J. van Klinken

Double scattering of electrons

Abstract The Mott asymmetry function for electrons with energies from 26 to 261 keV has been investigated with an improved arrangement for double scattering of accelerated electrons. First, a beam of singly scattered electrons with a reproducible and well defined degree of transverse polarization was obtained by performing a double scattering experiment with identical scattering conditions for polarizer and analyser. In this experiment scattering angles of 105° and gold foils of a standard thickness were used. The beam of singly scattered electrons was then used for calibration of a rotatable Mott analyser, consisting of a second scattering foil and four scintillation counters. Two counters were set at large scattering angles for detection of the asymmetry due to polarization and two were set at 45° for simultaneous observation and elimination of instrumental asymmetries. It is shown that the 45°-zero measurement is very effective in reducing the influence of several instrumental effects. The calibrated analyser can be ussed for polarization detection of other electron beams with a relatively large efficiency. The ratio of the numbers of incident and scattered electrons is about 0.6×10 −3 . The instrument yields the absolute degree of transverse polarization of the incoming electrons with an accuracy of better than 1% in favourable cases. Measurements are reported of the energy dependence of the Mott function S at 45° and 105° and of its angular dependence at 121 keV. The results agree with those of Mikaelyan et al. 9 ) but have higher accuracy. Qualitatively the present results show the energy and angular dependence of S predicted by theory. Measurements at small angles show a change of sign, as do the calculations. However, quantitative agreement is often poor. Especially at low energies and small angles the calculated S values are significantly larger than the observed values.

Some absolute beta polarization measurements

Abstract The longitudinal beta polarization of several transitions has been investigated for energies varying from 46 to 261 keV. The beta polarization was first converted from longitudinal to transverse by deflection of the electrons in an electric field, after which the transversely polarized electrons were analysed with the aid of a calibrated Mott analyser. Results are reported for beta rays from 32P, 60Co, 114In, 147Pm, 147Nd, 153Sm and 198Au. For energies larger than 100 keV, no large deviations from the expected value P = −v/c have been observed. At lower energies there exist deviations, which may partly be explained as an effect of screening, according to calculations of Buhring. Much attention has been given to the effect of depolarization in sources of finite thickness, the influence of which increases towards lower energies.

Average multipolarities of quasi-continuum transitions following the (α, 4n) reaction from measured internal conversion coefficients

Abstract The quasi-continuum of electromagnetic transitions following the 160 Gd(α,4n) 160 Dy reaction exhibits predominant E1 character. This is condluded from conversion electrons observed with a mini-orange spectrometer and corresponding γ-rays, both being selected by coincidences with discrete ground-state band transitions in 160 Dy.

Beta- and gamma-ray measurements on the decay of 38Cl

The decay of 37 min 38Cl has been reinvestigated with improved methods and accuracy. It was shown that the β-ray transition to the ground state of 38Ar has a unique shape. This confirms the theoretical predictions that the ground state of 38Cl is 2− and removes a contradiction arising from earlier measurements. After correction for the unique shape, the residual shape factor 1 + aW is found to be close to one: a = −0.005±0.004. This result must be compared with other measurements of unique transitions, which often give more pronounced negative values for a. The observed end-point energy is Emax(β0−) = 4913±5 keV. A comparative shape measurement with 42K yielded: a = −0.005±0.003, Emax(β0−) = 3519±4 keV. The energies of the three known γ-ray transitions were measured with a 20 cm3 Ge(Li) detector; γ1: 1642.16±0.13 keV (73.8±0.9); γ2: 2167.60±0.16 keV (100) and γ3: 3810±2 keV (0.061±0.005), with relative intensities given in parentheses. In an appendix the assignment 12− to 95Y resulting from the use of 38Cl for calibration is mentioned.

