ThePhysicsofRadiologychapter51放射物理英文版教学课件

1、Measurement of Radiation: Instrumentation&TechniquesPart OneBME College Sherman Sheen(辛学刚)The Physics of Radiology第1页，共20页。saturation in ion chambers 第2页，共20页。Mobilityin inert gases M negative ions M positive ions in air第3页，共20页。Recombination Coefficient number of recombinations per unit volume in t

2、ime, t = C+ C- t charge lost to recombination per unit volume in time, t = recombinationcoefficientC+ , C-Ions concentration第4页，共20页。Q is the charge concentration at time t, and Q0 is the initial concentration. 第5页，共20页。Example. In an air ion chamber there are present initially 10 pC of positive ion

3、s and an equal number of negative ions in each cm3. Determine the charge lost to recombination in the first ms.第6页，共20页。Calculation of Efficiency of Ion CollectionPulsed Radiation 第7页，共20页。Calculation of Efficiency of Ion CollectionPulsed Radiation Pulsed Radiation第8页，共20页。第9页，共20页。Example. Find the

4、 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% of the ions? 第10页，共20页。yielding a p of about 100 To

5、 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 6第11页，共20页。Calculation of Efficiency of Ion CollectionContinuous Radiation 第12页，共20页。Charge lost to recomb.per second in width dx 第13页，共20页。第14页，共20页。Example. A parallel plate i

6、on chamber with 1.20 cm gap is placed in a continuous radiation field, which produces 10.0 nC of charge/cm3s. The potential across the chamber is 150 volts. Estimate the fraction of ions collected. What conditions would allow for collection of 99.9% of the ions? 第15页，共20页。To collect .999 of the ions

7、, Figure shows that c should be about 30第16页，共20页。Cylindrical and Spherical Chambers第17页，共20页。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 continuous 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.第18页，共20页。第19页，共20页。Summary Saturation in ion chambersCalculation of