J.S. Geiger

The precision determination of some γ-ray energies using a β-spectrometer

Abstract The energies of several γ-rays which are frequently used as calibration standards in nuclear spectroscopy have been determined with an accuracy of ∼ 4:105 using the methods of internal and external conversion in conjunction with the Chalk River iron-free π 2 β-spectrometer. The effects on the positions of internal and external conversion lines arising from the finite thickness of the radioactive sources and radiators and from natural width and angular correlation phenomena have been investigated in some detail. The β-spectrometer has been calibrated with the internal and external conversion lines of the 411.795±0.009 keV γ-ray in Hg198. The γ-ray energy values obtained using the 1963 least-squares adjusted physical constants are: Pb208 583.139±0.023 keV; Pb208 2614.47±0.10 keV; Ni60 1173.23±0.04 keV; Ni60 1332.48±0.05 keV; Mg24 1368.53±0.04 keV and Mg24 2753.92±0.12 keV. The energy values of seven γ-rays in Pt192 are also reported.

Measurements of M1 and E2 transition probabilities in Te125, I127, Xe129, Cs133, La139 and Pr141

Abstract The mean lives of the first excited states of Xe129, I127, Cs133 and La139 have been measured electronically using a time-to-amplitude converter. The percentage E2 admixture in the dominantly M1 radiations de-exciting these states have been determined from measurements of the L subshell conversion line intensity ratios for the transitions. The percentage E2 admixture in the 145.43 keV γ-transition in Pr141, the 165.84 keV transition in La139 and the 35.48 keV transition in Te125 were determined in the same way. Summaries have been prepared of the presently available information on the M1 and E2 transition rate for the L forbidden M1 transitions of the classes represented by the above transitions ( d 3 2 ⇄ s 1 2 and g 7 2 ⇄ d 5 2 ; Z ≈ 50) .

A one-meter-radius iron-free double-focusing π√2 spectrometer for β-ray spectroscopy with a precision of 1:105

Abstract This high resolving power, high luminosity π√2 spectrometer employs a novel coil geometry which leaves ample room for large, accessible source and detector assemblies. The exact coil diameters and spacings used to achieve axial focusing up to the fourth order were specified by Lee-Whiting after a detailed theoretical study of the electron optical focusing properties of the coil arrangement. Four pairs of precisely manufactured coils generate the cylindrically symmetrical field whose shape closely approximates a 1 √r form near the optic circle, ro = 100 cm. The largest pair has 560 turns per coil and a mean diameter of 395.1 cm. The temperature control arrangements for this all-aluminum spectrometer ensure a constancy in the calibration ratio, gauss cm per ampere, of better than 1 part in 105 for electron energies up to 4 MeV. At resolution settings of 0.01%, 0.1%, and 1.0%, transmission values of 0.08%, 0.26%, and 1.1% respectively, can be used. At resolution settings of 0.1%, or better, the luminosity (source area times transmission) is larger than the luminosities of other precision spectrometers. The degaussing system, iron-free building, source, baffle and detector arrangements, current-regulating and measuring systems, automatic controls, and readout system are discussed briefly. Several examples of the performance at resolution settings in the range 0.1% to 0.01% are given which illustrate this instruments luminosity, ultimate resolution in momentum of 1 part in 104, and inherent precision of 1 part in 105.

A determination of the absolute energy of the Hg198 412 keV γ-radiation

Abstract The energy of the Hg198 412 keV γ-ray has been compared with the energy of the narrow (no Doppler spread) component of the annihilation radiation emitted from completely thermalized positonium in ice. This comparison was made with the Chalk River π√2 β-spectrometer using external conversion electron techniques. The momentum of the Hg198 412 keV LIII external conversion electron line from a 0.67 mg/cm2 uranium radiator was compared with the momentum of the annihilation radiation K external conversion electron line from the same radiator at a momentum resolution of ≈0.09%. The momentum ratio of these conversion lines was found to be 0.998761±0.000010. The Hg198 γ-ray energy has been deduced from this momentum ratio using both the 1955 and the 1961 (preliminary) sets of least-squares adjusted fundamental constants; the resulting Hg198 γ-ray energy values are 411.770±0.007 keV, respectively.

On the electromagnetic separation method of preparing radioactive sources for precision β-spectroscopy

Abstract With the recently developed techniques of high-resolution β-spectroscopy, the precision that can be realized is limited by source quality rather than the performance of the β-spectrometer. It is therefore of interest to consider in detail how source quality might be improved. Throughout the present work an inert gas has been used so as to study the problems of source preparation under the most difficult and general conditions. First, the general effect of source quality is examined experimentally by studying the shapes of the K 54.96 and the K 188.4 conversion lines from a wide variety of Xe 125 sources using momentum-resolution settings of ∽ 0.05% and ∽ 0.1%. Techniques for decelerating ions and thus minimizing source thickness are then described, followed by experimental measurements of the ranges of Na 24 , K 42 , Rb 86 , Cs 137 , and Xe 133 ions in Al and of Xe 133 ions in Au. Experiments relating to the diffusion and to the long-term retention of inert-gas atoms in a variety of metal targets are described. Finally, a number of effects which tend to limit source strength are considered, including beam current, beam scattering and neutralization, target collection, source thickness, and sticking factors.

