Γ-ray Anisotropy Research Articles

Nuclear orientation and parity-mixing studies of the decays of 171Er and 169, 175Yb

Angular distributions of γ-radiations emitted by 171Er and 169, 175Yb have been studied from radioactive samples polarized at low temperatures. The magnetic hyperfine field of Er in ZrFe 2 was deduced to be 8.3±1.0 MG, based on the anisotropies of the 171Tm γ-rays. The magnitudes of the 159Yb and 175Yb magnetic moments were deduced to be 0.63±0.09 μ N and 0.40±0.05 μ N, respectively, based on γ-ray anisotropies following the decay of samples polarized in Au. Gamma rays from 177Lu were observed to be emitted isotropically, indicating the lack of Lu polarization in Au. The E2/M1 and M2/E1 multipole mixing ratios were deduced for most of the mixed γ-transitions in 169, 171Tm and 175Lu. Investigations of the forward-backward asymmetry of the angular distributions of a number of strongly hindered γ-transitions in 169, 171Tm and 175Lu revealed no effect attributable to parity mixing in the nuclear states.

Effect of a magnetic field on the melting curve of3He?Addendum

An additional analysis of our low-temperature and high magnetic field measurements of the3He melting curve is presented, based on the assumption that the highest field (63.6 kG) theoretical melting curve is correct rather than the lowest field curve as assumed in the original paper. Between31.8 and63.6 kG there is reasonable agreement between experiment and theory, although a discrepancy remains at low fields that we assume is to be attributed to inaccuracies in the reduction of γ-ray anisotropies to temperatures. It is also shown that the measurements are not sufficiently precise to observe the effect seen in the(∂P/∂T) V,H measurements on solid3He by Kirk and Adams.

Nuclear polarization and NMR of 74As in Fe

Nuclear magnetic resonance has been detected in radioactive 74As nuclei polarized in Fe. The magnetic moment of the 74As ground state ( I π = 2 −) is | μ| = 1.597 ± 0.003 n.m. and the sign of the internal field at As nuclei in Fe is positive. The measured γ-ray anisotropies require the predominance of j β = 0 in the β-transitions to the first 2 + states in 74Se and 74Ge.

The Magnetic Moment of 59Fe

We have attempted to measure γ-ray anisotropies from 59Fe nuclei oriented in pure iron. The negative result allows us to put a limit of 0.9 n.m. on the magnetic moment.

Effect of a magnetic field on the melting curve of3He

The melting curve of3He was measured below 25 mK and in magnetic fields of 2.12, 6.36, 31.8, and 63.6 kG. The compressional cooling technique was used with a capacitive pressure gauge and a54Mn-in-iron γ-ray anisotropy thermometer. A depression was observed in the melting curve, indicating nuclear spin ordering in the solid3He. The effect is larger in magnitude and develops more rapidly with field than the theoretically expected results.

Levels ofMn53from theCr53(p, nγ)Mn53Reactions

In a study of the Cr53(p, nγ)Mn53 reactions, levels of Mn53 have been found at 377.5, 1288.4, 1440.1, and 1619.1 keV, with uncertainties of ±0.3 keV. The observation of γ-ray anisotropies and (p, n) cross-section ratios permits unique spin assignments to three of the levels. Analysis of the (p, n) cross-section ratios using the Hauser-Feshbach formalism is consistent with the results of earlier experiments for the two lower levels. The spins of the above levels are, in the order given above, 52, 32, 112, and 92. Mixing ratios for the 377.5- and 1619.1-keV transitions are measured to be -0.6±0.3 and -3.2±0.8, respectively.

Messung der γ-Winkel-Verteilung nach dem Neutroneneinfang an polarisierten Tb159-Kernen

The anisotropy of high-energy capture γ-rays from polarized Tb159 nuclei was measured. Polarization of Tb159 was achieved by cooling single crystals of Terbium-Ethylsulfate toT=0.35 °K in an external field of 4 kOe. With the results of this measurement, it was possible to deduce the spins of 11 levels in Tb160 between 64 keV and 767 keV. Furthermore theQ-value of the reaction Tb159(n, γ) Tb160 was determined to beQ=(6,374.0± 1.5) keV.

Nuclear alignment in a Kondo system as a very low temperature thermometer

Cu doped with less than 1 ppm Mn54 appears to offer a number of advantages as a γ-ray anisotropy thermometer in the temperature range 1 to 20 mK. In order to use such a thermometer it is necessary to know how the hyperfine field at the Mn54 nuclei varies with temperature and with applied field. We have studied this γ-ray anisotropy and find that the hyperfine field is independent of temperature in the range 4 mK <T<10 mK. Further, we have determined the hyperfine field to ±5% as a function of applied field in the range 60 Oe<Ha<1200 Oe. The latter data may be used to determine a Kondo temperature of 64±2 mK for this system.

Observation of NMR in an isomeric state following a nuclear reaction

The isomeric state in 73As ( 426 keV, 9 2 +·τ 1 2 = 6 μs ) was produced and aligned by the 71Ga( α, 2n) reaction in a liquid Ga metal target. The resulting γ-ray anisotropy was attenuated by inducing NMR transitions with a 1 MHz rf field. The value g = +1.146 ± 0.007 was determined.

Spin-lattice and cross relaxation of 54Mn nuclei in dilute paramagnetic crystals

The relaxation of nuclei of radioactive 54Mn in crystals of lanthanum magnesium nitrate is studied as a function of Ce concentration, magnetic field and temperature. The nuclear orientation is derived from the γ-ray anisotropy. Nuclear spin-lattice relaxation times of 10 1 to 10 5s were measured and interpreted by electronic spin-lattice relaxation in conjunction with forbidden transitions, i.e. nuclear spin flips. Generally a Ce concentration of more than 1% is required in order to influence this relaxation significantly. When the sample is cooled at a fast rate, the Ce spins can induce forbidden transitions ( e.g. Sz = − 5 2 , Iz = +3 → Sz = − 3 2 , Iz = +2 ) in the Mn spin system, which proces generates a population of the nuclear levels which strongly deviates from thermal equilibrium as was shown by the observation of anomalous nuclear orientation. An analysis of this process is given.

THE LOW-TEMPERATURE ORDERED STATE OF CERIUM MAGNESIUM NITRATE AND ITS INFLUENCE ON NUCLEAR ORIENTATION

The method of Luttinger and Tisza for minimizing the dipole–dipole interaction energy is applied to cerium magnesium nitrate, and an antiferromagnetic ordering of the cerium spins at 0 °K is found. Using this configuration, the magnetic field at the divalent ions is calculated. Next, the anisotropy of γ rays from Co60 aligned in this salt is calculated for temperatures below 0.003 °K. Qualitative agreement is found between these calculations and measurements of γ-ray anisotropy reported in the literature.

NUCLEAR ORIENTATION IN ANTIFERROMAGNETIC SINGLE CRYSTALS

Mn54 and Co60 have been successfully oriented in five antiferromagnetic single crystals (MnCl2∙4H2O, MnBr2∙4H2O, CoCl2∙6H2O, Co(NH4)2(SO4)2∙ 6H2O, and MnSiF6∙6H2O) and the orientation was detected by the anisotropy of the emitted γ rays. Only in the case of Co60 in MnBr2∙4H2O was no γ-ray anisotropy seen. It is concluded that antiferromagnetism can be used as a means of producing nuclear orientation. Attempts to orient Br82 and 1131 in the manganese halides by superexchange were unsuccessful.

γ-Ray Anisotropy ofCo60Nuclei Polarized by Paramagnetic Resonance Saturation

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