Measurement of In-Band Transition Probabilities at High Spin

Abstract

Quantitative measurements have been made of correlations in the y-ray continuum emitted by rotational nuclei populated in heavy-ion compound-nucleus reactions. The experimentally deduced intensity of the "ridge" structure in the correlation spectrum implies an average probability for inband stretched E2 decay of < 20% at spins above 30# in contrast to traditional expectations. PACS numbers: 21.10. Re, 21.10.Ma, 25.70.Gh, 27.60. +j It is important to understand the mechanisms of yray decay from the entry region following heavy-ion compound-nucleus evaporation reactions because they are sensitive to nuclear structure at high spin and excitation energy and also an aid to interpreting and planning high-spin y-ray spectroscopy experiments. Conventional models 1 of the deexcitation of the residual nuclei from the entry region assume first the emission of several "statistical" y rays, most likely dipole, of comparatively high energy. The resulting "cool" system decays further towards the ground state mainly by stretched El transitions in many rotational bands parallel, or nearly so, to the yrast line. Eventually all the intensity feeds the observable discrete-line transitions. It has been further assumed 2 that the unresolved transitions in these yrastlike cascades are mainly in band (as in the region in which discrete-line spectroscopy is possible). This Letter reports measurements of the in-band transition probability at high spin. Preliminary reports of the data have already been given. 3 A suitable tool for studying the unresolved transitions is the elegant method of "y-y correlations." 4 y-y coincidence data are sorted into a matrix of E yX vs E y2 . Coincidences between transitions in rotational bands will form well-defined "ridges" parallel to a "valley" around E y i = E y2 in this matrix. Such patterns are expected even in cases where alignment effects are important. 5 Moments of inertia ^band (Bohr and Mottelson 6 ) may be deduced from the ridge separation. Until recently 7 correlation studies have been bedeviled by problems with the background and often by poor statistics or bad resolution. They have needed to use rather unsatisfactory backgroundsubtraction methods 1 to see correlated structure at all; these methods destroy information on the intensity distribution. In the work of Hjorth et al., % however, an approximate unfolding was performed of the correlation spectrum obtained with poor Nal resolution from a reaction including at least twenty exit channels, no channel selection, and rather low-input angular momentum. After a rather complicated analysis the authors concluded that "at least half of all E2 y rays showed no rotational correlations."The TESSA2 spectrometer 9 at the Daresbury Nuclear Structure Facility has enabled us to overcome many of the previous experimental problems. TES-SA2 comprises six Compton-suppressed Ge detectors and a fifty-element calorimeter. It has low background, high resolution and efficiency, and the ability to select the entry region in reaction y-ray total energy and mult...

Determination of the Spreading Width for the Collective Transition Strength at Finite Temperature

“…The ridge structure is found to be weak and the valley is seen to be very shallow particularly at higher y-ray energies in accordance with Ref. 5. Attempts to explain the uncorrected intensity by simply introducing a spread in moments of inertia require an unreasonably large range of values to wash out the ridge-valley structure.…”

“…4 Since the small residual background is rather flat it is a simple operation to unfold the response functions of the detectors and to observe true y-ray intensities in twodimensional coincidence spectra. Such procedures have recently been carried out by Love et al, 5 who found that the ridge structure at higher y-ray energies, where the continuum predominates, is only a few percent of the total intensity, and that the B(E2) strength spreads over an energy range much greater than 15 keV.…”

“…Firstly, there is a range of possible transition energies related to the magnitude of the width r rot . Secondly, since the transition probabilities are weighted by (Ej -E f ) 5 , higher transition energies are favored, leading to a rotational cooling of the nucleus compared to the situation in the absence of a spreading width. The spreading width was maintained at a constant value for all energies above the yrast line greater than the critical value U 0 , T TOt being made to fall rapidly to zero for U < U 0 .…”

Effective moment of inertia in132Ce,134Nd and136Nd

“…For example ~b,,~ measurements at high rotational frequency remain difficult because of low intensity in the ridges of the valley which are set on a large background. Love et al [9] have shown that in 13~ the relative intensity of coincidences constituting the ridges relative to that of the 4+12 + coincidence varies from 22% to 6% when the spin goes from 30 to 40 h and that the low in-band probability is due to a rotational B(E2) strength spread over an energy range much greater than 15 keV. Then Bacelar etal.…”

Study of Electromagnetic Properties of Nuclei and Structural Changes at High Spins in Yb Isotopes

“…The component of the average E2 strength function with a width less than some tens of keV must be rather small, of the order of loyo, because a t best only weak ridge-valley structures are observed in E,, -IC,, correlation plots [13].…”