Prompt neutrons 7 rays

This can result in a radioactive product from the A(n, t)A reaction where A is the stable element, n is a thermal neutron, A is the radioactive product of one atomic mass unit greater than A, and y is the prompt gamma ray resulting from the reaction. A is usually a beta and/or gamma emitter of reasonably long half-life. Where access to a nuclear reactor has been convenient, thermal neutron activation analysis has proven to be an extremely valuable nondestructive analytical tool and in many cases, the only method for performing specific analyses at high sensitivities... 

Of these neutron interactions, those that produce prompt-7 rays were evaluated as the most feasible for mine detection. As discussed in the Introduction, we define a prompt 7-ray as one which is produced as a direct result of the primary neutron interaction, usually within nanoseconds of that initial event. Such reactions are obviously attractive because they can best satisfy the desired rapid sweep rate over a minefield. The three specific neutron-prompt gamma reactions that were intensively investigated by the US Army are listed below ... 

PGNAA involves the measurement of the prompt y rays emitted when the nuclei first absorb the neutrons. By energy and intensity analysis of the prompt y spectrum it is possible to reveal the elements present and determine their concentrations. Because the analysis is carried out during the irradiation, the method is quicker than delayed NAA, but it needs special equipment on site at the reactor to record the y spectrum whilst the sample is being irradiated (Glascock et al. 1984, Glascock 1994). It can, however, measure some elements more efficiently than delayed NAA (e.g., Pb), since it does not require the daughter nucleus to be radioactively unstable. 

Glascock, M.D., Spalding, T. G., Biers, J. C., and Corman, M.F. (1984). Analysis of copper-based metallic artefacts by prompt gamma-ray neutron activation analysis. Archaeometry 26 96-103. 

Ouri, Y., Shirari, N. and Ebihara, M. (2003) Chemical composition of Yamato (Y)980459 and Y000749 Neutron-induced prompt gamma-ray analysis study. Antarctic Meteorite Research, 16, 80—93. 

Prompt y-ray emission competes with or follows the last stages of prompt neutron emission. These photons are emitted in times from 10 15-10 7s. Typical y-ray multiplicities of 7-10 photons/fission are observed. These photons, as indicated earlier, cany away 7.5 MeV. This y-ray yield is considerably larger than one would predict if y-ray emission followed neutron emission instead of competing with it. Because of the significant angular momentum of the fission fragments ( 7-10 h) even in spontaneous fission, photon emission can compete with neutron emission. The emitted y rays are mostly dipole radiation with some significant admixture of quadrupole radiation, due to stretched El transitions (J/= 7, — 2). 

The prompt gamma-rays emitted following neutron or charged particle interactions with the target nuclide may be used as a basis for non-destructive analyses. The important advantage of this technique is that the determination does not depend in any manner on the half-life of a product radionuclide. In fact, using this technique, the product nuclide need not even be radioactive. Many conventional activation determinations are limited in their sensitivities by short half-life product radionuclides, or the fact that the most abundant or highest cross section isotope of the element to be determined leads to a stable product on irradiation. 

INAA and thermal neutron capture prompt y-ray activation analysis (PGAA) were used to this end. Unopened units were shown to have stable dry weight bases and stable mass fractions. The concentrations measured in this study for Al, As, B, Br, Ca, Cl, Co, Cu, Fe, K, Mg, Mn, Mo, Na, Rb, S, Se, and Zn were in very good agreement with the certified or consensus values, which justifies their continued use for QC purposes. 

Fission of heavy nuclei always results in a high neutron excess of the hssion products, because the neutron-to-proton ratio in heavy nuclides is much larger than in stable nuclides of about half the atomic number, as already explained for spontaneous hssion (Fig. 5.15). The primary fission products formed in about 10 " s by fission and emission of prompt neutrons and y rays decay by a series of successive / transmutations into isobars of increasing atomic number Z. The final products of these decay chains are stable nuclides. 

