Spectroscopy of Neutron Rich Nuclei using a Charged Heavy Ion Detector (CHICO) plus Gammasphere

M. W. Simon, C. Y. Wu, D. Cline, R. W. Gray

Nuclear Structure Research Laboratory, University of Rochester

M. A. Stoyer

Lawrence Livermore National Laboratory

K. Vetter, A. Macchiavelli, K. Gregorich, R. W. Macleod, S. Asztalos, J. Gilat

Lawrence Berkeley National Laboratory

C. T. Zhang, P. Bhattacharyya

Department of Chemistry and Physics, Purdue University

The technique of studying neutron-rich nuclei produced by spontaneous fission (SF) with large $\gamma$-ray arrays can be extended by using a thin source, which allows the detection of fission fragments in kinematic coincidence with the $\gamma$-rays. This method yields more information on the $\gamma$-rays than similar experiments with thick sources, and also provides additional selectivity in the the analysis of the $\gamma$-ray spectra. A high-statistics experiment was performed recently at Gammasphere using a thin ²⁵²Cf SF source and the Rochester heavy ion detector array (CHICO). The time-of-flight measurement provides a means of measuring the masses of the fission fragments to within  8 amu and, along with the detection of the fission axis direction, allows a Doppler correction to be performed on an event-by-event basis. The Doppler correction enables unambiguous assignment of the $\gamma$-ray to the heavy/light fission partner. Since the fragments recoil freely, the Doppler-broadened line shape problems of sealed source experiments are eliminated, allowing observation of levels with lifetimes comparable to, and shorter than, the stopping time. For example, many rotational bands have been extended to spin $\sim$20 $\hbar$, corresponding to an excitation energy of $\sim$7 MeV. The geometry of the particle detector also provides a method for studying isomeric nuclei and measuring lifetimes of transitions with $\tau_{\frac{1}{2}}\geq$ 10 ns.