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Experiments on nuclear orientation at low temperatures Lamarche, Gilles


At the low temperatures attainable by the technique of adiabatic demagnetization, It is possible to produce under certain favorable conditions an almost complete orientation in space of the nuclear spins. Observation of the gamma radiation emitted by such an assembly of oriented radioactive nuclei often shows an anisotropy in the intensity with respect to the direction of observation. From this anisotropy it la possible to deduce the multipole order of the electromagnetic radiation, the change in the angular momentum occuring in the transition that precedes it, and the value of the nuclear magnetic moment of the emitter. Using Bleaney's method for nuclear alignment, two isotopes were studied. Firstly, Yb¹⁷⁵ in cerium magnesium nitrate in the temperature range from 1.2° K. to 0.005° K. The same isotope was also investigated in Oxford in ytterbium ethyl sulphate and the object of our experiment was to find out if, given two salts suitable for nuclear alignment, the interactions in the crystal could affect the anisotropy. It was found to be the case. While the Oxford experiment revealed a sizeable anisotropy, none was observed in our own. The absence of anisotropy is attributed in our case partly to magnetic interaction between the paramagnetic ions and partly to a relatively long lifetime of the excited state of Lu¹⁷⁵ from which the γ-ray is emitted: a deorientation of the nucleus before emission ruins the effect. This is not the case in the ethyl sulphate. The second isotope Pr¹⁴² was also investigated by Bleaney's method from 4.2° K. to 0.005° K. The possible values of anisotropy to be expected were 0.33, 1.00, or -1.00. The experiment clearly showed that there was no anisotropy. It is almost certain that the details of the decay schemes so far proposed are incorrect. Either the spin of Pr¹⁴² is 0, not 2 as has been suggested or the decay chain which ends in the γ-ray observed includes a state of spin 0 before the emission of the γ-ray. An explanation which must be considered, however, is that the properties of the praseodymium ion in this crystal lattice are not fully understood. Finally, a series of experiments have been performed in an attempt to detect calorimetrically the nuclear quadrupole specific heat of I and Br in covalent compounds. This investigation was a contribution to the problem of nuclear alignment by Pound's method, and was part of a research program initiated by J. M. Daniels at Oxford. The observation of the effect was rendered impossible because the nuclear-ionic relaxation time for the establishment of an equilibrium in temperature of the nuclear spin system was larger than expected.

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