Production, Separation, and Decay Spectroscopy

Principal Investigator: Krzysztof Rykaczewski, HRIBF

Half-life measurements

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At the HRIBF, we produce very neutron rich nuclei using proton-induced fission of 238U. A schematic of the ISOL method for producing and separating beams is shown in Fig 1. Protons of 50 MeV energy and beam intensity up to 1013 protons/s accelerated in the Oak Ridge Isochronous Cylotron (ORIC) are creating around 1011 fission events per second in about 7 grams of 238UCx target. The further handling of this very high radioactivity involves the ionization and release from the target-ion source system and a first magnetic pre-separation (mass resolution ΔM:M ~ 10-3). The pre-selected, initially positive ions, are converted into the negative ones in the charge-exchange cell. The efficiency of the charge-changing process is typically about 1-20%. Note that some ions, eg. Zn, do not form negative ions. Further mass separation is achieved in the high resolution injector magnet (ΔM:M ~ 10-4). Finally, radioactive ions beams (RIBs) are accelerated to a few MeV/nucleon by the HRIBF Tandem. We are able to produce pure samples of individual neutron-rich isotopes and to deposit them in front of our detector setup measuring the decay properties. These measurements include the β-delayed neutron branching ratio and other properties of β decay.

1. Ranging out

A unique "ranging-out" method was developed [1] for separating the nuclei of interest from the isobaric cocktail of post-accelerated neutron rich beams. We pass the accelerated and mass-separated beam through a gas-filled ionization chamber which acts as an ion energy degrader, see Fig. 1. The most neutron rich component of the beam is transmitted through the chamber while the unwanted products are stopped, see Fig. 2. While studying the decays of the resulting pure activities[2,3]. We already found out that in several Cu and Ga isotopes the βn-branching ratios are two to three times higher than previously reported [4].

Fig. 1 - An example of the ranging-out process as applied to 79Cu.

Fig. 2 - The two-dimensional spectrum of the ion energies recorded in the gas-filled ionization chamber (IC) for mass A=76 ions. The total energy loss is the same for all A=76 isobars (horizontal axis) while the energy deposition in the last sector of the gas cell (vertical axis) is the largest for the lowest-Z component of the beam. The result is that the Z=29 76Cu ions deposit the largest energy at the end of the ionization chamber. Ions of 76Cu can be selectively transmitted through the ion chamber to the decay spectroscopy setup with a proper adjustment of the gas pressure. The charge exchange cell used at the HRIBF setup removed Zn ions from the mass-separated beam.

2. LeRIBSS

The Low-energy Radioactive Ion Beam Spectroscopy Station LeRIBSS can improve our measurements by providing higher yields and a larger range of isotopes. LeRIBSS is a new beamline positioned before the tandem accelerator but after the high resolution separator. As such, this station can receive beams whose charge is determined by the ion source and the beam does not require acceleration. Thus, we expect beams to be a factor of 10 more intense due to the lack of acceleration and by not requiring charge exchange, some beams will be improved by an additional factor of 5-100. See the link above to find out more about this new station and the equipment associated with it.

References

[1] Gross et al., Eur. Phys. Jour. A 25, s01, 115, 2005.
[2] Winger et al., Acta Phys. Pol. B 39, No 2, 2008.
[3] Ilyushkin et al., World Scientific, 2008.
[4] Pfeiffer, Kratz, Moeller, Progr. in Nucl. Energy, Vol. 41, 39, 2002.

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This file last modified Friday March 14, 2008