High-energy neutron spectrometer onboard aircraft and spacecraft

Transcription

1 High-energy neutron spectrometer onboard aircraft and spacecraft M.Takada 1, I.Awaya 2, S.Iwai 3, M.Iwaoka 4, M.Masuda 2, T.Kimura 2, S.Takagi 3, O.Sato 3, T.Nakamura 5 and K.Fujitaka 1 1 National Institute of Radiological Sciences, 4-9-1, Anagawa, Inage-ku, Chiba , Japan 2 Mitsubishi Heavy Industries, Itd, Kobe Shipyard & Machinery Works, 1-1-1, Wadamisaki-cho, Hyogo-ku, Kobe , Japan 3 Mitsubishi Research Institute, INC, 2-3-6, Chiyoda-ku, Tokyo , Japan 4 High-Reliability Components Corporation, 8-1, Higashi-arai, Tsukuba , Japan 5 Cyclotron and Radioisotope Center, Tohoku University, Aoba-ku, Sendai , Japan m_takada@nirs.go.jp Abstract. In an aircraft and a space station, neutrons, gamma rays and protons mainly contribute to an exposure mainly and dose equivalent from neutron accounts for half in an aircrafts and over 10% in the space station. Neutron dosimetry is very important for aircrews and astronauts, but has not been acquired satisfactorily yet because of its difficulty. We have developed neutron detector to be able to measure neutron spectrum from a few MeV to above 100 MeV on board these crafts for radiation protection. This detector is required to be safe and compact. This detector consists of the liquid scintillator covered by the slow plastic scintillator. This liquid scintillator keeps high safety and has never attacked the plastic scintillator chemically for over 5 years. Short photomultiplier was developed to be used on board crafts. Data acquisition system of pulse shape with fast ADC has been developed. The neutron detection system is operated with a battery. Experiments for particle discrimination were done at NIRS-cyclotron and HIMAC. This detector can distinguish neutron events from gamma-ray and proton events by utilizing different pulse shapes. Especially, tail component of proton pulse shape produced by the slow plastic scintillator is different from that of neutron. New technique to discriminate neutron pulse shape from proton and gamma ray has been done by fitting the particle pulse shape. By using this neutron detector and multi-spherical neutron detector, neutron spectra from thermal to over 100 MeV can be measured on board crafts, and this acquired spectra are necessary for radiation protection. 1. Introduction Frequent flyers and astronauts are exposed to cosmic rays on average 2 msv/year [1] and about 1 msv/day [2], respectively. The radiation environments at high altitude, 10-20km and inside the spacecrafts consist of gamma rays, neutrons and charged particles, mainly of protons, but neutron dose equivalents are especially contributed to about 50% at high altitude [3] and from 5 to 30 % inside the spacecrafts [4]. Behind a thick craft wall in the space station, neutron dose equivalent may increase up to 70%. Especially neutron above 10 MeV contributes to approximately 50% of neutron dose equivalent. Also for semiconductors in the electrical units, neutron component is important, because the semiconductor in the IC chip is damaged by neutrons, and the single event upset (SEU) and the latch up are caused. Neutron measurement inside the crafts is very important for crews and electrical unit, but the neutron measurements are very scarce. Neutron instruments are required to discriminate neutrons from charged particles in the radiation environment that is highly complex due to the coexistence of charged and neutral particles. Until now, integral energy spectra were measured using the nuclear emulsions [5], and neutron doses were measured using 6 Li TLD [6] and activation foils [7]. Low-energy neutron energy spectra inside the crafts were measured using the multi-sphere moderator neutron detectors[8]. It is still strongly needed to measure the neutron energy spectrum above 10 MeV is necessary for radiation protection. We have manufactured two real-time neutron detector systems to measure neutron energy spectrum, from thermal to 100 MeV that covers main part of neutron energy inside the crafts. One is the low-energy neutron detector system to measure the neutron energy spectrum from thermal neutron to a few MeV, as shown in the left side of Fig. 1. The detector system is based on the Bonner sphere neutron detectors [9]. Another is the high-energy neutron detector system covering neutron energy from a few MeV to 100 MeV, as shown in the right side of Fig. 1. The detector system is based on the phoswich neutron detector [10]. This phoswich neutron detector was developed to measure neutrons in the neutron and charged-particle mixed radiation fields. The detector showed good performance to distinguish neutron events from charged-particle events, and the performance to obtain neutron energy spectrum in the neutron and charged-particle mixed radiation fields with accelerator beam experiments [11].

