Key Takeaways & Executive Findings
- •• A novel method produces a neutron beam with Maxwellian energy distribution at kT=22 keV using a 3.4 MeV proton beam on a LiF target. • The neutron beam has a narrow angular spread (65.0°±2.6°), enabling direct measurement of neutron capture cross-sections for s-process nuclides. • This technique overcomes limitations of previous methods that suffered from broad angular distributions, improving accuracy in stellar nucleosynthesis studies. • The method was validated at the China Institute of Atomic Energy, demonstrating agreement with Maxwellian distribution within experimental uncertainties.
Abstract
To generate a neutron beam exhibiting a Maxwellian energy distribution with narrow emission angles for measuring the neutron capture reaction rates of the s-process nuclides, a monoenergetic 3.4 MeV proton beam produced by the tandem-accelerator in the China Institute of Atomic Energy was utilized. The proton beam was first transmitted through a 60.5 μm aluminum foil and then impinged on a natural LiF target to produce neutron beam via 7Li(p, n)7Be reaction. The quasi-Gaussian energy distribution of protons in the LiF target resulted in neutron energy spectra that agreed with a Maxwellian energy distribution at kT=(22±2) keV, which was achieved by integrating neutrons detected within an emission angle of 65.0°±2.6° using a 6Li glass detector positioned at 65° relative to the proton beam direction. The narrow angular spread of the Maxwellian-distributed neutron beam enables direct measurement of neutron capture cross-sections for most s-process nuclides, overcoming previous experimental limitations associated with broad angular distributions.
1. Introduction
Approximately half of the elements heavier than iron are synthesized through the slow neutron capture process (s-process). The s-process nucleosynthesis path progresses along the neutron-rich side of the valley of stability, where the neutron capture and β decay rates are the critical inputs of nuclear physics. These captured neutrons mainly originate from the 13C(α,n)16O and 22Ne(α,n)25Mg reactions occurring within the He and C burning shells of massive stars.
In these environments, temperatures typically range from 90 million Kelvin (MK) to 1 billion Kelvin (GK), enabling rapid thermalization of neutrons in the dense stellar plasma. This thermal equilibrium ensures that the neutron energy spectrum follows a Maxwell-Boltzmann distribution characteristic of the ambient stellar conditions. Therefore, the energy distribution of neutrons can be described in the form of Maxwell-Boltzmann distribution: φ(E) ∼ E_n exp(−E_n/kT), where k is the Boltzmann constant and T is the absolute temperature in Kelvin.
To obtain the stellar reaction rate, the Maxwellian-averaged cross section (MACS) is defined as ⟨σ⟩_kT = (2/√π) * (1/(kT)^2) * ∫_0^∞ σ(E_n) E_n exp(−E_n/kT) dE_n, where σ(E_n) is the energy dependent capture cross section. The MACS can be easily determined at each stellar temperature when neutron capture cross sections are measured over the energy range from thermal to about 500 keV. These measurements are typically conducted at time-of-flight (TOF) facilities employing pulsed neutron beams. However, because of the crowded narrow resonances and small capture cross sections, the TOF-derived neutron capture data have relatively larger uncertainties. Consequently, these experimental uncertainties propagate into stellar reaction rate calculations, with some rates exhibiting uncertainties exceeding 20%.
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HOU Jianglin, YAN Shengquan, LI Yunju, ZHANG Weijie, LI Ertao, WANG Youbao, SHEN Yangping, WANG Zhiqiang, LIU Yina, GUO Bing (2025). A New Method to Obtain Neutrons with Maxwellian Energy Distribution for Nuclear Astrophysics Study. SinoTechIntel Verified Research. https://doi.org/10.7538/yzk.2025.youxian.0341
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Frequently Asked Questions
What is the significance of generating a neutron beam with Maxwellian energy distribution?
A neutron beam with Maxwellian energy distribution mimics the neutron spectrum in stellar environments, allowing direct measurement of neutron capture cross-sections relevant to the s-process. This improves accuracy in stellar nucleosynthesis models.
How was the Maxwellian neutron beam produced in this study?
A 3.4 MeV proton beam from a tandem accelerator was passed through an aluminum foil and then impinged on a natural LiF target, producing neutrons via the 7Li(p,n)7Be reaction. The proton energy spread in the target resulted in a neutron spectrum that matched a Maxwellian distribution at kT=22 keV when integrated over a narrow angular range.
What are the advantages of the new method over previous techniques?
The new method produces a neutron beam with a narrow angular spread (65.0°±2.6°), enabling direct measurement of neutron capture cross-sections for most s-process nuclides. This overcomes limitations of previous methods that had broad angular distributions, leading to reduced uncertainties in reaction rates.
What is the Maxwellian-averaged cross section (MACS) and why is it important?
MACS is the energy-averaged neutron capture cross-section weighted by a Maxwell-Boltzmann distribution at a given stellar temperature. It is crucial for calculating stellar reaction rates in s-process nucleosynthesis, as it directly relates to the probability of neutron capture in stellar environments.
What are the potential applications of this neutron beam?
This neutron beam can be used for direct measurements of neutron capture cross-sections for s-process nuclides, improving nuclear data for astrophysical models. It may also be applied in other fields requiring well-characterized neutron beams with specific energy distributions.
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