New Physics Opportunities at Gammasphere
New Physics Opportunities at Gammasphere
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Gammasphere 的新物理机会
DOI：
10.1080/10506899609411095
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发表时间：
1996
影响因子：
--
通讯作者：
Frank Stephens
中科院分区：
文献类型：
--
作者：
R. Janssens;Frank Stephens
我们对电磁辐射的光谱有一定的了解，其能量范围大约在30个数量级--从无线电波（大约eV）到宇宙射线（至少达到10 * eV）。这种辐射在很多方面对我们都非常重要。我们用来看东西的光和我们用来交流的无线电和电视波就是明显的例子。在科学中，我们也有许多这样重要的例子：最近的可能是宇宙微波背景辐射，这是宇宙大爆炸起源的最有力证据之一，以及相对论重离子碰撞发出的高能伽马射线，这是为数不多的直接从核物质最热的相互作用区携带信息的探测器之一，边远地区的改造。此外，我们对原子、分子和核能级的了解，大部分来自于研究系统从一种状态跃迁到另一种状态时发出的电磁辐射。许多类型的电磁辐射探测器已经建成，这取决于感兴趣的能量范围从无线电接收器到由铅和合适的闪烁体交替层组成的大型探测器。对于由被限制在已知体积内的给定质量的（带电）粒子组成的系统，电磁辐射的特征能量范围可以根据测不准原理（mAv）（Ax）I h来估计，该原理给出了系统零点能量的粗略值，从而给出了能级之间的分离。对于原子核中的质子来说，这个能量的数量级是106 eV（1 MeV）。因此，为了研究原子核，我们需要设计和建造针对这一能量范围的探测器。通常，伽马射线探测器的最重要的特性是：1）在探测入射伽马射线方面的高效率; 2）导致单能伽马射线的非常窄的能量峰值的高分辨率; 3）全能量与部分能量事件的高比率;以及4）定位单个伽马射线的高粒度。对于1 MeV范围内的伽马射线，迄今为止，这些特性的最佳组合是由高纯度锗（Ge）晶体制成的半导体。目前商业上可以生产的最大的这种晶体是直径约10 cm和10 cm长的圆柱体，对于约30%的入射1 MeV单能伽马射线，其产生全能量峰值，其最大高度的一半处的全宽度为约2 keV。为了获得更好的全能量与部分能量事件的比率（称为峰总比，或P/T比），Ge探测器被密集的散射器包围-锗酸铋（BGO）是通常的散射器-其检测Ge晶体中康普顿散射的伽马射线，然后电子抑制Ge探测器中留下的部分能量脉冲。这导致1.3MeV伽马射线的P/T比从裸晶体的约0.3提高到抑制时的约0.7。为了提高效率和粒度，这种康普顿抑制探测器被组装成阵列。第一个这样的阵列，称为TESSA，于1980年在丹麦的Rise建立，由五个探测器组成;而目前的阵列，如
We have some familiarity with the spectrum of electromagnetic radiation over an energy range of something like thirty orders of magnitude--from radio waves (about eV) to cosmic rays (up to at least lo1* eV). This radiation is enormously important to us in many ways. The light by which we see and the radio and television waves through which we communicate are obvious examples. In science, we also have many examples of this importance: recent ones might be the cosmic microwave background radiation, which constitutes one of the strongest pieces of evidence for the big bang origin of the universe, and the high-energy gamma rays emitted from relativistic heavy ion collisions, which are one of the very few probes that carry information directly from the hottest interaction zones of nuclear matter without subsequent modification in the outlying regions. In addition, much of what we know about atomic, molecular and nuclear energy levels has come from studying the electromagnetic radiation emitted when the system makes a transition from one state to another. Many types of detectors have been built for electromagnetic radiation which differ widely depending on the energy range of interest-from radio receivers to massive detectors composed of alternating layers of lead and a suitable scintillator. The characteristic energy range for electromagnetic radiation from a systemcomposed of a (charged) particle of agiven mass confined to a known volume can be estimated from the uncertainty principle,(mAv)(Ax) I h, which gives a rough value for the zero-point energy of the system and thereby for the separation between energy levels. For a proton in a nucleus the order of magnitude for this energy is lo6 eV (one MeV). Thus for the study of nuclei, we need to design and build detectors aimed at this energy range. Generally, the most important properties of a gamma-ray detector are: 1) high efficiency in detecting incident gamma rays; 2) high resolution resulting in very narrow energy peaks for mono-energetic gamma rays; 3) high ratio of full-energy to partialenergy events; and 4) high granularity to localize individual gamma rays. For gamma rays in the one MeV range, by far the best combination of these properties is given by semiconductors made of high purity germanium (Ge) crystals. The largest such crystals that can currently be produced commercially are cylinders about 10 cm in diameter and 10 cm long, which, for about 30% of the incident one MeV mono-energetic gamma rays, produce a full-energy peak with a full width at half its maximum height of about 2 keV.There are some ways to optimize the performance of such detectors. In order to achieve a better ratio of full-energy to partial-energy events (called the peak-to-total, or P/T, ratio), the Ge detectors are surrounded by a dense scintillator-bismuth germanate (BGO) being the usual one-which detects gamma rays Compton-scattered out of the Ge crystal and then suppresses electronically the partial energy pulse left in the Ge detector. This results in an improvement in the P/T ratio for a 1.3 MeV gamma ray from about 0.3 for the bare crystal to about 0.7 when suppressed. To increase both the efficiency and the granularity, such Compton-suppressed detectors are assembled into arrays. The first such array, called TESSA, was set up in Rise, Denmark, in 1980 and consisted of five detectors; whereas, current arrays like