GRETA NEWSLETTER
Number 1: January 2003
![[GRETA Logo]](greta_logo.png){kind=logo}
GRETA NEWSLETTERNumber 1: January 2003 |
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Message from the GRETA Steering Committee
The Gamma Ray Energy Tracking Array
Current Status of the Project and Work Done to Date
Next Steps in the GRETA Development
Digital Electronics Development
Prototype Detector Development
Organization of the GRETA Project
The GRETA Steering Committee
The Gamma-Ray Tracking Coordination Committee
The GRETA Users Community
GRETA Working Groups
GRETA: A brief history of the project
Welcome to the first GRETA newsletter! In it we hope to inform you of the current status of GRETA, tell you about some of the significant recent happenings, and take a peep into the future of the project. We hope to keep you updated on a regular basis but don't forget to visit the official GRETA homepage at http://greta.lbl.gov/ once in a while for more complete details.
Certainly we were all overjoyed with the strong support given to GRETA in the 2002 NSAC Long Range Plan. The latter combined with positive results from the Gamma-Ray Tracking Coordination Committee have led to the first 3-cluster module being ordered this past November from Canberra Eurysis, with a delivery date in late 2003. Exciting work is also continuing on other fronts, such as digital electronics development, and further tests of the 36-fold segmented prototype, see below. Another very important development is that we are presently preparing (with your help!) a proposal to DOE for "GRETINA", which would consist of a quarter shell of GRETA. Such a system would be very powerful and compact allowing it to attack a broad range of important physics questions. If the proposal (and future R&D) is successful, construction of GRETINA could begin in 2005, with completion in 2008. Obviously this is a key milestone on route to the full 4pi GRETA system.
We hope you enjoy reading the Newsletter and please don't forget that anyone interested in gamma-ray tracking and GRETA 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.
GRETA, the Gamma-Ray Energy Tracking Array, will significantly advance gamma-ray spectrometer technology. The GRETA concept uses highly segmented Ge detectors such that all the individual interactions of every gamma ray in the detector can be resolved. Gamma-ray tracking will then identify which interactions belong to a particular gamma ray. Tracking determines the sequence of the gamma ray interactions from the energy and position of the interaction with the help of the energy-angle relation of the Compton scattering and the characteristics of pair production. The gamma-ray energy is obtained by summing only the interactions belonging to that particular gamma ray and the problem of summing two gamma rays is avoided. In addition, by obtaining the scattering sequence the position of the first and second interaction can be determined enabling an improved Doppler-correction capability and increased polarization sensitivity.
Compared to existing gamma-ray spectrometers GRETA will provide
The principle of the proposed gamma-ray energy-tracking array has been successfully demonstrated recently.
The 2002 NSAC Long Range Plan for Nuclear Science identified such a 4pi gamma-ray tracking array as an important new initiative, and made the following statement: "The detection of gamma-ray emissions from excited nuclei plays a vital and ubiquitous role in nuclear science. The physics justification for a 4pi tracking array that would build upon the success of Gammasphere is extremely compelling, spanning a wide range of fundamental questions in nuclear structure, nuclear astrophysics, and weak interactions. This new array would be a national resource that could be used at several existing stable and radioactive-beam facilities as well as RIA". With RIA construction now a priority, it is time for the national (and international) gamma-ray spectroscopy community to come together to design and build this powerful next generation gamma-ray spectrometer facility.
GRETA is envisioned as a national facility. As was the case with Gammasphere, the entire community needs to be involved in the design and construction of GRETA and it is envisioned that GRETA will go where the physics drives it, to both stable and radioactive beam facilities including RIA.
To date considerable progress has been made on the R&D necessary to design and build a tracking array.
The GRETA web page http://greta.lbl.gov provides a succinct summary of the project to date. Some significant milestones are sketched here:
Prototype Detector: A 36 fold segmented GRETA prototype detector was successfully manufactured and extensively tested at LBNL. Tests include source and in-beam measurements. For details see http://radware.phy.ornl.gov/greta/greta_html.pdf.
Position Resolution: A three-dimensional position resolution of better then 1mm at an energy of 374keV was measured by analyzing the pulse shapes of the segment signals. Factors such as noise and crystal orientation effects and their impact on the obtainable position resolution have been studied. For details see http://radware.phy.ornl.gov/greta/pulse_sens_html.pdf.
Signal Decomposition: Several different decomposition algorithms have been developed to determine amplitudes and positions of multiple interactions.
Tracking algorithms: Tracking algorithms that take into account Compton scattering as well as pair production have been developed to identify and separate multiple gamma rays, and to determine the scattering sequence of the individual gamma rays. For details see http://greta.lbl.gov/greta_docs/pdf/tracking.pdf.
The first cluster module: The first 3-cluster module has been ordered; delivery is expected in late 2003.
In summary, the "proof-of-principle" for a gamma-ray tracking array has been established.
Design and construction of a Pulse Shape Digitizer Board was begun early in 2002. It has 8 input channels, and uses 100 MHz 12-bit ADCs.
This project was undertaken to:
This board meets the specifications outlined at the digital electronics workshop held in Argonne (2001) for a general-purpose digital signal processing board for the low-energy nuclear physics community.
