Portable radiation detection device for operation in intense magnetic fields

Abstract

A portable radiation detection device with a detector unit comprising a scintillator with an array of avalanche photodiodes allows to reliably detect incident ionizing radiation or radiation contamination in the presence of intense magnetic fields of 0.1 Tesla and above.

Claims

1. A portable radiation detection device (10), comprising: a detector unit (12) comprising a scintillator (16) and a photomultiplier (18) coupled to said scintillator (16), said photomultiplier (18) comprising an array of avalanche photodiodes; a control unit (14) comprising control circuitry (38, 40, 50, 56) electrically coupled to said photomultiplier (18); wherein said control unit (14) comprises a high-voltage power supply unit (50), wherein said high-voltage power supply unit (50) comprises a voltage multiplier circuit, and wherein said detector unit (12) comprises a low-drop voltage regulator unit (22) coupled to said voltage multiplier circuit.

2. The device (10) according to claim 1, wherein said detector unit (12) and/or said control unit (14) does not comprise ferromagnetic material.

3. The device (10) according to claim 1, wherein said photomultiplier comprising said array of avalanche photodiodes is a silicon photomultiplier (18).

4. The device (10) according to claim 1, wherein said detector unit (12) and/or said control unit (14) are adapted to detect ionizing radiation at an energy range of 45 keV or more, and/or at an energy range of 1.3 MeV or less.

5. The device (10) according to claim 1, wherein said scintillator (16) comprises Lanthanum bromide, LaBr.sub.3.

6. The device (10) according to claim 1, wherein said detector unit (12) comprises an amplifier circuit (28) electrically coupled to an output of said photomultiplier (18).

7. The device (10) according to claim 6, wherein said detector unit (12) further comprises a filter unit (30) coupled to an output of said amplifier circuit (28), said filter unit (30) in particular comprising an AC-coupled low-pass filter.

8. The device (10) according to claim 7, wherein said detector unit (12) further comprises a discriminator unit (32) electrically coupled to an output of said filter unit (30).

9. The device (10) of claim 8, wherein said discriminator unit (32) comprises a digital pulse stretcher.

10. The device (10) of claim 7, wherein the AC-coupled low-pass filter is a Sallen-Key filter.

11. The device (10) of claim 6, wherein said amplifier unit (2) is a current-feedback operational amplifier (28).

12. The device (10) of claim 6, wherein the amplifier circuit (28) is electrically coupled to the output of said photomultiplier (18) with a gain in the range of 3 to 30 dB.

13. The device (10) according to claim 1, wherein said voltage multiplier circuit comprises a voltage multiplier cascade, in particular a voltage multiplier ladder network comprising capacitors and diodes.

14. The device (10) of claim 13, wherein the voltage multiplier circuit is a Cockcroft-Walton circuit.

15. The device (10) according to claim 1, wherein said high-voltage power supply unit (50) is battery-powered.

16. The device (10) according to claim 1, wherein said control unit (14) comprises a display unit, wherein said display unit comprises both an LCD display unit (42) and an LED display unit (44), wherein said LCD display unit (42) and said LED display unit (44) are electrically coupled to said control circuitry, and wherein said control circuitry is adapted to actuate either said LCD display unit (42) or said LED display unit (44) to display an output signal representative of the incident radiation.

17. The device (10) according to claim 16, wherein said LCD display unit (42) or said LED display unit (44) is actuated selectively depending on a detected presence of an external magnetic field.

18. The device (10) according to claim 1, wherein said control unit (14) comprises an external data interface (46).

19. A portable radiation detection device (10), comprising: a detector unit (12) comprising a scintillator (16) and a photomultiplier (18) coupled to said scintillator (16), said photomultiplier (18) comprising an array of avalanche photodiodes; a control unit (14) comprising control circuitry (38, 40, 50, 56) electrically coupled to said photomultiplier (18); wherein said control circuitry comprises a counter unit (38) electrically coupled to said photomultiplier (18), and a microcontroller unit (40) coupled to said counter unit (38).

Description

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(1) The features and numerous advantages of the present invention will best become apparent from a detailed description of preferred embodiments with reference to the accompanying drawings, in which:

(2) FIG. 1 is a schematic circuit diagram of a portable radiation detection device according to an embodiment of the present invention;

(3) FIG. 2 shows counting statistics obtained with a portable radiation detection device according to an embodiment of the present invention both with and without a magnetic field; and

(4) FIG. 3 shows counting statistics obtained with a portable radiation detection device according to an embodiment of the present invention in magnetic fields of varying strengths.

