ThePhysicsofRadiology-chapter5-1 放射物理 英文版 教学课件.pptVIP

Measurement of Radiation: InstrumentationTechniques Part One Summary * * BME College Sherman Sheen(辛学刚) The Physics of Radiology saturation in ion chambers Mobility in inert gases M negative ions M positive ions in air Recombination Coefficient number of recombinations per unit volume in time, t = C+ C- t charge lost to recombination per unit volume in time, t = recombinationcoefficient C+ , C- Ions concentration Q 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 of negative ions in each cm3. Determine the charge lost to recombination in the first ms. Calculation of Efficiency of Ion Collection —Pulsed Radiation Calculation of Efficiency of Ion Collection —Pulsed Radiation Pulsed Radiation 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 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? yielding a p of about 100 To collect 99.5% of the ions requires p ≈ 100 reduce d by a factor of about 2 and increase the potential V by a factor of about 6 Calculation of Efficiency of Ion Collection —Continuous Radiation Charge lost to recomb. per second in width dx Example. 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 across the chamber is 150 volts. Estimate the fraction of ions collected. What conditions would allow for collection of 99.9% of the ions? To collect .999 of the ions, Figure shows that c should be about 30 Cylindrical and Spherical Chambers 1 roentgen = 3.335×10-10 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 rad