GRETINA NEWSLETTER

Number 3: July 2004

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Table of Contents:

Welcome from the GRETINA Advisory Committee

Exciting news! Three-Crystal Cluster Prototype arrives in Berkeley

Other Recent Developments:

GRETINA passes CD1 Milestone (Feb 04); Outline of the planned schedule for the project.
GRETINA and GRETA feature heavily in the RIA Facility Workshop at MSU
Summary from the Detector Working Group Meeting, ORNL, 19-20 March 2004.
Summary from the Software Working Group Meeting, LBNL, 21-23 June 2004.
Planned meeting of the Electronics Working Group, ANL, 24-25 July 2004.
Results from recent in-beam tests show a position resolution of 2.4 mm for 36-fold segmented detector
Latest news on the prototype digitizer module
Good news regarding the effect of neutron damage on detector signal shapes
New GRETINA website
Pioneering Ge Array Builders Peter Twin and Bent Herskind win the 2004 EPS Lise Meitner Prize

The GRETINA Users Community and the new GAC Charter

Working Groups

A brief history of the project so far

Useful Links

Welcome from the GRETINA Advisory Committee

Welcome to our third newsletter! Once again we would like to update you on recent happenings pertaining to the GRETINA project since last Fall's CD-0 approval. With the official start of the GRETINA project in October 03, I.Y. Lee was selected as the Contractor Project Manager and a new GRETINA Advisory Committee was formed. (Details about the membership of the latter committee together with its official Charter are given below. Information about the management plan were contained in the previous newsletter.) As always there has been a good deal of activity in the past 6 months or so. There have been several official reviews of GRETINA, first by LBNL, and then another by a DOE appointed committee. In addition, a couple of important workshops have taken place, devoted either to various development aspects of GRETINA, or in which GRETINA(GRETA) have heavily featured. Other similar workshops are planned for later this year. We also include the latest news regarding developments in digital electronics, and important results from an in-beam test with the old 36-fold segmented single-crystal detector.

But to start the newsletter we want to bring you the BIG NEWS and that is The Three-Crystal Cluster Prototype has arrived in Berkeley. This is of course incredibly exciting and marks another major milestone in the project.

We hope you enjoy reading the Newsletter and please don't forget that anyone interested in gamma-ray tracking and GRETINA and/or GRETA is welcome to join the GRETINA-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 GRETINA Users Community meeting at the upcoming Fall DNP meeting in Chicago.

The first newsletter can be found here. Newsletter #2 can be found here.

Three-Crystal Cluster Prototype arrives in Berkeley

On June 3rd, 2004, the three-crystal cluster prototype landed on US soil. Below we see it being carefully unboxed. On the right you can see the three crystals and their segmentation pattern. The third picture shows it on the bench. Obviously this is all very exciting and there are many important tests planned. We should have a lot to tell in time for the next newsletter. Stay tuned!

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Recent Developments

GRETINA passes CD-1 Milestone: Other recent review information and the plan ahead.

Following CD-0, the Conceptual Design Report (CDR) and the Project Execution Plan (PEP) were completed and submitted for technical review. Two reviews were completed in late 2003, a LBNL project vetting review in November and the DOE technical, cost, and schedule review in December. In February 2004, DOE gave the CD-1 (preliminary baseline range) approval. The estimated cost of GRETINA is $17M. The funding profile and schedule of critical decisions are shown below.


Funding profile of GRETINA

CD0 : Mission need     Aug. 2003
CD1 : Preliminary Baseline Range Feb. 2004
CD2A/CD3A : Performance Baseline
           range for long lead time items 		April 2005
CD2B/CD3B : Start Construction Sept. 2007
CD4 : Start of Operation May 2010
Schedule of critical decision timeline for GRETINA

GRETINA and GRETA feature heavily in the RIA Facility Workshop at MSU

A RIA Facility Workshop was held on March 9-13, 2004 at Michigan State University. It was very cold but we were all too busy having fun discussing the future of RIA to notice the weather. Although the workshop was announced with fairly short notice, over 160 participants registered. It was an important opportunity for the machine builders of RIA to interact with the future experimentalists of RIA and discuss their needs and constraints. It was a very practical meeting with operational scenarios, such as delivering beams to simultaneous users, typical lengths of runs, and time required for beam changes being discussed. Other important issues included identifying and specifying the floor space and utility requirements for the experimental instrumentation. With regard to the latter it was no surprise to see GRETINA (updated to GRETA) featured as an essential component for a very large number of core experimental programs at RIA. In fact a great deal of time was spent trying to figure out the best way to move the array from end station to end station, and from experimental hall to experimental hall.

