Neutron irradiation reactions

Fig. 1. Nuclear reactions for the production of heavy elements by intensive slow neutron irradiation. The main line of buildup is designated by heavy...

Production-Scale Processing. The tritium produced by neutron irradiation of Li must be recovered and purified after target elements are discharged from nuclear reactors. The targets contain tritium and He as direct products of the nuclear reaction, a small amount of He from decay of the tritium and a small amount of other hydrogen isotopes present as surface or metal contaminants. 

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. 

Thermal oxidation of graphite moderators is significant in several contexts. In the early air-cooled reactors the moderator temperature was low and hence the thermal oxidation rate was acceptable. However, the rate increased as the graphite became damaged by neutron irradiation. Moreover, the heat produced from the exothermic reaction... 

E. Fermi (Rome) demonstration of the existence of new radioactive elements produced by neutron irradiation and for the related discovery of nuclear reactions brought about by slow neutrons. 

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... 

The study of the radiochemical reactions of arsenic atoms in benzene solution was carried further by comparing the product spectra of neutron irradiated ASCI3 solutions and GeC solutions which have undergone beta decay. The product spectra were found to be remarkably similar, especially when considered only as to the number of As-0... 

Baumgartner and Reichold prepared carrier-free Mo(CO)g in high yield by neutron irradiation of powdered mixtures of UjOg and Cr(CO)g. As with their preparation of ° RuCp2, the Cr(CO)g acted only as a catcher for fission-product molybdenum (and for its precursors niobium and zirconium). The yield of 60% found for Mo(CO)6 is higher than the fractional chain yield of Mo in fission, so that the reaction must be partly thermal, starting with molecular fragments which survive j8 decay. 

Maddock and Sutin (56) observed the formation of numerous oxygenated products in neutron-irradiated AsPhj which, they argued, must have come from reaction of AsPh radicals with oxygen or water during the separation. The annealing effect of heating at 45° led to increased yields of AsPhj, while AsPhj products first increased and then decreased. This was interpreted as showing the involvement of phenyl radicals in a series of consecutive reactions ... 

Formation of FeCp2 is also noted in neutron-irradiated [CpFe(CO)2]2, as will be described later. An important difference here is that the radioferrocene seems to be formed in the dimeric carbonyl only by very fast solid-state reactions. 

The radiochemistry of ruthenocene has been studied by Baumgartner and Reichold (9) and by Harbottle and Zahn (29). It is found that neutron irradiation of crystalline RuCp2 yields about 10% of the radioactive ruthenium as RuCp2- More specifically, an isotopic difference in the radiochemical yield is found Ru, 9.6 0.1% Ru, 10.7 0.2% and Ru, 9.9 0.2% (29). In liquid solution the isotopic effect is much more pronounced, although the yields are lower. This was suggested by Harbottle as a general principle the greatest isotope effects are associated with the lowest yields. While this principle has not yet been substantiated, it seems reasonable since any thermal reactions which may increase the yields would not likely show any isotope effect. 

The several polymeric metal carbonyls studied have led to some surprisingly high yields [e.g., Fe3(CO),2 and Ruj(CO)j2 in Table IV] but to no substantiated mechanisms. The 17% yield of Fe3(CO),2 in neutron-irradiated Fe(CO)j was interpreted as a reaction of Fe(CO)4 with the Fe(CO)5, but no further evidence is available. The study of Mn2(CO),o has been fruitful (44, 46). The insensitivity of the parent yield MnMn(CO),o to heat indicates that the molecule is formed by a reaction quite early in the sequence, perhaps epithermal. The discovery (46) of a species which reacts rapidly with I2 and exchanges with IMn(CO)5 led to the conclusion that the Mn(CO)5 radical is produced prominently (4.5%) by nuclear reactions in the solid decacarbonyl. The availability of this labeled Mn(CO)5 has made possible several interesting observations about the exchange properties of this radical in the solid (45) and in solution (42). 

Other workers have also made tributyltin-113 labelled compounds for environmental and metabolic studies. For instance, Brown and coworkers37 prepared bis(tributyltin-l 13) oxide by first refluxing tin-113, which was produced by neutron irradiation of metallic tin, in a bromine-chloroform solution for four hours. The resulting tin-113 tetrabromide was subsequently converted into tributyltin-113 bromide by reaction with three equivalents of unlabelled tetrabutyltin for four hours at 220 °C. The bis(tributyltin-113) oxide was finally obtained by hydrolysing the tributyltin-113 bromide with a KOH-95% ethanol solution... 

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. 

Table 11.6 Interfering nuclear reactions during neutron irradiation of mineral samples. Boldface principal reaction for " °Ar/ Ar dating procedure (from Brereton, 1970).

Also, Am-243 can be made from Pu-242, which can be prepared either by very intense neutron irradiation of Pu-239, or from Am-241 resulting from successive neutron-capture reactions. 

This, in turn is produced by successive slow neutron irradiation of curium-244 Californium-254 may be produced by thermonuclear explosion resulting in the reaction of uranium-238 with intense neutron flux followed by a sequence of p- decays (Cunningham, B. B. 1968. In Encyclopedia of Chemical Elements, ed. Clifford A. Hampel, New York Reinhold Book Co.)... 

Heavier isotopes Es-253, Es-254 and Es-255 can be produced in a nuclear reactor by multiple neutron capture reactions that may occur when uranium, neptunium and plutonium isotopes are irradiated under intense neutron flux. These and other isotopes also are produced during thermonuclear explosions. 

In practice, dating using 4°K-4°Ar system often uses a special method called 4°Ar- Ar method, which is well developed and widely applied. In this method, part of is converted into Ar by the following reaction (neutron irradiation) in a nuclear reactor ... 

---