Workshop of Equipment for RIA:

Gamma-ray Detection

Oak Ridge March 2003

Gamma-ray Tracking Detectors, March 2003The

The Workshop on Equipment for RIA was held at Oak Ridge, 18-22 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


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. The proposal for the funding of Gretina will be submitted to DOE May 2003

A triple-crystal detector module was ordered in September 2002, and delivery is expected at the end of 2003. 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.

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.