Demetrios G. Sarantites

The effect of gamma-ray competition in statistical-model calculations of isomer yield ratios☆

Abstract The distributions in angular momentum in the formation of compound nuclei in 4 He ion bombardments and in their de-excitation by particle or gamma-ray emission were investigated in a compound-statistical model calculation of excitation functions and particle evaporation spectra. The calculation was performed by the Monte Carlo technique and from the distributions in angular momentum in the reaction products, isomer yield ratios were calculated and compared with experiment. The importance of gamma-ray emission in competition with particle evaporation is discussed in detail. It is shown that the effect of the preferred population of the higher angular momentum states in the (α, n) reaction product over that for the (α, 2n), termed “spin fractionation”, depends critically on the strength on the gamma-ray competition. Agreement with experimental isomer ratios is obtained with model parameters which are consistent with the predictions of the Fermi gas model and which give agreement between calculated and experimental excitation functions.

Line shapes of doppler shifted γ-rays from coincidence and singles spectra in measurements of nuclear lifetimes

Abstract A procedure is described for extracting nuclear lifetimes from the analysis of line shapes of γ-rays from either coincidence or singles spectra obtained from Doppler-shift attenuation experiments. The nuclear and electronic stopping powers in a given slowing down medium are independently adjusted to give the line shape and lifetimes of “standard” Doppler-shifted γ-rays, the lifetimes of which have been determined by other independent methods. The stopping power thus calibrated is then used to determine the lifetimes of other transitions from Doppler-shift attenuation experiments in the same stopping medium. Line shapes are obtained by including the effects (a) of the recoils that escape the target, (b) of the decay of other levels into the state in question, (c) of the γ-or particle-detection angles and their corresponding solid angles, (d) of the integration over the distribution of recoils in the singles experiments including angular correlation effects to a good approximation for angles of observation ≤30° to the beam direction, and (e) of the detector response assumed to be gaussian. Lifetimes are obtained from comparison with experiment using both a line shape fitting technique and the F ( τ ) or centroid-shift technique.

A “nuclear-spin spectrometer”: Theory and general characteristics of a 4 π gamma-ray multidetector system for investigations of nuclear structure and reactions at high angular momentum

Abstract The theory and properties of a new type of spectrometer for studying the properties of high-spin states in nuclei and the mechanisms of heavy-ion induced nuclear reactions are presented in some detail. The spectrometer consists of a high efficiency 4π NaI multidetector system with individual tightly packed detector elements. This instrument is capable of recording on an event-by-event basis: (a) the γ-ray multiplicity; (b) the pulse heights and under certain conditions the energies of individual γ-transitions; (c) the total pulse height and the associated γ-ray multiplicity; (d) the associated neutron multiplicity; (e) the angular correlations among most of the γ-rays in the cascade; and (f) the time correlations among the various groups of γ-rays in each cascade. This information is obtained simultaneously thus permitting many correlations to be made among several relevant physical parameters that will enable us to learn more about the properties of nuclei at high-spin and about the role of angular momentum on the mechanism of many types of nuclear reaction.

γ-ray multiplicities from nuclear reactions: Theory, instrumentation and analysis

Abstract Expressions for interpreting γ-ray multiplicity measurements with a Ge(Li) and N NaI detectors are derived. A scattering chamber designed to allow measurements of multiple coincidences between a Ge(Li) detector and/or particle detectors and up to 12 NaI detectors is described. A special multiplicity recording unit has been constructed. It employs the strobed overlap coincidence technique and it is suitable for routing a multichannel analyzer or for interfacing to a computer. Details of the analysis procedure are discussed. The application of a computer code to multiplicity calculations is described.

Angular momentum effects in the compound-statistical model for nuclear reactions: (II). The importance of the Fermi gas model parameters on calculated excitation functions

Abstract The Monte Carlo technique has been employed in calculations of excitation functions in the framework of the evaporation model with detailed dependence of the emission probability on angular momentum. Comparison of calculated excitation functions in the 107 Ag+ 4 He ion system with experiment is made. The dependence of the calculated excitation functions on the Fermi gas model parameters for the level density expression is discussed. The effect of gamma-ray competition for de-excitation on the calculated excitation functions is examined. Values of the level density parameter a in the range ≈ A /12.5 to 1 15 A and of moments of inertia of the rigid body provide satisfactory agreement with experiment. A significant fraction of de-excitation by gamma-ray emission needs to be assumed in order to obtain better agreement with experiment.