(5) FIG. 1 is a schematic circuit diagram of a portable radiation detection device 10 according to an embodiment of the present invention. The detection device 10 may be a survey meter comprising a hand-held detector unit or probe unit 12 and a control unit or readout unit 14. In the embodiment of FIG. 1, the detector unit 12 and the control unit 14 are separate units that are connected by electric cable connections for data transfer and voltage supply. In this configuration, the detector unit 12 may be used as a probe with which a user scans apparatuses or surfaces for radioactivity, whereas the bulkier control unit 14 may be worn at a user's belt. However, the invention is not limited to devices with separate units, but may as well comprise a survey meter 10 in which the detector unit 12 and the control unit 14 are integrally formed in a common body.

(6) The detector unit 12 comprises a scintillator crystal 16 coupled to a silicon photomultiplier 18 comprising an array of avalanche photodiodes (not shown) that are sequentially connected on a common silicon substrate (not shown). The silicon photomultiplier 18 is provided in direct physical contact with the scintillator crystal 16. The coupling between the scintillator crystal 16 and the silicon photomultiplier 18 may be established by means of a micro-package made of plastic resin that covers and protects the silicon chip while allowing optical coupling, but other ways of coupling between the scintillator crystal 16 and the silicon photomultiplier 18 may be employed as well.

(7) Incident ionizing radiation 20, which is represented by a flash arrow in FIG. 1, will generate photons in the scintillator crystal 16, and the generated photons will impinge on a fraction of avalanche photodiodes in the array of photodiodes in the silicon photomultiplier 18, where they will generate electrons by means of the photoelectric effect. Every avalanche photodiode in the silicon photomultiplier operates in the Geiger mode and they add their current pulses to produce an accumulated signal at the output.

(8) In a preferred embodiment, the scintillator crystal 16 is made from Lanthanum bromide, LaBr.sub.3. The inventors found that this material is very well-suited for use in a portable radiation detection device 10 due to the low natural activity of the lanthanum, the fast decay time equal to 16 ns, and the high light yield of 65 photons per deposited keV of energy. The emission peak of LaBr.sub.3 is around 380 nm. Lanthanum bromide is hydgroscopic and should hence be encapsulated.

(9) However, the invention is not limited to lanthanum bromide as a scintillator material, and other suitable materials may be used as well.

(10) Silicon photomultipliers 18 that can be used in the context of the present invention are commercially available from, among others, Hamamatsu, SensL, FBK and STMicroelectronics.

(11) As an advantage, the small dimensions of the silicon photomultiplier 18 permit a compact, light and robust design of the detector unit 12. In one exemplary configuration, the inventors chose a silicon photomultiplier 18 with dimensions of about 4 mm4 mm comprising 3600 individual avalanche photodiodes. The microcell pitch is 60 m, whereas the microcell capacitance amounts to 170 fF and the quenching resistance amounts to approximately 350 k. This exemplary silicon photomultiplier 18 has a breakdown voltage of approximately 28 V and a dark current in the range of 200 pA before breakdown. The dark current denotes the average current value flowing in the silicon photomultiplier 18 even in the darkness of the device due to single microcell breakdown or leakage current. The occurrence rate of the microcell firing is the dark current rate which may depend on the bias voltage of a breakdown, the overvoltage OV, and on the temperature.

(12) The detector circuitry will now be described with reference to FIG. 1. The detector unit 12 comprises a bias voltage regulator 22 that provides a bias voltage in the range of 28 to 33 V for the silicon photomultiplier 18. The bias voltage is provided at a bias voltage input 24 of the photomultiplier 18. At a photocurrent signal output 26, the silicon photomultiplier 18 provides the current signal generated by amplification of the detector signal in the chain of avalanche photodiodes. The current signal is provided to an amplifier 28, which may be a low noise and large bandwidth amplifier with a gain in the range of 3 to 30 dB. The amplifier 28 may be a current-feedback operational amplifier, such as an AD8000 with an amplifier bandwidth of about 300 MHz. The inventors found that DC- and AC-coupled large bandwidth amplifiers may largely improve the signal-to-noise ratio.

(13) As can be further taken from the circuit diagram of FIG. 1, the output of the large-bandwidth amplifier 28 is provided to a filter 30, which may be an AC-coupled low-pass filter with three real coincident poles and a time constant as short as 30 ns. The inventors found that good results can be obtained with a second-order Sallen-Key filter and a passive differentiator.

