Eur. Phys. J. A 17, 49-56 (2003)
DOI: 10.1140/epja/i2002-10139-0

Fission lifetimes of Th nuclei measured by crystal blocking

S.A. Karamian¹, J.S. Forster², J.U. Andersen³, W. Assmann⁴, C. Broude⁵, J. Chevallier³, J.S. Geiger², F. Grüner⁴, V.A. Khodyrev⁶, F. Malaguti⁷ and A. Uguzzoni<sup>7

1</sup>  FLNR JINR, 141980 Dubna, Russian Federation
²  Départment de Physique, Université de Montréal, Montréal, Quebec, H3C 3J7 Canada
³  Department of Physics and Astronomy, University of Aarhus, DK-8000 Aarhus C, Denmark
⁴  Sektion Physik, Universität München, 85748 Garching, Germany
⁵  Physics Department Weizmann Institute of Science, Rehovot, Israel
⁶  Moscow State University, Moscow, Russian Federation
⁷  Department of Physics, University of Bologna and INFN Bologna, Italy

jua@ifa.au.dk

(Received: 2 October 2002 / Revised version: 1 January 2003 / Published online: 29 April 2003)

Abstract
Crystal blocking lifetime measurements have been made for highly excited Th nuclei with neutron number well removed from the stability line. Thin W crystals were bombarded with ³²S ions in the energy range 170-180 MeV and the yield of fission fragments was measured for emission close to a  axis. The fission blocking dips are compared to the appropriately scaled ones for elastic scattering of the ³²S beam ions and no significant difference is seen between the dips. This implies that the fraction of nuclei fissioning with lifetimes longer than 10 as is less than 2%. Fission lifetimes are increased by viscosity in the nuclear mass flow and comparison with a statistical model calculation indicates that the viscosity parameter, , must be lower than for Th and U nuclei near -stability. The effect of the N = 126 magic number is discussed.

PACS
24.75.+i - General properties of fission.
61.85.+p - Channeling phenomena (blocking, energy loss, etc.).

© Società Italiana di Fisica, Springer-Verlag 2003