Nuclear Structure Group

Gamma-ray Detector Systems

Just as the spectroscopic study of emitted photons from excited atoms played a crucial role in understanding atomic structure, gamma-ray spectroscopy is essential for understanding the structure of atomic nuclei. For nearly four decades, advances in germanium detector technology have led to revolutionary increases in the sensitivity of gamma-ray spectroscopy and the study of the structure of atomic nuclei (see below). In 1962, the first Li-drifted Ge detectors were introduced opening the field of high-resolution gamma-ray spectroscopy, and by 1970 such detectors were being used to determine the coincidence relationships between gamma rays emitted from the nucleus in order to construct detailed schemes of the nuclear states. In 1971, larger volume germanium detectors (which had a greatly increased efficiency) were based on high-purity crystals. Further gains were made by decreasing the background in experiments by surrounding the Ge detector by a shield of scintillator material (such as sodium iodide or bismuth germanate) to suppress events in which only part of the gamma-ray energy is deposited before it Compton scatters out of the germanium crystal. By 1980 small arrays of such Compton-suppressed detectors were being widely used.

Current state-of-the-art detector arrays, such as Gammasphere in the U.S. (presently based at the Argonne National Laboratory) comprise approximately 100 individual Compton-suppressed Ge detectors. They have an efficiency of about 10% for detecting the full energy of a 1MeV gamma ray in a Ge crystal. The corresponding maximum efficiency, obtained from 4 p coverage with Ge detectors (a Ge shell), is theoretically limited to about 60% due to losses from the finite size of the Ge crystals, gaps, and absorption in material required to mount the crystals. The drawback of this arrangement (and the reason why a shell built purely of Ge is unreasonable) comes from events involving the simultaneous emission of multiple gamma rays. For such events it becomes impossible to distinguish when two different gamma rays hit two different detectors or when one gamma ray scatters between the two detectors. To circumvent this double-hit or “add-back” problem, the number of detectors has to be increased to separate all interactions of each emitted gamma ray. Unfortunately, this approach is prohibitively expensive since it requires many individual detectors (>1000) to regain the maximum theoretical efficiency. The new approach is to “track” the interactions of all gamma rays emitted in an event.

Tracking takes advantage of the recent technological advances in the electrical segmentation of Ge crystals. It is now feasible to build an array of approximately 100 highly segmented Ge detectors, retaining high efficiency, but allowing a pulse-shape analysis of signals from each segment to be used to reconstruct the energy and three-dimensional positions of all gamma-ray interactions. This in turn allows the scattering of all the gamma rays from an event to be tracked and reconstructed. The concept of gamma-ray tracking is illustrated above. It is worth pointing out the potential improvements in other fields based on this ability to track gamma radiation in large volume Ge detectors. Unknown gamma-ray sources, for example in the environment, in geological formations, or in the human body, can be localized and identified much more efficiently than with current techniques, opening new possibilities in fields as diverse as planetary science or nuclear medicine.

To illustrate the power of this new concept for nuclear physics applications, we take three examples: gamma-ray spectroscopy of high-spin states in nuclei populated in heavy-ion fusion-evaporation reactions; the study of rare nuclei formed from reactions involving re-accelerated radioactive nuclei; the study of states in nuclei far-from-stability formed in fragmentation reactions and studied in flight. The first example is the most commonly employed reaction at current stable-beam facilities and will remain the “workhorse” for populating nuclear states with high-angular momentum. The second (`isotope separation online (ISOL)' or the `re-accelerated beam' method) and third (the `fast-beam fragmentation' reaction) are the two approaches employed by all existing and planned exotic beam facilities. Indeed, the push towards the development of a next generation facility for radioactive-ion beam production in the U.S., and similar facilities in Europe and Asia, promise the possibility of beams of rare, short-lived radioactive isotopes, opening up to study a whole range of exotic nuclei. This will have an impact on a broad range of science covering areas such as the physics of mesoscopic systems, nucleon-nucleon interactions, nucleosynthesis in the universe, new super-heavy elements, and physics beyond the Standard Model of elementary-particle physics. Gamma-ray spectroscopy will remain at the center of this diverse research program and also promises vast new gains in sensitivity due to the development of gamma-ray tracking arrays (such as the proposed GRETA - Gamma-Ray Energy Tracking Array - conceived at the Lawrence Berkeley National Laboratory). We will point out the unique features that make gamma-ray tracking the method of choice for gamma-ray spectroscopy in each case.

After a nucleus is formed in a heavy-ion fusion evaporation reaction it often has high angular momentum and decays via the successive emission of many gamma rays (20-30). A measure of the sensitivity of Ge arrays in these kinds of studies involves combining factors such as energy resolution, efficiency, response, granularity, and counting rate to yield an appropriate “resolving power”. For a tracking array, the huge increase in efficiency from the near 4 p coverage of Ge combined with the high granularity and the ability to recover full-energy events by tracking, will lead to an unprecedented gain of 2-3 orders-of-magnitude in sensitivity for studying high-multiplicity cascades.

In ISOL methods, rare isotopes are produced at rest in the laboratory by target fragmentation, fission, or spallation and are then extracted from the source, separated from other products, and re-accelerated. These beams of rare isotopes can be trapped and studied, or used in reactions on targets. The intensities of the rare nuclei produced by this method will be very low in comparison to beams of stable ions and it is clear that the large efficiency of a tracking array is extremely important. However, a factor that will largely determine the feasibility of studying such nuclei is the ability to isolate the rare decays from the large background of other radioactive decay in the environment. Here tracking plays another crucial role since it is possible to identify and suppress gamma rays that arrive in a direction other than that of the source itself.

In fast-beam fragmentation reactions the nuclei of interest are formed when a highly energetic beam collides with a target, fragmenting nuclei in the beam, the most exotic fragments of which (with very large neutron excess, for instance) can then pass through electromagnetic separators and be identified. In such reactions, which may often be the only way of forming some nuclei, not only will the nuclei of interest be rare but they will also have very high velocity (typically @ 30% the speed of light). The gamma-ray decay will take place faster than it is possible to stop the nuclei, and therefore must be studied in-flight. Gamma rays emitted from sources moving with such relativistic velocities suffer a Doppler shift in energy that depends on the angle of emission. Here the most crucial feature of tracking arrays is their ability to localize the first interaction point in a detector thereby defining the angle of emission of that gamma-ray from the moving nucleus and reducing the measured energy spread. It has been demonstrated that the first interaction point can be determined to ~ 2 mm in a GRETA-type detector. This translates to a vast improvement in the angular resolution.

The new gamma-ray tracking arrays hold the potential to have as profound an impact on the field of nuclear physics as occurred when the first Li-drifted detectors were introduced or the first multi-detector Compton-suppressed arrays came online. Whatever techniques are used to populate excited states in nuclei, whether using standard approaches at current stable beams facilities or new methods for production and use of rare radioactive species, gamma-ray spectroscopy using advanced Ge detector technology will remain at the heart of nuclear physics experimentation.

For more information on the state-of-the-art Ge detector arrays please follow the links below:

Gammasphere

GRETINA and GRETA