Last updated: 22 March 2004


The Gamma-Ray Tracking Coordinating Committee [GRTCC] was appointed, on 21 January 2002, by the Directors of the Nuclear Science Divisions at the Argonne National Laboratory, Lawrence Berkeley National Laboratory, and Oak Ridge National Laboratory, at the request of the DOE Division of Nuclear Physics. The purpose was to promote the development of γ-ray tracking detector technology in nuclear structure research. The goal is to help organize the γ-ray tracking community, to provide widespread support and to provide an effective plan for the future. The DOE Division of Nuclear Physics uses this committee to obtain timely advice on issues and proposals in γ-ray tracking.


Cyrus Baktash Oak Ridge National Laboratory
Doug Cline (Chair) University of Rochester
Teng Lek Khoo Argonne National Laboratory
Richard Kroeger Naval Research Laboratory
Augusto Macchiavelli Lawrence Berkeley National Laboratory
Mark Riley Florida State University
Michael Thoennessen Michigan State University
Kai Vetter Lawrence Livermore National Laboratory


A National Plan for Development of Gamma-Ray Tracking Detectors in Nuclear Science, July19, 2002:

The GRTCC was asked to prepare a report outlining the physics justifications for gamma ray tracking, establish performance goals, examine current efforts, and formulate a national R&D plan. The report, prepared in response to this charge, is entitled "A National Plan for Development of Gamma-ray Tracking Detectors in Nuclear Science" and was submitted to DOE on 19 July 2002. A copy of this report is available on the web both as a pdf file at or as a Word document at

The GRTCC report outlines the physics opportunities provided using gamma-ray tracking, lists functionality and performance goals for a national 4π tracking array, summarizes current efforts in gamma-ray tracking as of March 2002, and presents a national R&D plan for tracking detectors. The report makes the following recommendations:

  1. A 4π Gamma-Ray tracking facility is an important new initiative within the 2002 NSAC Long Range Plan. This committee unanimously recommends a shell of closely packed coaxial Ge-detectors as outlined in the GRETA conceptual design for this 4π gamma-ray tracking facility. We strongly recommend that DOE support this effort with highest priority.
  2. R&D necessary to demonstrate the full functionality of this detector was identified and has to be addressed immediately. We note that a substantial fraction of this R&D effort is manpower that must be supported.
  3. The R&D phase, the subsequent final design, and the construction of GRETA should continue to be a community effort; in particular, it should involve significant participation by the low energy nuclear physics national laboratories and universities.
  4. Tracking with planar detectors is of interest to the nuclear science community and has a wide range of applications outside of nuclear physics. R&D efforts in this direction should be supported as part of the drive to develop tracking, as most of the electronics and software challenges are common to all tracking detectors.
  5. Gammasphere continues to be the premier national gamma-ray facility until GRETA becomes operational. This research facility must be supported to sustain the vitality of the field.

These recommendations were based on written responses to questions posed to the major gamma-ray tracking detector groups in the US and a fact-finding meeting held 29-30 March at Argonne that was attended by active participants in tracking detector development in this country, plus representatives from Gammasphere and Europe. The recommendations were supported unanimously by the GRTCC, by all attendees at the Argonne Fact-finding Meeting, and by all 65 members of the community who responded to a quick survey performed at the request of the DOE.

Major Gamma-ray Tracking Detector Initiatives for Nuclear Science:

1) GRETA/Gretina

The first conceptual design study for applying γ-ray tracking to a proposed major new detector for nuclear structure physics was done at LBNL in 1994 and an array named GRETA (Gamma-Ray Energy Tracking Array) was proposed. The first prototype 12-fold segmented coaxial Ge detector was tested in 1997; the first working tracking algorithm for Compton scattering was successfully developed and 2D sensitivity demonstrated at LBNL. This success led to the first workshop of GRETA Physics held at LBNL February 1998. A GRETA Advisory Committee was formed April 1998, which later became the GRETA Steering Committee. In 1999 the second prototype 6x6 fold segmented GRETA detector was tested. Pulse-shape analysis of transient and net charge signals was used to obtain three-dimensional position information of individual γ-ray interactions, and the realization of γ-ray tracking algorithms based on the two dominant interaction processes, the Compton and the photo-electric effects. This success demonstrated the crucial first proof of principle for γ-ray tracking in segmented detectors with regard to detector manufacture, signal processing, and tracking algorithms.

