New digital techniques applied to A and Z identification using pulse shape discrimination of silicon detector current signals

Abstract

Extending pulse shape discrimination (PSD) to digitized signals is one of the most promising methods to identify particles stopped in a detector. Using the CIME accelerator in the GANIL laboratory, a measurement campaign was done to collect data corresponding to different charges, masses and energies of implanted ions. These data are used to develop an algorithm capable to discriminate the different particles both in mass and charge. In this experiment, a $300\mathrm{\mu }\mathrm{m}$ n-TD reverse mounted Si detector was used. These studies on PSD are part of the FAZIA R&D, a research and development project aiming at building a new $4\pi$ array for isospin nuclear physics.

Introduction

With respect to the first $4\pi$ arrays devoted to charged particles conceived in the 1980s, progresses in detection apparatuses have permitted in the 1990s the advent of compact $4\pi$ powerful devices [1] which allowed to improve the experimental study of the multifragmentation of highly excited nuclear systems, possibly connected to a first order phase transition in nuclear matter [2]. With the rapidly expanding number of radioactive ion beam accelerators, the possibility is offered of studying also the isospin (N/Z) dependence of the nuclear equation of state (EOS). For this purpose, the range of the identified mass number A with a compact geometry has to be extended and low thresholds for A and Z identification are necessary; developments of techniques toward a third generation of $4\pi$ multidetectors are necessary [2]. One of the new proposed devices is FAZIA [3], a high granularity $4\pi$ apparatus for charged reaction products, planned to operate in the field of heavy-ion induced collisions below and around the Fermi energy (10–100 MeV/nucleon). FAZIA will be designed to study thermodynamics and dynamics of excited exotic nuclei, exploring for example the isospin, temperature and density dependence of the EOS symmetry energy term [2]. In order to reach the best performances, this detector will exploit the development of digital electronics. In fact, by using high frequency analog to digital converters, it is now possible to implement a fully digital processing of the signals produced by detected particles and perform (possibly on-line) identification by using digital signal processor techniques. With such components which can be integrated in a compact way, one expects to be able to build new detectors with better angular resolution, better mass discrimination and lower identification thresholds. Using digital electronics, the mass number (A) and atomic number (Z) identification via pulse shape discrimination (PSD) can be envisaged in a new and more complete approach. PSD is not a new technique (see, for example, Ref. [4] and the following studies in relation with its application inside a $4\pi$ silicon ball detector [5], [6], [7], [8]), but as recent studies have demonstrated [9], [10], [11], we have now the possibility to perform it in a fully digital way. Through the PSD in the first detection layer, we will be able to decrease the identification threshold with respect to the standard $\mathrm{\Delta }E-E$ telescope technique which requires that particles have enough energy to punch through the $\mathrm{\Delta }E$ detector. Simplifying and automating the calibration procedure is also essential as the number of detectors is becoming larger and larger for highest granularity and angular coverage. In order to study and possibly improve PSD algorithms, a measurement campaign was performed using the CIME cyclotron in the GANIL laboratory. In this paper, we report on new results concerning the mass number identification of ions stopped in a silicon detector by using pulse shape analysis on the current signal. We found that, at energies around $E/A=8\mathrm{MeV}$, it is possible to fully identify the mass number for carbon isotopes, while from argon up to krypton isotopes, the mass number resolution can be considered, at least for the moment, of about 2–3 mass units.