Detector and method of operation

Abstract

A method of operation of a scintillator detector includes a scintillator and a photodetector is described, together with a device embodying the method. The method includes the steps of: periodically producing a light pulse; impinging at least some of the light from a successive plurality of such light pulses onto a light-receptive part of the photodetector; measuring the electrical response of the photodetector; processing the electrical response of the photodetector to determine a pulse height and a variance of pulse height; numerically processing the pulse height and variance of pulse height so determined to obtain at least a first data item characteristic of the response of the photodetector.

Claims

1. A method of operation of a scintillator detector comprising a scintillator and a photodetector, the method comprising: periodically producing a light pulse; impinging at least some of the light from a successive plurality of such light pulses onto a light-receptive part of the photodetector; measuring the electrical response of the photodetector; processing the electrical response of the photodetector to determine a pulse height and a variance of pulse height; numerically processing the pulse height and variance of pulse height so determined to obtain at least a first data item characteristic of the response of the photodetector; wherein a light source is provided configured to emit light in the ultraviolet spectrum; periodically producing a light pulse including light in the ultraviolet spectrum; impinging at least some of the UV light from a successive plurality of such light pulses onto a light-receptive part of the scintillator; measuring the electrical response of the scintillator; processing the electrical response of the scintillator to obtain at least a second data item characteristic of the response of the scintillator; and the use of a visible light source impinging on the photodetector to obtain the first data item and thereby a measure of photodetector gain change and the use of a separate ultraviolet light source impinging on the scintillator to obtain the second data item and thereby a measure of scintillator brightness.

2. The method in accordance with claim 1, comprising verifying the electrical response of the photodetector by comparing at least the first data item against a predetermined reference response.

3. The method in accordance with claim 1, comprising outputting a control signal to the photodetector, which signal is modified in part responsive at least to the value of the first data item.

4. The method in accordance with claim 1, comprising numerically processing the pulse height and variance of pulse height so determined to obtain at least a first data item characteristic of the response of the photodetector comprises generating a first data item correlated to the total number of photons detected at the photodetector.

5. The method in accordance with claim 1, wherein the light source is a light-emitting diode (LED) source comprising at least one LED.

6. The method in accordance with claim 1, wherein the scintillator comprises a gamma ray scintillator.

7. The method in accordance with claim 1, wherein the scintillator comprises a solid state scintillator.

8. The method in accordance with claim 1, wherein the scintillator comprises an alkali metal halide scintillator optionally doped with an activator.

9. The method in accordance with claim 1, wherein the photodetector comprises a photomultiplier.

10. The method in accordance with claim 1, wherein the photodetector comprises a silicon photomultiplier.

11. The method in accordance with claim 1, comprising a light source temperature compensation step, wherein a measurement is taken of the temperature of the light source, and a correction factor is applied to the measured response of the photodetector, for example dynamically pulse by pulse, that corrects for the known variation of the light output of the light source with temperature.

12. The method in accordance with claim 1, comprising a photodetector temperature compensation step, wherein a measurement is taken of the temperature of the photodetector, and a correction factor is applied to the measured response of the photodetector, for example dynamically pulse by pulse, that corrects for the known variation of the photodetector response with temperature.

13. The method in accordance with claim 1, wherein a light source is provided configured to emit light in the visible spectrum.

14. The method in accordance with claim 1, employing a visible light LED source and a UV LED source.

15. The method in accordance with claim 14, wherein the visible LED source comprises a second UV LED source equivalent to first LED source but overcoated with a temperature stable fluorescent material to fluoresce in the visible spectrum.

16. The method in accordance with claim 1, comprising verifying the electrical response of the scintillator by comparing at least the second data item against a predetermined reference response.

17. The method in accordance with claim 16, comprising optionally additionally or alternatively outputting a control signal to the photodetector, which signal is modified in part responsive at least to the value of the second data item.

18. The method in accordance with claim 1, comprising outputting a control signal to the photodetector, which signal is modified in part responsive at least to the value of the second data item.

