HRIBF NEWS


Edition 5, No. 1 August 15, 1997 Price: FREE

Contents:

Editor: Carl J. Gross

Contributors: J. C. Batchelder, J. D. Garrett, C. J. Gross, D. C. Larson, M. S. Smith, D. W. Stracener


1. Call for New Proposals

Proposals are now being accepted for the next series of experiments at the HRIBF. Proposals are solicited for experiments with beams of radioactive 66Ga and 67Ga with energies between 3.4 and 360 MeV, 69As and 70As with energies between 3.4 and 380 MeV, 17F and 18F with energies between 3.4 and 210 MeV, and 56Ni with energies between 3.4 and 360 MeV. Users desiring higher or lower beam energies or beams of heavier gallium or arsenic isotopes should contact Jerry Garrett. THE DEADLINE FOR THE RECEIPT OF THESE PROPOSALS IS THURSDAY, SEPTEMBER 11, 1997. Twelve copies of each proposal should be sent to:

Jerry Garrett
Bldg. 6000, Mail Stop 6368
Oak Ridge National Laboratory
P.O. Box 2008
Oak Ridge, TN 37831-6368.
About 107 ions per second (i/s) of 66Ga, 67Ga, 69As, and 70As are now available. Somewhat more intense beams of these isotopes might be available late in the scheduling period. The 70As beam is highly contaminated with its isobar 70Ge. Flourine and 56Ni beams are being developed (see the following article). Users should anticipate up to about 108 i/s of 56Ni and 107 i/s of 17F or 18F late this year. The 56Ni beam will be contaminated with 56Co.

As in the past, users should provide information on the minimum beam required to obtain useful results from their proposed study. Such information is crucial for evaluating the requests for early experiments and should be carefully substantiated in the experimental writeup. The past policy of accepting proposals for specific experiments, and not for ongoing scientific programs, will be continued.

Requests for measurements using stable beams will be considered for:

(i) commissioning equipment;
(ii) experimental measurements associated with a radioactive beam project; and
(iii) measurements in which the experimental facilities at the HRIBF are uniquely suited for the proposed study.
Arguments substantiating that stable beam work falls into one of these categories should be contained in the experimental writeup. However, projects involving radioactive beams will be given preference over those based on purely stable beams. A list of stable beams which have been accelerated is given on the HRIBF web site. If other stable beams are desired, please contact Jerry Garrett or Carl Gross.

The status of experimental equipment available is discussed in an additional article in this newletter.

The cover pages of the HRIBF proposals and guidelines for their preparation are available on the HRIBF web site, "www.phy.ornl.gov".

2. Recent On-line Tests for the Production of 17F

We have recently completed a series of experiments to study the usefulness of alumina fibers (3-5 micron diameter) and alumina atomic clusters (1 micron diameter) for the production of 17F ions using the 16O(d,n)17F reaction. Using a low-intensity deuteron beam from the Tandem and the capabilities of the UNISOR separator, we tested samples of alumina from three manufacturers in an electron beam plasma (EBP) positive-ion source.

The alumina fibers are held together by a SiO2 binder to form a felt material. The amount of SiO2 varied with vendor. The sample with the best performance had the least amount of SiO2. Sodium was a major contaminant in one sample, but was found in negligible amounts in the sample with the best performance. The alumina atomic clusters showed some promise; a relatively high yield was observed at lower target temperatures, but within a few hours the yield decreased dramatically due to sintering of the target material.

The alumina fibers have proven to be the best target material investigated so far for the release of 17F ions. About 90% of the observed 17F activity extracted from the source was observed in a single form, the molecule AlF. The range of operating target temperatures is 1450 - 1600 C which is easily achieved with the source design we are using. At temperatures above 1600 C, the source efficiency is reduced due to a large Al current from the decomposition of the Al2O3. In addition, the target remains viable over a relatively long period. We recently investigated an alumina fiber target which had been maintained at a temperature above 1450 C for 16 days and found no decrease in the measured yield of 17F.

