measurement datum lineal energy measurement datum A measurement of the amount of energy deposited by ionizing radiation per unit length of a medium. . It represents the average energy transferred to the material per unit length along the path of the particle in a specified micrometric ( or smaller) site divided by the mean chord length of the site volume. Lineal energy is typically expressed in units of keV/?m (kilo-electron volts per micrometer) or MeV/cm (mega-electron volts per centimeter)