HRIBF Newsletter, Edition 14, No. 2, Aug. 2006

Feature Articles

• 1. HRIBF Update and Near-Term Schedule
• 2. Recent HRIBF Research - Isospin Dependence of Fusion Hindrance
• 3. Recent HRIBF Research - Crystal-blocking lifetime studies of heavy-ion induced fission
• 4. What's New at HRIBF - Injector for Radioactive Ion Species 2 (IRIS2)
• 5. What's New at HRIBF -Dan Bardayan Received PECASE Award
• 6. DOE Science and Technology Review in June 2006
• 7. Future Exotic Beam Facility
• 8. Japan-U.S. Theory Institute for Physics with Exotic Nuclei (JUSTIPEN)
• 9. Fifth Annual RIA Summer School on Exotic Beam Physics Held at HRIBF
• 10. NS'06 held in Oak Ridge in July 2006
• 11. HRIBF Workshop on Nuclear Measurements for Astrophysics to be Held on October 23-24, 2006
• 12. PAC-13 Calls for Proposals

Regular Articles

• RA1 - RIB Development
• RA2 - Accelerator Systems Status
• RA3 - Users Group News
• RA4 - Suggestions Welcome for New Beam Development
• RA5 - HRIBF Experiments, January - June 2006

2. Recent HRIBF Research - Isospin Dependence of Fusion Hindrance
(W. Loveland, Spokesperson)

The use of neutron rich projectiles is thought to enhance fusion due to the lowering of the fusion barrier for the neutron rich projectiles. Work done at GSI [1] in the 1980s, however, suggested a canceling effect in that it was deduced that as the fusing system became more neutron rich (decreasing fissility, x), the fusion hindrance, measured by the "extra push" energy needed to get the nuclei to fuse, increased in contradiction to most theoretical models (see Fig. 2-1). If this were a general trend, the advantage of using radioactive beams in synthesizing new heavy n-rich nuclei would be significantly diminished.

Figure. 2-1. Deduced extra push energies for the ¹²⁴Sn + ^90-96Zr reaction as a function of the fissility of the fused system [1] and the theoretical predictions for these energies [2].

Because of the importance of this effect, we directly measured the capture cross-section for the ¹²⁴Sn + ⁹⁶Zr reaction. (In [1], the capture and fusion cross sections had not been directly measured but were deduced from measured evaporation residue cross-sections). The experiment was performed twice with different experimental setups to check the results. In both experiments, coincident fission fragments were detected and distinguished from elastic and inelastic scattered particles by observed angle/energy correlations. In the first experiment, four silicon strip detectors were used while in the second experiment, a larger solid angle, annular segmented strip detector was used to detect the fragments

The data are shown in Fig. 2-2 along with current theoretical estimates of the capture and fusion cross-sections [3]. Our measured data are in agreement with the theoretical predictions [3]. The extra push energy implied by the calculated fusion cross section is very small, in disagreement with the analysis of [1]. (We now believe the analysis used in [1] was in error because of the assumption that shell effects on level densities are washed out at very low excitation energies).

Figure 2-2. Comparison of our measured capture cross sections for the ¹²⁴Sn + ⁹⁶Zr reaction with the theoretical predictions of [3].

In summary, there appears to be no anomalous fusion hindrance in the ¹²⁴Sn + ⁹⁶Zr reaction. We are continuing the studies to characterize the ¹³²Sn + ⁹⁶Zr reaction.

1. C.C. Sahm, et al., Nucl. Phys. A441, 316 (1985)
. 2. S. Bjornholm and W.J. Swiatecki, Nucl. Phys. A391, 471 (1982).
3. G. Giardina, et al., (private communication).

