MULTILAYER THERMAL NEUTRON DETECTORS BASED ON BORON NITRIDE CERAMICS
M. Roth, O. Khakhan, A. Fleider, E. Mojaev and E. Dul’kin
Department of Applied Physics, The Hebrew University of Jerusalem, Jerusalem91904, Israel
Abstract
Polycrystalline hexagonal boron nitride (BN) or mixed with boron carbide (BxC) embedded in an insulating polymeric matrix acting as a binder and forming a composite material as well as pure polycrystalline BN have been tested as thermal neutron converters in a multilayer neutron detector design. Metal sheet electrodes were covered with 20 - 50 microns thick layers of composite material and assembled in a multi-layer sandwich configuration. High voltage was applied to the metal electrodes to create in an interspacing electric field. The spacing volume could be filled with air, nitrogen or argon. Thermal neutrons were captured in converter layers due to the presence of 10B isotope. The resulting nuclear reaction produced -particles and 7Li ions which ionized the gas in the spacing volume. Electron-ion pairs were collected by the field to create an electrical signal proportional to the intensity of the neutron source. The detection efficiency of the multilayer neutron detectors is found to increase with the number of active converter layers. Pixel structures of such neutron detectors necessary for imaging applications and incorporation of internal moderator materials for field measurements of fast neutron flux intensities are discussed as well.
Introduction
Common neutron radiation detectors are mainly based either on 3He and BF3 gas proportional counters, cryogenic LiF single crystals or plastic scintillators containing boron (enriched with the 10B isotope) or lithium (enriched with the 6Li isotope) nuclides [1,2]. The detection is based on conversion of neutrons, through nuclear reactions, to -particles or other charged species and - rays. Nuclides, such as 10B, 7Li, 113Cd, 157Gd or 199Hg, have large cross-sections for capturing thermal neutrons, as shown in Table 1, but only the 10B and 6Li isotopes emit primarily -particles. Also, only one isotope, 6Li, does not generate -radiation. Interaction of neutrons with the 10B isotope does produce a limited amount of -photons, but these are not absorbed by materials composed of low-Z number atoms, such as boron nitride (BN) or boron carbide (e.g. B4C).
Table 1.Abundance, cross-section for interaction with neutrons and radiation energies emitted by neutron sensitive nuclides.
Nuclide / Abundance(%) / Cross-section
(barns) / Reactant energies of thermal neutrons
10B / 19.8 / 3840 / 7Li (1.02 MeV) + (1.78 MeV)
7Li (0.84 MeV) + (1.47 MeV) + (478 keV)
199 Hg / 17.8 / 2000 / (368 keV)
157Gd / 15.7 / 240,000 / () below 220 keV
6Li / 7.4 / 940 / Tritium (2.73 MeV) + (2.05 MeV)
113Cd / 12.3 / 20,000 / (9 MeV + 558 keV + 651 keV)
A detailed account of our experimental results with thermal neutron composite detectors based on pure BN, BN mixed with B4C and LiF as neutron-to-alpha (n,) converter materials is given in [3]. Composites were formed by embedding polycrystalline powders of the converter materials into organic or inorganic binders. The organic polymer binder comprises at least one polymer that can be selected from the group comprising polystyrene, polypropylene, Humiseal™ and Nylon-6. The inorganic binder can be selected from B2O3, PbO/B2O3, Bi2O3/PbO, borax glass, bismuth borate glass and boron oxide based glass [4]. The composite compound is sandwiched between an electrode assembly configured to detect the neutron and alpha particles interacting with the bulk of the active region. It is noteworthy that in the case of boron-based materials, the initial ~20% content of 10B (in the natural powders) was reduced to about 10% in the composite. This type of composite material can be deposited over large areas as described earlier for HgI2 polycrystalline detectors bound with polymeric-type binders as well [5]. The composite solid-state detector has definite advantages over other types of thermal neutron detectors, such as a better signal/noise ratio, weak sensitivity to gamma radiation, ability to produce detectors of any shape and size. In particular, a Monte Carlo simulation study has been employed [6] to compare the efficiency of 3He pressurized gas detectors with our BN-based composite detectors, and it has been found that for the same detector dimensions the BN is more efficient. The detection efficiency of a one inch diameter 3He detector, even pressured to three atmospheres, has been 64% as compared with ~72% for a 1 mm thick composite BN detector.
