A new Electronic Neutron Dosimeter (END) for reliable personal ...

A new Electronic Neutron Dosimeter (END) for reliable personal dosimetry † H. Inga, T. Cousinsb, H.R. Andrewsa, R. Machrafia, A. Voevodskiya, V. Kovaltchouka, E.T.H. Clifforda, M. Robinsa, C. Larssonb, R. Hugronb, and J. Brownb a Bubble Technology Industries Inc., Hwy. 17, Chalk River, Ontario, Canada K0J 1J0 b Defence Research and Development Canada, Ottawa, Ontario, Canada ABSTRACT Tests of existing electronic neutron dosimeters by military and civilian groups have revealed significant performance limitations. To meet the operational requirements of emergency response personnel to a radiological/nuclear incident as well as those in the nuclear industry, a new END has been developed. It is patterned after a unique commercial neutron spectral dosemeter known as the N-probe.* It uses a pair of small special scintillators on tiny photomultiplier tubes. Special electronics were designed to minimize power consumption to allow for weeks of operation on a single charge. The size, performance, and data analysis for the END have been designed to meet/exceed international standards for electronic neutron dosimeters. Results obtained with the END prototype are presented. Keywords: neutron, EPD, dose, doserate, pulse-shape discrimination

1. INTRODUCTION Bubble Technology Industries (BTI) has been involved in the development of the state-of-art neutron spectrometers for decades. Figs. 1 and 2 show experimental neutron spectrometers from the 1960’s and 1970’s based, respectively, on several hydrogen-recoil gas counters and a liquid scintillator. Both of these spectrometers were leading-edge technology at their time and used for various scientific measurements see e.g. 1 & 2.

Fig. 1. First generation gaseous hydrogen recoil spectrometer.

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Work supported by CRTI * N-probe is the trademark of Bubble Technology Industries Inc.

Chemical, Biological, Radiological, Nuclear, and Explosives (CBRNE) Sensing IX, edited by Augustus Way Fountain III, Patrick J. Gardner, Proc. of SPIE Vol. 6954, 695419, (2008) · 0277-786X/08/$18 · doi: 10.1117/12.782489 Proc. of SPIE Vol. 6954 695419-1 2008 SPIE Digital Library -- Subscriber Archive Copy

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ir Fig. 2. First generation liquid scintillation spectrometer.

Figs. 3 and 4 show modern versions of these spectrometers respectively. The major changes are reduction in size – made possible by the advances in microelectronics over the intervening years – and the improvements to user interface so that these instruments can be used by non-specialists. The development of these instruments has been driven mainly by military applications involving Defence Research and Development Canada, Ottawa (DRDC-O). However, these instruments are now used by many non-military groups world-wide. Descriptions of these instruments can be found in the open literature 3, 4, 5.

Fig. 3. Modern version of gaseous hydrogen recoil spectrometer (ROSPEC).

Proc. of SPIE Vol. 6954 695419-2

Fig. 4. Modern version of liquid scintillation spectrometer (N-probe)

The events of 9/11 had a significant impact on the development of radiation detection instrumentation. Many terrorist scenarios that require the detection of radiation in pre-event or post-event incidences gave rise to new detector specifications for new radiation detection devices. Among these was the need for an accurate electronic neutron dosimeter to be worn by first responders when involved in the aftermath of an event where neutron hazard is present. Such an instrument is necessary to allow the first responders to be cognizant of their own radiation exposures during the execution of their duties, relative to recommended and regulatory dose guidelines. Tests of the performance of available commercial dosimeters by military groups e.g. 6 have shown significant deficiencies in relation to NATO or TTCP (The Technical Co-operation Program) operational specifications. The observed poor performance of commercial wearable neutron dosimeters was not un-expected. Development of electronic personal (pocket) dosimeters (EPDs) for neutrons has been on-going for over 2 decades see e.g. 7. Despite much concerted effort more recently to develop such a product mainly for civilian nuclear application see e.g. 8, tests of resulting products along with other commercial products have also revealed many short-comings see e.g. 9-12. In the absence of any apparent technically-sound approach to remedy these short-comings, we embarked on a project to develop an alternative electronic neutron dosimeter (END) to address these limitations and to meet/exceed current international performance standards as specified in IEC 61526.

