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What is GRETINA ?
GRETINA is a new type of gamma-ray detector to study the structure and properties of atomic nuclei.
It is built from large crystals of hyper-pure germanium and will be the first detector to use the recently
developed concept of gamma-ray energy tracking. GRETINA consists of 28 highly segmented coaxial
germanium crystals. Each crystal is segmented into 36 electrically isolated elements and four crystals are
combined in a single cryostat to form a quad-crystal module. There will be 7 modules in total. The
modules are designed to fit a close-packed spherical geometry that will cover one quarter of a sphere.
GRETINA is the first stage of the full Gamma-Ray Energy Tracking Array (GRETA).
To find out more about the recent history of the project and its current status you are invited to browse through our Newsletters and other significant documents. They contain a great deal of information about recent progress of the project with many
hyperlinks to associated meetings, and other events of interest.
- Newsletter #1 (Jan 2003)
- Newsletter #2 (Oct 2003)
- Newsletter #3 (July 2004)
- Newsletter #4 (Sept 2005)
- Newsletter #5 (Nov 2006)
- Newsletter #6 (Jan 2008)
- Newsletter #7 (March 2009)
Newsletter #8 (Feb. 2010)
- GRETA White Paper for the 2007 NSAC Long-Range Plan
- "Summary and Addendum to the 2007 White Paper (FRIB Workshop, Feb 2010)"
- Summary of 2007 Richmond meeting
The Development and Current Status of the Project
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 has proceeded according to schedule and is planned to be completed in 2011. The dates of Critical Decision are shown in the following table.
Critical Decisions |
Date |
CD0 |
Mission Need |
August 2003 |
CD1 |
Preliminary Baseline Range |
February 2004 |
CD2A/CD3A |
Start Construction of
Long Lead Time Items |
June 2005 |
CD2B/CD3B |
Start of Construction |
October 2007 |
CD4 |
Start of Operation |
March 2011 |
A short summary of the status 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 the first GRETINA production module is shown below.
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 g-ray tracking, the positions and energies of the g-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 GRETINAs 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 g-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 g-ray multiplicity changed from 1 to 25. The current tracking algorithm needs ~10% of the planned computing power.
End-to-End tests
The three-crystal prototype detector has been subject to several end-to-end tests with all of the 111 segments instrumented with signal digitizers. Measurements were carried out using g-ray sources, and with g-rays produced in-beam at both the 88-Inch Cyclotron at LBL and the NSCL at MSU. At MSU, g-particle coincidence data were taken with fragments detected in the S800 spectrograph. The full data analysis process, including event building, signal decomposition, and tracking, was performed. Results indicated that a position resolution of about 2 mm (RMS in all three dimensions) can be achieved with the prototype system.
Performance of GRETINA
The performance of GRETINA with seven quad-crystal modules is shown in the following table.
Detector module |
Number of Ge crystals1 |
≥ 28 |
Number of segments |
6 longitudinal x 6 transverse |
Segment Energy resolution |
≤ 2.5 keV (FWHM) average, at 1.33 MeV |
Noise per segment |
≤ 7 keV (standard deviation) average at 35MHz bandwidth |
Time resolution |
≤ 10 nsec (FWHM) average, at 1.33 MeV |
Array peak efficiency |
≥ 7.2 % at 1.33 MeV |
Array peak-to-total ratio |
≥ 40% at 1.33 MeV |
Position resolution |
≤ 2 mm (standard deviation) average for Eint > 300 keV |
Digital Signal Processing Module |
Digitizer sampling rate |
≥ 75 MHz |
Digitizer resolution 2 |
≥ 12 bits |
Final integral nonlinearity3
(in Egamma) |
≤ ± 0 .1% over the top 99% of the dynamic range |
Final differential nonlinearity3
(in Egamma) |
≤ ± 1% over the top 99% of the dynamic range |
Final energy/gain stability3 |
≤ ± 0.2%/hour gain drift for ≤ ± 5°temperature drift |
Trigger and Readout |
Readout speed |
≥ 10 MB/s/crystal |
Additional functionality |
Accommodate auxiliary detectors in the trigger and the data stream |
Computation |
Data processing rate |
≥ 20,000 gamma/s total |
Data storage rate |
≥ 10 MB/s |
Performance following Signal Decomposition and Tracking |
Efficiency |
≥ 5.4 % at 1.33 MeV |
Peak-to-total |
≥ 55 % at 1.33 MeV |
[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
Users Group
The GRETINA/GRETA Users Community is an organization of scientists interested in the development, and eventual use, of GRETINA/GRETA. Membership of the Users Group is open to all practicing scientists interested in any or all aspects of gamma-ray tracking. You can sign up on the web at
http://radware.phy.ornl.gov/greta/join.html
Management structure
Members of the GRETINA Management Advisory Committee are
• James Symons (LBNL)
• Glenn Young (ORNL)
• Robert Janssens (ANL)
• Konrad Gelbke (MSU)
Members of the GRETINA Advisory Committee are
• Con Beausang (Univ. of Richmond)
• Doug Cline (Univ. Of Rochester)
• Dirk Weisshaar (MSU)
• Kim Lister (ANL)
• Augusto Macchiavelli (LBNL)
• David Radford, Chair (ORNL)
• Mark Riley (FSU)
• Demetrios Sarantites (Washington Univ.)
