|Edition 9, No. 1||Winter Quarter 2001||Price: FREE|
- 1. HRIBF Update and Near-term Schedule
- 2. Recent HRIBF Research - Shell Model Embedded in the Continuum
- 3. Recent HRIBF Research - Opportunities for Accelerator Mass Spectrometry at HRIBF
- 4. ISOL'01 to be Held March 11-14
- 5. PAC-6 Scheduled for June 14-15, 2001; Proposals Due May 4
Editors: C. J. Gross and W. Nazarewicz
Feature contributors: R. L. Auble, K. Bennaceur, A. Galindo-Uribarri
Regular contributors: M. R. Lay, M. J. Meigs, P. E. Mueller, D. W. Stracener, B. A. Tatum
Following completion of an experiment using neutron-rich RIBs in October, the RIB injector platform was reconfigured for the installation and testing of a multi-target batch-mode target/ion source for the production of long-lived radionuclides. The target wheel was loaded with four calcium fluoride targets for producing 18F via the 19F(p,pn) reaction, and four nickel targets for producing 56Ni via the the 58Ni(p,p2n) reaction. Testing of the batch-mode target/ion source began in November 2000 and was completed in January 2001. The results of the tests were very encouraging and measured 18F beam intensities were within a factor of 4-5 of the predicted values. The major cause of the lower intensity is believed to be due to the ORIC beam spot being significantly larger than the sputtered area. Experiments using this source have been postponed until the 18F beam intensity can be increased and the users have developed techniques to cope with the large 56Co contaminant which will accompany the 56Ni beam.
The batch-mode assembly was removed in early February 2001, and the RIB injector is being readied for installation of a UC-target/EBP ion-source for continuation of the n-rich RIB research program. The tandem electrostatic accelerator was shut down for scheduled maintenance in mid-January and is presently scheduled to resume operation the week of March 12. Conditioning of the tandem and operation with stable-ion beams is scheduled through mid-March to ensure that the tandem accelerator will be ready to operate at up to 24 MV as required for the n-rich RIB research program.
The n-rich RIB research program is presently scheduled to run from mid-March, following the ISOL'01 meeting, until late April. During this time, we expect to complete most of the approved n-rich RIB experiments. Following the n-rich research period, a kinetic-ejection negative-ion source, coupled to a hafnia target, will be installed on the RIB injector platform. This target/ion source combination will be similar to that used for the highly successful 17F/18F research program completed last year. During installation of the RIB target/ion source, the tandem accelerator will be available for stable-ion beam experiments in April-May. Operation with 17F/18F beams is presently planned to begin in mid-May and continue through June. Following the 17F/18F campaign, the accelerators will be shut down for scheduled maintenance.
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The recent advances in experimental nuclear physics make it possible to study nuclear systems far from the beta stability line. In these nuclei the residual interaction between discrete and continuum levels is expected to play a crucial role and cannot be neglected. The Shell Model Embedded in the Continuum (SMEC) is a new formalism that can be used to study both static properties (e.g. excitation spectra, radii, quadrupole moments) of exotic nuclei as well as simple reactions in which they are involved . The SMEC is mainly based on the Continuum Shell Model , with a more realistic description of the discrete levels, an essential condition for the study of low-lying excitations. In the SMEC formalism, the atomic nuclei is considered as an open quantum system, i.e. the subspaces Q of (quasi-)bound states and P of scattering states are not separated artificially. In our case, the P subspace contains the states with N-1 particles in bound orbits coupled to one particle that can occupy scattering states. We only consider states with asymptotically at most one particle in the continuum, so the projectors on the subspaces fulfill the condition: P+Q=1. The key element of our treatment is the realistic description of the discrete levels. We solve the standard Shell Model (SM) problem:
HQQ Pi = EiPiwhere QHQ=HQQ is identified with a realistic SM Hamiltonian. The eigenstates Pi are the N-particle (quasi-)bound wave functions. We believe that for a quantitative description of the low-lying states in the exotic nuclei one has to use as a starting point the accurate many-body wavefunctions provided by the SM with effective interactions. For the continuum states, we solve the coupled channel equations:
(E(+)-HPP)F E(+)=0where c denotes the different channels and HPP=PHP. The superscript (+) means that boundary conditions for outgoing scattering states are used. The solutions of these equations are the nonresonant part of the scattering states. The couplings between channels depend on the structure of the N and N-1 particle system and are determined microscopically. The third system of equations to be solved consists of the inhomogeneous coupled channel equations:
(E(+)-HPP)Wi(+)=HPQPiin which the source term is primarily given by the SM structure of the N-particle state Pi. The solution of this equation represents the continuation of the discrete state Pi in the P subspace, and describes the fact that a discrete level can decay by emitting a nucleon.
