Measuring correlated fission observables in Cf-252 with the VERDI spectrometer

VERDI (VElocity foR Direct particle Identification) is a double energy - double velocity (2E-2v) spectrometer under development at JRC-Geel. Both the physical setup and the analysis have been improved upon since it first became operational in 2015. Currently, the spectrometer is being steadily improved using a Cf-252(sf) source. Measuring the velocities allows computation of the pre-neutron masses and together with the kinetic energy measurements, also the post-neutron masses can be obtained. Knowing both pre- and post-neutron masses allow calculating the average neutron multiplicity as a function of fragment mass. Contrary to the 2E method, the 2E-2v method does not need prior information regarding prompt neutron emission. Therefore, VERDI presents an independent means to assess neutron multiplicity data correlated with other fission observables, helping to understand how the excitation energy is shared between nascent fragments as well as benchmarking nuclear de-excitation models. To date our results indicate a superior pre-neutron mass resolution compared to 2E data obtained, e.g., using ionisation chambers. By implementing a new energy calibration method, based on the commonly used parametrisation proposed by Schmitt et al., we have improved the overall agreement with previous Cf-252 data. In consequence new and lower values for the set of Schmitt parameters were obtained. As the overall agreement is convincingly good, still VERDI show some discrepancies, e.g., the pre-neutron mass yields are higher for several high yield nuclei. The pronounced structures are possibly due to the higher pre-neutron mass resolution reached with VERDI. However, at present stage the post-mass resolution remains a critical feature to improve upon, which will be one focus in our future work. We will present the successful proof-of-principle that does not only show the feasibility of the VERDI setup but also the benefits to obtain correlated fission-yield data independent from previous measurements.