Measurement of lifetimes in 152Sm, 154Gd and 156Gd by doppler-shlft recoil-distance techniques

Abstract The following lifetimes (in ps) have been measured for the states in 152 Sm : 4 g : 82.8 ± 1.2, 6 g : 14.6 ± 0.4, 8 g : 4.3 ± 0.65, 0 β : 8.92 ± 0.55; in 154 Gd: 4 g : 66.3 ± 2.2, 6 g : 11.2 ± 0.6, 0 β : 5.7 ± 0.9; and in 156 Gd : 4 g : 163 ± 7 and 6 g : 20.5 ± 1.75 . The Doppler-shift recoil-distance method was used in which the states were populated by Coulomb excitation using 100 MeV 35C1 ions. Gamma rays were detected in coincidence with back-scattered 35C1 ions. The results for the 0β lifetimes are discussed in terms of the monopole matrix elements between the ground state and β-vibrational bands. Deviations from rotational behaviour in the ground state band transition probabilities are found to be consistent with the simple band-mixing picture assuming the β-ground mixings as given by Rud, Nielsen and Wilsky.

Conversion electron spectrum from Ce144 decay

Abstract The internal conversion electron spectrum following the β decay of Ce144 has been studied in the Chalk River iron-free π√2 β spectrometer at a momentum resolution of ≈ 0.1%. The 33 conversion lines observed are identified with 7 transitions of energies (in keV) and multipolarities 33.57±0.03, M1; 40.93±0.03, M1; 53.41±0.03, M1; 59.03±0.03, M3; 80.12±0.03, Ml; 99.95±0.03, E2; and 133.53±0.03, M1. The multipolarity assignment for each transition was obtained by comparing the observed conversion line intensity ratios with the theoretical line intensity ratios of Sliv. The intensity of the K conversion line of the 133.53 keV transition has been measured relative to the total intensity of the Pr144 β spectrum; IK 133/IPr β sp. = 0.053±0.002. The momentum ratio of the Ce144 K 133.53 and Cs137 K 661.6 lines, corrected for electron energy loss in the Ce course, is BρK 133/BρK 661 = 0.31495±0.00003. Intensity limits are placed on all previously reported Ce114 conversion lines not presently observed. Relative quantum intensities and total transition intensities are deduced from the conversion line data. A decay scheme for Ce144 is given which satisfactorily accounts for all the results of this investigation and is compatible with the results of earlier workers when some line reassignments are made. Interpretations of the Pr144 levels, established in this work, are discussed in terms of both the shell model and the unified model. With δ ≈ +0.07, the unified model accounts for the experimental evidence in a simple but convincing fashion.

Negative parity levels in the even-even nucleus Dy160

Abstract The conversion electron spectrum of Tb160 has been studied in the Chalk River air-cored ρ√2 β-ray spectrometer at resolution settings of 0.04 % to 0.1 %. Gamma-conversion electron coincidence experiments have been made to establish the positions of the levels in Dy160. The level scheme based on the present results is 0(0+), 86.7 (2+), 283.6 (4+), 965.8 (2+), 1048.8 (3+), 1264.4 (2-), 1286.5 (3-), 1358.2 (2-), and 1398.5 keV (3-). This is similar to that of Nathan in the positive parity levels but differs in the negative parity levels. Particular attention has been paid to determining the intensities of weak transitions in the decay scheme. The relative quantum transition probabilities have been deduced from the conversion line measurements using Slivs theoretical conversion coefficients. The agreement between the observed transition probabilities from the positive parity levels and the theoretical predictions of the Unified Model is greatly improved if a small amount of K = 0 band is assumed mixed in the K = 2 gamma-vibrational band. The relative transition probabilities from the negative parity levels are not consistent with K being a good quantum number in discussing these levels. The possibility of interaction between the negative parity levels is briefly discussed.

Measurement of lifetimes for high spin states in 152Sm, 154Gd and 156Gd by the Doppler broadened lineshape method

Abstract Lifetimes of the ground band levels up to spin 10 + in 152 Sm, 154 Gd and 156 Gd have been measured by the Doppler broadened lineshape (DBLS) analysis of peaks observed following Coulomb excitation of enriched metallic targsts by 132–143 MeV 35 Cl beams. The γ-ray lineshapes were measured in time coincidence with backscattered ions and were analyzed with a computer program incorporating tabulated electronic stopping powers. The nuclear stopping power of Lindhard et al and multiple scattering treatment by Blaugrund were assumed. Renormalization of the electronic stopping powers given by Northclifie and Schilling was found necessary to reproduce the accurately known lifetime of the 6 + state in 152 Sm. Stopping powers for Sm in Sm inferred from the tabulated 4 He stopping power of Ziegler and Chu support this renormalization. The stretching parameter a derived from the lifetimes of the ground band are (2.0 ± 0.6) × 10 −3 , (1.85 ± 0.40) × 10 −3 and (0.0 ± 0.45) × 10 −3 , in 152 Sm, 154 Gd and 156 Gd, respectively.

High spin states in the ground and β-bands of 156Dy and the band-crossing interpretation of back bending

Abstract High spin states have been identified in 156 Dy using (HI, x n) reactions and γ-γ coincidence techniques. The ground state band does not show any “phase change” or “back bending” behaviour up to spin 18 + . However a second sequence, based on the β-vibrational band, does show back bending at spin 12 + . The two sequences intersect just below spin 16 + with the 16 + , 18 + and 20 + members of the β-band sequence becoming yrast levels. Branching ratios have been obtained for transitions above and below the bend at spin 12 + . The data are discussed in terms of a simple band-crossing model of back bending.