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-
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 108 7Be/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.