Improved emitter device for particles or radiations

Abstract

Device for emitting radiations and / or ionizing particles, preferably alpha particles, comprising : a detector of residual nuclei after radioactive decay; a readout / processing circuit electrically connected to the detector and configured to form an electrical signal based on the energy of the detected, e. g. captured, residual nucleus after decay; an electronic time or energy identification circuit receiving as input the electrical signal to generate a time output representative of the emission of the particle and / or radiation; wherein a plurality of radionuclides is applied on or in the detector such that, when the particle is emitted, the residual nucleus passes through the detector remaining trapped therein, so as to release its energy and generate the electrical signal via the readout circuit.

Description

[0001] IMPROVED EMITTER DEVICE FOR PARTICLES OR RADIATIONS

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to a device for emitting particles or radiations, i . e . alpha or beta particles or neutrons and gamma radiations, comprising a module for generating a time signal synchronized with the emission of the particle -or radiation, in particular of the single particle or radiation. In particular, the detector device in the case of beta or gamma decay and of neutrons must be particularly sensitive . The time signal is used to provide a trigger signal to the user equipment, which preferably comprises a detector for the primary radiation or for a secondary radiation caused by the primary radiation. The applications of the device include, but are not limited to, smoke sensors, level sensors, devices for the stoichiometric analysis of materials, devices for the calibration of detectors for basic and applied research, equipment for nuclear physics education. The uses are based on the variation of the time of flight of the particle or radiation due to the perturbation, e . g. smoke, liquid, chemical substance, etc . , with respect to the time of flight in a reference condition, e . g. in air . In some cases it is sufficient to determine whether there is a delay or not of the time of flight; in other cases, following a calibration, e . g. through a heuristic mathematical model, the time of flight is proportional to the quantity of interest . It is also possible to associate the signal generated by the detector with a spectroscopic signal processing circuit, slower and based on theintegration of the signal generated by the detector, in order to provide a more precise and complete output .

[0004] STATE OF THE ART

[0005] Radioactive sources that emit ionizing particles, e . g. alpha particles, are used in numerous research and industrial applications (level sensors, thickness gauges, smoke detectors, detector calibration, elemental analyzers) . Their use, however, finds limits in the random nature of radioactive decay, which has the effect of making it impossible to have a time reference for the emission of single particles or radiations and determines the need to use relatively high quantities of radioactive material that allow obtaining a signal that exceeds the environmental background. Considering the necessary radioprotection rules, the widespread use of devices based on the above-mentioned process is penalized. In principle, a device analogous to the one proposed could be realized by using a transit detector ( for example a microchannel plate in configuration with an electrostatic mirror) , but such a device is very bulky (scale lengths of tens of centimeters, with volumes of at least 20 liters) , very expensive and complex to operate due to the need to maintain the entire sensitive part of the apparatus under high vacuum.

[0006] SUMMARY AND OBJECTIVE OF THE INVENTION

[0007] The obj ective of the present invention is to provide a device and a measurement method capable of at least partially overcoming the drawbacks listed above .The obj ective of the present invention is achieved by means of a device comprising a detector of charged particles, neutrons and beta or gamma radiations in which a small quantity of radioactive material is incorporated, an electronic circuit that produces a fast time signal following the emission of particles resulting from radioactive decay. A slower electronic circuit, which may be used alternatively or in combination, provides a spectroscopic analysis . In particular, the particles, e . g. alpha particles, emitted are produced in the radioactive decay of unstable nuclei contained in the device itself and a time signal is associated with the emission of each of them. In this way it is possible to drastically limit the quantities of radioactive material required in the common applications of radioactive sources and, therefore, the problems connected with their possession and use, also obtaining a particularly compact configuration. Some applications, particular but of relevance, are also possible, based on the knowledge of the emission instant, such as, for example, the calibration of coincidence systems in composite detectors .

[0008] The time signal is obtained by detecting the residual nucleus produced in the same event that produces the alpha particle . The logic signal is therefore produced in a completely autonomous manner, without resorting to manipulations of the energy signal of the primary radiation, by acquiring and forming the electric charge produced directly by the recoil phenomenon following radioactive decay .The simplicity of the device makes it possible to realize portable versions powered by battery.

[0009] The emission of alpha particles occurs in a cone whose aperture can be limited, according to the requirements, by an iris collimator . The time signal identifying each emitted particle is used as a coincidence signal for the user equipment .

