The ICESPICE demonstrator for particle/$\gamma$-$e^{-}$ coincidence experiments at Florida State University

The ICESPICE demonstrator, a modular mini-orange spectrometer developed at Florida State University, successfully enables particle/gamma-electron coincidence measurements for low-energy nuclear structure studies, as validated by commissioning tests and its first in-beam application with the Super-Enge Split-Pole Spectrograph.

The Gist

Imagine the atomic nucleus as a tiny, energetic dance floor. Sometimes, after a big move, the nucleus needs to calm down and release extra energy. Usually, it does this by shooting out a flash of light (a gamma ray). But sometimes, instead of a flash, it kicks a nearby electron off the dance floor. This is called Internal Conversion, and the kicked-out electron is the star of this story.

Scientists at Florida State University wanted to study these "kicked-out" electrons to understand the secrets of the atomic nucleus. The problem? These electrons are tiny, fast, and hard to catch, especially when they are mixed in with a chaotic crowd of other particles and background noise.

To solve this, they built a new tool called ICESPICE (Internal Conversion Electron SPectrometer In Coincidence Experiments). Think of ICESPICE as a high-tech, magnetic bouncer designed specifically to catch these electrons while ignoring the unwanted guests.

Here is how the paper explains their work, broken down into simple concepts:

1. The Magnetic Funnel (The "Mini-Orange")

The core of ICESPICE is a device called a "mini-orange spectrometer." Imagine a ring of powerful magnets arranged in a circle around a central hole.

• The Analogy: Think of these magnets as a magnetic funnel. When the electrons are kicked out, they try to fly off in every direction. The magnets act like a curved slide that only lets electrons with a specific speed (energy) slide through to the detector, while pushing away everything else (like gamma rays or heavier particles).
• The Design: They didn't invent new magnets; they used standard, off-the-shelf permanent magnets (like the strong ones used in speakers) arranged in a clever pattern. They used computer simulations (like a video game physics engine) to figure out the perfect shape and spacing so that about 1 million electron-volts of energy (a common speed for these particles) would get caught efficiently.

2. The Catcher's Mitt (The Detector)

Once the magnets guide the electrons, they need to be caught. ICESPICE uses special silicon detectors called PIPS detectors.

• The Analogy: If the magnets are the funnel, the PIPS detector is the catcher's mitt. It's a very thin, sensitive sheet of silicon that stops the electron and records exactly how much energy it had.
• The Challenge: The team tested mitts of different thicknesses. They found that for high-speed electrons (around 1 MeV), you need a thick mitt (1000 micrometers) to catch the whole electron. If the mitt is too thin, the electron punches right through, and the detector only gets a partial signal, making the data messy.

3. The "Double-Check" System (Coincidence)

The paper highlights a key feature: Coincidence. This means the system looks for two things happening at the exact same time.

• The Analogy: Imagine trying to hear a specific whisper in a noisy room. If you only listen for the whisper, you might hear a cough that sounds similar. But if you have a friend standing next to you who also hears a specific sound (like a bell) at the exact same moment, you know for sure you heard the right thing.
• In the Lab: ICESPICE works with a gamma-ray detector (the "friend"). When the nucleus kicks out an electron, it often sends out a gamma ray at the same time. ICESPICE waits to see if the electron detector and the gamma detector both "ring" at the same time. If they do, the scientists know, "Yes, that was a real event from our experiment," and they can ignore the background noise.

4. The Big Test: The "In-Beam" Experiment

After building the tool, they had to test it in the real world. They took ICESPICE to the Super-Enge Split-Pole Spectrograph (SE-SPS), a giant machine that smashes particles together to study nuclei.

• The Experiment: They shot a beam of deuterons (heavy hydrogen) at a lead target. This reaction created excited nuclei that then decayed, kicking out electrons.
• The Result: They successfully caught these electrons while the beam was running. They saw a clear signal where the electrons and the tritons (another particle from the reaction) arrived at the same time. This proved that ICESPICE works as a "sidekick" detector for the main machine.

5. What They Learned (and What's Next)

• Success: The system worked. They could clearly see the relationship between the gamma rays and the electrons using a radioactive source (Bismuth-207) and then with the actual particle beam.
• Limitations: The current detectors are a bit small (like a small catcher's mitt). For very high-energy electrons, some punch through. The paper suggests that in the future, they might use larger, thicker detectors (like room-temperature Silicon-Lithium detectors) to catch even more of these high-speed particles.
• Refinement: They are still tweaking the magnetic field maps and the distances between the magnets and the detector to make the "funnel" even more efficient.

In Summary:
The paper describes the successful creation and testing of a new, modular, and cost-effective device that uses magnetic funnels to catch specific electrons from atomic nuclei. By pairing this with a gamma-ray detector, the scientists can filter out the noise and study the structure of atoms with much greater clarity. It's a successful "proof of concept" that shows this tool is ready to help solve nuclear physics puzzles.

