7. More Thoughts on a Driver Upgrade for HRIBF
(J. R. Beene)
Several articles in this Newsletter over the past few years have discussed our Integrated Strategic Plan for development of the HRIBF. The HPTL and IRIS2 projects, along with development of new experimental tools, are all part of this plan. In the February 2007 Newsletter, a brief discussion of the next step in this plan, a driver accelerator upgrade, was discussed. That article described a particular driver proposal involving a high-power electron beam accelerator to enable production of neutron-rich species by photo-fission (see the "HRIBF Initiatives" web page), which we refer to as the electron driver upgrade (EDU). Implementation of the EDU would lead to truly remarkable increases in the yield of very neutron-rich species. One important feature of any potential driver upgrade is the use of a commercial accelerator system which would allow us to focus the efforts of our small facility operations staff on ISOL technology, beam purification, post-accelerated beam quality, and reliability.
In the summer of 2008, we became aware of another commercial accelerator which could be very interesting as a driver at HRIBF: the C70 cyclotron, built by IBA in Belgium. This is a multi-species cyclotron capable of delivering up to 750 μA of protons at energies up to 70 MeV, a minimum of 50 μA of deuterons at energies up to 35 MeV, along with an alpha-particle beam at 70 MeV (fixed) and 35 μA. The proton- and deuteron beams are accelerated as negative ions, and extracted by stripping. In this negative-ion mode, the C70 also has a dual port extraction capability.
A White Paper exploring how the C70 might be employed to enhance the capability and reliability of the HRIBF was completed and submitted to the DOE Office of Nuclear Physics in early November 2008. This document is available on the "HRIBF Initiatives" web page. Some of the points raised in that document are outlined in the next paragraph. Please read the White Paper for details. For convenience, the C70 cyclotron-based option is referred to in this article as the HDU (Hadron Driver Upgrade).
Even though the commercial electron accelerator employed in the EDU concept is less expensive than the C70, the HDU total project cost (TPC) is substantially less than the EDU TPC. This is because the HDU makes effective use of all the basic infrastructure of the existing HRIBF, including the IRIS1 and IRIS2 production stations, with only minor modification, while the EDU cost is dominated by substantial new civil construction. The C70 would replace essentially all the current applications of ORIC. It would improve our proton-rich beam production as well as the neutron-rich production, while the EDU is strictly a neutron-rich driver and would depend on the 50-year-old ORIC for proton-rich production.
The completed HDU would be less expensive to operate than the current HRIBF. The size of staff required to operate the upgraded facility is estimated to be the same as that required for the present facility. The electric power usage of the C70 is one-fifth that of ORIC (400 kW vs. 2000 kW). Maintenance of the C70 would be contracted to IBA, reducing our need to purchase craft effort from ORNL.
The dual port extraction capability of the C70 offers two significant additional capabilities to HRIBF. We now purchase long-lived isotopes for batch-mode operation. The C70 would offer a cost-effective way to produce batch-mode isotopes in-house at HRIBF without interfering with normal ISOL operations. In addition, the ability to extract two beams simultaneously could enable us to produce isotopes of interest to medical (or other) research activities, again without interfering with ISOL operations. This capability may be becoming more important as the responsibility for the isotopes program within DOE has been transferred to the Office of Nuclear Physics in FY2009.
Finally, we consider the performance of the HDU-based facility for
production of very neutron-rich species. This is the strength of the
EDU and represents a critical piece of the science program at HRIBF
over the next few years. The most obvious and straightforward way to
make use of the HDU is simply to extend our present method of
producing fission fragment beams with the 238U(p, fission)
utilize the higher proton-beam currents and energy (70 MeV vs. 50 MeV)
available from the C70. The gains that can be made this way are
probably modest. We believe that the maximum 70-MeV proton current
that a conventional direct proton-induced fission target system can
sustain is not much more than 50 μA, a small fraction of the 750 μA
available from the C70. The total gain in neutron-rich beam yields
would probably be about a factor of 5 across the board compared to
current facility performance. This enhanced direct proton-induced
fission production could be implemented at HRIBF with essentially no
development effort, but the factor of five gain would be two to three
orders of magnitude less than the gains made by the baseline
expectations of the EDU. However, with additional development effort,
more complex production schemes can be employed which offer much
greater performance and might eventually make use of essentially the
full beam-current capability of the C70. A very promising production
method would be 238U(n, fission) using secondary neutrons produced by
high-current proton or deuteron beams. Because of the higher proton
energy available, and the very high proton currents available,
production of intense secondary beams of neutrons by proton
bombardment of thick targets of nuclei containing weakly-bound
neutrons is much more favorable than the use of deuteron breakup as a
neutron source. We believe an HDU baseline on the order of 50% to 75%
of the EDU baseline could be achieved with essentially the same
modest-scale fission targets planned for the EDU. The distribution of
the yield of neutron-rich fission fragments from this secondary neutron
fission is very similar to the production from photofission.
Calculated photofission beam yields can be used directly to estimate
RIB yields from secondary-neutron induced fission. For further
discussion of the potential performance of a C70-based facility,
please see the White Paper.