Integral cross section measurement of the ${}^{235}\mathrm{U}(n,n^{\prime }){}^{235m}\mathrm{U}$ reaction in a pulsed reactor

The integral measurement of the neutron inelastic cross section leading to the 26-minute half-life ${}^{235m}\mathrm{U}$ isomer in a fission-like neutron spectrum is presented. The experiment has been performed at a pulsed reactor, where the internal conversion decay of the isomer was measured using a dedicated electron detector after activation. The sample preparation, efficiency measurement, irradiation, radiochemistry purification, and isomer decay measurement will be presented. We determined the integral cross section for the ${}^{235}\mathrm{U}(n,n^{\prime }){}^{235m}\mathrm{U}$ reaction to be $1.00\pm 0.13\mathrm{b}$. This result supports an evaluation performed with talys-1.4 code with respect to the isomer excitation as well as the total neutron inelastic scattering cross section.

Figure 1

Total neutron inelastic cross section. The experimental data points are taken from Refs. [4, 5, 6, 7]. The line is an evaluation performed with the talys-1.4 code [8].

Figure 2

Schema of the electrostatic deflecting electron spectrometer.

Figure 3

Calculated electron transmission curve.

Figure 4

A typical time spectrum showing the ${}^{235m}\mathrm{U}$ isomer decay. The black line is the result of the fit of an exponential decay plus a constant background, from which a half-life of 25.33 minutes was obtained.

Figure 5

The ${}^{235m}\mathrm{U}$ isomer detection efficiency against the ${}^{235}\mathrm{U}$ mass for each sample is plotted. The box size around each data point represents the errors associated with that sample measurement. The solid line is a linear fit to the data and the dotted lines represent the ±1σ limits.

Figure 6

Calculated CALIBAN neutron spectra in the central cavity (solid line) and outside the core between two polyethylene blocks (dashed). Spectra were calculated with the tripoli 4.4 code [19].

Figure 7

An example decay curve (bottom) measured for an irradiated ${}^{235}\mathrm{U}$ sample after radiochemistry and electrodeposition. The solid line represents the fit to the data using the form given by Eq. (4). The residual between the data and the fit is shown in the top figure.

Figure 8

Total neutron inelastic cross section data points taken from experiments [4, 5, 6, 7] and a theoretical evaluation (full line), using talys-1.4 code [8]. The THALYS code was also used to calculate inelastic cross section leading to the population of the ${}^{235m}\mathrm{U}$ isomer (dashed line). The CALIBAN neutron spectrum is given by the dotted line.