Technical Summary

Technical Summary: The ICESPICE Demonstrator for Particle/γ–e⁻ Coincidence Experiments

Problem Statement
Internal Conversion Electron (ICE) spectroscopy is a critical tool for investigating low-energy nuclear structure, particularly for studying electric monopole (E0) transitions and shape coexistence in medium- and heavy-mass nuclei. While the "mini-orange spectrometer" (MOS) concept has historically offered a compact solution for in-beam ICE measurements by using permanent magnets to transport electrons while suppressing background, existing implementations often face limitations in resolution, size, or the ability to perform complex coincidence measurements. There is a specific need for a modular, cost-effective spectrometer system capable of operating in coincidence with both gamma-ray detectors and charged-particle spectrographs (specifically the Super-Enge Split-Pole Spectrograph, SE-SPS) at Florida State University (FSU) to facilitate particle–electron and gamma–electron coincidence studies.

Methodology
The authors developed the Internal Conversion Electron SPectrometer In Coincidence Experiments (ICESPICE) demonstrator. The system is built upon the mini-orange concept but features a modular design utilizing commercially available permanent magnets arranged in toroidal configurations around a central tantalum attenuator.

• Design and Optimization: The system was optimized using a multi-stage simulation approach:
  • SolidWorks: Used for initial geometric modeling of the spectrometers within the constraints of the SE-SPS sliding-seal chamber (specifically fitting within a 12-inch diameter, 10-inch height sleeve at backward angles).
  • COMSOL Multiphysics®: Used to simulate magnetic fields for various magnet shapes and strengths (specifically N42 SmCo magnets) to maximize magnetic transmission probability ($T_M$) for electrons around 1 MeV.
  • Geant4: Used for particle tracking and detector response simulations to evaluate transmission efficiency and energy deposition.
• Detector Configuration: The system employs room-temperature Passivated Implanted Planar Silicon (PIPS®) detectors of varying thicknesses (100, 300, 500, and 1000 µm). Simulations indicated that thicker detectors (1000 µm) were necessary to achieve significant full-energy deposition for ~1 MeV electrons, minimizing the low-energy continuum caused by backscattering.
• Experimental Setup:
  • Calibration: Tests were conducted using a calibrated $^{207}$Bi source in a vacuum chamber and later in the SE-SPS scattering chamber.
  • Coincidence Measurements:
    • γ–e⁻ Coincidences: Performed using a CeBr$_3$ detector from the CeBrA array in coincidence with PIPS® detectors.
    • Particle–e⁻ Coincidences: The first in-beam test utilized the $^{208}$Pb(d, t)$^{207}$Pb reaction. Tritons were detected by the SE-SPS, while conversion electrons were detected by ICESPICE.

Key Results

• Spectrometer Performance: The optimized configuration for ~1 MeV electrons utilized five 1" × 1" × 1/8" N42 magnets arranged around a 3/8" tantalum attenuator, positioned at a target-to-spectrometer distance ($f$) of 70 mm and a spectrometer-to-detector distance ($g$) of 25–35 mm.
• Detector Response:
  • The 1000-µm thick PIPS® detector demonstrated superior performance for high-energy electrons, depositing full energy for ~35% of 1 MeV electrons, compared to ~3% for the 500-µm detector and negligible amounts for thinner detectors.
  • Energy resolution (FWHM) of 8–10 keV was achieved for conversion electrons from the 1633-keV state in $^{207}$Pb.
  • Geant4 simulations generally reproduced experimental spectral trends, though they slightly overpredicted full-energy deposition, suggesting experimental backscattering losses are higher than modeled.
• Coincidence Capabilities:
  • γ–e⁻: Clear correlations were observed between CeBr$_3$ gamma-ray detections and PIPS® electron detections, successfully isolating specific decay paths of $^{207}$Bi (e.g., the 1064-keV transition in coincidence with the 569-keV gamma ray).
  • Particle–e⁻: Prompt coincidences were observed between tritons detected by the SE-SPS and electrons detected by ICESPICE in the $^{208}$Pb(d, t)$^{207}$Pb reaction. The timing resolution between the PIPS® and the SE-SPS focal-plane scintillator was determined to be approximately 92 ns (FWHM) for the prompt peak.
• Limitations Observed: In the in-beam test, the measured electron yields for specific excited states were lower than expected based on transmission simulations. The authors attribute this to the limited active area (50 mm²) of the available PIPS® detectors, the small cross-sections of the reaction, and background electrons scattered within the chamber.

Significance and Claims
The paper presents ICESPICE as a promising ancillary detector system for in-beam internal conversion electron spectroscopy at the FSU SE-SPS. The authors claim that the demonstrator successfully validates the feasibility of:

1. Using a modular, commercially magnet-based mini-orange design tailored to the geometric constraints of the SE-SPS.
2. Performing simultaneous particle–electron and gamma–electron coincidence measurements in a low-energy nuclear physics environment.
3. Detecting conversion electrons with energies up to ~1 MeV using room-temperature PIPS® detectors, a capability relevant for studying E0 transitions in actinides.

The authors maintain a modest tone regarding the current performance, noting that while the system works, future improvements are necessary. They suggest that increasing the active area of the detectors (potentially using room-temperature Si(Li) detectors with larger areas and greater thickness) and refining the magnetic field mapping and background suppression strategies are required to enhance sensitivity and efficiency for future experiments.