Description

(1) The invention will now be described by way of example only with reference to the accompanying drawings in which:

(2) FIG. 1 is an exploded partial view of a portable detector including a scintillator detector having a neutron detector module and a gamma ray detector module to which the principles of the invention may be applied;

(3) FIG. 2 is an exploded schematic of the processing modules of a portable detector such as illustrated in FIG. 1;

(4) FIG. 3 is an example embodiment of a gamma ray detector arrangement such as illustrated in FIG. 2 modified with visible LED sources for calibration in accordance with the principles of the invention;

(5) FIG. 4 is a simple schematic of the control electronics for a system embodying a blue LED for calibration in accordance with the principles of the invention;

(6) FIG. 5 is a simple schematic of the control electronics for a system additionally embodying a UV LED in accordance with the principles of the invention;

(7) FIG. 6 is a simple schematic of a suitable arrangement of data processing and feedback where a reference library is used;

(8) FIG. 7 shows the temperature dependence of the scintillation yield of typical known inorganic scintillators;

(9) FIG. 8 shows the temperature dependence of the gain of typical known silicon photomultipliers;

(10) FIG. 9 shows an incident isotropic free-in-air gamma-radiation fluence-rate spectrum for a NaI(Tl) detector;

(11) FIG. 10 shows the relative intensity shift with temperature of a known LED;

(12) FIG. 11 shows the spectral shift with temperature of a known LED.

(13) An example implementation of the invention is described with reference to a portable detector with dual functionality for gamma rays and thermal neutrons comprising a high sensitivity gamma ray scintillator and a thermal neutron detector in a single package. The embodiment uses a gamma detector module (11) using a 25 mm25 mm25 mm CsI(Tl) scintillator for gamma rays, and a neutron detector module (21) using a 25 mm100 mm .sup.6LiF:ZnS scintillator for thermal neutrons. The components are compactly associated together within a housing (31). The modules are shown with the housing partially open in FIG. 1.

(14) The light from each scintillator is detected by an array of Silicon Photomultipliers (SiPMs). A general schematic of the arrangement of scintillators, SiPMs and control electronics modules is shown in exploded view in FIG. 2. In the case of the CsI(Tl) gamma detector the SiPMs are mounted on a PCB and attached to the crystal. The .sup.6LiF:ZnS thermal neutron scintillator uses an acrylic light guide, not shown, to channel the light to the SiPMs. The exploded view shows respectively a gamma scintillator (12) and SiPM (13) arrangement with a gamma digital board (14) and gamma analogue board (15); and a neutron scintillator (22) and SiPM (23) arrangement with a neutron digital board (24) and neutron analogue board (25).

(15) FIG. 3 is an example embodiment of a gamma ray detector arrangement such as illustrated in the combined detector of FIG. 2 modified with visible LED sources for calibration in accordance with the principles of the invention. A 7 mm21 mm three SiPM arrangement has blue light LEDs at either end to provide a source of pulsed light in accordance with the principles of the invention.

(16) In the embodiment a compact and portable combined gamma and neutron detector is shown. In accordance with the FIG. 3 modification a gamma ray detector arrangement such as illustrated in the combined portable detector of FIG. 2 is modified by use of a puled LED source in accordance with the principles of the invention. This is by way of illustration. It will be appreciated that the principles of the invention are not limited to such a combination detector or to the modification of the gamma ray detector in such a combined portable detector.

(17) FIG. 4 is a simple schematic of the control electronics for a system embodying a blue LED for calibration in accordance with the principles of the invention. A single blue LED is shown. The control electronics comprises an ASIC that includes an LED pulsar module to cause the LED to produce a successive plurality of periodic light pulses that impinge on the light-receptive part of the SiPM-detector and modules to measure and process the electrical response of the photodetector as described in general above and in accordance with the principles discussed below.

(18) FIG. 5 is a simple schematic of the control electronics for a system additionally embodying a UV LED in accordance with the principles of the invention. The ASIC includes a further LED pulsar module to cause the UV LED to produce a successive plurality of periodic UV pulses that impinge on the light-receptive part of the scintillator.

(19) FIG. 6 is a simple schematic of a suitable arrangement of data processing and feedback logic where a reference library is used to verify the electrical response of the photodetector and/or scintillator by comparing a measured response against a predetermined reference response.

(20) As discussed above, accurate detector calibration and gain stabilisation are critical factors for the operational effectiveness of scintillation detectors. The gain, or response, of all scintillation detectors exhibits a degree of variability (or drift).

(21) FIGS. 7 and 8 show data for known systems that illustrate some of the underlying causes of the problem, taken from the manufacturer's published data in each case. FIG. 7 shows the temperature dependence of the scintillation yield of typical known inorganic scintillators (Harshwa/QS scintillation detector catalogueSaint Gobain Industries 1992) including the CsI(Tl) scintillator used in the embodiment. FIG. 2 shows the temperature dependence of the gain of SensL's FC series SiPMs.