The meeting webpage can be found at http://meetings.nscl.msu.edu/ria2004/talks.php and includes a list of participants and powerpoint or pdf files of all presentations. The summary discussion which included GRETINA (GRETA) can be found in the report section entitled Experimental Area. The organizers request that if you use any information from this site, you properly quote the original source.

GRETINA Detector Workshop, 19-20 March 2004, Oak Ridge

On 19-20 March 2004, a very successful meeting of the GRETINA Detector Working Group was held at ORNL. Topics discussed included:

It was an intense couple of days but we had a lot of fun, and all of the participants felt that this was a very productive meeting.

Copies of the program, the list of participants, slides presented at the workshop, and photos taken at the meeting, are available at http://www.pas.rochester.edu/~cline/Gretina_Det_Workshop/Workshop_Program.htm

GRETINA Software Working Group Meeting, LBNL, 21-23 June 2004

On 21-23 June 2004, a meeting of the GRETINA Software Group was held at LBNL. Once again this was a very productive meeting, with discussion topics including:

You can find details of the program, copies of the presentations, and even some photos of the workshop at http://radware.phy.ornl.gov/greta/SoftwareMeeting2004/

Planned meeting of the GRETINA Electronics Working Group, ANL, 24-25 July 2004

A meeting of the GRETINA Electronics Working Group will be held July 24-25, 2004 at ANL prior to “The Limits” international nuclear structure conference. As with the workshops above we expect this to be another very productive gathering. Issues to be discussed will include:

After the meeting, the list of participants, and slides presented at the workshop will be available via the GRETINA webpage.

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

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An in-beam test of the 36-fold segmented GRETA prototype was carried out at the 88-Inch Cyclotron at LBNL, using a 385 MeV 82Se beam 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. Example spectra with a Doppler shift correction of the 2055 keV gamma-ray line are shown below.

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The bottom spectrum shows the 2055 keV gamma line corrected only using the center of the segment position (28.3 keV FWHM) while the upper spectrum has been Doppler corrected using a first hit position determined by signal decomposition and shows a resolution of FWHM=14.5 keV corresponding to an excellent position sensitivity of 2.4 mm (rms). This is a very significant result indeed.

Latest news on the GRETINA prototype digitizer module

Digital electronics are an integral part of gamma-ray tracking detectors, such as GRETINA, as one is required to sample the preamplifier signal from each segment during charge collection to identify the location and charge deposited by gamma rays scattering through the HPGe Crystal. To accomplish this task, a prototype 8-channel, VME-based, signal processing board (see photo below) has been developed which continuously samples at a rate of 100 MHz with a 12-bit dispersion. The board was also designed to meet the more general requirements for a signal processor board as specified by the Argonne workshop on digital electronics (2001) for low-energy nuclear physics experiments.

This board is differs from most commercial digitizers in that it emulates, in a single large FPGA, much of the functionality found in analog systems for Ge detectors. Segment energies are calculated on the board from the trace obtained from the detectors preamplifier using a digitally implemented trapezoidal shaper. This means that only the part of the trace which contains information relevant to signal decomposition needs to be extracted, rather than the significantly longer trace required to recover proper energy resolution. This dramatically reduces the data which is transferred on the VME backplane and  allows the board to operate at standard Ge rates (several kHz). Constant fraction times, pile-up detection and windowing algorithms (which are responsible for extracting the relevant part of the trace) are also implemented on the board.

