scalar measurement datum ionization cluster size measurement datum A measure of the spatial extent or size of the ionized region created by the passage of radiation through a medium. It measures the number of ion pairs that are produced within a localized volume or track of the radiation particle. ionisation cluster size measurement datum In the case of ionization cluster size per unit length, the common unit is "ion pairs per micrometer" (ion pairs/µm) or "ion pairs per millimeter" (ion pairs/mm). This unit represents the average number of ion pairs produced within a length of one micrometer or one millimeter along the path of the ionizing particle. For ionization cluster size per unit volume, the unit may be "ion pairs per cubic micrometer" (ion pairs/µm³) or "ion pairs per cubic millimeter" (ion pairs/mm³). This unit represents the average number of ion pairs produced within a volume of one cubic micrometer or one cubic millimeter."