8. A Photo-fission Driver Upgrade for HRIBF
(J. R. Beene )
The HPTL is complete, and the IRIS2 project is underway and proceeding well. The next step in our strategic plan for improvement of the HRIBF is a driver accelerator upgrade. We will propose to improve our RIB production capability by installing a turn-key electron accelerator capable of delivering a 100-kW beam, at an energy in the range 25 to 50 MeV. This is the most cost-effective way to achieve leadership class capability in our neutron-rich beam program. With existing HRIBF target technology, and modest-sized targets (~180 g U), such a facility will be capable of generating 1013 photo-fissions per second. For our baseline design, the beam power required to reach this fission rate is 50 to 60 kW (depending on electron beam energy), the corresponding power deposited in the target is <10kW, and the maximum power density in the target is equivalent to that produced in current HRIBF targets. The 1013 fs-1 rate is ~25 times larger than the current HRIBF proton-induced fission rate, but since photo-fission is a much "cooler" process the yields of the most neutron-rich species are much more strongly enhanced. As an example, the yields of 132,134,138Sn yield will be ~300, 1000, and 12000 times larger than current HRIBF capability respectively. The comparatively modest increase in total fission rate makes planning for radioactive materials handling simpler. The 1013 fs-1 rate can be considered a baseline: expected improvements in target technology would lead to higher rates. Expected intensities for un-accelerated (~50 keV) beams, and for single-stripped post-accelerated beams are shown in Fig.8-1 for the baseline 1013 fs-1 rate.
Figure 8-1: Expected intensities for un-accelerated (~50 keV) beams, and for single-stripped post-accelerated beams for the baseline 1013 fs-1 rate.
This upgrade will provide world class neutron-rich beams, but will only impact our proton-rich capability indirectly. The chief competitors for delivery of these beams will be ISAC-2 and SPIRAL-2. ISAC-2 will begin the development of fission fragment beams after licensing issues are addressed. The timescale for this is unclear. Their 50 kW, 500-MeV proton beam incident directly on a uranium carbide target will produce ~5x1013 fs-1, however yield of the most neutron rich species will be substantially less than that produced by photofission at 1013fs-1. Undoubtedly, indirect targets will eventually be developed at ISAC-2 to produce cold fission with secondary neutrons; the ultimate fission rate achieved in this mode depends on geometry and target size. The goal of the ambitious SPIRAL-2 project is a fission rate of 1014 s-1, produced by ~18 MeV neutrons from breakup of a 5 mA beam of 40-MeV deuterons. The fission rate goal will require use of very large targets (~5 kg U), with attendant uncertainties in the decay losses from short-lived beams. A more conservative target, likely to be used in early implementation will produce ~1013 fs-1. Operation of SPIRAL-2 at full power levels will not occur before 2013.
In addition to the remarkable yields of neutron-rich species that can be achieved, a second driver accelerator at HRIBF will offer us the opportunity to undertake an upgrade of ORIC, which should improve our capability on the neutron deficient side of stability.
In December, technical studies and preliminary design concepts for the photo-fission upgrade were presented to the HRIBF Scientific Policy Committee. Their report was very favorable, encouraging us to proceed with development of a full proposal. Documentation of the preliminary design study can be found on the HRIBF website. It is far too early in the process to present a timeline, but it should be possible to be operational by 2012 at the latest.