Science Bulletin | 探索原子核质子半径的新途径 

研究背景

原子核的尺寸有多大？质子和中子在原子核中是如何分布排列的？这些是核物理学中非常基本且至关重要的问题。R. Hofstadter（1961年诺贝尔物理学奖获得者）等利用电子弹性散射方法，研究了稳定原子核内的电荷分布。然而，这一方法很难拓展到短寿命、不稳定的原子核。基于放射性核束装置，高精度测量原子核在质子、碳等靶上的反应截面，探索不稳定原子核内核子密度分布，一直是核物理学研究的前沿课题。不同于理论非常完善的电子散射方法，将强子、重离子作为探针研究原子核，面临着几个关键难题：当前的核反应理论是否完善？如何高精度计算强子-原子核碰撞、重离子碰撞中的强相互作用？可否从不同能量、不同实验靶的研究中得到自洽的结果？为了厘清这些问题，亟需在不同能量和靶核下，对同位素链开展系统测量，发展基于强子探针的实验方法。

成果介绍

近期，北京航空航天大学的孙保华教授和谷畑勇夫教授领导的核物理研究团队，联合国际合作者，利用德国GSI亥姆霍兹重离子研究中心的放射性核束装置FRS，在900A MeV能区（~0.86倍光速），系统测量了从锂至氧等24种同位素在碳和氢靶上的电荷改变反应截面。数据涵盖稳定至丰中子原子核，其中大多数为首次发布。

他们的研究表明，在电荷改变反应中，直接移除质子的过程平均贡献了总截面的约90%（碳靶）和75%（氢靶），但在各核素上的贡献并不相同。

他们发现并提出了一个唯象因子，标记在核核碰撞中先发生中子移除、然后级联质子蒸发的过程，为质子蒸发过程的存在提供了实验依据。考虑反应理论的唯象修正后，在统一的框架下，分别利用碳靶和氢靶数据，提取了碳同位素链(^12,14–19C)和氮同位素链(^{14,15,17–22}N)以及其他轻核的点质子分布半径。

发现随着中子数的增加，对于具有偶数质子的原子核，利用两个反应靶提取的半径吻合，但是对于具有奇数质子的原子核，从碳靶数据提取的半径逐渐倾向于大于从氢靶中得到的结果，这需要实验和理论上的进一步探索。 

Jichao Zhang, Baohua Sun, Isao Tanihata, Rituparna Kanungo, Christoph Scheidenberger, Satoru Terashima, Feng Wang, Frederic Ameil, Joel Atkinson, Yassid Ayyad, Soumya Bagchi, Dolores Cortina-Gil, Iris Dillmann, Alfredo Estradé, Alexey Evdokimov, Fabio Farinon, Hans Geissel, Giulia Guastalla, Rudolf Janik, Satbir Kaur, Ronja Knöbel, Jan Kurcewicz, Yury Litvinov, Michele Marta, Magdalena Mostazo, Ivan Mukha, Chiara Nociforo, Hooi Jin Ong, Stephane Pietri, Andrej Prochazka, Branislav Sitar, Peter Strmen, Maya Takechi, Junki Tanaka, Jossitt Vargas, Helmut Weick, John Stuart Winfield. A new approach for deducing rms proton radii from charge-changing reactions of neutron-rich nuclei and the reaction-target dependence. Science Bulletin 2024;69(11): 1647-1652

Scientists report on a new approach for deducing proton radii from charge-changing reactions (phys.org)

The charge-changing reaction (CCR) observed in the experiment consists of two parts. The direct proton removal process (σ^driect), represents the dominate part in CCR and can be calculated with a good precision with theoretical models. In this process, proton(s) are removed directly in the reaction, which dominates the CCRs. The other one, the proton evaporation process (σ^evap) after direct neutron removals, is a two-stage process. Only neutron(s) are removed in the direct reaction stage, leaving a residual nucleus in a highly excited state. The residual nucleus then undergos a cascade decay by evaporating charged particles (usually protons). Credit: Science China Press

A study systematically measured the charge-changing reaction cross section of 24 light nuclei on carbon and hydrogen targets at the GSI Helmholtz Centre for Heavy Ion Research in Germany.

The team concluded that the charge-changing reaction measurement should include an extra contribution from the proton evaporation process, besides the direct proton removing process, which can be described in the framework of the Glauber model. The findings explain the problem in charge-changing reaction studies, where experimentally measured cross sections are always higher than expected from theoretical models.

"In deducing nuclear charge radii from charge-changing reactions, can one consistently treat the experimental data on different reaction targets? What is still missing in the current model analysis? We addressed these questions with new accurate data at 900A MeV," Sun says.

The researchers found a robust correlation between the contribution to the measurement from the proton evaporation process right after the neutron removal process and the nucleon separation energy, an inherent property of the nucleus itself. This correlation is supposed to be valid for predictions of most exotic nuclear systems (at least for the p-shell nuclides of interest in the paper) since it is obtained by interpolation.

Linear relationships exist between the ratio of measured cross sections to theoretical predictions for the charge-changing reactions of different nuclei, i.e., the proportion of the proton evaporation process, and the nucleon separation energy of that nucleus (S₁). The colored symbols represent the stable nuclei with well-known charge radii. The radii of the unstable nuclei, represented by the semi-transparent symbols, can be extracted by interpolation, rather than extrapolation. The proton evaporation process right after the neutron removal, can account for 10% – 15% of CCR cross sections for the carbon target case and 20% – 30% for the hydrogen target case. Credit: Science China Press

This enabled the researchers to extract, for the first time in the same framework, the point-proton distribution radii of nuclei from data of charge-changing reactions on various reaction targets, particularly for the exotic nuclei, which were hardly accessible using other experimental approaches.

They obtained consistent results for the nuclides with even proton numbers. For the most neutron-rich nuclei with odd proton numbers, systematic differences seem to exist in radii extracted from two target data, i.e., the carbon target data give slightly larger radii than the hydrogen target data. This may point to the effect of different hadron probes or the point-proton distribution form of exotic nuclei.

The paper is published in the journal Science Bulletin, and this study was led by Prof. Baohua Sun (School of Physics, Beihang University) and Prof. Isao Tanihata (School of Physics, Beihang University and Research Center for Nuclear Physics (RCNP), Osaka University).

More information: Jichao Zhang et al, A new approach for deducing rms proton radii from charge-changing reactions of neutron-rich nuclei and the reaction-target dependence, Science Bulletin (2024). DOI: 10.1016/j.scib.2024.03.051

Provided by Science China Press 

Citation: Scientists report on a new approach for deducing proton radii from charge-changing reactions (2024, May 23) retrieved 13 November 2024 from https://phys.org/news/2024-05-scientists-approach-deducing-proton-radii.html

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