B. Martin

Construction and application of a π√(13)2 high-resolution β-ray spectrometer connected with a tandem accelerator

Abstract A high-resolution iron-free β-ray spectrometer with a focusing angle of π√(13) 2 =324.5° has been constructed and connected with a 6 MV tandem accelerator. The focusing principle of this “flat” spectrometer involves radial focusing and axial defocusing at the exit slit which is curved. The “image” of a point source is a curved line, and the image of a line source is identical with that of a point source. The extra degrees of freedom due to this focusing principle are used to obtain a resolving width independent of all aperture angle terms up to the third power, and the only fourth-order term is that with ϕ2r × ϕ2z (ϕr and ϕz are radi and axial aperture angles, respectively). The dispersion is D = 6.5. The stationary orbit has a radius of r0=30 cm. The magnetic field is produced by five pairs of coils. The maximum electron energy is 4.5 MeV and the corresponding electric power 45 kW. A typical pair of values for resolution (fwhm) η and fractional solid angle ω is η=2.2 × 10−4 and ω=0.6 × 10−2. The spectrometer is situated in an iron-free brick building nearby a 6 MV tandem accelerator whose beam can be focused at the source position of the spectrometer to a spot of 0.5 × 5 mm2. The spectrometer was widely applied to study conversion line energies and intensities both from radioactive sources and from Coulomb-excited nuclei. At favorite cases momentum ratios can be measured with 2 × 10−6 precision (internal error), and intensity ratios with 1 per cent. In this paper results are mostly given for energy values.

Kernspektroskopische untersuchungen AN 8Li, 42K, 86Rb, 141Ce, 144Ce, 144Pr und 203Hg

Zusammenfassung The half-lifeof 8 Li has been found to be T 1 2 = 0.854±0.008 sec. The 2 − → 2 + β-groups of 42 K and 86 Rb were measured with the Heidelberg intermediate-image spectrometer. The shape factors show large deviations from the statistical shape with large statistical errors, however, in the case of 86 Rb. The integral β-circularly polarized γ-correlations were measured by Compton forward scattering. For 42 K, rather uncertain best values for the six matrix elements were obtained. The β-circularly polarized γ-correlation of 141 Ce is characterized by an asymmetry parameter ω = −0.16 ± 0.05 , which favours a value of X in the range 4.5 X X is the matrix element ratio in the ξ-approximation. In the cases of 144 Ce and 203 Hg, the reduced anisotropy coefficient ϵ′ of the β-γ directional correlation was found to be positive and not strongly energy dependent ( 144 Ce), and negative and strongly energy dependent ( 203 Hg). The β-circularly polarized γ-correlations of 144 Ce, 144 Pr and 203 Hg were measured. The 144 Ce correlation has ω = 0.64 ± 0.25 . The 144 Pr results (two cascades) agree with the formerly established value of 0 − for the 144 Pr ground state. The 203 Hg result ω = −0.18 ± 0.04 confirms the negative sign of the β-transversely polarized conversion electron correlation. The results are discussed with respect to the ξ-approximation.

Experimental test of conversion coefficient ratios for M-, N- and O-shell electrons

Abstract Relative conversion-line intensities have been measured for five E2 transitions of energies from 81 to 412 keV in nuclei with Z ranging from 55 to 80 using the iron-free 1 2 π√ 13 β-ray spectrometer. The results for M-, N- and O-shells were found to agree with the calculations where screening is taken into account. The atomic screening and finite nuclear size effects in internal conversion of the N-subshell electrons were investigated. Two previously measured conversion electron spectra were analysed with four different computer codes and the consistency of the results was examined.

The decay of 131Cs

Abstract In a study of the level scheme of 131Cs, we have measured the γ-γ and the ce-γ directional correlations of the 496-123.7 keV and 486-133.5 keV cascades. The linear polarization at φ=90° was measured for the 496-123.7 keV and 486-133.5 keV cascades. The L-subshell ratios for the 123.7 keV transition were determined. The g-factor of the 133.5 keV level and the half-lives of the 78.7, 123.7, 133.5 and 216 keV levels were re-investigated. The spin and parity assignments derived for the 620, 133.5 and 123.7 keV levels are 1 2 + , 5 2 + and 1 2 + , respectively. The 620, 496, 486, 133.5 and 123.7 keV transitions are found to be of pure E2, pure M1, pure E2, (M1+(22±2)%E2) and pure E2 multipolarity, respectively. An upper limit of 192 ps for the half-life of the 216 keV level is obtained. The results are compared with theoretical estimates from the calculations of Kisslinger and Sorensen, of Freed and of Freed and Miles.

