Proton induced reactions on 118Sn target at energies up to 18 MeV

This study investigates proton-induced reactions on an enriched 118Sn target up to 18 MeV using the stacked-foil activation technique, reporting first-time cross-section measurements for specific channels and revealing discrepancies between experimental data and theoretical models regarding composite-particle emission.

The Gist

Imagine a tiny, high-speed billiard game, but instead of a cue ball and pool balls, we are shooting protons (tiny particles of energy) at tin atoms.

This paper is a report from a team of scientists in Armenia and the USA who decided to play this game with a very specific target: an enriched version of the tin isotope Tin-118. They wanted to see what happens when they hit these tin atoms with protons at different speeds (energies) up to 18 million electron volts.

Here is the story of their experiment, broken down into simple concepts:

1. The Setup: The "Sandwich" Stack

To test different speeds of protons in one go, the scientists didn't just use one piece of tin. They built a stack of foils, like a giant sandwich.

• The Ingredients: They alternated layers of pure Tin-118 with layers of Copper.
• The Trick: As the proton beam (the "cue ball") travels through the stack, it hits the copper and tin layers, losing a little bit of speed with each layer, just like a car slowing down as it drives through thick mud.
• The Result: By the time the protons reach the first tin layer, they are fast. By the time they reach the last tin layer, they are slower. This allowed the scientists to measure what happens at every speed in a single experiment.

2. The Game: Breaking the Tin

When the protons hit the tin atoms, they don't just bounce off; they smash into the nucleus and break it apart or change it into something new. The scientists were looking for four specific "scenarios":

• The Swap (p,n): A proton hits, and a neutron pops out. The tin turns into Antimony-118.
• The Double Swap (p,2n): A proton hits, and two neutrons fly out. The tin turns into Antimony-117.
• The Heavy Hit (p,α): A proton hits, and a chunk of the atom (an alpha particle, which is like a tiny helium nucleus) is knocked loose. The tin turns into Indium-115.
• The Mystery Hit (p,x): A proton hits, and the atom loses a proton and a neutron (or a deuteron). The tin turns into Tin-117 (a radioactive version).

3. The Measurement: The "Radioactive Glow"

After the beam was turned off, the scientists took the foils apart and put them near a super-sensitive camera (a Germanium detector) that can "see" invisible energy rays (gamma rays).

• Every time a new atom was created, it was unstable and started to glow with radiation.
• By measuring the color (energy) and brightness of this glow, the scientists could tell exactly which new atom was created and how many of them were made at each speed.

4. The Big Discovery: Computers vs. Reality

The most interesting part of the paper is the comparison between what the scientists measured and what computer models predicted.

Think of the computer models (like TENDL and JENDL) as weather forecasters. They use complex math to predict what will happen when protons hit tin.

• The Good News: For the simple crashes (where just one or two tiny particles fly out), the forecasters were pretty accurate. Their predictions matched the real experiment well.
• The Bad News: For the complex crashes (where bigger chunks like alpha particles or deuterons are knocked out), the forecasters were wrong.
  • The computers predicted these big chunks would only fly out at very high speeds.
  • The experiment showed they were flying out at lower speeds than expected.

5. Why Were the Computers Wrong?

The scientists have a theory. They think the computer models treat the atomic nucleus like a smooth, uniform ball of dough. But in reality, the nucleus might be more like a cluster of grapes.

Inside the tin nucleus, protons and neutrons might be huddled together in little groups (clusters). When a proton hits, it might grab one of these pre-formed "grape clusters" and knock it out easily. The current computer models don't know about these "grape clusters," so they underestimate how often these big chunks get knocked out.

The Bottom Line

This paper is important because:

1. New Data: It provides the first-ever accurate measurements for some of these specific reactions.
2. Medical & Energy Use: Understanding these reactions helps us make better medical isotopes (for treating cancer) and manage nuclear waste.
3. Better Models: It tells scientists that their computer models need an upgrade. They need to teach the computers to recognize that atomic nuclei have "clusters" inside them, not just a smooth soup of particles.

In short: We shot protons at tin, measured the debris, and realized our computers need to learn that atoms are a bit more "clumpy" than we thought.

Technical Summary

1. Problem Statement

• Data Gap: While extensive experimental data exists for neutron-induced reactions, information on charged-particle (proton) induced reactions on enriched tin isotopes is limited, particularly in the energy range above 9 MeV where excitation function maxima often occur.
• Modeling Challenges: Evaluated nuclear data libraries (such as TENDL and JENDL) rely heavily on theoretical models (e.g., TALYS) in the absence of experimental data. However, these models often struggle to accurately predict cross-sections for reactions involving composite-particle emission (e.g., alpha particles, deuterons) compared to simple nucleon emission.
• Application Needs: Accurate cross-section data for tin isotopes is critical for:
  • Nuclear Waste Management: Understanding the transmutation of tin isotopes (fission products) in spent fuel.
  • Medical Isotope Production: Generating isotopes like $^{117}\text{Sb}$, $^{119}\text{Sb}$, and $^{124}\text{Sb}$ for therapeutic and diagnostic use.

