Gas proportional detectors Gas proportional detectors can be adapted to detect neutrons. While neutrons do not typically cause
ionization, the addition of a
nuclide with high
neutron cross-section allows the detector to respond to neutrons. Nuclides commonly used for this purpose are
helium-3,
lithium-6,
boron-10 and
uranium-235. Since these materials are most likely to react with
thermal neutrons (i.e., neutrons that have slowed to equilibrium with their surroundings), they are typically surrounded by
moderating materials to reduce their energy and increase the likelihood of detection. Further refinements are usually necessary to differentiate the neutron signal from the effects of other types of radiation. Since the energy of a thermal neutron is relatively low, charged particle reactions are discrete (i.e., essentially monoenergetic and lie within a narrow bandwidth of energies) while other reactions such as gamma reactions will span a broad energy range, it is possible to discriminate among the sources. As a class, gas ionization detectors measure the number (
count rate), and not the energy of neutrons.
3He gas-filled proportional detectors Helium-3 is an effective neutron detector material because it reacts by absorbing thermal neutrons, producing a 1H and 3H ion. Its sensitivity to gamma rays is negligible, providing a very useful neutron detector. Unfortunately the supply of 3He is limited to production as a byproduct from the decay of tritium (which has a 12.3 year half-life); tritium is produced either as part of weapons programs as a booster for nuclear weapons or as a byproduct of reactor operation.
BF3 gas-filled proportional detectors As elemental boron is not gaseous, neutron detectors containing boron may alternately use
boron trifluoride (BF3) enriched to 96% boron-10 (natural boron is 20% 10B, 80% 11B). Boron trifluoride is highly toxic. The sensitivity of this detector is around 35-40 CPS/nv (counts per second per neutron flux) the sensitivity of boron-lined detectors is approximately 4 CPS/nv. This is because in boron-lined detectors, neutrons react with boron to produce ion pairs inside the boron layer and so charged particles produced (alpha particles and lithium ions) may lose some of their energy inside that layer. This means that low-energy charged particles may be unable to reach the
ionization chamber's gas environment resulting in a lower number of ionizations produced. In BF3 gas-filled detectors, on the other hand, neutrons react with 10B atoms inside the detector gas volume, so charged particles produced are more likely to deposit their energy in the gas volume, producing more ionizations and therefore higher signal.
Boron-lined proportional detectors Boron-lined gas-filled proportional counters react similarly to BF3 gas-filled proportional detectors, but instead of containing boron-rich gas, the walls are coated with 10B with another fill gas. In this design, since the reaction takes place on the surface, only one of the two particles will escape into the proportional counter.
Scintillation neutron detectors Scintillation neutron detectors include liquid organic scintillators, crystals, plastics, glass and scintillation fibers.
Organic Scintillators Organic scintillators are materials that have the property to emit light (scintillate) when exposed to ionizing radiation. They generally have fast response time to radiation and can detect a wide range of radiation (from gamma to neutrons). As the name suggests, they are usually made of organic materials, mostly carbon-based materials. The organic scintillators comes in many forms such as plastic scintillators, liquid scintillators, and crystal scintillators. The plastic scintillators are made from polymers like polyvinyltoluene (PVT) or polystyrene. The Liquid scintillators are simply made by dissolving the organic scintillators in suitable solvents. The crystal scintillators are made of solid crystals like
anthracene or stilbene. When a scintillator is exposed to ionizing radiation, the molecules within them are excited to a higher energy state through the interaction with the incoming radiation. These excited molecules try to become stable (return to ground energy state) by emitting a photon. The emitted light can be detected by amplifying it in a photo-multiplier and converted to an electric signal. These signals are proportional to the energy of the radiation, which helps in identifying the radiation that was exposed to. The neutrons can be detected by the organic scintillators using the Pulse Shape Discrimination (PSD) technique, which can differentiate between the radiation based on their pulse/signal shape.