From our current positive-ion EBP source, the best efficiency measured for 17F (in the form of AlF+) was 0.1%, which corresponds to a yield of 17F from the ion source of 6 x 106 i/s/uA of deuterons. Assuming 20 uA of deuterons from ORIC, this would correspond to approximately 2 x 106 i/s of 17F on target when corrected for charge exchange (8%) and tandem transmission efficiencies (20%).

We have developed and are completing off-line tests of a negative surface ionization source with a negative-ion efficiency for 17F, which is more than an order of magnitude larger than that measured for the positive-ion EBP source and charge-exchange cell combination. We should be able to deliver at least 107 17F with i/s on target for radioactive ion beam experiments.

3. Experimental Equipment Status

Several short descriptions on the status of the HRIBF experimental endstations are given below. More detailed information, such as expected efficiencies and installation completion dates, is given in articles 13-15 of this newsletter. More information may be obtained by contacting the mentor of each device directly or Carl Gross and Jerry Garrett.


Device Mentor email address

Astrophysics Michael Smith msmith@mail.phy.ornl.gov
Reactions Jorge Gomez del Campo gomez@mail.phy.ornl.gov
RMS and Focal Plane Detectors Carl Gross cgross@mail.phy.ornl.gov
RMS Target Arrays Cyrus Baktash baktash@mail.phy.ornl.gov
UNISOR Separator Ken Carter carter@mail.phy.ornl.gov

Astrophysics endstation:
The astrophysics endstation is centered around the Daresbury Recoil Separator, which is presently undergoing commissioning tests with stable beams. It is anticipated that this system will be ready for user experiments in Spring 1998.

Reaction Endstation:
The reactions endstation is centered around the Enge Split-pole spectrometer (Enge) which is presently undergoing refurbishing. A new focal plane detector system has been designed and should be tested in the fall. It is hoped that the Enge will be available for experiments by the end of 1997.

Nuclear Structure Endstation:
The nuclear structure endstation is centered around the recently commissioned recoil mass spectrometer (RMS). Experiments which have been run successfully with the RMS and important dates are:

Spin Spectrometer (SS):
The SS, 70 NaI detectors in a 4-pi geometry, has not been used for several years and will need a few months for preparation before an experiment may be scheduled.

UNISOR Isotope Separator:
UNISOR is presently being used for on-line testing of RIB target-ion sources. It has three beamlines for laser studies, beta-decay tape stands, and a He-dilution refrigerator.

All-Purpose Beamline (APB):
The APB may be used for a user-supplied chamber or test stand. It is ideal for testing of detectors or simple experimental setups.

4. First HRIBF RIB Experiment Completed

HRIBF's first radioactive ion beam experiment, the Coulomb excitation of radioactive 69As was completed June 6, 1997. This experiment was led by Charles Barton of Clark University in collaboration with experimenters from Yale and Brookhaven National Laboratory. The HRIBF provided radioactive beams of 69As and 67Ga for 76 hours in the time period June 1-6 for this experimental study.

5. HRIBF Users Group Meeting at Whistler

The annual HRIBF Users Meeting will take place at the Autumn Division of Nuclear Physics Meeting being held this year at Whistler, British Columbia. The HRIBF Users meeting is scheduled from 5 to 6 p.m. Monday October 8 in Empress Room A at Chateau Whistler, the conference location. Some refreshments will be provided.

6. Observation of Ground State Proton Emission from 145Tm

Proton emission from the 145Tm ground state has been observed for the first time using the Recoil Mass Spectrometer with a double-sided Si strip detector at its focal plane. The 145Tm detected were populated using the 92Mo(58Ni,p4n) reaction. The energy and halflife of 145Tm were determined to be ~1.7 MeV and ~3.5 us, respectively. This is the shortest groundstate proton decay yet measured.

The halflife of 145Tm is sufficiently short for proton emission to dominate its decay resulting in a nearly 100% proton decay branching ratio. Therefore, the usual uncertainties in extracting spectroscopic factors from proton-decay data associated with uncertanties in the competition between proton and positron decay is circumvented in this case. Such uncertainties are usually the largest contributor to the assigned errors for the spectroscopic factors.

