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Experimental model of the device for detection of nuclear cycle materials by photoneutron technology
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 261 (2007) 360–364 www.elsevier.com/locate/nimb
Experimental model of the device for detection of nuclear cycle materials by photoneutron technology A.M. Bakalyarov, M.D. Karetnikov *, K.N. Kozlov, V.I. Lebedev, E.A. Meleshko, B.A. Obinyakov, I.E. Ostashev, N.A. Tupikin, G.V. Yakovlev Russian Research Center ‘‘Kurchatov Institute’’, Scientific and Technical Division ‘Electronics’, 123182 Kurchatov sq., Moscow, Russia Available online 3 April 2007
Abstract The inherent complexity of sea container control makes them potentially dangerous for smuggling nuclear materials. The experts believe that only active technologies based on recording the products of induced radiation from sensitive materials might solve the problem. The paper reports on the experimental model of the device on the basis of the electron LINAC U-28 for detection of nuclear materials by photonuclear technology. The preliminary numerical optimization of output units (converter, filter, collimator) for shaping the bremsstrahlung was carried out. The setup of experimental device and initial results of recording the prompt and delayed fission products are discussed. 2007 Elsevier B.V. All rights reserved. PACS: 25.20. x; 25.85. w; 29.17.+w Keywords: Photonuclear technology; Nuclear materials; Electron accelerator; Neutron detector
1. Introduction The inherent complexity of sea container control makes them potentially dangerous for smuggling illicit materials and items in them, including nuclear materials (NM). The experts believe that only active technologies based on recording the induced radiation from sensitive materials might solve the problem [1]. The photonuclear technology seems very promising for detection of NM [2]. It provides high sensitivity, wide range of detectable materials, accuracy of localization, low activation of inspected object. The other things being equal, the rate of induced fission of uranium by bremsstrahlung produced by 10 MeV, 10 lA electron beam, is
*
Corresponding author. Tel.: +7 495 1969042; fax: +7 495 1966032. E-mail address: [email protected] (M.D. Karetnikov).
0168-583X/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.03.070
• Close to the fission rate generated by neutron generator with 5 · 1011 1/s intensity; • 30 million times higher than 235U spontaneous fission; • 500 thousand times higher than 239Pu spontaneous fission. The photoneutron technology can be implemented in the following way. The pulsed electron beam is scanned in a vertical direction and incident on a bremsstrahlungproduction converter. At each moment, a very narrowangle bremsstrahlung of the same maximum energy as the electron beam energy is generated. Due to the vertical scanning of electron beam, the averaged (through the scanning period) directional diagram of bremsstrahlung has the ‘‘knife’’-type shape extended in the vertical direction along the beam-line. The object moves along the device in horizontal direction so all zones of the object are irradiated during the process of control. The threshold of photofission (c, f) and photoneutron (c, n) reactions for fissile and some other NM (deuterium, lithium-6) is relatively low
A.M. Bakalyarov et al. / Nucl. Instr. and Meth. in Phys. Res. B 261 (2007) 360–364
(<6 MeV) but significantly higher (>10 MeV) for structural materials. The excited fission products also emit the delayed radiation by beta-decay of fission fragments. Thus, the sensitive materials can be revealed by prompt and delayed radiation induced by photonuclear reactions. A part of bremsstrahlung passed through the object is recorded by the gamma-detector array. This ‘‘transmitted’’ image of the controlled object can be used for advancing the revealing capabilities of the photonuclear technology. 2. Description of experimental model of the device The electron beam is produced by the U-28 electron linear accelerator (LINAC) [3]. The output beam parameters are as follows: Peak beam energy, MeV Beam current, mA Pulse repetition rate, 1/s Pulse duration, ls
6–12 0–200 5–400 1–2
The basic components and dimensions of the experimental model are given in Fig. 1. The bremsstrahlung is produced at the converted and shaped by the collimator to the narrow beam. The rest of electron beam is absorbed by the filter. The signals from neutron detectors are processed by the data acquisition and control (DAC) system and transmitted to the computer (PC). 3. Numerical design of experimental model The material and geometry of the converter, collimator and beam filter have a great impact on the performance of the device. For their proper choice, the numerical simulation of gamma-neutron transfer and recording was carried out. The real apparatus and items of experimental setup were simulated by parallelepipeds, cylinders, rings. The GEANT3.21 [4] program package has been used. The per-