This detector was mentioned as a desirable future development project in the February 1996 Long Range Plan for Nuclear Science. It was identified in the 2002 Long Range Plan for Nuclear Science (LRP) as a major new initiative for the field that will provide new capabilities needed to address the exciting science opportunities that exist in nuclear structure, as well as nuclear astrophysics and weak interactions, at existing facilities and at RIA. Moreover, as mentioned above, the March 2003 report of the Ad-hoc Facilities Subcommittee of NSAC identified GRETA as one of three facilities having the highest ranking with regard to science and readiness of all of the large facilities reviewed.

The current design of GRETA is based upon a geodesic configuration, consisting of 120 hexagons plus 12 pentagons arranged in close-packed 4π geometry. It is envisioned that three 36-fold segmented encapsulated hexagonal detectors will be mounted in a common cryostat as a compromise to minimize both the dead layers between and the complications that arise in sharing a common vacuum by many detectors. GRETA will comprise a total of 40 cluster modules plus some single pentagon detectors.

Gretina is the first phase of a staged approach to GRETA. Gretina will have 30 highly-segmented coaxial germanium crystals and is ¼ of the full GRETA.  It will have the capability to determine the energy (with high-resolution) and position (within 1-2mm) of each gamma-ray interaction point and to track multiple gamma-ray interactions using the energy-angle relationship given by the Compton scattering formula. Gretina therefore will have all the intrinsic capabilities of the full 4π array GRETA, albeit with reduced efficiency, and will supercede the capabilities of many existing state-of-the-art arrays.

A triple-crystal detector module was ordered in September 2002, and delivery is expected at the March 2004. The design of this prototype integrates all the technology needed for a complete Gretina/GRETA detector module. Such a module consists of three encapsulated Ge detectors, each with 36 segments, placed in a single cryostat. Each crystal has a partially tapered shape, which maximizes the distance from the source to the detector allowing more space for auxiliary detectors in the target chamber, and optimizing the Ge coverage. Each crystal gives 37 signals (from the 36 segments and one central electrode) amplified with cold FETs mounted in the cryostat. Since such a module may be regarded as the ‘fundamental unit’ from which GRETA will be constructed, by accepting the order the manufacturer has indicated that there are no fundamental fabrication issues for the full array.

The Gretina proposal for the funding of Gretina was submitted to DOE June 2003. The GRTCC was asked to review the Gretina Proposal and DOE signed the CD-0 for Gretina on 21 August 2003. A copy of the GRTCC review of the Gretina Proposal is available. The Gretina Conceptual Design Report was submitted in November 2003, was reviewed by an DOE review panel in December 2003 followed by DOE approval of the CDR in February 2004. A Gretina/GRETA meeting will be held at Oak Ridge on 19-21 March to discuss Ge detector geometry optimization, neutron damge, crystal orientation, impurities distributions, testing procedures, support structure, and target chamber.

More information about GRETA and Gretina is available at the web page as well as at the RIA Workshop talk by I-Y Lee.

2) NSCL SeGA array:

 The National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University has developed the SeGA 32-fold segmented germanium detector array optimized for in-beam γ-ray spectroscopy experiments with fast (v/c = 0.3-0.5) exotic beams. The SeGA array consists of 18 individual detectors. Each cylindrical crystal is 8 cm long and 7 cm in diameter and made of n-type high-purity germanium. The outer p-type ion-implanted contact of the crystal is vertically segmented into four longitudinal segments and horizontally segmented into eight transverse segments. The detectors are placed “side-on” to the gamma-ray source (target) and the transverse segments provide the angular resolution for Doppler correction.

The array is in operation and the design goals have been achieved. The central contact energy resolution varies between 2.5 keV and 2.8 keV. The average resolution of the 32 side channels is 2.5 keV for all but the four segments at the front where the average resolution is 3.3 keV (all resolutions were measured at 1332 keV). Each crystal is about 75% efficient relative to a 3”x 3” NaI detector. Peak-to-total values range from 0.210 to 0.216. The time resolution ranges from 7.0 ns to 9.0 ns (FWHM) for energies greater than 100 keV from a 60Co source. The flexible design of the array was optimized for fast beam experiments. The detectors can be arranged in several configurations with distances to the target varying from 10 cm to 100 cm.

The SeGA array demonstrates that it is possible to manufacture a large number of reliable highly segmented coaxial germanium detectors. It also provides an opportunity to develop signal decomposition and tracking algorithms and this future effort will be coordinated with the development of digital signal processing for Gretina/GRETA and other gamma-ray tracking detectors.