2 In this work, the detector system is improved to carry it in the aircrafts and spacecrafts. We manufactured the compact, safe and easy handling detector system. Fig. 1 Neutron detector systems onboard aircrafts. Left (A) and right (B) pictures show the neutron detector to measure low-energy and high-energy neutron energy spectra, respectively. 2. Neutron detector system The particle discrimination of neutron events from charged-particle events is necessary for a neutron detector, because there are high background events of charged particles in the aircrafts and spacecrafts. If a normal neutron detector is used to measure neutrons there, charged particles are detected as neutron events and correction of charged-particle events is needed. The phoswich neutron to be able to distinguish neutrons from charged particles is installed as the high-energy neutron detector system. For the data acquisition (DAQ) system, a fast ADC is used to acquire pulse shapes from the detector. The detector system is operated using an aircraft outlet and a rechargeable battery Neutron spectrometer The neutron detector is installed in the measurement unit box as shown in Fig. 1-(B). The neutron detector consists of an organic liquid scintillator, EJ surrounded by a slow plastic scintillator EJ coupled to a short photomultiplier tube (PMT), XD1374. Both the scintillators were manufactured by ELJEN Technology, TX, and the PMT was manufactured by Hamamatsu Photonics. Fig. 2 show the photo and configuration of the neutron detector of 38 cm length by 19 cm diameter. The organic liquid scintillator is cm length with cm diameter to measure 100 MeV neutrons. The thickness of the plastic scintillator is 1.5 cm to reject charged-particle events. The clear acrylic plastic linear is inserted between the liquid and the plastic scintillator to provide an inert barrier between both scintillators. The scintillator component is supported by a pressure ring support to absorb shock and is coupled to the single PMT via an acrylic right guide.

3 Fig. 2 Photo (left) and sketch (right) of the phoswich neutron detector installed in the neutron measurement unit box. Though neutrons are detected with only the liquid scintillator, protons are detected with both the liquid and the plastic scintillators. Since the liquid and the plastic scintillators produce signals of 3.5 nsec (fast) and 280 nsec (slow) decay time, respectively, neutron and charged-particle signals have different pulse shapes, mainly in tail components, as shown in Fig. 3. By using the different pulse shapes, neutrons can be discriminated from charged particles [10]. Fig. 3 Schematic models of signals produced from the phoswich neutron detector. (A) and (B) show the energy deposition process (left) and pulse shapes (right) produced by neutron and proton, respectively. In order to improve the safety of the neutron detector, the organic liquid scintillator, the widely-used NE213 is exchanged for EJ due to chemical toxicity. The EJ is improved with its chemical formulation to provide very low solvent action and low flammability characteristics. The improved liquid scintillator shows a great advantage of low chemical toxicity and biodegradability. The EJ scintillator, of course, provides excellent pulse shape discrimination (PSD) property between neutrons and gamma rays. The EJ scintillator has never attacked an EJ plastic scintillator chemically for over 5 years. The basic physical properties of EJ are almost equivalent to BC501A scintillator as shown in table I, but the light yield of the EJ scintillator should be studied. It was measured using a direct proton beam of energy from 45 MeV to 70 MeV at the NIRS-cyclotron facility [12] in National Institute of Radiological Sciences (NIRS). The preliminarily measured light outputs are almost the same as those of an NE213 scintillator [13] as