[0010] BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The present invention will now be described according to a non-limiting embodiment, the description of which is supported by the following drawings, in which:

[0012] Fig. 1 refers to a diagram of a device according to a first embodiment of the present invention;

[0013] Fig. 2 refers to a diagram of a device according to a further embodiment of the present invention;

[0014] Fig. 3 illustrates curves showing the impact on the width of the coincidence window of the time resolution of the signal processing circuits and of the distance between the detectors;

[0015] Figs . 4 and 5 illustrate respective circuit diagrams of an embodiment for processing the signal output from the detector of the device of Fig. 1 or 2 ; and

[0016] Fig. 6 is a functional diagram of a spectrometer comprising the present invention.

[0017] DETAILED DESCRIPTION OF THE INVENTIONThe energy available in an alpha radioactive decay is distributed between the emitted particle, e . g. an alpha particle, and the residual nucleus, i . e . what remains of the radioactive nucleus following the emission of the alpha particle . Due to its small mass, the alpha particle acquires the greater part of the energy, typically of the order of a few MeV. The residual nucleus retains an energy of around 100 keV, too low to be used in the most common applications . The idea underlying the invention is to temporally mark the emission of the particle or radiation through the detection of the residual nucleus . Considering the small amount of energy of the residual nucleus and its high specific energy loss, which makes its collection in the active volume of a detector difficult, in the device forming the subj ect matter of the invention the radionuclides are carried by the detector, so as to make any intermediate layers between the detector and the radionuclide minimal or null . For example, implantation of the radionuclides directly in the detector is provided, where possible, or on the detector, e . g. on its entrance window. The possibility of using detectors of the most common and commercially available types (scintillation, gaseous and semiconductor) has been evaluated in order to increase implementation flexibility in different applications while keeping costs on the lower spectrum. In all cases, the possibility of producing a pulse to be transformed into a time signal associated with the emission of the alpha particle has been verified.

[0018] In particular, when semiconductor / gaseous detectors are used, the particle ionizes and the electric charge is collected by means of an electric f ield / capacitor ; in scintillation detectors, theresidual nucleus induces the generation of light, collected by photomultipliers, e . g. a photocathode .

[0019] The diagram of Fig. 1 illustrates the operating principle of the invention. The emitter device comprises a detector / generator comprising a detector, on the base of which, facing a user apparatus, there have been incorporated, e . g. by implantation, preferably by ionic implantation, or by deposition, preferably by electrodeposition, atoms of the desired emitting radioactive nucleus . The user apparatus is any target of the particles emitted by the emitter device : thanks to the device of the invention, each emitted particle will be accompanied by the temporal information generated by means of a signal generated by the detector activated by the decay residue .

[0020] In particular, the decay produces a residual nucleus, emitted in the direction opposite to that of the radiation, therefore towards the inside of the detector . This produces a signal which is processed through an electronic device and made available to the user . In this way, the radiations emitted by the emitter device of the invention are temporally marked.

[0021] Preferably, the electronic device is a signal processing circuit that has a threshold above the electronic noise and, when such threshold is exceeded by the signal coming from the detector, a time signal having its own resolution is generated, e . g. 3 nanoseconds . Figure 2 illustrates a diagram of an embodiment example of the emitter device incorporated in a measuring unit . An alpha emitter2 1 2Po) electrostatically collected on a silicon detector is used as the source .

[0022] The thickness of the sensitive volume of the detector must be sufficient to completely stop the path of the residual nuclei, which is easy to obtain due to the low energies of the residual nuclei . In this way, the entire energy of the residual nucleus can be used for the generation of the corresponding electrical pulse .

[0023] The absence of inactive layers is preferable, or at least the reduction of their thicknesses to micrometric values, i . e . not exceeding 10 microns . The solution to the problem has been found in creating, on the sensitive surface of the detector, a distribution of the radioactive atoms of interest, and this is done either by implantation or by deposition depending on the type of detector . The distribution of atoms could also take place on a thin support which is then placed in front of the detector and in contact with it . The choice of the type of detector is also multiple : in addition to the semiconductor detector (e . g. silicon) , or a scintillating crystal (e . g. CsI, cesium iodide) , it is possible to resort to a gas ionization chamber inside which, or better, on the entrance window, on the internal side, the plurality of atoms of the radionuclide of interest is applied, either directly on the window wall or on a support .

