Transmission measurements of 235U fission fragments in Mg, Al, Fe, Ni, Ag

Transmission measurements of the fission fragments arising in 235U thermal-neutron-induced fission were performed by irradiating a back-to-back fission chamber in the thermal column of the RB-2 reactor of Montecuccolino (Bologna) Italy. The back-to-back fission chamber is a gas-flow counter utilizing argon containing 2 per cent nitrogen. The electrode spacing was 10 mm and the gas pressure was adjusted slightly above ambient pressure. The operating voltage was 100 V positive applied to each anode. The source of fission fragments consists of a thin layer of natural uranium (∼0.2mg/cm2) evaporated under vacuum on a Pt disc. This uranium deposit is located in one of the two fission chambers of the counter. Irradiations were made in a neutron flux of ∼ 109 cm-2 s-1. During irradiation the fission fragments, which a r e able to penetrate a thin sheet of variable thickness (1-10 mg/cm2 ) of the element investigated, are directly counted. By repeating the measurements with different thickness of the sheets, one obtains a transmission curve of the 235U fission fragments for each element investigated. Another natural uranium deposit located in the second fission chamber acts as a monitor for the neutron flux for the different sorts of irradiation. From the transmission curves it is possible to derive the relative effective stopping power of the different targets, and the ranges of the fission fragments in the different elements investigated. Actually the ranges a r e obtained by coupling the measured relative stopping powers with the absolute value of the range of 235U fission fragment in Al, measured by Segre and Wiegand, by an independent experiment using the activation technique. The results obtained a r e compared with theoretical calculations concerning the loss of energy of fission fragments in material. (author)