361
formance criterion was the photo fission rate of the hypothetical spherical 10 g solid target from pure 235U located in the center of an empty 20-feet sea-cargo container. All possible processes were taken into account: (c, n), (c, 2n), (c, p), (c, f). For each run, 2 · 108–109 electrons were diced. Traditionally, the converter is made of tungsten or gold. However, such converter is a source of prompt neutron background due to the low threshold and high cross-section of photoneutron (c, n) reactions for these materials. At the moderate beam energy (8–12 MeV) it is more reasonable to use copper due to the higher (c, n) reaction threshold (9.9 MeV). The comparison of time dependence of counting rate of prompt neutron background in cases of tungsten and copper converters is given in Fig. 2(a). When using the tungsten converter, the maximum rate of photofission of 235U is only 1.4 times higher than is case of copper one (Fig. 2(b)); however, the background is more than five times higher. Thus, when measuring the prompt photoneutrons from the sensitive materials, copper is preferable as a material for converter and collimator as far as it gives much better effect/background ratio compared to heavy metals (tungsten, gold, lead). The delayed neutrons from fission fragments are recorded between the LINAC pulses with the delay (several ms) between end of irradiation and start of counting to avoid overlapping with the prompt neutron background. In this case, good results can be achieved using the tungsten converter and collimator [2]. In this work, the decay of counting rate of prompt neutron background was less than 1 ms as a result optimization of neutron detector. It may be seen from Fig. 2(b), the dependence of fission rate on the converter thickness is comparatively flat with the slight maximum in the range of 0.5–10 mm. Another important matter is the growth of radiation dose (in the container) with the converter thickness (Fig. 3(a)). Thus, the converter thickness of 0.8 mm is a reasonable compromise between efficiency of the converter and low radiation dose.
Fig. 1. Basic components of the experimental module.
362
A.M. Bakalyarov et al. / Nucl. Instr. and Meth. in Phys. Res. B 261 (2007) 360–364
Fig. 2. Counting rate of (a) background neutrons and (b) relative rate of photo fission of 235U versus converter thickness in case of tungsten (upper curve) and copper (lower curve) converters.
Fig. 3. Exposure doses from (a) bremsstrahlung versus converter thickness, (b) bremsstrahlung and electrons versus filter thickness.
The filter is used for absorption of the rest of electron beam after passing the converter. Aluminum was chosen as filter material as far as it simultaneously meets the following requirements: • low atomic number to minimize the additional dose of bremsstrahlung produced in the filter; • good heat conductivity for filter cooling; • high threshold of photonuclear reaction (to avoid generation of photoneutrons in the filter). The optimal filter width was assessed by the ratio of exposure doses from Bremsstrahlung and electrons (Fig. 3(b)). This ratio grows with the filter thickness as far as electrons transform their energy to the radiation and ionization. At the chosen filter thickness of 2.5 cm, the electron beam is almost entirely absorbed. The collimator is used for shaping the Bremsstrahlung to the ‘‘radiant beam’’ with the certain divergence angle. By the set of parameters (manufacturability, high cross-section of gamma-ray absorption, low neutron background), copper was chosen as the collimator material. The numerical experiments on assessment of quality of collimation at
various widths of collimator show that the quality of collimation is sufficient at the width of 15–20 cm. The results displayed in Figs. 2 and 3 are calculated for the 9.5 MeV electron beam. The numerical simulations at the 8–12 MeV electron beam energy lead to the similar conclusions. 4. Testing of the experimental model The neutrons were recorded by four CNM-18 neutron counters (helium-filled, 320 mm in length, 32 mm in diameter) mounted in the polyethylene moderator. The moderator was shielded by cadmium foil and borated polyethylene to reduce the background of thermal and epithermal stray photoneutrons. The thicknesses of the front and back wall of the moderator were optimized by the maximum efficiency of counting the prompt fission neutrons. The preamplifier unit is assembled on the top of each counter. The signals from preamplifiers are transmitted through the twisted pairs to the multichannel data acquisition and control (DAC) system. Each recorded event is represented by codes of number of electron beam pulse, number of counter, pulse amplitude, time of pulse registra-
