Influence of secondary neutrons on alpha-particle induced reaction cross section measurement below the Coulomb barrier

This study demonstrates that unexpectedly high alpha-particle induced reaction cross sections for the $^\mathrm{nat}$Pt($\alpha$,x)$^\mathrm{195m}$Pt reaction below the Coulomb barrier are primarily caused by secondary neutrons rather than the direct alpha interaction, highlighting the critical need to account for such neutron-induced backgrounds in low-energy charged-particle activation measurements.

The Gist

The Big Picture: The "Ghost" in the Machine

Imagine you are trying to measure how hard it is to push a heavy boulder (a platinum atom) with a specific type of ball (an alpha particle). You set up a test where you shoot these balls at a stack of boulders.

You expect that if you shoot the balls slowly (low energy), they won't have enough force to knock anything loose because of a "force field" (the Coulomb barrier) surrounding the boulders. You expect the results to show almost nothing happening at low speeds.

But here's the mystery: When the scientists ran the experiment, they found that even at very low speeds, they were still creating a specific type of "debris" (a radioactive isotope called $^{195m}\text{Pt}$). It was as if the boulders were breaking apart even though the balls didn't have enough energy to do it.

The paper asks: "Is our measurement wrong, or is there a hidden factor we missed?"

The Culprit: The "Side-Effects" (Secondary Neutrons)

The scientists realized that when the alpha particles hit the platinum, they didn't just bounce off or stop. They also created a spray of invisible, tiny particles called neutrons. Think of these neutrons like confetti or shrapnel flying off when the main ball hits the target.

These "confetti" neutrons are fast and energetic. Even though the main alpha balls were moving slowly, these secondary neutrons were flying around inside the stack of foils and hitting other platinum atoms on their own.

The Analogy:
Imagine you are trying to knock over a row of dominoes by gently tapping the first one with a feather (the slow alpha particle). You expect nothing to happen. But, the feather tap accidentally knocks a small pebble loose (the neutron). That pebble flies across the table and hits the other dominoes, knocking them over.

If you only look at the feather, you think, "Wow, this feather is incredibly strong!" But really, it was the pebble (the secondary neutron) that did the work.

How They Solved the Puzzle

The scientists used a super-powerful computer simulation called PHITS (think of it as a "virtual reality" physics lab) to figure out exactly how many of these "pebbles" (neutrons) were flying around.

1. The Simulation: They modeled the entire experiment on the computer, calculating how many neutrons were created at every step of the stack.
2. The Correction: They then calculated how many of the "debris" ($^{195m}\text{Pt}$) would be created just by those neutrons hitting the platinum.
3. The Result: When they subtracted the "neutron contribution" from their total measurements, the "ghost" effect disappeared! The remaining data made perfect sense. The "unexpectedly high" results were actually just the neutrons doing their job.

The "Extra Foils" Trick

To prove this, the scientists looked at the very end of their stack of metal foils. The main alpha particles (the feathers) stopped before reaching the last few foils. So, those last foils should have been empty.

However, they found that those last foils were radioactive. Why? Because the "pebbles" (neutrons) flew all the way to the end and hit them. This confirmed that the neutrons were indeed traveling through the whole stack and causing reactions even where the main beam couldn't reach.

What About Other Particles?

The scientists also checked if other "shrapnel" like protons or deuterons (other types of tiny particles) were causing trouble. They found that these were like dust motes compared to the "pebbles" (neutrons). Their effect was so tiny (less than 0.2%) that they could be safely ignored.

The Takeaway

This paper teaches us a valuable lesson for physics experiments: Don't just look at the main event; watch the background noise.

• The Problem: Scientists were confused why their low-energy experiments were producing more results than physics said they should.
• The Solution: They realized "secondary neutrons" (the side-effects) were the real cause.
• The Lesson: When measuring how particles interact at low energies, you must account for the "confetti" (neutrons) flying around, or your results will be misleading.

In short, the "unexpectedly high" results weren't a mystery of new physics; they were just a case of secondary neutrons doing the heavy lifting while the main beam took a backseat.

Technical Summary

1. Problem Statement

The authors investigated a discrepancy in previously measured alpha-particle induced reaction cross sections for the natPt(α,x)195mPt reaction.

• The Anomaly: Measurements taken at RIKEN using stacked foil methods at 30 MeV and 50 MeV alpha-particle beams revealed unexpectedly high production cross sections for the metastable isotope 195mPt in the sub-Coulomb barrier region (energies below ~23 MeV).
• The Expectation: Theoretically, the cross section should drop sharply below the Coulomb barrier. However, the experimental data showed weak energy dependence and systematically higher values for the 50 MeV beam compared to the 30 MeV beam in this low-energy region.
• The Hypothesis: The authors hypothesized that secondary neutrons generated by the primary alpha-particle interactions within the thick platinum foil stack were inducing the natPt(n,x)195mPt reaction, thereby artificially inflating the measured yield attributed to the alpha-induced reaction.

2. Methodology

The study employed a multi-step approach combining particle transport simulations and nuclear data evaluation to quantify the secondary neutron contribution.

