DEVELOPING RADIATION RESISTANT THERMAL NEUTRON DETECTORS FOR THE E_LIBANS PROJECT: PRELIMINARY RESULTS

DEVELOPING RADIATION RESISTANT THERMAL NEUTRON DETECTORS FOR THE E_LIBANS PROJECT: PRELIMINARY... Abstract Radiation-resistant, gamma-insensitive, active thermal neutron detectors were developed to monitor the thermal neutron cavity of the E_LIBANS project. Silicon and silicon carbide semiconductors, plus vented air ion chambers, were chosen for this purpose. This communication describes the performance of these detectors, owing on the results of dedicated measurement campaigns. INTRODUCTION E_LiBANS project (2016–18) is funded by INFN National Scientific Committee 5 (Technological, inter-disciplinary and accelerators research), San Paolo Company and C.R.T. foundation. One of its objectives is achieving intense thermal neutron fields in a macroscopic closed cavity (size can be varied up to a maximum of ~30 × 30 × 10 cm3)(1). The thermal field is obtained from a medical 18 MV LINAC ELEKTA SLi/PRECISE through a specially designed photo-neutron converter and moderating assembly. The field in the cavity is highly thermalized and homogeneous. The thermal neutron fluence rate inside the cavity may reach ~106 cm−2s−1 and is accompanied by a photon field up to tens of mGy h−1 in terms of air kerma rate. The LINAC beam has pulsed time structure, and this clearly reflects on the neutron and photon fields. THREE THERMAL NEUTRON DETECTORS HAVE BEEN EMPLOYED - Thermal neutron rate detector(2, 3) (TNRD), silicon-based and prone to radiation damage when exposed to fluence higher than 1011 cm−2. - Silicon carbide, expected to be highly radiation resistant. - vented ion chambers, expected to be highly radiation resistant. All detectors were sensitized to thermal neutron through a 6LiF deposition. A special deposition process allows to precisely deposit multiple detectors at the same time. This work shows how detectors were calibrated and their main performance when exposed in the mixed photon-thermal neutron field in the E_LiBANS cavity. READOUT ELECTRONICS All detectors can operate in current mode. This relies on a dedicated ultralow current analog board that drives the radiation-induced current (tens of fA or higher) to a resistor, making it measurable as a voltage drop. By changing this resistor, different amplification values (labeled 1× and 0.1×) can be chosen, according to the different sensitivity of the tested devices. Silicon carbide sensors also operate in pulse mode, through a traditional nuclear spectrometry chain formed by a charge sensitive preamplifier and a Gaussian shaping amplifier based on CREMAT components. Commercial digitizers are used in both current and pulse modes to transfer the information to a PC. DETECTION TECHNIQUES TNRD The TNRD(2) was developed within INFN project NESCOFI@BTF. It is an easy-to-use, small device. Its gamma sensitivity is minimal and works over a wide range of thermal neutron fluence rate values(3). Due to its degree of validation and long operating history, the TNRD was used as reference in this work. Nevertheless, it is made of silicon, so it may suffer radiation damage when exposed to a thermal fluence higher than 1011 cm−2. Large exposures affect the reticular structure of the Silicon, thus compromising the detector’s response(2). Silicon carbide sensors With the aim of finding a better radiation resistant material, silicon carbide (SiC) detectors have been studied. This material was an excellent candidate for high fluence thermal neutron fields, due to its energy gap being higher than Silicon’s (about three times(4)). SiC has been used already for nuclear reactor in-core instruments(4). Its minimal dimensions (sensitive area mm2 or lower) cause very low electronic noise. Its leakage current is smaller than 1 pA when the bias voltage is 20 V