R. M. Diamond

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.

A γ-ray tracking algorithm for the GRETA spectrometer

Abstract We discuss a γ-ray tracking algorithm that has been developed for the proposed gamma-ray energy tracking array (GRETA). This algorithm has been designed so as to maximize the resolving power for detecting high-multiplicity γ-ray events. The conceptual basis for this algorithm will be presented. In addition, Monte Carlo simulated data will be used to assess performance over a large range of relevant parameters. A discussion of the potential γ-ray polarimeter performance of GRETA is also presented.

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.

Quadrupole collectivity and shapes of OsPt nuclei

Abstract E2 collective properties for the low-lying states in 186,188,190,192 Os and 194 Pt have been investigated experimentally by means of Coulomb excitation using 3.3–4.8 MeV/nucleon 40 Ca, 58 Ni, 136 Xe and 208 Pb beams. The deexcitation γ rays following Coulomb excitation were detected in coincidence with the scattered particles. Levels with excitation energies up to 3–4 MeV of the ground-state, γ and 4 + collective bands as well as excited 0 + states were populated in each nucleus studied. A semiclassical Coulomb-excitation least-squares search code GOSIA was used to extract E2 matrix elements from the measured γ-ray yields. For each nucleus studied, a unique and almost complete set of E2 matrix elements for the low-lying states has been determined, which includes both the magnitudes and signs of the transitional and diagonal matrix elements. The completeness of the set of measured E2 matrix elements makes it possible to determine the intrinsic quadrupole deformation for the low-lying states in these nuclei via a model-independent method. The results indicate clearly that the E2 properties for the low-lying states in these nuclei are correlated well using only the quadrupole collective degrees of freedom. The extracted E2 matrix elements are compared with the prediction of various collective models such as the asymmetric rigid rotor model, the γ-soft model of Leander, and the IBA-2 model. These particular models do not reproduce the data satisfactorily, however the general trends of the data are consistent with the descriptions of γ-soft type collective models through a prolate to oblate shape-transition region. That the enhanced B (E2) values between the I ,quasi- K π = 4,4 + state and members of the I ,quasi- K π = 2,2 + band are well reproduced by the γ-soft model is consistent with the interpretation of the I ,quasi- K π = 4,4 + state being a two-phonon γ-vibration excitation.

Spectroscopy of Rn, Ra and Th isotopes using multi-nucleon transfer reactions

Abstract High-spin spectroscopy of Rn, Ra and Th isotopes has been performed. The nuclei have been populated using multi-nucleon transfer reactions involving a 232 Th target and a 136 Xe projectile. This type of reaction offers the only mechanism for populating high-spin states in many of these nuclei. Interleaving bands with opposite parities have been observed to high spin ( ∼28 h ) in 218,220,222 Rn, 222,224,226,228 Ra and 228,230,234 Th. A systematic study of the rotational alignment properties of octupole bands in radon, radium and thorium isotopes reveals information concerning the role of the octupole phonon and the onset of stable octupole deformation with increasing rotational frequency. Measurement of the magnitude of the intrinsic electric dipole moment, D 0 , provides additional information concerning the strength of octupole interactions in these nuclei.

Different structure collective bands in the N = 87 nuclei 149Sm, 151Gd and 153Dy

Abstract The (α, 3nγ) reaction was used to study the N = 87 isotones 149 Sm, 151 Gd and 153 Dy with 35 to 45 MeV α-beams. The measurements included γ-ray singles excitation function, angular distribution, and linear polarization studies as well as γγ coincidence measurements. The high-spin level patterns are remarkably similar for the three isotones. The predominant γ-decay proceeds via E2 transitions through several parallel ΔI = 2 bands associated with the i 3 2 , h 9 2 and f 7 2 intrinsic configurations. Also observed in each nucleus is a weakly populated ΔI = 1 band, dominated by Ml radiation and built upon the h 11 2 intrinsic state. From the observed band structures we conclude that the h 11 2 configurations are strongly deformed, in contrast to the other intrinsic configurations. A simple explanation for the strongly deformed h 11 2 configurations in these otherwise weakly deformed nuclei is based upon the Nilsson diagram and shown to be related to the well known ground-state shape change occurring between 88 and 90 neutrons. Other cases, recently observed, where the same mechanism is applicable are surveyed.

Multiphonon vibrational states in 106,108Pd

Abstract The nuclei 106 Pd and 108 Pd have been Coulomb excited using beams of 16 O, 58 Ni and 208 Pb. The data determined 25 and 31 E2 matrix elements in 106 Pd and 108 Pd, respectively. The experimental data are qualitatively in agreement with the spherical-harmonic quadrupole vibrational model, but quantitatively there are large discrepancies, even for the states generally accepted to be the two-phonon vibrational states. A sum-rule method was applied to the data, the result of which implies that the vibrational quadrupole strengths of the 0 + and 4 + two-phonon states are fragmented and shared between several E2 matrix elements. The E2 matrix elements imply appreciable quadrupole triaxiality with a γ-centroid of about 20°. A systematic comparison is made with neighbouring Pd-isotopes and with 114 Cd.

Transient magnetic fields for large-Z atoms recoiling through gadolinium

We have utilized Coulomb excitation of the 52− first excited state in 207Pb, whose g-factor is well known, to calibrate the transient field for Z = 82 recoils in ferromagnetic gadolinium at 77 K over a wide range of velocities, 2.4Aν0 ≦ ν ≦ 10.2ν0. The transient field is found at first to increase with increasing recoil velocity, and then to assume a nearly constant value of ≈ 6.2kT for ν ≳ 5.5ν0. The precession measurements in 207Pb, 232Th and 238U are combined with the results of other recent experiments to examine the atomic number dependence of the transient field. We find a roughly linear increase of the transient field with Z of the recoil. The observed transient fields for Pb recoils were interpreted as arising from single ns electrons. Calculations that use simple estimates for the single vacancy lifetimes and for the cross sections for electron capture and loss indicate a dominant contribution to the transient field from the 4s shell, with a polarization ξ, ≈ 0.09.

MEASUREMENT OF ULTRA-FAST GAMMA-RAY TRANSITIONS FROM HEAVY-ION COMPOUND NUCLEUS REACTIONS

A method for correcting target-thickness-induced Doppler broadening in gamma-ray spectra from weakly populated, very short-lived (femtosecond) states is presented. The method is illustrated by an a ...