4 shown in Fig. 4. Light attenuation in the scintillator was also measured, and found that the scintillation produced at the top of the scintillator was attenuated about 13% near the PMT window. Fig. 4 Proton light output of an EJ scintillator (dots) compared with that of an NE213 scintillator (lines) [13]. Table I Physical property of liquid scintillators, EJ and BC501A [14] Specifications EJ BC-501A Light output (% of Antracene) 60% 78% Wavelength of max. emission 425 nm 425 nm Decay time of short component 3.5 nsec 3.2 nsec Density (g/cm 3 ) Flash point H:C ratio The PMT, XD1374 was made in a small size, based on 12.7 cm diameter PMT, R4144. The length of XD1374 PMT including a base, 16.5 cm is less than a half of that of R4144 PMT, 39 cm. The XD1374 PMT has a wide dynamic range that a 2% pulse linearity keeps up to 140 ma in order to measure 100 MeV neutrons without pulse-height saturation. Although the rise time of 4.4 nsec and the signal width of 10.7 nsec become worse than those of R4144 PMT as shown in Table II, the PSD shows still a good performance with the XD1374 PMT. High voltage supplied to the PMT is determined at 1050 V to keep the pulse height of 140 MeV proton signals below the maximum value, 4.0 V. Table II Property of photomultiplier tube, PMT [15] Specifications XD1374 R4144 Length 16.5 cm 39 cm Rise time 4.4 nsec 1.5 nsec Signal width 10.7 nsec 3.75 nsec Gain (-1420 V) (-2000 V) 2% pulse linearity 140 ma 160 ma 2.2 Data acquisition system Pulse shape is acquired directly using a fast ADC to obtain much information of a signal. It can increase count rate providing successful discrimination and has no dead time in the traditional sense. Also, the original pulse shape can be reconstructed from the pile-up signal, and some types of PSD can be applied. Using a DAQ board, signals are acquired with 10 nsec/point sampling rate and 8 bits code. The DAQ starts with trigger level -0.1 V, and the pulse shape is captured every 10 nsec during 200 nsec with 20 points. Pulse height is obtained to integrate a signal during 100 nsec after a trigger time. Pulse height and pulse shape data are stored in a memory, labelled with a trigger time stamp. One

5 event needs to be measured during 1 µsec, including time to discriminate a particle simply and to store the data into a memory. The count rate can increase over 100 khz, and the counter in the system can measure the high rate event of about 10 MHz. Neutron pulse height is obtained with the off-line analysis. This DAQ was updated for a better PSD. Pulse shapes of signals from the detector were acquired using a digital storage osilloscope (DSO), DS4374 with the bandwidth of 500 MHz manufactured by Iwatsu Test Measurement Co., in order to measure real pulse shapes of neutrons and protons and to optimize the parameters for the PSD at the same time. The DSO includes an analog-to-digital conversion (ADC) system that samples a waveform in a read time mode at rates of 2G samples per second which is equivalent to 0.5 nsec/pt with 8 bits dynamic range. The waveform traces with each 1000 segment were stored to PC ATA card. Signal tail voltage indicates much smaller than that of signal peak voltage. To acquire signal tail with high resolution, signals are divided to three different gained channels, 10, 50 and 200 mv/div. in the DSO. Original pulse shapes are rebuilt at the off line analysis. Using this technique, pulse shapes with wide range was obtained with excellent pulse-height resolution. This data measurement technique is applicable to any radiation measurements, and shows an advantage of compact, easy, cheap cost and no dead-time DAQ system. 3. Particle pulse shapes 3.1 Pulse shape Proton and neutron pulse shapes from the detector were acquired using the DSO at the NIRS-cyclotron facility and the Heavy Ion Medical Accelerator in Chiba (HIMAC) [16] in NIRS. The NIRS-cyclotron facility and HIMAC accelerated 70 and 160 MeV protons, respectively. Proton energies were decreased using aluminum plates to measure the dependence of pulse shapes on incident proton energy. Protons were irradiated directly to the detector, and neutrons were produced from the full-stopping length acrylic plate (PMMA) bombarded by 160 MeV protons. Typical neutron and proton pulse shapes are shown in Fig. 5-(A) and (B), respectively. In Fig. 5-(B), two kind of pulse shapes are found. Signals of high pulse height and short tail are produced by protons that deposited their energies in both the liquid and the plastic scintillators, but signals of low pulse height and long tail are produced by protons that enter into only the plastic scintillator. Neutron signals only show pulse shapes with short tail. It was clarified that both neutron and proton signals show different pulse shapes. Fig. 5 Neutron (left) and proton (right) pulse shape from the phowish neutron detector measured using the DSO. Neutron pulses were acquired at the 400 mv trigger level. Proton energy is 150 MeV. Neutron and proton pulse shapes were normalized at the signal peak for comparison, as shown in Fig. 6. In Fig. 6, Neutron and proton pulse shapes were acquired with three different threshold levels, and at the six different proton energies from 74 to 140 MeV. Pulse tails magnified at signals from 0.2 to 0 indicates a clear difference between neutron and proton signals at 100 to 300 nsec after a trigger time,