• Kai Vetter (LBNL/UC Berkeley)
The chairpersons of the working groups are
Meetings
Detector Workshop, ORNL, March 19-20, 2004,
http://www.pas.rochester.edu/~cline/Gretina_Det_Workshop/Workshop_Program.htm
Software Working Group, LBNL, June 21-23, 2004
http://radware.phy.ornl.gov/greta/SoftwareMeeting2004/
Electronics Working Group, ANL, July 24-25, 2004
http://nucalf.physics.fsu.edu/~mriley/GRETINA_ElecWG_July04/Workshop_Program.htm
Software Working Group, FSU Tallahassee, May 31– June 2, 2005
http://fsunuc.physics.fsu.edu/~gretina/Signal_Decomp_Workshop_FSU_June05/Program.htm
Auxiliary Detector Working Group, Washington University, St Louis, January 28-29, 2006
http://fsunuc.physics.fsu.edu/~mriley/GRETINA_Aux_Det_Workshop_Jan06/
Electronics and Trigger, LBNL, April 2006.
Physics Working Group Meeting, FSU Tallahassee August 18-19, 2006
http://fsunuc.physics.fsu.edu/gretina
Physics Working Group, University of Richmond, Richmond VA, Oct. 14-15, 2007
Electronics Working Group, LBNL, May 28-29, 2008
NSCL User Workshop, Michigan State University, June 2, 2008.
http://meetings.nscl.msu.edu/userworkshop2008/
Science Workshop, LBNL, April 23-24, 2009.
Workshop on Physics Opportunities with GRETINA Joint APS/JPS DNP meeting Waikoloa, Hawaii, Oct 13-17, 2009
http://www.physics.fsu.edu/gretina/hawaii2009/workshop.pdf
Publications
I.Y. Lee, Nucl. Instrum. Methods Phys. Res. A422, 195 (1999).
G. J. Schmid et al., Nucl. Instrum. Methods Phys. Res. A430, 69 (1999).
M.A. Deleplanque et al., Nucl. Instrum. Methods Phys. Res. A430, 292 (1999).
K. Vetter et al., Nucl. Instrum. Methods Phys. Res. A452, 105 (2000).
K. Vetter et al., Nucl. Instrum. Methods Phys. Res. A452, 223 (2000).
G.J. Schmid et al., Nucl. Instrum. Methods Phys. Res. A459, 565 (2001).
I.Y. Lee et al., Rep. Prpg. Phys. 66 (2003) 1095
M. Descovich et al., Nucl. Instrum. Methods Phys. Res. B241, 931 (2005).
M. Descovich et al., Nucl. Instrum. Methods Phys. Res. A545, 199 (2005).
M. Descovich et al., Nucl. Instrum. Methods Phys. Res. A553, 535 (2005).
M. Cromaz et al., Nucl. Instrum. Methods Phys. Res. A597, 233 (2008).
J. Anderson et al., IEEE Trans. Nucl. Sci. 56, 258 (2009)
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