TY - JOUR
AU1 - Bielecki, J
AU2 - Drozdowicz, K
AU3 - Dworak, D
AU4 - Igielski, A
AU5 - Janik, W
AU6 - Kulińska, A
AU7 - Marciniak, Ł
AU8 - Scholz, M
AU9 - Turzański, M
AU1 - Wiącek, U
AB - Abstract Plastic organic scintillators such as the blue-emitting BCF-12 are versatile and inexpensive tools. Recently, BCF-12 scintillators have been foreseen for investigation of the spatial distribution of neutrons emitted from dense magnetized plasma. For this purpose, small-area (5 mm × 5 mm) detectors based on BCF-12 scintillation rods and Hamamatsu photomultiplier tubes were designed and constructed at the Institute of Nuclear Physics. They will be located inside the neutron pinhole camera of the PF-24 plasma focus device. Two different geometrical layouts and approaches to the construction of the scintillation element were tested. The aim of this work was to determine the efficiency of the detectors. For this purpose, the experimental investigations using a neutron generator and a Pu–Be source were combined with Monte Carlo computations using the Geant4 code. INTRODUCTION Plastic organic scintillators such as the blue-emitting BCF-12 are versatile and inexpensive tools. They are applied in various fields, e.g. in radiation therapy of cancer patients for the precise real-time dose rate measurements(1) or in high energy physics for the construction of active targets for detection of fast charged particles(2). Recently, BCF-12 scintillators have been also foreseen for investigation of the spatial distribution of 2.5 MeV neutrons emitted from dense magnetized plasma(3). For this purpose, small-area detectors based on BCF-12 scintillation rods and Hamamatsu photomultiplier tubes (PMTs) were designed and constructed in the Institute of Nuclear Physics Polish Academy of Sciences (IFJ PAN), Poland. The main objective of the detector design and construction was to separate the scintillation element from the associated PMT in order to avoid an influence of the interaction of electromagnetic wave produced during a plasma discharge with the PMT and data acquisition system. After detailed characterization, the BCF-12 scintillation detectors will be serving as a detection system of the neutron pinhole camera dedicated to the PF-24 plasma focus (PF) device (see Materials and Methods). The main aim of this work was to determine the efficiency of the small-area BCF-12 detectors dedicated for the pinhole camera. The precise knowledge of the efficiency and general performance of the detectors is crucial for the determination of the spatial distribution of neutrons emitted from fusion plasma. In turn, the knowledge of the spatial distribution, location and dynamics of neutrons emitted form DD reaction in a PF device is of great importance for the understanding of the fusion reaction character in a plasma pinch(4). In order to determine the efficiency and general performance of the pinhole camera detectors, the experimental investigations using a neutron generator and a Pu–Be source were combined with MC computations using the Geant4 code(5). Since two different approaches to the construction of the scintillation element and to scintillator-to-optical fiber bounding technique were explored, both cases were studied experimentally and numerically. MATERIALS AND METHODS PF-24 source and the neutron pinhole camera PF-24 at IFJ PAN is a medium-scale PF device(6). During the experiments, PF-24 operates with deuterium gas at a pressure of ~200 Pa and a capacitor bank is charged to 17 kV. The duration of a single plasma discharge ranges up to a few microseconds. The neutron yield is measured by a beryllium activation counter(7). Recently, in order to extend the possibility of plasma diagnostics, the neutron pinhole camera has been designed and constructed(3). The camera features four linearly arranged BCF-12 scintillation detectors coupled via 25 m long optical fibers to PMTs. The geometrical parameters of the camera were established using the principles of geometrical optics and verified by means of MCNP calculations. The detailed description of the camera is presented in reference(3). Figure 1 shows the camera installed on its alignment stand. Figure 1. View largeDownload slide The neutron pinhole camera with the BCF-12 detectors installed. Figure 1. View largeDownload slide The neutron pinhole camera with the BCF-12 detectors installed. Description of the detection system with BCF-12 scintillators The BCF-12 scintillators (Saint-Gobain Crystals, France) dedicated as detection elements