AGATA - The Ultimate Gamma-ray spectrometer

AGATA/先进伽马跟踪阵列--将成为世界上研究物质飞秒尺度结构的卓越设备。通过以前所未有的灵敏度测量原子核发射的伽马射线的性质，AGATA将提供对核和亚核行为的新见解，并将解决诸如核存在的限制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接近稳定的原子核的行为截然不同。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一小部分。因此，需要开发新的实验方法，以进一步研究原子核的稳定性。例如，放射性离子束加速器现在正在变得可用。它们的使用带来了许多新的挑战；低束流强度和高本底计数将需要新的、超灵敏的实验技术。伽马射线能谱是研究核结构的最重要的技术之一。为此，多年来在伽马射线探测方面取得了许多技术进步。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能谱仪。这种探测器的一个问题是，如果伽马射线在没有储存全部能量的情况下散射出探测器，光谱响应就会受到损害。作为补救措施，使用了逃逸抑制方法，即锗探测器被第二个探测器--抑制屏--包围，该探测器否决了散射的伽马射线。虽然这种方法显著提高了光谱的质量，但屏蔽占据了4(Pi)立体角的一个有价值的部分，限制了整体探测效率。在20世纪90年代，发展达到顶峰的是两台大型光谱仪：Euroball(欧洲)和Gammasphere(美国)，每个光谱仪都由大约100个逃逸抑制的锗探测器组成。取消屏蔽，只用锗探测器建造伽马射线光谱仪，将是向前迈出的一大步。可以从一个探测器跟踪到另一个探测器，而不是否决或失去散射伽马射线。这是阿加塔的基本原则。虽然跟踪听起来很简单，但实际上很复杂，需要记录每个伽马射线相互作用的能量和位置，以便跟踪从一个探测器到另一个探测器的散射伽马射线，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的敏感度将比它的前身高1000倍以上。因此，伽马射线追踪处于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极大地提高分辨率。AGATA将由来自12个国家的物理学家组成的大型欧洲合作组织开发和建造。英国是合作的重要组成部分，利用其在该领域的多年领导地位，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。该项目将分阶段实现；这项请求涵盖2008至2012年的阶段，目标是建造整个阵列的四分之一。最初，将建造一个由15个探测器组成的子阵--阿加塔演示器；虽然它的主要目的是证明跟踪的可行性，但它本身将是一个强大的装置。AGATA将继续扩大，并将在2012年前在三个欧洲实验室运营，每个实验室都有不同的特点：最初在意大利莱格纳罗的稳定束设施，后来在法国GANIL和德国的GSI的放射束设施。在这一授权期之后，完整的AGATA光谱仪将在2015年前建成。

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.