Measurement of high-energy prompt fission gamma rays in ²³⁵U(n_th,f)

We have measured the energy spectrum of prompt gamma-rays produced in thermal-neutron induced fission of ²³⁵U, up to 20 MeV. The measurement was performed at the PF1B beamline of the Institut Laue Langevin using a detection system composed of two multi-wire proportional counters (MWPCs) and two LaBr₃(Ce) scintillation detectors (thickness: 12.7 cm and radius: 15.9 cm), placed on the opposite sides of the ²³⁵U target facing the neutron beam. The large LaBr₃(Ce) scintillation detectors were employed to achieve high detection efficiency at high energy. The target thickness was 117 mg/cm², and the neutron flux at the target position amounted to 1.0x10⁸ neutrons/cm²/s. The gamma-rays from fission fragments, intercepted by the LaBr₃(Ce) detectors were recorded in coincidence with fragments detected  in the MWPCs, for the total duration of 437 hours. By unfolding the response of LaBr₃(Ce) scintillation detectors, the gamma-ray energy spectrum was obtained.

The resulting raw gamma-ray energy spectrum shows an exponential decline up to 10 MeV, with humps at 4 MeV and 6 MeV, similar to the spectrum from spontaneous fission of ²⁵²Cf in earlier studies. Above 10 MeV, the spectrum exhibits a broad bump that we tentatively attribute to the giant dipole resonance.

Compared to the literature data for prompt gamma-rays from the thermal neutron induced fission of actinides, whose energy spectra are known only up to 8 MeV, we have significantly increased the sensitivity and revealed the structure of the spectrum between 10 and 20 MeV. Our approach would open new studies on the structure of neutron-rich fission fragments as well as lead to better understanding of fission mechanism.