The decays of 70aCu, 70bCu and 67Ni

Abstract Three activities observed after 14 MeV neutron irradiation of 70 Zn have been assigned to two is omers of 70 Cu( 70a Cu, T 1 2 = 5 ± 1 s , and 70b Cu , T 1 2 = 42 ± 3 s ) and to 67 Ni (T 1 2 = 18 ± 4 s ) . Ge(Li) single γ-ray spectra, NaI-Ge(Li)γγ coincidences, plastic scintillation single β-ray spectra and plastic-Ge(Li) βγ coincidences were measured. Levels in 70 Zn are proposed at 884.8 (2 + ), 1786.5 (4 + ) and 3038.2 (> 3, − ) keV on the basis of coincidence spectra. Evidence suggests spins and parities of (1 + ) and (5 − ) for 70a Cu and 70b Cu, respectively. A probable level in 67 Cu is suggested at 1072.2 keV from the decay of 67 Ni. Many additional γ-rays were observed but were too weak for coincidence studies and could not be placed in the decay schemes. Some results of reaction studies are discussed.

The decays of 82a+82bAs and 79Ge

Abstract The decays of two states of 82 As are reported: 82a As with T 1 2 = 19±1.5 and 82b As with T 1 2 = 13.0±0.6 s . Information on the decay of 79 Ge with T 1 2 = 40±4 s was also obtained. Spectra of single β - and γ - rays , γγ and βγ coincidence measurements and fast chemical separations were used to identify and to study the decays of these activities, particularly of 82a+82b As. The two isomers of 82 As probably have spin and parity l + (a) and 5 − (b). From their decays levels are proposed in 82 Se at: 654.8(2 + ), 1731.2(2 + ), 1735.4(4 + ), 2550.8(3 + ), 2894.2(4 − ), 3454(5 − ), 4091, (4321) and (4565) keV. An unusually large hindrance factor of the allowed β-decay from the 1 + state of 82a As is noted. The partial decay scheme of 79 Ge has levels in 79 As at 230.4 and 772.9 keV and possibly at 985.9 keV.

A new system for depth-selective conversion electron mössbauer spectroscopy

Abstract A conversion electron Mossbauer spectrometer has been constructed in which a mini-orange magnetic filter focuses conversion electrons emitted by a moving absorber onto a thin window silicon surface barrier detector. This detector serves as an electron spectrometer. A special multi-scaler is used to record a number of Mossbauer spectra, corresponding to different conversion electron energy groups, simultaneously. A large reduction in counting time compared to other methods of energy differential conversion electron Mossbauer spectroscopy is achieved by the relatively high transmission of the mini-orange (∼2%), the high efficiency of the detector and the feature of simultaneous recording. Depth selectivity is mainly limited by the resolution (600–900 eV) of the Si-detector. The system has been tested with a sandwich absorber of 57Fe evaporated onto stainless steel enriched in 57Fe irradiated by a 57Co Rh source.

Average multipolarity of continuum transitions in nuclei at high angular momentum

The multipolarity of continuum transitions deexciting high-spin states has been deduced from measured conversion coefficients. The investigated 146Nd(20Ne, 4n or 5n) 162,161Yb reactions were selected by gating on discrete lines. The average multipolarity gradually changes from E2 at 0.5 MeV to E1 above 1.5 MeV.

The beta decay of 147Nd

The beta components with maximum energies of 806 and 364 keV in the decay of 147Nd have been studied. The coefficient a in the expression C(W) = k(1+aW) for the shape factor is found to be −0.07±0.01 for the 806 keV transition and −0.20±0.15 for the 364 keV transition. The previously reported degree of longitudinal polarization, measured for an electron energy of 261 keV, is −(1.000±0.025) v/c yielding b = 0.00±0.05 for the 806 keV component and b = 0.00±0.16 for the 364 keV component in the expression P = −(v/c)(1−b/W). The coefficients Rand e in the expression ϵ = (p2/W)(R+eW)/C′(W) for the β-γ angular correlation are R = −0.008±0.006, e = 0.002±0.003 for the (806β, 91γ) cascade and R = +0.06±0.05, e = −0.04±0.03 for the (364β, 531γ) cascade. The very low intensity of the beta component to the ground state of 147Pm (<0.2% of the 806 keV component) is briefly discussed.

Remark on the status of non-conservation of parity in nuclear beta-decay☆

Equality of β-decay coupling constants C′A/CA = C′V/CV = 1, implying V - A interaction, is less verified than commonly assumed. Deducing limits 0.52 < C′V/CV < 1.93 and 0.82 < C′A/CA < 1.22 we discuss a possible V+A admixture with a mass ratio m(WV−A)/m(WV+A) < 0.56 for intermediate bosons WV±A.