for the pinhole camera, have a form of 5 mm × 5 mm × 60 mm rods. They consist of a polystyrene based core and a PMMA cladding. The scintillating core contains a combination of fluorescent dopants selected to produce the desired scintillation, optical and radiation–resistance characteristics. Although the scintillation efficiency of BCF-12 scintillators is equal to 2.4% (scintillator yield of ~8000 photons per MeV from a minimum ionizing particle), the trapping efficiency, determined by refractive indices of the core (1.60) and cladding as well as the cross section of the scintillator, permits the collection of <4% of the photons for passage down the scintillator. The collected light is transported via 25 m long fiber optics to the PMTs. The separation of the PMTs and data acquisition (DAQ) system from the scintillators is required in order to reduce the influence of background noise present due to the electromagnetic wave produced during PF discharges. Figure 2 shows a block diagram of the measurement setup used in the experiments presented in this article. For the testing purposes, scintillators were mounted in the aluminum tubes of 10 mm diameter and a length of 80 mm. Figure 2. View largeDownload slide Block diagram of the measurement setup used to determine the efficiency of the scintillation detectors. Figure 2. View largeDownload slide Block diagram of the measurement setup used to determine the efficiency of the scintillation detectors. Two types of detectors have been prepared. In both types, the BCF-12 scintillators were attached to the optical fibers SH 8001 (Mitsubishi Cable Industries, Ltd) using an optical glue that matches the differences in a reflexive index between the scintillators’ core and the fiber (1.49). The 25 m long optical fibers made from an acrylic glass with a core diameter of 2 mm were used. In the first detector type, the fiber was introduced into the scintillator at a depth of 2 mm. In the second type, the fiber was butted with one end of the scintillator which had previously been sanded (with 400- to 2500-grit sandpaper) to form a quasi-conical shape. Regardless of the detector’s type, each scintillation rod was painted with a white reflective diffusive BC-620 (Bicron) paint employing a special grade of titanium dioxide. This paint was applied to (1) enhance photon collection by minimizing the loss of photons in the optical system and to (2) eliminate an optical crosstalk between closely packed scintillators inside the pinhole camera. The details on the construction of the scintillators and fiber optics bounding techniques are presented in Figure 3. Figure 3. View largeDownload slide The construction details of the two types of detectors. Figure 3. View largeDownload slide The construction details of the two types of detectors. The individual light flashes produced in the scintillator are transmitted via optical fiber to a PMT. The H3164-10 PMT with a magnetic shield operates in the wavelength range from 300 to 650 nm and its maximum efficiency (420 nm) is close to the wavelength of light emitted by the scintillator (435 nm). Moreover, it is characterized by high threshold sensitivity and high gain coefficient of 1 × 106. This PMT is designed to optimally operate at ~−1250 V. Short scintillation decay time of 3.2 ns ensures high temporal resolution of the detection system. Raw pulses measured directly from the detector on a high-speed (1 GHz) Tektronix DPO5104B oscilloscope were stored digitally for further post-processing. Experimental set-up with Pu–Be source In the initial stage of investigations, the experimental set-up featuring a Pu–Be source with a neutron yield of 5.3 × 105 n/s was used. The source was placed 300 mm from the front face of the single scintillator enclosed in an Al tube. This part of the BCF-12 detector (containing the scintillator) was set inside a 10 cm × 10 cm × 5 cm lead box whose side faces were additionally covered by borated polyethylene shielding bricks. A 2 cm thick Bi plate was placed in front of the lead box to attenuate hard X-ray and gamma radiation emitted from the source. Moreover, the optical fibers were shielded by a PCV pipe covered by a 5.0 mm thick single layer of lead in order to reduce the interaction of emitted gammas with the fibers. Experimental set-up with ING-14 neutron generator The second experimental set-up used the pulsed neutron generator (ING14 at IFJ PAN) where the 14.1 MeV neutrons are produced by accelerating deuterium onto a T/Ti target (tritium in titanium