3. Recent HRIBF Research - Crystal-blocking lifetime studies of heavy-ion induced fission
(J.S. Foster, Spokesperson)

Blocking of elastically scattered ions and fission fragments were measured at HRIBF with a position-sensitive gas counter by bombardment of thin tungsten crystals with 240-255 MeV 48Ti. The purpose is to study the time scale of fission for very heavy compound systems at high temperature. There is strong evidence from measurements of pre-fission neutrons [1] and giant-dipole gamma rays (GDR) [2] that fission times are rather long, and it is now generally accepted that the nuclear mass flow in fission at high temperature is very viscous (like molasses rather than water). However, the experimental evidence is indirect and the interpretation is model dependent [1-4]. In contrast, crystal blocking gives direct information on the recoil of the united nucleus before fission and thereby on the time delay of the fission.

In an earlier experiment we measured blocking dips by bombardment of tungsten crystals by 170-180 MeV ³²S ions and found no sign of a delay. The fission dips were virtually identical to dips in elastic scattering after an appropriate scaling of angles [5]. In contrast, the new data indicates an average recoil distance of order 0.1 A and a corresponding delay of about 2x10-18 sec, as seen in the figure Fig. 3-1 showing blocking dips along a <111> axis. The agreement with calculations is good except for an additive contribution from crystal defects to the minimum yield. The dip in elastic scattering should be narrower than a fission dip without recoil by a factor of 1.8 owing to the higher energy and lower atomic number of scattered beam particles, and the elastic dips have been scaled up in angle by this factor. The angular resolution in the experiment is limited by the size of the beam spot on the crystal, and the resolution is poorer for the narrower dip. The fission dip should rather be compared with an elastic dip with the same resolution, as given by the dotted curve.

Figure 3-1:

These results give clear confirmation of the picture of overdamped nuclear mass flow in fission of very heavy nuclei at high temperatures. The lifetime is at least an order of magnitude longer than inferred from measurement of neutron multiplicity and of GDR gamma rays. The null result for 32S bombardment is consistent with observations of very low intensities of GDR gamma rays in this reaction [2] and could be due to an influence of the magic neutron shell at N=126, as speculated in [5]. It is interesting that the difference between the minimum yields in the elastic and fission dips is less than predicted by the calculations. The increase comes mainly from the exponential tail of long recoils and its absence suggests a more well defined, classical delay, depending on the angular momentum of the united nucleus, rather than an exponential distribution or a sum of exponentials.

1. D.J. Hinde, D. Hilscher, H. Rossner et al, Phys. Rev. C 45, 1229 (1992).
2. Peter Paul, Annu. Rev. Nucl. Sci. 1994, 44, 44.
3. K. Siwek-Wilczynska, J. Wilczynski et al, Phys. Rev. C 51, 2054 (1995).
4. H. Hofman and F.A. Ivanyuk, Phys. Rev. Lett. 90, 132701 (2003).
5. S.A. Karamian, J.S. Forster, J.U. Andersen et al, Eur. Phys. J. A 17, 49 (2003).

4. What's New at HRIBF - Injector for Radioactive Ion Species 2 (IRIS2)
(B. A. Tatum)

The IRIS2 Project commenced at the beginning of this year. Accounts were opened with an initial allocation of $1.4M following approval of the Project Management Plan. Initial priority was placed on finalizing the beam optics design for the injector beam transport line, and determining the configuration of the high-voltage platform system for both the target and instrumentation rooms. Both tasks have been completed and the high-voltage platform system has been ordered. Procurement of the injector beamline RIB analysis magnet system is now under way. This system includes a 25-degree rectangular pre-separator magnet and two 60-degree sector dipoles that will be operated in a series electrical configuration. In addition, design efforts have begun for the 250-kV high- voltage conduits that will carry cables and utilities through the 9.5-foot wall between the target and instrumentation room platform systems. A ten-foot extension of the HPTL remote handling system bridge crane has been ordered, along with an automation package upgrade. Localized shielding and support structure design is also proceeding well. Design of an HVAC system that will provide both temperature and humidity control for the target room is nearing completion. The IRIS2 Project is on schedule and on budget.

