Ch6 Measurement of Ionizing Radiation

Introduction Chemical and Biologic effect Skin erythema dose (SED) AD 1928 , ICRU , Roentgen Exposure(X)

Free-Air ionization chamber Correction for (a) air attenuation (b) recombination of ions (c) the effects of temperature, pressure, and humidity (d) scattered photons

Limitations : High energy photons Non-unique electric field and greater ion recombination

Thimble chambers The inner cavity is equal to the maximum range of electrons

Effective of Atomic Number

Chamber Calibration (A) too few electrons are generated in the wall (B) reduced because if increased attenuation of the beam in the wall

Practical Thimble Chambers Condenser Chambers

Stem Effect

Farmer Chamber Condenser chamber--- lower energy beams (<=2MV) The design of stem and excessive stem leakage create dosimetric problems

String Electrometer

Other Electrometer

Special chamber- Extrapolation Chamber

Ion Collection Incomplete ion collection at low voltages

Collection Efficiency Two voltage testing techniques Ion recombination correction

Chamber Polarity effects Compton current Extracameral current Environmental conditions Exposure calibration factor(Nc):NIST Ctp:Temperature and pressure Cs:Recombination Cst:Stem leakage correction