等时性质量谱仪中N=Z核质量测量的方法探索

N=Z核的质量数据对于研究rp-和νp-过程至关重要。此外,N=Z原子核的质量数据将会帮助我们解决与核结构有关的许多关键问题。结合碎片分离器的等时性质谱仪（Isochronous mass spectrometry,IMS）是十分快速有效而且高分辨的质量测量工具。由于N=Z核的m/q值非常接近,目前国际上的储存环质量谱仪CSRe/IMP和ESR/GSI还无法实现对N=Z核运用飞行时间谱进行鉴别,因而无法对它们进行质量测量。在日本理化学研究所的仁科加速器中心新建了专用的等时性质谱仪（IMS）,即稀有放射性同位素储存环"Rare-RI Ring"（R3）,以确定短寿命的放射性原子核的质量,其目标质量相对精度为10-6。R3质谱仪与高分辨的碎片分离器BigRIPS的组合,运用束流线的高分辨的离子鉴别,使得R3上的IMS方法对N=Z核进行质量测量成为可能。本文设计了专用的等时性束流光学设置,模拟了124Xe的主束经过碎裂反应产生的N=Z核在束流线中穿过各焦平面的径迹、能量、速度等信息,同时检验了这些次级束在环内的飞行时间相对于动量的变化关系。模拟的结果表明:当储存环的等时性光学设置在某一个N=Z核时,所有其它N=Z核在环内的回旋时间也与动量弥散无关,说明了这些核也满足等时性条件。基于N=Z核的这种等时性的特点,本文对R3上刻度N=Z核的方法也进行了讨论。

Study of Mass-measurement Method for N=Z Nuclei with Isochronous Mass Spectrometry

Nuclear mass data of N=Z nuclei is crucial for the investigation of the rp-and the νp-processes. In addition, access to the nuclei on the N=Z line will help us solve key questions relating to many open questions of nuclear structure. Isochronous mass spectrometry (IMS) combined with a fragment separator is a very fast, efficient and high resolution mass measurement tool. Since the m/q values of the N=Z nuclei are very close, the current storage ring mass spectrometers CSRe/IMP and ESR/GSI cannot realize the identification of the N=Z ions of different species via revolution time spectrum, so it is impossible to realize mass measurements on them. An IMS using a newly constructed storage ring named the ‘Rare-RI Ring’ (R3) has been implemented at the RIKEN Nishina Center to determine the masses of short-lived rare nuclei with a relative precision of the order of 10-6. With the R3 operated as an IMS coupled to the high-resolution beam-line BigRIPS employed as a fragment separator, high-resolution particle identification of the N=Z nuclei on an event-by-event basis with the beam-line before their injection to R3 makes the mass measurements possible. Monte Carlo simulation studies of beam tracking, high-resolution particle identification and selection of the secondary beams have been carried out with a dedicated ion-optics design. The results show that the revolution time of all the N=Z nuclei are independent of momentum dispersion in the storage ring when we set one species of N=Z nucleus in an isochronous condition. The mass calibration method for the N=Z nuclei has also been discussed in this report based on the simulation.