or lower. Its capacitance is in the order of hundreds of pF. From capacitance–voltage (C–V) measurements it was possible to get information about the depletion layer thickness, resulted in ~2 or 3 μm (Figure 1). This is confirmed by tests with alpha particles from an 241Am source (Figure 2). The sensitive layer of the detector is too thin to stop alpha particles (~5.5 MeV), so that only a part of their energy is released and collected in the detector. The spectrum in Figure 2 shows a main peak, due to alphas normally impinging the detector, plus a structure on the right, due to oblique particles. These release larger energy than normally colliding ones. Figure 1. View largeDownload slide Measurement of depletion layer thickness (~3 μm) of SiC detector (sensitive area 7.6 mm2) thanks to the elaboration of capacitance–voltage measurements. Figure 1. View largeDownload slide Measurement of depletion layer thickness (~3 μm) of SiC detector (sensitive area 7.6 mm2) thanks to the elaboration of capacitance–voltage measurements. Figure 2. View largeDownload slide Alpha particle of 241Am source spectra obtained with SiC detector at different bias voltage values. Figure 2. View largeDownload slide Alpha particle of 241Am source spectra obtained with SiC detector at different bias voltage values. Vented ion chambers Air Ion chamber with parallelepipedal sensitive volume of ~5 cm3 (2.5 cm × 2.5 cm × 0.8 cm) were manufactured. The optimal value of the saturation bias voltage (200 V) was found by X-ray irradiation. These chambers work in pairs: one is coated with 6LiF, whilst the other (uncoated) is used to subtract the residual photon signal. Results TNRD TNRD detectors have been calibrated using the radial thermal column of the ENEA Casaccia TRIGA reactor(5). This facility is equipped with a remotely controlled beam shutter and, according to the reactor power, can stably provide thermal neutron fluence rates from 102 up to 1.5 × 106 cm−2s−1. The thermal neutron fluence rate per unit reactor power is 1.59 ± 0.03 cm−2s−1 W−1. This value was measured through gold foils activation technique. As the neutron fluence rates in the E_LIBANS cavity are very similar to these values, the TRIGA thermal column proved to be a valuable calibration tool. According to the calibration experiment, the TNRD calibration factor (Table 1) was 0.249 ± 0.005 μV cm2 s (output voltage level per unit fluence rate. All thermal fluence rates in this paper are expressed in terms of the quantity sub-Cadmium thermal fluence rate in the Westcott convention(6)). Table 1. Calibration factors of the devices operating in current mode. All values are reported to 0.1× amplification. Device Calibration condition at TRIGA reactor power (MW) Calibration coefficient (μVcm2 s) TNRD 1 (0.249 ± 0.005) SiC + 6LiF 0.5 and 1 (3.16 ± 0.07)10−4 SiC bare 1 (1.10 ± 0.03)10−5 Ion chamber + 6LiF 0.1 and 1 (4.45 ± 0.08)10−2 Ion chamber bare 1 (2.06 ± 0.06)10−5 Device Calibration condition at TRIGA reactor power (MW) Calibration coefficient (μVcm2 s) TNRD 1 (0.249 ± 0.005) SiC + 6LiF 0.5 and 1 (3.16 ± 0.07)10−4 SiC bare 1 (1.10 ± 0.03)10−5 Ion chamber + 6LiF 0.1 and 1 (4.45 ± 0.08)10−2 Ion chamber bare 1 (2.06 ± 0.06)10−5 Table 1. Calibration factors of the devices operating in current mode. All values are reported to 0.1× amplification. Device Calibration condition at TRIGA reactor power (MW) Calibration coefficient (μVcm2 s) TNRD 1 (0.249 ± 0.005) SiC + 6LiF 0.5 and 1 (3.16 ± 0.07)10−4 SiC bare 1 (1.10 ± 0.03)10−5 Ion chamber + 6LiF 0.1 and 1 (4.45 ± 0.08)10−2 Ion chamber bare 1 (2.06 ± 0.06)10−5 Device Calibration condition at TRIGA reactor power (MW) Calibration coefficient (μVcm2 s) TNRD 1 (0.249 ± 0.005) SiC + 6LiF 0.5 and 1 (3.16 ± 0.07)10−4 SiC bare 1 (1.10 ± 0.03)10−5 Ion chamber + 6LiF 0.1 and 1 (4.45 ± 0.08)10−2 Ion chamber bare 1 (2.06 ± 0.06)10−5 The TNRD was exposed in the E_LIBANS cavity with size (20 cm × 20 cm × 5 cm) and the Monitor Unit rate of the LINAC was varied from ~100 up to 600 MU min−1, thus allowing to demonstrate the linearity of the detector’s response (Figure 3). Figure 3. View largeDownload slide Linearity test of TNRD using different LINAC dose rate (thermal neutron fluence rate at 500 MU min−1 is ~1.5 × 106 cm−2 s−1). Figure 3. View largeDownload slide Linearity test of TNRD using different LINAC dose rate (thermal neutron fluence rate at 500 MU min−1 is ~1.5 × 106 cm−2 s−1). The pulsed structure of the field in the E_LiBANS cavity was appreciated by increasing to 0.25 MHz the sampling rate of the TNRD digitizer. In the example shown in Figure 4 the LINAC Monitor Unit rate was 200 min−1. The pulse duration was ~5.2 ms and the pulse repetition rate was ~0.21 kHz. Figure 4. View largeDownload slide LINAC pulsed structure measured in the cavity with TNRD (sampling frequency 0.25 MHz). Figure 4. View largeDownload slide LINAC pulsed structure measured in the cavity with TNRD (sampling frequency 0.25 MHz). Silicon carbide Figures 5 and 6 show the pulse height distributions (spectra) obtained with the SiC detectors (sensitive area 1 mm2), deposited with 6LiF or bare, respectively. As in the case of alpha particles (Figure 2), the peak broadening in Figure 5 is related to the very thin depletion layer of the detector. Alphas and Tritons from the 6Li neutron captures release only a fraction of their energy. This fraction increases as the particles directions change from perpendicular (events on the left) to oblique (right tail), with respect to the detector surface. Interestingly, a similar distribution (with much lower integral) is obtained from the bare detector. This could be attributed to residual fast neutrons interacting with Carbon or Silicon, or to boron impurities in the substrate. Figure 5. View largeDownload slide Spectrum obtained during a measurement in the center of the E_LiBANS cavity (size 20 cm × 20 cm × 5 cm) with a SiC + 6LiF (measurement time: 180 s). Figure 5. View largeDownload slide Spectrum obtained during a measurement in the center of the E_LiBANS cavity (size 20 cm × 20 cm × 5 cm) with a SiC + 6LiF (measurement time: 180 s). Figure 6. View largeDownload slide Spectrum obtained during a measurement in the center of the cavity the E_LiBANS cavity (size 20 cm × 20 cm × 5 cm) with a bare SiC detector (measurement time: 180 s). Figure 6. View largeDownload slide Spectrum obtained during a measurement in the center of the cavity the E_LiBANS cavity (size 20 cm × 20 cm × 5 cm) with a bare SiC detector (measurement time: 180 s). After depositing the 6LiF on the SiC, a preliminary test with thermal neutrons was performed in the HOTNES(7, 8) thermal neutron facility (ENEA–INFN Frascati), where the thermal neutron fluence can be varied from 600 to 1000 cm−2 s−1 and its value is known through standard gold foil measurements. The calibration factor for SiC operating in pulse mode was 3686 ± 133 cm−2 (fluence rate per unit count rate). The calibration coefficients of SiC + 6LiF or bare SiC in current mode at TRIGA reactor are shown in Table 1. The linearity test performed in the E_LIBANS cavity, by varying the LINAC Monitor Unit rate, is shown in Figure 7. By subtracting the bare SiC signal from the SiC + 6LiF one, the contribution due to the 6LiF layer only can be achieved (‘difference’ line in Figure 7). This is ~97% of the SiC + 6LiF signal. Figure 7. View largeDownload slide Linearity test of SiC detectors studied by varying the LINAC Monitor Unit rate. By subtracting the bare SiC signal from the SiC + 6LiF one, the contribution due to the 6LiF layer only can be achieved (thermal neutron fluence rate at 500 MU min−1 is ~1.5 × 106 cm−2 s−1). Figure 7. View largeDownload slide