Performance of the Particle-Identification Silicon-Telescope Array Coupled with the VAMOS++ Magnetic Spectrometer

TL;DR

PISTA introduces a high-granularity, two-stage silicon-telescope array designed to identify target-like recoils and reconstruct the excitation energy of fissioning systems on an event-by-event basis in inverse kinematics, in concert with the VAMOS++ spectrometer. The system achieves isotopic separation up to oxygen with a mass resolution of about $1.1\%$ (FWHM) and an excitation-energy resolution of $870$ keV (FWHM) when the beam position on target is known, representing a significant improvement over prior SPIDER-based approaches. By coupling with VAMOS++ and EXOGAM, PISTA enables simultaneous access to entrance-channel properties and fission-fragment isotopics, enabling fission probabilities and yields to be studied as a function of $E^{*}$ across diverse multi-nucleon-transfer channels. The work details detector design, electronics, and data flow, and demonstrates performance with $^{238}$U+$^{12}$C at approximately $5.95$ MeV/nucleon, including Doppler-corrected gamma coincidences and missing-mass excitation-energy reconstruction. These capabilities open new pathways for precision fission studies in the actinide region and exotic systems produced in transfer reactions.

Abstract

The Particle-Identification Silicon-Telescope Array (PISTA) is a new detection system designed for high-resolution studies of the fission process induced by multi-nucleon transfer in inverse kinematics. It is specifically optimized for experiments with the VAMOS++ magnetic spectrometer at GANIL (Grand Accélérateur National d'Ions Lourds). The array comprises eight trapezoidal $Δ$E-E silicon telescopes arranged in a corolla configuration. Each telescope integrates two single-sided stripped silicon detectors, enabling target-like recoil identification, energy loss measurements, and trajectory reconstruction. Positioned in close proximity to the target, PISTA's compact geometry achieves high-efficiency tracking of target-like recoils produced in multi-nucleon transfer reactions at Coulomb barrier energies. The spatial segmentation of the array allows precise determination of the mass and charge of the target-like nucleus, and excitation energy of fissioning systems. This work presents the particle identification and excitation energy reconstruction performances for the interactions of $^{238}$U beam with $^{12}$C target. An excitation energy resolution of 870 keV (FWHM) was determined together with mass resolution of 1.1\% (FWHM). The combination of PISTA and VAMOS++ magnetic spectrometer enables unprecedented investigations of the fission process as a function of the excitation energy of the fissioning nucleus, particularly for exotic systems produced in transfer-induced reactions.

Figures (9)

• Figure 1: Calculations for the reactions in inverse kinematics (a) $^{12}$C($^{238}$U,$^{238}$U)$^{12}$C, (b) $^{12}$C($^{238}$U,$^{240}$Pu)$^{10}$Be, and (c) $^{12}$C($^{238}$U,$^{243}$Am)$^{7}$Li, where a $^{238}$U beam at 5.95 MeV/A is bombarding a $^{12}$C target. The distinct curves illustrate the laboratory energy-angle correlation of the recoiling target-like nucleus for reactions resulting in the ground state (solid line) and excited states at $E^{\ast}=5$ MeV (dotted line) and $E^{\ast}=20$ MeV (dashed line) in the beam-like heavy ejectile. The vertical lines denote the angular range encompassed by PISTA, while the horizontal lines correspond to the energy constraint necessary for complete identification (i.e., events where the target-like ions are stopped in the second stage of the silicon telescope).
• Figure 2: Three-dimensional representation of the PISTA detection system is depicted, positioned around the target location in front of the VAMOS++ spectrometer Rejmund2011. Three EXOGAM HPGe clover detectors EXOGAM are placed at backward angles.
• Figure 3: Drawing of the front side of (a) $\Delta E$ (FFF4 design) and (b) $E$ (FFF3 design) manufactured by Micron Semiconductor Ltd.
• Figure 4: Schematic top view of the detection systems surrounding the $^{12}$C target. The PISTA array, comprising $\Delta E-E$ telescopes (illustrated in yellow), and the DPS-MWPC (illustrated in green) are depicted. A representative reaction scenario is presented for the $^{12}$C($^{238}$U,$^{236}$U)$^{14}$C transfer reaction, followed by the fission of $^{236}$U into two fragments, specifically $^{94}$Rb and $^{142}$Cs, and the detection of $^{14}$C in PISTA. The inset provides a photograph of the PISTA experimental setup.
• Figure 5: Overview of the PISTA hardware connections. On the left, a diagram illustrates the detector array, showing a single $E$ detector connected and for half of the front side. The front-side readout is achieved via a flat cable (brown), while the rear side is connected using a single coaxial cable (red), which also serves as the bias voltage input. A schematic representation of one electronic flange is provided, including a $\Delta E$ MMR and a $E$ MMR module and a preamplifier (Pre-Amp). Each module is annotated with typical operating values. On the right, the VME crate dedicated to signal processing is shown, containing all essential modules used for the acquisition and digitization of the detector signals. Additionally, in the bottom right corner, the crates responsible for generating the trigger logic are displayed, along with the associated electronic modules.