HRIBF NEWS

Edition 8, No. 3	Summer Quarter 2000	Price: FREE

Feature Articles

• 1. Call for Proposals
• 2. Update of RIB Delivery Plans
• 3. Recent HRIBF Research - Decay of a Resonance in ¹⁸Ne by the Simultaneous Emission of Two Protons
• 4. Temporary Staffing Changes
• 5. ISOL'01 Conference Reminder
• 6. RIA Workshop Held July 24-26 in Durham, NC

Regular Articles

• RA1 - RIB Development
• RA2 - Accelerator Systems Status
• RA3 - Experimental Equipment - Brief Update of the Major Endstations
• RA4 - Users Group News
• RA5 - HRIBF Experiments, May-July 2000

Editors: C. J. Gross and W. Nazarewicz

Feature contributors: R. L. Auble, J. Gomez del Campo
Regular contributors: M. R. Lay, M. J. Meigs, D. W. Stracener, B. A. Tatum, R. F. Welton

1. Call for Proposals

The HRIBF is now accepting proposals for experiments. A list of radioactive ion beams and their intensities may be found at http://www.phy.ornl.gov/hribf/users/beams/. The Program Advisory Committee will consider and prioritize proposals for any of the many radioactive ion beams now available for research.

We have decided to use electronic submission in order to save costs and to accelerate the processing of the proposals. The review process will proceed almost immediately after the deadline. We must receive your proposal as an e-mail attachment on or before October 13. The address to send your proposal is liaison@mail.phy.ornl.gov. Portable Data Format (pdf) is the preferred format, although we will accept postscript format as well. Should we have difficulties in the postscript-to-pdf conversion, the spokesperson will be notified and requested to correct the problem. More information, including locations of software which converts postscript to pdf, may be found at http://www.phy.ornl.gov/hribf/users/prop_guide_elec.html. A proposal submission form from our web site must also be submitted.

Information concerning experimental equipment may be found on our web site and in this newsletter. The Program Advisory Committee is headed by John Hardy of Texas A&M and will meet in November.

As always, proposals involving stable ion beam (SIB) will be accepted on the basis of the criteria:

• A research effort directed to studying nuclei far from stability.
• The experiment commissions equipment or develops new techniques.
• The experiment requires a unique capability of the HRIBF.

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2. Update of RIB Delivery Plans

Starting in mid-July, measurements were made to determine the intensities of neutron-rich RIBs produced by high-intensity proton bombardment of a uranium carbide target coupled to an electron beam plasma (positive-ion) ion source. The results of these measurements are given elsewhere in this newsletter. These measurements were made with 42-MeV protons from ORIC, with proton beam intensities up to 5 microamperes. The target/ion-source and recirculating charge-exchange cell have performed up to expectations. Operation with the present target/ion-source system is expected to continue until mid-September, and we remain optimistic that at least one pilot experiment utilizing a neutron-rich RIB will be completed by then. In parallel, we will also investigate the use of a negative-surface-ionization source for production of group VII elements, in particular the bromine isotopes, in an effort to increase the available beam intensity. The present goal is to complete testing of this source before the end of CY 2000.

From mid-September until early October, the tandem is scheduled to provide stable-ion beams for three experiments which required advance scheduling to accommodate outside collaborators. The present plan is to use this time to remove the EBP-source/uranium carbide target and install the batch-mode source to produce a ⁵⁶Ni beam. Following the ⁵⁶Ni campaign, if there is sufficient interest from the researchers, targets may also be installed in the batch-mode source to allow production of a ¹⁸F beam with, possibly, higher intensity than was provided during the previous ^17,18F campaign. Operation with the batch-mode source is expected to continue until the next accelerator maintenance period currently scheduled for December.

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3. Recent HRIBF Research - Decay of a Resonance in ¹⁸Ne by the Simultaneous Emission of Two Protons

In a recent experiment done at HRIBF with collaborating institutions of ORNL Physics Division, ORISE, and UNAM (Mexico), clear evidence for simultaneous two-proton emission from the 6.15-MeV state (J^p=1^-) in ¹⁸Ne was obtained with the radioactive beam-induced reaction ¹⁷F+ ¹H. Because of limited angular coverage, the experiment could not differentiate between the two possible modes of simultaneous decay: diproton (²He) emission or three body decay.

