Nuclear asrophysics with low energy beams of short-lived nuclei

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We have investigated the possible use of liquid helium as an effective ion-source for beams of radio-active nuclei, which open up studies of nuclear synthesis via reactions associated with unstable nuclei under environments of very high temperature and high density such as in novae. An ion introduced into the liquid helium gathers many helium atoms around itself due to the electrostriction and the ion is trapped in the cluster of helium atoms. This charged cluster is referred to as "snowball" When the ions of radioactive nuclei produced in nuclear reactions are stopped and trapped in very low temperature liquid helium, the extracted beams from the liquid will have very small emittance.The feasibility of such ion-source relies on achieving (i) the high efficiency in forming the snowball, (ii) long lifetime of the snowball and (iii) high efficiency in extracting the ions from the liquid. We have examined the characteristics of the snowball in the following experiments; ions of ^<14>N with energy of 90 MeV are implanted into the liquid helium, and the ion currents have been measured under the electric fields applied in correlation with the beam pulse. The results we obtained are as follows:(1) The lifetime of the snowball was found to be about 700 ms, which is much longer than in the other methods of ion trapping.(2) The ion current depended much on the electron density around the ion in the liquid. This implies that the efficiency in forming the snowball may be increased by decreasing the implantation energy of the ions and by increasing applied field.(3) The mobility of the snowball was much larger in the superfluid helium than in the normal helium. Therefor more rapid control of the snowball is possible in the superfluid. In summary the liquid helium ion-source for the radio-active nuclear beams is quite promising.