ThePhysicsofRadiologychapter51放射物理英文版教學課件

1、Measurement of Radiation: Instrumentation&TechniquesPart OneBME College Sherman Sheen(辛學剛)The Physics of Radiology第1頁，共20頁。saturation in ion chambers 第2頁，共20頁。Mobilityin inert gases M negative ions M positive ions in air第3頁，共20頁。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頁，共20頁。Q is the charge concentration at time t, and Q0 is the initial concentration. 第5頁，共20頁。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頁，共20頁。Calculation of Efficiency of Ion CollectionPulsed Radiation 第7頁，共20頁。Calculation of Efficiency of Ion CollectionPulsed Radiation Pulsed Radiation第8頁，共20頁。第9頁，共20頁。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頁，共20頁。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頁，共20頁。Calculation of Efficiency of Ion CollectionContinuous Radiation 第12頁，共20頁。Charge lost to recomb.per second in width dx 第13頁，共20頁。第14頁，共20頁。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頁，共20頁。To collect .999 of the ions

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