Basic properties of the neutron

The neutron is a nuclear particle with a mass, \(m_{\rm n}\), rather close to that of the proton

\begin{equation}\label{dummy782923085} m_{\rm n} = 1.67493 \cdot 10^{-27} \, {\rm \,kg} . \end{equation}

The neutron does not exist naturally in free form, but decays into a proton, an electron, and an anti-neutrino. The neutron lifetime, \(\tau = 886 {\rm \,s}\), is much longer than the time a neutron spends within a scattering experiment, which is merely a fraction of a second. Hence, neutron decay can typically be neglected in experiments.

The neutron is electrically neutral but still possesses a magnetic moment

\begin{equation}\label{eq:intro_moment} \mu = \gamma \mu_{\rm N} ,\, \end{equation}

where \(\gamma= - 1.91304\) is the neutron magnetogyric ratio and the nuclear magneton is given by \(\mu_{\rm N} = e \hbar / (2 m_{\rm p}) = 5.05078 \cdot 10^{-27} \,{\rm J}/{\rm T}\). The neutron magnetic moment is coupled antiparallel to its spin, which has the value \(s=1/2\).

The neutron interacts with nuclei via the strong nuclear force and with magnetic moments via the electromagnetic force. Most of this text deals with the consequences of these interactions; i.e. the scattering and absorption of neutrons by atoms and nuclei inside materials, as well as reflection from surfaces and interfaces.