ThePhysicsofRadiologychapter42放射物理英文版教学课件

1、Measurement of Radiation: DosimetryPart TwoBME College Sherman Sheen(辛学刚)The Physics of Radiology第1页，共17页。Determination of Absorbed Dose Using an Absolute Ion Chamber 第2页，共17页。第3页，共17页。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页，共17页。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页，共17页。Dose to water = 0.792 Gy 1.111 = 0.890 Gy 第6页，共17页。CORRECTION FACTORS FOR THE FINITE SIZE OF THE ION CHAMBER 第7页，共17页。Effects of Temperature and Pressure on Ionization Measurements 第8页，共17页。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页，共17页。Standard Air Chamber 第10页，共17页。第11页，共17页。

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页，共17页。Practical Ion ChambersThe Thimble Chamber 第13页，共17页。Determination of Absorbed Dose in “Free Space” 第14页，共17页。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页，共17页。the absorbed dose in free space at a point in air: 第16页，共17页。Summary Determination of Absorbed Dose Using an Absolute Ion ChamberEffects of Temperature and Pressure on Ionization MeasurementsExposure-The RoentgenStandard Ai