M. W. Simon

CHICO, a heavy ion detector for Gammasphere

A 4π position-sensitive heavy-ion detector system, CHICO, has been developed primarily for use in conjunction with the 4π γ-ray facility, Gammasphere. The CHICO detector comprises an array of 20 Parallel Plate Avalanche Counters (PPACs) covering 12°<θ<85° and 95°<θ<168° and 280° in φ. The PPACs have segmented delay-line cathode boards, measuring the polar scattering angle θ to 1°, and segmented anodes, measuring the azimuthal angle φ with 9° resolution, while measuring the time-of-flight difference with 500 ps resolution. For binary reactions the kinematics can be reconstructed from the measured information, allowing identification of the target- and projectile-like nuclei with a mass resolution of Δm/m≈5%. The measured masses, recoil velocities, and recoil angles allow correction for Doppler shift and assignment of individual γ-rays to decay of the correct reaction product. This paper describes the design, operation and performance of the CHICO detector. The powerful combination of CHICO plus Gammasphere provides new research opportunities for the study of nuclear structure and reactions.

Violations of K‐Conservation in 178Hf

Coulomb excitation of Kπ=6+(t1/2=77 ns), 8−(t1/2=4.0 s) and 16+(t1/2=31 y) 178Hf isomers has led to the measurement of a set of Eλ matrix elements, coupling the isomer bands to the γ‐ and ground state bands. The resulting matrix elements, derived using a coupled‐channel semiclassical Coulomb excitation search code, have been used to probe the K‐components in the wave functions and revealed the onset and saturation of K‐mixing in low‐K bands, whereas K‐mixing is negligible in the high‐K bands. The implications can be applied to other quadrupole‐deformed nuclei. An upper limit on the Coulomb depopulation yield of the 16+ isomer was calculated based on the present set of matrix elements.

A kinematic-coincidence detector system for sub-barrier heavy-ion reaction studies

A detector system has been developed to study sub-barrier heavy-ion transfer reactions by means of the coincident detection of both reaction products. Scattered ions are momentum-analyzed by an Enge split-pole magnetic spectrometer, and the recoils are detected in a 3.5 m long time-of-flight arm. Ion trajectories through the spectrometer are determined by an angle measurement at the focal plane, which together with the measured time-of-flight through the spectrometer provides independent measurements of the momentum and velocity of the scattered particles. This raytracing method allows the spectrometer to be operated with a large (10.3 msr) entrance solid angle without significantly degrading the velocity and momentum measurements. Two-body kinematic coincidence is used to measure the mass of the recoil and the reaction Q-value distribution. This detector system has been used to measure the cross sections and Q-value distributions for sub-barrier quasielastic transfer reactions in the system 58Ni + 162Dy. Mass resolution of better than 0.8 u (FWHM) is obtained, resulting in adequate channel separation. The Q-value resolution is approximately 1.4 MeV (FWHM) for ≈ 50 μg/cm2 thick Dy targets. Prospects for the improvement of the performance of such detector systems are discussed.