prior-art method of long integration time in an environment with sufficient radioactivity present, while Fig. 9b shows the noisy output of a fast plateau test according to an embodiment of the present invention. Figs 9c and 9d show the outputs of both plateau tests (standard and fast, respectively) processed in accordance with the present method as described above. The vertical line in Figs 9b and 9d marks an optimal value 91 determined using the present method, which clearly would be difficult to read of directly from the noisy data shown in Fig. 9b. In contrast, an optimal voltage value 92 is easily picked out in Fig. 9a where the data is well behaved. The derived voltage value may be used to adjust the high voltage control setting used to power the PMT. Alternatively, a number of plateau tests can be performed, and the voltage value used to adjust the high voltage control setting may be a statistics of a predetermined number of derived voltage values obtained from the plateau tests to obtain an improved setting value that further compensates for noise. In addition, or alternatively, the derived voltage value can be compared to previous derived voltage values and assessed against pre-determined threshold limits. Preferably the change in the high voltage setting is applied as a ramp change and not a step change. In particular, by adjusting the high voltage setting gradually instead of as a stepchange, the risk of damage to the PMT, or of erroneous results, caused by abrupt changes to the PMT can be minimised. The method of the present embodiment advantageously allows for an automatic plateau testing and optimisation of the HV of a gamma radiation detection and measurement device, with digital control over the testing parameters, and with the test being performed on much a shorter timescale than traditional plateau testing. As such, calibration can be performed more efficiently, with less down time, and with more control of the testing parameters. Improvements and modifications may be made without departing from the scope of the invention defined in the appended claims and their equivalents.

Claims

1. A computer implemented method of calibrating a gamma radiation detection and measurement device, comprising a scintillator module, an optical transducer module coupled to the scintillator module, and a comparator for comparing the output of the optical transducer module with an operating pulse energy threshold for detecting gamma radiation;wherein the device has an operating voltage for powering the optical transducer module and wherein the device is switchable between a first mode of operation for detecting gamma radiation and a second mode of operation for calibrating the said operating voltage for use in the first mode;the method comprising:setting a calibration voltage for powering the optical transducer module to a predetermined first voltage value, wherein the first voltage value is below the operating voltage;in the second mode, increasing the calibration voltage by a predetermined magnitude and measuring a pulse count rate detected by the device over a predetermined period of time, wherein the step of increasing the calibration voltage is performed until the measured pulse count rate is greater than a predetermined target pulse count-rate value; andwhen the measured pulse count rate reaches the predetermined target pulse count-rate value, determining an adjusted operating voltage based on the value of the calibration voltage;and wherein, optionally, the method further comprises at least one of the following steps:switching the device from the first mode of operation to the second mode of operation, optionally the switching to the second mode comprising the step of setting the calibration voltage for powering the optical transducer module to the predetermined first voltage value;changing the operating voltage for use in the first mode based on the adjusted operating voltage; andswitching the device from second mode of operation to the first mode of operation, optionally the switching to the first mode comprising the step of changing the operating voltage for use in the first mode.

2. The method of claim 1, wherein the predetermined period of time is less than a minute.

3. The method of claim 2, wherein the predetermined period of time is less than 10seconds.

4. The method of any preceding claim, wherein determining the adjusted operating voltage comprises ignoring pulse readings that fall above a first error threshold and below a second error threshold.

5. The method of claim 4, wherein the first error threshold is a predetermined percentage above of the average sample count rate and the second error threshold is a predetermined percentage below the average sample count rate.

6. The method of any preceding claim, wherein determining the adjusted operating voltage comprises finding the centre of a plateau mode of operation of the optical transducer module.

7. The method of any preceding claim, wherein determining the adjusted operating voltage comprises determining a portion of a best-fit curve of the pulse count-rate readings with a minimum gradient.

8. The method of claim 7, wherein determining the adjusted operating voltage comprises obtaining a derivative of the best-fit curve of the pulse count-rate readings.

9. The method of claim 8, wherein the derivative is a first derivative or a second derivative.

10. The method of any preceding claim, comprising comparing the adjusted operatingvoltage to previously obtained adjusted operating voltage value(s).