Linearity test of SiC detectors studied by varying the LINAC Monitor Unit rate. By subtracting the bare SiC signal from the SiC + 6LiF one, the contribution due to the 6LiF layer only can be achieved (thermal neutron fluence rate at 500 MU min−1 is ~1.5 × 106 cm−2 s−1). Ion chambers The calibration coefficients for the Ion chamber + 6LiF or bare Ion chamber, operating in current mode, are shown in Table 1. The 6LiF-loaded chamber responds, in the TRIGA neutron field, ~2000 times more than the bare one. The linearity was verified at thermal power level of ~100 and 500 kW and the calibration coefficients are shown in Table 1. CONCLUSIONS Three thermal neutron detection techniques have been developed for monitoring a high fluence rate (106 cm−2 s−1 or higher) closed thermal neutron cavity (adjustable size, up to 30 cm × 30 cm × 10 cm) in the framework of the INFN project E_LIBANS. In addition to the well-established TNRD, two new sensors with higher radiation resistance have been developed, namely Silicon carbide (sensitive area 1 mm2) and small volume (5 cm3) ion chambers. In all cases, the devices are made sensitive to thermal neutrons to 6LiF deposition. The preliminary tests described in this work suggest that the new devices may be suited for the purposes of the E-LIBANs project. However, further experiments to complete their characterization are still needed, such as radiation resistance studied in very intense neutron fields. ACKNOWLEDGEMENTS This work has been supported by the E_LIBANS project (INFN National Scientific Committee 5—Technological, inter-disciplinary and accelerators research), San Paolo Company and C.R.T. foundation. The staff of TRIGA reactor at ENEA Casaccia are greatly acknowledged. Special thanks to Ruo Redda director, Veltri Radiodiagnostics section director from A.O.U. San Luigi di Orbassano, Dr V. Rossetti, Dr U. Nastasi, Dr E. Madon from A.O.U. Città della Salute and Scienza di Torino, Dr A. Zanini from INFN, sr. S. Petruzzi director of the logistics department of the Univeristy of Torino and ELEKTA s.p.a. technical division. REFERENCES 1 Costa , M. et al. . Neutron sources based on medical Linac . Il Nuovo Cimento 38C 180 , ( 2015 ). 2 Bedogni , R. et al. . Experimental characterization of semiconductor-based thermal neutron detectors . Nucl. Instrum. Methods Phys. Res. A 780 , 51 – 54 ( 2015 ). Google Scholar CrossRef Search ADS 3 Bedogni , R. , Bortot , D. , Pola , A. , Introini , M. V. , Gentile , A. , Esposito , A. , Gómez-Ros , J. M. , Palomba , M. and Grossi , A. A new active thermal neutron detector . Radiat. Prot. Dosim. 161 , 241 – 244 ( 2014 ). Google Scholar CrossRef Search ADS 4 Gerhardt , R. Properties and applications of Silicon Carbide, chap.13. InTech Europe ( 2011 ). 5 Palomba , M. , Carta , M. , Falconi , L. , Iorio , M. G. , Bedogni , R. , Pola , A. , Treccani , M. et al. . Testing newly developed thermal neutron detectors at the ENEA TRIGA RC‐1 research reactor. Rotterdam (NL), RRFM2017 European Research Reactor Conference, poster (14−18 May 2017 ). 6 NPL report DQL RN008 . Thermal fluence and dose equivalent standards at NPL (March 2005 ). 7 Bedogni , R. , Pietropaolo , A. and Gómez-Ros , J. M. The thermal neutron facility HOTNES: theoretical design . Appl. Radiat. Isot. 127 , 68 – 72 ( 2017 ). Google Scholar CrossRef Search ADS PubMed 8 Bedogni , R. , Sperduti , A. , Pietropaolo , A. , Pillon , M. , Pola , A. and Gómez-Ros , J. M. Experimental characterization of HOTNES: A new thermal neutron facility with large homogeneity area . Nucl. Instrum. Methods Phys. Res. A 843 , 18 – 21 ( 2017 ). Google Scholar CrossRef Search ADS © The Author(s) 2018. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices) http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Radiation Protection Dosimetry Oxford University Press

