******************************************************************************* Edition 2, No. 5 December 21, 1994 Price: FREE *******************************************************************************Contents:
Editor: Carl J. Gross, Scientific Liaison, cgross@orph01.phy.ornl.gov Contributors: Jim Bailey, Matthew Brinkman, Bill Milner, Michael Smith, Charles Jones David Olsen.******************************************************************************* *******************************************************************************1. Town Meeting on Radioactive Ion Beams.
As mentioned in our previous announcements, the Nuclear Structure, Low Energy Nuclear Physics, and Radioactive Ion Beam Town Meeting will be held in Durham, NC, on January 19-21, 1995. The meeting is designed to provide "grass roots" input to the Nuclear Science Advisory Committee (NSAC) which will outline recent nuclear science achievements to the DOE and NSF and set priorities in this field for the next decade. In particular, NSAC is charged with developing the next Long-Range-Plan which should be submitted to DOE and NSF in the fall of 1995. We encourage you to attend this meeting not only to indicate your support of RIB Physics, but also to provide detailed input into planning the future of American nuclear science. We also wish to emphasize that accelerator-based nuclear astrophysics is to be included in this meeting.
This town meeting is being organized by NSAC members Richard F. Casten (BNL), Calvin Howell (Duke University/TUNL), and Witold Nazarewicz (University of Tennessee). The meeting will be held at the Sheraton University Center, which is located near the Duke University campus and the Triangle Nuclear Physics Laboratory. For reservations call (800) 633-5379 or (919) 383-8575. To obtain the special rate of $69 per night, state that you are attending the "Nuclear Physics Long-Range-Plan Town Meeting" when you make reservations. The special $69 rate is for single or double occupancy.
2. Election of New Officers to the Users Executive Committee.
Two new members of the HRIBF Users Executive Committee have been elected. Carrol Bingham of the University of Tennessee and Joe Hamilton of Vanderbilt University will take office on January 1, 1995, and will serve three-year terms. Carrol and Joe replace Cyrus Baktash of ORNL and Bill Walters of the University of Maryland and will join Rick Casten (BNL), Art Champagne (UNC), Cary Davids (ANL), and Ed Zganjar (LSU) on the committee. Art Champagne will serve as the 1995 Chairman.
During the past year a new Users Charter has been written for review and approval by the 1995 Executive Committee. This year's committee has also recommended possible candidates to serve on the HRIBF Program Advisory Committee (see Sept. 30, 1994, HRIBF Newsletter); advised on the pros and cons of Letters of Intent to do research at HRIBF and on target/ion source issues; and provided guidance on other topics of concern to the Users.
HRIBF wishes to thank Cyrus and Bill for their service. Many thanks are also given to outgoing chairman Ed Zganjar for his work during the transition period between the demise of HHIRF and its rebirth as HRIBF.
3. Are Bunching and Pulsing of RIBs Necessary?
Although the original RIB proposal did not address the question of bunched or pulsed beams, the possibility to use such beams at HRIBF will be available. The most obvious disadvantages with the bunching of RIBs are the loss in intensity which is on the order of 25-50% and the additional complexity to an already complex system. The intensity is further reduced by a factor of two or more if pulsing is required. Pulsing a bunched beam, which requires a chopper, provides a much cleaner beam comprised of narrow pulses with little or no beam between the pulses (dark current). The dark current with bunched beam is substantial, the other 25-50% of the beam, but it is spread out between the bunches. The advantage of beam bunching is the reduction in the background due to non-beam-related events and the ability to do time-of-flight measurements. In addition, measurements related to time such as isomer lifetimes are feasible.
The current feeling among the local experimenters is that beam bunching is highly desirable if no significant loss of intensity can be achieved. Although no plans exist at present to upgrade the existing system, it should be possible to design a buncher which is 75-80% efficient. For most experiments, the overall bunching requirements are fairly loose with pulse widths of 5 ns or less and pulse separations of about 200 ns. Dark current effects can be gated out electronically using TACs or TDCs.