7. Summary of the Workshop on the Science for an Advanced ISOL Facility

A Workshop on Physics with an Advanced ISOL Facility, organized by Argonne National Laboratory and Oak Ridge National Laboratory, was held July 30 - August 1 at Ohio State University in Columbus, OH. Though the workshop was organized quickly during the traditional summer vacation time, it was attended by over 150 participants. About 60 contributions were given on scientific opportunities which could be addressed using a National ISOL Facility, with a wide variety of intense radioactive beams.

The ideas presented at this workshop will be incorporated in a White Paper to be written by a committee composed of Cyrus Baktash, Jim Beene, Rick Casten (chair), John d'Auria, Jerry Garrett, Gregers Hansen, Walter Henning, Masayasu Ishihara, I-Yang Lee, Witek Nazarewicz, Peter Parker, Ernst Rehm, Guy Savard, and Rolf Siemssen. This White Paper will be submitted to the Nuclear Physics Program Office at DOE, where it will be used to substantiate a "Mission Needs" document for the construction of an advanced ISOL Facility in the U. S.

8. New Scientific Personnel at the HRIBF

Several new researchers have joined the scientific staff of the Physics Division and will be working on HRIBF programs. New scientific staff members include Krystztof Rykaczewski from Warsaw University and Alfredo Galindo-Uribarri and David Radford both from AECL, Chalk River. Felix Liang and Shashi Paul have recently assumed postdoctoral appointments and are working with scientific programs at the HRIBF. Felix was a postdoc at the University of Washington, Seattle, and Shashi is a recent graduate of Bombay University.

9. Thielemann Appointed Visiting Distinguished Scientist at ORNL

Fredrich "Friedel" Thielemann, Professor of Astrophysics at the University of Basel, in Basel, Switzerland, has accepted an appointment as a Visiting Distinguished Scientist in the Physics Division at Oak Ridge National Laboratory. In such a position Friedel will maintain an active involvement in the nuclear physics and nuclear astrophysics programs at ORNL.

10. Users Group Executive Committee

The newly elected members of the Users Group Executive Committee are Noemie Koller from Rutgers University and Michael Smith from ORNL. The committee members had a conference call meeting in late January and elected Michael Smith as vice-chairperson. Michael will succeed Joe Hamilton next year as chair.

11. Training for Experimental Work at HRIBF

All nuclear physics facilities operated by the DOE require training to mitigate hazards associated with the facility. At HRIBF, we have developed a Training Requirements Matrix to associate tasks with training. For most experimental work, modules on Unescorted Access, Experiment Safety, Radiation Safety, Hazard Communication, and Electrical Safety will suffice. The good news is that we have developed a Division-level procedure on Radiation Safety designed to meet needs of most users. This seven-page training module allows users to enter and work in areas posted as a Radiological Buffer Area or Radiation Area, use sealed radioactive sources (including thin-window alpha sources), and generally handle experiment materials after exposure to beam. However, if your experiment at HRIBF requires a) work in other radiological areas (for example, a posted contamination area), b) performance of activities expected to generate radioactive contamination, or c) handling unsealed radioactive sources, you must have (or arrange to take) DOE Radiological Worker II training. We believe this new module will be a significant time-saver by shortening training time for many HRIBF Users.

Each training module has an associated test, with a required grade of 80% to pass, and must be completed prior to beginning experimental work. Determining training requirements for users is the responsibility of the HRIBF Scientific Director, and your input on expected activities will help ensure proper training is obtained. Study guides can be obtained through the Scientific Director prior to your arrival, and the tests will be administered locally after you arrive.