5. Recent HRIBF News - Dan Bardayan Received PECASE Award

6. DOE Science and Technology Review in June, 2006
(G. R. Young)

The DOE Office of Nuclear Physics held a science and technology review of the HRIBF on June 6-7, 2006, at ORNL. The review was chaired by Dr. Eugene Henry of the DOE Office of Nuclear Physics, with Dr. Dennis Kovar, Dr. Jehanne Simon-Gillo, Dr. Manouchehr Farkhondeh and DR. Wlodek Guryn of the Office of Nuclear Physics also participating, and with four external consultants forming the committee; Dr. Robert Laxdal (TRIUMF), Dr. Christopher Lister (Argonne National Laboratory), Professor Bradley Sherrill (Michigan State University) and Dr. Thomas Roser (Brookhaven National Laboratory). The committee heard one day of plenary presentations on June 6 and on the second day toured HRIBF and the HPTL, observing posters and talking with staff and guests, prior to hearing numerous research talks in plenary and parallel sessions. The closeout listed several findings and comments and made three specific recommendations, which are noted below together with deadlines for response. This is expected to be a recurring review, one of four held annually by the Office of Nuclear Physics to assess HRIBF, ATLAS, RHIC and CEBAF.

The talks addressed most aspects of the program at HRIBF. The first morning talks included an overview of the HRIBF and the Physics Division, a discussion of scientific direction and strategic plan for HRIBF, responses made to recommendations of the previous S&T review held in November 2004, the publication record for HRIBF, the operation of the PAC and SPC, scientific output from HRIBF and its relation to the overall goals put forth for the next several years for all of nuclear physics by DOE-ONP, interactions of HRIBF with its user community and the broader scientific community, the operations and development activities of the accelerator systems of HRIBF, and a description of R&D on ISOL systems. The afternoon session addressed safety and regulatory issues, noting HRIBF's continued good safety record and the excellent participation in that by outside users, and then moved to a series of talks on science results. The science talks covered experimental astrophysics, the Be-7 program, the decay spectroscopy program, reaction measurements with neutron-rich beams, and the master plan for instruementation development over the next several years at HRIBF. Talks on the second morning covered nuclear structure, notably with odd-A beams, theoretical astrophysics work in the division and that work's links to HRIBF experiments, and observation of the very rare alpha decay chain Xe-109 -> Te-105 -> Sn-101. Further talks addressed the relationship of (d,p) and (n,gamma) reactions, the Stewardship program, new results on the decay of Ho-141, laser purification of RIBs, in particular the work at ISTF-2, and improvements being made to the isobar separator.

The formal report from the review has been received and lists three recommendations from the DOE. HRIBF staff and the user community will work to respond to these recommendations, all of which represent opportunities for the facility.

The first recommendation is to generate a risk-based analysis of the reliability of the existing facility and laboratory infrastructure, including the assessment of the present scheduled maintenance procedures and the adequacy of staffing levels, with the report due to ORNL management and DOE by the end of CY2006.

The second recommendation is to vet the plans for future facility upgrades, in particular the choice of a second driver accelerator, in a peer review format by the end of CY2007. This review should cconsider scientifici merti, technical approach, proposed cost and schedule.

The third rcommendation is to generate a resource-loaded strategic plan which articulates and justifies the resources needed to meet scientific goals, operate the facility, and implement upgrades, with this last report due to DOE by January 31, 2007.

7. Future Exotic Beam Facility
(G. R. Young)

DOE has charged NSAC, at the July 21, 2006, meeting, to develop a plan for an exotic beam facility which would cost approximately half the estimated cost of RIA, which was about $1.1B. This facility should be placed in the international context of facilities in this area of science. One would expect it to have world-leading capabilities in its areas of emphasis and to complement other facilities extant or under consruction. The charge letter notes that to begin engineering and design work in FY2011 requires the design be defined already in FY2007. A subcommittee is being set up to address this question and report to NSAC by March 2007. The report from this subcommittee would be expected to be reflected in the Long Range Plan which NSAC was also asked at this same meeting to update by late 2007. The report should also define the expected facility well enough that the agency could move forward with a request for proposals, which means the facility must be well-enough defined that a proposal to build it could be written, which in turn implies a definite design, proposed approach, cost and schedule. DOE has indicated all options are open to create such a facility. One can thus consider heavy-ion drivers, ISOL facilties based on light-ion drivers, or alternative approaches.