The present HRIBF buncher, which was originally designed for injecting beam into ORIC, is a variable frequency, highly efficient buncher which can operate from 16 MHz (62.5 ns) to 4 MHz (250 ns). Typically, bunched beams are available with approximately 50% of the original beam intensity in the bunch, while pulsing the bunched beam to remove dark current reduces this intensity another 50%. The pulse width of the beam is 1/60th of the rf frequency, i.e., at 16 MHz the beam width is approximately 1 ns and at 4 MHz is 4 ns. In addition, with the low intensities expected for RIBs, the standard capacitive pickup circuit which provides timing signals will not be effective. In some cases, it may be necessary for the experimenter to provide a signal from the target which can provide the necessary timing information.
Are there experiments which require more stringent timing characteristics? If so, please contact the User Liaison Officer. For more information on the current system at HRIBF, the appropriate references are: N.F. Ziegler, et al., Nucl. Inst. Methods Phys. Res. A268, 509 (1988); W.T. Milner, et al., IEEE Trans. Nucl. Sci. NS-26, 1445 (1979).
4. Gamma-Ray Detection Array Upgrade.
A proposal to upgrade the HRIBF gamma-ray detection array to be used in conjunction with the Recoil Mass Separator has been sent to DOE for technical review. This proposal calls for the purchase of 11 modified EUROGAM-type Clover detectors. Each "Clover" detector will consist of four 35% high-purity Ge (HPGe) crystals arranged into a clover-leaf pattern using a single cryostat and sharing a common BGO Compton-suppression shield.
Each of the HPGe crystals are electrically isolated and can be treated as a separate detection element. A large enhancement in photopeak efficiency can be achieved, however, by operating in add-back mode where the entire Clover is treated as a single detector. Initial results from EUROGAM Phase II indicate a 44% enhancement in photopeak efficiency for Clovers operated in add-back mode even for high-multiplicity events for which the probability of two distinct gamma rays entering the same Clover detector is not negligible. Other advantages of Clover detectors include better energy resolution than can be achieved using 70% HPGe detectors (as found in GAMMASPHERE) and the ability to use Clover detectors as polarimeters for undertaking linear polarization measurements. A prototype Clover detector has been ordered from EURISYS MESURES (formerly, Intertechnique) in Strasbourg, France. This prototype is expected to arrive in April of 1995.
The 11 modified Clover detectors will be added to 12 of the existing 25% Compton-Suppressed Spectrometer (CSS) elements. This detector arrangement will consist of six rings of detectors. The rings will consist of 1) 8 CSS at 58 degrees, 2) 6 Clovers at 90 degrees, 3) 4 CSS at 122 degrees, 4) 3 Clovers at 132 degrees, and 5) 2 Clovers at 155 degrees. When complete, the upgraded gamma-ray detection array will provide an absolute total photopeak efficiency of slightly greater than 2.6% for 1.33-MeV gamma rays, and maximal sensitivity for measuring both transition multipolarities and linear polarizations.
For further information on Clover Detectors, see F.A. Beck et al., Proceedings: Workshop on Large Gamma-Ray Detector Arrays, Chalk River, p. 364, May 1992.
5. Training for Experimenters.
Pursuant to DOE regulations, after April, 1995 all persons desiring unescorted entry to Radiation Areas or Radioactivity Storage Areas containing sealed sources must have appropriate training. For example, this requirement must be fulfilled if the experimenter wishes to handle sealed radioactive sources, handle a target that has been in beam, or even enter a target room after beam has entered but before the room has been surveyed by personnel from the ORNL Office of Radiation Protection (ORP). In effect, if you wish to be able to control your experiment, you (and your colleagues) must be trained in radiological safety. More information about HRIBF training requirements will be provided as they become available.
6. Happy Holidays!
We at HRIBF want to wish you and your families a very happy and safe holiday season. We thank you for your support this past year and hope you will continue that support next year!
****************************************************************************** ****************************************************************************** Jerry D. Garrett, Scientific Director |Email: garrett@orph01.phy.ornl.gov Mail Stop 6368 |Tel: (615) 576-5489 Carl J. Gross, Scientific Liaison |Email: cgross@orph01.phy.ornl.gov Mail Stop 6371 |Tel: (615) 576-7698 Holifield Radioactive Ion Beam Facility |Tel: (615) 574-4113 Oak Ridge National Laboratory |Fax: (615) 574-1268 Oak Ridge, TN 37831