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

This paper presents the performance evaluation of the new PISTA silicon-telescope array coupled with the VAMOS++ magnetic spectrometer, demonstrating its capability to achieve high-resolution particle identification and excitation energy reconstruction (800 keV FWHM) for studying fission processes induced by multi-nucleon transfer reactions in inverse kinematics.

The Gist

Imagine trying to understand how a fragile glass vase shatters when hit. To do this, you need to know exactly how hard it was hit, what angle it was hit from, and what pieces flew off. In the world of nuclear physics, scientists want to understand how heavy atomic nuclei (like Uranium) split apart, a process called fission.

This paper introduces a new, high-tech tool called PISTA (Particle-Identification Silicon-Telescope Array) designed to act like a super-precise camera and speed trap for these tiny, exploding atoms.

Here is a simple breakdown of what the scientists built, how it works, and what they found.

1. The Problem: A Blurry Snapshot

Previously, scientists used an older tool (called SPIDER) to study these nuclear explosions. Think of the old tool like a camera with a slightly out-of-focus lens. It could tell you that a vase broke, but it couldn't tell you exactly how much energy was in the hit, or distinguish clearly between different types of broken shards. This made it hard to study the "rules" of how these atoms break apart.

2. The Solution: The "Lampshade" Camera

The team built PISTA to fix this.

• The Shape: Imagine a lampshade made of eight trapezoidal silicon detectors arranged in a circle around the target. This shape is crucial because it catches particles flying out at different angles without blocking the view of the main explosion.
• The Layers: Each "lampshade" piece is actually a sandwich of two silicon detectors.
  • Layer 1 (The Speed Bump): A thin layer that measures how much energy a particle loses as it passes through (like a speed bump slowing down a car).
  • Layer 2 (The Stopping Pad): A thicker layer that catches the particle and stops it, measuring its total remaining energy.
• The Magic: By comparing how much energy was lost in the first layer versus how much was left in the second, the system can identify exactly what kind of atom is flying by (like telling the difference between a ping-pong ball and a marble based on how they bounce).

3. How the Experiment Worked

The scientists fired a beam of heavy Uranium atoms (like a cannon) at a thin sheet of Carbon (the target).

• The Collision: When the Uranium hit the Carbon, they didn't just bounce off; they swapped pieces of themselves (a process called "multi-nucleon transfer").
• The Result: Sometimes, this swap gave the Uranium so much "excitement" (energy) that it immediately split apart (fissioned).
• The Catch: The Uranium split into two big pieces (fission fragments) that flew forward into a giant magnet called VAMOS++. Meanwhile, the tiny Carbon piece (now a "target-like recoil") flew backward toward the PISTA array.

4. What PISTA Actually Did

PISTA caught the tiny Carbon piece flying backward. Because PISTA is so precise, it could tell the scientists:

1. Exactly what the Carbon piece was: Was it a normal Carbon-12? Or did it lose a few neutrons and become Carbon-10?
2. Exactly how fast it was going: This allowed them to calculate the energy of the collision.
3. The "Missing" Energy: By knowing exactly what the Carbon piece was and how fast it was moving, they could use math (the "missing-mass method") to figure out exactly how much energy the Uranium had before it split.

5. The Results: Sharper Than Ever

The paper claims that PISTA is a massive upgrade over the old tools:

• Crystal Clear Identification: It can distinguish between different isotopes (versions of elements) with a precision of 1.1%. The old tool was only about 8% precise. It's like going from being able to tell "a car" from "a truck" to being able to tell a "2020 Ford" from a "2021 Ford."
• Better Energy Resolution: It can measure the energy of the split with a resolution of about 800 keV (a very specific unit of energy). The old tool was about three times blurrier (2.7 MeV).
• No Damage: The design is smart enough to let the big, dangerous fission fragments fly through the center hole without hitting the delicate silicon sensors, while catching the smaller, safer pieces.

6. Why This Matters (According to the Paper)

The paper states that this new clarity allows scientists to study fission in a way they never could before. Specifically, they can now see how the probability of an atom splitting changes depending on exactly how much energy it has.

They tested this by looking at Uranium hitting Carbon. They found that:

• They could identify the tiny Carbon pieces perfectly.
• They could calculate the energy of the Uranium split with high accuracy.
• They even checked if the Carbon pieces were "excited" (wiggling) by looking for gamma rays (light) they emitted, confirming their calculations were correct.

In short: PISTA is a new, high-resolution "speed trap" for atomic particles. It lets scientists see the exact details of nuclear fission events, removing the blur that existed with previous equipment, allowing for a much clearer understanding of how heavy atoms break apart.

