Delayed gammas

In terms of atomic spectrometry, NAA is a method combining excitation by nuclear reaction with delayed de-excitation of the radioactive atoms produced by emission of ionising radiation (fi, y, X-ray). Measurement of delayed particles or radiations from the decay of a radioactive product of a neutron-induced nuclear reaction is known as simple or delayed-gamma NAA, and may be purely instrumental (INAA). The y-ray energies are characteristic of specific indicator radionuclides, and their intensities are proportional to the amounts of the various target nuclides in the sample. NAA can thus... 

Figure 2.13 Schematic diagram of the nuclear processes involved in neutron activation analysis. Prompt gamma neutron activation analysis (PGNAA) occurs within the reactor delayed gamma NAA (DGNAA) occurs at some remote site. (After Glascock, 1994 Fig. 1. John Wiley Sons Limited. Reproduced with permission.)...

The basic principle of the experiment of Canter, Mills and Berko (1975) was to collide low energy positrons with a surface and to look for coincidence between a Lyman-a photon and a delayed gamma-ray arising from the subsequent annihilation of a 13S positronium. The presence of the Lyman-a signal was verified by the use of three interference filters with pass bands centred on, just above, and just below, 243 nm. An enhanced coincidence rate was found with the 243 nm filter in place. A similar Lyman-a gamma-ray technique has been adopted by all subsequent workers in this field (e.g. Laricchia et al., 1985 Hatamian, Conti and Rich, 1987 Ley et al., 1990 Schoepf et al., 1992 Steiger and Conti, 1992 Hagena et al., 1993 Day, Charlton and Laricchia, 2000). 

Of the five stable Isotopes of zinc, only three ( Zn, Zn, and Zn) can be measured with (delayed gamma) Neutron Activation Analysis and hlgh-resolutlon gamma spectrometry. The required methodology for the measurement of Zn is the most stringent of the three isotopes and the necessary Radiochemical Neutron Activation Analysis has been developed and described in detail elsewhere (.8), The salient features of neutron activation of these three Isotopes are tabulated in Table li and from these, two points emerge First, with the availahility of thermal neutron... 

Following the primary fission reaction, the radioactive fission products undergo radioactive disintegration, yielding beta particles and delayed gamma rays and ending up as stable fission products. Since the radioactive fission products have half-lives ranging from fractions of a... 

Gammas. About seven gammas are emitted at the time of fission. These are called prompt gammas. At later times, about seven to eight more gammas are released, called delayed gammas. Photons carry a total of about 15 MeV per fission. 

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. 

Why is delayed gamma-ray emission most often used in NAA ... 

For short half-lives, C approaches zero and the counting increases linearly with the measurement time. This is automatically the case when prompt radiation is detected instead of decay radiation. For this reason, prompt gamma activation analysis is inherently more sensitive than delayed gamma activation analysis. 

The prompt gamma-ray cross sections were derived from a comparison of measurements with the neutron beam at the Budapest Reactor and other data from the literature (Molnar 2004 Choi et al. 2007). In some cases, these values were renormalized to better agree with the adopted total radiative cross section from all measurements. Delayed gamma-ray cross sections were derived from a comparison of Budapest Reactor measurements, corrected for activation and decay times, values derived from the ko database, and values adopted from total radiative cross sections and emission probability values from either the Table of Radionuclides (Be et al. 2004) or the ENSDF (2010) nuclear structure and decay database (O Table 36.3). 

Campbell, L.W., Smith, L.E., and Misner, A.C. (2011). High energy delayed gamma spectroscopy for spent nuclear fuel assay, IEEE Trans. Nucl. Sci. 58, 231-240. 

CONTROLLED RADIOACTIVE AND EXCITED NUCLEI RADIATIVE GAMMA DECAY BY THE DELAYED GAMMA-GAMMA COINCIDENCE METHOD... 

The aim of the present section is connected with direct observation and examination of the law of controlled gamma decay of radioactive nucleus at resonant screen presence bythe delayed gamma-gamma coincidence... 

Experimental layout for detecting of the effect of controlling the time of radiation nuclear decay based on the delayed gamma-gamma coincidence method for cases thin resonant Fe screen is situated near the controlled radioactive source (a) thin Fe resonant and thick Pb nonresonant screens are situated near the controlled radioactive source (b). 

Activation methods are based on the measurement of the radioactivity or radiation produced in samples when they are irradiated with neutrons or charged particles, such as hydrogen, deuterium, or helium ions. An overview of the most common type of neutron activation is shown in Figure 32-6. Here, a neutron is captured by the target nucleus to form an excited compound nucleus. The compound nucleus de-excites almost instantaneously by emission of one or more characteristic prompt gamma rays. In many cases a new radioactive nucleus is formed, which can undergo /3 decay to an exited product nucleus with the emission of another characteristic delayed gamma ray. 

With respect to the time of measurement, NAA falls into two categories (1) prompt gamma-ray neutron activation analysis (PGNAA), where measurements take place during irradiation, or (2) delayed gamma-ray neutron activation analysis (DGNAA), where the measurements follow radioactive decay. The latter operational mode is more common. 

DGNAA Delayed gamma-ray neutron activation analysis... 

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