12. Second Oak Ridge Symposium on Atomic and Nuclear Astrophysics

December 2-6, 1997, ORNL will host the Second Oak Ridge Symposium on Atomic and Nuclear Astrophysics. This Symposium will focus on stellar atmospheres, stellar evolution, stellar explosions (novae, supernovae, and x-ray bursters), pregalactic and galactic chemical evolution, the interstellar medium, and atomic and galactic chemical evolution, the interstellar medium, and atomic and nuclear data for astrophysics. The symposium will consist of invited presentations, invited posters, and contributed posters covering observations, modeling, and the atomic and nuclear physics foundations (data, experiments, and theories) essential to understanding these astrophysical objects and events.

Correspondence concerning the symposium should be directed to:

Ms. Althea Tate
Symposium Secretary
Physics Division
Bldg. 6003, MS 6373
P.O. Box 2008
Oak Ridge, TN, 37831-6373

Tel. (423) 574-4576
email: astrosym@mail.phy.ornl.gov.
Additional information can be obtained at the symposium web site "www.phy.ornl.gov/workshops/2orsymp/2orsymp.html".

13. Astrophysics Endstation

Daresbury Recoil Separator (DRS):
Installation of the DRS components was completed in June 1997, and these components, the upstream beamline, and two detector systems (a carbon-foil microchannel plate and a gas ionization counter, described below) are currently being commissioned with stable beams. At Daresbury Laboratory, the DRS had measured acceptances of 6.5 msr in solid angle, +/- 1.2 % in A/Q, +/- 2.5 % in velocity, and +/- 5 % in energy. It had a measured mass/charge resolution at the focal plane of 1/300 with a dispersion of 0.1 %/mm. These parameters are not expected to change in the current installation at ORNL. However, for measurements of capture reactions in inverse kinematics, the DRS will be tuned differently to collect one mass (the capture recoils) at the focal plane and to maximize the rejection of scattered beam particles. The two DRS velocity filters are performing as expected, and their electrostatic plates have been conditioned to voltages of over +/- 150 kV.

Carbon-foil Microchannel Plate Detector:
This detector, built and used at Daresbury Laboratory, uses a microchannel plate backed by a position-sensitive (resistive-film) anode to derive fast timing and position information on incident recoils - from electrons ejected when recoils pass through a thin (20 ug/cm2) carbon foil. The timing and position resolution were measured at Daresbury Laboratory to be 0.2 ns and 0.2 mm, respectively. The carbon foil intercepts the beam at a 30-degree angle, closely aligned with the DRS focal plane.

Ionization Chamber:
This detector, also built and used at Daresbury Laboratory, has a thin (50 ug/cm2) polypropylene window, a cathode, a Frisch grid, and a three-segment anode structure. The detector typically operates with 20 torr of isobutane gas. The three anode segments (of lengths 50, 50, and 100 mm) allow two energy-loss measurements and one residual energy measurement, respectively, of the incident recoils. The energy loss and residual energy information are used to identify the nuclear charge of the recoils, and the two energy loss measurements are used to reject events in which recoils scatter off detector gas molecules. The window assembly includes a biased grid of thin wires which keeps the entrance window flat under pressure and which maintains an even electric field gradient near the detector entrance. Some ray tracing capability will be available between the gas ionization counter and the microchannel plate detector.

Silicon Detector Array:
We are constructing a new annular array of 128 Si detectors (of thickness 300 um and an array diameter of 13 cm) in a second, larger DRS target chamber for measurements of scattering and transfer reactions in inverse kinematics. The target chamber for the array is currently under construction and is expected to be completed in September. Some of the electronic components are also under construction. We anticipate assembly of the array in Fall 1997 and commissioning tests with stable beams until Spring 1998.

Future Systems:
Future detector systems that will be used with the DRS include the ORNL-MSU-TAMU Array of Barium Fluoride detectors around the DRS target chamber, a moving tape system at the DRS focal plane, and a second timing detector at the focal plane which will enable time-of-flight measurements for better particle identification. These systems will not be available during this proposal period.