A.M. Bakalyarov et al. / Nucl. Instr. and Meth. in Phys. Res. B 261 (2007) 360–364
tion (measured from each pulse of the electron beam). The programmable interlock during 0–20 ms (3.3 ls step) after the electron beam pulse can be set to avoid overload of control system just after beam pulse. The numerical simulations (Fig. 2(a)) show that the counting rate of prompt neutrons falls down rapidly (the characteristic time is around 40 ls). However, the neutron detector is exposed to the radiation dose as high as several mrad/pulse. It leads to a very long (more than 100 ls) dead time after an electron beam pulse required a neutron counter to recover. The dead time was decreased down to 30 ls by undertaking the following measures: • using counters filled by pure helium (without quenching organic additions); • shielding the neutron counters by iron and concrete bricks; the dose was reduced by 2 orders of magnitude; • double differentiation of the signals from the neutron counter [2]. The sensitive material was imitated by 100 g of natural uranium. The series of experiments included measurements of neutron yield from (a) unshielded uranium, (b) shielded uranium, (c) shielding without uranium, and (d) background. Five centimeter lead bricks, 6 cm iron bars/ and 15 cm water cans were used for shielding. The pulse repetition rate was 20 1/s, the duration of each measurement was 180 s. The prompt neutrons were recorded in the interval 0.03–0.3 ms, the start of delayed neutron counting was delayed by 3 ms. The distance between the output of electron accelerator and the holder of sensitive material is 170 cm. Table 1 displays the results of experiments. The data are normalized by the number of detectors and charge of electron beam I Æ t (in mC) where I – averaged beam current, t – duration of measurement. The background associated with delayed neutron recording is less than 2 count/mC. It can be seen that the detection of sensitive material by prompt photoneutrons is effective in case of low background (e.g. water or iron shielding). When the sensitive material is covered by lead, the delayed neutrons are more
363
informative due to the better signal/noise ratio. The higher yield of prompt neutrons in case of lead compared to lead shielded uranium can be explained by some instability of the accelerator parameters. 5. Future plans In experimentations, the dead time of a neutron counter after an electron beam pulse was decreased mostly by high shielding of a neutron detector against bremsstrahlung. However, the design considerations show that this way is hardly available for commercial control system. The gating of neutron counter during the electron beam pulse seems very promising for sufficient reduction of the dead time and advancing the prompt neutron recording. The preliminary experiments show that the dead time can be reduced down to 10–15 ls by short-time removal of HV from the counter during the electron beam pulse. Currently, the extensive numerical simulations of application of photonuclear technology for detection of sensitive materials in the sea cargo containers are being carried out. Various options for container load and position of sensitive material in the container are considered. The efficiency of recording the prompt and delayed neutrons and delayed gamma-rays are examined. A special attention is devoted to the decision making algorithm. The following problems will be considered: • Determination of the most informative time windows for recording the prompt and delayed radiation; • Choice, qualitative comparison and ranking of detectors of different types for optimization of the detection system. Analysis of the complex multidetector system. • Using the data of the container content obtained by other detection technologies (e.g. radiography), custom declaration, statistical model of the container for advancing the revealing capabilities of the photonuclear technology. The works will also address calculation of induced activity and assessment of potential harmful impacts of radiation (such as damage of photo and video materials,
Table 1 Averaged number of counts recorded by 1 neutron detector normalized by electron beam charge (in mC) Type of shielding
Energy (MeV) 8
Unshielded 6 cm iron 12 cm iron 5 cm lead 10 cm lead 15 cm water 30 cm water
9
10.5
11.5
NB
NPN
NB
NPN
NB
NPN
NDN
NB
NPN
NDN
63.5 41.3 39.4 203 254 43.7 43.0
455 127 45.0 – – 174 156
87.1 101 85.2 – –
843 440 104 – – – –
527 525 600 27095 33520 597 550
5877 1575 887 27387 33420 1205 792
87.4 15.2 7.5 10.0 3.3 7.6 5.0
782 2504 5110 35445 43052 – –
8147 4270 5509 36247 43100 – –
130 25.0 15.2 17.7 4.5 – –
–
NB: background, NPN: prompt neutrons, NDN: delayed neutrons.