MeV) and of prompt y-ray photons. The number of prompt neutrons emitted by the primary fission fragments depends mainly on their excitation energy. It increases with the mass number of the fissioning nuclei (Table 8.2). In Fig. 8.17 this number is plotted as a function of the mass of the fission fragments. It is relatively low for fragments with filled neutron shells N = 50, = 82). 

In general, the activity of the radionuclide produced by the nuclear reaction is measured after irradiation, either directly or after chemical separation. However, the prompt y-ray photons emitted in (n,y) reactions may also be counted on the site of their production. For that purpose, the samples must be irradiated outside the nuclear reactor or by another neutron source, and the y-ray photons are recorded by means of a y-ray spectrometer. The intensity I of the y rays produced by the nuclear... 

C. Yonezawa, Prompt y-Ray Analysis of Elements Using Cold and Thermal Reactor Neutrons, Anal. Sci.9, 185 (1993). 

Figure 1. Histogram of detection limits for neutron activation analysis (NAA) and neutron-capture prompt gamma-ray activation analysis (PGAA). Data from Reference 15.

Neutron-capture prompt-gamma ray activation analysis (PGAA) is a recent addition to the nuclear analytical arsenal. In this technique the instantaneous gamma ray emission from a sample is measured as it is irradiated in a flux of reactor neutrons (33,3, 35). Because the sample must be several meters from either the core of the reactor or (less commonly) from the detector, the sensitivity of this technique is generally poorer than in conventional NAA. However, it is possible to measure small quantities of many elements which do not give radioactive neutron-capture products, notably 0.01 mg of H, 50 ng B, and 1 mg P in an electronics context. 

Glendenin (87) attempted to explain the high yields of the 133 and 134 mass chains by assuming that nuclei with 83 neutrons (which has already emitted the usual number of prompt neutrons) would have a high probability of boiling off a prompt neutron to form the most stable 82 neutron configuration rather than emit or y rays as in the ordinary case. According to this scheme, the 134 mass chain would be increased by the reaction... 

Andeeson DL (2000) Neutron capture prompt gamma-ray activation of meat homogenates. J Radioanal Nud Chem 244 225 - 229. 

Neutron activation analysts (NAA) can be accomplished by measurement of the prompt gamma rays during irradiation or by measurement of the delayed... 

FIGURE 32-6 Overview of the neutron activation process. The incident neutron is captured by the target nucleus to produce an excited compound nucleus, which de-excites with emission of a prompt gamma ray. The radioactive nucleus formed decays by emitting a beta particle. If an excited product nucleus is formed, a delayed gamma ray can be emitted. If decay is directly to the ground state of the product nucleus, no gamma ray is emitted. 

The neutrons emitted have an average kinetic energy of 2 MeV. For the average of 2.5 neutrons emitted in fission of U by thermal neutrons, about 5 MeV of the fission energy is required. The emission of y-rays in the act of fission, prompt y-rays, accounts for another 6-8 MeV. The largest part of the fission energy is observed as the kinetic energy... 

Two elemental analyzer systems have been developed, the "Continuous On-line Nuclear Assay of Coal", CONAC, (Science Application, Inc., Palo Alto, CA) and "The Elemental Analyzer" (MDH Industries, Inc., Monrovia, CA). Both of these units are based upon the measurement of prompt gamma rays that are emitted from a nucleus following the capture of a neutron. This technique is commonly known as prompt neutron activation analysis, PNAA. 

If the spin of the initial state is very different from that of the capturing state, we should expect to find that the intensity of a low energy y-ray depends markedly on which of the two spin states is responsible for capture. For prompt y-rays this phenomenon has not yet been studied, but the corresponding effect on the yield of isomeric states has been noted by several authors. It seems to have been first detected by Capron and Verhoeve-Stokkink. In more detail it has been studied by Sailor, who has examined the activation of the isomeric state of In produced by neutron capture in the first two resonances in targets of In, and by Wood in studies of the activation of the isomeric state of Eu 2... 

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