MASS AND RANGE OF PLUTONIUM FISSION FRAGMENTS

The range of fragments from nuclear fission can be determined by various methods. For this purpose a Wilson chamber and an ionization chamber of variable pressure are used, or the activity of aluminum foils of various thicknesses surrounding the substance undergoing fission is investigated.

In the work being reviewed[^1] the authors used a device consisting of a beryllium tube 225 mm long and 25 mm in diameter. At one end of the tube, perpendicular to its axis, plutonium foil about 0.1 mg/cm² thick was placed. At a distance of 100 mm from the plutonium foil, at intervals of 3 mm from one another and perpendicular to the axis of the tube, 14 identical films of zapon lacquer were placed. The thickness of the films was 8 μg/cm². The tube was enclosed in a neoprene jacket and filled with dry air at a pressure of 120 and 140 mm of mercury. Neutron irradiation was carried out in the Los Alamos boiler for 30 minutes or 14 hours. After irradiation the films, except for the first and last, were dissolved in acetone and subjected to radiochemical analysis. Almost all the slowing down of the plutonium fission fragments was caused by the air, and after stopping they settled on the nearest film. The activity of each film was examined over one or two half-life periods. In some cases the decay curves were examined for two components. The investigations were carried out for 20 different isotopes of fission fragments with masses from 83 to 157. The activity of each film for a given isotope gave a curve of differential ranges. The total activity of all the films—

… after this yielded the curve of integral ranges. With the aid of this curve the extrapolated range was determined. In the group of light masses from 83 to 117, the extrapolated range varied from 2.9 cm of air (under normal conditions) to 2.26 cm. In the group of heavy masses from 127 to 157, the extrapolated range varied from 2.25 cm to 1.95 cm. The graph gives the range of the fragments as a function of their mass.

Graph labels:
Vertical axis: Extrapolated range
Horizontal axis: Mass numbers
Legend: ○ Reliable values; □ Possible values; × Values obtained from range measurements

Initially^2 the fission fragments have an ionization multiplicity of about 20 and thus retain the greater part of the atomic electrons. The decrease in the velocity of the fragments is almost linear, which is a consequence of the decrease in charge during slowing down. The observed spread in the ranges of fragments of a given mass depends on variations of the initial charge and energy, statistical fluctuations in the collisions of the fragments with electrons and nuclei, and experimental conditions. None of the curves of differential ranges obtained is Gaussian. They all have a steeper slope at distances close to the plutonium foil. It is indicated that the method can be used to study short-lived fission products.

K. D. Tolstov

CITED LITERATURE

1. S. Katcoff, J. Miskel and C. Stanley, Phys. Rev. 74, 631 (1948).
2. N. O. Lassen, Phys. Rev. 69, 137 (1946).