(22) As discussed above, the use of natural radiation background as a source for calibration is known. However the method can be difficult to automate due to the variability in rate and the potential presence of other background sources. FIG. 7 shows a typical background trace for a NaI(Tl) detector (John E. Pattison, Enhancement of natural background gamma-radiation dose around uranium microparticles in the human body, DOI: 10.1098/rsif.2009.0300, Published 23 Sep. 2009). The characteristic peaks at 1460 keV due to 40K and at 2610 keV due to 208Tl (in the 232Th series) are apparent.

(23) As discussed above, an alternative approach is to employ a stable pulsed LED light source to produce a repeatable signal in the detector. There are however two problems with this method. First, although the light pulse will characterised the photo-detector and subsequent signal processing stages, it does not reveal variation in light output from scintillating crystal which as shown in FIG. 7, can be up to 20% for standard Alkaline Halide scintillators over a temperature range of 20 to +50 C. Second the light source used, typically LEDs, are themselves subject to variation with temperature. FIG. 10 is an illustrative example of LED relative intensity shift over temperature. Normalized at 25 C. Avago Technologies. FIG. 11 is an illustrative example of LED spectral shift over temperature. Chromaticity shift duv <0.005. Avago Technologies.

(24) The invention seeks to mitigate some of the problems associated with prior art pulsed LED source methods and systems to provide for more effective calibration and in particular gain stabilisation of a gamma ray scintillator radiation detector such as that in FIGS. 1 and 2. Accordingly an LED light source (such as shown in the example representations of FIGS. 3 to 5) for example a visible LED light source, may be incorporated within the housing and positioned to supply pulsed illumination to the gamma ray scintillator and associated SiPM for example on power up or during use.

(25) The method thus involves the general known principle of the use of a stable pulsed LED to illuminate the photodetector and the numerical processing of the response of the photodetector to that pulsed illumination. The key characterising feature of the method is that it involves measuring not only mean channel excited by the pulsed LED, but variance of pulse height (or resolution). This enables the number of photons captured to be calculated using the formulas below. This largely removes LED variability from the calibration.

(26) Where detector resolution is defined as the Full Width Half Maximum (FWHM) or E divided by peak energy E, the contribution of the different noise sources, to overall resolution R, can be described as follows

(27) ${\left(\frac{E}{E}\right)}^2=R^2=R_{\mathrm{inh}}^2+R_p^2+R_{\mathrm{DN}}^2$

(28) Where, R.sub.inh is the inherent resolution in the crystal (which to a first approximation is fixed), R.sub.DN is contributed by dark noise in the photo-detector (while this is proportional to temperature it is readily characterised, and for the latest generation of SiPMs will be a relatively small factor at normal operating temperatures). This leaves the term R.sub.p as the main variable, which is due to photo collection statistics in the photo-detector, calculated as follows.
R.sub.p=2.35{square root over ((1+Var(M))/N.sub.p)}

(29) Where Var(M) is the gain variance of the photo-detector, and N.sub.p is the number of photons collected. Hence from E and E the number of photons captured during calibration can be calculated.

(30) A burst of LED pulses injected into the photo-detector (e.g. on power up) will verify photon sensitivity, and can be made relatively insensitive to LED luminosity.

(31) A number of additional techniques can be employed to mitigate further the limiting factors by additionally compensating for temperature drift in LEDs based on known techniques from established methods based on pulsed LED systems.

(32) These include for example direct temperature compensation, whereby the temperature at the LED is monitored using a suitable temperature measuring device (not shown) and used to compensate for variation in intensity, and using the ratio of pulse height from an LED with two or more bias voltages.

(33) In a further refinement such as illustrated by the example schematic embodiment in FIG. 5, in addition to a visible LED light source, a UV LED light source may be incorporated within the housing and positioned to supply pulsed illumination to the gamma ray scintillator. By exciting luminescence in the scintillator directly this may enable characterisation of the complete system including the scintillator. The use of UV LEDs can further verify integrity of the scintillator crystal and light collection path, including cladding, optical coupling etc.

(34) Thus, this preferred embodiment combines:

(35) using variance in pulse height (for a visible LED) to determine the number of photons detected, thus making the system relatively independent of LED brightness;

(36) using a UV LED to interrogate scintillator brightness as well as photo-detector gain change;

(37) employing a dual LED (visible and UV) to combine the benefits of the above two in a compact portable device.