Neutron detector

Since neutrons are uncharged, they do not produce direct ionization as does a charged particle. The detection of neutrons, therefore, depends on their first being made to take part in some reaction which produces charged particles, which may then be detected in the conventional manner. One of the commonly chosen reactions is the production of a particles following neutron capture in 

Scintillation methods offer the possibility of high-efficiency detectors with a more rapid time response than the BF3 counter. As mentioned in the previous section, the basis of the scintillation detector is the conversion, in a suitable crystal, such as thallium-activated sodium iodide, Nal(Tl), of the kinetic energy of the charged particle to light, which can be amplified by a photomultiplier tube to provide an electrical pulse. Again, the neutron has to interact to produce either a charged particle or a 7 ray, the latter of which may in turn interact to produce ionizing particles. 

Scintillators which have hydrogen as a constituent, such as organic liquids for example, may be used for fast neutron detection, since the protons produced by fast neutron collisions create the ionization required to operate the detector. In order to adapt a sodium iodide scintillator for the detection of slow neutrons, a small concentration of boron may be distributed in the crystal, giving a particles on neutron capture as discussed above. Alternatively, it is possible to add a neutron absorber which emits 7 rays following the (n, y) capture reaction. Another possibility is the use of lithium iodide (Lil) which, in addition to its own suitability as a scintillator, interacts with neutrons through the reaction 

The efficiency of the detector may be improved by increasing the isotopic ratio of the Li, which is present as only 7.4% in natural lithium. Owing to its sensitivity to y radiation, a scintillation detector is not generally suitable where a strong background of high-energy y rays is present. 

Where it is required to measure neutrons in the presence of a significant y-ray background, it is possible to make use of the properties of certain scintillators to distinguish the pulses produced by neutrons from those due to y rays. This is known as a pulse-shape discrimination (PSD) system. In stilbene and some organic liquid scintillators, the pulse rise time for the fluorescence caused by the secondary electrons from a y-ray interaction is considerably shorter than that due to the recoil protons produced by neutron scattering. By the use of fast timing discriminators, it is possible to separate the pulses caused by neutrons from those due to the y rays. 

Main attributes of various neutron detector systems are listed in O Table 63.5. 

Neutrons can only be detected by indirect methods, e.g., via nuclear reactions producing charged particles. The electrical signal produced by the resulting charged particles can then be processed by the detection system. 

BF3 detectors tree occasionally used based on the B(n, a) Li reaction. BF3 detectors are less sensitive to gamma radiation fields but are less efficient. Recently, solid-state neutron radiation devices with boron carbide diodes have been developed, which demonstrate very promising potential for future applications such as miniaturized handheld neutron detection devices. 

System Detector Measurement task Typical performance values (%)  

BNCN Pu in fast critical assembly fuel plates 5 

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It was found that that in the case of soft beta and X-ray radiation the IPs behave as an ideal gas counter with the 100% absorption efficiency if they are exposed in the middle of exposure range ( 10 to 10 photons/ pixel area) and that the relative uncertainty in measured intensity is determined primarily by the quantum fluctuations of the incident radiation (1). The thermal neutron absorption efficiency of the present available Gd doped IP-Neutron Detectors (IP-NDs) was found to be 53% and 69%, depending on the thicknes of the doped phosphor layer ( 85pm and 135 pm respectively). No substantial deviation in the IP response with the spatial variation over the surface of the IP was found, when irradiated by the homogeneous field of X-rays or neutrons and deviations were dominated by the incident radiation statistics (1). 

The fractional loss in energy when a neutron collides with an atom is greatest for the H atom. Thus, by passing a beam of fast neutrons thru a series of samples, of the same material, but with varying moisture contents, a relationship is observed between moisture content and measured thermal neutron intensity. Using a small. radioisotope fast neutron source and a lithium iodide thermal neutron detector, this neutron... 

Solid state neutron detector of boron phosphide which is arefractory semiconductor with a wideband gap.l" ... 

The worst operating condition in a common design practice consists of overly conservative assumptions on the hot-channel input. These assumptions must be realistically evaluated in a subchannel analysis by the help of in-core instrumentation measurements. In the early subchannel analysis codes, the core inlet flow conditions and the axial power distribution were preselected off-line, and the most conservative values were used as inputs to the code calculations. In more recent, improved codes, the operating margin is calculated on-line, and the hot-channel power distributions are calculated by using ex-core neutron detector signals for core control. Thus the state parameters (e.g., core power, core inlet temper-... 

Gas-filled detectors are used, for the most part, to measure alpha and beta particles, neutrons, and gamma rays. The detectors operate in the ionization, proportional, and G-M regions with an arrangement most sensitive to the type of radiation being measured. Neutron detectors utilize ionization chambers or proportional counters of appropriate design. Compensated ion chambers, BF3 counters, fission counters, and proton recoil counters are examples of neutron detectors. 

Another method for detecting neutrons using an ionization chamber is to use the gas boron trifluoride (BF3) instead of air in the chamber. The incoming neutrons produce alpha particles when they react with the boron atoms in the detector gas. Either method may be used to detect neutrons in nuclear reactor neutron detectors. 

