Abstract
Spectra of γ rays following neutron capture at isolated resonances of 6 stable Gd isotopes were measured with highly segmented BaF 2 detector DANCE at the Los Alamos LANSCE spallation neutron source. The main emphasis was put on studying the γ -cascade decay of neutron resonances to get unique information on photon strength. An analysis of the accumulated γ -ray spectra within the extreme statistical model leads to an inescapable conclusion that scissors mode resonances are built not only on the ground-state , but also on excited levels in all product nuclei studied. The results on summed B(M1)↑ strength and energy of the scissors mode are compared with systematics of scissors mode parameters for the ground-state transitions deduced from nuclear resonance fluorescence measurements. A specific feature of our experiments is the investigation of scissors mode of odd nuclei, for which the nuclear resonance fluorescence provides only limited information.
Highlights
In medium and heavy mass nuclei detailed spectroscopic information exists only for levels at low excitation energy near the ground state and often for resonances above the neutron separation energy S n
It is believed that γ decay of the nucleus in the quasicontinuum is described by the extreme statistical model in terms of the nuclear level density and a set of photon strength functions (PSFs) for different multipolarities
The multistep cascade γ-ray spectra for different multiplicities were used to test the validity of various PSF models
Summary
In medium and heavy mass nuclei detailed spectroscopic information exists only for levels at low excitation energy near the ground state and often for resonances above the neutron separation energy S n. The combination of the pulsed neutron beam at LANSCE (Los Alamos Neutron Science CEnter) and the highly segmented, highly efficient γ calorimeter DANCE (Detector for Advanced Neutron Capture Experiments) provides an ideal opportunity to study these γ-ray spectra In this contribution we describe a measurement of the series of stable Gd isotopes using the DANCE calorimeter. A major goal of these experiments is to use γ-ray spectra measured for various multiplicities to determine the appropriate PSFs with special interest in the behavior of the scissors mode which is expected to play an important role in γ-decay of Gd nuclei
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