# How the neutron magnetic moment was measured?

1. How was the neutron magnetic moment measured?

2. Was the antineutron magnetic moment measured too?

• One way is by doing a Stern-Gerlach type experiment on neutrons, but far more precise methods employ nuclear magnetic spin resonance, see e.g. journals.aps.org/prd/abstract/10.1103/PhysRevD.20.2139 – CuriousOne Oct 12 '14 at 20:44
• Indeed that is the classic paper, 35 years old in a few weeks... – Jon Custer Oct 13 '14 at 15:58

The best measurement of the neutron magnetic moment was reported in 1979 by Greene and collaborators. That measurement used nuclear magnetic resonance (NMR) to measure the rate at which the spins of polarized neutrons precess around a magnetic field. The magnetic field was measured by flowing water through the same volume as the neutrons and also performing NMR on the protons in the water, so the experiment gave a high-quality (0.25 parts per million uncertainty) measurement for the ratio $μ_\mathrm{n}/μ_\mathrm{p}$ of the magnetic moments of the neutron and proton. That measured ratio is now one of the many inputs to the biannually-updated CODATA fit of all the known measurements of fundamental constants.
I don't believe there has ever been a direct measurement of the antineutron magnetic moment. It's just too hard to make antineutrons. For example, the Greene et al. measurement took place on a cold neutron beamline with a rate of about $10^5\,\mathrm{n/s}$, which was extracted from a reactor with a (hot) neutron flux of about $10^{14}\,\mathrm{n\,cm^{-2}\,s^{-1}}$. This sort of fabulous inefficiency is pretty typical for cold neutron sources. The very first measurement of the neutron magnetic moment (Alvarez and Bloch, 1940) was based on NMR measurements on a population of a few million (hot) neutrons, which seems like a reasonable floor for the number that you'd need. By contrast, the best current measurement of the neutron mass (Cresti et al, 1986) is based on 59 antineutron events from a sample of a few thousand candidates. So the antineutron population in a top-notch antineutron mass experiment thousands of times smaller than the neutron population in a minimal neutron magnetic moment experiment. Furthermore, if you tried to produce an antineutron beam for doing NMR, that beam would contain at least half neutrons (since every $\bar n$ is created with an $n$, and they cannot be separated by their charge).
• No, the neutron is absolutely not its own antiparticle (read the Cresti paper or its references to see what antineutrons do to neutrons). What I meant is that you'll always make $n,\bar n$ pairs, and you can't steer them in different directions because it's hard to steer neutrons. – rob Nov 8 '14 at 6:35