The three distinct functions are then used to express the many-body scattering solutions of the problem and the discrete states wave functions modified by the couplings to the continuum. The latter are related with the effective Hamiltonian
HQQeff=Hwhere GP(+) is the Green function for the motion of single particles in the P subspace. This effective Hamiltonian has complex energy-dependent eigenvalues Ei-iGammai/2. These eigenvalues at energy Ei(E)=E determine the energies and widths of the nuclear states.
Fig. 2-1 - In the left part of the figure, the astrophysical S factor for the reactions 16O(p,gamma)17F [Jp=5/2+] and 16O(p,gamma)This formalism is fully symmetric and consistent in treating the continuum and discrete part of the solutions, the continuum states being modified by all the discrete states, and the discrete levels being modified by the coupling to the continuum.
17F [Jp=1/2+] are plotted as a function of the center of mass energy. The parameters of the residual interaction between P and Q and the effective interaction in Q are discussed in ; the experimental values are taken from . The right part represents the phase-shifts for the p+16O elastic scattering as a function of the proton energy for different partial waves. The experimental points are from .
The SMEC has been applied for the study of the properties of the mirror nuclei 8B and 8Li, and the radiative capture reaction 7Be(p,gamma)8B, which is the key reaction for the understanding of the solar high energy neutrino flux .
As an example, we report here some results concerning the study of 17F and the reactions of elastic scattering p+16O and radiative capture 16O(p,gamma)17F. The spectroscopic properties of 17F reported in [3,6] are well reproduced by our model. We have shown that a realistic description of the structure of 16O and 17F (including the 2 particles - 2 holes correlations) are required in order to reproduce in a consistent way both the spectroscopy of 17F, the 16O(p,gamma)17F astrophysical S factor, and the phase shift of the elastic scattering p+16O (see Fig. 2-1).
The SMEC give the possibility to investigate the static properties of exotic nuclei and the reactions in which they are involved within a single microscopic formalism. It can easily be used for other reactions not discussed here, such as inelastic scattering or proton emission. An extension for systems with more than one particle in the continuum, although not straightforward, is possible. We are currently working on the description of 18Ne and reactions involving 17F. We are also revisiting the radiative capture reaction 7Be(p,gamma)8B by taking into account the influence of the target excitations.
*This work is being developed by a collaboration of researchers from GANIL, the Institute of Nuclear Physics of Krakow, the Laboratory of Theoretical Physics of the University of Strasbourg, the University of Tennessee, and the Oak Ridge National Laboratory.
 K. Bennaceur et al., Nucl. Phys. A651, 289 (1999).
 H. W. Barz, I. Rotter and J. Höohn, Nucl. Phys. A275, 111 (1977).
 K. Bennaceur et al., Phys. Lett. B488 (2000) 75.
 R. Morlock et al., Phys. Rev. Lett. 79, 3837 (1997).
 R.A. Blue and W. Haeberli, Phys. Rev. 137, B284 (1965).
 K. Bennaceur et al., Nucl. Phys. A671, 203 (2000).
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We have started to investigate the feasibility of using the 25 MV Tandem from the HRIBF for Accelerator Mass Spectrometry (AMS). AMS is one of the analytical techniques with the highest sensitivity known in physics. The technique of AMS is used to perform measurements of rare isotopes in samples placed in the ion source of an accelerator system. AMS uses a particle accelerator in conjunction with ion sources, magnets and detectors to separate out interferences and count single atoms in the presence of up to 1 x 1016 stable atoms. These radionuclides can be used for a wide variety of dating and tracing applications in environmental and biological monitoring, in the study of ocean circulation patterns, in radioactive waste, nuclear safeguards and nuclear physics. One of the best known applications is radiocarbon 14C dating. Two features make the tandem accelerator the preferred instrument for AMS. First, the requirement of negative ion beams for injection eliminates the interference of some stable isobars (14N, 36Ar, 129Xe) which do not form negative ions that would impair the detection of the radioisotopes of interest (14C, 36Cl, 129I). Second, the stripping of electrons at the high voltage terminal eliminates molecular interference.
The most interesting radioactive nuclear beams (RIBs) produced for astrophysics and nuclear physics involve generally rather short-lived species such as 17F (T1/2=64.5 s) and 18F (T1/2=110 m). By contrast, one of the primary objectives of AMS is the measurement of long-lived radioisotopes produced in natural materials by the interaction of cosmic rays (half-lives from a few years to millions of years). At first sight it would seem that AMS and RIB production are very different fields. However, there are a number of similarities between AMS and RIBs. The removal of interfering isobars is one of several common challenges of both AMS and RIB production. A facility such as HRIBF has a variety of equipment choices for beam transport and analysis and for rejection of unwanted species. Both AMS and RIBs will benefit from the use of the most efficient techniques for production, isobar separation, transport, and detection.