DEVELOPING RADIATION RESISTANT THERMAL NEUTRON DETECTORS FOR THE E_LIBANS PROJECT: PRELIMINARY RESULTS

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Abstract

Abstract Radiation-resistant, gamma-insensitive, active thermal neutron detectors were developed to monitor the thermal neutron cavity of the E_LIBANS project. Silicon and silicon carbide semiconductors, plus vented air ion chambers, were chosen for this purpose. This communication describes the performance of these detectors, owing on the results of dedicated measurement campaigns. INTRODUCTION E_LiBANS project (2016–18) is funded by INFN National Scientific Committee 5 (Technological, inter-disciplinary and accelerators research), San Paolo Company and C.R.T. foundation. One of its objectives is achieving intense thermal neutron fields in a macroscopic closed cavity (size can be varied up to a maximum of ~30 × 30 × 10 cm3)(1). The thermal field is obtained from a medical 18 MV LINAC ELEKTA SLi/PRECISE through a specially designed photo-neutron converter and moderating assembly. The field in the cavity is highly thermalized and homogeneous. The thermal neutron fluence rate inside the cavity may reach ~106 cm−2s−1 and is accompanied by a photon field up to tens of mGy h−1 in terms of air kerma rate. The LINAC beam has pulsed time structure, and this clearly reflects on the neutron and photon fields. THREE THERMAL NEUTRON DETECTORS HAVE BEEN EMPLOYED - Thermal neutron rate detector(2, 3) (TNRD), silicon-based and prone to radiation damage when exposed to fluence higher than 1011 cm−2. - Silicon carbide, expected to be highly radiation resistant. - vented ion chambers, expected to be highly radiation resistant. All detectors were sensitized to thermal neutron through a 6LiF deposition. A special deposition process allows to precisely deposit multiple detectors at the same time. This work shows how detectors were calibrated and their main performance when exposed in the mixed photon-thermal neutron field in the E_LiBANS cavity. READOUT ELECTRONICS All detectors can operate in current mode. This relies on a dedicated ultralow current analog board that drives the radiation-induced current (tens of fA or higher) to a resistor, making it measurable as a voltage drop. By changing this resistor, different amplification values (labeled 1× and 0.1×) can be chosen, according to the different sensitivity of the tested devices. Silicon carbide sensors also operate in pulse mode, through a traditional nuclear spectrometry chain formed by a charge sensitive preamplifier and a Gaussian shaping amplifier based on CREMAT components. Commercial digitizers are used in both current and pulse modes to transfer the information to a PC. DETECTION TECHNIQUES TNRD The TNRD(2) was developed within INFN project NESCOFI@BTF. It is an easy-to-use, small device. Its gamma sensitivity is minimal and works over a wide range of thermal neutron fluence rate values(3). Due to its degree of validation and long operating history, the TNRD was used as reference in this work. Nevertheless, it is made of silicon, so it may suffer radiation damage when exposed to a thermal fluence higher than 1011 cm−2. Large exposures affect the reticular structure of the Silicon, thus compromising the detector’s response(2). Silicon carbide sensors With the aim of finding a better radiation resistant material, silicon carbide (SiC) detectors have been studied. This material was an excellent candidate for high fluence thermal neutron fields, due to its energy gap being higher than Silicon’s (about three times(4)). SiC has been used already for nuclear reactor in-core instruments(4). Its minimal dimensions (sensitive area mm2 or lower) cause very low electronic noise. Its leakage current is smaller than 1 pA when the bias voltage is 20 V or lower. Its capacitance is in the order of hundreds