14. Reactions Endstation - Enge Split-pole Spectrometer

The Enge was fabricated by Scanditronix from the design of Spencer and Enge (NIM 49, 181 (1967)). It has a maximum design field of 16 KG (typically ~14 KG) with Rho(max) of 90.9 cm and Rho(min) of 30 cm. A focal plane detector is being developed and consists of a Position Sensitive Avalanche Counter (PSAC) which is 36 cm long. Behind the PSAC, a plastic scintillator of the same length will be used to stop the reaction products and provide an energy signal. It can be operated in a high vacuum mode (10-7 torr) or in a gas-filled mode (~10 torr). Computer programs are available to help in the setup of the Enge. At present, the old hybrid counter is on loan to another institution, but may be recalled if needed. Detectors may be placed in the 18-inch-diameter scattering chamber in fixed or movable mounts.

15. Nuclear Structure Endstation

Recoil Mass Spectrometer (RMS):
The RMS can be run in two modes: diverging and converging mass solutions. The best understood mode is the diverging solution. In this mode the energy acceptance of the device is +/-10%; a mass resolution (reaction-dependent) of M/dM=450 has been observed; and the A/Q acceptance is +/-4.9%. Typical RMS production/detection rates are 3% per charge state for pure nucleon evaporation channels and 2% per charge state for those channels involving alpha particle emission.

In the converging mass mode, the mass resolution has been measured to be M/dM=350 and the A/Q acceptance can be as high as +/-4%. The differences between modes come from the variable strength of the quadrupole doublets and reversed polarity of the last two quadrupoles. More tests need to be done on this solution before experiments requiring this mode are scheduled.

Ge Array:
The new Ge support structure is in place and can be loaded with 24 Ge detectors. Currently, 6 Clover Ge detectors and 10 BGO Compton-suppressed, 25% Ge detectors (CSS) are available for use in experiments. Each Clover detector has an efficiency of 0.3% in add-back mode, and the 10 CSS detectors together contribute 0.5%. Although in the early part of the time period, electronics shortages may not permit the entire array to be used in a particular experiment, a total Ge efficiency of approximately 2% at 1.3 MeV should be available. Five of the Clover detectors are segmented and provide a position resolution of ~2 cm.

New electronics for the Clover Ge have been designed and prototypes are being fabricated. These new modules, based on the GAMMASPHERE design, are CAMAC-based with fast FERA readout. It is hoped that these modules will become fully operational in Spring 1998.

New Scattering Chamber:
This chamber has a forward funnel to allow stopping of the radioactive ion beams away from the Ge detectors and to accommodate the forward charged-particle detectors of the Hyball. This chamber is designed and is presently undergoing fabrication. It is anticipated that this chamber will become available in early 1998, before the CsI part of the Hyball becomes operational. This chamber (or some similar chamber) is required for in-beam gamma-ray radioactive ion beam experiments,

Ionization Chamber (IC):
The ionization chamber has been tested with the reaction 58Ni(28Si,xpyn) at an effective beam energy of 203 MeV. Discrimination between Z=39 (Y) and Z=40 (Zr) recoils was achieved while using a position-sensitive gas counter in front of the detector. Analysis of the data from this reaction indicated that, of all the events reaching the IC, less than 50% were from scattered beam (no fingers or collimation at the achromat were used). Using time-of-flight coincidences (recoil-gamma), the amount of data taken to tape attributed to this scattered beam was less than ~5% of the total data.

Strip Detector System (DSSD):
The strip detectors are fully operational at the focal plane position. Two square double-sided silicon strip detectors of 40 strips, 1 mm wide, and 60 microns thick have been used. In addition, a thick silicon detector and external x-ray detector may also be used in coincidence with the DSSD. The system has been used to detect ground-state proton emission with T1/2~3 us and to distinguish isotopes with Z~55.

HYBALL:
The CsI detectors are in the design stage and the electronics have been received. Funds for the construction of the fast, forward detectors that cover angles less than 25 degrees are requested for FY1998.

To accelerate the implementation of the HYBALL the new electronics will be tested with the 44-element miniball particle-detector array of the 8-pi collaboration which will be available through January 1998. The array fits in a 10.6 cm-diameter plastic vacuum vessel made of Delrin.