A. Particle Transport Simulation (PHITS)

• Code: The Particle and Heavy Ion Transport code System (PHITS) version 3.35 was used to simulate the interaction of alpha particles with the stacked platinum foils.
• Models: Two approaches were compared to characterize the secondary neutron spectra:
  1. Intranuclear Cascade (INC) Model: Specifically the Liège Intranuclear Cascade (INCL) model, suitable for high-energy processes.
  2. Evaluated Nuclear Data Library (NDL): Using the TENDL-2023 library (processed via NJOY2016) for energies up to 200 MeV.
• Validation: The simulation models were first validated against existing experimental data for double-differential thick target neutron yields from 209Bi(α,n+x) reactions at 30 MeV. Both INC and NDL options showed similar performance in the critical low-energy region (~1 MeV).
• Geometry: Simulations modeled the specific foil stacks used in the 30 MeV (9 Pt foils) and 50 MeV (19 Pt foils) experiments, including aluminum and titanium interleaving foils.

B. Cross Section Renormalization

• Library: The JENDL-5/A library was used for neutron-induced reaction cross sections.
• Issue: The original JENDL-5/A cross sections for 194Pt(n,γ), 195Pt(n,n'), and 196Pt(n,2n) reactions significantly underestimated experimental data.
• Correction: The authors applied a least-squares renormalization to the JENDL-5/A cross sections using 43 experimental data points from the EXFOR library. This generated optimized scaling factors for the three isotopic reactions to better match experimental elemental cross sections.

C. Yield Calculation

• The secondary neutron fluence ($\phi$) calculated by PHITS was combined with the renormalized cross sections ($\sigma$) to calculate the yield ($Y$) of 195mPt produced solely by secondary neutrons in each foil.
• This yield was converted into an equivalent cross section ($\sigma_n$) to be compared directly with the experimental alpha-induced cross sections ($\sigma_{exp}$).

3. Key Contributions

• Quantitative Correction Method: Developed a rigorous method to quantify the "background" contribution of secondary neutrons in low-energy charged-particle activation measurements, a factor often neglected in standard stacked-foil analyses.
• Data Library Improvement: Renormalized the JENDL-5/A library cross sections for Pt isotopes, significantly improving agreement with experimental data for neutron-induced 195mPt production.
• Mechanism Elucidation: Demonstrated that the "anomalous" high cross sections below the Coulomb barrier are not due to a failure of nuclear reaction theory for alpha particles, but rather an experimental artifact caused by secondary neutrons.
• Secondary Charged Particle Analysis: Systematically evaluated the contribution of secondary charged particles (protons, deuterons, tritons, alphas, 3He) and confirmed their influence is negligible (<0.2% of the neutron contribution).

4. Results

• Spectral Characteristics: The simulations revealed a prominent peak in the secondary neutron spectrum around 1 MeV, produced primarily via compound nucleus processes (isotropic emission). High-energy neutrons (>1 MeV) were produced via intranuclear cascades (forward-peaked).
• Agreement with Experiment:
  • The calculated secondary neutron contribution ($\sigma_n$) matched the experimental cross sections ($\sigma_{exp}$) remarkably well in the sub-barrier region (below 23 MeV).
  • For the 50 MeV experiment, the last three foils (17th, 18th, 19th) were placed beyond the range of the primary alpha beam. In these foils, the measured activity was entirely due to secondary neutrons. The calculated $\sigma_n$ for these foils agreed with the measured values within a few percent.
• Overestimation at Very Low Energies: In the deepest foils of the 30 MeV stack (where alpha energy is very low), the calculated $\sigma_n$ slightly exceeded $\sigma_{exp}$ (by up to 40%). This suggests that while secondary neutrons explain the bulk of the signal, other minor factors or uncertainties in the very low-energy alpha cross sections may exist.
• High-Energy Neutrons: The contribution of neutrons with energies >20 MeV (outside the JENDL-5/A range) was found to be negligible (<1% for 50 MeV irradiation).
• Charged Particles: The ratio of 195mPt production from secondary charged particles to that from secondary neutrons was found to be less than 0.2%, confirming they can be ignored in this context.

5. Significance

• Re-evaluation of Low-Energy Data: This study highlights that secondary neutron effects are not negligible even in low-energy charged-particle activation measurements. Previous measurements of alpha-induced cross sections below the Coulomb barrier may require re-evaluation if secondary neutron contributions were not accounted for.
• Practical Correction Strategy: The authors propose a practical experimental solution: adding "extra" foils at the end of a stack (beyond the particle range). The activity measured in these foils provides a direct measure of the secondary neutron background, which can be subtracted from the total yield of the active foils.
• Impact on Nuclear Data: The work underscores the need for nuclear data evaluators to systematically adjust evaluated libraries (like JENDL) against experimental elemental cross sections to ensure accuracy for applications such as medical isotope production (e.g., 211At production via Bi targets, which also involves secondary neutrons).
• Methodological Benchmark: The successful application of PHITS with renormalized libraries provides a validated framework for correcting similar artifacts in other charged-particle induced reaction measurements.