L. G. Kostova

Neutron-core excitations in the N=50 nucleus 89Y

Abstract High-spin states of the N=50 nucleus 89Y up to 31 2 h at 8.7 MeV have been investigated in the (α,2n) reaction by using in-beam γ-ray spectroscopy. Besides singles γ-ray spectra, excitation functions and γγ-coincidences, the angular distribution and linear polarization of the γ-rays have been measured. For 23 of the observed 28 levels, mean lifetimes or limits have been determined by means of the Doppler-shift attenuation of γ-r.f. methods. In order to calculate the stopping of recoils in the two-component RbCl target a new procedure had to be developed. On the basis of all these experiments several new levels as well as many spin/parity values could be uniquely assigned. The most surprising results are the observed high level density at spin 23 2 and 25 2 and the identification of several fast M1-transitions at high spin. Both results may be explained by the inclusion of neutron particle-hole excitations across the closed N=50 shell. No collective E2 transitions have been found. The experimental results are compared with shell model calculations.

Isomeric transitions between shape coexisting states in iodine isotopes withA=115, 119 and 121

Applying delayedγ- r.f. coincidences and generalized centroid-shift analysis, nanosecond isomers in some odd-A iodine isotopes are studied in-beam. The reactions106Cd(12C,p2n) and116,118Sn(6Li,3n) have been used. The following half-lives were determined:T1/2(565 keV)=0.50±0.10 ns andT1/2(837 keV) =0.90±0.20 ns in115I,T1/2(687 keV)=5.50±0.50 ns andT1/2(2418 keV)=4.0±0.50 ns in119I as well asT1/2(812 keV)=0.35±0.20 ns andT1/2(445 keV)<0.10 ns in121I. A systematics of the absolute probabilities for the transitions deexciting theπg9/2 9/2+ andπ11/2 11/21− band heads in115–125I is presented and discussed.

The half-life of the 1437 keV 11/2− state in97Mo

Using the (α,n) reaction, the half-life of the first 11/2− level in97Mo has been determined as T1/2(1437.0 keV)=2.5±0.3 ns by means of the pulsed beam method. The experimentally obtainedB(M2) value between theh11/2 andg7/2 configurations in97Mo has been satisfactorily described within the quasiparticle-phonon model usinggseff=0.6gsfree. Similar calculations and comparisons with experminental data have been performed also for95,99Mo as well as for99Ru and101Pd.

p-n Multiplets and band-like structures in106Ag

The half-life of a 3− excited state in106Ag was measured as T1/2 (364.4 keV)=0.9±0.1 ns using the103Rh(α, n)106Ag reaction. The intramultiplet transition rate B(M1, 3−→2−) indicates that the configuration π(g9/2)0−2π(p1/2)−1 vd5/2 can be assigned. A low-lying 3+ isomer, as known in108,110Ag, was not found in106Ag, what is explained by the raising energy of the vd3/2 shell with decreasing neutron number. For the 6− bandhead of a rotational-like structure T1/2 (765.2 keV)< 0.2 ns was derived in contradiction to an earlier prediction (200 ns). Indirect evidence for a negative sign of the magnetic moment of105Ag (g.s.) is obtained.

Systematical Investigations of Cs - 137 Concentration in Soils in Bansko - Razlog Region

Systematical investigations of Cs‐137 concentration in soil in Bansko — Razlog region have been performed for the first time on a total area of about 40 km2. By means of high‐resolution gamma‐ray spectroscopy the radio‐nuclide content of soil samples has been determined. The Cs‐137 deposition density following the Chernobyl accident has been estimated and compared with results obtained in other countries. The additional dose rate caused by this fallout has been estimated, too.

Decay properties of the 19/2− isomers in113,117Sb

By means of the generalized centroid-shift method, the following half-lives were determined:T1/2(3045 keV)=3.7±0.3 ns in113Sb using the reaction104Pd(12C,p2n) as well asT1/2 (1322.8 keV)=3.5±0.3 ns,T1/2(2779.8 keV) =0.50±0.15ns,T1/2(2874.9 keV)<0.2 ns andT1/2(3072.9 keV)<0.1 ns in117Sb using the116In(α, 2n) reaction. Three-quasiparticle configurations of the type πd5/2ν(h11/2d3/2) in113,117Sb are found to dominate in the wave functions of the 19/2− states at 3045 and 2780 keV in113Sb and117Sb, respectively. TheB(E2, 19/2− → 15/2−) values in113–119Sb are discussed.

Lifetime measurements and particle-core coupling calculations in144Sm

Applying the generalized centroid shift method in the142Nd(α, 2n)144Sm reaction the following half-lives have been measured:T1/2(3,376 keV)=(1.6±0.2) ns,T1/2(3,460 keV) =(0.5±0.2) ns andT1/2(5,150 keV)≦0.3 ns. Level energies and electromagnetic transition rates have been calculated within a particle-core coupling approach that is based on the shell model with configuration mixing. The persistence ofl-forbiddenness of M1 transitions between rather complex configurations could be confirmed.