[0024] It is possible to employ any radioactive species that emits alpha particles with energy of the order of MeV, to which correspond residues whose energy ranges from 90 to 150 keV. All other conditions being equal (costs, production techniques) , nuclei corresponding tohigher residue energies are to be preferred. In the table that follows, characteristic data of some alpha-emitting radionuclides are shown.

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[0030] support in turn decays producing two other alpha -emit ting radionuclides, with formation of the corresponding residues . Consequently, the integrated flux of radiations in the detector increases : this fact does not prevent the operation of the device, but to some extent varies its characteristics, since the energy of the particles emitted by the different radionuclides is significantly different, with an impact on performance that depends on the particular application. For this reason, in general, a radionuclide that decays into a stable nucleus (e . g.2 1°Po) or into one with a long mean life (e . g.z 44Cm) represents the best choice .

[0031] In principle, isotopes that emit radiations other than alpha particles can also be used, provided that there is sufficient energy transferred to the recoil to allow its detection. In particular, the other usable particles or radiations are beta, gamma and neutrons . Furthermore, it is possible to apply radionuclides of different species on the detector, selecting however species havingsufficiently different residual energies of the residual nucleus so that the identification of each species is possible and thus subsequent analyses, e . g. coincidence analyses, are enabled.

[0032] The signal of the residual nucleus is collected by the detector and, by means of a charge-sensitive preamplifier, a current pulse proportional to the charge lost by the residual nucleus in the volume of the detector is generated through a suitable signal processing circuit . The signal thus obtained is shaped by an amplifier (Figures 4 and 5) . Furthermore, a time identification circuit, e . g. a time to analog converter or a time to digital converter coupled to the processing circuit, generates a time signal corresponding to the measurement of the energy of the residual nucleus .

[0033] According to one embodiment (Figure 2 ) , the emitted alpha particle is collected by another silicon detector in order to detect the reception and the transmission of the time signal between the departure and arrival of the alpha particle, i . e . the time of flight, is enabled when a coincidence analysis, carried out by a suitable circuit, provides a positive result . The detector of the user apparatus, e . g. of the alpha particle, also has its own processing circuit and a time circuit . When the time of flight has the same order of magnitude as the time resolution of the signal processing and coincidence circuits, the use of a delay or timing circuit is preferable, which delays one of the two signals, preferably that of the recoil detector because it is more subj ect to random coincidences, by a time e . g. 2-3 times greater than the time resolution of the discrimination circuit, i . e . of the circuit for coincidence analysis that forms the time signal representative ofthe time of flight . In fact, the detection of particles or radiations, which has a high percentage of true coincidences, provides a greater probability of a positive outcome of the coincidence analysis . Therefore, in order to occupy the coincidence analysis circuit for a time having high probabilities of providing a positive outcome of the analysis, such circuit is activated when the time signal of the target detector is generated and the coincidence analysis is carried out, with the corresponding coincidence window, towards the delayed signal of the residual nucleus starting from the reception signal of the particle or radiation. In this way there are no "negative times" potentially due, if such delay were not present, to the fact that, due to a combination of the circuit dynamics of creation of the time signal, the signal of the received particle or radiation is generated before that of the residual nucleus that originated the particle or radiation. Furthermore, a delay less than or equal to 3 times the time resolution of the electronic circuit (s) is a good compromise to reduce the dead time of the coincidence analysis circuit, so that it is available for subsequent analyses . The time resolution of the time circuits that form the time signal of the radiation, i . e . of the residual nucleus, and of the user apparatus, i . e . of the particle or radiation, and of the coincidence circuit is the factor that more than all the others determines the width of the coincidence window, as shown in Figure 3a, where simulated time spectra are compared considering two different time resolutions (30 and 100 ns, respectively) . Figure 3b presents the rather modest impact of the variation of the distance of the source from the user apparatus ( from5 to 100 mm, respectively) , the second parameter in importance in determining the coincidence interval .

[0034] A preferred but non-limiting embodiment example of circuit diagrams connected to the detector / emitter and to the particle or radiation detector for signal processing are shown in Figures 4 and 5. The signals generated by the two detectors, one of which is suitably delayed as discussed, are sent to a timing circuit to verify the coincidence between the two signals and thus enable the user apparatus to accept the data .

