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Re-Accelerated Beam Yields (0 - 15 MeV/u)
Intensities for secondary acceleration: The estimates are for yields of beams that are stopped, ionized, and then accelerated.* For low energies and for light beams these are the intensities to be expected for experiments, for higher energies and for heavier beams the intensities may be somewhat lower.
What is assumed: The tables include estimates of efficiencies in extraction and ionization, but not of subsequent losses that may be caused by stripping to achieve the given final energy. Such stripping losses are expected to lower the intensities by a factor of 3-10, depending on the species and desired energy. The beam power is assumed to be 100 kW. (RIA is envisioned to be able to deliver 100 kW beams to 2 or more targets simultaneously, or conceivably several 100 kW to one target, if that proves feasible).
Accuracy: These are approximate estimates, the accuracy is perhaps at the order-of-magnitude level, probably somewhat worse at the limits of extrapolation.
Mechanisms The number listed corresponds to the highest yield from one of the following four mechanisms that have been considered in some detail so far:
a) The Standard ISOL mechanism, is known to be able to produce a number of isotopes of the volatile and chemically not too active elements, with proton beams. Here the observed yields at ISOLDE were taken and scaled for the RIA beam power, using the same target thickness as was used at ISOLDE. For elements where the mechanism has been shown to work, it was extrapolated to additional isotopes, correcting for the diffusion time out of the target in cases where the lifetimes are expected to be comparable or shorter than the diffusion times.
b) The Two-Step Fission mechanism is similar, but originates from neutrons where a favorable geometry has been developed using a deuteron beam on a liquid-lithium cooled porous tungsten target surrounded by a uranium carbide secondary target. This method has important advantages since the beam power is not dissipated in the same target from which the isotope needs to be extracted and thus conditions are more readily controlled. Yields are calculated with the LAHET code package supplemented by empirical n-induced fission branching data. The same corrections were made as for the Standard ISOL mechanism for diffusion times, and yields for this mechanism are included only for elements where the above mechanism has been shown to work. For low intensities, where the Monte Carlo nature of the LAHET calculations could not produce a number, the trends were extrapolated -- with some assumptions about lifetimes and diffusion times, and thus increased uncertainties.
c) Fragmentation reactions of heavy beams can produce useful yields of isotopes when a heavy beam impinges on a target, assumed to be liquid lithium. These beams are selected in a system of magnets with a momentum acceptance of ± 9% and an angular acceptance of ± 50 mrad, suitable for stopping in a helium gas cell. The efficiencies for the slowing down, the stopping as charged particles, and the transporting of the ions to the acceleration section, are included. The EPAX II code, an empirical parameterization of the yields observed (largely at GSI) has been used to estimate the yields. Estimates for the attenuation of the secondary beam in the thick production target are included. For isotopes near the limit, secondary processes (involving two fragmentation reactions) can be significant and have been added. Estimates of losses in the energy degrader are also included. These rates have been limited to 109 ions/s to insure that the operation of the gas cell is not affected by space-charge effects. The same limit has been adopted, somewhat arbitrarily, for the in-flight intensities -- here the limitation is likely to be one of beam contamination. The very high intensities occur for fragments close in mass to the primary beam and the tails of the primary beam may be a problem. Both these limitations may be improved in the future and an additional order of magnitude gained for beams where the intensity in the table is now limited to 109/s.
d) In-Flight Fission is the same as the fragmentation technique, except that the primary beam is uranium, which fissions in the target, introducing a larger momentum and angular spread in the products. Estimates of these yields are based on measurements at GSI and are extrapolated when necessary. Other losses and estimates of efficiencies are the same as for Fragmentation above.
Note that other mechanisms (e.g. compound nucleus formation) are not included in these tables. This mechanism is likely to be the optimal one for heavier proton-rich nuclei.
* For stopped beams the intensities are similar or somewhat higher, depending on the production mechanism, the species, and in what form and isotopic purity the stopped nuclei are desired.
The estimates for individual elemental production rates are shown graphically here:
