Surrogate neutron-capture studies with fission detection in inverse kinematics at the ESR storage ring

This paper reports on the successful implementation and performance of a new fission-fragment detection system within the NECTAR experiment at the ESR storage ring, which enabled the first simultaneous detection of $\gamma$-decay residues, multi-neutron-emission residues, and fission fragments during surrogate neutron-capture studies using a stored $^{238}$U beam and a deuterium target.

The Gist

Imagine you are a chef trying to figure out exactly how a specific, rare ingredient (let's call it "Super-Heavy Uranium") reacts when you add heat. The problem is, this ingredient is so unstable and radioactive that you can't just put it in a pot and watch what happens. If you try to cook it directly, it might explode or vanish before you can measure anything.

This is the challenge nuclear physicists face when trying to understand how heavy atoms react to neutrons. To solve this, they use a clever trick called a "Surrogate Reaction."

Think of it like this: Instead of trying to cook the unstable ingredient directly, you use a "stand-in" or a "surrogate" ingredient that behaves very similarly. You mix the stand-in with a different, easier-to-handle ingredient (in this case, a gas jet of deuterium) and watch what happens. By studying the stand-in, you can mathematically predict how the real, unstable ingredient would behave.

The Setting: The High-Speed Race Track

The experiment took place at GSI in Darmstadt, Germany, inside a massive machine called the ESR storage ring. Imagine this ring as a high-speed racetrack for atoms.

• The Racers: Instead of cars, they shot a beam of Uranium atoms (specifically $^{238}$U) around the track at incredibly high speeds.
• The Crash: Inside the track, they introduced a tiny cloud of deuterium gas. As the Uranium atoms zoomed past, they collided with the gas atoms.
• The Result: These collisions created excited, unstable versions of Uranium. The scientists wanted to see how these excited atoms "calmed down." They could calm down by:
  1. Spitting out gamma rays (light).
  2. Spitting out neutrons.
  3. Splitting in half (this is called fission).

The Problem: Missing the Split

In previous experiments (like the 2022 version of this study), the scientists had great cameras to see the light (gamma rays) and the neutrons. But they were missing a crucial camera: they couldn't see the pieces when the atom split apart.

It's like trying to study a car crash where you can see the smoke and the debris flying, but you can't see the two halves of the car that crashed. Without seeing the split, you can't get the full picture of the crash.

The Solution: The New "Fission Camera"

This paper describes the successful installation of a brand-new fission-fragment detection system. Think of this as installing high-speed, ultra-sensitive cameras all around the crash zone to catch the two halves of the splitting atom.

Here is how they built it:

1. The Challenge: The racetrack (storage ring) is a vacuum tube. You can't just stick a big camera inside; it would ruin the vacuum and stop the race.
2. The Fix: They built special "pockets" (little protective boxes) behind thin windows. Inside these pockets, they placed three new detectors:
   • One above the track.
   • One below the track.
   • One to the side (which could be pulled back like a retractable telescope when the race started, so it wouldn't get hit by the beam).
3. The Physics: When the Uranium splits, the two pieces fly out mostly in the forward direction (like a shotgun blast). These new detectors were positioned perfectly to catch them.

What Did They Find?

The team ran the experiment with the new setup and it worked perfectly.

• The "Split" was Caught: They successfully detected the fragments when the Uranium atoms split.
• The "Light" and "Neutrons" were Caught: They also continued to detect the gamma rays and neutrons.
• The "Stand-in" Worked: They could clearly see the different ways the excited Uranium settled down. They could distinguish between atoms that just lost a little energy (gamma decay), lost a neutron, or split apart.

Why Does This Matter?

This is a huge breakthrough. For the first time, scientists can use this "Surrogate" method to measure all three ways a heavy nucleus can decay: Gamma rays, neutrons, and fission.

The Big Picture:
Understanding how heavy atoms react to neutrons is critical for:

• Nuclear Energy: Designing safer and more efficient reactors.
• Nuclear Waste: Figuring out how to clean up radioactive waste.
• Astrophysics: Understanding how heavy elements (like gold and uranium) are forged inside exploding stars.

By building this new "fission camera," the NECTAR experiment has given scientists a complete toolkit to study the most unstable ingredients in the universe, even when they are too dangerous to touch directly. It's like finally getting a full 360-degree view of a car crash, allowing engineers to design much safer cars for the future.

Technical Summary

1. Problem Statement

Determining neutron-induced reaction cross-sections for short-lived or highly radioactive heavy nuclei is critical for nuclear astrophysics and reactor design but is often impossible via direct measurement. The surrogate reaction method offers an alternative by populating the same compound nucleus via a different reaction mechanism (e.g., deuteron-induced) and measuring its decay probabilities.

However, previous implementations of this method faced significant limitations:

• Kinematics: Most surrogate experiments use direct kinematics, requiring complex efficiency corrections and background subtraction.
• Incomplete Decay Channel Coverage: Specifically, the NECTAR (Nuclear rEaCTions At storage Rings) experiment at GSI Darmstadt, which pioneered surrogate studies in inverse kinematics using heavy-ion storage rings, previously lacked a dedicated fission-fragment detection system.
• Consequence: Without fission detection, the NECTAR setup could not measure the complete decay probability (specifically the fission channel) for fissionable systems, limiting the ability to constrain fundamental nuclear properties like fission barrier heights.