Recently, we have pointed out [7] that the detection of thermal neutrons with composite large bandgap semiconductor materials, such as boron compounds, is not based on charge collection within the semiconductor, but rather on registering discharges between the contacts of the converter material. As a result of the following reactions [1]:
, [1]
induced by thermal neutrons (Q is the amount of energy released) heavy charged particles (alpha particles and 7Li ions) are emitted in opposite directions from the interaction point [1]. These particles ionize air or an inert gas in the spacing volume and generate positive air ions and electrons which are collected at the electrodes by the applied electric field creating electrical bursts (pulses). The electrical signal (number of electron-ion pairs) is proportional to the energy of the initial heavy charged particles. Based on the new detection mechanism, we have proposed thereby a new design of the boron-based composite detectors, namely multilayer detectors aimed at substantial enhancement of the thermal neutron detection efficiency.
In the current report we present the results of further advancement of the multilayer boron-based composite thermal neutron detector design and performance. Unlike other types of detectors, they have a simple construction and are low cost. The number of layers and the active converter area are not limited, and there is also a possibility of realizing a pixel geometry of electrodes for imaging applications and. A detector of such type can be successfully operated both in the counting regime (in the case of low intensity neutron fields) and the current regime (high intensity neutron fields). Multilayer thermal neutron detectors may be applied as image detectors for controlling combustible, explosive materials and drugs (in solid or liquid state) inside a metallic container and for detecting neutron radiation in transportation and industry. A design of a detector with an embedded neutron moderator is suggested for field operation.
Experimental
The composite detector materials were fabricated by mixing BN and B4C or other materials with polymeric binders, mainly polystyrene, as reported elsewhere [4,8]. A 98% purity hexagonal BN polycrystalline powder with an average grain size of 2 m was used as the composite ingredient. In order not to reduce the natural abundance of the neutron-active 10B isotope from ~20% to 10%, with the binder, we frequently used a <1 m size h-BN powder suspension in toluene, and painted it on metal electrode plates. The commercially purchased B4C had a very high dark current, but a XRD analysis has determined that the powder was basically composed of 50% B4C and 50% B6.5C. One way to reduce the dark current and to enable application of higher biases was to mix the B6.5C compound with BN in 1:1 ratio. Alternatively, laser ablated B4C revealed low enough dark currents.
Multilayer detector structure design in this study were based on the 4-layer units described earlier [7] by stacking them in a “sandwich” configuration. It comprises three parallel plates mounted one above the other on an electrically insulating frame with a space between the plates of about 3-6 mm. The top and bottom plates were made either of metal or of plastic coated on the inside with a metal foil. In the latter case, the metal foil was coated with thin layers of the thermal neutron absorber, namely BN or B4C. The center plate was made of plastic, and it was also covered from top and from bottom by the copper foil. These layers of foil (on the central plate) could be divided into isolated regions, which we name as pixels, to produce a prototype pixel detector. One of such detectors prepared for the current study contain four pixel of 23 cm2 area each, and its general layout is shown in Fig. 1. The central plates were also covered with the neutron converter materials from both sides. The thicknesses of these active layers varied from 20 to 50 m. Even in the case of the use of the suspension of submicron BN particles, their adhesion to the metal after drying was very good.
Fig. 1. Design of a 4-layer “pixel” neutron detector.