The experiment used the recently developed ¹⁷F radioactive beam, at a bombarding energy of 44 MeV with an intensity of 1.2 x 10⁵ fully-stripped ions per second. The thick target techinque, described elsewhere [1,2], was used in the bombardment of a 40-micron CH₂ foil which stops the ¹⁷F beam but allows the recoil protons to escape. A E-DE solid state telescope was placed behind the target to detect the recoiling protons. The DE detector consisted of a 300-micron-thick, double-sided silicon strip detector (DSSD) subtending a laboratory angle of 15^o. The E detector consisted of a 100-micron, 900-mm² surface barrier detector subtending an angle of 10^o. The two-proton events were defined as two coincident signals between nonadjacent strips of the DSSD. Every two-proton event consists of six parameters: two polar angles (theta), two azimuthal angles (phi), and two energies E₁, E₂. With these six parameters, distributions of the opening angle, the relative kinetic energy, and initial center-of-mass energy can be obtained. The distributions of the opening angle and relative kinetic energy distributions, when compared to theoretical predictions, are not distinct enough to differentiate between the two mechanisms responsible for the simultaneous emission of the two protons. However, it is possible to rule out contribution from sequential emission by a detailed analysis of the E₁ vs E₂ correlation. Although the maximum excitation energy in ¹⁸Ne is 6.3 MeV (3 MeV in ¹⁷F) and no discrete state exists in ¹⁷F for which sequential emission can proceed, it is possible that highly excited states in ¹⁷F whose widths are large enough could result in some sequential emission. In particular, the excited state in ¹⁷F at 5.1 MeV (8.4 MeV in ¹⁸Ne) has a width of 1.5 MeV and could produce small amounts of sequential emission of the two protons through the tail of the 5.1-MeV state. However, the E₁ vs E₂ correlation would be asymmetric since the first proton decaying from ¹⁸Ne to ¹⁷F will have a very small energy.

Fig. 1. Two-dimensional plot of the energy in the lab in one strip versus the coincident signal in a nonadjacent strip of the DSSD. Numerous gating conditions have been applied to the data to remove background. The gate drawn is the result of a Monte Carlo simulation of decay of ²He.

Results for the distribution of the initial center-of-mass energies are shown in fig. 2 and basically represent a two-proton excitation function. In the top panel of the figure we show the 2p excitation function for the full angular coverge of the detector (15^o). The bottom panel shows the excitation function derived for the angular interval of 0^o to 10^o and corresponds to the same events plotted in fig. 1.

Fig. 2. Experimental 2p excitation functions for the full coverage of the DSSD detector (top panel) and for the 0^o to 10^o detector-telescope region (bottom panel), extracted from the recoil protons for the reaction ¹⁷F + ¹H. The solid curves are the MULTI [3] calculation coupled to a Monte Carlo simulation.

More information will be available in a forthcoming paper [4].

References

[1] A. Huerta-Hernandez et al., Nucl. Instrum. Methods Phys. Res. B 143, 569 (1998).
[2] A. Galindo-Uribarri et al., Nucl. Instrum. Methods Phys. Res. B, in press.
[3] R. O. Nelson, E. G. Bilpuch, and G.E. Mitchell, Nucl. Instrum. Methods Phys. Res. A 236, 128 (1985).
[4] J. Gomez del Campo et al., submitted to Phys. Rev. Lett.

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4. Temporary Staffing Changes

There has been a temporary change in personnel at the HRIBF. Facility Director Jim Beene is redirecting his efforts for the next year to work on the Oak Ridge Laboratory for Neutrino Detectors (ORLaND). This initiative will take advantage of the neutrinos produced at the Spallation Neutron Source and is a collaborative effort by many institutions headed by ORNL and ORAU. During this period, Ron Auble will serve as acting Facility Director and Ray Juras will assume the duties of Operations Group Leader. We wish them success in their new responsibilities.

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5. ISOL'01 Conference Reminder

The ISOL'01 Conference will be held March 11-14, 2001, in Oak Ridge, TN. Sponsored by the ORNL Physics Division, ORISE, UNIRIB, and JIHIR, the conference will celebrate the 20-year anniversary of the Holifield and RIB physics. More information, including a list of invited speakers and hotel information, may be found at the ISOL'01 home page. The next circular calling for contributions will be sent out shortly. If you have not received any of the previous circulars or require more information, contact Carl Gross at isol01@mail.phy.ornl.gov.