It is anticipated that the CsI detectors of the HYBALL will be available for experiments by Spring 1998. Use of the miniball in some early commissioning experiments not requiring the higher granularity of the HYBALL may be possible. For further information on the possible use of the miniball, please contact Alfredo Galindo-Uribarri at uribarri@mail.phy.ornl.gov.

Neutron Detectors:
Five NE-213 neutron detectors are currently being tested. These detectors should be available for use in early 1998. A support stand, needed to house these detectors, will be designed and built when needed.

Tape System:
The LSU Moving Tape Collector (MTC) was tested earlier this year. Excessive noise was observed in the silicon detectors used for the pair spectrometer. The chamber and motor have been modified to correct this problem, and the MTC is ready for another test. It may be used for decay spectroscopy (including internal conversion and pair spectroscopy) with halflives on the order of several hundred milliseconds. For shorter lifetimes, the MTC can be used to remove activity from the collection point. This system is expected to be available for general use early next year.

16. Accelerator Physicists Sought

Two accelerator physicist positions are being advertised by ORNL. The first position is a senior accelerator physics staff position being advertised by Physics Division for operating and improving HRIBF and for preparing plans for the construction of the National ISOL Facility at ORNL. The announcement of this position is appended below.

The second position is offered jointly by the Physics Division and the National Spallation Neutron Source. The formal announcement of this position will be available soon. Further information for either of these positions can be obtained by contacting Jim Beene (jrb@ornl.gov or 423-574-4622)


OAK RIDGE NATIONAL LABORATORY - ACCELERATOR PHYSICIST

The Oak Ridge National Laboratory's Physics Division invites applications for a staff position in accelerator physics with the Holifield Radioactive Ion Beam Facility (HRIBF). This unique, newly commissioned facility uses two accelerators, the k=100 Oak Ridge Isochronous Cyclotron and the 24 MV tandem electrostatic accelerator, to produce accelerated beams of short-lived radioactive species, which are then used for research in nuclear structure physics and nuclear astrophysics. Operation, optimization, and improvement of the existing facility are now the primary missions of the HRIBF staff. ORNL's Physics Division will compete for the next-generation radioactive ion beam facility, proposed for future funding in the 1995 DOE/NSF planning document, Nuclear Science: A Long Range Plan. The Physics Division offers an excellent environment for research and development, including access to state-of-the-art computational facilities and opportunities for collaboration with guest scientists at the Joint Institute for Heavy Ion Research.

The successful candidate must have a Ph.D. or equivalent experience in Physics or Engineering with 5+ years' professional experience in accelerator physics; demonstrated record of accomplishments in accelerator design and development; excellent communication skills; the desire to work in a team environment on technically challenging problems; and a working knowledge in such areas as: magnet technology, beam transport, RF systems, and superconducting cavities. Project leadership and facility management experience are desired.

Qualified applicants are invited to send a current resume and arrange for 3 letters of evaluation to be sent to:

Dr. James R. Beene
Oak Ridge National Laboratory
P. O. Box 2008
Oak Ridge, TN 37831-6368
email: jrb@ornl.gov.

For more information about ORNL, the Physics Division, and the HRIBF, please visit our web sites at: http://www.ornl.gov, http://www.phy.ornl.gov, and http://www.phy.ornl.gov/hribf/hribf.html.

ORNL, a multipurpose research facility managed by Lockheed Martin Energy Research Corp. for the U.S. Department of Energy, is an equal opportunity employer committed to building and maintaining a diverse work force.





Additional copies of the newsletter and more information about HRIBF can be found on the World Wide Web at www.phy.ornl.gov. You may contact us at the addresses below.

Jerry D. Garrett Carl J. Gross
Scientific Director Scientific Liaison
Mailstop 6368 Mailstop 6371
garrett@mail.phy.ornl.gov cgross@mail.phy.ornl.gov
+1-423-576-5489 +1-423-576-7698

Holifield Radioactive Ion Beam Facility
Oak Ridge National Laboratory
Oak Ridge, Tennessee 37831 USA
Telephone: +1-423-574-4113
Facsimile: +1-423-574-1268