BEARS: radioactive ion beams at Berkeley
Introduction
This paper describes Berkeley Experiments with Accelerated Radioactive Species (BEARS), a project that adds a light-isotope radioactive ion-beam capability to the 88-in. Cyclotron at Lawrence Berkeley National Laboratory. The system couples existing equipment for relatively minor cost. After two years of development and construction, BEARS has recently achieved its first steady radioactive beam: 11C with a maximum on-target intensity, at 120 MeV, of .
The basic BEARS system involves isotope production in a gas target at a low-energy proton cyclotron, transport via a 350-m-long gas capillary to the 88-in. Cyclotron, cryogenic separation of the activity from the target and carrier gases, and injection into the 88-in. Cyclotron's ion source for ionization and subsequent acceleration.
The production accelerator, normally used to support Positron-Emission Tomography and other medical research, is a 40-microamp, 11-MeV proton cyclotron belonging to the Berkeley Isotope Facility (BIF) [1]. It is capable of producing several light proton-rich isotopes with half-lives long enough to utilize, in particular , and 18F, all via (p,n) and (p,α) reactions on low-Z targets. Initially, we have focused on the production of and , both produced from a nitrogen gas target. The maximum thick-target production yields are approximately 1×1011 atoms/s of 11C and of 14O [2].
The 88-in. Cyclotron, located about 350 m from the BIF accelerator at LBNL, is a K=120, sector-focused cyclotron [3], fed by one of two Electron–Cyclotron Resonance (ECR) ion sources. These sources, particularly the upgraded Advanced ECR source (AECR-U) [4] can reliably achieve good ionization efficiencies at high-charge states. However, they require vacuums of less than 10−6 Torr to operate. Therefore a central technical challenge of BEARS is the coupling of a 350-m-long gas transport system to an ECR ion source.
Section snippets
Development tests
Prior to construction of an activity transfer line between the two accelerators, tests were carried out entirely at the 88-in. Cyclotron [5]. A nitrogen-gas cell was bombarded by an 11-MeV proton beam to produce small quantities of 11C and 14O. The target gas flowed continuously from the target, through a plastic capillary of 3-mm inner diameter (i.d.), to an area next to the ion source where it was passed through a cryogenic trap consisting of a stainless-steel coil submerged in liquid
Transfer time tests
The two accelerators in the BEARS system are located more than 300 m apart and a short gas-transport time is required, both to reduce decay losses suffered for short-lived isotopes like 71-s 14O, and to minimize radiation levels outside the two accelerator buildings. Tests were carried out, without radioactivity, by using bursts of helium gas in an overall flow of nitrogen. The differing response of a thermal-convection vacuum gauge to helium and nitrogen allowed for timing measurements to be
Final BEARS configuration
The following sections describe the entire BEARS system along with its typical operation for the production of 11C beams.
Conclusion
During the first successful test of the full BEARS system an on-target 11C beam of better than was achieved at an energy of 120 MeV. This beam was immediately put to use, collecting additional data for the 197Au(11C,xn) experiment [7]. Since that time, the BEARS 11C beam has been used in the support of three other experiments: the measurement of 11C fusion/fission on gold and platinum isotopes, 11C+proton elastic scattering, and a comparison of 49Fe yields from 40Ca targets via the (
Acknowledgements
We would like to particularly thank G.J. Wozniak and G. Rech for their assistance in the construction and commissioning of BEARS. This work supported by the U. S. Department of Energy, under Contracts DE-AC03-76SF00098 and DE-AC02-98CH10886.
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