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1、hePhysicsofRadiology-chapter5-1放射物理英文版教学Measurement of Radiation:Instrumentation&TechniquesPart OneBME College Sherman Sheen(辛辛学刚学刚)The Physics of RadiologyhePhysicsofRadiology-chapter5-1放射物理英文版教学saturation in ion chambers hePhysicsofRadiology-chapter5-1放射物理英文版教学MobilitykVandkVin inert gases M negat
2、ive ions M positive ions)(8.1airforVcmscmkkin airhePhysicsofRadiology-chapter5-1放射物理英文版教学Recombination Coefficient number of recombinations per unit volume in time,t =C+C-t charge lost to recombination per unit volume in time,t =tQQerecombinationcoefficientC+,C-Ions concentrationhePhysicsofRadiology
3、-chapter5-1放射物理英文版教学dtQedQ2tQeQQ00)(1Q is the charge concentration at time t,and Q0 is the initial concentration.hePhysicsofRadiology-chapter5-1放射物理英文版教学Example.In an air ion chamber there are present initially 10 pC of positive ions and an equal number of negative ions in each cm3.Determine the cha
4、rge lost to recombination in the first ms.100.010101060.1106.13311191360scmCCscmtQe3309.9100.0110cmpCcmpCQhePhysicsofRadiology-chapter5-1放射物理英文版教学Calculation of Efficiency of Ion CollectionPulsed Radiation hePhysicsofRadiology-chapter5-1放射物理英文版教学Calculation of Efficiency of Ion CollectionPulsed Radi
5、ation Pulsed RadiationdVkktdxxdx/)()()(2kkVdtrhePhysicsofRadiology-chapter5-1放射物理英文版教学021210306.0QdVphePhysicsofRadiology-chapter5-1放射物理英文版教学Example.Find the fraction of the ions collected in a parallel plate ion chamber exposed to radiation that liberates 10 pC per cm3 per pulse when the plate sepa
6、ration is 1.5 cm and 300 volts is applied to the chamber.What voltage would be required to collect 99.5%of the ions?.893.008.408.41025.230010306.1010;5.1;3001112110iscollectedfractionthepForpCpCQandcmdvoltVhePhysicsofRadiology-chapter5-1放射物理英文版教学yielding a p of about 100 To collect 99.5%of the ions
7、requires p 100 reduce d by a factor of about 2and increase the potential V by a factor of about 6hePhysicsofRadiology-chapter5-1放射物理英文版教学Calculation of Efficiency of Ion CollectionContinuous Radiation AdQIcIAdVkdQ)(VkdQdQc2)(hePhysicsofRadiology-chapter5-1放射物理英文版教学xVkdQQc)()(xdVkdQxQcCharge lost to
8、recomb.per second in width dx 222)()(VkkdxxdxdQeAAdxQQechePhysicsofRadiology-chapter5-1放射物理英文版教学hePhysicsofRadiology-chapter5-1放射物理英文版教学cQdVc2610219.1Example.A parallel plate ion chamber with 1.20 cm gap is placed in a continuous radiation field,which produces 10.0 nC of charge/cm3s.The potential ac
9、ross the chamber is 150 volts.Estimate the fraction of ions collected.What conditions would allow for collection of 99.9%of the ions?hePhysicsofRadiology-chapter5-1放射物理英文版教学cmdvoltVCnCQc20.1;150;100.10827.11044.1115010219.146c.%62.;617.collectedareeif To collect.999 of the ions,Figure shows that c s
10、hould be about 30hePhysicsofRadiology-chapter5-1放射物理英文版教学Cylindrical and Spherical ChambershePhysicsofRadiology-chapter5-1放射物理英文版教学1 roentgen=3.33510-10 C/cm3 for air at NTP Example.A cylindrical air wall chamber of radius 0.5 cm with a central electrode of radius 0.05 cm is used to measure continuo
11、us radiation from a cobalt unit with an exposure rate of 240 R/min with a collection voltage of 150 volts.Find the efficiency of ion collection.hePhysicsofRadiology-chapter5-1放射物理英文版教学997.5.171065.3)535(.15010219.1535.;150;1065.31034.1310335.34/4min/240exp535.0)05.05.0(19.119.11005./5.0/5265310310fccmdvoltVQscmCscmCQsRRrateosurelengthgapequivalentKbaratiocccylhePhysicsofRadiology-chapter5-1放射物理英文版教学Summary Saturation in ion chambersCalculation of efficiency of ion collection-pulsed radiationCalculation of efficiency of ion collection-continuous radiation
hePhysicsofRadiologychapter51放射物理英文版教学课件
