ThePhysicsofRadiologychapter51放射物理英文版

1、Measurement of Radiation: Instrumentation&TechniquesPart OneThe Physics of Radiologysaturation in ion chambers MobilitykVandkVin inert gases M negative ions M positive ions )(8 . 1airforVcmscmkkin airRecombination Coefficient number of recombinations per unit volume in time, t = C+ C- t charge l

2、ost to recombination per unit volume in time, t = tQQerecombinationcoefficientC+ , C-Ions concentrationdtQedQ2tQeQQ00)(1Q is the charge concentration at time t, and Q0 is the initial concentration. Example. In an air ion chamber there are present initially 10 pC of positive ions and an equal number

3、of negative ions in each cm3. Determine the charge lost to recombination in the first ms.100. 010101060. 1106 .cmCCscmtQe3309. 9100. 0110cmpCcmpCQCalculation of Efficiency of Ion CollectionPulsed Radiation Calculation of Efficiency of Ion CollectionPulsed Radiation Pulsed RadiationdVkkt

4、dxxdx/)()()(2kkVdtr021210306. 0QdVpExample. 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 separation is 1.5 cm and 300 volts is applied to the chamber. What voltage would be required to collect 99.5%

5、 of the ions? .893. 008. 408. 41025. 230010306.1010;5 . 1;3001112110iscollectedfractionthepForpCpCQandcmdvoltVyielding a p of about 100 To collect 99.5% of the ions requires p 100 reduce d by a factor of about 2and increase the potential V by a factor of about 6Calculation of Efficiency of Ion Colle

6、ctionContinuous Radiation AdQIcIAdVkdQ)(VkdQdQc2)(xVkdQQc)()(xdVkdQxQcCharge lost to recomb.per second in width dx 222)()(VkkdxxdxdQeAAdxQQeccQdVc2610219. 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 pot

7、ential across the chamber is 150 volts. Estimate the fraction of ions collected. What conditions would allow for collection of 99.9% of the ions? cmdvoltVCnCQc20. 1;150;100 .10827. 11044. 1115010219. 146c.%62.;617.collectedareeif To collect .999 of the ions, Figure shows that c should be about 30Cyl

8、indrical and Spherical Chambers1 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 continuous radiation from a cobalt unit with an exposure rate of 240 R/min with a collection voltage of 1

9、50 volts. Find the efficiency of ion collection.997.5 .171065. 3)535(.15010219. 1535.;150;1065. 31034.1310335. 34/4min/240exp535. 0)05. 05 . 0(19. 119. 11005. /5 . 0/5265310310fccmdvoltVQscmCscmCQsRRrateosurelengthgapequivalentKbaratiocccylSummary Saturation in ion chambersCalculation of efficiency of ion collection-pulsed radiationCalculation of efficiency of ion collection-continuous radiationCalculation of Efficiency of