Detection of neutrons

The most common neutron detectors are of the proportional gas type. Since neutrons themselves have no charge and are non-ionizing, they are harder to detect than X-rays. Detection relies on the absorption of the neutron by an atomic nucleus with the simultaneous emission of a y-ray photon, often referred to as an (n,y) reaction. Since the absorbing material must absorb neutrons and be capable of existing in gaseous form, the choice of substances is limited. The most common is He gas, which relies on the reaction  

Another suitable gas is BF3, which uses the absorption properties of the isotope °B, releasing an energetic a particle and Li  

BF3 is usually isotopically enriched as normal boron consists of only 20% of the strongly absorbing isotope. For thermal neutrons the He gas detector is considered superior, but for long-wavelength neutrons BF3 is preferred, but is rarely used nowadays because of the corrosive and toxic nature of the gas. Since the capture cross-section of gases is small, He detectors are usually operated above atmospheric pressure, 5-10 bar. They are typically 10-15 cm long and 2-5 cm in diameter. 

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J.J. Antel, Identification of Hydrogen in Materials by Resonance Detection of Neutrons , Watertown Ars, Mass TR 73-8 (1973)... 

When judging by communications available in the open literature, none of effects 1 through 5 could so far be observed repeatedly and reproducibly under rigorously controlled conditions. Provisionally, all instances of published experimental confirmation can be placed into two groups (1) the observation of sporadic sufficiently pronounced manifestations, and (2) the observation of more invariant but very weak effects (as a rule, at the level of background noise, particularly in the detection of neutrons and tritium). However, there were far fewer confirmations than infirmations (i.e., work in which the successful experiments could be carefully reproduced or the method used to determine the products was analyzed and shown to be in error). Such work has been of exceptional value in the area of advancing the methods and techniques used in experimental studies. 

Secondary ionization caused by the capture of neutrons is important in the detection of neutrons. Neutrons will interact with B-10 to produce Li-7 and He-4. 

To a limited degree, the fill-gas will determine what type of radiation the proportional counter will be able to detect. Argon and helium are the most frequently used fill gases and allow for the detection of alpha, beta, and gamma radiation. When detection of neutrons is necessary, the detectors are usually filled with boron-triflouride gas. 

Using the Chart of the Nuclides as a guide, estimate the sensitivity (minimum quantity that can be detected) of neutron activation analysis for europium using a thermal neutron flux of 3 x 1012 n/cm2-s. Assume no irradiation may last more than 1 h and the minimum detectable activity is 10 dpm. 

Fig. 6. Large neutron counter with 3He counting tubes for the detection of neutron bursts emitted in the spontaneous fission of superheavy nuclei. Reproduced from R.L. Macklin et al. [43], Copyright (2002), with permission from Elsevier Science.

Detection of neutrons is based on ionization processes caused by the products of their interactions (nuclear reactions or collisions) with nuclei. 

A variation of the detector described above is the so-called drift chamber. The drift chamber determines the position from the time it takes the electrons produced by the incoming particle to drift to the nearest anode wire. A two-dimensional MWPC has also been constructed for detection of neutrons scattered from biological samples. It is a He gas-filled counter that detects neutrons through the (n, p) reaction. 

Detection of neutrons by proton recoil is based on collisions of neutrons with protons and subsequent detection of the moving proton. Since neutrons and protons have approximately the same mass, a neutron may, in one collision, transfer all its kinetic energy to the proton. However, there is a possibility that the struck proton may have any energy between zero and the maximum possible, as a result of which the relationship between a neutron energy spectrum and a pulse-height distribution of the struck protons is not simple. It is the objective of this section to derive a general expression for this relationship. The sections that follow show its application for specific detectors. 

The neutron bubble detector (trade name BD-IOOR) is a reusable, passive integrating dosimeter that allows instant, visible detection of neutron dose. The bubble detector consists of a glass tube filled with thousands of superheated liquid drops in a stabilizing matrix. When exposed to neutrons, these droplets vaporize, forming visible permanent bubbles in an elastic polymer. The total number of bubbles formed is proportional to the neutron dose equivalent H. The bubbles can be counted manually or by a machine. Figure 16.15 shows the response of the bubble detector as a function of neutron energy. 

Fleischmann, M. and Pons, S. (1992) Concerning the detection of neutrons and /-rays from cells containing palladium cathodes polarised in heavy water. Nuovo Cimento della Societa di FisicaA Nuclei, Particles and Fields, 105A, 763. 

The detection of neutrons with an ion chamber requires some type of special feature within the detector since neutrons are not directly ionizing particles. Which of the following is the most commonly used special feature to allow ion chamber neutron detection ... 

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... 

Fig. IIl.B.l Schematic layout of the (H, d)atom experiment. The detection of neutrons from the reaction (E ,d)atom H + n will be the signature of H particle production.

Generally, the efficiency of detection of neutrons of wavelength, A, by a detector of thickness x, containing a number density, n, of absorbing atoms with neutron absorption cross-section, may be... 

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