Fluctuation analysis of rotational decay from excited nuclei

Abstract

A statistical method to analyze the gamma decay of excited, rapidly rotating nuclei is presented. It is based on the fact that the cooling path of these systems goes through regions of both suppressed and enhanced fluctuations typical of large and low level density, respectively. The method is applied to the study of the high-spin quasicontinuum of ,67,68 Yb.PACS numbers: 24.60. Ky, 2U0.Re, 23.20.Lv, 27.70.+q Nucleons can organize their motion in nuclei, leading to quadrupole deformed shapes of the average field. These nuclei can be produced at finite excitation energies and large angular momenta in fusion reactions induced by heavy-ion collisions. The resulting system cools down by particle and photon emission.The associated continuum y spectrum is mostly composed of stretched £2 transitions typical of rotational spectra. However, the correlation in the energy of two successive y transitions is much weaker than that associated with the decay of a perfect rotor. For rotational bands, two transitions cannot have the same energy, whereas the experimental two-dimensional coincidence spectrum only displays a shallow valley running along the diagonal at E 7l s=s E rr This observed deviation from the regular pattern is probably caused by the fact that the dominant part of the enhanced electric quadrupole decay passes through regions of high level density, where rotational bands become mixed, with mixing coefficients which change as a function of the spin. As a consequence, the E2 decay from a state of spin / will not go to a single state of spin / -2, but to a range of states [1-5], implying damping of the rotational motion. These transitions can be correlated with the intensities observed in the valleys, while the ridges are built out of discrete electric quadrupole transitions between states belonging to rotational bands close to the yrast line (cf. Fig. 1). Even if this picture seems to be fairly well established, central questions still remain to be answered: (i) How many levels are associated with the decay in the damped region, and how many in the case when the decay proceeds along discrete bands? (ii) What is the excitation energy above the yrast line which defines the onset of rotational damped motion, and to what extent is this a sharp transition? (iii) What is the shape of the strength function associated with the stretched quadrupole decay in the damped region?The subject of the present paper is to study how these

Rotational damping in a multi-j shell particles-rotor model

“…Here, the number of rotational bands corresponds to the experimental effective number of paths which can be obtained from E γ × E γ spectrum by the fluctuation analysis method [8,9].…”

“…Experimentally rotational damping has been studied through the analysis of quasi-continuum spectra [3,4,5,6,7]. Recent experimental progress in high precision three-dimensional gamma-ray correlation measurements makes it possible to study various features of collective rotational motion in the regions of discrete rotational bands and damped rotational motion [8,9,10,11,12,13,14,15]. In particular, the newly developed fluctuation analysis method [8,9] has presented the number of rotational bands existing in a rare-earth nucleus is only about 30 at a given angular momentum, thus confirming the occurrence of rotation damping.…”

Onset of rotational damping in superdeformed nuclei

“…Since different configurations respond differently to the Corioli force due to different single-particle alignments, the configuration mixing results in loss of collectivity or damping of the collective rotational motion. Experimental data on the damping of the rotational motion are accumulating for normally deformed rare-earth nuclei from the analysis of the quasi-continuum part of gamma-ray spectra containing gamma-rays emitted from the highly excited levels [3,4]. In particular, a fluctuation analysis of the quasi-continuum points to the presence of about 30 rotational bands in a nucleus, thus confirms occurrence of the rotational damping [3].…”

Microscopic structure of rotational damping

“…For analyses of the rotational damping, it is useful to look at two coincident gammarays emitted in the E2 decay cascade [3,4]. If there is no rotational damping, the rotational E2 transition takes place along the rotational bands, and the energy difference between two consecutive gamma's for I + 2 → I and for I → I − 2 forms a sharp peak at E γ1 − E γ2 = 4/J because of the rotational correlation E γ ∼ 2I/J + const.…”

“…The collective rotation of deformed nuclei becomes a damped motion as the nuclei are thermally excited [1,2,3,4]. It is known that the levels near the yrast line form rotational band structures based on simple configurations which are well described by cranked mean-field models.…”