Detectors were operated by applying high voltage to the upper and to the lower metal contact plates. The actual electric field in air gaps between the outer and central plates varied from 100 to 200 V/mm. In the case of the pixel detector, each pair of the metal foil sections (pixels) located above and below the central plate was connected to a separate input of the preamplifier equipped with low-noise field-effect transistors (2N4416). The signals from the preamplifier were fed into the spectroscopy amplifier with a typical shaping time of 10s and led to the multi-channel analyzer (MCA) which had a capability of threshold discrimination for filtering out the electronic noises. The detector together with the preamplifier was mounted in a metal box to protect against electromagnetic interferences. The neutron source was placed on top of this metal box. The entire detector/source assembly was embedded into two paraffin blocks with dimensions of Ø24×10 cm each (see Fig. 2) moderating the fast neutrons emitted by the source and turning them into thermal neutrons.
Fig. 2. Basic configuration of the neutron detection measurements.
We used an AmBe (241Am+9Be) alloy as a low flux (nominally about 2 ×102 cm-2∙s-1) source of thermal neutrons. The 241Am isotope activity was 100 mCi. This source emitted about 220,000 fast neutrons per second in a 4 steradian angle. Fast neutrons slow down in the paraffin blocks, and a “cloud” of thermal neutron is formed around the detector as a result. We performed actual measurements of the thermal neutron flux impinging the detector surface using a calibrated BF3 detector and found that it was of the order of 150 cm-2∙sec-1. The source was surrounded by a 4 cm diameter Pb ring, 2 mm thick, which absorbed the detrimental 59.6 keV radiation emitted by the 241Am isotope almost completely.
Results and discussion
We will first refer to the 4-layer “pixel” detector already shown in Fig. 1. Detailed neutron response measurements have been performed using such detector with a relatively large active area, namely 92 cm2 if we sum up all four surfaces. Stainless steel was used as the upper and bottom contact material, while copper was used as the central electrode. The distance between the electrodes was 5 mm. Uniform natural BN converter layers were brushed onto the electrode surfaces. The average thickness of these layers was 50 m. A 600 V bias was applied to each pair of electrodes, and the neutron pulse height response has been registered. The neutron spectrum is shown in Fig. 3. It has just one resolvable maximum corresponding to the 1.47 MeV energy of the majority of -particles produced by the thermal neutrons (see eq. 1). In fact, the appearance of a peak in the neutron spectrum taken by a solid state detector is not necessarily expected. The -particles are generated by neutrons at various depths, they diffuse variable distances in the solid and thus loose different amounts of energy before emerging in the air gap between the electrodes. This experiment has been conducted in air.
Fig. 3. Pulse height response of the 4-layer “pixel” thermal neutron detector. Overall amplification of the electronic system - 500,000. Threshold discriminating channel - 25.
The measurement, or pulse collection, time was 3000 s. Signals from channels below 25 of the MCA were discriminated to obtain a reasonable S/N ratio at a highest possible pulse count rate. The numbers of counts per second have been measured with and without the source. The latter gives an estimate of the noise level, and the result has been just 6 pulses/s without the source. The count rate with the source, namely the neutron signal, has been 680 s-1. This gives an excellent S/N ratio of 113. We will also note that each pixel collects ionization charges mainly above (or below) its area in spite of the top and bottom electrodes being continuous. This has been confirmed experimentally by covering up different parts of the large detector surface and is of fundamental importance for future attempts to obtain good spatial resolution neutron imaging.
We have also extended our investigation of multilayer neutron detectors, beyond the 4-layer level. Measurements have been carried out both in air and inert gas. It is well known that BN-based detectors operated in argon and nitrogen yield higher signal amplitudes (higher pulse heights) and better signal-to-noise ratios [7]. The largest number of converter layers in one devise has been 16. All measurements were executed in two steps, as above: count rate measurements with and without the presence of the AmBe neutron source for identical periods of time to have an accurate account of the noise level.
Fig. 4. 16-layer thermal neutron detectors with electrical interconnects.