2. DESIGN OF THE END Existing neutron pocket dosimeters are poor in two main technical areas: detection efficiency and energy dependence. Both deficiencies are associated with the fundamental properties of the neutron sensors that were chosen for the products. We decided to use similar types of neutron sensors for the END as in the well-proven BTI N-probe 5: a proton-recoil scintillator for fast neutrons and a specially-shielded thermal neutron detector for thermal and epi-thermal neutrons. Calculations showed that small versions of such sensors are amply adequate to meet desired performance specifications. The other technical challenges were to minimize power consumption and device size to meet mechanical specifications. Initial estimates showed these were achievable using the latest low-power electronics along with special designs of critical subsystems such as high-voltage power supply and analogue front-end pulse-processing circuitry.


Fig. 5 shows the two sensors used in the END. The fast neutron sensor is a special 1 cm3 scintillator with excellent n/γ pulse-shape discrimination properties used with a tiny PMT. The thermal/epi-thermal sensor is a small 6LiI scintillator used with a similar PMT. Like in the N-probe, this sensor is normally embedded within a specially-designed, thick, boron shell in order to alter the response of the 6LiI detector. Fig. 6 shows the calculated effective response of the thermal/epi-thermal sensor when it is used as a detector assembly. The shell “flattens” the response so that thermal neutrons do not overwhelm the response to epi-thermal neutrons.

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Fig. 5. The fast and thermal neutron sensors chosen for END.

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Fig. 6. Response of thermal/epi-thermal detector with and without boron shell.


3. END DEVELOPMENT Fig. 7 shows the prototype of the END, used for testing its radiological performance. In its final version, the electronic boards will be reduced in size by about 40% and the device will be packaged in its final enclosure. The current design of the final enclosure is shown in Fig. 8. The final device is expected to be well within desired mechanical specifications.

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Fig. 7. Prototype of END used for testing performance.

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Fig. 8. Current concept of final END enclosure.


Tests have been done on the sub-systems of the END and with the prototype. Fig. 9 (top) shows the response of the fast neutron sensor to monoenergetic neutrons of various energies from 400 keV to 1.3 MeV. The bottom figure shows that the response with neutron energy is linear, after correcting for scintillation efficiency, as expected. Of course, the detection of higher energy neutrons up to more than 20 MeV poses no concern for the chosen scintillator size. The main challenge is the detection of neutrons below 1 MeV due to possible gamma-ray signal interference.

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Fig. 9. Response of fast neutron sensor to monoenergetic neutrons (above) and linearity analysis

Fig. 10 shows a two dimensional display of the pulse-shape verses energy (light output) produced by the fast neutron sensor signals. The separation between neutron signals (top line) and gamma (bottom line) is clean to below 500 keV. The END performs neutron/gamma discrimination using digital analysis of these signals.


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Fig. 10. Two-dimensional n/γ plot showing good separation of neutrons to below 500 keV.

Fig. 11 shows the response of the thermal/epi-thermal detector to 137Cs and 60Co gamma rays as well as to 238PuLi and 252 Cf neutron sources. The main spectrum illustrates the excellent discrimination between gamma rays and neutrons (taken without the boron shell) based on pulse-height alone. The insert shows the spectrum (with the shell in place) taken in a separate experiment using a 252Cf source. The counts to the right of the thermal peak are due to epi-thermal neutrons. Based on the data in Figs. 10 and 11, we have chosen to use the fast neutron sensor to detect neutrons from 600 keV to 20 MeV and the thermal/epi-thermal sensor from thermal to 600 keV.

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Fig. 11. Response of thermal neutron scintillator to neutrons and gamma rays.


