Neutron Irradiation, activities

Most chemical iavestigations with plutonium to date have been performed with Pu, but the isotopes Pu and Pu (produced by iatensive neutron irradiation of plutonium) are more suitable for such work because of their longer half-Hves and consequendy lower specific activities. Much work on the chemical properties of americium has been carried out with Am, which is also difficult to handle because of its relatively high specific alpha radioactivity, about 7 x 10 alpha particles/(mg-min). The isotope Am has a specific alpha activity about twenty times less than Am and is thus a more attractive isotope for chemical iavestigations. Much of the earher work with curium used the isotopes and Cm, but the heavier isotopes... 

The neutron dose to graphite due to irradiation is commonly reported as a time integrated flux of neutrons per unit area (or fluence) referenced to a particular neutron energy. Neutron energies greater that 50 keV, 0.1 MeV, 0.18 MeV, and 1 MeV were adopted in the past and can be readily foimd in the literature. In the U.K., irradiation data are frequently reported in fluences referenced to a standard flux spectrum at a particular point in the DIDO reactor, for which the displacement rate was measured by the nickel activation [ Ni(np) t o] reaction [equivalent DIDO nickel (EDN)]. Early on, neutron irradiation doses to the graphite moderator were reported in terms of the bum-up (energy extracted) from imit mass of the adjacent nuclear fuel, i.e., MW days per adjacent tonne of fuel, or MWd/Ate. 

At PicArsn (Ref 19), the fast neutron activation approach for detection of expls in suitcases was extended to the activation of both nitrogen and oxygen using two 7-ray detector stations in sequence. After 14 MeV neutron irradiation, the baggage is first monitored for 6.1 MeV 7-rays from the l60(n,p),6N reaction (7.5 sec half-life), followed by measurement of the 10 min 13N. Because expls are also rich in oxygen and have characteristic ratios of N/O, it was felt that this approach would increase the probability of detection with a corresponding decrease in the false alarm rate... 

A significant step was made by neutron irradiation of ruthenocene. It was found that 20—25% of the ruthenium activity was recoverable as ruthenocene, and that also considerable rhodium activity was sublimed along with the ruthenocene. The rhodium was identified as being rhodium dicyclopentadienide, produced in high yield by the decay of ruthenocene. 

The early work of Sutin and Dodson (85) on neutron-irradiated ferrocene exemplifies the results and problems of recoil chemistry. After dissolving their samples in hexane and extracting with aqueous solutions they isolated, after further purification, radioactive FeCp2 and a species which emerged as ionic iron(III). Adsorbed on the walls of the glass vessels remained another species soluble in acetone which accounted for up to 50-60% of the radioactive iron. This species has not yet been identified. The FeCp2 activity accounted for some 10-12% of the Fe, which increased on standing several weeks at room temperature or 2-3 days at 110° C, as is shown in Table III. 

The existence and properties of Np(C5Hj)3Cl were first determined (5) by jS-decay synthesis. U(C5H5)3C1 served as the starting compound, being first converted by neutron irradiation to /8 -active 11(05115)301. Since the... 

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. 

The experimental technique devised in this work is based on a NAA method. Neutron irradiation of solid waste forms of simulated HLW (see Table II) produces activation of the elements in the sample. The activation products can be readily measured before and after leaching by radiochemical and/or instrumental techniques. In order to be useful for these purposes, the activation product must have a sufficiently energetic and abundant radiation (either 6 or y) to be easily detected, as well as a sufficiently long half-life several days or more) to be useful for relatively... 

What was the rate of production, in atoms per second, of 128I during a constant 1-h cyclotron (neutron) irradiation of an iodine sample if the sample was found to contain 2.00 mCi of 128I activity at 15 min after end of the irradiation ... 

The use of radioactivated discs of aluminum and steel to estimate fragment erosion in solid targets at very high velocities was found to be feasible. Activation of discs was accomplished by slow neutron irradiation in a nuclear reactor at a flux of 8 x 1012 neutrons per cm2 per sec for 3 days for the aluminum discs (4g, 2.5cm diam x 0.3cm thick), and for 4 hrs for the steel discs (5g, 2.5cm diam x 0.15cm thick). Gamma-ray spectrometry indicated the presence of 59Fe (half-life 46 days) and 51Cr (half-life 28 days) in ratios 0.5 for aluminum and 1.3 for steel. The radioactivities in the aluminum arose solely from impurities, whereas in the steel they were contributed by the major component, iron, and only supplemented by the chromium impurity. The radioactivity was found by successive acid soln determinations to be distributed evenly in both metals... 

Taylor, D. Neutron Irradiation and Activation Analysis. London Newnes 1964. 

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 cross sections for (n,y) reactions common in reactor thermal neutron activation generally decrease with increasing neutron energy with the exception of resonance-capture cross section peaks at specific energies. This reaction is, therefore, not important in most 14 MeV activation determinations. However, some thermalization of the 14 MeV flux may always be expected due to the presence of low Z elements in the construction materials of the pneumatic tubes, sample supports, sample vial, or the sample itself (particularly when the sample is present in aqueous solution). The elements Al, Mn, V, Sn, Dy, In, Gd, and Co, in particular, have high thermal neutron capture cross sections and thermal capture products have been observed in the 14 MeV neutron irradiation of these elements in spite of care taken to reduce the amount of low Z moderating materials in the region of the sample irradiation position 25>. 

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... 

Fig. 10. Ge(Li) gamma-ray spectrum for a thermal neutron irradiated tektite. A major portion of the la4Sb activity in this spectrum is due to antimony contamination in the quartz irradiation vial...

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. 

Taylor, D. Neutron Irradiation and Activation Analysis. Princeton, N. J. D. Van Nostrand Company, Inc. 1964. 

Neutron activation analysis (NAA) technique has also been used for determining low levels of barium in human blood (Olehy et al. 1966). This technique is based on the interaction of the nuclei of individual barium atoms with neutron irradiation, resulting in the emission of x-rays (photons). Detection limits of 7 pg barium/L of erythrocyte and 66 pg barium/L of plasma were obtained (Olehy et al. 1966). The advantages of the NAA technique are its nondestructive nature of sample and minimum sample manipulation. Disadvantages of this technique include its high costs and a nuclear reactor may not be readily available to many laboratories. 

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