Uranium fast-fission cross-section measurements

A remark on the fission cross section of uranium is in order here. U compounds are available to many workers and a large number of them has been examined at both reactor and spallation sources. Natural uranium has a fission cross section of 4 b, and,even depleted U contains a measurable quantity of What this means, of course, is that when a beam of slow or thermal neutrons is incident on the sample, the fission process gives rise to fast neutrons. These are difficult to stop, thermalize in a variety of ways, and contribute to the background not only on the instrument being used, but, more embarrassing, often on neighboring instruments as well The experimentalist should be aware of this problem, which becomes worse the lower the energy of the incident neutrons. In our experience care in the form of extra... 

The threshold reaction contributions to the total fission rate can be assumed small for the AGN-201 reactor, since its moderator-to-uranium volume ratio is appreciable and its fuel is enriched with the isotope. Very fast fission is normally accounted for in the four-factor formula by the factor e, the number of neutrons produced by all fissions divided by the number produced by thermal fission. In the AGN-201, nonthermal fission is predominately resonance fission, since has finite fission cross sections at all energies. The amount of epithermal fission can be determined by a simple cadmium-ratio measurement of AGN-201-type fuel. The fission product activity of a bare and cadmium-covered fuel sample can be counted on a proportional counter after two similar irradiations in the reactor core. Their ratio will yield the amount of nonthermal fission to total fission after proper corrections for differences of sample weight, irradiation times, and, power level have been made. The final expression for power level then becomes, . . f... 

Reactions with fast neutrons, such as (n, 2n), (n, p) and (n, a) reactions, are only of minor importance for production of radionuclides in nuclear reactors. However, special measures may be taken for irradiation of samples with high-energy neutrons. For instance, the samples may be irradiated in special fuel elements of ring-like cross section as shown in Fig. 12.1, or they may be irradiated in a receptacle made of enriched uranium. In both cases, the fast neutrons originating from the fission of enter the samples directly and their flux density is higher by about one order of magnitude than that at other places in the reactor. 

The opinion from the Carnegie may have been hardheaded, but it was based on more than prejudice. Roberts, Hafstad and fellow DTM physicist Norman P. Heydenburg had improved their measurements of cross sections for fast-neutron fission, scattering and capture in natural uranium. 

Szilard, also working at Columbia, became interested around this time in what is now called the fast effect. The fast effect, is the increase in the multiplication constant obtained by the emission of neutrons by which is induced to fission by the fission neutrons before they are slowed down. Szilard measured both the cross section of such fission neutrons to induce fast fission and also their inelastic cross section, i.e., the probability for their being slowed down below the fast fission threshold by an inelastic collision with uranium. He concluded on the basis of these measurements that one may obtain an increase of as much as 6-8% in the multiplication constant by using large and metallic lumps of uranium. Szilard was also somewhat discouraged by the low multiplication constant which Fermi s experiment gave but was far from giving up hope. 

The subcadmium activation distributions were used in conjunction with cross sections computed by Westcott to calculate values of the thermal utilization f and the thermal migration area L in the usual way. A base value of V was calculated from Westcott values, assuming the neutron flux spectrum in the moderator to be Maxwellian at 2(PC. This value was then modified for flux hardening effects >y comparing the ratios of the 1/v activations (Cu and Mn) and the U-235 activations at various locations. Values of the fast fission factor < were obtained by comparing the fission product activities of natural and depleted uranium foils according to the technique described by Futch . The neutron age r was measured to indium resonance from isolated fuel assemblies in DjO. Corrections were calculated for the age to thermal energy and for lattice effects. 

Fast-fission ratio, 6, (number of 0-238 fissions per 0-235 fission) was measured using natural OOj and depleted uranium-metal foils . The fission rates in a representative cross section of the fuel were compared to the rates in a fast-neutron facility with a known fast-fission ratio, as determined wiUi a back-to-back fission chamber. 

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