Speaker
Description
The particle spectrometer VERDI (VElocity foR Direct particle Identification) allows high-precision measurements of fission fragment (FF) mass distributions by using the 2E-2v technique. VERDI consists of two arms with 16 Silicon PIPS (Passivated Implanted Planar Silicon) detectors and a Micro Channel Plate (MCP) each. The MCPs provide the pick-off signal used to trigger the time-of-flight (ToF) measurement, and the Si detectors are used both for energy detection and as a stop signal. In silicon detectors, the signal amplitude and shape get affected by the formation of a plasma from the interaction between the heavy ions and the detector material. In PIPS detectors this plasma causes a signal delay, the Plasma Delay Time (PDT), with typical delays up to 5 ns for FFs, resulting in a smearing of the mass distribution as well as increasing the systematic errors [1]. Moreover, it can render faulty fission-neutron correlations, if not properly corrected. Existing studies [2-4] propose different parameterizations to calculate the PDT contribution. These studies are not in agreement with each other and are limited to certain ion species and regions of energy. Moreover, the PDT effect is believed to be detector-specific, which necessitates a dedicated investigation.
A characterization of the PDT in the silicon detectors used in VERDI was performed at the LOHENGRIN recoil separator of the Institut Laue Langevin, for a wide range of energies and masses, $E$ $\sim$ 40-110 MeV and $A$ $\sim$ 80-160 u, respectively. With LOHENGRIN, characteristic FFs from $^{239}$Pu(n,f) were selected based on their A/q and E/q ratios. A unique collaboration allowed the utilization of a MCP detector from the STEFF spectrometer [5] and 6 silicon detectors from VERDI. The measured ToF, between the MCP and silicon, was compared to the true ToF derived from LOHENGRIN. The signals were recorded in a digital acquisition system to completely exploit the offline analysis capabilities. All the PIPS detectors were fully characterized to study their individual response to the PDT effect. The data will provide a calibration procedure in which the PDT contribution is calculated relative to alpha particles and protons. The achieved combined timing and energy resolutions of our experimental setup are around 160 ps and 0.1 MeV (FWHM), respectively.
During the presentation, we will discuss the PDT trend as a function of both mass and energy of the FF, and we will relate them to the protons and alpha particles. In addition, we will discuss the impact of the PDT correction on the fission fragment data measured with the 2E-2v method.
[1] Jansson, K., Al-Adili, A., Andersson Sundén, E. et al. The impact of neutron emission on correlated fission data from the 2E-2v method. Eur. Phys. J. A 54, 114 (2018). https://doi.org/10.1140/epja/i2018-12544-0
[2] J. Velkovska and R.L. McGrath. Fission fragment mass reconstruction from Si surface barrier detector measurement. Nucl. Instrum. Methods Phys. Res. A. 430, 2-3 507-511 (1999). https://doi.org/10.1016/S0168-9002(99)00225-9
[3] W. Seibt, K.E. Sundström, P.A. Tove. Charge collection in silicon detectors for strongly ionizing particles Nucl. Instrum. Methods Phys. Res. A. 113, 3 317-324 (1973). https://doi.org/10.1016/0029-554X(73)90496-5
[4] H-O. Neidel, H. Henschel, H. Geissel, Y. Laichter. Plasma delay of $^{238}$U ions in surface barrier detectors Nucl. Instrum. Methods Phys. 212, 1-3 299-300 (1983). https://doi.org/10.1016/0167-5087(83)90705-6
[5] I. Tsekhanovich, J.A. Dare, A. G. Smith, et. al. A novel 2v2e spectrometer in Manchester: new development in identification of Fission Fragments Seminar on Fission pp 189-196 (2008). https://doi.org/10.1142/9789812791061_0018