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degradation of most of semiconductor elements, alteration of optical features and radiation and chemical transmutation). Acknowledgements This work has been supported by the International Scientific and Technical Center, project # 2637 [5]. The authors are grateful to the specialists of Moscow Engineering Physics Institute V.V. Kadilin, V.M. Lubkov, A.A. Makarov, A.A. Suharev for fruitful discussions and technical support.
References [1] PNL, Nuclear and radiological threat detection for vehicle cargo, Pacific Northwest National Laboratory, PIET-43741-QR-005, February 28, 2002. [2] J.L. Jones et al., Pulsed Photoneutron Interrogation: The GNT Demonstration System. WINCO-1225, Idaho, Idaho Falls, 1994. [3] O.A. Valdner et al. The Linear Electron Accelerator U-28. Debugging and Putting into Operation. In All-Union Seminar on Linear Accelerators. Kharkov, KhPhTI, 1976. [4] GEANT3.21 Detector Description and Simulation Tool, Manual, CERN Program Library, CERN Geneva, Switzerland, 1993. [5] http://tech-db.istc.ru/istc/db/projects.nsf/prjn/2637.
Nuclear Instruments and Methods in Physics Research B 261 (2007) 360–364 www.elsevier.com/locate/nimb
Experimental model of the device for detection of nuclear cycle materials by photoneutron technology A.M. Bakalyarov, M.D. Karetnikov *, K.N. Kozlov, V.I. Lebedev, E.A. Meleshko, B.A. Obinyakov, I.E. Ostashev, N.A. Tupikin, G.V. Yakovlev Russian Research Center ‘‘Kurchatov Institute’’, Scientific and Technical Division ‘Electronics’, 123182 Kurchatov sq., Moscow, Russia Available online 3 April 2007
Abstract The inherent complexity of sea container control makes them potentially dangerous for smuggling nuclear materials. The experts believe that only active technologies based on recording the products of induced radiation from sensitive materials might solve the problem. The paper reports on the experimental model of the device on the basis of the electron LINAC U-28 for detection of nuclear materials by photonuclear technology. The preliminary numerical optimization of output units (converter, filter, collimator) for shaping the bremsstrahlung was carried out. The setup of experimental device and initial results of recording the prompt and delayed fission products are discussed. 2007 Elsevier B.V. All rights reserved. PACS: 25.20. x; 25.85. w; 29.17.+w Keywords: Photonuclear technology; Nuclear materials; Electron accelerator; Neutron detector
1. Introduction The inherent complexity of sea container control makes them potentially dangerous for smuggling illicit materials and items in them, including nuclear materials (NM). The experts believe that only active technologies based on recording the induced radiation from sensitive materials might solve the problem [1]. The photonuclear technology seems very promising for detection of NM [2]. It provides high sensitivity, wide range of detectable materials, accuracy of localization, low activation of inspected object. The other things being equal, the rate of induced fission of uranium by bremsstrahlung produced by 10 MeV, 10 lA electron beam, is
*
Corresponding author. Tel.: +7 495 1969042; fax: +7 495 1966032. E-mail address: [email protected] (M.D. Karetnikov).