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6. RIA Workshop Held July 24-26 in Durham, NC

From July 24-26, approximately 215 nuclear physicists met in Raleigh-Durham, NC, for a Workshop devoted to the physics opportunities offered by an advanced exotic beam facility known as the Rare Isotope Accelerator (RIA). For the first time, a large group of scientists employed a consistent set of pre-established intensities ( http://www.phy.anl.gov/ria/ria_yields/yields_home.html) to design and discuss a variety of experiments covering the RIA science.

The RIA2000 workshop included a small number of plenary talks and over 12 hours of working group discussions. The latter comprised four areas of RIA science: Nuclear Structure, Nuclear Reactions, Astrophysics, and Fundamental Physics. RIA2000 was put together by the RIA steering group who represent all of the scientific and technical areas of RIA research. The local arrangements were carried out by the local Organizing Committee chaired by Werner Tornow.

The principal goals of RIA2000 were to organize the RIA community in anticipation of the upcoming Long Range Plan exercise and to sharpen the scientific case for RIA. The report from the Workshop, a short RIA White Paper, will summarize the key scientific opportunities in radioactive nuclei with RIA.

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RA1 - RIB Development

Neutron-rich RIB Development

We recently began tests on a new-design uranium carbide target that is built on a highly conductive graphite foam matrix. Due to the excellent thermal conductivity of the graphite structure, we believe that the temperature of the target will be more uniform than in the UC targets presently in use. This should help to increase the yield of RIBs extracted from this target/ion source by eliminating any cooler regions in the target thus enhancing the diffusion out of the material itself and improving the rate of effusion through the target matrix. Also, since the temperature along the beam axis will be somewhat lower due to the high conductivity, we should be able to increase the intensity of the production beam incident on the target and, possibly, raise the production rate by a factor of two or three.

The technique used to manufacture these new UC targets is both fast and cheap. The target that we have tested has a mass ratio of 3.5:1 (U:C) and a composite density of 1.73 g/cm³ with the UC₂ layer being approximately 10 microns thick. The density of the graphite foam substrate is 0.53 g/cm³. The initial tests show that some oxygen remained in the target from incomplete conversion of uranium oxide to the uranium carbide. This was observed as large currents of CO⁺ and O⁺ extracted from the ion source at target temperatures of 1200^oC while the operating temperature of these targets should be at least 1800^oC. Such large amounts of oxygen greatly reduce the efficiency and lifetime of the ion source. Fortunately, the levels of CO⁺ and O⁺ were significantly reduced after a few days of operation, and we will modify the manufacturing process to greatly reduce the amount of oxygen in the target before it is used. The results of on-line tests of this new-design UC target will be reported in the next newsletter.

As, Ga, and Se RIB Development

We have revisited the problem of trying to recirculate the vapors from a hot Ge target to allow higher target temperatures, which would result in faster diffusion times for the species produced in this target. Initial tests with a graphite chimney, which was designed to allow Ge vapors to condense and then flow back into the primary Ge reservoir, were unsuccessful. We then tested several different materials in an attempt to find a surface that Ge would wet but yet not interact with so strongly as to destroy the surface in a short period of time. In addition to graphite, we tested pyrolitic graphite, rhenium, tungsten, boron nitride, and aluminum oxide. The best candidate seems to be tungsten even though Ge forms a eutectic with W, which has a low melting point. However, even thin foils can survive for several hours, and we now have a thick W tube from which we will make a recirculating chimney to be used in further tests.

New Beams

Some of the initial tests that need to be done with any proposed target material are a high temperature test (does the material sinter or decompose) and an ion source test (does the material have high vapor pressure or does it interact with and destroy the ion source). Many tests of this nature were performed in the early days of the facility, and we are now continuing them with the goal of producing beams of chlorine and aluminum proton-rich isotopes. The materials we are presently investigating are powders of cerium sulfide for beams of ^33,34Cl and silicon carbide for ²⁵Al. The SiC powder was unaffected during the high temperature test (1700^o C) and looks promising as a target material. Silicon carbide fibers also perform well at high temperatures. The Ce₂S₃ powders readily sintered at 1800^o C and converted to CeS. This results in long diffusion paths and high vapor pressures in the ion source, either of which could limit the usefulness of this material. We are investigating the possibility of using a low-density tungsten matrix to separate the powders and prevent large crystals from forming. Also the problem of high vapor pressure may be solved if we can obtain or make some CeS powder, but it is not readily available.

