NSCL Directory Profile

Krzysztof Starosta Assistant Professor Experimental Nuclear Physics   PhD, Physics, Warsaw University 1996 Joined NSCL in August 2003 Phone (517) 324-8138 Fax (517) 353-5967 Office W107   starosta at nsclmsuedu Professional homepage

I am investigating structure of short-lived exotic states in nuclei far from stability. My experimental program at NSCL concentrates on in-beam studies which explore nuclear properties through electromagnetic excitations. The electromagnetic probe provides an excellent way to study nuclear systems bound by still poorly known effective nuclear interactions; it is well understood, and can be treated quantitatively in structure and reaction models. In particular, the electromagnetic transition matrix elements are observables that models can be examined against. These matrix elements can be obtained from transition rates or lifetimes of nuclear states.

Many important excited nuclear states that decay by γ-ray emission with lifetimes in the picosecond range can be reliably measured using Doppler shift methods. A significant fraction of my research program at NSCL is based on applications of the Recoil Distance Method (RDM) and Doppler Shift Attenuation Method (DSAM) which I have developed for fast beams from fragmentation reactions. In an RDM experiment the beam ions are excited on a movable target, emerge from the target and decay in flight after a distance related to the lifetime. A stationary degrader positioned downstream with respect to the target is used to further reduce the velocity of the excited nuclei. The resulting γ-ray spectra exhibit two peaks; one with a larger Doppler shift that corresponds to the gamma-rays emitted before traversing the degrader, and one with a smaller Doppler shift that corresponds to γ-rays emitted after the degrader. From the relative intensity of the peak areas as a function of target-degrader distance, the lifetime of the state can be inferred. The RDM was first applied at NSCL in experiments with the Segmented Germanium Array (SeGA), in measurements of the lifetimes of the first excited state in ⁶⁴Ge (produced by a single neutron knock-out reaction from ⁶⁵Ge) and in ¹¹⁴Pd (populated by intermediate-energy Coulomb excitation).

The scientific reach of the RDM and DSAM methods will be greatly enhanced by applications of the digital signal processing to γ-ray spectroscopy. I am currently leading a project to develop a fully digital, state-ofthe- art, 600-channel Digital Data Acquisition System (DDAS) for the SeGA array; one of the largest array of segmented detectors available for use with rare ion beams. The DDAS will have γ-ray tracking capability and will allow determination of the first interaction position of a γ-ray in a SeGA detector with a precision by a factor of two better than currently available. This will translate into significantly improved energy resolution from more precise Doppler correction, or increased efficiency available with more compact SeGA geometries. Lifetimes below 1 ps are expected to be accessible in digital SeGA experiments. Such RDM and DSAM studies of the products from fragmentation reactions have the promise of reaching far from stability and providing lifetime information for intermediate-spin excited states in a wide range of nuclei.