Thermal neutron irradiation

Tissues Freeze drying of samples subjection to thermal neutron irradiation chemical separation of elements NAA No data No data Hewitt 1988... 

As a result of slow (thermal) neutron irradiation, a sample composed of stable atoms of a variety of elements will produce several radioactive isotopes of these activated elements. For a nuclear reaction to be useful analytically in the delayed NAA mode the element of interest must be capable of undergoing a nuclear reaction of some sort, the product of which must be radioactively unstable. The daughter nucleus must have a half-life of the order of days or months (so that it can be conveniently measured), and it should emit a particle which has a characteristic energy and is free from interference from other particles which may be produced by other elements within the sample. The induced radioactivity is complex as it comprises a summation of all the active species present. Individual species are identified by computer-aided de-convolution of the data. Parry (1991 42-9) and Glascock (1998) summarize the relevant decay schemes, and Alfassi (1990 3) and Glascock (1991 Table 3) list y ray energy spectra and percentage abundances for a number of isotopes useful in NAA. 

Trace amounts of Tc are also determined in filter paper and vegetable samples by neutron activation analysis The procedure consists of the following major steps separation of technetium from the sample, thermal neutron irradiation of the Tc fraction to produce °°Tc, post-irradiation separation and purification of °°Tc from other activated nuclides, and counting of the 16 s Tc in a low-background P counter. The estimated detection limits for Tc in this procedure are 5 x 10 g in filter paper and 9 x 10 g in vegetable samples. 

Although Muraour and Ertand stated that they had substantiated the results of Bowden and Singh (Refs 28 35), a different environment was used. It should be noted that the former utilized reactor irradiation at higher dose rates and doses as compared to the slow thermal neutron irradiation for the latter. For example the thermal neutron dose rate for Pb azide was 4.2 x IQ9 compared to 2 x 107n/cm2/sec and the total dose was 3 x 1014 compared to 7.2 x 10lon/cm2 ... 

T. Waaler, S. A. Sande, B. W. Muller, G. Schuller-Lisether, The influence of thermal neutron irradiation on the in vitro characteristics of ASA oral dosage forms—Validation of neutron activation, Eur J Pharm Biopharm 43 159-164 (1997). 

The Ge(Li) gamma-ray spectrum of thermal neutron irradiated cigarette tobacco as obtained in our laboratory is shown in Fig. 8. In this case a 50 day decay period has eliminated the major portion of interferences from sodium, potassium and phosphorus activities. Assignments of photopeaks in this figure may be regarded as tentative, since half-lives of the individual peaks were not followed. As many as fifteen elements have been determined in tobacco products and biological standard kale... 

The application of Ge(Li) detectors to the determination of trace elements in rocks is beautifully illustrated by the work of Gordon et al. 6 who were able to determine instrumentally 23 elements in a wide variety of igneous rocks. A useful discussion of sensitivities and potential interferences may also be found in this paper. The Ge(Li) gamma-ray spectrum of thermal neutron irradiated standard granite G—1 as obtained in our laboratory is shown in Fig. 9. Again, the tentative assignment of photopeaks is based... 

Fig. 9. Ge(Li) gamma-ray spectrum for thermal neutron irradiated standard granite rock, G—1...

A discussion of the coincidence technique with some general applications has been published by Wahlgren, Wing and Hines 71>. Many of the early applications of the technique made use of the fact that 64Cu is one of the few radionuclides produced by thermal neutron irradiation for which the 0.511 MeV positron annihilation photopeak is a prominent feature of the spectrum. Copper has been determined in meteorites 72> and copper ores 73,74) ]-,y coincidence counting of 04Cu annihilation radiation. The rapid and selective nature of the determination may have important applications in the on-line sorting of copper ores. 

Fig. 15. Singles and coincidence spectra for a thermal neutron irradiated stony meteorite...

The use of 14 MeV neutron activation principally for major elements, Ge(Li) detectors for trace elements following thermal neutron irradiations, and gamma-gamma coincidence techniques for positron or cascade gamma-ray emitters as discussed in the previous sections, provide the analyst with powerful tools for devising schemes for non-destructive analysis. A few additional activation techniques which may be useful in special applications are discussed briefly below. In most of these cases rather sophisticated instrumentation is required. It is unlikely, therefore, that these techniques will be in routine use in a facility devoted principally to analytical applications. In some cases, however, arrangements may be made for part time use of a more extensive nuclear facility for a specific analytical problem. 

Szllard-Chalmers. In 1954 It was reported by Szilard and Chalmers that radioactive iodine produced by thermal neutron Irradiation of ethyl iodide could be extracted into an aqueous phase. Then by being reduced to iodide and precipitated as silver iodide, the concentrated. radioactive iodide could be obtained (255) ... 

The radioisotope 14C (j8 , 5570 years), which is widely used as a tracer, is made by thermal neutron irradiation of Li or A1 nitride, 14N(n,p)14C. It is available not only as C02 or carbonates, but also in numerous labeled organic compounds. Its formation in the atmosphere and absorption of C02 by living organisms provide the basis of radiocarbon dating. 