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RA2 - Accelerator Systems Status

ORIC Operations and Development

ORIC experienced a failure of the rf system's Beryl Industries 4648A power amplifier tube at the beginning of the period. Most of the period was required to replace and condition the tube and to repair peripheral rf components that were damaged in conjunction with the tube failure. In preparation for the ORIC control system conversion to EPICs, the last eln node was eliminated. Other control system upgrades included improvements to the rf tuning pages and the addition of remote control of the auxiliary main field power supply. ORIC operation resumed in July with 2-5 microamperes of 42-MeV protons delivered to the RIB Injector and UC₂ target for the production of neutron-rich beams.

Tandem Operations and Development

Most of this time period was spent with the tank open, doing scheduled maintenance. Scheduled maintenance was moved forward due to the failure of the ORIC rf tube and the wish to have both machines up and running at the same time. During this maintenance there was an attempt to install the recirculating gas stripper, but it again leaked at the epoxied, roughing-port joint. The pump was returned to the manufacturer for warranty repair and the tank was again closed without a recirculating gas stripper. We have discovered that other pumps from the same manufacturer purchased at the same time also have this failure. Two unscheduled tank openings were also necessary during this period. The first was due to a break of a nylon stud connecting two shorting rods. All of these studs have now been replaced with fiberglass ones of greater strength. The second unscheduled tank opening was to repair some electronics that had been damaged during one of the 24 MV sparks while conditioning.

Conditioning took 348 hours during this period. The machine was conditioned to run at operating voltages up to 23 MV before it was decided to move up the maintenance period. After the maintenance, it was necessary to redo the conditioning. The machine should now operate at voltages up to 23 MV. Due to the maintenance and conditioning, only 287 hours of beam were available for research. The machine ran at terminal potentials of 18.06 to 22.66 MV and ⁵⁸Ni, ⁸¹Br, and ¹H were provided.

A major maintenance job was completed which did not impact the schedule. The SF₆ vaporizer, which is used in transferring gas from storage to the accelerator, was replaced. The old vaporizer was leaking badly at the joints, and it was suspected that corrosion was the cause; but after replacement, it was found that the old gaskets were the problem. Therefore, the old vaporizer has been cleaned, painted, and stored to be used as a spare should we need it. The new vaporizer shows no leaks, which will help keep our SF₆ inventory in better shape.

RIB Injector Operations and Development

Over the summer, two on-line experiments were conducted involving the uranium carbide target described in the previous newsletter. The target consisted of nine, 2 mm thick disks of low-density reticulated vitreous carbon foam coated with ~12 microns of UC₂, forming a total target thickness of approximately 2 g/cm². This material was coupled to our standard electron beam plasma ion source. In the first experiment, 2 microamperes of protons were incident on the target material for ~24 hours. During this bombardment a survey of fission products extracted from the source was conducted. Neutron-rich isotopes of Ga, Ge, As, Se, Br, Kr, Rb, Sr, Y, Ag, Cd, In, Sn, Sb, Te, I, Xe, Cs, Ba, La and Ce were observed. The measured yields ranged from 10³-10⁸ ions/s. A complete table of yields can be found at http://www.phy.ornl.gov/hribf/users/beams/nrich_tab.html.

The second on-line experiment involved studying the yield of a single isotope, ⁸⁷Br, as a function of proton beam intensity incident on the target material. As the proton beam intensity was increased from 2 to 5 microamperes, we observed an approximately linear increase in yield, indicating constant production efficiency. A proton intensity of 5 microamperes was held on target for ~42 hours.

During this period, the capabilities of the RIB injector were also upgraded. A new tape system was bench tested, mounted and is presently under vacuum at the image location of the second-stage isobaric separator of the injector. The new charge exchange cell added to the RIB injector this spring is now operational.