The text of the charge letter can be found at http://www.science.doe.gov/np/nsac/docs/NSAC%20Charge_RIA.pdf.

More can be found at the JUSTIPEN web site. The JUSTIPEN efforts are beginning and will be featured in the ORNL Review soon. Shown below is the official opening picture taken in RIKEN.

This year, 55 students (out of 90 applicants) attended lectures by Thomas Papenbrock (ORNL/UT, Nuclear Theory), Daniel Bazin (MSU, Nuclear Reactions), Art Champagne (UNC, Nuclear Astrophysics), Brad Sherrill (MSU, RIA), Mike Carpenter (ANL, Nuclear Structure), Ian Towner (Queens, Weak Interactions), John Hardy (TAMU, Precision Measurements), Dan Stracener (ORNL, ISOL), David Schultz (ORNL, RIB Production Using Lasers), and Raph Hix (ORNL, Supernova Science). Lecture notes and photographs have been posted on the RIA Summer School website.

In the afternoon, the students worked with local scientists on experimental activities in the laboratory. These topics ranged from ISOL science and beam emittance to windowless gas cells and microchannel plate detectors to digital signal processing. In the evenings students presented their work in a series of talks and a poster session. Saturday was free time and many enjoyed a white water rafting trip down the Ocoee River. See the website for photographs.

11. HRIBF Workshop on Nuclear Measurements for Astrophysics to be Held on October 23-24, 2006

The Holifield Radioactive Ion Beam Facility (HRIBF) at Oak Ridge National Laboratory (ORNL) will hold a workshop on nuclear measurements for astrophysics with a special emphasis on the applications of radioactive beams to such studies on October 23-24, 2006. The purpose of this workshop, hosted by the HRIBF users' executive committee and the Joint Institute for Heavy Ion Research, is to bring together scientists who wish to study nuclear astrophysics with radioactive and stable beams and to acquaint participants with HRIBF's beams and experimental facilities available for such experimental studies.

A list of confirmed speakers is available at the workshop webpage. We are also soliciting contributions for the poster session to be held on the afternoon of the first day. There is no charge to attend the workshop, but those interested should register at the workshop website. The information obtained at registration is used to process entry to the laboratory. The registration deadlines are August 21, 2006 for foreign nationals and October 1, 2006 for U.S. citizens. We are asking even those holding current ORNL badges to register so an accurate count can be obtained for the lunches and coffee breaks. More information about the workshop is available at the workshop webpage.

12. PAC-13 Calls for Proposals

The HRIBF will accept proposals for consideration at the next meeting of the Program Advisory Committee (PAC-13) until November 1, 2006. The PAC, which meets on December 11-12, 2006 in Oak Ridge, will consider and prioritize proposals for any of the many radioactive ion beams now available for research. More information will be posted on our website and a reminder will be sent closer to the due date.

RA1. RIB Development
(D. W. Stracener)

A number of interesting radioactive beams, such as ¹⁷F, ³³Cl, and ⁵⁶Ni, can be efficiently ionized using negative-ion sources coupled to an ISOL production target. In each of the cases listed, an isobaric contaminant dominates, resulting in low-purity radioactive beams delivered to the experiment if no purification technique is available. In a previous HRIBF Newsletter we reported on one technique developed to purify a ⁵⁶Ni beam contaminated with⁵⁶ Co (see also Liu, Beene, Havener, and Liang, Appl. Phys. Lett. 87, 113504 (2005)). This technique utilizes selective nonresonant laser photodetachment to neutralize and substantially suppress the isobaric contamination. A laser beam having the appropriate photon energy and overlapping the ion beam can be used to selectively neutralize the contaminant if the electron affinity of the contaminant is lower than the electron affinity of the desired radioactive ions. In order to increase the laser-ion interaction time, and thus improve the photodetachment efficiency, the laser-ion beam overlap is made inside a gas-filled RFQ where the ion residence time can be a few milliseconds.