2. Methodology

The authors report on the design, implementation, and validation of a new fission-fragment detection system integrated into the existing NECTAR experimental setup at the ESR (Experimental Storage Ring).

• Experimental Configuration:

  • Beam: A stored beam of bare $^{238}\text{U}^{92+}$ ions at 17.24 MeV/u.
  • Target: An internal gaseous deuterium jet target.
  • Reactions: The beam interacts with the target to populate excited $^{239}\text{U}$ (via $^{238}\text{U}(d,p)$) and $^{238}\text{U}$ (via $^{238}\text{U}(d,d')$) nuclei.
  • Kinematics: Inverse kinematics, where the heavy projectile is the moving target-like species.

• Detector Systems:
  The upgraded setup combines three complementary systems housed in vacuum pockets behind 25 µm stainless-steel windows:

  1. Target-like Particle Telescope: Located at $\theta = 60^\circ$, consisting of a Double-Sided Silicon Strip Detector (DSSSD) for $\Delta E$ and a stack of silicon detectors for $E$. This identifies reaction channels (protons vs. deuterons) and reconstructs excitation energy.
  2. Beam-like Residue Detector: Located in a dispersive section of the ring to separate residues based on magnetic rigidity. It uses a DSSSD to identify $\gamma$-decay, neutron emission, and multi-neutron emission residues.
  3. New Fission-Fragment Detection System: The central upgrade, consisting of three DSSSDs positioned downstream of the target:
     • Top and Bottom Detectors: Custom BB36 DSSSDs ($80 \times 40 \text{ mm}^2$, 500 µm thick, 16x16 strips) placed 220 mm and 400 mm from the target.
     • Side Detector: A BB29 DSSSD ($122 \times 40 \text{ mm}^2$, 500 µm thick, 122x40 strips) mounted on a movable bellow system. It retracts during beam injection to prevent damage and inserts close to the beam axis during data taking.

• Simulation and Validation:

  • Monte Carlo simulations using g4beamline and the GEF code were employed to model the geometry and fission fragment kinematics.
  • Simulations predicted that due to the high beam velocity, fission fragments are strongly forward-focused (within a $\sim 17^\circ$ cone) in the laboratory frame.
  • The system was validated by comparing simulated hit distributions with experimental data.

3. Key Contributions

• First Simultaneous Detection: This work achieves, for the first time in a surrogate experiment, the simultaneous detection of $\gamma$-decay residues, multi-neutron-emission residues, and fission fragments.
• Novel Detector Integration: The successful integration of a retractable, multi-angle fission detection system into the ultra-high vacuum environment of a heavy-ion storage ring without compromising beam quality.
• Deuteron Breakup Identification: The setup uniquely allows for the identification of events originating from deuteron breakup (a background process in direct kinematics surrogate experiments). By identifying these events in the beam-like residue spectrum, their contribution can be subtracted from the compound nucleus decay probabilities, significantly improving data accuracy.
• Efficiency Determination: The authors determined a fission detection efficiency of approximately 64%, validated by the agreement between simulated and measured hit distributions (specifically the two-ring pattern of light and heavy fission fragments).

4. Results

• Fission Detection: The system successfully detected fission fragments from the asymmetric fission of $^{239}\text{U}$. The measured amplitude and hit distributions on the side detector matched simulations perfectly, confirming the geometric model and the 64% efficiency.
• Residue Identification:
  • The beam-like detector successfully separated different decay channels based on the excitation energy and horizontal position (magnetic rigidity).
  • Distinct bands for $\gamma$-decay, 1n, 2n, and 3n emission were observed.
  • A unique structure corresponding to deuteron breakup was identified at lower strip numbers (closer to the beam axis), a feature previously impossible to isolate in direct kinematics experiments.
• Transmission Efficiency: For residues undergoing $\gamma$-emission, 1n, and 2n emission, the transmission to the beam-like detector was 100%. Transmission dropped for 3n emission (due to wall collisions) and was negligible for 4n emission.

5. Significance

This development represents a crucial milestone in nuclear physics:

• Completeness: It enables the first complete determination of decay probabilities (including fission) for heavy nuclei in inverse kinematics surrogate experiments.
• Nuclear Data Constraints: The ability to measure fission probabilities alongside $\gamma$ and neutron emission allows for tighter constraints on nuclear level densities (NLD), fission barrier heights, and $\gamma$-strength functions.
• Model Improvement: These precise measurements serve as critical inputs for reaction models, leading to more reliable calculations of neutron-capture cross-sections essential for nuclear energy and astrophysics (e.g., the r-process).
• Technological Advancement: The successful deployment of this complex detector array in a storage ring demonstrates the feasibility of high-resolution, multi-channel surrogate studies for short-lived isotopes that cannot be studied via direct neutron capture.