6 as shown in Fig. 6-(B). From the difference of tail components of neutron and proton signals, neutron and proton events can be identified each other. Fig. 6 Neutron and Proton pulse shape normalized at the pulse peak (left). Signals are magnified at normalized signal, -0.2 to 0 (right). 3.2 Particle discrimination We have examined the best PSD method for the neutron detector among of three PSD methods. As a first technique, signal charges were integrated during two different value of duration and time. One is charge integration of the peak of signal (total component), and another is that of the tail of signal (slow component). The horizontal and vertical axes in Fig. 7 show the relative values of total component and ratio of slow to total components, respectively. Neutron and proton events are plotted in the lower and upper regions in Fig. 7 and can be clearly separated. For the second technique, voltages of signals normalized at peak voltage at any particular time elapsed from a trigger time are applied to separate neutron and proton events. It is possible to separate particles on line. For the last method, PSD is done to fit signals to a sum of three exponential formulas, F(t) = a 1 exp( t T 1 ) T 1 + a 2 exp( t T 2 ) T 2 + a 3 exp( t T 3 ) T 3 (1), where a 1, a 2 and a 3 are the normalization factors. T 1, T 2 and T 3 are the decay time constants which are fixed to 15, 70 and 300 nsec, respectively. Fig. 8-(A) shows an example of 140 MeV proton pulse shape fitted to the equation (1). The dots are the measured pulse shapes, and a thick line is the fitted result in Fig. 8-(A). This formula well agrees with the measured pulse shapes. Some pulse shapes are fitted using equation (1), and ratio of the a 3 factor to a sum of three normalization factors, a is plotted in Fig. 8-(B). Neutron and proton events are separated roughly. The PSD method needs to be studied in detail and be optimized for every signal.

7 Fig. 7 Scatter plot of signal charge integrated during 20 to 100 nsec (total) and ratio of signal charge integrated during 150 to 300 nsec to the total. Upper, medium and bottom regions show proton, neutron and gamma-ray events, respectively. Fig MeV proton pulse shape fitted using formula (1) (left). The ratio of third factor, a 3 to a sum of all factors, a 1 to a 3.(right). 4.Conclusion The biodegradable and compact neutron detector to measure neutron energy spectrum on board aircrafts was completed. Neutron and proton pulse shapes were acquired using the DSO. The difference of both pulse shapes was shown clearly to discriminate between neutron and proton events. We hope to measure neutron energy spectrum onboard aircrafts. Acknowledgements The authors wish to thank Dr. T.Sanami at the high energy accelerator research organization, (KEK) for advice about biodegrable scintillator. The authors are very grateful to the staff members for NIRS-cyclotron and HIMAC operation during the experiment. This work was financially supported by

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