On the role of γ-ray emission from nucleon unbound states in the mechanism of compound nuclear reactions

Abstract The compound statistical theory for nuclear reactions was employed to compute total cross sections for many levels in 61 Cu and 60 Ni populated via ( 4 He, pγ) and ( 4 He, 2pγ) reactions on 58 Ni. Angular momentum effects and photon emission from the continuum were included in the formalism. Decay to the well-known discrete states in 61 Cu and 60 Ni is treated separately and for higher excitations the level spectrum is represented by a level-density expression. The cross sections for many levels in 61 Cu and 60 Ni were calculated as a function of excitation energy in the 61 Cu compound nucleus from where the γ-cascades originate in competition with nucleon emission. Spectra of protons populating levels in 61 Cu and 60 Ni via ( 4 He, pγ) and ( 4 He, 2pγ) reactions were also calculated. Fermi-gas-model expressions for nuclear level densities were employed with nuclear moments of inertia approaching the rigid-rotor value with increasing excitation energy. The calculated level cross sections and particle spectra were compared with recent experimental results to determine the level-spacing parameter a as 1 10 A . By comparison with experimental independent yields ratios for many levels in 60 Cu the following quantities were deduced: (a) the moment of inertia as (0.7–0.8) I rig , for 8–12 MeV of excitation in 61 Cu and (b) the fraction of decay of the continuum states by quadrupole radiation. The γ-transition strength for E1 of M1 and E2 radiations were obtained from ( 4 He, pγ)/( 4 He, 2pγ) cross-section ratios and were found to correspond to E1 or M1 retardation factors of ≈ 350 or ≈ 7, respectively, and to E2 enhancement factors of ≈ 35 over the single-proton estimates. At the higher bombardment energies it is found that the 9 2 and 11 2 states in 61 Cu are populated via cascades through the yrast line with ≈ 25 % of the yield carried by dipole radiation. The states with J ≦ 7 2 are populated primarily by quadrupole transitions not involving the yrast line. The variation with bombardment energy of the independent yield ratios for the levels in 61 Cu and 60 Ni is well reproduced. The region in the EJ diagram for 61 Cu where γ-emission dominates is also deduced.

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.

Sub-picosecond lifetimes from Doppler-shift attenuation measurements following (α,2p) and (α,pn) reactions of 54Fe in singles experiments

Abstract A procedure is described by which the Doppler-shift attenuation technique is extended to measurement of nuclear-level lifetimes from single experiments following two-nucleon emitting reactions. It is shown that in reactions such as (α,pn) and (α,2p) it is possible to obtain sub-picosecond lifetimes from the careful measurement at several bombardment energies of the line shapes and centroid shifts of γ-rays near and above the threshold for the levels of interest. The values of (100±15) fs, (580±60) fs, and (180±30) fs, (490±50) fs were obtained for the mean lifetimes of levels at 3638.1, 4180.4 keV in 56 Co, and 3755.8, 5255.5 keV in 56 Fe, respectively.

Levels in 110Cd populated in the decay of 69 min 110gIn

Abstract The level structure of 110 Cd has been investigated from the decay of 69 min 110g In. Singles γ-ray spectra were taken with a large-volume, high-resolution Ge(Li) detector. Extensive γ-γ coincidence measurements were made with both NaI-NaI and NaI-Ge(Li) arrangements. From these data it was concluded that levels at 657.5, 1475.2, 1540.9, 1731.5, 1783.3, 1809.0, 2078.8, 2355.5, 2463.1, 2786.5, 2868.3, 2974.1, 3077.9, 3101.4, 3193.8, 3313.3, 3402, 3451.1, 3464.3, 3474, 3596, 3701 and 3770 keV in 110 Cd are populated in the decay of 69 min 110g In. From log ft values determined in this work and from γ-ray intensity information, limits have been placed for the J π values of many levels. The half-life of 110g In has been measured and found to be 69.1 ± 0.5 min.

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.