0168-583X/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.03.070
• Close to the fission rate generated by neutron generator with 5 · 1011 1/s intensity; • 30 million times higher than 235U spontaneous fission; • 500 thousand times higher than 239Pu spontaneous fission. The photoneutron technology can be implemented in the following way. The pulsed electron beam is scanned in a vertical direction and incident on a bremsstrahlungproduction converter. At each moment, a very narrowangle bremsstrahlung of the same maximum energy as the electron beam energy is generated. Due to the vertical scanning of electron beam, the averaged (through the scanning period) directional diagram of bremsstrahlung has the ‘‘knife’’-type shape extended in the vertical direction along the beam-line. The object moves along the device in horizontal direction so all zones of the object are irradiated during the process of control. The threshold of photofission (c, f) and photoneutron (c, n) reactions for fissile and some other NM (deuterium, lithium-6) is relatively low
A.M. Bakalyarov et al. / Nucl. Instr. and Meth. in Phys. Res. B 261 (2007) 360–364
(<6 MeV) but significantly higher (>10 MeV) for structural materials. The excited fission products also emit the delayed radiation by beta-decay of fission fragments. Thus, the sensitive materials can be revealed by prompt and delayed radiation induced by photonuclear reactions. A part of bremsstrahlung passed through the object is recorded by the gamma-detector array. This ‘‘transmitted’’ image of the controlled object can be used for advancing the revealing capabilities of the photonuclear technology. 2. Description of experimental model of the device The electron beam is produced by the U-28 electron linear accelerator (LINAC) [3]. The output beam parameters are as follows: Peak beam energy, MeV Beam current, mA Pulse repetition rate, 1/s Pulse duration, ls
6–12 0–200 5–400 1–2
The basic components and dimensions of the experimental model are given in Fig. 1. The bremsstrahlung is produced at the converted and shaped by the collimator to the narrow beam. The rest of electron beam is absorbed by the filter. The signals from neutron detectors are processed by the data acquisition and control (DAC) system and transmitted to the computer (PC). 3. Numerical design of experimental model The material and geometry of the converter, collimator and beam filter have a great impact on the performance of the device. For their proper choice, the numerical simulation of gamma-neutron transfer and recording was carried out. The real apparatus and items of experimental setup were simulated by parallelepipeds, cylinders, rings. The GEANT3.21 [4] program package has been used. The per-
361
formance criterion was the photo fission rate of the hypothetical spherical 10 g solid target from pure 235U located in the center of an empty 20-feet sea-cargo container. All possible processes were taken into account: (c, n), (c, 2n), (c, p), (c, f). For each run, 2 · 108–109 electrons were diced. Traditionally, the converter is made of tungsten or gold. However, such converter is a source of prompt neutron background due to the low threshold and high cross-section of photoneutron (c, n) reactions for these materials. At the moderate beam energy (8–12 MeV) it is more reasonable to use copper due to the higher (c, n) reaction threshold (9.9 MeV). The comparison of time dependence of counting rate of prompt neutron background in cases of tungsten and copper converters is given in Fig. 2(a). When using the tungsten converter, the maximum rate of photofission of 235U is only 1.4 times higher than is case of copper one (Fig. 2(b)); however, the background is more than five times higher. Thus, when measuring the prompt photoneutrons from the sensitive materials, copper is preferable as a material for converter and collimator as far as it gives much better effect/background ratio compared to heavy metals (tungsten, gold, lead). The delayed neutrons from fission fragments are recorded between the LINAC pulses with the delay (several ms) between end of irradiation and start of counting to avoid overlapping with the prompt neutron background. In this case, good results can be achieved using the tungsten converter and collimator [2]. In this work, the decay of counting rate of prompt neutron background was less than 1 ms as a result optimization of neutron detector. It may be seen from Fig. 2(b), the dependence of fission rate on the converter thickness is comparatively flat with the slight maximum in the range of 0.5–10 mm. Another important matter is the growth of radiation dose (in the container) with the converter thickness (Fig. 3(a)). Thus, the converter thickness of 0.8 mm is a reasonable compromise between efficiency of the converter and low radiation dose.
