Nuclear matter distribution of 56Ni measured with EXL
Nuclear matter distribution of 56Ni measured with EXL
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使用 EXL 测量 56Ni 的核物质分布
DOI：
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发表时间：
2015
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通讯作者：
M. Schmid
中科院分区：
文献类型：
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作者：
M. Schmid
利用质子弹性散射首次成功地测量了56 Ni的核物质分布和均方根物质半径。作为一个质子数和中子数相等的双魔核，56 Ni具有特殊的物理意义。由于它也是一个放射性原子核，实验必须在逆运动学中进行。因此，该实验是在GSI Helmholtzzentrum fuer Schwerionenforschung的ESR（实验储存环）上进行的，作为EXL（光离子诱导反应中研究的外来核）的第一次实验活动的一部分。将58 Ni束流在飞行中碎裂产生的56 Ni束流注入ESR中，与内氢靶相互作用。储存环的真空条件要求很高，因此有必要开发一种新型的探测器系统。这必须与超高真空（UHV）兼容，同时具有尽可能低的能量阈值，以便能够测量在低动量转移下散射的粒子。为了同样满足这两个条件，开发了一种无窗检测器系统，其中UHV通过硅条检测器与辅助真空分离。在辅助真空中，可以放置额外的检测器以及其他非UHV兼容组件。通过这种方式，建立了一个基于硅探测器的望远镜，它可以测量能量范围从几百keV到大约50 MeV的质子。在本工作的过程中，所采用的探测器进行了测试，并进一步发展广泛的实验室测试以及在束实验。
从测量的反冲质子的角分布推导出质子弹性散射的微分截面。为此，全面的蒙特-卡罗模拟的设置已被执行。然后，利用Glauber多重散射理论从截面中提取核物质分布。为此，密度分布参数化的现象分布，对称费米分布和模型无关的高斯求和（SOG）方法被使用。后者允许确定理论依赖的贡献的系统误差。最后，由物质分布计算出~（56）Ni的均方根物质半径为（3.76 ± 0.08）fm，与HFB和HF+BCS计算结果一致。整个方法的正确性，即逆运动学测量和应用的分析程序，通过与已知的58 Ni的核物质分布的比较得到了证明，其结果与文献值吻合得很好。
In the present work, the nuclear matter distribution and the RMS matter radius of 56Ni were successfully measured for the first time by exploiting elastic proton scattering. Being a doubly magic nucleus with an equal number of protons and neutrons, 56Ni is of particular physical interest. Since it is also a radioactive nucleus, the experiment has to be performed in inverse kinematics. Hence, the experiment was conducted at the ESR (Experimental Storage Ring) at the GSI Helmholtzzentrum fuer Schwerionenforschung as part of the first experimental campaign of EXL (EXotic nuclei studied in Light-ion induced reactions). The beam of 56Ni, which was produced by in-flight fragmentation of a 58Ni beam and selected by the FRagment Separator (FRS), was injected into the ESR and interacted with the internal hydrogen target. The demanding vacuum conditions of a storage ring made it necessary to develop a novel detector system. This had to be ultra-high vacuum (UHV) compatible and, at the same time, feature an energy threshold as low as possible to enable the measurement of particles scattered at low momentum transfer. To equally fulfil both conditions, a windowless detector system was developed in which the UHV is separated from an auxiliary vacuum by a silicon strip detector. In the auxiliary vacuum, additional detectors as well as other non-UHV compatible components may be placed. This way, a telescope based on silicon detectors was set up which makes the measurement of protons in an energy range starting at few hundreds of keV up to about 50 MeV possible. In the course of the present work the employed detectors were tested and further developed by extensive laboratory tests as well as in-beam experiments.
The differential cross section for elastic proton scattering was deduced from the measured angular distribution of the detected recoil protons. For this, comprehensive Monte-Carlo simulations of the setup have been performed. Then, the nuclear matter distribution was extracted from the cross section with the help of the Glauber multiple-scattering theory. For this purpose, the density distribution was parametrised by a phenomenological distribution for which a symmetrised Fermi distribution and the model-independent Sum-Of-Gaussians (SOG) method was used. The latter allows to determine theory-dependent contributions to the systematic error. Eventually, the RMS matter radius of 56Ni was calculated from the matter distributions to be (3.76+-0.08) fm which is in agreement with predictions by HFB and HF+BCS calculations. The correctness of the whole method, i.e. the measurement in inverse kinematics and the applied analysis procedure, was proven in comparison to an already known nuclear matter distribution of 58Ni of which the results are in a good agreement with the literature values.