GDR γ-ray decay in 156Dy∗ from regions selected on temperature and angular momentum

The strength distribution of the GDR built on highly excited states in a restricted temperature domain in Dy-156 and Dy-155 nuclei has been deduced by subtraction of gamma-ray spectra obtained for the decay of Dy-154* and Dy-156* from regions selected on angular momentum. The resulting difference spectra have been analyzed within the statistical model. The results show a large deformation (\beta\ similar to 0.51 +/- 0.29 and 0.35 +/- 0.14) for the angular-momentum regions with [J] similar to 32 ($) over bar h at T approximate to 1.8 +/- 0.2 MeV and [J] similar to 46 ($) over bar h at T approximate to 1.7 +/- 0.2 MeV respectively, in satisfactory agreement with calculations performed in the framework of Landau theory of shape transitions and statistical fluctuations. The deduced centroid energies are in agreement with the systematics of the GDR built on the ground state. The width of the GDR shows a systematic increase with increasing temperature

NONCOLLECTIVE DEGREES OF FREEDOM IN PB-192

Two positive-parity and two negative-parity groups of states of non-collective character with excitation energies ranging from 3 to 7 MeV and spins as high as 21 HBAR have been established in 192Pb. The main features of each group are described phenomenologically as four-quasiparticle configurations, arising from coupling of the 2(1)+, 4(1)+ and 6(1)+ states to a specific aligned neutron two-quasiparticle configuration. Supporting evidence for this interpretation and the importance of proton excitations in this singly closed-shell nucleus is obtained from the predictions of the quasiparticle multi-step shell model (QMSM). In addition, a four-quasiparticle structure of predominant neutron character is proposed on the basis of comparisons with the experimental data on 194Pb and with the results of the QMSM calculations

ANGULAR-MOMENTUM AND TEMPERATURE-DEPENDENCE OF THE GDR IN DY NUCLEI

The statistical gamma-ray decay of the GDR built on excited states in Dy-151-156 has been investigated as a function of angular momentum and temperature. The selection on angular momentum has been made by using the total gamma-ray energy and gamma-ray multiplicity. The temperature dependence in the range T almost-equal-to 1.0 MeV to T almost-equal-to 2.0 MeV has been measured by taking data for two reactions, i.e. Cd-114(40Ar,xn-gamma)Dy-154-x* and Cd-116(40Ar,xn-gamma)Dy-156-x* at beam energies of 180 MeV and 200 MeV, respectively, populating the same final nuclei. A systematic shift of the strength to lower energies has been observed at increasing angular momentum, which can be attributed to a change of the nuclear shape from prolate to oblate

Viscosity and fission time scale of Dy-156

In the fusion-fission reaction Ar-40+Cd-116-->Dy-156-->fission, performed at beam energies E(b) = 216 MeV and 238 MeV, gamma rays were measured in coincidence with fission fragments. The gamma-ray spectra are interpreted using a modified version of the statistical-model code CASCADE. From a comparison of the experimental and calculated spectra it is deduced that the nuclear viscosity is in the range 0.01 <gamma <4. The extracted fission time scale is of the order of 10(-19) s

Viscosity and fission time scale of Dy-156 at high excitation energies and large angular momenta

The reaction Ar-40 + Cd-116 --> Dy-156* --> fission as Studied at beam energies E(b) = 216 MeV and E(b) = 238 MeV, wherein gamma-rays were measured in coincidence with fission fragments. From these spectra the nuclear viscosity gamma and fission time scale were deduced by comparing to statistical model calculations yielding viscosities in the range 0.01 <gamma <4 and fission time scales of the order of 10(-19) s

High-sensitivity isobar-free AMS measurements and reference materials for 55Fe, 68Ge and 202gPb

Isobaric interference represents one of the major limitations in mass spectrometry. For a few cases in AMS with tandem accelerators, isobaric interference is completely excluded like the well-known major isotopes 14C, 26Al, 129I. Additional isotopes are 55Fe (t1/2=2.74years), 68Ge (t1/2=270.9days) and 202Pb (t1/2=52.5kyr), with 68Ge and 202Pb never been used in AMS so far. Their respective stable isobars, 55Mn, 68Zn and 202Hg do not form stable negative ions. The exceptional sensitivity of AMS for 55Fe, 68Ge and 202gPb offers important insights into such different fields like nuclear astrophysics, fundamental nuclear physics and technological applications. VERA, a dedicated AMS facility is well suited for developing procedures for new and non-standard isotopes. AMS measurements at the VERA facility established low backgrounds for these radionuclides in natural samples. Limits for isotope ratios of <10−15, <10−16 and ⩽2×10−14 were measured for 55Fe/56Fe, 68Ge/70Ge and 202Pb/Pb, respectively. In order to generate accurate isotope ratios of sample materials, AMS relies on the parallel measurement of reference materials with well-known ratios. A new and highly accurate reference material for 55Fe measurements with an uncertainty of ±1.6% was produced from a certified reference solution. In case of 68Ge dedicated neutron activations produced a sufficiently large number of 68Ge atoms that allowed quantifying them through the activity of its decay product 68Ga. Finally, for 202Pb, the short-lived isobar 202Tl was produced via neutron activation and served as a proxy for 202Pb AMS measurements. © 2012 Elsevier B.V