(14) The filtered signal is provided from the filter 30 to a discriminator 32, which is a fast comparator with hysteresis and doubles as a digital pulse stretcher. The discriminator 32 provides as an output a digital pulse train 34 that is sent via a data connection 36, such as a cable connection, to a counter unit 38 provided in the control unit 14. The counter unit 38 provides the data to a microcontroller 40 for further processing. The microcontroller 40 may be a commercially available microchip controller, such as a controller belonging to the PIC 18 family.

(15) The microcontroller 40 may process the acquired data to distinguish between different types of acquired radiation, or to convert the acquired counts or count rates into a dose or dose rate. The acquired data may be displayed at a display unit which may comprise both an LCD display unit 42 and an LED display unit 44. The inventors found that an LCD display unit 42 is usually preferable as a display due to its low power consumption. However, an LCD display unit may be unsuitable in high magnetic fields and may become frozen at magnetic inductions in the range of 1 Tesla or even less. The inventors found that an LED display unit 44 is insensitive to magnetic fields, and hence is well-versed to display radiation readings even in intense magnetic backgrounds.

(16) In an embodiment of the present invention, the LCD display unit 42 and the LED display unit 44 may be employed selectively and alternately, depending on whether or not the survey meter 10 is placed in high magnetic fields. For instance, the control unit 14 may comprise a magnetic detector unit (not shown) to detect and measure the strength of an external magnetic field. If the detected magnetic field is larger than a pre-determined threshold, the LCD display unit 42 may be deactivated automatically, and the LED display unit 44 may be used instead to display radiation readings. If the magnetic field falls below the pre-determined threshold, for instance if the user changes his position and moves away from the source of the magnetic field, the control unit 14 may automatically switch the display back from the LED display unit 44 to the LCD display unit 42 to reduce the power consumption.

(17) The microcontroller 40 is further connected with a data interface 46 via a data interface connection 48. The data interface 46 may be a USB port, or any other data interface that may be used to export the data collected in the control unit 14 to an external device.

(18) The control unit 14 further comprises a power supply unit 50 for power supply of both the various components of the control unit 14 and of the handheld detector unit 12, via a power cable connection 52. The power supply unit 50 is controlled by the microcontroller 40 via a battery monitor and recharger circuit 54.

(19) The power supply unit 50 is preferably battery-powered. The inventors found that Pb accumulators are particularly well-suited due to their insensitivity to high magnetic fields. Provision of the detector bias voltage (typically in the range of 28 to 33 V) then requires an up-conversion in the power supply unit 50, which may be achieved by means of a voltage multiplier circuit of the Cockcroft-Walton type. This is a charge pump-based power supply circuit which does not use any magnetic elements, such as inductors or transformers found in conventional DC-DC converters. Avoidance of magnetic elements and ferromagnetic material in the survey meter 10 reduces the sensitivity to external magnetic fields.

(20) The high-voltage power supply unit 50 comprising the Cockcroft-Walton voltage multiplier provides the output voltage to the handheld detector unit 12 via the power cable connection 52. The detector bias voltage for the silicon photomultiplier 18 is then obtained in the bias voltage regulator 22 by low-drop regulation.

(21) The control unit 14 further comprises a voltage regulator 56. The voltage regulator 56 supplies supply voltages +5V/5V for the analog circuits, and the supply voltage +5V for the digital circuits. The voltage regulator 56 further provides, via a voltage supply line 58, a programmable voltage reference to the bias voltage regulator 22. This allows an indirect gain control that the inventors found very useful in testing and calibrating.

(22) The combination of components as described above provides a survey meter 10 that allows to reliably detect and measure incident radiation in external magnetic fields of up to 1 Tesla or even more. In particular, by combining a semiconductor photomultiplier 18 that employs avalanche photodiodes instead of standard photomultiplier tubes in combination with charge pump-based power circuits, the device is remarkably insensitive to external magnetic fields. The use of Pb accumulators as an electrical power source in combination with high frequency quartz for oscillating circuits and an LED display in addition to a conventional LCD display, likewise contribute to the enhanced insensitivity to magnetic fields. In the survey meter 10 as shown in the circuit diagram of FIG. 1, ferromagnetic materials are almost completely avoided.

(23) Optionally, a spectrometer functionality can be added so that the survey meter 10 can perform gamma spectroscopy. As shown in FIG. 1, the spectrometer functionality can be implemented by means of a fast pulse stretcher 60 and an analog-to-digital converter 62, which provide a spectroscopy signal from the output of the AC-coupled low pass filter 30 to the microcontroller 40. The fast pulse stretcher 60 and the analog-to-digital converter 62 may be placed in the control unit 14; and may be provided as separate units or may be incorporated into the microcontroller 40.

