[Triumf-seminars] TRIUMF Seminar today at 14:00

[TRIUMF Seminars]

Date/Time: Thu 2011-06-30 at 14:00

Location:  Auditorium          

Speaker:   Philip Voss (Simon Fraser University)

Title:     Recoil Distance Method Lifetime Measurements Via Gamma-Ray and Charged-Particle Spectroscopy at the NSCL

Abstract: Recoil Distance Method (RDM) lifetime measurements provide a direct and model independent measure of electromagnetic transition rates. The deduced transition matrix elements lend valuable insight into the evolution of nuclear structure and theoretical models at extreme isospin. Neutron-rich 18C presents one such example, where a small B(E2; 2+1 -> 0+gs) represented a dramatic shift from the expected inverse relationship between the B(E2) and 2+1 excitation energy. To shed light on the nature of this quadrupole excitation, the RDM lifetime technique was applied with the Koeln/NSCL plunger at Michigan State University. States in 18C were populated by the one-proton knockout reaction of a 19N radioactive secondary beam. De-excitation gamma rays were detected with the Segmented Germanium Array in coincidence with reaction residues in the focal plane of the S800 Magnetic Spectrometer. The lifetime results were found to be well described by ab initio no-core shell model calculations utilizing the importance truncation scheme.

Additionally, a novel extension of RDM lifetime measurements via charged-particle spectroscopy of exotic proton emitters has been investigated. Substituting the reaction residue degrader of the Koeln/NSCL plunger with a thin silicon energy loss detector permits the study of short-lived nuclei beyond the proton dripline. Lifetime information of such nuclei, especially near the canonical rp-process waiting point nuclei 64Ge, 68Se, and 72Kr, is crucial for characterizing nucleosynthesis pathways. The mean lifetime of the two-proton emitter 19Mg was measured as a proof of concept. The results indicated a sub-picosecond lifetime, one order of magnitude smaller than the published results, and validate this new technique for lifetime measurements of charged-particle emitters.

The discussion will conclude with a summary of the development and experimental outlook of the TIGRESS Integrated Plunger (TIP) facility. The completion of TIP will deliver a new TRIUMF experimental program utilizing re-accelerated beams from ISAC-II and a variety of production mechanisms for electromagnetic transition studies of nuclei far from stability. The TIP facility will offer unmatched flexibility for transition rate measurements with the TIGRESS segmented germanium array and can be coupled to auxiliary detectors such as segmented annular silicon detectors, a CsI 4 array, the neutron detector array DESCANT, and the electromagnetic spectrometer EMMA.

Stimulants available 15 minutes before the talk.

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