Project Development
GRETINA Project Development
The concept of a gamma-ray tracking detector array was proposed in 1994, and after about ten years of R&D, the technology was in place to construct such a detector. The Department of Energy made the Critical Decision 0 (CD0) for GRETINA in August 2003 to construct a tracking detector covering one-fourth of the total solid angle. Since then the project proceeded according to schedule and was completed in 2011 on time and within budget. The dates of Critical Decision are shown in the following table.
A short summary of the major components of the GRETINA project is given below.
Detector
The critical detector technology is the manufacture of two-dimensionally segmented coaxial germanium detectors which provide signals with sensitivity for locating interaction points in three dimensions. In addition, the crystals should have large volume and be shaped into tapered irregular hexagon shapes to allow for close packing into a spherical shell with a high solid angle coverage. We have been working closely with the detector manufacturer to develop such a detector through several prototype stages. The geometric design of GRETINA uses 120 crystals packed in 30 cryostats. The first production 4-crystal detector module was ordered and was delivered at the end of 2006. A picture of a GRETINA production module is shown to the right.
Electronics
Determining the gamma-ray interaction position in three dimensions requires a detailed analysis of the pulse shapes. To accomplish this, the pulse shape from each segment needs to be recorded at a sampling rate of about 100 MHz and with a resolution of 14 bits. To reduce the amount of data that has to be stored on disc, online processing in the digitizer generates energy, time, and trigger information, as well as capturing the relevant portion of the pulse shapes for further signal decomposition by a computer farm in real time. A trigger and timing system will carry out complex trigger decisions and distribute the clock and trigger information to GRETINA and its auxiliary detectors. All of the digitizer and trigger modules were produced and tested in 2008, and some of them are in use.
Signal decomposition
In order to perform gamma-ray tracking, the positions and energies of the gamma-ray interactions in the Ge crystal must be accurately determined from the signal waveforms. Each gamma-ray typically interacts via several Compton scattering events, followed by photoelectric absorption. The procedure must handle cases where two or more interactions occur within one of the detector segments. An algorithm to perform this "signal decomposition" has been developed, by combining several methods such as Singular Value Decomposition, adaptive grid search, and constrained least-squares. It utilizes calculated signal waveforms, and incorporates such effects as the preamplifier response and two different types of cross talk. We have shown experimentally that this algorithm can achieve an average position resolution of at least 2 mm.
It is important that the signal decomposition be performed in real time, so that large quantities of wave-form data need not be stored. This requirement means that signal decomposition is expected to form the data acquisition bottleneck; computational speed and efficiency of the algorithm are therefore very important. On the current generation of 2 GHz processors, the algorithm requires less than 10 ms of CPU time per hit segment. With advances in processing power from multi-core CPUs, this performance will be sufficient to meet our requirements. The GRETINA's computer farm will consist of 40 eight-core processors.
Tracking
The tracking process uses the energies and positions of the interaction points produced by the signal decomposition to determine the scattering sequence for a particular gamma-ray. Algorithms have been developed to track events based on Compton scattering, pair-production and photo electric interactions. The tracking efficiencies achieved ranged from ~100% to 50% when gamma-ray multiplicity changed from 1 to 25. The current tracking algorithm needs ~10% of the planned computing power.
Performance of GRETINA
The performance of GRETINA with seven quad-crystal modules is shown in the following table.
Figure 1: A GRETINA quad module prepared for CMM analysis at LBNL. Each module houses 4 36-fold segmented tapered hexagonal crystals inside a common cryostat.
[1] Plus one preexisting module with 3 crystals
[2] Resolution refers to the nominal value, not the effective resolution or effective number of bits
[3] As measured in the final energy spectrum