Table 1 summarizes some of the important properties of the carbon isotopes. Note that only the rare ( 1%), naturally occurring, stable carbon isotope, namely, C, has a nuclear spin and is observable by NMR. The organic chemist is fortunate that 99% of natural carbon is the isotope C with no nuclear spin, so that proton and carbon-13 NMR spectra of organic compounds are not complicated by spin - spin splitting arising fi om adjacent carbon atoms. The radioisotope C is made by thermal neutron irradiation of lithium or aluminum nitride (equation 1). It decays back to stable yN by jS emission, with a half-life of 5570 years (equation 2). Cosmic rays generate thermal neutrons, which leads to the formation of C02 in the atmosphere (equation 1). Metabolism of... 

Experimental approaches have been reported by two groups. The group at Gunma University (25) synthesized a prototype of ladder polymers, poly(bicyclosilane)s. Polymers with three, four, and five ladder steps were synthesized, and the bathochromic shifts were measured, Ikehata et al, (26) attempted substitutional doping by thermal neutron irradiation of (SiH) ladder polymers synthesized through the reduction of trichlorosilane with lithium in THE (tetrahydrofuran). Thermal neutron capture by Si will yield P in this process for total flux amounts of up to 10 /cm. The concentration of phosphorus impurity is 0.1 ppm. No serious deterioration was caused by strong neutron irradiation. However, in this study, the detailed properties... 

Effects of Radiation. LA exposed to gamma radiation by Warren et al and by Kosen-wasser, as reported in Ref 139, exhibited post-irradiation gas evolution as measured by vac stab test appar. Bowden Singh (Ref 135) irradiated Pb, Ag and Cd azides with electrons, neutrons, fission prods and x rays. All azides were exploded by an intense 75-kv electron scream. Thermal neutron irradiation did affect the subsequent decompn of Li and Pb azides. Muraour Ercaud (Ref 129) also subjected LA to a neutron flux. Raney (Ref 158) reported that a total flux of 7.5 10 ... 

Since about 1930 a great number of papers concerned with radiation effects on catalysts have been produced, but very little work in comparison has been done with ion bombardment. The great majority of the papers dealt with 7-, X-ray, electron or thermal neutron irradiations. Some of the later studies using protons, deuterons and a-particles are to some extent comparable to irradiations with ions . In some cases enhancement of the catalytic activity was found, in others in-... 

The availability of high flux thermal neutron irradiation facilities and high resolution intrinsic Ge and lithium drifted germanium (Ge(Li)) or silicon (Si(Li)) detectors has made neutron activation a very attractive tool for determining trace elemental composition of petroleum and petroleum products. This analytical technique is generally referred to as instrumental neutron activation analysis (INAA) to distinguish it from neutron activation followed by radiochemical separations. INAA can be used as a multi-elemental method with high sensitivity for many trace elements (Table 3.IV), and it has been applied to various petroleum materials in recent years (45-55). In some instances as many as 30 trace elements have been identified and measured in crude oils by this technique (56, 57). 

Fid. 3. Mass spectrogram of gadolinium before and after thermal neutron irradiation (110). 

Cereals, food CRMs I Combust in quartz tube in train with fiir-nace/02, purify, trap and irradiate I2 in reactor [DA] Count gamma rays after thermal neutron irradiation of separated I2 in pneumatic facility of reactor [SEP/CONC-RNAA] [DA-SEP/ CONC-RNAA] Norman and Iyengar(1994)... 

Determination of a fission track age requires several further experimental steps to measure the uranium concentration. The uranium concentration is not measured directly, but a second set of fission tracks is created artificially in the sample by a thermal neutron irradiation. This irradiation induces fission in a tiny fraction of the atoms, which are present in a constant ratio to U in natural uranium. Knowing the total neutron fluence received during irradiation, the number of induced tracks provides a measure of the uranium concentration of the grain. Because the induced tracks are derived from a different isotope of uranium than the spontaneous tracks an important consideration in fission track dating is the assumption that the isotopic ratio of the two major isotopes of uranium, and is constant in nature. With the notable exception of the unique natural nuclear reactors of Oklo in Gabon (Bros et al. 1998), where this isotopic ratio is disturbed, this is a very safe assumption. Numerous measurements have shown that and are always present in their natural abundances of 0.73% and 99.27%, respectively. 

Kumashiro, Y, K. Kudo, K. Matsumoto, Y. Okada, and T. Koshira. 1987. Thermal neutron irradiation experiments on BP single crystal wafers. In Proceedings of the Ninth International Symposium on Boron Borides Related Compounds, ed. H. Werheit, pp. 371-372. Duisburg Duisburg University Press. 

Graphite structural components can be subject to thermal, neutron irradiation, and mechanical strains along with vibrational fatigue, chemical attack, and erosion. The potential modes of failure include loss of structural material, cracking, and ultimately, fracture of the component. 

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