The HRIBF Tandem Accelerator is by far the highest operating voltage electrostatic accelerator in the world. Tandem voltages up to 25.5 MV have been used in experiments. The HRIBF 25 MV Tandem Accelerator is capable of producing beams of 0.1-10 MeV per nucleon for light nuclei and up to 5 MeV per nucleon for mass 80. There are a number of isotopes of interest for AMS where the high energies achieved could represent a very important advantage for their detection. For example, the beam energies are sufficiently high to strip all electrons from a good portion of the ions up to about mass 50. Isobars are then produced in different charge states that can be separated by beam line analyzers. This guarantees a strong suppression (107) of background events originating from lighter isobars. Furthermore, the HRIBF offers a variety of equipment such as high-resolution momentum analyzers (including one internal to the machine), electrostatic analyzers, plus a vast array of particle detection devices. We have started to develop additional detection systems oriented to heavy isotopes. The higher energies achieved will facilitate the isobar separation for the heavy species. The various research tasks include the establishment of beam diagnostic procedures for the low- and high-energy ends of the machine and the study of the stability and reliability of operation of the tandem for extremely low beam intensities. In particular, the ion source, beam optics, terminal voltage stabilization, and detection systems are extremely important in AMS.
Specifically we are interested in the potential use of the facility for the detection of isotopes such as 36Cl, 44Ti, 90Sr, 99Tc, 129I, and 236U. Very recently, an AMS experiment was performed at HRIBF to detect 36Cl, for the first time in a sea water sample from the Scotia Shelf in Nova Scotia, Canada. Also groundwater samples from the Great Artesian Basin (the largest freshwater aquifer in Australia) taken from a well known as "Oodnadatta" and a salina sample from a drilling core from the "Salina Formation" in southwestern Ontario, Canada. The samples in the form of AgCl were placed in the ion source. The tandem was set at 22.5 MeV. The Cl7+ were selected and postripped with a 50 ug/cm2 carbon foil to Cl17+. The ions were then transported to the Enge Spectrometer and deflected to a silicon position sensitive detector in the focal plane. These initial results proved that the HRIBF Tandem is potentially the most powerful in the world for the measurement of chlorine-36.
The 25 MV Tandem Accelerator of the HRIBF offers a powerful tool for AMS. The necessary elements exist at ORNL to explore the establishment of an AMS program: 25 MV Tandem, negative ion-source technology, accelerator physics, mass spectrometry, detector development, and research interests. We will focus on areas where the high voltage of the tandem plus the specialized instrumentation we have (or will develop) can play a unique role in AMS. Our proof-of-principle tests and our results will indicate if it is possible to use HRIBF as a prototyping facility to aid in the development of new AMS methods. This work is currently supported by an ORNL SEED Money Award.
*Participants include A. Galindo-Uribarri, B. Fuentes, J. Doupe, G. D. Alton, R. L. Auble, J. R. Beene, J. Gomez del Campo, R. C. Juras, A. E. Litherland, J. F. Liang, M. J. Meigs, G. D. Mills, D. C. Radford, S. Raman, and D. W. Stracener and represent the Universidad Nacional Autonoma de Mexico, University of Toronto, and ORNL.
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The ISOL'01 Conference will be held March 11-14 in Oak Ridge, TN. Focusing on nuclear physics studies with radioactive ion beams at ISOL facilities, the conference will have over 50 oral presentations and one poster session. In addition, ISOL'01 is dedicated to the celebration of twenty years of science at the Holifield facility and its continuing leadership role in nuclear physics research. More information including our on-line registration form may be found on our web site at http://www.phy.ornl.gov/isol01/.
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The sixth Program Advisory Committee meeting has been scheduled for June 14-15, 2001. Proposals for experiments will be due on May 4 and must be submitted by email attachment to the address email@example.com. Proposals should be targeted to the radioactive ion beam sources which will be used in the last half of the year. At present, this would involve the neutron-rich LaB6 source targeted at isobaric contaminate-free beams of Br and I. More information, including other ion sources to be scheduled during this period, will be sent out in April. Approximately 100 shifts (8 hours) of RIBs and 100 shifts of SIBs will be allocated.