11. The method of any preceding claim, comprising compiling a statistics of a plurality of adjusted operating voltages and using the statistics to change the operating voltage used in the first mode.

12. The method of claim 11, wherein the statistics comprises at least one of a straight average, a moving average and a weighted average.

13. The method of any preceding claim, wherein switching to first mode comprises gradually changing the operating voltage over a period of time, the said period of time spanning the time taken to obtain a plurality of adjusted operating voltage values.

14. A computer-implemented method of calibrating a gamma radiation detection and measurement device, comprising a scintillator module, an optical transducer module coupled to the scintillator module, and a comparator for comparing the output of the optical transducer module with an operating pulse energy threshold for detecting gamma radiation;wherein the device is switchable between a first mode of operation for detecting gamma radiation and a second mode of operation for calibrating the operating pulse energy threshold;the method comprising:setting a calibration pulse energy threshold of the device to a predetermined first value, wherein the first value is above the operating pulse energy threshold;in the second mode, decreasing the calibration pulse energy threshold and measuring a further pulse count rate detected by the device, wherein the step of decreasing is performed until the measured further pulse count rate is greater than a further predetermined target pulse count-rate value; andwhen the measured further pulse count rate reaches the further predetermined target pulse count-rate value, determining an adjusted operating pulse energy threshold based on the value of the calibration pulse energy threshold;and wherein, optionally, the method further comprises at least one of the following steps: switching the device from the first mode of operation to the second mode of operation, optionally the switching to the second mode comprising the step of setting the calibration pulse energy threshold of the device to the predetermined first value;changing the operating pulse energy threshold for use in the first mode based on the adjusted operating pulse energy threshold;switching the device from second mode of operation to the first mode of operation, optionally the switching to the first mode comprising the step of changing the operating pulse energy threshold for use in the first mode.

15. The method of any preceding claim, wherein the optical transducer module comprises a photomultiplier tube (PMT).

16. The method of any preceding claim, wherein the device is located in a borehole.

17. The method of claim 16, wherein the method is performed during a drilling operation,the method comprising:receiving a signal from a sensor proximate to the device indicating that axial movement of a drill string drilling has ceased or been paused; andon receipt of the signal, switching to the second mode of operation.

18. A computer implemented method of calibrating a gamma radiation detection and measurement device comprising the steps of claim 1 and claim 14.

19. A computer program comprising instructions which, when executed by a controller coupled to a gamma radiation detection and measurement device, the device comprising a scintillator module, and an optical transducer module coupled to the scintillator module, cause the controller to perform a method according to any preceding claim.

20. A controller configured to control a gamma radiation detection and measurement device comprising a scintillator module, an optical transducer module coupled to the scintillator module, wherein the optical transducer module is powered by a voltage supply, and a comparator for comparing the output of the optical transducer module with a pulse energy threshold for detecting gamma radiation; wherein:the controller is configured to:provide an operating pulse energy threshold to the comparator in a first mode of operation for detecting gamma radiation; andadjust the operating pulse energy threshold provided to the comparator to determine the operating pulse energy threshold in a second mode of operation for calibrating the gamma radiation detection and measurement device.

21. The controller of claim 20, wherein the controller is further configured to:control the voltage supply to provide an operating voltage of a first magnitude to the optical transducer module in a first mode of operation for detecting gamma radiation; andcontrol the voltage supply to adjust the magnitude of the voltage provided to the optical transducer module to determine the magnitude of the operating voltage in a second mode of operation for calibrating the gamma radiation detection and measurement device.

22. The controller of claim 20 or 21, wherein the controller is configured to switch from the first mode of operation to the second mode of operation on receipt of a signal from a sensor.

23. A gamma radiation detection and measurement system comprising:a gamma radiation detection and measurement device, the device comprising a scintillator module and an optical transducer module coupled to the scintillator; andthe controller of any one of claims 20 - 22.

24. A gamma radiation detection and measurement device comprising:an elongate housing;a scintillator module mounted in said housing and arranged to respond to the presence of gamma radiation in the local environment in which the device is located, and said gamma radiation passing through a wall of the housing and into the scintillator module, said module having an optical output for transmitting an optical signal; andan optical transducer module mounted in the housing and having an input arranged to receive the optical output from the scintillator module, said optical transducer module being able to convert the instantaneous optical output into an electrical outputcorresponding to the energy and / or flux of the incoming gamma radiation as detected in the scintillator module;at least one dampening ring mounted on an outer periphery of at least one of said modules and contactable with an inside of the housing;wherein in a given radial location of the at least one dampening ring, the ring has at least two wings contactable with the inside of the housing.