Each channel of the board can be independently triggered and three triggering modes are provided; internal, external and validate modes. In internal mode a channel of the signal processing board is triggered by an internal, digitally implemented, leading edge discriminator. Each discriminator is accessible as an ECL logic pulse at the front-panel. In external mode, the channel is triggered by an externally provided ECL logic pulse input on the boards front panel.  In validate mode, the board is triggered if the leading edge discriminator fires followed by an externally provided ECL logic signal which validates the event.

Associated with each event produced by a valid trigger is a 48 bit timestamp, incremented at the digitization rate, which allows for event building between digitizer boards. Each board can be configured to operate from an external clock to synchronize these timestamps.

Each board contains a 1 MB output FIFO to store events before they are read out. These events contain an event header (which includes the energy, leading edge and constant fraction time, segment and board ID) and a section of the trace from the triggered channel.  Mechanisms to support both polled and interrupt driven acquisition systems are provided.  Board configuration is also done over the VME backplane where the integration time for the energy, the position of the trace windows, discriminator levels and constant fraction discriminator parameters can be set. Provision was also made to reprogram the FPGA over the VME backplane.

Several prototype signal digitizers were constructed and successfully used in source tests with clover detectors and in in-beam tests with the GRETA prototype--2, 36-segment detector.

Recently, the production of 15 boards to fully instrument the 3-crystal GRETINA module prototype has been completed and will be used to perform end-to-end detector testing.

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The latest GRETINA digitizer module

Good news regarding the effect of neutron damage on detector signal shapes

Fast neutrons damage the lattice structure of germanium radiation detectors by introducing hole traps. This results in a decrease of charge collection efficiency and a degradation of the energy resolution of the detector [1, 2].

For tracking detector such as GRETINA, it is important to understand if neutron damage degrades also the features of the pulse shape, limiting the position resolution and therefore the performance of the array. It has been estimated that for the GRETINA detector a position resolution of 1-2 mm could be achieved, which is limited by physics factors such as Compton profile (electron momentum) and the finite range of primary electron in germanium. Our interest is to know the amount of neutron damage corresponding to a position error of 1-2 mm.

The effect of neutron damage has been introduced in the pulse shape simulation code through a parameterization of the charge collection efficiency. The number of holes created by the gamma-ray interaction decreases [2] as the holes travel towards the collecting electrode as:

where n0 is the initial number of holes, lambdah is the hole mean free drift length and Delta-r is the distance traveled by the holes. No electron trapping is assumed in the model.

The mean free path depends on amount of traps, crystal type (n- or p- type), applied bias, detector temperature, and temperature history. It is expected to be inversely proportional to the neutron fluence and to vary with the electric field as:

where Ec is a normalization constant (Ec = 2000 V).

The relationship between neutron fluence and lambda has been experimentally measured [3]. At a temperature of 95 K the mean free path of holes corresponding to a fluence of 109 neutron/cm2 is about 100 cm.

The effect of hole trapping on energy and position resolution has been studied as a function of the trapping length in the range from 100 cm to 10 cm, using simulated pulse shapes. The GRETINA prototype detector was used as model in the simulations. It has the shape of a regular hexagon (8 cm diameter, 9 cm length, 10 degree tapering) and is 36-fold segmented. Pulse shapes from the 36 channels have been calculated for a number of interaction points (over 500) covering one segment of the detector, considered as representative segment. The energy is calculated from the signal amplitude; the position resolution is calculated from the difference between pulse shapes with a given lambda and reference pulse shapes, corresponding to the same interaction position. 

Hole trapping results in two effects: 1) reduction of the collection efficiency; 2) the charge induced on the neighboring segments does not integrate to zero; the net charge contribution from all the segments has to be added to the amplitude of the net charge signal.

The energy loss due to hole trapping is a simple function of the distance traveled by the hole. Therefore, knowing the position of the interaction, it is possible to use an empirical formula to recover the energy resolution of the detector.

Fig. 1 shows the energy resolution before and after correction and the position resolution as a function of the trapping length lambda. The energy resolution is given by the FWHM of the peak shape. Neutron damage degrades both the energy and the position resolution of the detector, but the worsening of the position resolution is much slower. If no correction is applied, the critical energy resolution (~ 3 keV, at this energy resolution annealing is required) is reached for lambda ≤ 70 cm. If the energy loss is corrected for the interaction position, DeltaE ~ 3keV for lambda ~ 30 cm, this means that the detector can be exposed to neutrons for a much longer time (more than a factor of 2).  The position resolution corresponding to a trapping length of 30 cm is 0.4 mm, i.e. still better than the specification.