Expected beam into Tandem |
(with 10 uA of protons)
To measure the release of 25Al (T1/2= 7.2 s) from the SiC target, an on-line run was performed using a low-intensity proton beam from the Tandem to bombard a target at the UNISOR facility. Due to problems with the source, no 25Al was observed. Since published diffusion rates of aluminum in silicon carbide vary widely, we decided to use a SiC target in powder form having an average particulate diameter of 1 micron to provide short diffusion lengths. At some point, possibly during the pump-down cycle, some of the SiC powder migrated into the Ta transfer line between the target and the ion source. This transfer line operates at 1800 to 2000 C so the SiC in this region dissociated completely causing a large amount of silicon to be released, destroying many of the Ta source parts and 'killing' the ion source efficiency.
There were, however, some positive aspects to this experiment. First, most (>90%) of the SiC powder was still in the target holder and had not sintered. Second, while there was evidence that the production beam did irradiate the target, no 26Al (T1/2= 7.1x105 years) was observed in the remaining SiC target material after the irradiation. This is evidence that the diffusion rate of aluminum in silicon carbide may be reasonably fast since the target temperature was reduced to less than 1000 C within two minutes after the end of the irradiation, effectively trapping any 26Al that remained in the target at that time. Some 26Al was found in the TIS enclosure deposited on cooler surfaces.
The next on-line test to look for the release of Al isotopes will be done with a SiC fiber target in order to eliminate problems inherent with a powder target. Even though the release efficiency of short-lived isotopes from the fibers (15 micron diameter) is likely to be lower than from the powder, we should be able to extract enough beam to optimize the ion source parameters. In parallel, a design for the target holder will be developed to accommodate the fine powder targets, since powders (if they do not sinter) have the largest surface to volume ratios.
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In the last newsletter, we reported that a shorting strap was removed from three tube sections which had been shorted for more than a year, and magnets were placed to try to alleviate any electron leakage into these tubes. The first conditioning period showed that these tubes were now able to be conditioned easily and to hold that conditioning for a long period of time. It is not known whether the magnets cured the problem or the problem went away with time. In any case, we now have the full column available and could have operated at 24+ MV before the current tank opening, but no experiment needed the corresponding energy. It required 132 hours of conditioning to reach this level, and we should be able to return with a minimal amount of conditioning after the tank is closed. The accelerator was conditioned to high voltage in anticipation of higher energies needed for neutron-rich beams.
New Alpha Omega oxygen monitors have been installed in place of the Beckman monitors, which had required a great deal of maintenance throughout the years. The new monitors will require very little maintenance and should not cause as many false alarms.
A microchannel plate detector has been installed in the diagnostics box at the image of the second-stage mass separator in beam line 12. The focal area of this magnet is instrumented with a rotating wire beam profile monitor, remotely operable horizontal and vertical slits, a Faraday cup, a moving tape collection system, and now this new type of beam profile monitor. This detector was developed as a transmission timing detector for use at energies greater than 1 MeV-A. However, we have found that it retains its excellent position sensitivity when used as a stopped beam imaging device at 200 keV. The device consists of a target foil that is inserted into the beam and a stationary microchannel plate that detects the secondary electrons emitted from the foil by the beam particles and is read out by a resistive layer anode. Both the foil and the microchannel plate are backed by magnets which guide the secondary electrons. The signals from the detector are processed and displayed to yield a real time updating two dimensional view of the beam. With a defocused beam incident on the object slits, we saw spectacular images of both the object and image slits. This new capability will help us understand the optics of the second-stage mass separator and improve our ability to separate isobars injected into the tandem accelerator.
The turbopump mounted on the beam line 9 diagnostics box providing the upstream pumping for the target ion source failed. Since it is necessary to pump on both sides of the target ion source (the turbopump mounted on the quadrupole box immediately after the target ion source provides the downstream pumping), the operating turbopump mounted on the diagnostics box in front of the first-stage mass separator was moved to the beam line 9 diagnostics box. This arrangement provides adequate pumping capability pending repair of the failed turbopump which was moved into the hood in RADLAB for bearing replacement. In addition to the turbopump work, the O-ring was replaced in the high vacuum valve isolating beam line 9 from the target ion source and the actuator for the viewer in the beam line 9 diagnostics box was removed. All of this work proceeded smoothly despite having to be performed in full anti-contamination clothing including respirators. Even in the post uranium carbide regime (where contamination levels on the extract electrode were 35 rad/hr (beta) a couple of weeks after fission-product production), we are still able to effectively conduct maintenance in C-111S.
The platform motor-generator shaft was overhauled because of excessive vibration. The platform and source motor-generators run continuously; the last shaft overhaul for both was over two years ago.