This technique is being developed using negative-ion beams of stable isotopes at the Ion Source Test Facility I (ISTF-1). The main focus of this off-line facility in recent years has been the development of negative-ion sources for RIB production. In the Ni/Co system, the Co contaminant in the negative-ion beam was reduced by a factor of 1000 while only 22% of the Ni ions were neutralized. This will result in an improvement in the 56Ni beam purity from less than 10% to about 99%.

The usefulness of this technique was recently demonstrated for suppressing oxygen contamination in fluorine beams and sulfur contamination in chlorine beams. The electron affinities for negative ions of these elements are: F (3.41 eV), O (1.43 eV), Cl (3.62 eV) and S (2.08 eV). The tests were made using a laser beam with a photon energy of 2.35 eV (523 nm) interacting with negative-ion beams of stable isotopes for each element. Initial results show that 95% of the negative oxygen ions and 99% of the negative sulfur ions were neutralized, while no photodetachment of fluorine and chlorine ions was observed. The negative-ion transmission through the gas-filled RFQ ranged from 27% up to 52%. Further tests will be needed to determine the optimum laser power, laser beam size, and laser-ion beam interaction time, and to improve the ion beam transmission.

RA2. Accelerator System Status

ORIC Operations and Development (B. A. Tatum)

ORIC was shut down during the first part of the reporting period for the development, installation, and testing of a new driver amplifier system for the rf system. This system previously consisted of four ENI A300, 300-watt, amplifiers which were coupled to the RCA 4648A power amplifier (PA) tube grid by means of an in-house-constructed combiner/divider unit. The ENI amplifiers served us well for many years, but they are no longer manufactured and most parts are unobtainable.

Consequently, four 500-watt replacement units were purchased from AR Kalmus. During the installation and testing period, problems were encountered with the combiner/divider because of high third-harmonic content in the output of the new amplifiers. Therefore, a new commercially-available combiner/divider unit with higher bandwidth was also purchased from Werlatone, Inc. It was also discovered that the output circuitry of the new amplifiers was vulnerable to rf system transients, but our staff worked with the manufacturer to add transformer coupling to the output stage to make them more robust. Unfortunately, the amplifiers had to be returned to the manufacturer on more than one occasion. After further development and design improvements, we have now been able to operate with two new amplifiers, a 2-1 commercial combiner, and a 1-4 commercial divider. This configuration allows us to have two spare AR amplifiers, as well as two old ENI amplifiers that will drive the system if necessary. Combined with the installation of new ceramic plate capacitors last December we look forward to more reliable rf system operation.

Progress has been made in upgrading portions of the utilities infrastructure. Facilities and Operations Directorate funds have been used to purchase a new vacuum breaker for the 13.8kV switchgear that is required for starting the main field MG-set. The old breaker has recently experienced several component failures. A new battery bank related to the MG-set and emergency lighting system has also been ordered. During the latter portion of the period, ORIC operation resumed with the delivery of 10-15 uA of 54Mev protons for the production of neutron-rich RIBs.

RIB Injector Operations and Development (P.E. Mueller)

During the week of 4 June 2006, the 25 MV Tandem Electrostatic Accelerator delivered a beam of

• 14.6 pps [17.5 MV 12+ terminal foil stripped] 228 MeV ⁷⁶Cu to general purpose Beam Line 21.

This beam was produced via proton induced fission of ²³⁸U by bombarding a uranium carbide coating on a reticulated vitreous carbon fiber target coupled to an Electron Beam Plasma (positive) Ion Source (EBPIS) with 8 uA of 42 MeV ¹H. Over one hundred times as much ⁷⁶Ga was additionally in the beam.