Plastic phosphors are made by adding scintillation chemicals to a plastic matrix. The decay constant is the shortest of the three phosphor types, approaching 1 or 2 nanoseconds. The plastic has a high hydrogen content therefore, it is useful for fast neutron detectors. 

Four other types of radiation detectors are the self-powered neutron detector, wide range fission chamber, flux wire, and photographic film. 

In very large reactor plants, the need exists to monitor neutron flux in various portions of the core on a continuous basis. This allows for quick detection of instability in any section of the core. This need brought about the development of the self-powered neutron detector that is small, inexpensive, and rugged enough to withstand the in-core environment. The self-powered neutron detector requires no voltage supply for operation. Figure 29 illustrates a simplified drawing of a self-powered neutron detector. 

There are two distinct advantages of the self-powered neutron detector (a) very little instrumentation is required—only a millivoltmeter or an electrometer, and (b) the emitter material has a much greater lifetime than boron or U235 lining (used in wide range fission chambers). 

One disadvantage of the self-powered neutron detector is that the emitter material decays with a characteristic half-life. In the case of rhodium and vanadium, which are two of the most useful materials, the half-lives are 1 minute and 3.8 minutes, respectively. This means that the detector cannot respond immediately to a change in neutron flux, but takes as long as 3.8 minutes to reach 63% of steady-state value. This disadvantage is overcome by using cobalt or cadmium emitters which emit their electrons within 10 14 seconds after neutron capture. Self-powered neutron detectors which use cobalt or cadmium are called prompt self-powered neutron detectors. 

A description of how self-powered neutron detectors, wide range fission chambers, flux wires, and photographic film detect radiation is summarized below. 

Two different types of information can be obtained by bombarding soil with neutrons. Fast neutrons are slowed when they interact with water and thus can be used to measure the amount of water present. This type of analysis is most often conducted in the field rather than in the laboratory. Figure 8.2 illustrates the use of a fast neutron source and a slow neutron detector to measure the moisture content of soil. This method depends on the interaction of neutrons with hydrogen and so it is not as useful in soils with significant or highly variable organic matter contents. 

B is a powerful neutron absorber and has been employed in reactor control rods, neutron detectors, and other applications. Cascades based on exchange distillation of boron-ether complexes have usefully large a s and were used for 10B/UB isotope separation by the US DOE. Exchange distillation takes advantage of the fact that condensed phase/vapor phase separation factors can be enhanced (as compared to liquid/vapor a s) by association/dissociation equilibria in one or the other phase. At the normal boiling point (173 K) the VPIE for... 

Neutron detectors are often separated into two types low-energy neutron and fast neutron detectors. Low-energy neutrons are typically detected through the use of a... 

Self-powered neutron detectors (SPNDs) use a material such as cadmium that has a high cross section for low-energy neutrons and produces copious gammas or... 

Fast neutron detection sometimes uses a hydrogenous moderator to slow down the neutrons and then employs a low-energy neutron detector as described above. One common fast neutron detector is a Bonner sphere. In this detector, a scintillator is placed in the center of a polyethylene sphere. Radiation transport calculations are used to produce efficiency curves that depend on the energy of the incident neutron. Another common fast neutron detector is a long counter. This detector uses a slow neutron detector (originally a BF3 chamber) at the center of a cylindrical moderator designed so that the detector is only sensitive to neutrons incident from one side. 

High energy neutron generators are particularly useful in well logging applications. In such applications one important factor is accurate knowledge of the neutron pulses that irradiate the surrounding formation. For example, it is desirable to accurately measure the neutron output, e.g., the number of neutrons emitted by the neutron detector. 

Owing to their neutron-absorbing properties, some benzo fused 1,2-dihydro-1,2-azaborines have been suggested for use as neutron sensitive material in neutron detectors (67MI12100). 

Intensity of scattered neutrons is measured as a function of scattering angle 20. The measured response of the neutron detector is the sum of the coherent scattered intensity of the sample particles (Eq.2), the scattering from the solvent, the scattering from the sample cell, and the electronic noise in the detector. To obtain the scattering from the sample particles, background scattering due to solvent and sample cell, and noise counts in the detector, must be subtracted from the experimental scattered intensity. The result is normalized to an... 

A simplified illustration of neutron gauging is shown in Fig 2. The essential components are an isotopic neutron source, the sample to be measured, a thermal neutron detector and appropriate nuclear counting instrumentation to record changes in thermal neutron count rates. Neutron gauging can be performed in two distinct modes. If a bare or unmoderated isotopic... 

The density and thermal neutron cross-section values in Table 6 pertain to the thermal neutron attenuation gauging process. In this method, advantage is taken of the large thermal neutron scattering cross-section of hydrogen as compared to most other elements. In its simplest form, when a beam of thermal neutrons of intensity IQ traverses a sample of thickness x, the intensity 1 of neutrons measured by a thermal neutron detector will be... 

A top cross-sectional sketch of the fast neutron gauge is shown in Fig 14. The sample is sandwiched between the neutron source capsule and the window of a shielded thermal neutron detector. The actual assembly includes either a 5 curie plutonium-beryllium capsule or a 4 microgram 252 Cf capsule as the source of fast neutrons the neutron output for each source is about 107 neutrons/sec. The sample container... 

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