of pF. From capacitance–voltage (C–V) measurements it was possible to get information about the depletion layer thickness, resulted in ~2 or 3 μm (Figure 1). This is confirmed by tests with alpha particles from an 241Am source (Figure 2). The sensitive layer of the detector is too thin to stop alpha particles (~5.5 MeV), so that only a part of their energy is released and collected in the detector. The spectrum in Figure 2 shows a main peak, due to alphas normally impinging the detector, plus a structure on the right, due to oblique particles. These release larger energy than normally colliding ones. Figure 1. View largeDownload slide Measurement of depletion layer thickness (~3 μm) of SiC detector (sensitive area 7.6 mm2) thanks to the elaboration of capacitance–voltage measurements. Figure 1. View largeDownload slide Measurement of depletion layer thickness (~3 μm) of SiC detector (sensitive area 7.6 mm2) thanks to the elaboration of capacitance–voltage measurements. Figure 2. View largeDownload slide Alpha particle of 241Am source spectra obtained with SiC detector at different bias voltage values. Figure 2. View largeDownload slide Alpha particle of 241Am source spectra obtained with SiC detector at different bias voltage values. Vented ion chambers Air Ion chamber with parallelepipedal sensitive volume of ~5 cm3 (2.5 cm × 2.5 cm × 0.8 cm) were manufactured. The optimal value of the saturation bias voltage (200 V) was found by X-ray irradiation. These chambers work in pairs: one is coated with 6LiF, whilst the other (uncoated) is used to subtract the residual photon signal. Results TNRD TNRD detectors have been calibrated using the radial thermal column of the ENEA Casaccia TRIGA reactor(5). This facility is equipped with a remotely controlled beam shutter and, according to the reactor power, can stably provide thermal neutron fluence rates from 102 up to 1.5 × 106 cm−2s−1. The thermal neutron fluence rate per unit reactor power is 1.59 ± 0.03 cm−2s−1 W−1. This value was measured through gold foils activation technique. As the neutron fluence rates in the E_LIBANS cavity are very similar to these values, the TRIGA thermal column proved to be a valuable calibration tool. According to the calibration experiment, the TNRD calibration factor (Table 1) was 0.249 ± 0.005 μV cm2 s (output voltage level per unit fluence rate. All thermal fluence rates in this paper are expressed in terms of the quantity sub-Cadmium thermal fluence rate in the Westcott convention(6)). Table 1. Calibration factors of the devices operating in current mode. All values are reported to 0.1× amplification. Device Calibration condition at TRIGA reactor power (MW) Calibration coefficient (μVcm2 s) TNRD 1 (0.249 ± 0.005) SiC + 6LiF 0.5 and 1 (3.16 ± 0.07)10−4 SiC bare 1 (1.10 ± 0.03)10−5 Ion chamber + 6LiF 0.1 and 1 (4.45 ± 0.08)10−2 Ion chamber bare 1 (2.06 ± 0.06)10−5 Device Calibration condition at TRIGA reactor power (MW) Calibration coefficient (μVcm2 s) TNRD 1 (0.249 ± 0.005) SiC + 6LiF 0.5 and 1 (3.16 ± 0.07)10−4 SiC bare 1 (1.10 ± 0.03)10−5 Ion chamber + 6LiF 0.1 and 1 (4.45 ± 0.08)10−2 Ion chamber bare 1 (2.06 ± 0.06)10−5 Table 1. Calibration factors of the devices operating in current mode. All values are reported to 0.1× amplification. Device Calibration condition at TRIGA reactor power (MW) Calibration coefficient (μVcm2 s) TNRD 1 (0.249 ± 0.005) SiC + 6LiF 0.5 and 1 (3.16 ± 0.07)10−4 SiC bare 1 (1.10 ± 0.03)10−5 Ion chamber + 6LiF 0.1 and 1 (4.45 ± 0.08)10−2 Ion chamber bare 1 (2.06 ± 0.06)10−5 Device Calibration condition at TRIGA reactor power (MW) Calibration coefficient (μVcm2 s) TNRD 1 (0.249 ± 0.005) SiC + 6LiF 0.5 and 1 (3.16 ± 0.07)10−4 SiC bare 1 (1.10 ± 0.03)10−5 Ion chamber + 6LiF 0.1 and 1 (4.45 ± 0.08)10−2 Ion chamber bare 1 (2.06 ± 0.06)10−5 The TNRD was exposed in the E_LIBANS cavity