Tandem Development (M. Meigs)

The Tandem Accelerator was operated for about 2580 hours since the last report.   The machine ran at terminal potentials of 1.27 to 23.6 MV and the stable beams ¹H, ¹²C, ¹⁴N, ¹⁶O, ¹⁷O, ¹⁸O, ⁴⁸Ti, ⁵⁴Fe, ⁵⁸Ni, ⁷⁶Ge, ⁷⁶Se, ⁸²Sr, ⁷⁹Br, ¹²⁴Sn, ¹²⁶Te, ¹³⁰Te and ¹⁹⁷Au were provided.  Radioactive beams of ⁷⁶Cu, and ⁸³Ga accounted for 348 hours of beam on target.  About 245 hours were spent on conditioning with the goal of operation up to 24.5 MV for the neutron-rich program.  The tank was still open for a few days for scheduled maintenance at the beginning of the period.  An obstruction over a light-link port caused another tank opening immediately after the tank was closed.  A wooden platform had been inadvertently placed over the port during tank closing.  Only one other tank opening was done in this period and that only to allow National Geographic to film inside the tank; no maintenance was done during this opening.  The Tandem has run for about six months with no maintenance!

In February, nanotube carbon foils were tested in the tandem terminal.  This is the first time that nanotube foils have ever been used for stripping beam in an accelerator.  These first tests showed that the nanotube foils do not last as long or have as good transmission as the laser-ablated carbon foils, but they are very easy to work with.  Very thin foils, 2-3 µg/cm², can be handled like 50 µg/cm² conventional foils.  If the homogeneity of the nanotube foils can be improved, they may begin to approach the laser-ablated foils lifetime and transmission.

RA3. HRIBF Users Group News
(C. J. Gross)

It is time to hold the biennial election for the Users Executive Committee. The nomination committee (Uwe Greife, David Radford, and Witek Nazarewicz) has selected the slate of candidates below.

Additional nominations may be submitted from the users group at-large by collecting the support of 10 members and forwarding the name of the nominee to Carl Gross at grosscj@ornl.gov. Deadline for at-large nominations is September 15. More information may be found in the Users Group Charter or by contacting Carl Gross. The candidate receiving the most votes for a given seat will take office January 1 and serve for 4 years.

The HRIBF will hold its annual Users Group Meeting at this year's DNP meeting in Nashville, TN on Thursday evening, October 26. As has become tradition, this meeting will be held jointly with the ATLAS, NSCL, GAMMASPHERE/GRETINA, and RIA Users groups. More details will be provided in future emails.

The Users Group Workshop on Nuclear Measurements Astrophysics is scheduled for October 23-24, 2006 and will end so that attendees can easily travel to Nashville for the DNP meeting. See the accompanying article and the workshop's web pages for more information.

RA4. Suggestions Welcome for New Beam Development

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 liaison@mail.phy.ornl.gov. 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.

• The tandem accelerates negative ions only.
• Positive ions may be charge-exchanged or used directly off the platform (E < 40 keV).
• ORIC presently produces up to 52-MeV of ¹H (12 uA); 49-MeV ²H (12 uA); 120-MeV ³He (not yet attempted, costly); 100-MeV ⁴He (3 uA). Higher currents may be possible.
• Typical reactions required to produce more than 10⁶ ions per second are n, 2n, pn, and alpha-n fusion-evaporation reaction channels and beam-induced fission products. More exotic reactions are possible if extremely low beam currents are all that is needed.
• Species release is strongly related to the chemistry between the target material and the beam species. It is best when the properties are different and the target is refractory. Thin, robust targets (fibrous, liquid metals, a few grams per square centimeter) must be able to withstand 1500 degrees Celsius or more.
• Minimum half-life is seconds unless chemistry is very favorable.
• Very long-lived species (T_1/2 > 1 h) are probably best done in batchmode, i.e., radioactive species are produced with ORIC beams and then transported to the ion source where beams are produced via sputtering. Sputter rates of the species and target substrates are important.
• Isobaric separation is possible for light beams (fully stripped ions), while isobaric enhancement may be possible for heavy beams.
• Beware of long-lived daughters or contaminant reaction channels.

RA5. HRIBF Experiments, January through June 2006
(M. R. Lay)