The European efforts in γ-ray tracking are focused on the AGATA project. AGATA, the Advanced Gamma-Tracking Array, is a 4π array of segmented coaxial detectors very similar to the proposed GRETA design. The original design consists of a geodesic tiling of a sphere with 12 regular pentagons and 180 hexagons.  To minimize inter-detector space losses while still preserving modularity, 3 hexagonal crystals are arranged in one cryostat. The pentagonal detectors are individually canned. The inner radius of the array is 17 cm. The total solid angle covered by germanium material is close to 80% and the photo peak efficiency is 50% for an individual 1 MeV γ-ray. The AGATA collaboration includes 38 institutions from Bulgaria, Denmark, Finland, France, Germany, Italy, Poland, Sweden and UK, and the estimated cost for the project is 40MEuros and the manpower 150 FTEs. It could be completed in 8 years.

A significant amount of R&D has been supported by the TMR Network project and included work on simulations and tracking, calculations of pulse shapes, signal decomposition and development of segmented detectors. These efforts resulted in the AGATA proposal. The AGATA results are consistent with those obtained by the GRETA collaboration. A segmented prototype detector with 6 azimuthal and 4 longitudinal segments plus an extra segment in the front was built by Eurisys for the Legnaro-Padova group. Using this detector the collaboration demonstrated that tracking can be used to locate the position of the first interaction point and correct for Doppler broadening. This result is an important milestone in the development of a gamma-ray tracking array. The full analysis of the experiment is still in progress.

The AGATA and Gretina/GRETA designs share a great deal of common technology and therefore the two projects have benefited and will continue to benefit from common developments in all areas of R&D. Currently the AGATA project is about to order a prototype  three-crystal module similar to the GRETA/Gretina prototype that was ordered last year. The recent status AGATA is available on the web.


The EXOGAM array at GANIL, France, comprises 16 Compton-suppressed 16-fold segmented Clover detectors with 4 crystals per Clover cryostat. This system is operational.


The MINIBALL array at REX-ISOLDE will comprise 40 six-fold segmented, encapsulated hexaconical detectors clusters in eight cryostats each holding 3 crystals plus four cryostats each holding 4 crystals. Currently eight three-crystal modules are operating.


The proposed TIGRESS array for TRIUMF Canada will comprise 16 Compton-suppressed 32-fold segmented Clover detectors with 4 crystals per Clover cryostat. A prototype has been tested.  

7)  ANL X-Array

Double-sided planar Ge strip detectors are being developed at Argonne. The initial goal was to build the proposed GARBO array but this now has evolved into the X-Array proposal. A description of current work is available at the RIA Equipment Workshop.

8) Majorana Project:

The Majorana project proposes to construct an array of segmented coaxial germanium detectors comprising 210 crystals, total mass 500kg, using 85% enrichment 76Ge. This detector array will be sited in a deep underground location with the goal of measuring the effective Majorana neutrino mass to as low as 0.02-0.07 eV.


Recent actions related to tracking detectors for nuclear science

A National Plan for Development of Gamma-Ray Tracking Detectors in Nuclear Science, July19, 2002

The recommendations listed in the GRTCC report are given above.

Town Meeting at the APS Fall Meeting held at MSU, 11 October 2002

The recommendations listed in the GRTCC report were briefly summarized by Doug Cline at the Town Meeting.

Gamma-ray Tracking Detector Meeting at DOE, 13 November 2002

A meeting was held at DOE Headquarters in Germantown on 13 November 2002 to discuss the next steps in the development of gamma-ray tracking detectors for nuclear science.  Attendees were Dennis Kovar, Jehanne Simon-Gillo, and Gene Henry of DOE; Doug Cline, Teng Lek Khoo and Mark Riley representing GRTCC, I-Yang Lee and Paul Fallon from LBNL, and Kim Lister and Mike Carpenter from ANL.  

The recommendations of the GRTCC National Plan for Development of Gamma-ray Tracking Detectors were outlined by Doug Cline. The DOE representatives commented that the report was excellent, thorough, informative, while the recommendations were crystal clear and were being folded into the planning for gamma-ray detector proposals.

I-Yang Lee presented a R&D Review of GRETA. He discussed possible cost reductions plus the costs for a staged approach to the GRETA project.  The staged approach was greeted with enthusiasm and it was suggested that a proposal for the first stage comprising a one quarter segment of GRETA be initiated. It was indicated that the cost of this first stage would make it feasible to be included within the near term DOE budget. Subsequently, this first stage, which will comprise 10 three-crystal modules, was given the name Gretina. Another result of this presentation was that the DOE agreed to supplemental funding to support provision of cold FET’s in the three-crystal prototype now on order. This will improve the performance when this three-crystal prototype is delivered in Fall 2003. 

