Abstract
This work aims at measuring prompt-fission neutrons at different excitation energies of the nucleus. Two independent techniques, the 2E-2v and the 2E techniques, are used to map the characteristics of the mass-dependent prompt fission neutron multiplicity, v(A), when the excitation energy is increased. The VERDI 2E-2v spectrometer is being developed at JRC-GEEL. The Fission Fragment (FF) energies are measured using two arrays of 16 silicon (Si) detectors each. The FFs velocities are obtained by time-of-flight, measured between micro-channel plates (MCP) and Si detectors. With MCPs placed on both sides of the fission source, VERDI allows for independent timing measurements for both fragments. 252Cf(sf) was measured and the present results revealed particular features of the 2E-2v technique. Dedicated simulations were also performed using the GEF code to study important aspects of the 2E-2v technique. Our simulations show that prompt neutron emission has a non-negligible impact on the deduced fragment data and affects also the shape of v(A). Geometrical constraints lead to a total-kinetic energy-dependent detection efficiency. The 2E technique utilizes an ionization chamber together with two liquid scintillator detectors. Two measurements have been performed, one of 252Cf(sf) and another one of thermal-neutron induced fission in 235U(n,f). Results from 252Cf(sf) are reported here.
Highlights
The evolution of the fragment-specific neutron-multiplicity, ν(A), as a function of excitation energy, is an important observable
It reflects the sharing of the total excitation energy (TXE) and how nuclear shell effects wash out at higher excitation energy
For low Total Kinetic Energy (TKE) values, the efficiency becomes as low as 40% while for the highest TKE values it reaches 100%
Summary
The evolution of the fragment-specific neutron-multiplicity, ν(A), as a function of excitation energy, is an important observable. The 2E-2v method has one obvious advantage, namely, that it does not need external input data regarding ν(A), Total Kinetic Energy (TKE) or incident neutron energy (En), during analysis. The Si detectors measure the kinetic energy of the fission fragments. Decent results were reported already using one MCP providing a common start signal Another MCP was installed recently in order to enhance the TOF precision, and to improve the calibration procedure. The pre-neutron emission mass distributions, integral and as a function of TKE, of the updated VERDI instrument were promising. The velocity plot was linearly fitted to provide the "true" velocity as a function of the measured one This method seems to contradict the PDT-systematics of Bohne et al [12]. We plan to refine this method by parameterizing the PDT consistently with the systematics of Ref. [12]
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