Chapter 6 External Photon Beams: Physical Aspects - IRSN

66.2 QUANTITIES USED IN DESCRIBING PHOTON BEAMS6.2.4 Exposure in air Collision air kerma in air and exposure in air X( )W= colairKairairX e(K col air) air( W / e)is the average energy required to produce an ion pair in dry air.air( W / e) = 33.97 J/C.airSpecial unit of exposure is the roentgen R(K air(1 R = 2.58 10 4 C/kg air)Ccol ) air= 2.58 10 4 33.97 J kg airC X = cGy0.876 R XIAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 6.2.4 Slide 1 (11/170)6.2 QUANTITIES USED IN DESCRIBING PHOTON BEAMS6.2.5 Dose to small mass of medium in airThe concept “dose to small mass of medium in air”D med also referred to as “dose in free space” is basedon measurement of air kerma in air. D med is subject to same limitations as exposure X andcollision air kerma in aircol ) air. Thus it is:(K air• Defined only for photons.• Defined only for photon energies below 3 MeV.IAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 6.2.5 Slide 1 (12/170)