(24) Contamination measurements can be achieved by a specific probe using an appropriate scintillator (such as CsI, BGO or plastic) and a windowless or thin window configuration.

(25) An automatic correction to compensate for the temperature drift of the detector response may likewise be provided. The bias voltage tracks the breakdown voltage drifts due to the detector temperature variations in order to keep the overvoltage (and the gain) independent of the temperature at the first order approximation.

(26) Metal filters may be provided at the scintillator crystal 16 to reduce the energy dependence of the detector response.

(27) The survey meter 10 is capable of detecting and measuring radioactivity in materials or detecting alpha, beta and gamma emissions from radioactive sources in the presence of a high magnetic field with very fast response rates. The inventors found that photon energies between 45 keV and 1.3 MeV and equivalent dose rates in the range of a fraction of Sv per hour to a few mSv per hour may be reliably detected, irrespective of the external magnetic field.

(28) Exemplary experimental results that illustrate the insensitivity to external magnetic fields are shown in FIGS. 2 and 3. FIG. 2 is a count rate histogram that shows the distribution of count rates obtained from a sample source. Curve 1 shows the counts in the absence of an external magnetic field, whereas curve 2 shows counts that have been obtained when both the handheld detector unit 12 and the control unit 14 are placed in a high magnetic field of approximately 1 Tesla. The intermediate curve 3 shows the count rates in a test in which only the handheld detector unit 12 is placed inside a high magnetic field of approximately 1 Tesla, whereas the control unit 14 is placed outside the magnetic field (0 Tesla).

(29) As can be taken from a comparison of curves 1, 2 and 3, the deviations of the sample rate are insignificant. These results confirm that the survey meter 10 is highly insensitive to an ambient magnetic field.

(30) FIG. 3 shows a similar counting rate histogram, but based on counting statistics acquired when the survey meter 10 is placed in an external magnetic field of varying field strengths. Curve 4 corresponds to a magnetic induction of 0.3 Tesla, whereas curve 5 corresponds to 0.6 Tesla and curve 6 again to 1 Tesla. The counting rate time base for the measurements shown in FIG. 3 are 1 second, and the measurement lasted 1200 seconds. Again, no significant variation in the count rate appears, which confirms that the survey meter 10 performs reliably over a wide range of different magnetic inductions.

(31) The portable radiation detection device according to the present invention is not only well-versed for radiation surveys at particle accelerators, such as those used in research and in industrial and medical applications. The device may also be used when conducting radiation surveys at medical positron emission tomography/magneto-resonance imaging scanners, where radioactivity is handled and administered to patients in the presence of high-magnetic fields.

(32) Another context for an application of the portable radiation detection device is a particle accelerator coupled to magnetic-resonance-imaging scanners for image-guided radiation therapy.

(33) Further applications may involve the magnetic separation in industrial mineral processing, where a separation of magnetic particles is required for various purposes in the processing of ferromagnetic materials. Natural radioactivity is present in these applications, and the potential presence of orphan sources may be a concern and require radiation monitoring.

(34) The invention can also be useful at scrap dealerships to detect the radioactive contamination of scrap metals with orphan sources.

(35) Fire brigades can use the portable radiation detection device according to the invention when called to intervene in unknown environments where radioactive sources may be present together with high magnetic fields.

(36) Radioactive contamination monitoring in continuous casting of steel and other metals where a magnetic field is used to ensure quality of the casting process, is another promising application. Radioactive contamination monitoring in magnetic convection control for the molten silicon in the crucible of a Czochralski crystal-growing process can also benefit from the survey meter of the present invention.

REFERENCE SIGNS

(37) 10 survey meter 12 handheld detector unit/probe unit of survey meter 10 14 readout unit/control unit of survey meter 10 16 scintillator crystal 18 Silicon photomultiplier 20 incident radioactive radiation 22 bias voltage regulator 24 bias voltage input of Silicon photomultiplier 18 26 photocurrent signal output of Silicon photomultiplier 18 28 large-bandwidth amplifier 30 AC-coupled low pass filter 32 discriminator 34 output pulse train 36 data connection 38 counter unit 40 microcontroller 42 LCD display unit 44 LED display unit 46 data interface 48 data interface connection 50 power supply unit 52 power cable connection 54 battery monitor and recharger circuit 56 voltage regulator 58 supply line 60 fast pulse stretcher 62 analog-to-digital converter