The charge number Z of a nucleus reduced by a factor depending on the relative velocity (velocity of the nucleus relative to the velocity of light) which takes account of the fact that the nucleus is not completely ionised at low velocities.
;
[math] Z^*=Z (1-exp(-125⋅βZ^{2/3})) [/math]
(ICRP Publication 123, 2013)
ICRP Glossary entry - June 2019