We present initially the results obtained with a 16-layer detector. It has been combined of four 4-layer units as shown in Fig. 4. This device has a particularly large active area, 40 cm2 in every layer. All metal electrodes are made of stainless steel, and the distance between the electrodes is 4 mm. The natural (20% 10B) abundance) BN converter layers are 20 m thin. The spectral measurements have been conducted in argon gas atmosphere. A 750 V bias has been applied to each unit, and the neutron response spectrum with a well developed 1.47 MeV peak is shown in Fig. 5. The pulse collection time was only 1000 s, but the electronic amplification and discrimination below channel #25 has remained as in the former experiment. The overall count rate, with the source, is now as high as 904 s-1, which is mainly due to the large number of layers. The S/N ratio obtained is about 300, which is particularly high. It is particularly interesting how the detector properties evolve with the increase in the number of active layers.
Fig. 5. Pulse height response of the 16-layer thermal neutron detector. Overall amplification of the electronic system - 500,000. Threshold discriminating channel - 25.
Table 2 lists a few technical parameters and the neutron detection efficiencies of devices with the number of layers varying from 2 to 16, but also with variable converter materials as well as single layer areas and thicknesses. The detection efficiency is defined as the ratio of the number of neutron registered to the overall number of neutrons impinging the detector surface. Naturally, as a general trend, the detection efficiency increases with the number of converter layers, since neutrons transmitted through the upper layers have more chances to be eventually absorbed. Of course, attention should be paid also to other parameters. Thus, the two 4-layer detectors have similar detection efficiencies in spite of the fact that one of them has a much smaller area, since the converter material is enriched with the 10B isotopes having the large cross-section for reacting with neutrons. Our rough estimates show that the 10B-enrichment of the converter layer to 97% can result in five times enhancement of the neutron detection efficiency. Therefore, detection efficiency in excess of 50% is feasible making the BN-based solid state detectors comparable in performance with the 3He proportional counter. An inert gas filled solid state detector is expected to yield still higher detection efficiency, since it displays a much better S/N ratio. We have also fabricated and successfully tested a 4-layer BN detector with a 15 mm polyethylene-filled spacing between the inner two converter layers, which serves as the fast neutron moderator. Such design is suitable for field applications, from simple measurements of fast neutron radiation intensities to imaging.
Table 2. Technical parameters and detection efficiencies of multilayer thermal neutron detectors.
Number of converter layers / Converter material / Single layer area [cm2[ / Active layer thickness [m] / Detection efficiency [%]2 / B4C / 24 / 50 / 2.9 (air)
4 / BN (enriched) / 16 / 20 / 5.6 (air)
4 / BN / 100 / 20 / 5.5 (air)
8 / BN / 50 / 50 / 8.0 (air)
12 / B4C / 16 / 30 / 10.0 (air)
16 / BN / 40 / 20 / 12.5 (argon)
Summary and conclusions
It has been shown that multilayer thermal neutron detectors based on thin layers of absorbers containing the 10B isotope can be fabricated successfully. The detector efficiency increases with the number of absorber, or converter, layers in the detector. Multilayer thermal neutron detectors developed by us are very close in their performance to the 3He gas proportional counters with respect to such basic parameters as detection efficiency, signal counting rate, signal amplitude, temporal stability, signal-to-noise ratio and low sensitivity to gamma radiation. In addition, they are 5 to 10 times cheaper and allow for unrestricted size and shape of the detector active area. The permanent shortage in the 3He gas should be considered as a limiting factor as well.
The conversion efficiency of a single layer detector based on natural boron compounds is about 6% for thermal neutrons [9,10]. Enrichment with the 10B isotope to above a 90% content is estimated to increase the multilayer B-based detector efficiency to the level of the 3He proportional counters. A very important advantage of our BN detector design is the possibility of individual pixels configuration, which provides the ability of acquiring real-time images of objects undergoing radiography under fluxes of neutrons emitted from the neutron generators or nuclear reactors. Finally, if a plastic moderator spacer (e.g. polyethylene) is placed between the B-based converter layers, the device can be used as a detector of fast neutrons with higher energies.
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