F.S. Stephens

GRETA: Utilizing New Concepts in Gamma-Ray Detection

We present a new concept for c-ray detector arrays. An example, called GRETA (Gamma-Ray Energy Tracking Array), consists of highly segmented HPGe detectors covering 4p solid angle. The new feature is the ability to track the scattering sequence of incident c-rays and in every event, this potentially allows one to measure with high resolution the energy deposited, the location (incident angle) and the time of each c-ray that hits the array. GRETA will be of order of 1000 times more powerful than the best present arrays, such as Gammasphere or Euroball, and will provide access to new physics. ( 1999 Published by Elsevier Science B.V. All rights reserved.

Three-dimensional position sensitivity in two-dimensionally segmented HP-Ge detectors

Abstract Measured- and simulated-pulse shapes in electrically segmented coaxial Ge detectors have been investigated. Three-dimensional position sensitivities have been determined experimentally and theoretically in a 36-fold segmented Ge detector. By using the two-dimensional segmentation in conjunction with pulse-shape analysis, a position sensitivity of better than 1 mm can be obtained in three dimensions at an energy of 374 keV. This is achieved by analyzing the shape of net charge signals of segments containing interactions and of transient image charge signals of neighboring segments. The ability to locate interactions in three-dimensions is one of the crucial properties in the proposed γ-ray energy tracking array (GRETA). The concept of γ-ray tracking will not only increase the efficiency in detecting γ radiation but also enables the localization and characterization of unknown γ-ray sources with much higher accuracy than is possible with current instruments.

ROTATIONAL STATES PRODUCED IN HEAVY-ION NUCLEAR REACTIONS

Abstract The de-excitation of the ground state rotational band in nine deformed even nuclei has been observed following heavy-ion nuclear reactions. The transitions from states up to spin 16 (on the average) were observed and their energies were measured with an accuracy of ±0.3%. The rotational spacings thus identified have been compared with several calculations, and much the best agreement is obtained with a simplified calculation by Davydov and Chaban taking into account the effects of centrifugal stretching of the nucleus.

Performance of the GRETA Prototype Detectors

Abstract A working, two-dimensionally segmented Ge detector is one of the crucial elements in the development of GRETA – a next-generation 4π germanium detector array that uses three-dimensional positions and energies to of individual interactions of γ rays in the detector to reconstruct the full energies and direction vectors of the individual γ rays by employing tracking algorithms. The three-dimensional position and the energy of interactions will be determined by using a two-dimensionally segmented Ge detector along with pulse-shape analysis of the signals. The current prototype is a 36-fold segmented HP-Ge detector in a closed-ended coaxial geometry. Preamplifiers with a compact design, low noise, and very good response properties have been built and implemented. An integrated noise level of about 5 keV has been measured for the segment channels. The average energy resolution of this detector was measured to be 1.14 and 1.93 keV at 60 and 1332 keV, respectively. Using pulse-shape analysis, a three-dimensional position sensitivity of 0.2 to 0.5 mm (R.M.S) has been obtained at 374 keV, dependent on the position and the direction. The results represent a major step towards the feasibility of a γ-ray tracking detector.

ANGULAR DISTRIBUTIONS OF GAMMA RAYS PRODUCED IN REACTIONS WITH HEAVY IONS.

Abstract Angular distributions of the stretched E2 gamma-ray transitions cascading from rotational or vibrational states, populated by heavy-ion, x n reactions, have been measured. The reactions were induced by 4 He, 11 B, 14 N and 19 F ions on targets in the mass range from 159 to 186. In one case the variation of anisotropy with bombarding energy was measured. Large anisotropies were observed indicating the strong alignments of the decaying states. The problems involved in a theoretical calculation of the alignments are discussed. The implications of the strong angular distributions for nuclear structure measurements with these reactions are considered.

Coulomb excitation of Tb159, Ho165 and Tm169 with O16 ions

Abstract The Coulomb excitation of Tb159, Ho165 and Tm169 has been studied using principally 60 MeV O16 ions as projectiles. Both gamma ray and conversion electron spectra were taken. Rotational bands observed at 514 and 687 keV in Ho165, at 580 (and possibly ≈ 1280) keV in Tb159 and at 570 (and possibly ≈ 1170) keV in Tm169 have been assigned as collective (gamma vibrational) bands. The bands at 361 keV in Ho165, at 348 and 971 keV in Tb159 and at ≈ 900 keV in Tm169 are ascribed to intrinsic (Nilsson) configurations. The properties of these bands are discussed in some detail. By multiple excitation 6 or 8 members of each ground state rotational band were excited. The analysis of the energies of these levels indicates the presence of higher-order Coriolis terms in the rotational energy formula in the cases Tb159 and Tm169.

A coupling scheme relevant to high angular momenta and intermediate nuclear deformations

Abstract Based on the particle-plus-rotor model, it is shown that a new coupling scheme should occur in nuclei under certain conditions. In this coupling scheme, j is quantized along the direction of I rather than along the symmetry axis. Simple approximate wave functions for this scheme have been developed and compared with the exact solutions of the particle-plus-rotor model.

Collective levels in light even Ce isotopes

Abstract The heavy-ion reactions ( 16 O, 4n) and ( 20 Ne, 4n) have been used to populate levels in the neutron-deficient even cerium isotopes 136 Ce- 128 Ce. Prominet γ-rays in the “in-beam” spectra have been assigned to transitions between the members of the ground-state quasi-rotational bands. The results show that as the number of neutrons decreases these isotopes tend towards rotational behaviour. The cross sections for the neutron evaporation reactions used are considerably reduced for the most neutron-deficient systems due to competition from charged-particle emission. Such competition prevents us from studying Ce isotopes lighter than 128 Ce by this method.

Isomeric levels in the light thallium isotopes

Abstract A study has been made of the metastable states produced in certain heavy-ion reactions by observing the gamma-ray and conversion-electron transitions occurring between the beam bursts of the HILAC at the Lawrence Radiation Laboratory. Targets and projectiles were chosen which produce thallium isotopes ranging from mass 200 to mass 191. New E3 isomers were found in Tl200, Tl199 and Tl193. The isomer in Tl200 is apparently a 7+ level, in close analogy with other known even-mass thallium isotopes, Considerable evidence was accumulated that in the odd-mass thallium nuclei, including the previously studied Tl197 and Tl195, the metastable state has spin and parity 9 2 − . Such a state is not expected to occur at low energies in these nuclei, and its nature, therefore, presents an interesting problem.

Energy-level systematics of odd-mass lanthanum isotopes a new coupling scheme

Abstract Heavy-ion reactions have been used to populate states in the odd-A La nuclei (A = 125 → 137). The γ-ray spectra indicate strong population of a band based on the 11 2 − state of each isotope. The character of this band indicates a new type of coupling, and can only be explained if the state has a prolate deformation. This is in contrast to earlier suggestions of oblate shapes for these nuclei.