Manufacture of a fast neutron detector using EJ-301 liquid scintillator

A fast neutron detector using the EJ301 scintillator was manufactured for study on detecting fast neutrons and gamma-rays. Detector characteristics include the energy linearity, the efficiency response and the neutron/gamma discrimination were guaranteed for neutron detection in the energy range from 50 to 3000 keVee. The ability discrimination of neutrons/gamma-rays of the detector was evaluated by the charge comparison (CC) method using an Cf source. The total efficiencies when measured on 22Na, 137Cs, 60Co and 252Cf sources were obtained 17.8%, 3.9%, 9.8% and 14.8%, respectively. The Figure of Merit (FoM) values of CC method were 0.4–1.55 for the range of energy 50–1000 keVee (keV electron equivalent). Keywords—EJ-301 liquid scintillator, fast neutron detector, pulse shape discrimination


INTRODUCTION
eutron detection is very important in research about the field of neutron, such as radiation safety, research material, scattering particles, particle physics, etc. The slow neutrons are commonly detected based on the nuclear reaction mechanism, while the fast-neutrons are detected based on elastic scattering mechanism with light nuclei such as hydrogenous, 4 He or organic scintillators [1,2]. Organic scintillator detectors are widely employed in studies with fast neutrons and gamma-rays by many good properties: the fast decay time, the relatively high light-output and a reasonably good efficiency for fast neutrons [1,3]. EJ-301 organic scintillator was manufactured by ElJen Technology (or its equivalent, NE213, BC501A), the yield curve consists of two exponential decaysthe fast and slow components of the scintillator light that depends on different kinds of radiation [1,4,5]. By coupling a photo multiplier tube (PMT) -to the scintillator, the light can be collected and converted into a voltage pulse, allowing for data acquisition/processing [1,6]. Those properties are commonly used to identify neutrons and gammarays by using pulse shape discrimination (PSD) techniques.
In the present study, a fast neutron detector was designed and manufactured using the EJ-301 liquid scintillator for neutron monitoring and training purposes. A preamplifier was also manufactured in order to make the suitable shaping pulse for data acquisition and processing. The qualities of the detector were assessed by the total efficiency, sensitivity and linearity with gamma-rays. The ability to distinguish between neutrons and gamma-rays was assessed through digital CC method. The CC method was implemented by a program in MATLAB software using the data that are digitized from the pulses of detector by a digital oscilloscope.

Detector manufacture
The designed layout of the detector is shown in Fig.1, which consist of a liquid scintillator container (cell), a photo-multiplier tube (PMT), a voltage divider, a shield cover and a preamplifier. The cell is a right cylinder made of aluminum with 34mm diameter 60mm length in size. The  The signals produced by the PMT have a very short rise time (less than 5 ns) because the fast decays component of EJ-301 is 3.2 ns [4], so that the signal is distorted when it is transmitted to the digitized block, which is usually placed away from the detector [1]. The preamplifier consists of four main stages because the anode pulses produced by the PMT are current pulses, the first stage converts the current pulses to the voltage pulses using the load resistance 50Ohms. The second stage amplifies the signal voltage from the first stage (gain of 30 times). The third stage is a filter using the second-order low-pass Sallenkey filter (f-3dB=33.8MHz, Butterworth=0.6). The final stage has matched impedance to match cable impedance 50 Ohms. The Preamp would shape the pulses which had the rise time of approximately 12 ns and fall time of approximately 31 ns for the pulse of gammarays. The total amplifier voltage gain of the Preamp is -17.85 V/V and the output amplitude at the Compton edge of the 137 Cs source is 344.7mV and the 60 Co source is 806.8mV, respectively. The total noise of preamplifier contribution to signal was 797.9±0.34µV, which is equivalent to 1.13keVee calculated a calibration energy scale of the detector.

Examined main characteristics of neutron detector
The preamplifier was designed for linear output voltages in the 0 to + 2.2V range, corresponds to range from 0 to 3100keVee. A test setup is shown in Fig. 2 The results of the signal-to-noise ratio (SNR), the gains, sensitivity and linearity of preamplifier were shown in Table 1   Because the light intensity of the EJ-301 liquid scintillator is good linearity on gamma sources [1,4], this study uses three 22 Na, 137 Cs and 60 Co standard sources to evaluate the linearity of the detector. The relation the height of pulse with energy at the Compton edge of the gamma sources was used that evaluate the linearity of the detector with energy. The maximum backscatter energy (E c ) was counted by equation (2) [1].
Where, E c , E , m e and c are maximum backscatter energy, the energy of gamma-ray, electron rest mass, and speed of light in vacuum, respectively. The Table 2 showed that measurements were performed with gamma-ray sources of 22 Na, 137 Cs and 60 Co, and each the measurement of those gamma sources were placed beside the monitor scintillation. Each the measurement of the pulse amplitude histogram was measured by the DPO7254C as the amplitude spectrum of the gamma source, respectively. The number of channels of the Compton edge corresponded to the Ec of the gamma source, respectively. Because the Compton edge of the 1137.2keV peak of 60 Co was obscured by the that of 1332keV peak, only the Conton edge of the 1137.2keV peak was not used in the calibration. The energy spectra of 60 Co, 22 Na and 137 Cs sources are shown in Fig. 4,that used the oscilloscope DPO7254C which was operated in spectrum mode. The total efficiency of the detector was evaluated by the schematic on Fig. 5. The total efficiency is defined as the ratio of the total number of events which are detected to the total number of gammaray incident on the detector. The total efficiencies of the detector were identified by 22 Na (activity on 12/2000 was 9µCi), 137 Cs (activity in 12/2001 was 11µC i ), 60 Co (activity in 12/2000 was 11µC i ), and 252 Cf (activity in 05/2011 was 11.6mCi) sources. The gamma sources are placed near the cell scintillator and placed 100cm from the 252 Cf source to the detector (see Fig. 5). The pulses in these processes include gamma source, 252 Cf and background were counted by the Multi-Channel-Analyzer (MCA) and spectrum analyzer software on a computer. The cross section of the liquid scintillator cell when decrease 5% by the air bubble was 19.4cm 2 .

