Program - Brookhaven National Laboratory

very unstable nuclei if the beta-decay T 1/2 and the Q-beta value are precisely known and if the relative
B(GT) values are studied in CE reactions for the mirror nucleus. In order to determine the degree of
precision of this method a campaign has been launched at several different laboratories to study the decays
of Tz=-1 and Tz=-2 nuclei in the fp shell with the aim of comparing the results with the results from
CE experiments on the corresponding Tz=+1 and Tz=+2 mirror nuclei carried out at RCNP (Japan).
Recent experimental developments have made this comparison effective. On the one hand there has been
a substantial improvement in resolution in studies of ( 3 He,t) CE reactions at RCNP. On the other hand
the use of fragment separators to create and identify short-lived nuclear species has improved experimental
accessibility to the beta decays of exotic nuclei (all nuclei studied here have T 1/2 of the order of 100 ms).
We will present here a summary of the results with special emphasis on the precise beta decay studies
obtained at the GSI and GANIL facilities. The experiments include DSSD implantation- beta- proton
detectors and Ge-gamma arrays. The experiments resulted in T 1/2 determinations for more than 20 nuclei
with one to two orders-of-magnitude improvement in accuracy and absolute branching ratios to excited
states up to 5 MeV excitation in nuclei where typically only the branching to the lowest state was known
previously. A special effort has been made to determine the strength of the g.s to g.s beta decay branch,
which is notoriously difficult to measure.
OD 2 2:00 PM
Half-life Measurements of Excited Levels in Fission Products Around a Mass Number of
150
Y. Kojima, Radioisotope Research Center, Nagoya University, Japan. Y. Shima, Graduate School of
Engineering, Nagoya University, Japan. H. Hayashi, Institute of Health Biosciences, The University of
Tokushima Graduate School, Japan. A. Taniguchi, Research Reactor Institute, Kyoto University, Japan.
M. Shibata, Radioisotope Research Center, Nagoya University, Japan.
Half-lives of nuclear excited levels relate γ transition probabilities and nuclear matrix elements, which are,
for example, used for investigating nuclear structure and for modifying nuclear models. Therefore, data on
level half-lives are one of the most essential properties in fields of nuclear science and technology. These
data have been accumulated through many sophisticated experiments, and evaluated for conveniences of
various applications. In spite of these efforts, experimental half-lives are known only for a limited number
of levels in β unstable nuclides, owing to difficulties both of measuring the half-lives and of producing the
nuclides. In order to measure level half-lives around 0.1-10 ns by means of the β-γ-γ delayed coincidence
technique, a new spectrometer has been installed to the on-line isotope separator at the Kyoto University
Reactor. As described in details in ref. [1], the spectrometer consists of a LaBr3, a plastic scintillator and
an HPGe detector. The LaBr3 detector is used as a fast γ detector, whereas the HPGe detector is used
to select a desired γ branch. A β signal from the 1-mm-thick plastic scintillator starts the functioning
of a time-to-amplitude converter module. Time resolutions (FWHM) were found to be 375 ps for the
1333-keV, and 601 ps for 99-keV γ rays. This means that our spectrometer is capable of measuring the
level half-lives down to sub-nanosecond range. Using this experimental apparatus, we have performed
half-life measurements for neutron-rich rare-earth nuclides around a mass number of 150. These nuclides
were produced by the thermal neutron-induced fission of 235 U, ionized by a thermal ion source, and massseparated
with a mass resolution of about 600. Until now, we successfully observed new level half-lives in
148 Pr [1] and 149 Nd (partly reported in ref. [2]). In this conference, we will present an overview of the
measuring setup as well as the latest experimental results including new level half-lives in 149,150 Pr. This
work was supported by a Grant-in-Aid for Scientific Research (no. 23540344) from Japan Society for the
Promotion of Science.
211