J-Dependence of Level Density in 180Hf and 178Hf from Statistical Properties of Series of Resonances with Assigned Spin
The spin cut-off parameter a of the level-density formula may be determined directly from counting of highly excited states, without depending upon particular assumptions 1) for the interpretation of the experimental results. Aim of this work was to evaluate σ from the densities of the two series of states with definite J excited by low-energy s-wave neutron capture in an odd-neutron nucleus. 177Hf(I=7/2) and 179Hf(I=9/2) were chosen because of the high spin and low p-wave strength; the probability that any of the observed resonances be p-wave was found to be negligible. The spin of the resonances was determined by the γ-cascade multiplicity method 2), and for 179Hf also by the low-level population method 3). The experiment was performed at the electron linac of Geel, with the time-of-flight technique. Isotopically enriched samples were used. The a value was deduced in each nucleus from the ratio R between the estimated mean spacings DJ of the two series of levels. To get a reliable estimate, either no level or the same percentage of levels must be lost in the two series. However, as the spins of the levels populated by s-wave capture differ by only one unit, the currently assumed statistical fluctuations of the level positions imply large errors also in the case in which no level is missed. For this reason we sought an experimental check of the more stringent conditions of the Dyson-Mehta theory4), in order to obtain estimates of σ affected by smaller errors. The influence of possible short-range anomalies of the level density was examined.
KeywordsLevel Density Experimental Check Level Density Parameter 178Hf Level Order Correlation Function
Unable to display preview. Download preview PDF.
- 5).Neutron Cross Sections, BNL-325, 2nd ed. Suppl. 2 (1966).Google Scholar