Paper 3.5: "First Measurement of Parity-Nonconserving Neutron-Spin Rotation: The Tin Isotopes," M. Forte, B. R. Heckel, N. F. Ramsey, K. Green, G. L. Greene, J. Byrne and J. M. Pendlebury, Phys. Rev. Lett. 45, 2088–2092 (1980)
Reprinted with permission from Physical Review Letters. Copyright 1980, The American Physical Society.
1964, F. C. Michel [Phys. Rev. 133, B329–B332 (1964)] published a thought-provoking paper on the possibility of there being a small parity-nonconserving spin rotation of a neutron passing through matter, but it appeared in the then new Physical Review B and was not seen by physicists who might have been interested in doing the experiment. In 1974 L. Stodolsky independently had the same idea at CERN, but experimentalists there considered the rotation angle to be much too small to be observed. Stodolsky told me of this work and I was immediately interested, for two reasons: my long term interest in parity non-conservation tests and my realization that our neutron electric dipole moment experiments were extremely sensitive tests of a small rotation of the neutron spin. I did a simple calculation and concluded that we could adapt our neutron electric dipole methods to measuring the proposed parity-nonconserving spin rotation. When M. Forte learned of our experiment, he urged that we first study 124Sn, since he had a theory according to which the spin rotation with 124Sn should be about ten times greater than with most materials. We, therefore, asked him to join our group and we constructed the apparatus described in Paper 3.5.
In our first run we measured the neutron spin rotation in 124Sn and used a measurement of isotopically unseparated Sn as a control. To our surprise, we found no spin rotation with 124Sn, but we did find a significant rotation with our control of normal Sn. This clearly indicated that the effect existed, but in a different Sn isotope. We later obtained a sample of separated 117Sn and found with this isotope a neutron spin rotation of (+36.7 ± 2.7) × 10-6 radians per cm of sample traversed. In later publications [Phys. Lett. 119B, 298–300 (1982) and Phys. Rev. C29, 2489–2492 (1984)], we reported values for the parity-nonconserving spin rotations of neutrons passing through isotopes of lead where the sign of the rotation was negative, and of lanthanum where the rotation was much larger. Currently (1997), Heckel and his associates are preparing experiments to measure the spin rotation of neutrons passing through liquid H2 and He from which they should be able to derive the neutron–neutron parity-nonconserving interaction.
