Plasma-delay studies on heavy ion detection using PIPS at the LOHENGRIN recoil separator

Abstract

The VERDI fission spectrometer is designed to measure fragment velocities and kinetic energies to achieve high-precision yield measurements. It consists of two time-of-flight (TOF) sections, each hosting a micro-channel plate (MCP) and up to 32 passivated implanted planar silicon (PIPS) detectors. The main challenge to achieve accurate fragment velocities is the so-called plasma delay time (PDT) phenomena in the PIPS detectors. In this work, we present a dedicated experimental campaign at the LOHENGRIN fission-fragment recoil separator, to solve the pending PDT challenges. The PDT effect was systematically investigated, as a function of mass and energy, using a dedicated time-of-flight setup. In addition, the pulse height defect (PHD) was determined simultaneously. The studies were conducted for five PIPS detectors, in energies and mass numbers ranging from 20 to 110 MeV and A = 85 to 149, respectively. Using digital signal processing, an excellent timing resolution was achieved, reaching as low as 60 ps (one \(\sigma \)) for the heavy ions. The PDT revealed a strong positive correlation with the ion energy and a weak negative correlation with the mass. The experimental PDT values determined from five detectors confirm a consistent systematic behavior with respect to mass and energy. Some systematic discrepancies were exhibited by two detectors, possibly due to the use of different pre-amplification chains. The PDT measurements ranged between 1 and 3.5 ns, for heavy ions relative to \(\alpha \)-particles. The PHD values showed also a strong correlation with the ion energy, and moreover with the ion mass. The PHD for heavy ions was found to range between 2 and 8 MeV, relative to \(\alpha \)-particles. Finally, a two-dimensional parameterisation was developed to model the experimental PDT data, as a function of mass and energy. This new model, which is valid in the fission fragment mass and energy regime, will be of benefit for heavy-ion velocity measurements, using silicon detectors, as done in VERDI.