Technical Summary

Technical Summary: Performance of the PISTA Array Coupled with VAMOS++

Problem Statement
A comprehensive understanding of the nuclear fission process requires precise measurements of fission probabilities and fragment yields as a function of the fissioning system's excitation energy ($E^*$). While multi-nucleon transfer reactions in inverse kinematics offer a unique pathway to study a wide range of compound nuclei in a single experiment, previous detection systems faced significant limitations. Specifically, the SPIDER silicon telescope array, previously used with the VAMOS++ magnetic spectrometer at GANIL, suffered from insufficient spatial granularity and non-uniform effective thickness. These design constraints resulted in limited mass resolution (8% FWHM) and a relatively poor excitation energy resolution (2.7 MeV FWHM), hindering the ability to perform event-by-event isotopic identification and precise reconstruction of the fissioning system's properties.

Methodology
To address these limitations, the Particle-Identification Silicon-Telescope Array (PISTA) was developed and coupled with the VAMOS++ magnetic spectrometer. The PISTA system consists of eight trapezoidal $\Delta E-E$ silicon telescopes arranged in a "lamp-shade" configuration around the target, covering laboratory polar angles from $30^\circ$ to $60^\circ$.

• Detector Design: Each telescope comprises two single-sided silicon stages separated by a 4.5 mm gap. The first stage ($\Delta E$) is a 100–108 $\mu$m thick detector segmented into 91 strips (535 $\mu$m wide), while the second stage ($E$) is a 1 mm thick detector segmented into 57 strips (1.17 mm wide). The detectors are oriented nearly perpendicular to the particle trajectory to minimize effective thickness variations with angle.
• Kinematics and Identification: The system operates in inverse kinematics (e.g., $^{238}$U beam on a $^{12}$C target). Target-like recoils (ranging from helium to oxygen) are stopped within the telescopes, allowing for isotopic identification via the $\Delta E-E$ technique. The fissioning system (the heavy beam-like partner) is not detected directly; instead, its properties (mass, charge, and excitation energy) are inferred event-by-event using the missing-mass method based on the measured energy and scattering angle of the target-like recoil.
• Electronics and DAQ: The system utilizes Mesytec modules for signal processing, including custom preamplifiers for the high-capacitance rear sides of the $\Delta E$ detectors. Data acquisition is synchronized with the VAMOS++ spectrometer (via DPS-MWPC tracking) and EXOGAM HPGe clover detectors using the GANIL NARVAL data flow system and a global trigger/synchronization system (GTS).

Key Contributions
This work presents the design, construction, and performance characterization of the PISTA array. The primary contribution is the demonstration of a detection system capable of high-precision isotopic separation and excitation energy reconstruction for target-like recoils in multi-nucleon transfer reactions. The paper details the mechanical assembly, electronic readout chain, and the specific calibration procedures required to achieve sub-MeV energy resolution.

Results
Experimental data were collected using a 5.95 MeV/A $^{238}$U beam on a $^{12}$C target. The performance metrics are as follows:

• Isotopic Identification: The array successfully identified target-like isotopes up to oxygen. A Deep Neural Network (DNN) applied to the $\Delta E-E$ correlation yielded a mass resolution of 1.1% (FWHM) for carbon isotopes, a significant improvement over the 8% achieved with the SPIDER array.
• Excitation Energy Resolution:
  • In the elastic scattering channel ($^{12}$C($^{238}$U, $^{238}$U)$^{12}$C), where the beam interaction position on the target could not be determined event-by-event, an excitation energy resolution of 518 keV ($\sigma$) (1.22 MeV FWHM) was determined.
  • For events in coincidence with fission fragments detected by VAMOS++, the beam interaction position could be reconstructed. Monte Carlo simulations and experimental analysis indicate an excitation energy resolution of $\sigma \approx 340$ keV (approximately 800 keV FWHM).
• Coincidence Measurements: The system successfully correlated target-like recoils with $\gamma$-rays detected by EXOGAM. This allowed for the measurement of the probability of exciting the target-like nucleus (measured at 10(3)% for $^{10}$Be) and validated the kinematic reconstruction of the fissioning system's excitation energy.

Significance
The paper claims that the combination of PISTA and VAMOS++ enables unprecedented investigations of the fission process as a function of excitation energy, particularly for exotic systems produced in transfer-induced reactions. By achieving a mass resolution of 1.1% and an excitation energy resolution of ~800 keV (FWHM), the system allows for the extraction of fission probabilities and isotopic fission yields with significantly higher precision than previous setups. This capability is crucial for constraining theoretical models regarding level densities, barrier heights, and shell effects in the actinide region. Furthermore, the high spatial segmentation of PISTA is noted to enhance the selectivity for reconstructing complex reaction channels involving multiple particles, such as the decay of unbound $^8$Be into two $\alpha$ particles.