[0035] Charge-sensitive preamplifiers (CSP) are often the optimal choice in the design of readout circuits for pulse detectors . The basic configuration of a charge-sensitive preamplifier is illustrated in Figure 2. A feedback capacitor, Cf, connected between the input and the output, stores the charge coming from the detector, determining a gain equal to 1 / Cf . Each current pulse generated by the detector produces at the output of the CSP a mathematical "step" function corresponding to the temporal integral of the detector current . The output appears as a potential difference and expresses the conversion of the particle energy in MeV into a signal in volts, the unit of measure for which will be : mV / MeV.

[0036] Figures 4 and 5 illustrate an embodiment that implements the integration of the electrical signal at the output of the detector and therefore realizes a logic based on energy. Such logic is more precise but has a higher characteristic time, i . e . a greater time resolution, since it is necessary to acquire all the energy released by the residual nucleus to the detector .In an embodiment not illustrated, e . g. to realize a "fast" circuit identified in the previous paragraphs, the electrical signal at the output of the detector when a residual nucleus is captured is processed by means of a derivative readout or processing circuit : in such case the measuring device is more prompt in identifying a potentially useful signal but, given the greater sensitivity of derivative circuits to noise, such an approach is less precise than that based on integrative circuits .

[0037] According to a further aspect of the present invention, schematically illustrated in Figure 6, the alpha particle emission device of the invention is portable and furthermore incorporates a Particle Induced Xray Emission - Rutherford Backscattering detector . In greater detail, an emitter device according to the present invention is arranged on the same side with respect to a sample as a detector, e . g. a silicon surface barrier detector, for Rutherford backscattering at backward angles RBS to estimate data relating to the masses of the nuclei impacted by the alpha particles that scatter backward. Preferably, the RBS detector surrounds the alpha particle emitter device . A further X-ray detector, e . g. an energy-dispersive detector, detects the rays emitted following the ionization produced by the alpha particles in order to determine, according to the Particle-induced X-ray emission technique, the chemical species of the sample . In this way, it is possible to realize a portable spectrometer configured to perform isotopic analyses in the field.

Claims

CLAIMS1 . Alpha particle emitter device, comprising:a detector (RN) of residual nuclei after radioactive decay; a processing circuit (CP) electrically connected to the detector and configured to form an electrical signal based on the energy of the detected, e . g. captured, residual nucleus after decay;an electronic time identification circuit (T) receiving as input the electrical signal to generate a time output representative of the emission of the alpha particle; wherein a plurality of radionuclides is applied on or in the detector such that, when the particle is emitted, the residual nucleus passes through the detector remaining trapped therein, so as to release its energy and generate the electrical signal via the processing circuit (CP) .

2. Emitter device according to claim 1, wherein the radionuclides are such that the residual nucleus after particle emission has an energy comprised between 90 and 150 keV.

3. Device according to any one of the preceding claims, wherein the time identification circuit (T) has a time resolution less than or equal to 10 nanoseconds .

4. Device according to any one of the preceding claims, comprising a further detector (RP) configured to detect the particle orradiation of the radionuclide and a corresponding further electronic processing circuit (CP' ) and a further electronic time identification circuit (T' ) configured to form a further electrical signal based on the energy released by the alpha particle to the further detector and to generate a further time output representative of the reception of the particle or radiation; and a coincidence circuit (CC) receiving as input the time output and the further time output and configured to send a time signal relating to the time of flight of the alpha particle if the coincidence analysis has provided a positive outcome .Device according to claim 4, wherein the coincidence circuit is configured to perform the coincidence analysis on the basis of the reception of said further time signal .Device according to one of claims 4 or 5, wherein the coincidence circuit comprises a delay circuit (CD) configured to delay one between the time output and the further time output .Device according to claim 6, wherein the coincidence circuit is configured to delay said time output .Device according to any one of claims 5 to 7, configured either to generate a binary signal, e . g. of presence or alarm, based on a time threshold, or to estimate a physical parameter, e . g. the thickness of an obj ect crossed by the particle or radiation during the time of flight, on the basis of said time of flight .

9. Device according to any one of the preceding claims, further comprising a Rutherford Backscattering (RBS) detector arranged on the same side of the emitter device with respect to a specimen so as to detect the backscattering induced in a sample by alpha particles, preferably to estimate data relating to the mass of the nuclei impacted by the alpha particles .

10. Device according to any one of the preceding claims, further comprising a Particle Induced X-ray Emission (PIXE) detector, preferably to estimate a chemical species of the s amp 1 e .

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