Ionizing radiation is measured in the SI (International System of Units, abbreviated “SI” from the French Le Système International d’Unités as the modern metric system of measurement)-derived unit of absorbed dose per mass unit, the gray (Gy), with 1 Gy equal to the net absorption of 1 J in 1 kg of material (water), compared to the previously used radiation unit “rad”, 1 Gy = 100 rad. The biological effectiveness of radiation largely depends on the local energy distribution in the target material, the Linear Energy Transfer (LET). The quality factor Q is the biological weighting function of ionizing radiation in relation to the LET of the radiation. The product of Q and the absorbed dose is the dose equivalent. The SI-derived unit for the dose equivalent is the sievert (Sv).