W. L. Talbert

Half-lives of some short-lived mass-separated gaseous fission products and their daughters

Abstract The half-lives of several short-lived isotopes of Kr, Rb, Sr, Xe, Cs and Ba have been measured using sources of isotopically pure Xe or Kr and multi-scaling selected transitions in the Ge(Li) gamma-ray spectra of their decays. Beta-ray multi-scaling was employed to confirm the gamma-ray multi-scaling results in several cases. The results of the measurements are (in sec) 89 Kr, 190.7±1.4; 89 Rb, 909±7; 90 Kr, 32.32±0.09; 90 Rb, 153±3 and 251±10; 91 Kr, 5.87±0.04; 91 Rb, 58.2±0.2; 92 Kr, 1.840±0.008; 92 Rb, 4.50±0.03; 93 Kr, 1.289±0.012; 93 Rb, 5.86±0.13; 93 Sr, 439±6; 137 Xe, 229.1±0.8; 138 Xe, 850±4; 138 Cs, 1932±12; 139 Xe, 39.68±0.14; 139 Cs, 556±3; 140 Xe, 13.60±0.10; 140 Cs, 63.7±0.3; 141 Xe, 1.720±0.013; 141 Cs, 24.7±0.4; 141 Ba, 1096±4; 142 Xe, 1.24±0.02; 142 Cs, 1.68±0.02 and 142 Ba, 639±7. The existence of an isomeric state in 90 Rb is indicated by the result spectrum multi-scaling. In most cases, these results compare favorably with recently published values of these half-lives.

Gamma-ray decay schemes of 109gPd, 111gPd and 111mPd

Abstract The levels of 109Ag and 111Ag excited in the beta decay of 109gPd, 111gPd, and 111mPd have been studied through the use of Ge(Li) and NaI(Tl) detectors, both in singles and in coincidence. Twenty-eight gamma rays from 109gPd have been placed into a decay scheme involving 11 excited levels in 109Ag. This scheme is very similar to those reported in previous studies of this decay. Approximately 150 gamma rays have been identified in the decay of 111gPd and 111mPd. More than 100 of these have been placed into a decay scheme involving 35 excited levels in 111Ag, of which 18 are populated in the decay of 111gPd abd 23 are populated in the decay of 111mPd. These levels are discussed in terms of several different model interpretations. Of particular interest is a group of levels in 111Ag that may represent a K = 1 2 + rotational band similar to bands reported recently in odd In isotopes. Other levels are not amenable to a simple rotationalnterpretation, although it is possible that the Ag nuclei are transitional, being rather soft toward deformation.

A plastic scintillation detector for beta-ray spectrum measurements☆

Abstract A scintillation detection system has been built for the determination of beta decay Q-values and ground state branching of short-lived fission products. The beta-ray response of the well-type plastic scintillators was determined by direct measurements of monoenergetic beta rays with energies up to 5 MeV. The beta-ray energy calibration, determined by continuous beta-ray spectra and internal conversion electrons, was found to have a partially quadratic energy dependence. A technique for using gamma-ray Compton spectra to obtain the energy calibration is described. The method of analysis of the distorted beta-ray spectra, which is successful in correcting the distortions down to one-tenth of the maximum beta-ray energy, allows the number of fit parameters to be reduced by application of constraints based on knowledge of the energy level scheme of the daughter nucleus. Sample results, which illustrate the quality of the fits to beta-ray spectra, are presented for 32P and 137Xe.

The Tristan on-line isotope separator facility☆

Abstract The Tristan facility consists of an isotope separator and associated nuclear spectroscopy equipment located at the Ames Laboratory Research Reactor. It is used for on-line studies of short-lived neutron-rich nuclides produced by fission of enriched 235U. The facility is first described in terms of the target and neutron irradiation system along with the associated transport line. Next, the isotope separator is described with emphasis placed on developments of components for on-line operation, and results obtained from tests for mass purity. Ion beam handling to several detector stations through the use of a switching magnet is discussed along with a description of the stations. Also included are the design considerations of a moving tape collector and its drive electronics; retention measurements on tape material; control unit for time sequencing of beam, data accumulation, and tape transport; detector station for study of delayed neutrons; and a high-resolution π 2 β- ray spectrometer. Some of the operational experiences over an eight-year period are discussed.