Neutron-sensitive scintillating glass fiber detectors Scintillating 6Li glass for neutron detection was first reported in the scientific literature in 1957 and key advances were made in the 1960s and 1970s. Scintillating fiber was demonstrated by Atkinson M.
et al. in 1987 and major advances were made in the late 1980s and early 1990s at
Pacific Northwest National Laboratory where it was developed as a classified technology. It was declassified in 1994 and first licensed by Oxford Instruments in 1997, followed by a transfer to Nucsafe in 1999. The fiber and fiber detectors are now manufactured and sold commercially by Nucsafe, Inc. The scintillating glass fibers work by incorporating 6Li and Ce3+ into the glass bulk composition. The 6Li has a high cross-section for thermal neutron absorption through the 6Li(n,α) reaction. Neutron absorption produces a tritium ion, an alpha particle, and kinetic energy. The alpha particle and triton interact with the glass matrix to produce ionization, which transfers energy to Ce3+ ions and results in the emission of photons with wavelength 390 nm – 600 nm as the excited state Ce3+ ions return to the ground state. The event results in a flash of light of several thousand photons for each neutron absorbed. A portion of the scintillation light propagates through the glass fiber, which acts as a waveguide. The fibers ends are optically coupled to a pair of photomultiplier tubes (PMTs) to detect photon bursts. The detectors can be used to detect both neutrons and gamma rays, which are typically distinguished using pulse-height discrimination. Substantial effort and progress in reducing fiber detector sensitivity to gamma radiation has been made. Original detectors suffered from false neutrons in a 0.02 mR gamma field. Design, process, and algorithm improvements now enable operation in gamma fields up to 20 mR/h (60Co). The scintillating fiber detectors have excellent sensitivity, they are rugged, and have fast timing (~60 ns) so that a large dynamic range in counting rates is possible. The detectors have the advantage that they can be formed into any desired shape, and can be made very large or very small for use in a variety of applications. Further, they do not rely on 3He or any raw material that has limited availability, nor do they contain toxic or regulated materials. Their performance matches or exceeds that of 3He tubes for gross neutron counting due to the higher density of neutron absorbing species in the solid glass compared to high-pressure gaseous 3He. This property makes neutron photon discrimination easier. Due to its high 6Li density this material is suitable for producing light weight compact neutron detectors, as a result LiCaAlF6 has been used for neutron detection at high altitudes on balloon missions. The long decay time of Eu2+ doped LiCaAlF6 makes it less suitable for measurements in high radiation environments, the Ce3+ doped variant has a shorter decay time but suffers from a lower light-yield.
NaIL Dual Detection Neutron-Gamma Scintillator Sodium Iodide crystal co-doped with Thallium and Lithium [NaI(Tl+Li)] a.k.a. NaIL has the ability to detect Gamma radiation and Thermal Neutrons in a single crystal with exceptional Pulse-shape Discrimination.The use of low 6Li concentrations in NaIL and large thicknesses can achieve the same neutron detection capabilities as 3He or CLYC or CLLB detectors at a lower cost.6Li (95% enriched) co-doping introduces efficient thermal neutron detection to the most established gamma-ray scintillator while retaining the favorable scintillation properties of standard NaI(Tl). NaIL can provide large volume, single material detectors for both gammas and neutrons at a low price per volume.
