GRETA NEWSLETTER

Number 2: October 2003

Welcome to the second GRETA newsletter! There has been a great deal of activity so far in 2003 and we would like to update you on this. We have some wonderful news to report regarding the approval of CD-0 for GRETINA, which means there will be changes (hence the “transitioning”) in the management and steering/advisory committee areas, see below. In addition we include the latest news regarding developments in electronics and computing, the status of the three-crystal prototype detector, an in-beam test with the present 36-fold segmented single-crystal detector, and other news of interest to our community.

We hope you enjoy reading the Newsletter and please don't forget that anyone interested in gamma-ray tracking and GRETA and/or GRETINA is welcome to join the GRETA Users Community. Also don't hesitate to contact us if you have any suggestions or concerns about any aspect of the project.

Don't forget the GRETA users community meeting at the upcoming Fall DNP meeting in Tucson, in the Madera room on Thursday, October 30th from 8:15pm to 8:30pm.

The first GRETA newsletter can be found here.

Wonderful News … CD-0 is signed for GRETINA!

In response to the submission of a proposal in June 03 by the GRETA Steering Committee, and years of hard work by a great many people, we are pleased to announce that on August 21^st, Raymond Orbach, Director of the Office of Science, signed approval of the Mission Need Statement for GRETINA as a Major Item of Equipment. This marks Critical Decision Zero (CD-0) for GRETINA, the Gamma Ray Energy Tracking In-beam Nuclear Array, and thus represents the official start to the project.

In the approval letter it was mentioned that GRETINA “will represent the next generation gamma-ray instrumentation, rotating among the Nuclear Physics low energy accelerator facilities, optimizing their scientific productivity and capitalizing on previous investments within these accelerator facilities. GRETINA will provide significant gains in sensitivity in addressing several high priority scientific topics highlighted in the NSAC Long Range Plan, including how weak binding and extreme proton-to-neutron asymmetries affect nuclear properties and how the properties of nuclei evolve with changes in excitation energy and angular momentum. A successful implementation of this detector represents a major advance in gamma-ray tracking detector technology that will impact nuclear science, as well as detection techniques in homeland security and medicine.”

[Gretina]

GRETINA will consist of 30 highly segmented coaxial germanium crystals (10 triple-crystal modules) 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 will be a unique and extremely powerful detector system, easily surpassing the capabilities of existing detector arrays in many critical areas, and is an important step toward GRETA.

A copy of the GRETINA proposal which includes detailed discussion of physics topics, technical developments, geometry, and management issues, is available here.

Management Plan for GRETINA

While the final details of the management plan for GRETINA are still being formulated, a preliminary version is contained in the GRETINA proposal. Two of the senior committees will be the Management Advisory Committee and the GRETINA Advisory Committee.

Lawrence Berkeley National Laboratory has been designated as the Host Laboratory for GRETINA and we expect that I-Yang Lee will be appointed as the Contract Project Manager.

The Management Advisory Committee (MAC) will serve as an oversight committee to the project. It will include the Physics Division Directors of the DOE National Laboratories directly involved in the construction of GRETINA (ANL, LBNL, and ORNL), the Director of National Superconducting Cyclotron Laboratory at MSU, and the chair of the GRETINA Advisory Committee. It will ensure that the different tasks of the project at the non-host sites are proceeding on schedule and on budget, as well as providing general oversight at the host site. It will arrange such reviews of the project as it deems necessary to fulfill these responsibilities. It will also be responsible for the appointment of the GRETINA Advisory Committee.

The GRETINA Advisory Committee (GAC) represents the interests of the community. Specifications for GRETINA will be developed in concurrence between the GRETINA Advisory Committee and the Contract Project Manager. The GRETINA Advisory Committee will meet regularly with the Contract Project Manager, to discuss the scientific and technical issues. This committee will also maintain the GRETINA Users Group and facilitate communication between the project management and the Users Group.

Recent R&D Developments

Since the last newsletter there has been a great deal of progress made in several important aspects of the project.

Computing

Work has been carried out at ORNL to optimize the signal decomposition algorithm that locates interaction points in the crystal from the signals induced on the detector contacts. This optimization involved several code improvements and the replacement of the generalized commercial constrained minimization algorithm with a least-squares fitting algorithm that is highly optimized for the specific GRETA decomposition problem. A large speed-up factor of about 300 was achieved. These modifications to the decomposition algorithm have also improved the quality of the solutions, giving lower chi-squared fits for 70% of the cases.

Digital Electronics

The design of the LBNL 8-channel digitizer board for the GRETINA module test has been completed and three prototype boards produced. A comprehensive set of tests was carried out at LBNL on these boards both with standard sources using clover detectors and in an in-beam test with the 36-fold segmented GRETA prototype detector. The digitizer boards had both energy and time resolution comparable to current analog systems. Synchronization tests between boards also demonstrated the global timestamp mechanism required to correlate events between crystals works well. Currently one of the prototype digitizer boards is at ORNL for testing and a second will be sent to NSCL shortly in keeping with the agreement made at the Digital Electronics Workshop (Argonne, 2001) to share R&D carried out for GRETINA with the wider low-energy nuclear physics community.

From the tests carried out this year several fixes and improvements necessary for the GRETINA module test have been identified and implemented into the design of a second prototype board which is currently undergoing final layout. Fifteen of these boards will soon be produced to provide the 120 high-speed digital channels required to instrument the GRETINA module prototype.

Concurrent with the digitizer tests, a high-speed data acquisition system has been developed for the module test. This VME based system employs a single Power-PC based embedded processor card running vxWorks and has a maximal data rate of 15 Mb/s. This system was designed to be simple yet have the high data throughput necessary to carry out the module test at realistic data rates. Data will be stored directly to disk via standard network protocols to enable rapid analysis of the data collected. The data acquisition system was employed in the both source tests and in-beam tests and was found to perform well.

Recent In-Beam Test with the 36-fold Segmented Detector

An in-beam test of the 36-fold segmented GRETA prototype was carried out at the 88-Inch Cyclotron at LBNL in July 2003. A 385 MeV ⁸²Se beam was used to bombard a ¹²C target. This reaction was chosen to maximize the Doppler shift, and it produced ⁹⁰Zr nuclei with a recoil velocity of 0.087c. The detector was placed at a distance of 4 cm from the target, at an angle of 90 degrees to the beam direction. Pulse shape data from 24 of the 36 segments were taken with three 8-channel digitizer modules described above. Preliminary Doppler shift correction of the 2055 keV gamma-ray line, using only the angle to the center of segment, has shown an energy resolution improvement consistent with expectations. Full data analysis is underway using the recently optimized signal decomposition program (discussed above) and the existing tracking program.

The Three-Crystal Cluster Prototype

The latest news from Canberra-Eurysis is that they are hoping to deliver the three-crystal cluster prototype by the end of November this year.

The GRETA Users Community

The GRETA Users Community is an organization of scientists interested in the development, and eventual use, of GRETA (and now GRETINA.) Membership of the GRETA Users Community is open to all practicing scientists interested in any or all aspects of gamma-ray tracking and GRETA.

You can sign up on the web at http://radware.phy.ornl.gov/greta/join.html

Working Groups

Various working groups have been organized with the aim of updating the physics case, and developing working collaborations to actively design and construct various elements of the array including geometry, detectors, front-end electronics, data acquisition etc.

Please contact one of these persons if interested in joining up and helping in any aspect of the project.