Fig. 1. Basic components of the experimental module.
362
A.M. Bakalyarov et al. / Nucl. Instr. and Meth. in Phys. Res. B 261 (2007) 360–364
Fig. 2. Counting rate of (a) background neutrons and (b) relative rate of photo fission of 235U versus converter thickness in case of tungsten (upper curve) and copper (lower curve) converters.
Fig. 3. Exposure doses from (a) bremsstrahlung versus converter thickness, (b) bremsstrahlung and electrons versus filter thickness.
The filter is used for absorption of the rest of electron beam after passing the converter. Aluminum was chosen as filter material as far as it simultaneously meets the following requirements: • low atomic number to minimize the additional dose of bremsstrahlung produced in the filter; • good heat conductivity for filter cooling; • high threshold of photonuclear reaction (to avoid generation of photoneutrons in the filter). The optimal filter width was assessed by the ratio of exposure doses from Bremsstrahlung and electrons (Fig. 3(b)). This ratio grows with the filter thickness as far as electrons transform their energy to the radiation and ionization. At the chosen filter thickness of 2.5 cm, the electron beam is almost entirely absorbed. The collimator is used for shaping the Bremsstrahlung to the ‘‘radiant beam’’ with the certain divergence angle. By the set of parameters (manufacturability, high cross-section of gamma-ray absorption, low neutron background), copper was chosen as the collimator material. The numerical experiments on assessment of quality of collimation at
various widths of collimator show that the quality of collimation is sufficient at the width of 15–20 cm. The results displayed in Figs. 2 and 3 are calculated for the 9.5 MeV electron beam. The numerical simulations at the 8–12 MeV electron beam energy lead to the similar conclusions. 4. Testing of the experimental model The neutrons were recorded by four CNM-18 neutron counters (helium-filled, 320 mm in length, 32 mm in diameter) mounted in the polyethylene moderator. The moderator was shielded by cadmium foil and borated polyethylene to reduce the background of thermal and epithermal stray photoneutrons. The thicknesses of the front and back wall of the moderator were optimized by the maximum efficiency of counting the prompt fission neutrons. The preamplifier unit is assembled on the top of each counter. The signals from preamplifiers are transmitted through the twisted pairs to the multichannel data acquisition and control (DAC) system. Each recorded event is represented by codes of number of electron beam pulse, number of counter, pulse amplitude, time of pulse registra-
A.M. Bakalyarov et al. / Nucl. Instr. and Meth. in Phys. Res. B 261 (2007) 360–364
tion (measured from each pulse of the electron beam). The programmable interlock during 0–20 ms (3.3 ls step) after the electron beam pulse can be set to avoid overload of control system just after beam pulse. The numerical simulations (Fig. 2(a)) show that the counting rate of prompt neutrons falls down rapidly (the characteristic time is around 40 ls). However, the neutron detector is exposed to the radiation dose as high as several mrad/pulse. It leads to a very long (more than 100 ls) dead time after an electron beam pulse required a neutron counter to recover. The dead time was decreased down to 30 ls by undertaking the following measures: • using counters filled by pure helium (without quenching organic additions); • shielding the neutron counters by iron and concrete bricks; the dose was reduced by 2 orders of magnitude; • double differentiation of the signals from the neutron counter [2]. The sensitive material was imitated by 100 g of natural uranium. The series of experiments included measurements of neutron yield from (a) unshielded uranium, (b) shielded uranium, (c) shielding without uranium, and (d) background. Five centimeter lead bricks, 6 cm iron bars/ and 15 cm water cans were used for shielding. The pulse repetition rate was 20 1/s, the duration of each measurement was 180 s. The prompt neutrons were recorded in the interval 0.03–0.3 ms, the start of delayed neutron counting was delayed by 3 ms. The distance between the output of electron accelerator and the holder of sensitive material is 170 cm. Table 1 displays the results of experiments. The data are normalized by the number of detectors and charge of electron beam I Æ t (in mC) where I – averaged beam current, t – duration of measurement. The background associated with delayed neutron recording is less than 2 count/mC. It can be seen that the detection of sensitive material by prompt photoneutrons is effective in case of low background (e.g. water or iron shielding). When the sensitive material is covered by lead, the delayed neutrons are more
