AGATA (Advanced Gamma-ray Tracking Array) is a new generation high-resolution γ-ray spectrometer providing unprecedented Doppler-correction capabilities thanks to a combination of fine detector segmentation, efficient pulse-shape analysis algorithms, and implementation of an innovative γ-ray tracking concept.
Gamma-ray detection using semiconductor materials, such as high-purity germanium typically used for high-resolution γ-ray spectroscopy, is possible thanks to three mechanisms in which γ rays interact with matter, namely photoelectric effect, pair creation and Compton scattering. In the two former processes the total energy of a γ ray is transferred to an electron, or to an electron-positron pair, which subsequently generate a cloud of electron-hole pairs along their path in the detector material. These pairs are collected at the detector electrodes and form a signal, which amplitude is proportional to the γ-ray energy. In contrast, when the Compton scattering occurs, an electron gains only a part of the γ-ray energy, and a new photon is emitted that carries its remainder. Through a series of subsequent Compton-scattering events concluded by a photoelectric absorption the entire energy of the γ ray can be deposited in the detector. However, it is possible that a Compton-scattered photon escapes from the detector volume, and thus the registered energy will be lower than the γ-ray energy. To prevent this, so-called anti-Compton shields are typically used in high-resolution γ-ray spectroscopy, which surround the germanium detector, but are protected from a direct irradiation by thick metal collimators. Thus, if any signal is registered by the shield, it must originate from Compton scattering inside the detector, which implies that the coincident signal in the detector should be rejected. Such configuration, presented in figure (a) on the right, obviously limits the solid angle W which can be covered by germanium detectors, and, consequently, the total detection efficiency ε.
AGATA relies on a different concept, schematically presented in figure (b) on the right. The anti-Compton shields with related collimators are eliminated, and instead the entire solid angle covered by the spectrometer is filled by high-purity germanium detectors. Thanks to their fine segmentation (each germanium crystal is electrically segmented into 6 radial sectors and 6 longitudinal slices, i.e. 36 individual signals are read out) and application of pulse-shape algorithms (PSA), the 3D position of each γ-ray interaction point can be determined with a precision of ~5mm. Using the list of interaction points for each event and the corresponding deposited energies, resulting from the PSA procedure, the trajectory of the γ ray through the entire spectrometer can be reconstructed with help of sophisticated tracking algorithms. This includes possible scattering from one crystal to another, and thus events that would have been rejected by anti-Compton shields in a standard γ-ray spectrometer are recovered here, greatly increasing the detection efficiency. This innovative solution, made possible through advances in germanium detector technology, digital electronics, and pulse-shape analysis, provides also a much more precise determination of the direction of the γ-ray emission than what is achievable with conventional set-ups, which is particularly important for experiments using fast ion beams of hundreds MeV per nucleon.
Schematic presentation of two types of gamma-ray spectrometers: (a) standard configuration with anti-Compton shields (gray) and collimators (black), (b) AGATA. Gamma-ray interaction points that contribute to the energy measured by the spectrometer are marked in green, those that are “missed” by the detectors are in red. Maximum solid angle coverage omega and efficiency epsilon for each configuration are indicated.
The AGATA demonstrator consisted of five triple clusters (3 asymmetric Ge detectors in a common cryostat). The surface of each diode is electrically segmented into 36 segments (i.e. 5*3*36= 540 segments for the whole and 540+15=555 measurement channels).