Quasicontinuum Lifetimes and Recent Progress at UC Berkeley

Leo Kirsch, Lawrence Berkeley National Laboratory
Thursday, Jul 20, 3:00 PM - Special Seminar
1300 Auditorium (FRIB Tower 3)

Abstract:  This presentation explains a new experimental method to determine the absolute magnitude and energy dependence of Nuclear Level Density and the Gamma Strength Function employing an extension of the Doppler shift attenuation method in 56Fe(p,p') using the GRETINA array coupled to a fast phoswich particle detector. The Gamma Ray Energy Tracking In-beam Nuclear Array (GRETINA) is an 1152-segmented germanium detector array that has the capabilities to extract quasi-continuum lifetimes (QCtau) from minuscule shifts in Doppler shift as a function of outgoing particle energy. High resolution gamma-ray spectroscopy provides access to lifetimes of discrete nuclear excited states via changes in gamma-ray energy with angles respective to the initial nuclear recoil vector. Gates on the outgoing particle energy isolate events where decay from the QC precedes a low lying discrete transition. This gives the recoiling excited nucleus more time to slow down and reduces Doppler shift of subsequent gamma-rays. The relationship between QCtau, Nuclear Level Density, and the Gamma Strength Function complements both the traditional Oslo method [1], the Two Step Cascade method developed by Becvar et. al. [2], and the direct reaction two-step cascade method developed by Wiedeking et al. [3]. This presentation also delves into ongoing experimental physics campaigns at UC Berkeley's Nuclear Engineering department. Recently commissioned, the High Flux Neutron Generator (HFNG) is a quasi-monoenergetic 2.5 MeV DD neutron source. The HFNG science program is focused on pioneering advances in the 40Ar / 39Ar dating technique for geochronology, new nuclear data measurements for medical isotopes and reactors, basic nuclear science, and education. The HFNG also operates as a user facility for researchers in industrial applications. This section of the talk will summarize publications including neutron cross section measurements and resolved technical issues with electron backstreaming. References: [1] Guttormsen, M. et. al., Radiative strength functions in 93-98Mo, Phys. Rev. C, 71(4):044307, 2005 [2] Becvar, F. et. al., Test of photon strength functions by a method of two-step cascades, Phys. Rev. C 46 (1992) 1276–1287. [3] Wiedeking, M., et. al., Low-Energy Enhancement in the Photon Strength of 95Mo, Phys. Rev. Lett., 108(16):162503, 2012