2. Methodology

• Experimental Setup:
  • Target: A stacked-foil arrangement consisting of 12 alternating layers of natural copper ($^{nat}\text{Cu}$) and enriched $^{118}\text{Sn}$ (98% enrichment). The tin foils ranged from 23 to 54 $\mu$m in thickness.
  • Beam: An 18 MeV proton beam from the IBA Cyclone 18/18 cyclotron (Yerevan, Armenia) with a current of 1 $\mu$A.
  • Technique: The stacked-foil activation technique was used. As the proton beam traversed the stack, its energy degraded, allowing the measurement of excitation functions across a continuous energy range (approx. 6–18 MeV) in a single irradiation.
  • Monitoring: Copper foils served as energy degraders and monitor foils. The proton flux and energy were determined using the ratio of cross-sections for monitor reactions ($^{nat}\text{Cu}(p,x)^{62,63,65}\text{Zn}$) against IAEA-recommended data.
• Data Acquisition & Analysis:
  • Detection: High-purity Germanium (HPGe) detectors measured $\gamma$-spectra periodically over several days post-irradiation.
  • Energy Determination: The authors compared two methods for calculating the effective energy in each layer:
    1. Average Energy ($E_{av}$): Simple mean of incoming and outgoing energies.
    2. Effective Energy ($E_{eff}$): A weighted integral considering the proton energy distribution (from SRIM simulations) and the energy-dependent cross-section.
  • Cross-Section Calculation: Standard activation equations were used, incorporating corrections for decay constants, detector efficiency, and target thickness.
• Theoretical Comparison: Experimental data were compared against:
  • Previous experimental results (converted from natural tin targets).
  • TENDL-2023 and TENDL-2025 (TALYS-based).
  • JENDL-5 (Japanese Evaluated Nuclear Data Library).

3. Key Contributions

• First-Time Measurements: This study reports the first experimental cross-sections for the following reactions on enriched $^{118}\text{Sn}$:
  • $^{118}\text{Sn}(p,x)^{117m}\text{Sn}$
  • $^{118}\text{Sn}(p,\alpha)^{115m}\text{In}$
• Extended Energy Range: Provided data up to 18 MeV, filling a gap in the region of excitation function maxima which was previously under-studied (most prior data was <9 MeV).
• Methodological Validation: Validated the use of simple average energy calculations ($E_{av}$) for stacks of homogeneous foils (tens of micrometers thick), showing they differ by only 0.3–3% from complex effective energy calculations ($E_{eff}$).
• Comprehensive Benchmarking: Provided a rigorous benchmark for evaluating the predictive capabilities of TENDL-2023/2025 and JENDL-5 for tin isotopes.

4. Key Results

• Measured Reactions: Excitation functions were successfully determined for:
  • $^{118}\text{Sn}(p,n)^{118m}\text{Sb}$
  • $^{118}\text{Sn}(p,2n)^{117}\text{Sb}$
  • $^{118}\text{Sn}(p,\alpha)^{115m}\text{In}$
  • $^{118}\text{Sn}(p,x)^{117m}\text{Sn}$ (via $p,pn$ and $p,d$ channels)
• Agreement with Theory:
  • $^{118}\text{Sn}(p,2n)^{117}\text{Sb}$: All theoretical libraries (TENDL and JENDL) showed good agreement with experimental data.
  • $^{118}\text{Sn}(p,n)^{118m}\text{Sb}$: JENDL-5 showed the best agreement (deviations <29%), while TENDL versions differed significantly.
  • Composite Particle Emission ($p,\alpha$ and $p,pn/p,d$): Significant discrepancies were observed.
    • TENDL-2023 and TENDL-2025 failed to reproduce the experimental data well.
    • JENDL-5 performed better but still showed an average deviation of ~66% for the $(p,\alpha)$ channel.
    • Systematic Shift: For the $(p,pn)+(p,d)$ channel leading to $^{117m}\text{Sn}$, all theoretical models were systematically shifted toward higher incident energies compared to the experimental results.

5. Significance and Conclusions

• Model Limitations: The study highlights that current nuclear reaction models (specifically TALYS-based libraries) adequately describe simple two-nucleon emission channels but struggle with composite-particle emission (alpha, deuteron) at lower proton energies.
• Physical Insight: The discrepancies suggest that standard statistical models may underestimate the probability of composite-particle emission. The authors propose that nuclear cluster correlations (specifically $\alpha$-cluster structures at the nuclear surface of Sn isotopes) play a significant role. These correlations are not explicitly included in standard reaction codes, leading to the observed underestimation of cross-sections.
• Impact on Nuclear Data: The new experimental data serves as a critical benchmark for refining nuclear reaction models and improving evaluated data libraries (TENDL, JENDL). This is essential for accurate simulations in nuclear fuel cycle studies and the optimization of medical isotope production routes.
• Future Directions: The authors conclude that further refinements are needed in modeling composite-particle emission, potentially requiring the inclusion of cluster degrees of freedom in reaction codes to accurately predict cross-sections for tin isotopes.