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Mapping the boundaries of the seniority regime and collective motion: Coulomb excitation studies of 208Po and 210,212Rn

Addendum to IS506T. Grahn

University of JyväskyläINTC meeting 1.2.2012

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The neutron-deficient trans-Pb region

p1h9/2

n2f5/2

208Rn206Po

N=122

210,212Rn 208Po N=126

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The neutron-deficient trans-Pb region

Around the Z=82 and N=126 shell closures level patterns resembling seniority n=2 structure have been observed.

Relative high-j proton single-particle orbital (j=9/2)dominate the structure - seniority n 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 1h9/2 protons. This implies that the seniority can be preserved.

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N=124,126 isotones in the trans-Pb regionLevel-energy systematics

Isomeric 8+ states

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N=124,126 isotones in the trans-Pb regionOpen questions: The microscopic structure of the proposed Dn=2 2+

states. – Seniority scheme predicts parabolic behaviour of the B(E2)

values across the ph9/2 shell.– Neutron states and configuration mixing?

The proton-neutron interaction in this region should not be strong due to the large Dn and Dl between the available proton and neutron orbitals.– Different trend for the B(E2) values across the ph9/2 shell

expected (flat as the p-n interaction weak).

THIS STUDY ADDRESSES FUNDAMENTAL NUCLEAR STRUCTURE MATTERS AT AND

NEAR THE CLOSED SHELLS.

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N=124,126 isotones in the trans-Pb regionMinutes of the 38th 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.

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Proposed experiments

Objectives: to measure the B(E2;0+→2+) values in the 208Po and 210,212Rn through Coulomb excitation at ISOLDE.

Post-accelerated beams of 208Po and 210,212Rn (3 MeV/u) will be Coulomb excited in the 114Cd target at MINIBALL target position. De-exciting g rays will be detected with the MINIBALL g-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.

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Proposed experiments

From the IS506 data

Ene

rgy

(MeV

) 114Cd208Rn

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Proposed experiments

208Rn on 114Cd

208Rn on 120Sn

From the IS506 data

Preliminary, Doppler correction for 208Rn

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Production and purity of the 210,212Rn 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. 208Rn ~ 5.6∙105 pps at the MINIBALL target position (TAC comment).

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Production and purity of the 208Po beam

Possible isobaric contaminants in the 208Po beam can be suppressed by using the RILIS laser ion source (c.f. experiment IS479) and HRS in the high-resolution mode.

TAC note: 208Fr contaminant strong ⇒ 208Po t1/2 ≈ 3 y ⇒possibility to suppress (208Fr t1/2 ≈ 1 min) in proton on/off experiment (c.f. IS465).

Yield estimate conservative, possible reduction by a factor of 2 is not crucial.

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Production and purity of the 208Po beam

IS465, courtesy of L. P. Gaffney

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Count rate estimates and beam time request

ISOLDE primary yields: 208Po 5·106 mC-1 and 210,212Rn similar to 208Rn.

Assume: 1% post-acceleration efficiency, 7% efficiency of MINIBALL, 2 mg/cm2 target thickness, the CD angular coverage of 16°≤θ≤53° and 0⟨ +||Ô(E2)||2+ = ⟩0.407 eb (204Pb).

Conservative estimate (2+ to 0+ transitions): 200 g rays in 3 shifts (208Po), 9000 g rays in an hour (210,212Rn).

Beam time efficient study, in total 4 shifts of beam time is requested.

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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)