M. Jääskeläinen

The spin spectrometer: Design, instrumentation and response characteristics of 4πγ-ray multidetector system

Abstract Construction of a new type of spectrometer for the investigation of nuclear structure at high angular momentum and of the mechanisms of heavy-ion induced reactions is reported. The spectrometer consists of 72 NaI(Tl) detectors closely packed in a 4π arrangement. It is capable of recording on an event-by-event basis for each detector that fires (1) the identification number, (2) the pulse height, (3) the time of flight, and (4) the pulse width. From these data we can obtain for each event: the γ-ray multiplicity, the total pulse height and thus the total γ-ray energy, the neutron multiplicity, the angular correlations of the emitted γ-rays, and the time relationships between the γ-rays in a cascade. Pulse height spectra of the γ-rays and of the neutrons associated with the above parameters can also be obtained. The geometric arrangement, the performance of the individual detectors, the associated electronics, the data acquisition system, the response of the entire system to incident γ-cascades of given multiplicity and total energy (including the effect of coincidence summing and detector-to-detector scattering), and the response to incident neutrons are discussed for various approximate and realistic inputs. The unfolding of populations of total pulse height and coincidence fold to yield populations in excitation energy and γ-ray multiplicity is described. The unfolding of pulse-height spectra observed in individual detectors to yield γ-ray energy spectra is also discussed.

Population of the entry states in heavy-ion fusion reactions☆

Abstract The excitation energy and angular momentum dependence of the population of the entry states following fusion of 136 MeV 20Ne with 146Nd has been measured with a new type of instrument. Statistical-model calculations reproduce the main features of the data. Structure effects for J⪆45 are evident in the entry lines. The entrance-channel orbital angular momentum distribution leading to fusion has been deduced.

Evolution of the decay modes of 158Yb with spin

Abstract The decay of the entry states in 158 Yb populated in the reactions of 149 MeV 20 Ne with 144,146 Nd has been investigated with the Spin Spectrometer gated by a Ge counter. The average excitation energy, the γ -ray spectra and the angular distributions as a function of multiplicity suggest a region of non-collective behavior at low spin. At higher spin collective characteristics are apparent, but between I ≈40 and 49 a strong stretched dipole component is seen. Above I ≈49 no additional dipole transitions are observed. These characteristics are discussed in terms of recent calculations.

High Spin Phenomena in the Mass 100-200 Region Seen Through the Crystal Ball

The average properties of the gamma ray entry region and the decay from it are studied systematically, for 49 nuclear systems, in the spin spectrometer. Preliminary results are given for the mass the neutron number dependence of the gamma ray fold distribution and of unresolved γ spectra. The possibility of gating simultaneously on narrow regions of fold and excitation energy is exploited.

Evolution of nuclear shapes in 157–161yb as a function of spin and neutron number

Abstract The population distributions of the entry states, the entry lines 〈E∗> versus multiplicity, the energy spectra and the angular distributions of the continuum γ-rays as a function of multiplicity in 157–161yb from the reactions of 136 MeV and 149 MeV 20Ne with 144Nd and 146Nd have been investigated with a 4π multidetector system gated with a Ge detector. The observed entry lines and γ-decay modes indicate changes in the nuclear structure with increasing spin. This systematic change for N = 87– 91 Yb isotopes suggests an evolution of nuclear shapes from prolate at low spins to particle aligned oblate structure for 157yb and 158yb followed by the onset of high-K bands built on largely deformed oblate states at increasingly higher spin between 38 and 50 kh with increasing neutron number for 157–161yb. Furthermore evidence for an evolution to triaxial shape at I ⋍ 50 for 158yb was found.

Heavy-Ion Reaction Mechanisms Studied with the Spin Spectrometer

Experimental data and statistical-model calculations for xn and αxn products of the reaction 20Ne + 146Nd at 136 MeV are shown to be in generally good agreement, indicating that equilibrium processes are dominant. Preliminary results on the heavy-ion ejectiles from 19F + 159Tb are presented.

Research highlights with the spin spectrometer

Abstract The excitation energy and angular momentum dependence of the entry states in fusion reactions measured with the spin spectrometer is discussed. A new decay mode involving the onset of localized stretched dipole radiation at half the accompanying stretched E2 collective radiation is found in 157–161 Yb. The appearance of this mode correlates smoothly with neutron number and spin. Possible interpretations are presented in terms of the evolution of the nuclear shapes from prolate to aligned-quasiparticle oblate to collective oblate and then to triaxial. Evidence for nuclear deformation that increases with spin at very high excitation is presented based on α-particle angular distributions measured relative to the spin direction, using a new method for deriving the spin alignment.