target, deposited on a copper substrate): H2+H3→n(14.1MeV)+H4e(3.5MeV). (1) In order to obtain the 2.45 MeV neutrons a D/Ti target was used. In this case, the following reactions occur with almost equal probability: H2+H2→n(2.45MeV)+H3e(0.82MeV), (2) H2+H2→p(3.02MeV)+H3(1.01MeV). (3) The neutron yield was monitored by a BCF-720 probe (1.6% efficiency at 19 MeV) and was ~1 × 104 n/s during the experiment, while pulse extraction voltage and acceleration voltage were set to 2.66 kV and 80.2 + 30.2 kV, respectively. The BCF-12 detectors have been placed one by one within 50 mm from a back wall of the target vacuum chamber. Monte Carlo calculations Efficiency of the detectors was investigated by means of Geant4 (version 9.3.p02) computations. Two kinds of calculations related to the experiments described in the previous section were performed: (1) modeling of detection of neutrons emitted from the Pu–Be source and (2) modeling of 14.1 and 2.45 MeV neutron detection with the neutron generator. The BCF-12 models related to both types of detectors’ geometry shown in Figure 3 were constructed. The models were composed of: the scintillator core, the cladding, the optical fiber and a titanium paint layer. The Ti paint layer was simulated by defining a dielectric-metal optical border between the cladding and the air layer in the detector construction class. A scintillator material is characterized by its photon emission spectrum, its rise time, and its exponential decay time components. These quantities were sampled in 7 energy bins (from 2.25 to 3.1 eV) for the polystyrene core and in 50 energy bins (from 2.00 to 3.47 eV) for the PMMA cladding. For the core scintillation material, the optical properties were assumed after(8) and optical properties of PMMA cladding were taken from the NIST database. A characteristic light yield Y = 8000/MeV of the scintillator was assumed after(8). Besides general, hadronic and electromagnetic processes, Geant4 optical processes (G4OpticalPhysics) were included into the physics list class in order to model the production and transport of optical photons. The neutron high precision (HP) data transport model used the standard G4NDL4.3 neutron data library that is distributed with the Geant4 package. In order to simulate detection of 2.45 and 14.1 MeV neutrons emitted from the neutron generator, isotropic sources of 2.45 and 14.1 MeV neutrons, respectively, were located 5 cm away from the front face of the detector. This layout mimicked the experimental conditions of the tests with the neutron generator. In case of the simulations with Pu–Be source, a uniform source of neutrons with energy distribution shown in Figure 4(9) was constructed. To estimate the efficiency of the detectors, a custom-made routine has been developed that counts photons generated and transported to the optical fiber. The computations were performed using a dedicated high performance computing cluster. Figure 4. View largeDownload slide Neutron emission spectrum of Pu–Be source(9)—black line and points. The neutron energy distribution taken to the MC simulation—in green. Figure 4. View largeDownload slide Neutron emission spectrum of Pu–Be source(9)—black line and points. The neutron energy distribution taken to the MC simulation—in green. RESULTS AND DISCUSSION Table 1 presents the results of the experimental and MC BCF-12 detectors efficiency determination. The calculated efficiency of 2.5 MeV neutrons detection is ~0.5% for both types of the detectors, while the experimental values are ~50% lower the calculated ones. However, the experimental results feature high measurement uncertainties. The calculated efficiency for detection of 14.1 MeV neutrons is around 1.5% for both types of the detector, while the experimental results give ~0.5% for both types of the detectors. Again, the experimental results feature quite high measurement uncertainty. The experimental and computational results of efficiency determination for the detection of neutrons of Pu–Be source spectrum are consistent. In this case, the efficiency is ~0.9%. The experimental uncertainty is much lower in this case due to the precise knowledge of the source activity. Additionally, in order to test the capability of the detectors with PF-24 source, several experiments have been performed. Figure 5 shows an example of X-ray and neutron time traces collected using BCF-12 detectors for a PF-24 discharge. The total neutron yield measure by the Be counter for the discharge was 1.2 × 1010 n. Three scintillators were