Ch 6 Measurement of Ionizing Radiation Introduction Chemical

Ch 6 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 (<=2 MV) • 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