2011 YÜksek lisans tez özetleri

Discrete and Resonance Structures Observed in Gamow-Teller Transitions

Gamow-Teller (GT) transitions are one of the important weak processes and they are observed in β decays. They are caused by the spin-isospin flip operator στ_± and charactarized by ΔL=0 and ΔS=1. It is important to know the reduced transition strength B(GT) of GT transitions for the understanding of nuclear structure, astrophysics and neutrino physics. The most direct information is obtained by a β_± decay study, but accessible excitation energies are limited by the decay Q values. The GT transitions are also studied in hadron charge-exchange reactions (CE) such as (p,n) and (n,p) reactions performed at intermediate energies. The CE reaction can access GT_± transitions at higher energies. In the (p, n) reactions performed in the past, the achieved energy resolution was of the order of 200-300 keV. The difficulty to realize high energy resolution has been overcome by the use of (³He,t) reactions.

It was shown that (³He,t) reactions at scattering angle 0° and at intermediate incident energy of 140 MeV/nucleon are good probes of GT transition strengths owing to the close proportionality between the cross sections and the B(GT_±) values. Once the "standard" B(GT_±) value is obtained from a β-decay study, the unit cross section GT_±) can be derived. Therefore, B(GT_±) values for the transitions to higher excitation energies can be determined from the measured cross sections. In order to study precise GT_- strength distributions, (³He,t) experiments have been performed at the Research Center for Nuclear Physics (RCNP), Osaka. With the complete beam matching techniques, high energy and scattering-angle resolutions have been achieved.

Studying Gamow-Teller transitions from isotopes of Zr are interesting since the neutron g_9/2 orbit is occupied and proton g_9/2 orbit is empty. Therefore, we have a long term project to study the GT transitions from all of the stable Zr isotopes. The nucleus T_z=5 ⁹⁰Zr has 40 protons and 50 neutrons, where N=50 is a magic number and at Z=40 a semi-magic structure is expected. From ⁹⁰Zr, we can expect two configurations of υg_9/2→πg_9/2, υg_9/2→πg_7/2 contribute in the GT transitions. In this thesis GT_- transitions from T_z=6 spherical nucleus ⁹²Zr to T_z=5 ⁹²Nb nucleus were studied. Since ⁹²Zr has two more neutrons we expect four configurations of υg_9/2→πg_9/2, υg_9/2→πg_7/2, υg_7/2→πg_9/2 and υg_7/2→πg_7/2. Our interest is how the nuclear structure develops when we add neutrons on top of N=50 shell closure of ⁹⁰Zr. For this purpose ⁹²Zr(³He,t)⁹²Nb reaction experiment was performed at RCNP. A high resolution of 30 keV was achieved. For the 0-0.5, 0.5-0.8, 0.8-1.2, 1.2-1.6, 1.6-2.0 angle cuts energy spectra were obtained. Discrete states were observed up to 7 MeV and GT resonance structure was observed in the 10-15 MeV region. In deriving the B(GT_-) values on this target nuclei, the systematics of so-called R² values for mass 92 was used. Strength distribution of the deduced B(GT) values are presented.

UYSAL Hüseyin

Danışman : Doç. Dr. Y. Gürkan Çelebi

Anabilim Dalı : Fizik

Programı : Genel Fizik

Mezuniyet Yılı : 2011

Tez Savunma Jürisi : Doç. Dr. Y. Gürkan Çelebi

Doç. Dr. Deniz Değer Ulutaş

Doç. Dr. Nevin Kalkan

Yrd. Doç. Dr. Saffettin Yıldırım

Yrd. Doç. Dr. Baki Altuncevahir