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A record production of ultra-pure radioactive beams by laser ion source is obtained at ALTO

To push the studies of the structure of exotic nuclei further and further from stability, it is necessary to have recourse to more and more elaborate purification techniques.

For these studies, the very short-lived nuclei are actually produced from very minority channel reactions. It is necessary to isolate them from the other produced nuclei in order to reach a S/N ratio allowing to identify them unambiguously and to obtain data of quality. To produce exotic nuclei with the ISOL method at ALTO, we have developed a laser ion source allowing the ionization of one element only, that is selecting the nucleus with its Z charge number.

This selection is obtained by setting the laser wavelengths to the energies of the atomic transition characteristic of the studied atom. A single charge ion is thus formed by absorption of many photons until the ionization continuum. Without any lasers, the atom is simply ionized by the heating of the ion source : it is the surface ionization. The more the ionization tube is hot the most atoms are ionized. But it’s with a non-selection and with a minimal efficiency.

By adding the laser ionization, one increases the efficiency of the ion source by a factor of ten to 50.

The ALTO laser ion source is now more reliable and an online experiment on indium radioactive nuclei has been successfully completed. At medium heating temperature of the source, we obtained an unequaled laser ionization efficiency up to now.

Counting the same time without laser (surface ionization only) and with laser (surface ionization + laser), we record the radioactive decay spectra of 127In below (nucleus far from 13 masses of stability). A source efficiency multiplied by 50 is obtained thanks to the lasers.

Caption : Gamma emission spectra characteristic of 127In radioactive decay obtained at ALTO. In blue by surface ionisation and in red by adding the laser. The surface ratio of the line stamped “127In“ gives a direct measurement of the gain obtained in the production rate.

This performance allowed us to isolate the very exotic nucleus 134In (containing 19 neutrons more than the last stable isotope 115In) produced in the photo-fission of 238U and to study its decay properties. This nucleus is located at the current boundaries of our nuclear structure knowledge in the doubly magic 132Sn region and has just been studied through an experiment on TETRA at ALTO as part of the BESTIOL collaboration.

 This work is part of a joint R&D with GANIL concerning the installation of the future laser ion source at S3. This should be universal, ie combining the laser sources of GANIL and IPN on the RILIS model at ISOLDE / CERN.

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UMR 8608 - CNRS/IN2P3

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