In particular, removal of the restrictions imposed by parity conservation brought order to the theoretical chaos that existed with regard to subatomic particles. The "elementary" particles seen in cosmic rays and particle accelerator experiments were understood to be manifestations of the strong and weak nuclear interactions. The better understanding of their characteristics has led to a more unified theory of the fundamental forces.
The beta-decay experiments were carried out by C. S. Wu of Columbia University and NBS staff members Ernest Ambler, Raymond W. Hayward, Dale D. Hoppes, and Ralph P. Hudson. The Bureau's low temperature laboratory was chosen for the experiments because of its previous experience in low-temperature alignment of atomic nuclei,2 an essential feature of the beta-decay study.survey3 of experimental information on the question of parity. They concluded that the evidence then existing neither supported nor refuted parity conservation in the "weak interactions" responsible for the emission of beta particles, K-meson decay and such. They thus proposed that the K-meson itself may have definite parity, and the observed opposite parity of the two systems of decay products may be the manifestation of parity non-conservation in its decay. They also proposed a number of experiments on beta decays and hyperon and meson decays that would provide the necessary evidence for or against parity conservation in weak interactions. One of the proposed experiments involved measuring the directional intensity of beta radiation from oriented cobalt-60 nuclei. At the suggestion of Professor Wu of Columbia, arrangements were made to carry out this experiment in the Bureau's low temperature laboratory.
The magnetic polarity of the nucleus is determined by its direction of spin, and, under the influence of a magnetic field, most of the cobalt-60 nuclei align themselves so that their spin axes are parallel to the field.
As cobalt-60 is radioactive, its nuclei continuously emit beta and gamma rays. If parity is conserved in such interactions, then the intensity of the beta emission should be the same in either direction along the axis of spin. This, of course, was the critical question in the cobalt-60 experiments. It was resolved by measuring the intensity of beta emission in both these directions, i.e., along and against the field direction.
The cobalt-60 was located in a thin (50 µm) surface layer of a single crystal of cerium magnesium nitrate. The crystal was placed in an evacuated flask which in turn was immersed in liquid helium within a Dewar flask surrounded by liquid nitrogen. An inductance coil on the surface of the inner flask was used to measure the temperature of the crystal in terms of its magnetic susceptibility.
A major experimental problem was the location of a radiation counter within the evacuated flask for detection of beta particles. This problem was solved by placing a thin anthracene crystal inside the chamber to serve as a scintillation counter. The anthracene crystal was located about 2 cm above the cobalt-60 source. Scintillations caused by beta particles striking the crystal were transmitted through a glass window and a 120 cm lucite tube acting as a light pipe to a photomultiplier at the top of the flask. The resulting pulses were counted on a 10-channel pulse-height analyzer.
Cooling to the low temperature necessary for nuclear alignment was accomplished by the process of adiabatic demagnetization using a magnetic field of about 2.3 tesla (23,000 gauss). This process involved successive magnetization and demagnetization of the paramagnetic salt, cerium magnesium nitrate, which supported the cobalt-60 specimen. The heat produced by magnetization was removed by the boiling off of liquid helium in the surrounding Dewar. The specimen was then thermally isolated and upon demagnetization the temperature fell to about 0.003 K.
A vertical solenoid was then raised around the lower end of the outer Dewar to provide a magnetic field for polarization of the cobalt-60 nuclei. After the beta emission had been measured for this condition, the direction of the magnetic field was reversed, and the beta emission again measured for the nuclei now polarized in the opposite direction. It was found that the emission of beta particles is greater in the direction opposite to that of the nuclear spin. Thus, a spinning cobalt-60 nucleus has a beta emission distribution that is not the same as that of its mirror image. This result unequivocally demonstrated that parity is not conserved in the emission of beta particles by cobalt-60.
The first entries below the date were made by Ambler. The first of two successful runs began at 12:04 (middle of page). Hudson's notation "Field on" (directly under "Demag II") refers to the magnetic field produced by the solenoid. An initially high counting rate of β particles (emitted by the cobalt-60 nuclei as polarized by this field) was observed to decrease to the value for randomly oriented nuclei as the polarization decreased because of the gradual warming of the cobalt-60 nuclei ("β counts decrease," boxed for emphasis).
After again cooling the crystal and then polarizing the cobalt-60 nuclei in the opposite direction (by reversing the direction of the solenoid current and thus the magnetic-field direction), the NBS physicists observed the opposite behavior of the β-particle counts with time -- the counts gradually increased from an initially low value to the value for randomly oriented nuclei. These two results were what they had been looking for. Hudson later added "PARITY NOT CONSERVED!" at the top of the page.
A second experiment was then performed using cobalt-58, which is a positron emitter. In this case the opposite effect was observed, namely that β+ particles are preferentially emitted along the direction of the nuclear spins. This provided additional confirmation of the theory and constituted a convincing demonstration of the basic validity of the experiments.
The further developments of the theory, together with a large number of follow-up experiments, has led to the unification of the weak and electromagnetic interactions. A description of both the history and the physics is available in the Nobel lectures by Weinberg, Salam, and Glashow.4
- An experimental test of parity conservation in beta decay, by C. S. Wu, E. Ambler, R. W. Hayward, D. D. Hoppes, and R. P. Hudson, Phys. Rev. 105, 1413 (1957).
- Low-temperature alignment of radioactive nuclei, NBS Tech. News Bul. 40, 49 (April 1956); see also E. Ambler, R. P. Hudson, and G. M. Temmer, Phys. Rev. 97, 1212 (1955) and 101, 1096 (1956).
- Question of parity conservation in weak interactions, by T. D. Lee and C. N. Yang, Phys. Rev. 104, 254 (1956).
- S. Weinberg, Rev. Mod. Phys. 52, 515 (1980); A. Salam, p. 525; and S. L. Glashow, p. 539.