Calculation of power-consumption by the END prototype has been made under conditions of maximum power usage. With the chosen batteries, the END is expected to operate in excess of a month on a full charge. Preliminary tests on the END prototype have been made using Am-Be and Cf sources that are normally used for detector calibration. The doserates measured are in good agreement with expected values. More stringent measurements must yet be made with monoenergetic neutrons, produced using a Van de Graaff accelerator, in the presence of a strong gamma-ray source. More work is on-going on the display software and user-interface. The current display consists of 2 screens. The lower button of the END shown in Fig. 8 alternates the displays. One display shows the neutron doserate and the other shows the accumulated neutron dose from a pre-set start time. The current screen display format is shown in Fig. 12. These displays are not yet finalized and may be modified due to end-user input.

Fig. 12. END displays as currently configured. The displays are alternated by the push of a button.


4. END STATUS At the time of writing, the software for the END is being finished. Testing of the prototype is on-going at BTI and, when finished, the prototype will be given to DRDC-O and LANL for independent testing. Any suggested modifications from these tests will be implemented. Then the final components will be incorporated into an enclosure resembling that in Fig. 8 and final product testing will be done by BTI before product release.

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Cross, W.G. and Ing, H., [The Dosimetry of Ionizing Radiation, Vol 11], Academic Press Publishers, New York, Chapter 2 (1987) Ing, H., Cross, W.G. and Bunge, P.J. “Spectrometers For Radiation Protection At Chalk River Nuclear Laboratories”, Radiat. Prot. Dosim. 10, 137-145 (1985). Ing, H., Clifford, T., McLean, T., Webb, W., Cousins, T. and Dhermain, J., “ROSPEC – A Simple Reliable High Resolution Neutron Spectrometer”, Radiat. Prot. Dosim. 70, 273-278 (1997). Ing, H., Djeffal, S., Clifford, T., Li, L., Noulty, R. and Machrafi, R., “Modification Of ROSPEC To Cover Neutrons From Thermal To 18 MeV”, Radiat. Prot. Dosim., Advanced Access published on May 23, (2007) doi:10.1093/rpd/ncm 073. Ing, H. Djeffal, S., Clifford, T., Machrafi, R. and Noulty, R., “Portable Spectroscopic Neutron Probe”, Radiat. Prot. Dosim., Advanced Access published on June 16 (2007) doi:10. 1093/rpd/ncm 049. CBR AG48 [TTCP CBR AG48 Experiment, Evaluation of Technical Performance Characteristics of Neutron Dosimeters] conducted Nov. 18-22 (2002) at DRDC-O, Ottawa, Canada; report unpublished. Gibson, J.A.B., “Personal Alarm Neutron Dosemeters”, Radiat. Prot. Dosim. 10, 197-205 (1985). Rannon, A., Clech, A., Devita, A., Dollo, R. and Pescaye, G., “Evaluation of Individual Neutron Dosimetry By A Working Group In the French Nuclear Industry”. Radiat. Prot. Dosim. 70, 181-186 (1997). Luszik-Bhadra, M., “Electronic Personal Dosimeters: The Solution To Problems of Individual Monitoring In Mixed Neutron/Photon Fields?” Radiat. Prot. Dosim. 110, 747-752 (2004). Luszik-Bhadra, M., “Compliance of Electronic Personal Neutron Dosemeters With The New International Standard IEC61526” Radiat. Prot. Dosim. 125, 15-18 (2007). Fiechtner, A., Boschung, M. and Werli, C., “Performance of a PADC personal neutron dosimeter at simulated and real workplace fields of the nuclear industry”, Radiat. Prot. Dosim., Advance Access published on June 19 (2207) doi: 10. 1093/rpd/ncm 065. Lindborg, L., Bolognese-Milsztajn, T., Boschung, M., Coeck, M., Curzio, G., d’Errico, F., Fieshtner, A., Hallforth, D., Lievens, B., Lillhok, J. E., Lovefors-Daun, A., Lacoste, V., Luszik-Bhadra, M., Reginatto, M., Schumacher, H., Tanner, R. and Vanhavere, E., “Application of Workplace Correction Factors To Dosemeter Results For The Assessment Of Personal Doses At Nuclear Facilities” Radiat. Prot. Dosim. 124, 213-218 (2007).
