Direct measurement of the 103Rh(n,gamma) and 103Rh(gamma,n) cross section up to stellar temperatures at the CSNS Back-n and SSRF SLEGS

This study presents direct measurements of the 103Rh(n,gamma) and 103Rh(gamma,n) cross sections at the CSNS Back-n and SSRF SLEGS facilities, respectively, revealing new resonance structures and resolving discrepancies with evaluated libraries to provide critical benchmarks for stellar nucleosynthesis, nuclear medicine, and detector applications.

The Gist

Imagine the universe as a giant, cosmic kitchen where stars are the chefs. These chefs cook up heavy elements (like gold, silver, and rhodium) using two main recipes: the Slow Cook (s-process) and the Flash Fry (p-process).

For a long time, scientists trying to understand these recipes had a problem: their cookbooks (nuclear data libraries) disagreed on the exact ingredients and temperatures needed. Specifically, they were confused about Rhodium-103, a key ingredient in the cosmic kitchen.

This paper is like a team of master chefs going back into the kitchen with brand-new, ultra-precise measuring tools to finally settle the debate. They did this in two very different ways, using two massive "kitchen labs" in China.

Part 1: The Slow Cook (Neutron Capture)

The Goal: To measure how Rhodium-103 catches a neutron (like a sponge soaking up water) to become heavier. This happens in the "Slow Cook" recipe inside stars.

The Tool: They used the Back-n facility at the Chinese Spallation Neutron Source. Think of this as a machine that fires a stream of neutrons at the Rhodium sample. It's like a high-speed camera that takes a picture of the neutrons at different speeds (energies) to see exactly when and how the Rhodium grabs them.

The Discovery:

• Finding Hidden Clues: In the past, the cookbooks said there were "resonances" (special moments where Rhodium is super eager to catch a neutron) at specific energies. The new measurements showed that some of these old entries were actually mistakes caused by tiny impurities in the old samples (like finding a speck of platinum in a rhodium crystal).
• New Map: They found brand new resonance structures that no one knew about before. It's like discovering new secret passages in a maze that everyone thought was fully mapped.
• The Result: They created a much more accurate "heat map" of how Rhodium behaves, which helps astronomers calculate exactly how much Rhodium should exist in the universe.

Part 2: The Flash Fry (Photoneutron Reaction)

The Goal: To measure what happens when a high-energy light beam (gamma ray) hits Rhodium and knocks a neutron out of it. This is part of the "Flash Fry" recipe in exploding stars.

The Tool: They used the SLEGS facility at the Shanghai Synchrotron Radiation Facility. Imagine a laser that shoots a beam of light so pure and focused it's almost like a single color of light (quasi-monochromatic). This beam hits the Rhodium, and the team uses a special "net" (a flat-efficiency detector array) to catch the neutrons that fly out.

The Discovery:

• Clearing the Fog: Previous experiments were like trying to see through a foggy window; the results were blurry and disagreed with each other. Some said the reaction was strong, others said it was weak.
• The New Lens: By using their new "laser net" and a clever math trick (unfolding the data), they cleared the fog. They found that the reaction is actually slightly weaker than some old theories predicted, but it matches perfectly with the most recent, high-quality data from other labs.
• Why it Matters: This helps scientists understand how stars explode and create rare elements. It also helps doctors who want to use Rhodium isotopes for targeted cancer therapy, as they need to know exactly how to produce them.

The Big Picture: Why Should You Care?

Think of this paper as updating the GPS for nuclear physics.

1. For Astronomers: It helps them understand how the universe got its heavy elements. If the "ingredients" (cross-sections) are wrong, their models of how stars live and die are wrong.
2. For Engineers: Rhodium is used in nuclear reactors to detect radiation. Knowing exactly how it reacts to neutrons makes these detectors safer and more sensitive.
3. For Medicine: Rhodium isotopes are being looked at for treating cancer. Precise data means doctors can produce the right amount of medicine more efficiently.

In short: The scientists went to two of the most advanced labs in the world, used high-tech lasers and neutron beams, and finally got the Rhodium story straight. They corrected old mistakes, found new secrets, and gave the scientific community a reliable "gold standard" to build the future of nuclear science upon.

Technical Summary

1. Problem Statement

The study addresses critical gaps and inconsistencies in nuclear data regarding Rhodium-103 (103Rh), a key isotope in nuclear astrophysics, reactor physics, and medical applications.

• Astrophysical Context: 103Rh is a crucial node in the stellar s-process (slow neutron capture) and p-process (photoneutron reaction) nucleosynthesis. Accurate cross-sections are required to model heavy element synthesis and the origin of p-nuclides.
• Reactor & Medical Context: 103Rh is used in self-powered neutron detectors (SPNDs) for reactor monitoring and as a precursor for the medical isotope 103mRh (used in targeted radiotherapy and SPECT imaging).
• Data Discrepancies:
  • Neutron Capture (n,𝛾): Existing data in the Resolved Resonance Region (RRR, 0.3–4 keV) is sparse and low-resolution. Evaluated libraries (ENDF/B-VIII.1, JEFF-3.3, TENDL-2023) show significant discrepancies, particularly regarding resonance structures at 11.9 eV and 33.0 eV, and varying Maxwellian-Averaged Cross Sections (MACS) at stellar temperatures.
  • Photoneutron (𝛾,n): Experimental data for the 103Rh(𝛾,n) reaction (critical for the p-process and 103mRh production) shows large inconsistencies between historical datasets (e.g., Saclay vs. NewSUBARU) and evaluated libraries, particularly regarding the Giant Dipole Resonance (GDR) peak position and width.

2. Methodology

The authors employed two distinct experimental setups to measure the cross-sections across the relevant energy ranges.