β-zerfall des J130 und ξ-approximation

The β decay I130→Xe130 has thoroughly been investigated. The β and e− spectra were measured with a double lens spectrometer, the γ-ray spectrum with a scintillation spectrometer, four β−γ directional correlations with a scintillation counter set-up, and two β-(circularly polarized γ) correlations via the method of Compton forward scattering. The β-ray spectrum consist of three groups: 1.78 MeV (0.4%), 1.04 MeV (48%), and 0.62 MeV (52%). The γ-ray spectrum shows five lines: 0.419 MeV (36% transition intensity per decay; 0.5% E2+99.5% M1), 0.538 MeV (100%; E2), 0.669 MeV (100%; E2), 0.743 MeV (87%; 89% E2+11% M1), and 1.15 MeV (12%; E2). The Xe130 level spectrum consist of the ground state (0+) and excited states of 0.54 MeV (2+), 1.95 MeV (5+), and 2.73 MeV (6+). The ground state of I130 is 5−. The three β-ray transition populate the three upper levels and, hence, are non-unique first forbidden. The two strong groups show allowed shape and very small though non-vanishing energy-independent reduced anisotropy coefficients e‘ while the weak group has an isotropic correlation with the succeding γ radiation. Therefore, the β-(circularly polarized γ) correlations may well be interpreted in terms of the ξ appro ximation which is reviewed. The coefficient eβ− of the 0.62 MeV β group has, within the experimental error, the value uniquely determined by the ξ approximation and the measured spins while ωβ− of the 1.04 MeV group implies nX=−CA1i∫σ·rCA∫σ·r÷ Cvi∫r=−1.0−0.9′+0.6 nSummarizing one may state that the β decay of I130 exhibits a nice example for the validity of the ξ approximation.

The Heidelberg MP tandem Van de Graaff

Abstract Properties and improvements of the Heidelberg MP tandem are reported. The upgrading to terminal voltages of 13 MV and the installation of new components are discussed in detail. The performance of the machine and projects in future such as the installation of a pelletron chanrging device and a nanosecond beam pulsing system are described.

Nuclear structure effect in internal conversion of the 57.54 keV E1 transition in 180Hf

Abstract Measurements have been performed on relative conversion line intensities of the highly K -forbidden E1 transition (8 − → 8 + ) of 57.54 keV in 180 Hf. Anomalies observed for the L, M, N subshell and O total intensity ratios were explained with the nuclear current parameter η = −7.0 ± 0.3 and nuclear charge parameter ξ = 10 ± 50.

Einfluß von festkörpereffekten auf die β-γ-Richtungskorrelation des 166Ho

Abstract The β-γ directional correlation of 166 Ho has been measured in metal, oxide and chloride sources, in order to study the influence of solid state effects on the correlation. In metal sources an energy-independent attenuation factor G 2 = 0.356 ± 0.005 was found. The oxide and chloride sources showed attenuation factors which varied with the β-ray energy. Between 0.5 and 1.8 MeV G 2 was found to increase from 0.16 to 0.25 (oxide), and from 0.25 to 0.37 (chloride evaporated in vacuo).

Internal conversion investigations on the 42 keV E3 transition in 191Ir

Abstract Relative conversion line intensities of the 41.85 keV E3 transition in 191Ir have been measured for twelve atomic subshells, using the iron-free 1 2 π √13 β-ray spectrometer. The transition energy of 41.85 ± 0.03 keV and multipolarity E3 + (0.001 ± 0.01)% M4 were determined. Internal conversion coefficients were calculated in different physical approximations. Precise calculations were performed for three atomic models. Experimental data were found in agreement with recent tabulations and present calculations based on the relativistic potential.

Nuclear structure effect in internal conversion of the isomeric transition in 191Os

Abstract Measurements have been performed on relative conversion line intensities of the hindered 74.38 keV isomeric transition in 191Os with the 1 2 π√13 iron-free β-ray spectrometer. Anomalies were observed for the L, M and N subshell intensity ratios and were explained with a nuclear structure parameter λ = 9.4 ± 0.5 and a mixing ratio ¦δ( E4/M3 )¦ = (3.0 ± 0.3) × 10 −3 .