The Wu experiment is the first experiment to prove parity violation of the weak interaction. In the experiment, ultra-cold cobalt-60 atoms are aligned in a magnetic field. The cobalt-60 atoms decay to excited nickel-60 atoms by the weak force under the emission of an electron and an electron-antineutrino. The excited nickel-60 atoms emit photons. The angular distribution of the photons is used as a reference for the angular distribution of the emitted electrons.

The Wikipedia article on the Wu experiment claims:

The resulting nickel nucleus, however, is excited and promptly decays to its ground state by emitting two gamma rays (γ)


..., the distribution of the emitted electrons could be compared to the distribution of the emitted gamma rays to compare whether they, too, were being emitted isotropically. In other words, the distribution of the gamma rays acted as a control for the distribution of the emitted electrons.

At the same time, the Wikipedia article on the Gamma ray states that the two photons are emitted sequentially:

Atomic transition of nickel-60

How are these two statements compatible?

The emission of a photon in an atomic transition follows an angular distribution. For the simple hydrogen atom, the angular distribution is the product of the harmonic functions of the initial and final state times the spatial coordinate expressed in spherical coordinates. For the nickel-60 transition, we would expect a more complex radiation pattern because of electron-electron interactions.

In both cases the emission direction is non-deterministic. It is not clear if there is any correlation between the first 1.17 MeV and the second 1.33 MeV gamma ray emission in terms of the angular distribution.

Furthermore, how is angular momentum conserved if the photon has spin one, but according to the Wikipedia article on the Wu experiment the excited state of the nickel-60 atom loses only one unit of total angular momentum during the photon emissions?

Why is it still. valid to use the angular distributions of the two gamma rays for normalization of the electron anisotropy?


1 Answer 1


The angular correlation for 4-2-0 quadrupole-quadrupole transitions is:

$$ W(\theta) = 1 + \frac 1 8 \cos^2{\theta}+ \frac 1 {24} \cos^4{\theta}$$

where $\theta$ is the angle is between the 2 photons' momenta, and $W(\theta)$ is the detection rate at $\theta$ relative to that rate at 90 degrees. (See:D. R. Hamilton, Physical Review 58, 122 (1940).)

In electromagnetic nuclear decay, if the initial state has spin $J_i$ and the final state is $J_f$, then the photon can carry away $l$ units of angular momentum, with $l \ne 0$ and:

$$ |J_i-J_f|\le l \le (J_i+J_f) $$

The radiations is referred to a $2^l$-pole transition, so in this case, both transitions are $l=2$, or quadruple radiation.

  • $\begingroup$ so the gamma-ray emission in the Wikipedia article about gamma rays is a dipole transition whereas the Wu experiment uses a quadrupole transition? I would assume that the dipole transition is over some orders of magnitude more probable than the quadrupole transition. How did Wu prevent dipole photons to dominate the measurements? $\endgroup$
    – bodokaiser
    Commented May 18, 2020 at 15:17

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