with size (20 cm × 20 cm × 5 cm) and the Monitor Unit rate of the LINAC was varied from ~100 up to 600 MU min−1, thus allowing to demonstrate the linearity of the detector’s response (Figure 3). Figure 3. View largeDownload slide Linearity test of TNRD using different LINAC dose rate (thermal neutron fluence rate at 500 MU min−1 is ~1.5 × 106 cm−2 s−1). Figure 3. View largeDownload slide Linearity test of TNRD using different LINAC dose rate (thermal neutron fluence rate at 500 MU min−1 is ~1.5 × 106 cm−2 s−1). The pulsed structure of the field in the E_LiBANS cavity was appreciated by increasing to 0.25 MHz the sampling rate of the TNRD digitizer. In the example shown in Figure 4 the LINAC Monitor Unit rate was 200 min−1. The pulse duration was ~5.2 ms and the pulse repetition rate was ~0.21 kHz. Figure 4. View largeDownload slide LINAC pulsed structure measured in the cavity with TNRD (sampling frequency 0.25 MHz). Figure 4. View largeDownload slide LINAC pulsed structure measured in the cavity with TNRD (sampling frequency 0.25 MHz). Silicon carbide Figures 5 and 6 show the pulse height distributions (spectra) obtained with the SiC detectors (sensitive area 1 mm2), deposited with 6LiF or bare, respectively. As in the case of alpha particles (Figure 2), the peak broadening in Figure 5 is related to the very thin depletion layer of the detector. Alphas and Tritons from the 6Li neutron captures release only a fraction of their energy. This fraction increases as the particles directions change from perpendicular (events on the left) to oblique (right tail), with respect to the detector surface. Interestingly, a similar distribution (with much lower integral) is obtained from the bare detector. This could be attributed to residual fast neutrons interacting with Carbon or Silicon, or to boron impurities in the substrate. Figure 5. View largeDownload slide Spectrum obtained during a measurement in the center of the E_LiBANS cavity (size 20 cm × 20 cm × 5 cm) with a SiC + 6LiF (measurement time: 180 s). Figure 5. View largeDownload slide Spectrum obtained during a measurement in the center of the E_LiBANS cavity (size 20 cm × 20 cm × 5 cm) with a SiC + 6LiF (measurement time: 180 s). Figure 6. View largeDownload slide Spectrum obtained during a measurement in the center of the cavity the E_LiBANS cavity (size 20 cm × 20 cm × 5 cm) with a bare SiC detector (measurement time: 180 s). Figure 6. View largeDownload slide Spectrum obtained during a measurement in the center of the cavity the E_LiBANS cavity (size 20 cm × 20 cm × 5 cm) with a bare SiC detector (measurement time: 180 s). After depositing the 6LiF on the SiC, a preliminary test with thermal neutrons was performed in the HOTNES(7, 8) thermal neutron facility (ENEA–INFN Frascati), where the thermal neutron fluence can be varied from 600 to 1000 cm−2 s−1 and its value is known through standard gold foil measurements. The calibration factor for SiC operating in pulse mode was 3686 ± 133 cm−2 (fluence rate per unit count rate). The calibration coefficients of SiC + 6LiF or bare SiC in current mode at TRIGA reactor are shown in Table 1. The linearity test performed in the E_LIBANS cavity, by varying the LINAC Monitor Unit rate, is shown in Figure 7. By subtracting the bare SiC signal from the SiC + 6LiF one, the contribution due to the 6LiF layer only can be achieved (‘difference’ line in Figure 7). This is ~97% of the SiC + 6LiF signal. Figure 7. View largeDownload slide Linearity test of SiC detectors studied by varying the LINAC Monitor Unit rate. By subtracting the bare SiC signal from the SiC + 6LiF one, the contribution due to the 6LiF layer only can be achieved (thermal neutron fluence rate at 500 MU min−1 is ~1.5 × 106 cm−2 s−1). Figure 7. View largeDownload slide Linearity test of SiC detectors studied by varying the