Measurement of B(M1) values in the crossing region of shears band 1 in lead-197 and interpretation in the semiclassical model .

摘要

In the Tilted Axis Cranking (TAC) model, angular momentum is generated within the band by reducing the angle between the angular momentum vectors of the proton and neutron currents. This resembles the closing of a pair of shear, and thus the bands are referred to as shears bands [Ba94]. An important prediction of the shears mechanism is that certain transition matrix elements, called B(M1) values, decrease as the angular momentum increases. This is experimentally verified for several shears bands in 193–197 Pb [Cl98a].; However, an interesting counter example is observed in shears band 1 of 197Pb [Cl98a]. The emphasis of this work is to test the proposed mechanism by measuring the nuclear level lifetimes and thus B(M1) values in a well understood crossing region (i.e., the A11−/ABC11 − crossing) of shears band 1 in 197Pb.; The measurement of the lifetimes and thus B(M1) values required the construction of a plunger—a device for measuring nuclear level lifetimes that are on the order of 1 ps to 1000 ps using the recoil-distance method.; The reaction 176Yb(26Mg,5n)197Pb was used and a 130 MeV beam was delivered by the ATLAS linear accelerator. About 1.5 × 108 4-fold or greater gamma coincidences were measured for each of 10 distances between electrical contact and 32 μm. Lifetimes were extracted using the Differential Decay Curve Method for 8 levels below and in the crossing region. The resulting B(M1) values are in agreement with the mechanism proposed to explain the previously measured values.; Further quantitative comparison was made with the semi-classical model (including core rotation), which was extended to allow for different lengths of the proton and neutron angular momentum vectors. The model was applied using three band mixing to shears band 1 of 197Pb. A good fit to the energy levels was found, and then the B(M1) values were calculated. The data and model calculations indicate that the B(M1) values first decrease with increasing spin as expected until the backbend begins. When the second configuration begins to mix and the B(M1) values increase, confirming the characteristic patters expected from the semi-classical model. Furthermore, the data and calculations provided the first indication for the existence of levels with shears angles larger than 90°. (Abstract shortened by UMI.)