During this reporting period we also replaced the air motor for the coupling mechanism between the target ion source and beam line 9, moved the controls for the roughing pumps in C111S to C111N, and installed a backing vacuum accumulator for the turbopump in beam line 12 to minimize the running of its associated roughing pump. For the first time we remotely fixed a leak in the previously mentioned coupling mechanism by adjusting (from C111N) the air pressure to its pneumatic clutch.
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A new vacuum chamber has been installed directly behind the RMS target chamber and before the first quadrupole of the RMS. The chamber is a 6-way, 8-inch inner-diameter cross and mounts directly to the target chamber. This arrangement will allow the placement of detectors and catcher foils some 46 cm from the target. Such arrangements will be ideal in the study of nanosecond isomers, prompt gating of reaction products with a micro-channel plate plus thin foil detector, and possibly time-of-flight measurements with other detectors. The present micro-channel plate detector subtends approximately +/-2.5 degrees with respect to the beam.
Fig. RA3-1 - The new 6-way cross located between the RMS target chamber and the first quadrupole is shown. The distance from the target to the center of the cross is approximately 46 cm. At present, a micro-channel plate detector is inserted in the upper section of the cross. The picture may be enlarged by "clicking" on the image. Other images are also available.This new measuring position is available for user experiments. Space constraints exist when used in combination with the CLARION array and the cross is not compatible when the forward array (Si detector) part of the HyBall is in place.
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The new Users Executive Committee held their first telephone conference call with the facility in mid-January. A brief status report of the facility was presented and discussions ranged from the results of PAC-5, the problems of isobaric beam contamination, and the location of the annual HRIBF Users Meeting. A poll of the users has been conducted and it has been decided to declare that the ISOL'01 conference, which includes a tour of the HRBIF, will satisfy the annual meeting requirement mandated in the Users Charter. We hope to return to our traditional annual meetings at the DNP next year.
Other business conducted at the meeting was the election of the vice-chair of the committee. We are happy to announce that Ani Aprahamian will serve in this capacity for this year and will become chair of the committee in 2002. Congratulations Ani!
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HRIBF welcomes suggestions for future radioactive beam development. Such suggestions may take the form of a Letter of Intent or an e-mail to the Liaison Officer at firstname.lastname@example.org. In any case, a brief description of the physics to be addressed with the proposed beam should be included. Of course, any ideas on specific target material, production rates, and/or the chemistry involved are also welcome but not necessary. In many cases, we should have some idea of the scope of the problems involved.
Beam suggestions should be within the relevant facility parameters/capabilities listed below.
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|RIB-000||Commissioning of the RMS||Rykaczewski/ORNL||12/7-8/00|
|RIB-012||As+Ti sub-barrier fusion||Gomez del Campo/ORNL||11/14-15/00|
|RIB-013||Commissioning of the DRS||Bardayan/ORNL||12/11/00|
|RIB-014||RIB development on UNISOR||Stracener/ORNL||1/23/00|
|RIB-024||Decay studies at the proton drip line in the 100Sn region with a 56Ni radioactive beam||Rykaczewski/ORNL||10/30-11/3/00|
|RIB-035||Target ion source development (actinide targets)||Stracener/ORNL||11/10/00|
|RIB-039||High voltage injector development||Mueller/ORNL||12/13-14/00|
|RIB-040||Beam diagnostics development||Shapira/ORNL||11/6/00|
|RIB-051||Search for the decay of 16+ isomer in 96Cd||Grzywacz/ORNL||11/13/00|
|RIB-054||(p,t) reactions with SIDAR detector array||Bardayan/ORNL||1/24-25/01|
|RIB-060||Accelerator mass spectrometry||Doupe/Univ Toronto
|RIB-068||HYBALL charged particle detector system response||Galindo-Uribarri/ORNL||12/4-5/00|
|RIB-069||Decay study of a short-lived proton emitter 103Sb||Rykaczewski/ORNL||1/8/01|
|RIB-075||Identification of excited states in the odd-odd, self-conjugate nucleus 70Br||Piechaczek/ORNL||11/27-12/1/00|
|RIB-076||Structure and ultra-fast dynamics of electrons in nano-structures and bulk solids by particle induced X-ray spectroscopy||Vane/ORNL||12/15/00|
|Scheduled maintenance/operator training||1/2-5/01|
|Unscheduled maintenance, tandem injector||12/21/00|
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|Witek Nazarewicz||Carl J. Gross|
|Deputy Director for Science||Scientific Liaison|
|Mail Stop 6368||Mail Stop 6371|
|Holifield Radioactive Ion Beam Facility|
|Oak Ridge National Laboratory|
|Oak Ridge, Tennessee 37831 USA|