Doppler-shift attenuation lifetime measurement of the Ar 36 21+ level

At TRIUMF, the TIGRESS Integrated Plunger device and its suite of ancillary detector systems have been implemented for charged-particle tagging and light-ion identification in coincidence with gamma-ray spectroscopy for Doppler-shift lifetime studies and low-energy Coulomb excitation measurements. As a test of the device, the lifetime of the first 2(+) excited state in Ar-36 was measured from the gamma-ray line shape of the 2(1)(+) -> 0(g.s.)(+) transition using the Doppler-shift attenuation technique following Coulomb excitation. The line-shape signatures, vital for precision lifetime measurements, were significantly improved by enhanced reaction-channel selectivity using a complementary approach of kinematic gating and digital rise-time discrimination of recoiling charged particles in a silicon PIN diode array. The lifetime was determined by comparisons between the data and simulated line shapes generated using our TIGRESS Coulomb excitation code as an input to the Lindhard method, which was then extended and included as a class in GEANT4. The model-independent lifetime result of 490 +/- 50 fs corresponds to a reduced quadrupole transition strength of B(E2; 2(1)(+) -> 0(g.s.)(+)) = 56 +/- 6 e(2)fm(4) and agrees well with previous intermediate energy Coulomb excitation measurements, thereby resolving reported discrepancies in the 2(1)(+) level lifetime in this self-conjugate nucleus.