LINAC Monitor Unit rate. By subtracting the bare SiC signal from the SiC + 6LiF one, the contribution due to the 6LiF layer only can be achieved (thermal neutron fluence rate at 500 MU min−1 is ~1.5 × 106 cm−2 s−1). Ion chambers The calibration coefficients for the Ion chamber + 6LiF or bare Ion chamber, operating in current mode, are shown in Table 1. The 6LiF-loaded chamber responds, in the TRIGA neutron field, ~2000 times more than the bare one. The linearity was verified at thermal power level of ~100 and 500 kW and the calibration coefficients are shown in Table 1. CONCLUSIONS Three thermal neutron detection techniques have been developed for monitoring a high fluence rate (106 cm−2 s−1 or higher) closed thermal neutron cavity (adjustable size, up to 30 cm × 30 cm × 10 cm) in the framework of the INFN project E_LIBANS. In addition to the well-established TNRD, two new sensors with higher radiation resistance have been developed, namely Silicon carbide (sensitive area 1 mm2) and small volume (5 cm3) ion chambers. In all cases, the devices are made sensitive to thermal neutrons to 6LiF deposition. The preliminary tests described in this work suggest that the new devices may be suited for the purposes of the E-LIBANs project. However, further experiments to complete their characterization are still needed, such as radiation resistance studied in very intense neutron fields. ACKNOWLEDGEMENTS This work has been supported by the E_LIBANS project (INFN National Scientific Committee 5—Technological, inter-disciplinary and accelerators research), San Paolo Company and C.R.T. foundation. The staff of TRIGA reactor at ENEA Casaccia are greatly acknowledged. Special thanks to Ruo Redda director, Veltri Radiodiagnostics section director from A.O.U. San Luigi di Orbassano, Dr V. Rossetti, Dr U. Nastasi, Dr E. Madon from A.O.U. Città della Salute and Scienza di Torino, Dr A. Zanini from INFN, sr. S. Petruzzi director of the logistics department of the Univeristy of Torino and ELEKTA s.p.a. technical division. REFERENCES 1 Costa , M. et al. . Neutron sources based on medical Linac . Il Nuovo Cimento 38C 180 , ( 2015 ). 2 Bedogni , R. et al. . Experimental characterization of semiconductor-based thermal neutron detectors . Nucl. Instrum. Methods Phys. Res. A 780 , 51 – 54 ( 2015 ). Google Scholar CrossRef Search ADS 3 Bedogni , R. , Bortot , D. , Pola , A. , Introini , M. V. , Gentile , A. , Esposito , A. , Gómez-Ros , J. M. , Palomba , M. and Grossi , A. A new active thermal neutron detector . Radiat. Prot. Dosim. 161 , 241 – 244 ( 2014 ). Google Scholar CrossRef Search ADS 4 Gerhardt , R. Properties and applications of Silicon Carbide, chap.13. InTech Europe ( 2011 ). 5 Palomba , M. , Carta , M. , Falconi , L. , Iorio , M. G. , Bedogni , R. , Pola , A. , Treccani , M. et al. . Testing newly developed thermal neutron detectors at the ENEA TRIGA RC‐1 research reactor. Rotterdam (NL), RRFM2017 European Research Reactor Conference, poster (14−18 May 2017 ). 6 NPL report DQL RN008 . Thermal fluence and dose equivalent standards at NPL (March 2005 ). 7 Bedogni , R. , Pietropaolo , A. and Gómez-Ros , J. M. The thermal neutron facility HOTNES: theoretical design . Appl. Radiat. Isot. 127 , 68 – 72 ( 2017 ). Google Scholar CrossRef Search ADS PubMed 8 Bedogni , R. , Sperduti , A. , Pietropaolo , A. , Pillon , M. , Pola , A. and Gómez-Ros , J. M. Experimental characterization of HOTNES: A new thermal neutron facility with large homogeneity area . Nucl. Instrum. Methods Phys. Res. A 843 , 18 – 21 ( 2017 ). Google Scholar CrossRef Search ADS © The Author(s) 2018. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices)

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Radiation Protection DosimetryOxford University Press

Published: Aug 1, 2018

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