From these results, it can be concluded that before neutron damage begins to negatively impact the capability of reconstructing the position of the interaction from the pulse shape signals it will be necessary to anneal the detector to regain adequate energy resolution for operation. This again is very good news.

[1]  L. S. Darken et al., Nucl. Inst. Meth. 171 (1980) 49.
[2]  W. Raudorf and R. Pehl, Nucl. Inst. Meth. A 255 (1987) 538.
[3]  E. L. Hull, P.hD Thesis, Indiana University, Department of Physics (1998).

New GRETINA Website

A website devoted to all aspects of the GRETINA project has been set up. This site is very much still “Under Construction” but obviously builds upon and is closely linked to the GRETA website. Thus to find out the latest developments in the GRETINA project one must now refer to the new GRETINA site, at http://grfs1.lbl.gov/.

Pioneering Ge Array Builders Peter Twin and Bent Herskind win the 2004 EPS Lise Meitner Prize

Two of the founding fathers of large Ge arrays and modern gamma-ray spectroscopy have been awarded the 2004 Lise Meitner Prize in Nuclear Science from the European Physical Society. The citation honoring  Peter Twin and Bent Herskind reads

"The Prize is awarded for their pioneering development of experimental tools, methods of analysis and experimental discoveries concerning rapidly spinning nuclei, in particular the discovery of superdeformed bands in wide regions of the periodic table."

You can find more details, plus read a more detailed citation letter (motivation.doc) at
http://www.kvi.nl/~eps_np/organization/activities/meitner/2004/meitner_2004.html

We congratulate Peter and Bent on this well deserved recognition.

The GRETINA Users Community, the GRETINA Advisory Committee and its Charter

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

You can contribute or keep up to date with developments by signing up on the web at http://radware.phy.ornl.gov/greta/join.html

The GRETINA Advisory Committee or GAC represents the interests of the GRETINA Users Community. Its present members include Con Beausang (cwb@galileo.physics.yale.edu), Thomas Glasmacher (glasmacher@nscl.msu.edu), Kim Lister (lister@anlphy.phy.anl.gov), David Radford (Chair, (radforddc@ornl.gov), Mark Riley (mriley@nucmar.physics.fsu.edu), Doug Cline (cline@nsrl.rochester.edu), Kai Vetter (kvetter@llnl.gov), Augusto Macchiavelli (AOMacchiavelli@lbl.gov) and Demetrios Sarantites (dgs@wustl.edu).

Remember I.Y. Lee (IYLee@lbl.gov) is now the official Contractor Project Manager for GRETINA. A charter has been composed for the GAC and is listed below:

Charter for the Gretina Advisory Committee

The GRETINA Advisory Committee (GAC) represents the interests of the GRETINA community. The GRETINA Advisory Committee will meet regularly with the Contractor Project Manager, usually by phone conference, to discuss scientific and technical issues. The Committee may meet by itself, or with consultants, if the occasion warrants. It may be consulted by the Management Advisory Committee (MAC) to discuss  problems under review.

Composition
The Committee shall be composed of representatives from the three DOE national laboratories (ANL, LBNL, and ORNL), the NSCL at MSU, and other institutions  engaged in GRETINA.  If necessary, replacement members will be appointed by the Director of the Nuclear Science Division at LBNL subject to approval by the Management Advisory Committee.  Efforts must be made to ensure that the composition of the Committee reflects the interests of the GRETINA scientific community, and no more than one member shall be from any one institution.  The Contractor Project Manager is not a formal member of the Advisory Committee, but will generally participate in Advisory Committee activities.  The Committee shall elect its own chairman who will serve as an ex-officio member of the MAC.

Responsibilities

Please contact any member of the GAC if you have any suggestions, comments or ideas about the GRETINA project.

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.

The chairpersons of the working groups are

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

A brief history of the GRETINA-GRETA project so far

Useful Links

July  2004.