Semiconductor neutron detectors There are two basic types of semiconductor neutron detectors, the first being electron devices coated with a neutron reactive material and the second being a semiconductor being partly composed of neutron reactive material. The most successful of these configurations is the coated device type, and an example would be a common planar Si diode coated with either 10B or 6LiF. This type of detector was first proposed by Babcock et al. The concept is straightforward. A neutron is absorbed in the reactive film and spontaneously emits energetic reaction products. A reaction product may reach the semiconductor surface, and upon entering the semiconductor produces electron-hole pairs. Under a reverse bias voltage, these electrons and holes are drifted through the diode to produce an induced current, usually integrated in pulse mode to form a voltage output. The maximum intrinsic efficiency for single-coated devices is approximately 5% for thermal neutrons (0.0259 eV), and the design and operation are thoroughly described in the literature. The neutron detection efficiency limitation is a consequence of reaction-product self-absorption. For instance, the range in a boron film of 1.47 MeV α particles from the 10B(n,α) 7Li reaction is approximately 4.5 microns, and the range in LiF of 2.7 MeV tritons from the 10B(n,α) 7Li reaction is approximately 28 microns. Reaction products originating at distances further from the film/semiconductor interface can not reach the semiconductor surface, and consequently will not contribute to neutron detection. Devices coated with natural Gd have also been explored, mainly because of its large thermal neutron microscopic cross section of 49,000 barns. However, the Gd(n,γ) reaction products of interest are mainly low energy conversion electrons, mostly grouped around 70 keV. Consequently, discrimination between neutron induced events and gamma-ray events (mainly producing Compton scattered electrons) is difficult for Gd-coated semiconductor diodes. A compensated pixel design sought to remedy the problem. Overall, devices coated with either 10B or 6LiF are preferred mainly because the energetic charged-particle reaction products are much easier to discriminate from background radiations. The low efficiency of coated planar diodes led to the development of microstructured semiconductor neutron detectors (MSND). These detectors have microscopic structures etched into a semiconductor substrate, subsequently formed into a pin style diode. The microstructures are backfilled with neutron reactive material, usually 6LiF, although 10B has been used. The increased semiconductor surface area adjacent to the reactive material and the increased probability that a reaction product will enter the semiconductor greatly increase the intrinsic neutron detection efficiency. The MSND device configuration was first proposed by Muminov and Tsvang, and later by Schelten et al. It was years later when the first working example of a MSND was fabricated and demonstrated , then having only 3.3% thermal neutron detection efficiency. Since that initial work, MSNDs have achieved greater than 30% thermal neutron detection efficiency. Although MSNDs can operate on the built-in potential (zero applied voltage), they perform best when 2-3 volts are applied. There are several groups now working on MSND variations. The most successful types are the variety backfilled with 6LiF material. MSNDs are now manufactured and sold commercially by Radiation Detection Technologies, Inc. Advanced experimental versions of double-sided MSNDs with opposing microstructures on both sides of a semiconductor wafer have been reported with over 65% thermal neutron detection efficiency, and are theoretically capable of over 70% efficiency. Semiconductor detectors in which one of more constituent atoms are neutron reactive are called bulk semiconductor neutron detectors. Bulk solid-state neutron detectors can be divided into two basic categories: those that rely on the detection of charged-particle reaction products and those that rely on the detection of prompt capture gamma rays. In general, this type of neutron detector is difficult to make reliably and presently are not commercially available. The bulk materials that rely upon charged-particle emissions are based on boron and lithium containing semiconductors. In the search for bulk semiconductor neutron detectors, the boron-based materials, such as BP, BAs, BN, and B4C, have been investigated more than other potential materials. Boron-based semiconductors in cubic form are difficult to grow as bulk crystals, mainly because they require high temperatures and high pressure for synthesis. BP and Bas can decompose into undesirable crystal structures (cubic to icosahedral form) unless synthesized under high pressure. B4C also forms icosahedral units in a rhombohedral crystal structure, an undesirable transformation because the icosahedral structure has relatively poor charge collection properties which make these icosahedral forms unsuitable for neutron detection. BN can be formed as either simple hexagonal, cubic (zincblende) or wurtzite crystals, depending on the growth temperature, and it is usually grown by thin film methods. It is the simple hexagonal form of BN that has been most studied as a neutron detector. Thin film chemical vapor deposition methods are usually employed to produce BP, BAs, BN, or B4C. These boron-based films are often grown upon n-type Si substrates, which can form a p–n junction with the Si and, therefore, produce a coated Si diode as described at the beginning of this section. Consequently, the neutron response from the device can be easily mistaken as a bulk response when it is actually a coated diode response. To date, there is sparse evidence of boron-based semiconductors producing intrinsic neutron signals. Li-containing semiconductors, categorized as Nowotny–Juza compounds, have also been investigated as bulk neutron detectors. The Nowotny–Juza compound LiZnAs has been demonstrated as a neutron detector; however, the material is difficult and expensive to synthesize, and only small semiconductor crystals have been reported. Finally, traditional semiconductor materials with neutron reactive dopants have been investigated, namely, Si(Li) detectors. Neutrons interact with the lithium dopant in the material and produce energetic reaction products. However, the dopant concentration is relatively low in Li drifted Si detectors (or other doped semiconductors), typically less than 1019 cm−3. For a degenerate concentration of Li on the order of 1019 cm−3, a 5-cm thick block of natural Si(Li) would have less than 1% thermal-neutron detection efficiency, while a 5-cm thick block of a Si(6Li) detector would have only 4.6% thermal-neutron detection efficiency. Prompt gamma-ray emitting semiconductors, such as CdTe, and HgI2 have been successfully used as neutron detectors. These detectors rely upon the prompt gamma-ray emissions from the 113Cd(n, γ)114Cd reaction (producing 558.6 keV and 651.3 keV gamma rays) and the 199Hg(n, γ) 200Hg reaction (producing 368.1 keV and 661.1 keV gamma rays). However, these semiconductor materials are designed for use as gamma-ray spectrometers and, hence, are intrinsically sensitive to the gamma-ray background. With adequate energy resolution, pulse height discrimination can be used to separate the prompt gamma-ray emissions from neutron interactions. However, the effective neutron detection efficiency is compromised because of the relatively small Compton ratio. In other words, the majority of events add to the Compton continuum rather than to the full energy peak, thus, making discrimination between neutrons and background gamma rays difficult. Also, both natural Cd and Hg have relatively large thermal-neutron (n,γ) cross sections of 2444 b and 369.8 b, respectively. Consequently, most thermal neutrons are absorbed near the detector surface so that nearly half of the prompt gamma rays are emitted in directions away from the detector bulk and, thus, produce poor gamma-ray reabsorption or interaction efficiency.
Neutron activation detectors Activation samples may be placed in a neutron field to characterize the energy spectrum and intensity of the neutrons. Activation reactions that have differing energy thresholds can be used including 56Fe(n,p) 56Mn, 27Al(n,α)24Na, 93Nb(n,2n) 92mNb, & 28Si(n,p)28Al.
Fast neutron detectors Fast neutrons are often detected by first moderating (slowing) them to thermal energies. However, during that process the information on the original energy of the neutron, its direction of travel, and the time of emission is lost. For many applications, the detection of "fast" neutrons that retain this information is highly desirable. Typical fast neutron detectors are liquid scintillators, 4-He based noble gas detectors and plastic detectors. Fast neutron detectors differentiate themselves from one another by their (1) capability of neutron/gamma discrimination (through pulse shape discrimination) and (2) sensitivity. The capability to distinguish between neutrons and gammas is excellent in noble gas based 4-He detectors due to their low electron density and excellent pulse shape discrimination property. In fact, inorganic scintillators such as zinc sulfide has been shown to exhibit large differences in their decay times for protons and electrons; a feature that has been exploited by combining the inorganic crystal with a neutron converter (such as polymethyl methacrylate) in the Micro-Layered Fast-Neutron Detector. Such detection systems are capable of selectively detecting only fast neutrons in a mixed neutron-gamma radiation field without requiring any additional discrimination techniques such as pulse shape discrimination. Detection of fast neutrons poses a range of special problems. A directional fast-neutron detector has been developed using multiple proton recoils in separated planes of plastic scintillator material. The paths of the recoil nuclei created by neutron collision are recorded; determination of the energy and momentum of two recoil nuclei allow calculation of the direction of travel and energy of the neutron that underwent elastic scattering with them. == Applications ==