ThePhysicsofRadiologychapter42放射物理英文版教學(xué)課件

1、Measurement of Radiation: DosimetryPart TwoBME College Sherman Sheen(辛學(xué)剛)The Physics of Radiology第1頁，共17頁。Determination of Absorbed Dose Using an Absolute Ion Chamber 第2頁，共17頁。第3頁，共17頁。1、the air cavity (radius a) is small enough to produce negligible perturbation of the radiation field、the shell of

2、wall material has essentially the same atomic number and density as the medium第4頁，共17頁。Example. Suppose a carbon ion chamber with a 1 cm3 air cavity is placed in a water phantom 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 a

3、re slightly thicker thanthe range of electrons. 第5頁，共17頁。Dose to water = 0.792 Gy 1.111 = 0.890 Gy 第6頁，共17頁。CORRECTION FACTORS FOR THE FINITE SIZE OF THE ION CHAMBER 第7頁，共17頁。Effects of Temperature and Pressure on Ionization Measurements 第8頁，共17頁。Exposure-The Roentgen 1 R 2.5810-4 C/kg of air Or 1 C

4、/kg = 3876 R Since air at STP has a density of 0.001293 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 第9頁，共17頁。Standard Air Chamber 第10頁，共17頁。第11頁，共17頁。

5、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 exposure X at point P. Exposure in R .02166 387

6、6 R83.9 R 第12頁，共17頁。Practical Ion ChambersThe Thimble Chamber 第13頁，共17頁。Determination of Absorbed Dose in “Free Space” 第14頁，共17頁。X: the exposure at point P the ratio of averaged mass energy absorption coefficients for the material of m to air the attenuation factor the absorbed dose at point P 第15頁，共17頁。the absorbed dose in free space at a point in air: 第16頁，共17頁。Summary Determination of Absorbed Dose Using an Absolute Ion ChamberEffects of Temperature and Pressure on Ionization MeasurementsExposure-The RoentgenStandard Ai