Unlike some of the commercially available ADC boards, which perform only waveform digitization, the GRETA prototype board is capable of performing real-time digital signal processing with a functionality equivalent to standard analog electronic systems for Ge detectors.
Currently implemented functions include:
Also provided on the board are three trigger modes (internal, external, and combined) for each channel to allow maximum flexibility. The VME readout of the board is designed for a sustained counting rate of 10 kHz allowing testing of the prototype at counting rates typical of in-beam experiments.
Design of the 8-channel board began at LBNL in Jan. 2002. Full simulations of the complex VHDL code, which implement the above functions, have been carried out successfully. The layout of the board was completed in September and three cards have now been manufactured and populated. The design engineers are currently carrying out tests of the analog and digital sections. Preliminary tests of the board with Ge detectors will be carried out soon. Following successful tests, 15 of these boards will be fabricated and integrated in a 120-channel acquisition system required for the three-crystal Greta module prototype.
Workshops are planned in early 2003 for both the GRETA electronics and software working groups to further familiarize the wider GRETA community with the details of the operation of these boards. These workshops will also serve as catalysts to begin discussion of the full 40-channel signal processing boards required for the full GRETA array which will greatly benefit from the experience gained from the 8-channel unit we have produced.
![[8-channel digitizer board]](8ch_board.jpg)
In June 2002, the GRETA Steering Committee finalized the design of the next detector prototype, a triple-crystal detector module. After the bidding process, the order was placed in September 2002. Delivery is expected in one year.
The goal of this prototype is to develop all the technology needed for the GRETA detector module. With this module, we will test tracking gamma rays across crystal boundaries. After testing, this module could be used in a number of experiments under different conditions including high spin, high-energy gamma ray and high recoil velocity. If this module functions as expected, two more triple-crystal modules with irregular shaped crystal will be acquired. This array of 3 three-crystal modules will have a photo peak efficiency of up to 3% and a P/T ratio of about 50% at 1.33 MeV.
This module consists of three encapsulated Ge detectors, each with 36 (6x6) segments, placed in one cryostat. Each crystal, before it is shaped, has a diameter of 8 cm and a length of 9 cm. To simplify the production of this first module, a regular tapered hexagonal shape was chosen. As shown in the figure, the crystal has a taper angle of 10 degrees between the axis and the center of a flat surface, and the apex of the tapered surface is 15 cm from the front surface of the crystal. Thus, only the first 5 cm of the crystal has a cross section of a regular hexagon (fully tapered), the next 3 cm is partially tapered, and the last 1 cm of the crystal retains the original cylindrical shape of the crystal (not tapered). This shape maximizes the distance from the source to the detector, thus allows for more space for auxiliary detector in the target chamber, and optimizes the Ge coverage of the shell space between 15 and 26 cm. Six longitudinal segmentation lines are placed at the centers of the flat faces. The transverse segmentation separates the crystal into 6 unequal "layers" with thickness of 0.8, 1.4, 1.6, 1.8, 2.0, and 1.4 cm respectively. The crystals are packed closely in one cryostat with minimal gamma-ray absorbing material. The distance between the crystals is 3.5 mm and the distance between the crystal and the cryostat wall is 4.5 mm. Each crystal gives 37 signals (36 segments and one central electrode) amplified with cold FET's (a total of 111 signals) mounted in the cryostat.
![[Triplet cluster prototype]](greta_proto.png)
The GRETA prototype module consists of three tapered Ge detectors, each with 36 (6x6) segments, closely packed in one cryostat.
In order to have broader community participation and to move the project forward more effectively, the following steps have been taken:
The GRETA Steering Committee is comprised of one member from each of the national laboratories, plus four university representatives of the wider community.
The current members of the GRETA Steering committee are:
The Gamma Ray Tracking Coordinating Committee, GRTCC, was formed January 2002 by the directors of the National Laboratories to promote, coordinate and advise on the development of gamma-ray tracking detector technology in nuclear structure research. The membership of the GRTCC comprises:
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 at www.pas.rochester.edu/~cline/GRTCC-report.pdf. The report also is available at http://greta.lbl.gov/ where it is listed under "GRETA Documents" as the first internal report.
The report makes the following recommendations:
A 4pi 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 4pi gamma-ray tracking facility. We strongly recommend that DOE support this effort with highest priority.
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.
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.
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.
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 the attendees at the Argonne Fact-finding Meeting, and by all 65 members of the community who responded to a recent survey.
The GRTCC report outlines the physics opportunities provided using gamma-ray tracking, lists functionality and performance goals for a 4pi tracking array, summarizes current efforts in gamma-ray tracking as of March 2002, and presents a national R&D plan for tracking detectors. Recent DOE funding of R&D for GRETA and other gamma-ray tracking detectors has followed the GRTCC recommendations.
The GRETA Users Community is an organization of scientists interested in the development, and eventual use, of GRETA. 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
Various working groups have been organized with the aim of updating the GRETA physics case, already first outlined in the Berkeley conference of 1998, and developing working collaborations to actively design and construct various elements of the array including geometry, detectors, front-end electronics, data acquisition etc.
The chairpersons of the working groups are
Please contact one of these persons if interested in joining up.
8 Jan 2003.