Cross bombardment determination

Indirect methods such as half-life systematics, excitation functions for the production reactions, and cross bombardments have been used to reinforce this information. In order to positively identify the atomic number of a spontaneously fissioning nuclide from detection of the fragments, the atomic numbers of both primary fragments from the same SF event must be determined in coincidence and added together to determine the Z of the new, unknown fissioning nuclide. Detection of only SF decay has resulted in much controversy concerning discovery and identification of the transactinide elements. 

The relevant nuclear reaction for tellurium is primarily Sb-121(p,4n)Te-118 with some contribution from the (p,6n) reaction on Sb-123 (42.7% abundance). The nuclear excitation functions for these reactions have not been measured. A series of stacked foil irradiations is planned to determine thin target cross sections. This will allow selection of optimal bombardment parameters for thick target irradiation at the BLIP. A calculated excitation function for the (p,4n) reaction is shown in Figure 8. This calculation is based on the interpolation method of Munzel et al. (11) and should allow prediction of thick target yields to within a factor of 2 or 3. 

The first detailed investigation of [n, p) and [n, 2n) reactions with neutrons of 9 Mev and above, was made by Waffler. He measured the cross sections for the n, p) process for a number of elements irradiated by the neutrons emitted in the forward direction from Li and B, bombarded by 500 kev deuterons. In the forward direction the former reaction, in addition to producing many low energy neutrons, produces also neutrons of 10 and I3 Mev the latter reaction produces neutrons of 9 and 13-5 Mev. The flux of neutrons was determined by a photographic plate method. The cross sections were found by measuring the radioactivities of the product nuclei. Since the neutron sources produced a rather varied energy spectrum, Waffler s results represent average cross sections, and. a strict comparison with theoretical values is difficult. His results, however, show that the n, p) process is consistently more probable than theory will allow. 

Since the lowest levels of even-even nuclei almost always have the same parity as the ground state and spin 2, it is generally assumed that Coulomb excitation in heavy elements is mainly an electric quadrupole process and that the cross section observed experimentally measures the E 2 transition probability. Similar assumptions are made for odd-A nuclei. In the majority of nuclei this assumption is justified. However, as Bjerregaard and Huus have shown, the multipole order can be determined directly by measurements with bombarding particles with different specific charge, e.g., protons and deuterons (or a-particles). They have verified that the multipole order is indeed two for the excitation of the lowest excited states in the even-even wolfram nuclei. 

Even a boron isotope effect has been determined (B vs B ) for the cleavage of some boronic acids by mercuric chloride (Matte-son et al., 1964). The boron-carbon bonds cleaved with isotope effects of 2-3%. A noteworthy point is the unusual method used to determine the isotope ratios. The method took advantage of the widely differing cross sections of B " and B to neutron bombard-... 

The product radioisotopes are easily detected qualitatively by means of their characteristic decay curves and estimated quantitatively by extrapolation to the time at the end of bombardment. The concentration of the parent isotope can be calculated from the cross section of the nuclear reaction, the time of bombardment, and the flux and energy of the bombarding particles. Corrections have to be applied for the time elapsed between the end of bombardment and the counting of the product and for the counting and separation efflciencies. This absolute method is rarely used except to determine the most favorable conditions and as a rough check of the results. 

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