Mean volume reflection angle.

The mean volume reflection angle of a high-energy charged particle passing through a bent crystal is expressed as an integral involving the effective interplanar potential over a single crystal period. Implications for positively and negatively charged particles, and silicon crystal orientations (110) and (111) are discussed. A generic next-to-leading-order expansion in the ratio $E/R$ of the particle energy $E$ to the crystal bending radius $R$ is given. For positively charged particles, the dependence of the mean volume reflection angle on $E/R$ proves to be approximately linear, whereas for negatively charged particles the linear behaviour is modified by an $E/R$-dependent logarithmic factor. Up-to-date experimental data are confronted with predictions based on commonly used atomic potentials.