Chap 6 Measurement of Ionizing Radiation By Dr

Chap 6. Measurement of Ionizing Radiation By: Dr. Ali Jomehzadeh Assistant Professor in Medical Physics 1

Chap 6. Measurement of Ionizing Radiation z The roentgen z Free-air ionization chamber z Thimble chambers z Practical thimble chambers z Electrometers z Special chambers z Ion collection z Chamber polarity effects z Environmental conditions z Measurement of exposure 2

The roentgen z Unit of exposure z X = d Q/d m z d Q is the absolute value of the total charge of the ions of one sign produced in air when all the electrons liberated by photons in air of mass dm are completely stopped in air z 1 R = 2. 58 x 10 -4 C/kg air z electronic equilibrium ( figure 6. 1) 3

Free-Air Chamber 4

Electron Equilibrium 5

Free-Air Chamber 6

Free-air ionization chamber z Standard chamber z used only for the calibration of secondary instruments z. Xp = z XD = Q . Ap. L X X 1 2. 58 x 10 -4 roentgens 7

Thimble chambers(1)-chamber wall z Effective atomic number is the same as that of air; air equivalent z Wall materials: graphite, Bakelite; the atomic number is closer to that of carbon (Z=6); inside layer: conducting layer of graphite or of a conducting mixture of Bakelite and graphite 8

Thimble Ionization Chamber 9

Thimble chambers(2)-effective atomic number z Z = ( a 1 Z 1 2. 94 + a 2 Z 2 2. 94 + a 3 Z 3 2. 94 + …. . . + an. Zn 2. 94 ) 1/2. 94 z a 1, a 2, a 3, …an: fractional contributions of each element to the total number of electrons in the mixture z Z air = 7. 67 10

Thimble chambers(3)-chamber calibration z. X= Q . x 1 A z A = the fraction of the energy fluence transmitted through the air-equivalent wall of equilibrium thickness z The factor A is slightly less than 1. 00 11

Wall Thickness VS. Chamber Response 12

Thimble chambers(4)-Desirable chamber characteristics z Minimal variation in sensitivity over a wide range of photon energies z The sensitivity ( charge measured per roentgen ) is directly proportional to the chamber sensitive volume z Minimal variation in sensitivity with the direction of incident radiation z Minimal stem leakage z Minimal ion recombination losses 13

Farmer Chamber 14

Practical thimble chambers(1)-Condenser chambers: (a) chamber sensitivity z Q = X. air. z V= Q x C 3. 33 x 10 -4 X. C z C = total capacitance of the chamber; V = the voltage drop V X = 3. 33 x 10 -4 C Chamber sensitivity( voltage drop per roentgen) 15

Practical thimble chambers(1)-Condenser chambers: (b) stem effect z Measurable ionization in the body of the stem z Ionization of the air between the end of the chamber and the metal cap z Measuring exposure rate in air for relatively lower-energy beams (<= 2 Me. V ) z Stem correction 16

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Geometry Correction 18

Ion Chamber & Electrometer 19

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Electrometers(b)-other electrometers z Operational amplifiers: chamber remains connected to the electrometer during exposure 21

Special chambers z The detector must be very thin so that there is no dose gradient across its sensitive volume z The chamber must not significantly perturb the radiation field z (A) extrapolation chamber : measuring surface dose in an irradiated phantom z (B) parallel-plate chambers : minimizes cavity perturbations in the radiation field; important in the dosimetry of electron beams 22

Extrapolation Ion Chamber 23

Ion collection: (a) Saturation z The chamber should be used in the saturation region so that small changes in the voltage do not result in changes in the ionic current 24

Ion collection: (b) Collection efficiency Efficiency = The number of ions collected The number of ions produced (1) Two-voltage testing technique: one given working voltage and the other much lower voltage--theoretical formula by Boag and Currant (2) Determining ion recombination correction factor (P ion)--measure ionization at V 1 and V 2; V 1=2 V 2 ; the ratio of the two readings is related to P ion 25

Recombination Corre. Factors 26

Chamber polarity effects z Ionic charge collected changes in magnitude as the polarity of the collecting voltage is reversed z Polarity effects relatively more severe for measurements in electron beams than photon beams; the effect increases with decreasing electron energy z Should be less than 0. 5 % for any radiation beam quality 27

Environmental conditions z Temperature or Pressure the density or the mass of air z The chamber reading for a given exposure will increase as the temperature decreases or as the pressure increases z The correction C T, P = 760 P x 273 + t 295 28

Measurement of exposure ( Last one !! ) z X = M. N C. C T, P. Cs. C st z M = reading data z N C = the correction for any perturbation produced in the beam by the chamber z Cs = correction for loss of ionization as a result of recombination z C st = stem leakage correction 29