Circular polarization measurements of gamma rays following the beta decay of 124Sb and 134Cs

Circular polarization measurements of gamma rays following allowed beta transitions in 134Cs and 124Sb were made using forward Compton scattering of the protons from partially polarized electrons available in magnetized iron. Stable fast-slow coincidence circuitry and an accurately timed 32 sec polarization analyser magnet switching cycle were used. From the observed coincidence counting rates for opposite orientations of the magnet, the value obtained for the 134Cs circular polarization asymmetry parameter was A = −0.077<+mn;0.013. Measurements were performed relative to 60Co for which the theoretical value A = −12 was assumed. From the theoretical relation (computed assuming the (V-A) beta interaction) between A and the Fermi-to-Gamow-Teller matrix element ratio Y = CVMF/CAMGT, the corresponding matrix element ratio for the 134Cs beta transition was determined to be Y = 0.220<+mn;0.019. The asymmetry parameter measured for the allowed 0.621 MeV beta transition of 124Sb was A = <+;0.035<+mn;0.030 with the corresponding ratio Y = -0.173<+mn;0.031. The value obtained for the 124Sb asymmetry parameter agrees in sign with that of the only known measurement previously reported but is greater in magnitude.

Target developments at TRISTAN

Abstract Developments at TRISTAN to make available non-gaseous fission-product activities have progressed along three approaches: UF 4 samples to replace the traditional stearate sample, He-jet system connected to the separator ion source, and modification of the ion beam optics to allow placement of the ion source in the neutron beam. The status of each approach is presented in the light of the resulting availability of non-gaseous fission products of interest for systematic studies of nuclear structure of neutron-rich nuclei far from stability.

A seven-detector angular-correlation apparatus for the study of short-lived nuclei

Abstract A seven-detector γ-γ angular-correlation apparatus has been constructed to study short-lived fission product nuclei produced by the TRISTAN on-line isotope separator system. The apparatus employs either seven NaI(Tl) detectors or six NaI(Tl) detectors and one Ge(Li) detector. A 16 384-channel analyzer may be used in conjunction with a series of single-channel discriminators to measure correlations simultaneously on all cascades involving a single γ-ray transition. Alternatively, a buffer tape system may be used to record data simultaneously on all cascades in a decay scheme. The apparatus has been tested on cascades in 138 Ba from the decay of 138 Cs. The results for three direct and two skip cascades are consistent with the known spins of 2 + , 4 + , 4 + , and 3 + for the levels at 1436, 1899, 2308, and 2446 keV in 138 Ba, respectively. In addition mixing ratios of 0.01±0.03, 0.05 −0.12 +0.20 , and 0.10±0.03 have been obtained for the transitions at 1009, 409, and 547 keV in this nucleus.

Tristan II: Extension of the Tristan facility to the study of non-gaseous activities☆

Abstract The TRISTAN on-line isotope separator facility was modified in the summer of 1976 by installing an integrated target/ion source to provide non-gaseous fission product activities. The new capabilities of the facility are presented, along with plans for future studies.

The decay of 85mKr

Abstract The allowed beta spectrum and internal conversion electron spectrum in the decay of 85m Kr were measured with the high resolution π 2 magnetic spectrometer of the TRISTAN on-line isotope separator system, and the 85m Kr gamma-ray spectrum was measured with a high resolution Ge(Li) detector. The half-life of 85m Kr was found to be 4.480 ± 0.008 h. The allowed beta decay of 85m Kr was found to have an end-point energy of 840 ± 2 keV, an intensity of 78.8 % and a log ft of 5.15. Using the PBS method for calculating internal conversion coefficients, the 150.99 ± 0.05 keV transition in 85 Rb was found to be of pure M1 multipolarity with a K-shell internal conversion coefficient α k of 0.0400 ± 0.0008 and a K/L/M ratio of 89/10/2.2. The 304.47 ± 0.05 keV isomeric transition in 85 Kr was found to be a pure M4 multipole with α k = 0.432 ± 0.020 and a K/L/M ratio of 67/10/2.0. The gamma-ray energies were determined by both Ge(Li) and spectrometer measurements.

Decay of mass-separated /sup 138/I

>The levels of /sup 93/Zr populated in the BETA decay of sup 93/Y have been deduced from Ge(Li) -Ge(Li) coincidence and Ge(Li) singles gamma -ray spectra. 22 of the 23 observed gamma -ray transitions have been placed in a level scheme for /sup 91/Zr with 11 excited states. These results are compared with those of previous investigations. A number of corrections, clarifications, and expansions of decay schemes previously reported resulted from the use of high- resolution detectors and a mass-separated source strong enough to permit coincidence studies of the weak as well as the intense transitions. Spin-parity assignments of the /sup 91/Zr levels have been deduced using relative gamma -ray transition rates, BETA -decay log(ft) values, and results of nuclear-reaction studies. Possible interpretations of some of the levels are presented, based on shell-model descriptions. (auth)