Examined the ability of neutron-gamma discrimination
In order to assess the ability to discriminate of the detector, this study used the 252 Cf source, which was placed at 100cm from the detector (Fig. 5). The detector was biased high voltage of - 1200 V by the High Power Supply (Canberra 3002D); the detector's pulses were acquired by the DPO7254C which was set at 12bit resolution, the bandwidth of 2.5GHz and at a sampling rate of 1 GSPS. The pulses were transferred to the PC for offline analysis by the PSD CC method. The program of PSD CC method was performed on MATLAB software and the results of the graph and FoMs were calculated by the Originlab 8.5 software.

Fig. 6. Typical neutron and gamma-ray pulses in one sampling
The typical neutron and gamma -ray pulses with the same amplitude of the EJ-301 detector were shown in Fig. 6. The neutron pulses exhibited a larger decay time to the baseline, so with the same amplitude neutron/gamma pulses the area of the tail of the neutron pulse was greater than that of the gamma pulse. The digital PSD method chosen for comparison consists of integration techniques were applied to digitized pulses, where each pulse was integrated twice, using two different ranges [7][8][9][10]14]. The total integral was calculated for full pulse that began is at the start point (t1) to an optimal point at the tail pulse (t 3 ). The tail integral was calculated in range begins at a fixed position after the pulse maximum (t 2 ) and also extended to the last data point chosen in the total integral range (t 3 ). The survey data indicate that the separation was the best where t 2 was 20ns and t 3 was 210ns after the pulse maximum. The PSD parameters could be created using the ratio values between the tail and total integrals. The PSD parameter of neutron pulses was larger than that of gamma pulses.
3 RESULTS AND DISCUSSION The measured data with a neutron source 252 Cf and 60 Co were analyzed by the PSD CC method. The scatter plots of the neutron-gamma separation with an energy threshold of 50keVee by the CC method are shown in Fig. 7 (a) and (b), respectively. In the region of the energy survey shown that the threshold over 200keVee the ability to distinguish between neutrons and gamma-rays very well. While below the 200keVee threshold the ability to distinguish between neutrons and gamma-rays was not good and at the threshold 50keVee the discrimination was not clear for neutron and gamma. The statistical chart of the CC method at energy threshold 300keVee was shown that the ability to distinguish between neutrons and gamma-rays was very clear (FoM = 1.22).  Fig. 9 showed the FoM values as a function of threshold in a range of energy from 50 to 1100keVee. The FoMs were approximately 0.43 at 50keVee and greater than 1.0 at 200keVee energy threshold. At the 83keVee energy threshold, the FoM was measured 0.7 and its reached the value 1.15 at the 200keVee energy threshold. At the 1000keVee energy threshold, the FoM increased of 1.55. These results were similar as the presented in Ref. [7,8,11]. The results of the total efficiency of the detector were surveyed by 22 Na, 137 Cs, 60 Co and 252 Cf sources ( Table 3). The survey values showed that the total efficiency was maximum for the 22 Na source. The events of both 511 and 1274.5keV peaks were used for canculated total efficiency. The total efficiency on the 252 Cf reached 14.8% that was measured with both neutron and gamma events. Determining exactly the efficiency of the EJ-301 was quite complex by the inadequate standard sources and the bad resolution of the EJ-301 liquid scintillator. This issue is still being studied by the authors and will be published in another time.
4 CONCLUSION A scintillation detector using the EJ-301 liquid scintillator has been designed and built for fastneutron measurements. The detector is designed to measure in the 50 to 3000keVee energy range corresponding to an output voltage of 35.8mV to 2200mV, which was compatible with the input voltage range of the high speed ADCs that it could directly interconnect. The sensitivity of the detector was 707mV/MeV. The most important characteristic of the neutron detector was the ability to discriminate between neutrons and gamma-rays to eliminate gamma-rays noise in fast-neutron measurements that have been evaluated by the PSD CC method. Those results showed that the EJ-301 detector could be used in system fast-neutron measurements by digital technology.