3. Recent HRIBF Research - Recent Progress with ⁷Be Beams
(J. C. Blackmon, A. E. Champagne, U. Greife, and D. W. Stracener, Spokespersons)

An intense beam of ⁷Be has been developed by a collaboration between the University of North Carolina,  the Colorado School of Mines, and the HRIBF.  Samples of lithium metal were activated at Triangle Universities Nuclear Laboratory (TUNL) using a 10-MeV beam of protons from the FN tandem accelerator, producing ⁷Be by the (p,n) reaction.  Up to 50 mCi of ⁷Be per day were produced using proton beam intensities of about 10 microAmps.  The activated lithium slugs were shipped to ORNL, where a simple wet chemical process was performed to separate the ⁷Be from the lithium.  The separated ⁷Be was added to a metal powder matrix, coverted to an oxide (⁷BeO), and pressed into a cathode designed for a multi-sample cesium sputter source.  The influence of the cathode geometry, chemical composition and ion source parameters on the production of ⁷BeO^- beam was studied in a series of tests at the OLTF.  The ⁷BeO^- beam currents measured as a function of time from two of the better cathodes are shown in Fig. 3-1.  Both cathodes contained 1 mCi of activity within a copper metal matrix. Average beam intensities of about 1-2 million ⁷BeO^- /s/mCi were typical under good operating conditions, and a single cathode produced good beam output for about a week with a total efficiency of about 0.5-1.0% for production of ⁷BeO^- beam.

So far three experiments have been performed using accelerated ⁷Be beams from the HRIBF.  The ⁷Be(d,t)⁶Be reaction (RIB-129) was studied in the fall of 2004 by bombarding a 1 mg/cm² CD₂ target with a 100-MeV beam of pure ⁷Be.  A 30 mCi cathode was used producing an average beam intensity of about 1.5 million ⁷Be/s on target.  Tritons from the (d,t) reaction were detected and cleanly identified using the SIDAR array operated in telescope mode with a 100 micron layer backed with a 500 micron layer.  The energy spectrum of tritons is currently being analyzed to study the properties of possible unbound states in ⁶Be that might have important influences on the ³He(³He,2p)⁴He reaction.  This experiment is part of the Ph.D. dissertation of Andy Chae from the University of Tennessee.

The primary motivation for development of the ⁷Be beam was to improve our understanding of the ⁷Be(p,&gamma)⁸B reaction.  A very accurate value for the ⁷Be(p,&gamma)⁸B cross section is needed to interpret measurements of the solar neutrino flux because this reaction determines the ratio of neutrinos emitted from ⁷Be and ⁸B decay.  Excitation functions for ⁷Be+p elastic and inelastic scattering were measured in one set of measurements to help better constrain extrapolations of ⁷Be(p,&gamma)⁸B to solar energies (RIB-109).  Both the ⁷Be+p s-wave scattering lengths and broad resonances at higher energies could have small but still significant influences on the shape of the S-factor for ⁷Be(p,&gamma)⁸B.  In the HRIBF measurements ⁷Be beams at 19 different energies ranging between 4 and 27 MeV bombarded thin CH₂ targets.  Reaction products were detected in the SIDAR array.  Targets with gold backing were used in some cases to obtain accurate absolute normalizations.   The data is currently being analyzed by Jake Livesay from the Colorado School of Mines as part of his Ph.D. dissertation.

We are also measuring the ⁷Be(p,&gamma)⁸B cross section directly at the HRIBF (RIB-049).  The development of the experimental technique and the initial measurements comprised the Ph.D. dissertation of Ryan Fitzgerald from the University of North Carolina (2005).  In our first measurement conducted in March, a 12-MeV beam of ⁷Be (with contamination of the ⁷Li isobar) was produced from a 130 mCi cathode achieving an average intensity of 15 million ⁷Be/s on target.  The ⁷Be beam bombarded the windowless hydrogen gas target, operating at a pressure of 5 Torr with a thickness equivalent to 10 micrograms/cm².  Recoiling ⁸B nuclei were collected and separated from the ⁷Be beam by the Daresbury Recoil Separator.  The ⁸B recoils were detected by a gas ionization chamber at the focal plane and cleanly distinguished.  A particle identification spectrum from the ion chamber is shown in Fig. 3-2 for an accumulated time of 2.6 days of beam on target.  A total of 22 recoiling ⁸B ions were observed corresponding to a cross section of 1.1 microbarns.  The target thickness, incident beam current, and recoil detection efficiency were determined by independent techniques with a precision much less than counting statistics (21%).

We are currently working to improve the experimental setup to provide  even more robust control on potential systematic uncertainties.  Most importantly, we are currently implementing improvements to the ion source and cathodes that we expect will produce at least a factor 4 improvement in ⁷Be beam intensity.  At that level, we would be able to achieve a measurement of the ⁷Be(p,&gamma)⁸B cross section with a precision of about 5% in a 10 day experimental run.  We are currently constructing a new chamber that will allow for ⁷Be production using the ORIC cyclotron, and ⁷Be beams with intensities of up to 10^{8 7}Be/s on target should be routinely available at the HRIBF in the near future.  Look for future articles on the results from the measurements described here as they become available.