I.Y. Lee

Lepton Number Violation in Softly Broken Supersymmetry

Abstract We study the implications of explicit lepton number violating soft operators in a general low-energy effective theory with softly broken supersymmetry. We find that nonzero but relatively small VEVs of scalar neutrinos could exist. In the case where the VEVs of scalar neutrinos vanish, the lepton number violating effects are naturally suppressed.

Reduced transition probabilities for high-spin states in 232Th

Abstract The lifetimes and reduced transition probabilities for spins 6 + − 18 + in the ground-state rotational band of 232 Th have been measured by recoil-distance, Doppler-broadened lineshape, and multiple Coulomb excitation methods using 84 Kr and 136 Xe projectiles. Within the limits of experimental uncertainty, transition probabilities from all of the methods are consistent with those of a good rotor for spins 6 + − 18 + . The descrepancy with the prediction of simple β-band mixing and centrifugal stretching models is discussed.

High-spin states of 174, 176Yb studied in Coulomb excitation with Kr AND Xe beams

Abstract Rotational states up to spin 20+ (18+) in 174Yb (176Yb) have been Coulomb excited using beams of 136Xe. Lifetimes up to and including the 14+ state have been measured using Doppler-broadened lineshape techniques with 136Xe and 86Kr beams. An annular gas-scintillation counter has been developed in order to perform particle-γ coincidence studies. Moments of inertia in 174, 176Yb behave very regularly, showing no signs of backbending effects. The measured lifetimes are in agreement with the rotational model predictions, and the measured cross sections for Coulomb excitation of the high-spin states are in agreement with the semiclassical Winther-de Boer calculation.

Structure of exotic nuclei near and above 208 Pb populated via deep-inelastic collisions

High-spin states have been studied in neutron-rich nuclei produced in deep-inelastic collisions between a pulsed beam of Pb ions and a U target. The structure of nuclei such as Hg, Pb and Bi has revealed new information on the couplings of single-particle states to each other and to the octupole-vibrational phonon, essential input to the nuclear shell model. In addition, the first observations of high-spin states in U, U and U are compared to recent results for neighbouring plutonium nuclei.

K+→π++γ̃γ̃ in spontaneously broken supersymmetric models

Abstract We study the decay K + →π + γ γ in models with spontaneous supersymmetry breaking and find that it is generally suppressed relative to the decay K + Λπ + ν ν of the conventional model, except possibly for a class of models where the squark masses are generated by radiative corrections from a much larger supersymmetry breaking scale. For a small range of squark and photino parameters, the cascade process K + →π + π 0 →π + γ γ will become dominant over the ν ν mode. We also comment on the possibility of probing the neutrino mass through the K + →π + π 0 →π + ν ν cascade decay.

Positron production in heavy-ion collisions☆

Abstract An experimental method has been developed which can predict the shape and intensity (within a factor of two) of the internal pair conversion positron spectra in heavy-ion nuclear reactions, independent of the production mechanism. To check the method, positron spectra have been measured from bombardments of 624 MeV 136Xe on targets of Th, Au and A1.

Isometric trapping following Coulomb excitation of high spin states in 178Hf

Abstract Coulomb excitation of states in 178, 180 Hf has been observed to 16 + and 12 + , respectively, with Kr and Xe ions. A strong up bend in the moment of inertia above the 12 + level is seen in 178 Hf and is well reproduced by cranked shell model calculations. For the first time in Coulomb excitation, population of the lowest K π = 8 - isomer in 178 Hf was observed.

Experimental study of 136Nd at high angular momentum

Abstract The γ-ray multiplicities following 100 Mo ( 40 Ar , x n γ) Nd have been studied by several methods. The measured width of the multiplicity distribution and the shape of the multiplicity spectrum as a function of transition energy suggest a broad region of feeding into the 4n channel. The dependence of the multiplicity on transition energy, for the sum of all reaction channels and for only the 4n reaction channel, is well reproduced by a calculation which suggests that the products become deformed rotors at spins above 30ħ.

Subcoulomb fission induced by Xe and Kr ions

Abstract Excitation functions have been measured for fission of 232 Th and 238 U induced by 86 Kr and 136 Xe beams at energies around the Coulomb barrier. Fission events were detected in coincidence with the back-scattered projectiles. The results suggest that Coulomb fission is occuring in the Xe + U reaction.

Search for superheavy nuclei with lifetimes longer than 2 ps

Abstract The reaction 469 MeV 86 Kr + 208 Pb was used to search for superheavy elements which decay at rest by fission. The technique searches with equal sensitivity over a lifetime range from 2 ps (the stopping time of the fused system) to many hours and thus covers a broad spectrum of lifetimes not yet examined for superheavy element production. No evidence for such formation was found; the upper limit on the cross section is 1.5 μb.