Görgen, A

Siem, S

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CERN-INTC-2009-035/INTC-CLL-00619/10/2009                                                                    1 Letter of Clarification   A. Görgen (CEA Saclay), S. Siem (Univ. Oslo) 19.10.2009   In its February 2009 meeting the INTC has evaluated our proposal “Study of oblate nuclear shapes and shape coexistence in neutron-deficient rare earth isotopes” (CERN – INTC – 2009 – 009/P – 257). In its report,  the Committee endorsed the physics case and requested a Letter of Clarification which addresses the issue of contamination suppression needed and tests of RILIS schemes before recommendation for approval by the Research Board.  In this letter of clarification we discuss the technical progress for the development of beams of rare earth elements made over the last months and the consequences for the feasibility of the proposed experiment. The physics case comprises studies of four isotopes of rare earth elements with neutron number N=78: 138Nd, 140Sm, 142Gd, and 144Dy. Theoretical calculations predict a transition from prolate shape in 140Sm to oblate in 142Gd, and it was proposed to study these two nuclides using the technique of projectile Coulomb excitation. As such beams have never been produced at ISOLDE, testing and development work was required. The chemical properties of the rare earth elements are very similar, so that resonant laser ionization is mandatory. In particular the question of beam contamination due to surface ionization needed to be addressed in order to evaluate the feasibility of the project.   An improvement in the reduction of unwanted surface ionization in the RILIS was expected to come from the use of a new high-temperature, low work function cavity of gadolinium hexaboride (GdB6). Tests with such a cavity have been performed on the GPS separator in June 2009 over a period of three days. Currents have been measured for different mass settings and the beam composition has been analyzed by measuring the beta and gamma decay of the ions. During the tests the laser ionization scheme for Nd atoms has been applied and laser on/off measurements were performed. An enhancement factor of 6 was found between the measurements with laser on and off for Nd beams at temperatures of 2000°C and 1800°C for the target and transfer line, respectively. The tests show that surface ionization in the GdB6 cavity is significantly suppressed compared to standard W or Ta cavities. In this way it was possible to produce a 139Nd beam with approximately 50% purity, with the main contaminants being 139Pm and 139Sm. The tests also yielded additional experimental data on production cross sections for various nuclides in the region of interest. More details and results from the tests can be found in a technical report prepared by V. Fedosseev and T. Stora.   The test has shown that it is possible to produce beams of rare earth elements with intensities and purities that are sufficient for a successful Coulomb excitation experiment. The results are very encouraging for the proposed experiment since the suppression of unwanted surface ionization should work equally well for the production of other rare earth elements. Despite the positive results for the production of Nd beams we see no strong motivation to start the experimental program with a measurement on 138Nd. Among the four nuclei under consideration 138Nd is the least interesting to understand the evolution of shapes in this region                                                                     2 of the nuclear chart. The production of a mass A=138 beam is furthermore likely to suffer from contamination of the stable 138Ba isobar. The production of a 140Sm beam seems in fact easier in comparison to the 138Nd case. The only stable A=140 isobar is 140Ce, which was shown to be unproblematic in the tests of June 2009. We therefore adhere to our original plan to start the experimental program with a measurement on 140Sm.   After the positive results of the test with the GdB6 cavity we feel confident that a 140Sm beam of sufficient purity and intensity can be produced. Nevertheless, exact numbers for the beam purity can only be given after an on-line test of the laser ionization scheme for Sm. Such a scheme exists, but remains to be tested at ISOLDE. With this letter we would like to ask that a test of the Sm RILIS scheme be scheduled with high priority early in 2010. If this test confirms our expectations, a Coulomb excitation experiment with 140Sm beam could be scheduled in the Miniball campaign of 2010. Based on the data obtained during the Nd test, we expect a beam intensity of better than 105 particles per second after acceleration. Originally we had expected 4⋅105 pps based on old yield measurements obtained with the SC driver. To achieve the goals described in the proposal for 140Sm we estimate that 15 shifts of beam time are needed. More precise numbers for the beam intensity and purity will be available after a test of the RILIS scheme.  The continuation of the program with a measurement on 142Gd requires more testing and development work. We feel that this part of the experimental program should only be attempted after a successful production run with 140Sm. We therefore propose to postpone this part of the program until first results are available for 140Sm.      

Study of oblate nuclear shapes and shape coexistence in neutron-deficient rare earth isotopes

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