Kim Lister presented a talk entitled HpGeDSSD Tracking R&D and the X-Array. The aim of this work is to continue development of planar germanium detector technology at Argonne as recommended in the GRTCC Report. The goal of the Argonne group is to use the planar detectors for an efficient gamma-ray detector system for the FMA which they will call the X-Array. 

Report of the Ad-hoc Facilities Subcommittee of the Nuclear Science Advisory Committee.

 The Ad-hoc Facilities Subcommittee of the National Science Advisory Committee, chaired by C. Glashausser, was formed at the request of Dr. Ray Orbach (Office of Science and Technology). The charge was for NSAC to consider new and upgraded nuclear physics facilities costing above $50M within the timeframe of the next 20 years. The proposed National 4π Gamma-ray Tracking Detector Array, GRETA, was one of these proposed facilities (along with RIA, the 12 GeV upgrade of JLab, etc). A written report on GRETA was submitted by the GRETA Steering Committee. Also presentations were given regarding the science and readiness of the project at the subcommittee meeting held in February at Rutgers University. The NSAC Subcommittee report selected GRETA, along with RIA and the CEBAF 12 GeV upgrade, as being the projects that have the highest possible ranking both for the science and the readiness. A full copy of the subcommittee presentation given to NSAC in March is available.

Workshop of Equipment for RIA; Gamma-ray Tracking Detectors, March 2003

The gamma-ray detector program for the Workshop on Equipment for RIA was split into three parts. The first part included five talks addressing the physics and functional requirements for gamma detection at RIA. The second part included 10 talks that focused on advances in the development of high-resolution tracking γ-ray detectors since it is obvious that this powerful new technology will play a key role in γ-ray detection for RIA. This section is an excellent review of the current status of high-resolution gamma-ray tracking detectors. The third part entitled “New Concepts in γ-ray Detection” discussed newly developed scintillators and wide band-gap semiconductors. The workshop summary on gamma detectors for RIA was split into the physics plus functional requirements, and then a summary of the two advanced technical sessions plus a summary of the equipment needs for γ-ray detection at RIA. The Workshop reaffirmed that γ-ray detection will be used for a very large fraction of the RIA program. Providing sufficient γ-ray detector facilities to handle simultaneous beams and experiments at many different experimental stations is a problem still to be resolved.  The program is listed below with links to the slide reports for each talk.

Physics Requirements:

Gamma-ray detection for experiments with fast beams     Thomas Glasmacher
Gamma-ray detection for experiments with slow beams Mark Riley
Gamma-ray detection for experiments with ISOL beams  Dave Radford
Physics Opportunities and Functional Requirements for "Offline" γ-Ray Spectrometers Kim Lister
Giant resonances studies with radioactive beams   Michael Thoennessen

High-resolution Tracking Gamma-ray Detectors:

GRETA   I-Yang Lee
GRETINA Simulations Martina Descovich
AGATA Dino Bazzacco
Detector characterization in Europe  Andy Boston
Tigress Helen Scraggs
CNS Ge Array for spectroscopy of fast moving exotic nuclei Susumu Shimoura
Source tests of a Germanium Strip Detector  Sean Freeman & Kim Lister
Digital Pulse-shape Discriminator & Detector Segmentation for the Majorana Project   Craig Aalseth
Role of particle detection in nuclear structure studies with GRETA Demetrios Sarantites
4π Heavy-ion Detector for γ-ray Spectroscopy  C.Y. Wu et al


New Concepts in Gamma-ray Detection

Advanced Gamma-ray Detection Systems  Lynn Boatner
3D Position-sensitive CdZnTe Gamma-ray Spectrometers Zhong He
Cadmium Zinc Telluride (CZT) Detectors Lee Sobotka & Walter Reviol
Mercuric Iodide and Gas Detectors   Ben C. Perry



Physics and Functional Requirements Cyrus Baktash
Gamma-ray Detectors for RIA Doug Cline


Gretina Proposal:

The Gretina proposal for the funding of Gretina was submitted to DOE June 2003. The CD0 was approved August 2003. The GRTCC was asked to review the Gretina Proposal. A copy of the GRTCC review of the Gretina Proposal is available. The Conceptual Design Review presentation occurred at the LBNL in December 2003 followed by DOE approval in early February 2004.

Gretina Detector Workshop, March 19-20, 2004

The Gretina Detector Workshop was held March 19-20, 2004 at the Oak Ridge National Laboratory. This successful workshop reviewed recent developments and led to a list of recommendations for further R&D work. Copies of slides presented at the Workshop are available at the Gretina Detector Workshop web page.