Because of this it is necessary to slow the neutrons down to increase the probability of interaction. Gas proportional detectors such as these are efficient only for thermal (low energy) neutrons for high energy neutrons their capture cross sections are very small, making it very unlikely that a neutron will interact with the fill gas and cause the necessary detection reaction. They also provide a cost effective and stable means of constructing detectors for a wide range of applications. They offer high detection efficiency with excellent gamma discrimination. Gas filled proportional counters offer low noise amplification of the ionization event producing a charge pulse processed by the attached nucleonics chain. Quench gas is also added to control the ionization process.Īnother common method uses BF 3 filled detectors that utilize the fission of the 10B atom to provide the charged particle.Ī typical instrument configuration that can be used with either of these detectors can be seen in Figure 1.48.įigure 1.48 - Neutron Counter Electronics Where both the proton and the triton are detected by a gas filled proportional counters using 3He fill gas. The most common reaction used for high efficiency thermal neutron detection today is: These charged particles are then directly detected and from them the presence of neutrons is deduced. This means that neutron detectors must rely upon a conversion process where an incident neutron interacts with a nucleus to produce a secondary charged particle. Because of this they cannot directly produce ionization in a detector, and therefore cannot be directly detected. Neutrons have mass but no electrical charge. Like SF neutrons, they have a broad energy spectrum and are time-correlated. ![]() In spent fuel Cm and Cf isotopes may be significant.įissions can be induced in 239Pu, 235U, and 238U by neutron interrogation of the sample with an external neutron source. Uranium isotopes and oddnumbered plutonium isotopes spontaneously fission at a much lower rate (0.0003 to 0.006 SF/gram-second). SF neutrons are time-correlated (several neutrons are produced at the same time), with the average number of neutrons per fission being between 2.16 and 2.26. Like (α,n) neutrons, SF neutrons have a broad energy spectrum. The even-numbered isotopes of plutonium ( 238Pu, 240Pu, and 242Pu) spontaneously fission (SF) at a rate of 1100, 471, and 800 SF/gram-second respectively. Other α-emitting nuclides can also make important contributions, for example 241Am. Neutrons from (α,n) reactions are produced randomly (not time-correlated) and they exhibit a broad energy spectrum. The yield depends upon the chemical composition of the matrix and the alpha production rate for plutonium and uranium. The alpha particle is absorbed by the nuclei of the low atomic number elements (Li, B, Be, O, F, C, Si, etc.) and a neutron is produced. Plutonium and uranium isotopes decay by alpha particle emission. There are several methods by which neutrons may be produced in the fuel cycle principle amongst these are:
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