The AGATA Spectrometer

AGATA -先进伽马跟踪阵列-将是世界上最杰出的设备，用于研究物质的飞秒尺度结构。通过以前所未有的灵敏度测量原子核发射的伽马射线的特性，AGATA将为核和亚核行为提供新的见解，并将解决核存在的极限和宇宙中元素的起源等基本问题。最近的实验结果已经开始表明，远离稳定的原子核的行为可能与其接近稳定的邻居非常不同。为了全面了解原子核的结构，我们需要了解所有原子核的行为，而不仅仅是接近稳定的小子集。因此，需要开发新的实验方法，以研究离稳定性更远的原子核。例如，放射性离子束加速器现在已经可以使用。它们的使用提出了大量新的挑战;低光束强度和高背景计数将需要新的超灵敏实验技术。γ射线能谱是研究原子核结构的重要技术之一。由于这个原因，多年来在伽马射线探测方面取得了许多技术进步。在20世纪80年代，英国核物理学家率先开发了由锗探测器阵列组成的伽马射线光谱仪。这种探测器的问题在于，如果伽马射线散射出探测器而没有沉积其全部能量，则光谱响应受损。作为补救措施，使用逃逸抑制的方法，其中锗探测器被第二个探测器-抑制屏蔽-包围，该屏蔽否决散射的伽马射线。虽然这种方法显著地提高了光谱的质量，但是屏蔽占据了4（π）立体角的有价值的部分，限制了整体检测效率。在20世纪90年代，两个大型光谱仪的发展达到了顶峰：Euroball（欧洲）和Gammaside（美国），每个都由大约100个逃逸抑制锗探测器组成。一个巨大的进步将是放弃屏蔽，并建立一个伽马射线光谱仪完全由锗探测器。散射的伽马射线可以从一个探测器追踪到另一个探测器，而不是否决和丢失。这是AGATA的基本原则。虽然跟踪听起来很简单，但实际上它是复杂的，需要记录每个伽马射线相互作用的能量和位置，以便从一个探测器到另一个探测器跟踪散射的伽马射线，从而通过事件重建确定其全部能量。然而，这种复杂性是值得的，因为AGATA的灵敏度将比其前身高出1000倍以上。因此，伽马射线追踪是全世界核物理研究的前沿。跟踪在其他领域也很重要，例如，在核医学成像中，伽马射线能量的重建将大大提高分辨率。AGATA将由来自12个国家的物理学家组成的大型欧洲合作团队开发和建造。英国是合作的重要组成部分，利用其在该领域多年的领导地位，在几个关键领域拥有专业知识。最终AGATA将由180个探测器组成。该项目将分阶段实施;本请求涵盖2008年至2012年这一阶段，目标是建造全部阵列的四分之一。最初，将建造一个15个探测器的子阵列-AGATA演示器;虽然其主要目的是演示跟踪的可行性，但它本身将是一个强大的设备。AGATA将继续扩大，并将在2012年之前在三个欧洲实验室运行，每个实验室都有不同的特点：最初在意大利的Legnaro稳定束设施，后来在法国GANIL和德国的GSI放射性束设施。在此资助期之后，完整的AGATA光谱仪将于2015年建成。

AGATA - the Advanced GAmma Tracking Array - will be the world's pre-eminent device for studies of the femtoscale structure of matter. By measuring the properties of gamma rays emitted by atomic nuclei with unprecedented sensitivity, AGATA will provide new insights into nuclear and sub-nuclear behaviour and will address fundamental issues such as the limits of nuclear existence and the origin of the elements in the universe. Recent experimental results have begun to suggest that nuclei far from stability may behave very differently from their near-stable neighbours. For a complete understanding of nuclear structure, we need to understand the behaviour of all atomic nuclei, not just the small subset close to stability. New experimental methods therefore need to be developed, to study nuclei ever further from stability. For example, radioactive-ion beam accelerators are now becoming available. Their use presents a wealth of new challenges; low beam intensities and high background counts will require new, ultra-sensitive experimental techniques. Gamma-ray spectroscopy is one of the foremost techniques for studying nuclear structure. For this reason, many technological advances in gamma-ray detection have been made over the years. In the 1980s, UK nuclear physicists pioneered the development of gamma-ray spectrometers made up of arrays of germanium detectors. A problem with such detectors is that the spectral response is impaired if a gamma ray scatters out of the detector without depositing its full energy. As a remedy, the method of escape suppression is used, whereby the germanium detector is surrounded by a second detector - a suppression shield - which vetoes scattered gamma rays. Although this method significantly improves the quality of the spectra, the shield occupies a valuable fraction of the 4(pi) solid angle, limiting the overall detection efficiency. In the 1990s developments culminated in two large spectrometers: Euroball (Europe) and Gammasphere (USA) each made up of ~100 escape-suppressed germanium detectors. A giant step forward would be made by dispensing with shields, and building a gamma-ray spectrometer solely from germanium detectors. Instead of vetoing, and losing, scattered gamma rays, they could be tracked from one detector to another. This is the underlying principle of AGATA. Although tracking sounds straightforward, in practice it is complex / it is necessary to record the energy and position of every gamma-ray interaction, in order to track a scattered gamma ray from one detector to another, and thereby determine its full energy by event reconstruction. The complexity however pays off as AGATA will have sensitivity over 1000 times better than its predecessors. Gamma-ray tracking is thus at the forefront of nuclear-physics research throughout the world. Tracking is also important in other fields, for example, in nuclear medical imaging where the reconstruction of gamma-ray energies will vastly improve resolution. AGATA will be developed and built by a large European collaboration of physicists from over 12 countries. The UK is a major part of the collaboration, exploiting its many years of leadership in the field, with expertise in several key areas. Ultimately AGATA will consist of 180 detectors. The project will be realized in phases; this request covers the phase from 2008 to 2012, where the aim is to build a quarter of the full array. Initially, a 15-detector sub-array - the AGATA Demonstrator - will be built; although its main purpose is to demonstrate the feasibility of tracking, it will be a powerful device in its own right. AGATA will be continually expanded, and will be operated at three European laboratories before 2012 each with different characteristics: initially at the stable-beam facility at Legnaro in Italy, and later at radioactive-beam facilities at GANIL in France and GSI in Germany. Following on from this grant period, the complete AGATA spectrometer will be built by 2015.