A. Neutron Capture (103Rh(n,𝛾)) at CSNS Back-n

• Facility: Chinese Spallation Neutron Source (CSNS), Back-n white neutron facility.
• Technique: Time-of-Flight (TOF) method using a double-bunch proton beam mode (41 ns pulse width) to enhance time resolution.
• Detection: C6D6 liquid scintillator detectors utilizing the Pulse Height Weighting Technique (PHWT) to eliminate dependence on cascade gamma-ray energy.
• Sample: High-purity (99.95%) 103Rh target (30 mm diameter, 1 mm thick). Backgrounds were subtracted using Carbon and Lead targets; Gold (197Au) was used for normalization.
• Analysis:
  • RRR (0.3 eV – 4 keV): Fitted using the SAMMY R-matrix code to extract resonance parameters.
  • Unresolved Resonance Region (URR, 4–1000 keV): Averaged cross-sections were calculated using the TALYS nuclear reaction code.
  • MACS Calculation: Derived for stellar temperatures ($kT = 5–100$ keV).

B. Photoneutron (103Rh(𝛾,n)) at SSRF SLEGS

• Facility: Shanghai Synchrotron Radiation Facility (SSRF), Shanghai Laser Electron Gamma Source (SLEGS).
• Technique: Laser Compton Scattering (LCS) to generate quasi-monochromatic $\gamma$-rays (9.32–16.76 MeV) via inverse Compton scattering of a 10.64 $\mu$m CO2 laser with a 3.5 GeV electron beam.
• Detection:
  • Neutrons: A new Flat Efficiency Detector (FED) array consisting of 26 $^3$He proportional counters embedded in a polyethylene moderator.
  • Gamma Rays: A BGO detector measured the incident $\gamma$-spectrum after attenuation to reconstruct the energy distribution.
• Analysis:
  • Unfolding: An iterative unfolding method was applied to deconvolve the measured folded cross-sections from the broad $\gamma$-ray energy distribution to obtain the monochromatic cross-section.
  • Efficiency: Neutron detection efficiency was calibrated using GEANT4 simulations and the ring-ratio technique.

3. Key Contributions

1. High-Precision RRR Data: Provided the first high-resolution measurement of the 103Rh(n,𝛾) cross-section from 1 eV to 1 MeV, identifying multiple new resonance structures (e.g., at 26.5, 79.9, 86.8, 102.3, 941.9, and 976.8 eV) previously unreported in libraries.
2. Resolution of Historical Controversies:
   • Demonstrated that the previously reported 11.9 eV and 33.0 eV resonances in ENDF/B-VIII.1 are likely artifacts of impurities (195Pt and 108Pd) rather than intrinsic 103Rh properties, as the high-purity sample showed no such peaks.
   • Reconciled conflicting 103Rh(𝛾,n) data by providing a new dataset with <5% total uncertainty, bridging the gap between Saclay and NewSUBARU results.
3. Novel Detector Application: Successfully utilized a new Flat Efficiency Detector (FED) array at SLEGS, demonstrating its capability for precise photoneutron measurements with flat efficiency across the evaporation spectrum.
4. Comprehensive Nuclear Data: Generated a complete dataset including resonance parameters, average URR cross-sections, and MACS values, serving as a benchmark for nuclear data evaluation.

4. Key Results

• Neutron Capture (n,𝛾):
  • RRR: Identified new resonances and corrected the resonance parameters. The data aligns well with ENDF/B-VIII.1 in the URR but reveals discrepancies in specific resonance regions.
  • MACS: The calculated MACS at $kT=30$ keV is 799 ± 80 mb. This value is consistent with KADoNiS 0.3 and ENDF/B-VIII.1 but slightly lower than KADoNiS 1.0 and significantly lower than TENDL-2023 (which underestimates the cross-section).
  • Impurity Analysis: Confirmed that previous "resonances" at 11.9 eV and 33.0 eV were caused by Pt and Pd impurities in older samples, not 103Rh itself.
• Photoneutron (𝛾,n):
  • Measured the cross-section from 9.57 MeV to 16.81 MeV with a total uncertainty of <5%.
  • The results show a GDR peak shape and magnitude that are generally lower than TENDL-2023 and slightly lower than previous LCS data (NewSUBARU) and Saclay data.
  • Integral Ratios: The integrated cross-section ratio ($\sigma_{int}^{SLEGS} / \sigma_{int}^{Others}$) ranges from 0.88 to 0.93, confirming the new data is systematically lower than many existing evaluations but consistent with high-precision LCS measurements.
  • The data validates the NewSUBARU results and suggests TENDL-2023 significantly overestimates the cross-section below 13.5 MeV.

5. Significance

• Nuclear Astrophysics: Provides reliable benchmarks for s-process and p-process nucleosynthesis models, improving the accuracy of predicted elemental abundances in stars.
• Nuclear Engineering: Offers precise data for optimizing Rhodium-based Self-Powered Neutron Detectors (SPNDs), crucial for reactor safety and monitoring (e.g., in HPR1000 reactors).
• Medical Applications: The refined understanding of the (𝛾,n) vs. (𝛾,𝛾') channel ratios is essential for optimizing the production yield of 103mRh, a promising isotope for targeted Auger electron therapy and SPECT imaging.
• Data Evaluation: The study resolves long-standing discrepancies between experimental datasets and evaluated libraries (ENDF, JEFF, TENDL), providing a solid foundation for future nuclear data updates and theoretical model optimization.

In conclusion, this work represents a significant advancement in nuclear data precision for 103Rh, utilizing state-of-the-art facilities (CSNS and SSRF) to resolve historical ambiguities and provide high-quality benchmarks for both fundamental physics and practical applications.