1. UNIVERSITY OF JYVÄSKYLÄ Mapping the boundaries of the seniority regime and collective motion: Coulomb excitation studies of 208 Po and 210,212 Rn Addendum to IS506 T. Grahn University of Jyväskylä INTC meeting 1.2.2012

2. UNIVERSITY OF JYVÄSKYLÄ The neutron-deficient trans-Pb region  1h 9/2 2f 5/ 2 208 Rn 206 Po N=122 210,212 Rn 208 Po N=126

3. UNIVERSITY OF JYVÄSKYLÄ The neutron-deficient trans-Pb region Around the Z=82 and N=126 shell closures level patterns resembling seniority =2 structure have been observed. Relative high-j proton single-particle orbital (j=9/2)dominate the structure - seniority  can be regarded as a good quantum number. In the trans-Pb nuclei with 120 ≤ N ≤ 128 the neutrons occupy high n, low l orbitals and therefore they should have weaker interactions with the 1h 9/2 protons. This implies that the seniority can be preserved.

4. UNIVERSITY OF JYVÄSKYLÄ N=124,126 isotones in the trans-Pb region Level-energy systematics Isomeric 8 + states

5. UNIVERSITY OF JYVÄSKYLÄ N=124,126 isotones in the trans-Pb region Open questions: The microscopic structure of the proposed  =2 2 + states. –Seniority scheme predicts parabolic behaviour of the B(E2) values across the  h 9/2 shell. –Neutron states and configuration mixing? The proton-neutron interaction in this region should not be strong due to the large  n and  l between the available proton and neutron orbitals. –Different trend for the B(E2) values across the  h 9/2 shell expected (flat as the p-n interaction weak).

6. UNIVERSITY OF JYVÄSKYLÄ N=124,126 isotones in the trans-Pb region Minutes of the 38 th meeting of the INTC: “The committee took the view that although the proposed measurements would be insufficient for a complete systematic study, it recommends for approval…” ⇒ More systematic experiments are proposed. ISOLDE is the only facility where the trans-Pb RIBs are available While nuclear structure near N=126 may seem well understood, experimental B(E2;0 + →2 + ) values are missing. ⇒ No direct experimental information on the transition matrix elements available.

7. UNIVERSITY OF JYVÄSKYLÄ Proposed experiments Objectives: to measure the B(E2;0 + →2 + ) values in the 208 Po and 210,212 Rn through Coulomb excitation at ISOLDE. Post-accelerated beams of 208 Po and 210,212 Rn (3 MeV/u) will be Coulomb excited in the 114 Cd target at MINIBALL target position. De-exciting  rays will be detected with the MINIBALL  -ray spectrometer in coincidence with outgoing particles detected in MINIBALL CD. Ideally to be run back-to-back with the other Rn and Po experiments, REX easily scalable by users.

8. UNIVERSITY OF JYVÄSKYLÄ Proposed experiments From the IS506 data Energy (MeV) 114 Cd 208 Rn

9. UNIVERSITY OF JYVÄSKYLÄ Proposed experiments 208 Rn on 114 Cd 208 Rn on 120 Sn From the IS506 data Preliminary, Doppler correction for 208 Rn

10. UNIVERSITY OF JYVÄSKYLÄ Production and purity of the 210,212 Rn beams Rn as noble gas can be purified using the cooled transfer line (c.f. IS506 July 2011). The yield sufficient for the proposed studies, c.f. 208 Rn ~ 5.6∙10 5 pps at the MINIBALL target position (TAC comment).

11. UNIVERSITY OF JYVÄSKYLÄ Production and purity of the 208 Po beam Possible isobaric contaminants in the 208 Po beam can be suppressed by using the RILIS laser ion source (c.f. experiment IS479) and HRS in the high-resolution mode. TAC note: 208 Fr contaminant strong ⇒ 208 Po t 1/2 ≈ 3 y ⇒ possibility to suppress ( 208 Fr t 1/2 ≈ 1 min) in proton on/off experiment (c.f. IS465). Yield estimate conservative, possible reduction by a factor of 2 is not crucial.

12. UNIVERSITY OF JYVÄSKYLÄ Production and purity of the 208 Po beam IS465, courtesy of L. P. Gaffney

13. UNIVERSITY OF JYVÄSKYLÄ Count rate estimates and beam time request ISOLDE primary yields: 208 Po 5·10 6  C -1 and 210,212 Rn similar to 208 Rn. Assume: 1% post-acceleration efficiency, 7% efficiency of MINIBALL, 2 mg/cm 2 target thickness, the CD angular coverage of 16°≤θ≤53° and 0 + ||Ô(E2)||2 + = 0.407 eb ( 204 Pb). Conservative estimate (2 + to 0 + transitions): 200  rays in 3 shifts ( 208 Po), 9000  rays in an hour ( 210,212 Rn). Beam time efficient study, in total 4 shifts of beam time is requested.

14. UNIVERSITY OF JYVÄSKYLÄ Collaboration CERN-ISOLDE (E. Rapisarda, D. Voulot, F. Wenander) STFC Daresbury Laboratory (M. Labiche, J. Simpson) TU Darmstadt (S. Bönig, S. Ilieva, Th. Kröll, M. Scheck, M. Thürauf) University of Jyväskylä (T. Grahn, A. Herzan, U. Jakobsson, R. Julin, J. Pakarinen, P. Rahkila) KU Leuven (N. Bree, J. Diriken, M. Huyse, N. Kesteloot, E. Rapisarda, S. Sambi, P. Van Duppen, K. Wrozek-Lipska) University of Liverpool (P. A. Butler, L. P. Gaffney, D. T. Joss, D. O’Donnell, R. D. Page) University of Rochester (D. Cline) University of Warsaw (P. Napiorkowski) Yale University (V. Werner) University of York (D. G. Jenkins)