《hePhysicsofRadiology-chapter4-2放射物理英文版教学课件》由会员分享，可在线阅读，更多相关《hePhysicsofRadiology-chapter4-2放射物理英文版教学课件（17页珍藏版）》请在金锄头文库上搜索。
1、Measurement of Radiation: DosimetryPart TwoBME College Sherman Sheen(辛辛学学刚)The Physics of RadiologyhePhysicsofRadiology-chapter4-2放射物理英文版教学Determination of Absorbed Dose Using an Absolute Ion Chamber hePhysicsofRadiology-chapter4-2放射物理英文版教学hePhysicsofRadiology-chapter4-2放射物理英文版教学1、the air cavity (ra
2、dius a) is small enough to produce negligible perturbation of the radiation field、the shell of wall material has essentially the same atomic number and density as the mediumhePhysicsofRadiology-chapter4-2放射物理英文版教学Example. Suppose a carbon ion chamber with a 1 cm3 air cavity is placed in a water phan
3、tom and exposed to Co-60 gamma rays, which produce a charge of 3 10-8 C. Find the dose to the water. Assume the carbon walls are slightly thicker thanthe range of electrons. hePhysicsofRadiology-chapter4-2放射物理英文版教学Dose to water = 0.792 Gy 1.111 = 0.890 Gy hePhysicsofRadiology-chapter4-2放射物理英文版教学CORR
4、ECTION FACTORS FOR THE FINITE SIZE OF THE ION CHAMBER hePhysicsofRadiology-chapter4-2放射物理英文版教学Effects of Temperature and Pressure on Ionization Measurements hePhysicsofRadiology-chapter4-2放射物理英文版教学Exposure-The Roentgen 1 R 2.5810-4 C/kg of air Or 1 C/kg = 3876 R Since air at STP has a density of 0.0
5、01293 g/cm3, 1 kg of air has a volume of 103/.0012937.734 105 cm3 and 1 roentgen is equivalent to:to produce 1 coulomb of charge by the ionization of air requires an energy absorption of 33.85 joules hePhysicsofRadiology-chapter4-2放射物理英文版教学Standard Air Chamber hePhysicsofRadiology-chapter4-2放射物理英文版教
6、学hePhysicsofRadiology-chapter4-2放射物理英文版教学Example. In a standard air chamber the limiting diaphragm has an area of 0.500 cm2 and the length of the sensitive electrode is 8.00 cm. In an irradiation a charge of 1.12 10-7 coulombs is collected. Air is at STP (density 1.293 kg m-3 ). Determine the exposu
7、re X at point P. Exposure in R .02166 3876 R83.9 R hePhysicsofRadiology-chapter4-2放射物理英文版教学Practical Ion ChambersThe Thimble Chamber hePhysicsofRadiology-chapter4-2放射物理英文版教学Determination of Absorbed Dose in “Free Space” hePhysicsofRadiology-chapter4-2放射物理英文版教学X: the exposure at point P the ratio of
8、averaged mass energy absorption coefficients for the material of m to air the attenuation factor the absorbed dose at point P hePhysicsofRadiology-chapter4-2放射物理英文版教学the absorbed dose in free space at a point in air: hePhysicsofRadiology-chapter4-2放射物理英文版教学Summary Determination of Absorbed Dose Using an Absolute Ion ChamberEffects of Temperature and Pressure on Ionization MeasurementsExposure-The RoentgenStandard Air ChamberPractical Ion ChambersThe Thimble ChamberDetermination of Absorbed Dose in “Free Space”hePhysicsofRadiology-chapter4-2放射物理英文版教学