The table below shows a few selected isotopes along with estimates of the intensity of the accelerated RIB on target. Estimates are based on the measured yields, measured charge exchange efficiencies (with the exception of Ag which is estimated to be 5%) and transmission through the tandem accelerator assuming single stripping (10% for light fragments and 5% for heavy fragments).

Expected accelerated RIB intensity on target assuming single stripping

Isotope*	Intensity (ions/s)	Contaminant Isotopes	Contaminant Intensity (ions/s)
87Br	3x104
89Rb	1x105
91Rb	1x104	91Sr	5x105
93Rb	1x104	93Sr	2x105
117Ag	5x105
130Sn	4x105	130Sb	4x106
132Sn	1x105	132Sb, 132Te	1x106, 1x107
133Te	5x106	133I	5x105
* Other isotopes are tabulated at www.phy.ornl.gov/hribf/users/beams/nrich_tab.html. Contaminant isotopes will be present in most beams and can be estimated from the tabulated intensities.

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RA3 - Experimental Equipment - Brief Update of the Major Endstations

Daresbury Recoil Separator (DRS)

The DRS is a mass separator based on velocity selection followed by momentum analysis. The focal plane is outfitted with a position-sensitive transmission microchannel plate detector and a gas ionization counter. A silicon strip detector array, SIDAR, is available for use at the target position which can be arranged in a number of configurations. The ionization counter can be positioned just behind the target chamber for coincidence measurements with the SIDAR and radioactive ion beam diagnostics. A windowless hydrogen gas target is presently under construction and should be available in late 2001. The research program at the DRS has focused on measuring properties of low-energy resonances which dominate the fusion of radioactive heavy ions with protons. The reactions studied have been those that drive stellar explosions. For more information see the DRS home page or contact Michael Smith.

Enge Spectrograph

The Enge has a small (36 cm) gas avalanche counter and plastic scintillator combination at its focal plane. Able to operate in vacuum or gas-filled modes, the Enge focal plane detectors may be operated in coincidence with double-sided silicon strip detectors and multi-channel plate plus thin foil detectors within the target chamber and in the beamline. Total fusion cross-section measurements, resonance reactions, and breakup reactions are a few of the experiments which have been done. Beam counting with multichannel plate plus thin foil detector systems is available. For more information see the Enge home page or contact Felix Liang.

Recoil Mass Spectrometer (RMS)

The RMS consists of a momentum separator and a mass separator which is compatible with many detector systems located at the target, achromatic focal plane, and the final mass-dispersed focal plane. All three regions may be used for experiments. In-beam g-ray spectroscopy may be performed with the CLARION and HyBall Arrays coupled to the RMS (CHARMS). Decay spectroscopy experiments using double-sided silicon strip detectors, the moving tape system, and clover Ge detectors are also possible. We are working on techniques suitable for low-intensity RIBs. We have used a position-sensitive avalanche counter directly behind the target and have some capability of counting individual beam particles. For more information see the RMS home page or contact Carl Gross (RMS, focal plane detectors), Kris Rykaczewski (focal plane detectors), David Radford (CLARION), and Alfredo Galindo-Uribarri (HyBall).

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RA4 - User Group News

The annual HRIBF Users Group meeting will be held at the DNP meeting in Williamsburg, VA, on Thursday, October 5. Due to the large interest in RIB physics and the preparations for next year's Long Range Plan, the HRIBF, ATLAS, 88-Inch, and GAMMASPHERE Users Groups Meetings will be held jointly. The ordering of the groups has not yet been finalized.

It is time to hold the annual election for the Users Executive Committee. A nomination committee has selected four nominees:

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RA5 - HRIBF Experiments, May-July 2000

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You may contact us at the addresses below.

Witek Nazarewicz	Carl J. Gross
Deputy Director for Science	Scientific Liaison
Mail Stop 6368	Mail Stop 6371
witek@mail.phy.ornl.gov	cgross@mail.phy.ornl.gov
+1-865-574-4580	+1-865-576-7698
Holifield Radioactive Ion Beam Facility
Oak Ridge National Laboratory
Oak Ridge, Tennessee 37831 USA
Telephone: +1-865-574-4113
Facsimile: +1-865-574-1268