25. A device as claimed in claim 24, wherein the two wings are a wing pair and there is a plurality of wing pairs spaced radially around the ring, such as more than three pairs.

26. A device as claimed in any one of claims 24 and 25, wherein the wings are resilient and configured to flex in response to an external shock then return the at least one module to a rest position.

27. A device as claimed in any one of claims 24 to 26, wherein there are up to twelve wing pairs.

28. A device as claimed in any one of claims 24 to 27, wherein the outer diameter of the at least one dampening ring, measured at the extremity of the wings, is larger than the inner diameter of the elongate housing.

29. A device as claimed in any one of claims 24 to 28, wherein at least two pairs of wings are discontinuous around the perimeter of the ring.

30. A device as claimed in any one of claims 25 to 29, wherein, the at least one dampening ring also defines a plurality of further apertures in addition to a central aperture of the said ring.

31. A device as claimed in claim 30, wherein a space(s) defined between one pair of wings and a next pair of wings is said further aperture(s) which are also, in part, defined by an inside of the elongate housing.

32. A device as claimed in any one of claims 24 to 31, wherein the wings are at an angle of 10-120 degrees relative to an axis through the main plane of the ring, optionally 10-40 degrees, further optionally 40-80 degrees, or further optionally 80-120 degrees.

33. A device as claimed in any one of claims 24 to 32, wherein the thickness of the wings (defined as the depth in the direction of reflection) is in the range of 1.2-2.0 mm.

34. A device as claimed in any one of claims 24 to 33, wherein at least one dampening ring is provided around the scintillator module.

35. A device as claimed in claim 34, wherein at least two dampening rings are provided and each module has at least one dampening ring.

36. A gamma radiation detection and measurement device which comprises: an elongate housing;a scintillator module mounted in said housing and arranged to respond to the presence of gamma radiation in the local environment in which the device is located and said gamma radiation passing through a wall of the housing and into the scintillator module, said module having an optical output for transmitting an optical signal; andan optical transducer module mounted in the housing and having an input arranged to receive the optical output from the scintillator module, said optical transducer module being able to convert the instantaneous optical output into an electrical output corresponding to the energy and / or flux of the incoming gamma radiation as detected in the scintillator module;wherein the optical transducer module is mounted in a container, with a support sleeve therebetween.

37. A device as claimed in claim 36, wherein the support sleeve comprises apertures.

38. A device as claimed in claim 36 or 37, wherein a split ring with an inverted conical seat retains the support sleeve.

39. A device as claimed in claim 38, wherein the split ring is urged radially outwards.

40. A device as claimed in claim 38 or 39, wherein a spring urges the split ring towards the support sleeve.

41. A device as claimed in any one of claims 24 to 40 wherein the scintillator module and the optical transducer module are rigidly coupled together.

42. A device as claimed in any one of claims 24 to 41 wherein the container comprises a rubber bump stop between each end of a combined unit of said modules, and other components in the housing.

43. A device as claimed in any one of claims 24 to 42 comprising a fluid expansion piston mounted for sealed movement within said housing in order to provide compensation for thermal expansion of optical fluid in service.

44. A device as claimed in any one of claims 24 to 43, wherein the scintillator module comprises a glass vacuum tube.

45. A device as claimed in any one of claims 24 to 44, wherein a space is defined inside the housing between scintillator module and the optical transducer module and is filled with an optical fluid to improve the transmission of the optical signal to the input of the optical transducer module and / or damping movement of either or both of said modules within said housing.

46. A drilling tool comprising the device as claimed in any one of claims 24 to 46, comprisinga drill string and, optionally, a drill bit.

47. A method of any one of claims 1 to 18, wherein the gamma radiation detection and5 measurement device is the device of any one of claims 24 to 46, optionally, wherein thedevice is part of the gamma radiation detection and measurement system as claimed in claim 23.

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