363
informative due to the better signal/noise ratio. The higher yield of prompt neutrons in case of lead compared to lead shielded uranium can be explained by some instability of the accelerator parameters. 5. Future plans In experimentations, the dead time of a neutron counter after an electron beam pulse was decreased mostly by high shielding of a neutron detector against bremsstrahlung. However, the design considerations show that this way is hardly available for commercial control system. The gating of neutron counter during the electron beam pulse seems very promising for sufficient reduction of the dead time and advancing the prompt neutron recording. The preliminary experiments show that the dead time can be reduced down to 10–15 ls by short-time removal of HV from the counter during the electron beam pulse. Currently, the extensive numerical simulations of application of photonuclear technology for detection of sensitive materials in the sea cargo containers are being carried out. Various options for container load and position of sensitive material in the container are considered. The efficiency of recording the prompt and delayed neutrons and delayed gamma-rays are examined. A special attention is devoted to the decision making algorithm. The following problems will be considered: • Determination of the most informative time windows for recording the prompt and delayed radiation; • Choice, qualitative comparison and ranking of detectors of different types for optimization of the detection system. Analysis of the complex multidetector system. • Using the data of the container content obtained by other detection technologies (e.g. radiography), custom declaration, statistical model of the container for advancing the revealing capabilities of the photonuclear technology. The works will also address calculation of induced activity and assessment of potential harmful impacts of radiation (such as damage of photo and video materials,
Table 1 Averaged number of counts recorded by 1 neutron detector normalized by electron beam charge (in mC) Type of shielding
Energy (MeV) 8
Unshielded 6 cm iron 12 cm iron 5 cm lead 10 cm lead 15 cm water 30 cm water
9
10.5
11.5
NB
NPN
NB
NPN
NB
NPN
NDN
NB
NPN
NDN
63.5 41.3 39.4 203 254 43.7 43.0
455 127 45.0 – – 174 156
87.1 101 85.2 – –
843 440 104 – – – –
527 525 600 27095 33520 597 550
5877 1575 887 27387 33420 1205 792
87.4 15.2 7.5 10.0 3.3 7.6 5.0
782 2504 5110 35445 43052 – –
8147 4270 5509 36247 43100 – –
130 25.0 15.2 17.7 4.5 – –
–
NB: background, NPN: prompt neutrons, NDN: delayed neutrons.
364
A.M. Bakalyarov et al. / Nucl. Instr. and Meth. in Phys. Res. B 261 (2007) 360–364
degradation of most of semiconductor elements, alteration of optical features and radiation and chemical transmutation). Acknowledgements This work has been supported by the International Scientific and Technical Center, project # 2637 [5]. The authors are grateful to the specialists of Moscow Engineering Physics Institute V.V. Kadilin, V.M. Lubkov, A.A. Makarov, A.A. Suharev for fruitful discussions and technical support.
References [1] PNL, Nuclear and radiological threat detection for vehicle cargo, Pacific Northwest National Laboratory, PIET-43741-QR-005, February 28, 2002. [2] J.L. Jones et al., Pulsed Photoneutron Interrogation: The GNT Demonstration System. WINCO-1225, Idaho, Idaho Falls, 1994. [3] O.A. Valdner et al. The Linear Electron Accelerator U-28. Debugging and Putting into Operation. In All-Union Seminar on Linear Accelerators. Kharkov, KhPhTI, 1976. [4] GEANT3.21 Detector Description and Simulation Tool, Manual, CERN Program Library, CERN Geneva, Switzerland, 1993. [5] http://tech-db.istc.ru/istc/db/projects.nsf/prjn/2637.