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To resolve the reported anomalous behavior of the a-decay rates of 186 -188 ' 1 90,i92p^ we determined more accurate a-decay branches for these isotopes. Our data result in a-decay reduced widths whose dependence on TV is similar to that observed for other elements. However, contrary to the expectation of a shell effect at Z = 82, this new information indicates lead nuclei to be less hindered toward a decay than mercury isotopes. It appears that midway between N = 82 and N = 126 the proton number of 82 is not magic.

Background An assessment done under the auspices of the Nuclear Energy Agency in 2007 suggested that the β decays of abundant fission products in nuclear reactors may be incomplete. Many of the nuclei are potentially affected by the so called Pandemonium effect and their β-γ decay heat should be restudied using the total absorption technique. The fission products 137 I and 137 Xe were assigned highest priority for restudy due to their large cumulative fission branching fractions. In addition, measuring β-delayed neutron emission probabilities is challenging and any new technique for measuring the β-neutron spectrum and the β-delayed neutron emission probabilities is an important addition to nuclear physics experimental techniques.Purpose To obtain the complete β-decay pattern of 137 I and 137 Xe and determine their consequences for reactor decay heat andνe emission. Complete β-decay feeding includes ground state to ground state β feeding with no associated γ rays, ground state to excited states β transitions followed by γ transitions to the daughter nucleus ground state, and β-delayed neutron emission from the daughter nucleus in the case of 137 I. MethodWe measured the complete β-decay intensities of 137 I and 137 Xe with the Modular Total Absorption Spectrometer at Oak Ridge National Laboratory. We describe a novel technique for measuring the β-delayed neutron energy spectrum, which also provides a measurement of the β-neutron branching ratio, Pn.Results We validate the current ENSDF evaluation of 137 Xe β decay. We find that major changes to the current ENSDF assessment of 137 I β-decay intensity are required. The average γ energy per β decay for 137 I β decay (γ decay heat) increases by 19%, from 1050 keV to 1250 keV, which increases the average γ energy per 235 U fission by 0.11%. We measure a β delayed neutron branching fraction for 137 I β decay of 7.9 ± 0.2(f it) ± 0.4(sys)% and we provide a β-neutron energy spectrum. ConclusionsThe Modular Total Absorption Spectrometer measurements of 137 I and 137 Xe demonstrate the importance of revisiting and remeasuring complex β decaying fission products with total absorption spectroscopy. We demonstrate the ability of the Modular Total Absorption Spectrometer to measure β-delayed neutron energy spectra.

The neutron-deficient isotopes ' " Tl have been studied using collinear fast atom beam laser spectroscopy with mass-separated beams of 7&(10 to 4&( l0' atoms per second. By laser excitation of the 535 nm atomic transitions of atoms in the beam, the 6s 7s S1&2 and 6s 6p P3/2 hyperfine structures were measured, as were the isotope shifts of the 535 nm transitions. From these, the magnetic dipole moments, spectroscopic quadrupole moments, and isotopic changes in mean-square charge radii were deduced. A large isomer shift in '"Tl was observed, implying a larger deformation in the -, isomer than in the -'+ ground state. The " ' '' 'Tl isotopes have deformations that increase as the mass decreases. A deformed shell model calculation indicates that this increase in deformation can account for the drop in energy of the -' bandhead in these isotopes. An increase in neutron pairing correlations, having opposite and compensating effects on the rotational moment of inertia, maintains the spacing of the levels in the -, strong-coupled band. Results for ' Tl differ from previously published values, but are consistent with the 190, 192Tlm d