placed at first outside the camera at the distance of 1 m from the center of the anode. The delay between the peaks in Figure 5 is due to the difference in the time-of-flight between neutrons and X-rays. The X-ray signal was attenuated by placing a 5 cm thick Pb block in the front of the detectors. The results clearly show the capability of short neutron pulse detection. For comparison, also a spare optical fiber connected only to a PMT without any scintillator (blue curve ‘OF’ in Figure 5) was used. The signal from the spare optical fiber provides an assessment of expected signal-to-noise ratio in the final arrangement of the pinhole camera. Table 1. Results of BCF-12 detectors efficiency determination Detection efficiency (%) Source/energy (MeV) Calculated Experimental Type 1 Type 2 Type 1 Type 2 0.49 ± 0.08 0.51 ± 0.06 0.19 ± 0.06a 0.29 ± 0.09a NG (2.45) 1.6 ± 0.1 1.7 ± 0.1 0.53 ± 0.14a 0.59 ± 0.14a NG (14.1) 0.9 ± 0.04 0.94 ± 0.05 0.90 ± 0.06 0.93 ± 0.05 PuBe Detection efficiency (%) Source/energy (MeV) Calculated Experimental Type 1 Type 2 Type 1 Type 2 0.49 ± 0.08 0.51 ± 0.06 0.19 ± 0.06a 0.29 ± 0.09a NG (2.45) 1.6 ± 0.1 1.7 ± 0.1 0.53 ± 0.14a 0.59 ± 0.14a NG (14.1) 0.9 ± 0.04 0.94 ± 0.05 0.90 ± 0.06 0.93 ± 0.05 PuBe aHigh experimental uncertainty due to low efficiency of the reference probe. Table 1. Results of BCF-12 detectors efficiency determination Detection efficiency (%) Source/energy (MeV) Calculated Experimental Type 1 Type 2 Type 1 Type 2 0.49 ± 0.08 0.51 ± 0.06 0.19 ± 0.06a 0.29 ± 0.09a NG (2.45) 1.6 ± 0.1 1.7 ± 0.1 0.53 ± 0.14a 0.59 ± 0.14a NG (14.1) 0.9 ± 0.04 0.94 ± 0.05 0.90 ± 0.06 0.93 ± 0.05 PuBe Detection efficiency (%) Source/energy (MeV) Calculated Experimental Type 1 Type 2 Type 1 Type 2 0.49 ± 0.08 0.51 ± 0.06 0.19 ± 0.06a 0.29 ± 0.09a NG (2.45) 1.6 ± 0.1 1.7 ± 0.1 0.53 ± 0.14a 0.59 ± 0.14a NG (14.1) 0.9 ± 0.04 0.94 ± 0.05 0.90 ± 0.06 0.93 ± 0.05 PuBe aHigh experimental uncertainty due to low efficiency of the reference probe. Figure 5. View largeDownload slide X-ray and neutron time traces collected using BCF-12 detectors for a PF-24 discharge. Figure 5. View largeDownload slide X-ray and neutron time traces collected using BCF-12 detectors for a PF-24 discharge. CONCLUSIONS Two types of small-area BCF-12 scintillation detectors have been studied in order to determine the detection efficiency. There is no statistically significant difference in efficiency of the two types of detectors. However, the trend shows that possibly the second approach to the construction is more efficient. High experimental uncertainties are present due to relatively low detection efficiency of the reference probe. The discrepancies between experimental and MC results are caused by high uncertainties of the experimental results as well as by the simplified model of the detectors in MC calculations. The detectors can be applied in the neutron pinhole camera to investigate shots with the neutron yield above 2 × 1010 n/discharge(3). Further investigations, using a dedicated reference detector (e.g. a diamond detector), can help to improve experimental uncertainties. Moreover, the plastic optical fibers could be exchanged into the quartz ones to eliminate the contribution from the X-rays to the registered signal without necessity of shielding the fibers. REFERENCES 1 Beierholm , A. , Andersen , C. E. , Lindvold , L. R. , Kjær-Kristoffersen , F. and Medin , J. A comparison of BCF-12 organic scintillators and Al2O3:C crystals for real-time medical dosimetry . Radiat. Meas. 43 , 898 – 903 ( 2008 ). Google Scholar CrossRef Search ADS 2 Macdonald , J. et al. . Apparatus for a search for T-violating muon polarization in stopped-kaon decays. Nucl. Instrum. Methods Phys. Res A 506 , 60 – 91 (2003). 3 Bielecki , J. , Wójcik-Gargula , A. , Wiacek , U. , Scholz , M. , Igielski , A. , Drozdowicz , K. and Woźnicka , U. A neutron pinhole camera for PF-24 source: conceptual design and optimization . Eur. Phys. J. Plus 130 , 145 ( 2015 ). Google Scholar CrossRef Search ADS 4 Zakaullah , M. , Akhtar , I. , Murtaza , G. and Waheed , A. 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TI - EXPERIMENTAL AND MONTE CARLO INVESTIGATIONS OF BCF-12 SMALL‑AREA PLASTIC SCINTILLATION DETECTORS FOR NEUTRON PINHOLE CAMERA
JF - Radiation Protection Dosimetry
DO - 10.1093/rpd/ncx277
DA - 2018-08-01
UR - https://www.deepdyve.com/lp/oxford-university-press/experimental-and-monte-carlo-investigations-of-bcf-12-small-area-1LSwOszLHk
SP - 427
EP - 431
VL - 180
IS - 1
DP - DeepDyve
ER -