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Experiments with Oriented Nuclei
The angular distribution of the radioactive radiation of a sample containing unoriented nuclei has spherical symmetry; in contrast, the radioactive radiation of oriented nuclei is anisotropic.
It was shown theoretically^1 that, first, anisotropy in the angular distribution of radioactive $\alpha$-, $\beta$-, or $\gamma$-radiation occurs for nuclear spin $I > 1$; second, the intensity of radioactive radiation per unit solid angle at an angle $\vartheta$ to the quantization axis of the nuclear spins depends only on $\cos^2 \vartheta$ (the intensity $F(\vartheta)$ is given by a polynomial of the following form: $F(\vartheta)=a_0+a_2\cos^2\vartheta+a_4\cos^4\vartheta+\cdots$, where the coefficients $a_2$, $a_4$ depend on the nuclear quadrupole polarization $g$ and vanish when $g=0$).
In one of the papers reviewed^2, the angular distribution of the $\gamma$-radiation of Ni^60 nuclei was studied (the Ni^60 nuclei were obtained from oriented Co^60 nuclei by means of $\beta$-decay).
As the sample under investigation, the authors used a single crystal of diluted Tutton salt $Rb_2(1\%\,Co,\ 12\%\,Cu,\ 87\%\,Zn)(SO_4)_2\cdot6H_2O$
Copper ions play the role of cooling agents (under adiabatic demagnetization); zinc is added to reduce the local magnetic field at the Co ends (the zinc ion is diamagnetic); thanks to the reduction of this local field, considerably lower temperatures are reached upon demagnetization than in the case of a salt not diluted with zinc.
The orientation of Co\({}^{60}\) nuclei is achieved in the present work by means of the method of anisotropic spin–spin interaction (see the preceding abstract). Adiabatic demagnetization of the above-mentioned salt is carried out from an initial
\[ \frac{H}{T}=30\,\frac{\text{kilo-oersted}}{{}^{\circ}\mathrm{K}} \]
to zero field. The final temperature thereby obtained is approximately \(0.02^\circ\mathrm{K}\).
Before describing the experiments under consideration, let us briefly review some data on the crystalline structure of Tutton salts. The elementary cell of Tutton salts contains two divalent ions. Crystallographic studies and analysis of experimental data on paramagnetic resonance absorption indicate that each of them is situated in an electric field (caused by neighboring ions) possessing approximate tetragonal symmetry. The angle between the tetragonal axes of the two ions is approximately \(75^\circ\) (for rubidium–cobalt Tutton salt; the angle will apparently be approximately the same also for the salt used in the experiments under consideration).
Thus the sample used by the authors contains two inequivalent sets of Co nuclei with two different axes of quantization of the nuclear spin. The quantization axis of the Co nuclear spin is the tetragonal symmetry axis of the electric field at the given nucleus.
Study of the magnetic properties of Tutton salts shows that one of the principal directions of the magnetic-susceptibility tensor (we shall denote this direction by \(K_1\)) is the bisector of the angle between the two tetragonal axes, while the second principal direction (\(K_2\)) is perpendicular to the plane containing the tetragonal axes. Thus, the radiation intensity along \(K_1\) gives a quantity proportional to \(F(\alpha)\) (where \(\alpha=37.5^\circ\)), while the intensity along \(K_2\) gives a quantity proportional to
\[ F\left(\frac{\pi}{2}\right). \]
Let us now turn to the description of the experiments themselves. A single crystal of the salt weighing \(4\ \mathrm{g}\), containing \(70\) microcuries of Co\({}^{60}\), is placed in a cryostat in which adiabatic demagnetization is carried out. On both sides along the directions \(K_1\) and \(K_2\) one Geiger counter is placed, which measure the intensity of \(\gamma\)-radiation (for each pair of oppositely placed counters an averaging is performed) \(F(\alpha)\) and \(F\left(\frac{\pi}{2}\right)\).
The counts were taken every 10 seconds; the susceptibility of the salt was measured in parallel (to determine the temperature). The temperature of the salt rose continuously as a result of heating by the energy released in \(\beta\)-decay. The sample remained at a sufficiently low temperature only for 5 minutes (after demagnetization).
As a result of the experiments the authors determined the dependence of
\[ \frac{F\left(\frac{\pi}{2}\right)}{F(\alpha)} \]
on \(T\) down to a temperature of about \(0.02^\circ\mathrm{K}\). It turned out that for \(T>0.2^\circ\mathrm{K}\) this quantity is equal to unity (i.e. the radiation is isotropic); with further decrease of the temperature it increases, reaching a value of \(1.45\) at a temperature of about \(0.01\text{--}0.025^\circ\mathrm{K}\).
We note that the γ-rays are emitted not directly by cobalt, but according to the following scheme:
\[ \mathrm{Co}^{60} \xrightarrow{\ \beta\ } \mathrm{Ni}^{60} \xrightarrow{\ \gamma\ } \]
If a change of nuclear spin takes place in the β-transition, this will cause some decrease in the degree of orientation; however, the authors indicate that theoretical consideration shows the smallness of this effect.
Similar experiments, on which we shall not dwell, are described in another paper³.
Of greatest interest would be reactions with oriented nuclei and polarized neutrons. The results of such experiments would provide much valuable information on the spin dependence of nuclear forces, on the angular momenta of compound nuclei, etc. Such experiments have been carried out⁴, but so far with negative results.
G. Kh.
References Cited
- J. A. Spiers, Nature 161, 807 (1948).
- J. M. Daniels, M. A. Grace and F. N. H. Robinson, Nature 168, 780 (1951).
- C. G. Gorter, O. S. Poppema, M. J. Steenland and J. A. Bean, Physica 17, 1050 (1951); C. G. Gorter, H. A. Tolhoek, O. J. Poppema, M. J. Steenland and J. A. Bean, Physica 18, 195 (1952).
- M. H. C. Pryce, Proc. int. conf. low temp., Oxford, 1951, p. 155; C. G. Gorter, ibid., p. 158.