10 chap 06 measurement of ionizing radiation

3,226 views

Published on

Published in: Technology, Health & Medicine
0 Comments
7 Likes
Statistics
Notes
  • Be the first to comment

No Downloads
Views
Total views
3,226
On SlideShare
0
From Embeds
0
Number of Embeds
9
Actions
Shares
0
Downloads
265
Comments
0
Likes
7
Embeds 0
No embeds

No notes for slide

10 chap 06 measurement of ionizing radiation

  1. 1. Chapter 6 Measurement of Ionizing Radiation6.1 IntroductionIn the early days of x-ray usage, skin erythema was used as ameasure of radiation effects.In the orthovoltage era, the skin was the limiting organ to thedelivered dose. This is no longer true after megavoltagemachines were introduced with its skin-sparring properties.In 1928, the roentgen (denoted by R) was adopted as the unitof measuring x and γ radiation exposure. 1
  2. 2. 6.2 The Roentgen The roentgen is a unit of exposure. dQ The exposure is defined as: X = , dm where dQ is the amount of charges (either + or −, but not both) produced in air when all the electrons liberated by photons in air of mass dm are completely stopped in air. 1R = 2.58 × 10 −4 C / kg airElectron equilibrium: + + + +in a volume,ionization loss = ionization gain + _ _ _ + source e- + e- e- _ _ + + _or, + + _ _electrons entering the volume +_= electrons leaving the - - - - + 2volume
  3. 3. 6.3 Free-Air Ionization ChamberUsed in primary national labs (NIST) to measure exposure according toits definition. collecting volume Lead-lined box + + + + diaphragm Ap + _ _ _ + source e- + D P L - - - - collecting guard To electrometer electrode electrode ∆Q ∆Q Xp = 1 ρ • A p • L 2.58×10 −4 (R) XD = 1 ρ • AD • L 2.58×10 −4 (R) 3
  4. 4. 6.4 Thimble Chamber Solid Air shell air shellAir cavity Air cavity Central electrode Thimble wall (air-equivalent) (graphite or aluminium) Air cavity insulator Most of the ionization produced in the cavity air arises from electrons liberated in the wall. 4
  5. 5. 6.4 Thimble Chamber (effective atomic number)The effective atomic number of the wall material and thecentral electrode should be such that the system as a wholebehaves like a free-air chamber. Most commonly used wallmaterial is graphite and electrode is either graphite oraluminum.The effective atomic number Z of a mixture is the atomicnumber of an element with which photons interact the sameway.Z = aZ( 1 1 2.94 + a2 Z 2.94 2 +  + an Z n ) 2.94 1 / 2.94Where a1, a2, …, an are the fractional contribution of eachelement to the total number of electrons in the mixture. 5
  6. 6. 6.4 Thimble Chamber (effective atomic number)Example: calculation of Z for air:Composition by weight - N: 75.5%, O:23.2%, Ar:1.3% N AZNumber of electrons/g of air: × (fraction by weight) Aw 6.02 ×10 23 × 7N= × 0.755 = 2.27 ×10 23 a1 = 2.27/3.01 = 0.754 14.007 6.02 ×10 23 × 8 a2 = 0.70/3.01 = 0.233O= × 0.232 = 0.7 ×10 23 15.999 6.02 ×10 23 ×18Ar = × 0.013 = 0.04 ×10 23 a3 = 0.04/3.01 = 0.013 39.94Number of electrons/g of air: 3.01×1023Z air = ( 0.754 × 7 2.94 + 0.233 × 8 2.94 + 0.013 × 18 ) 2.94 1 / 2.94 = 7.67 6
  7. 7. 6.4 Thimble Chamber (chamber calibration)A thimble chamber can be used directly to measureexposure if (a) it were air equivalent, (b) its cavity volumewere accurately known, and (c) its wall thickness wassufficient to provide electronic equilibrium. Q 1The exposure X is given by: X = • ρ •v AQ is the ionization charge chamber responseliberated in the cavity air ofdensity ρ and volume v;A is a correction factoraccounting for theattenuation in the wall. wall thickness 7
  8. 8. 6.4 Thimble Chamber (chamber calibration)In practice, however, a chamber is calibrated against thefree-air chamber for x-rays up to a few hundred kilovolts,and against a standard cavity chamber for higherenergies. This is because:1. The chamber is not exactly air-equivalent.2. The cavity volume is not precisely known.The chamber calibration factor provided by the standardlaboratory already includes the wall correction factorand other perturbation factors. 8
  9. 9. 6.4 Thimble Chamber (desirable chamber characteristics)1. Minimal variation in sensitivity or exposure calibration factor with photon energies.2. Suitable volume to allow expected range of exposures.3. Minimal variation in sensitivity with the direction of incident radiation.4. Minimal ‘stem’ leakage.5. Calibrated for exposure against a standard instrument for all radiation qualities of interest.6. Minimal ion recombination loss (sufficient voltage, typically 300 V). 9
  10. 10. 6.5 Practical Thimble Chambers (condenser) Metal shield air Air equivalent – – – – – – – – – – – wall ––––– ++++++++++ +++++ ++++++++++ +++++insulator ––––– – – – – – – – – – – – The chamber is initially fully charged. When it is exposed to radiation, ions are produced in the air cavity and collected, leading to a reduction of charge on the electrodes. The reduction in charge is proportional to the exposure. 10
  11. 11. 6.5 Practical Thimble Chambers (condenser)Chamber sensitivity is defined as voltage drop perroentgen, it is proportional to the air cavity volume v, andinversely proportional to the capacitance C.Charge Q produced in chambervolume v due to exposure X:Voltage dropoff V over chambercapacitance C: V = Q/CChamber sensitivity: V / X = ρ air • v C 11
  12. 12. 6.6 Electrometers (string electrometer)The string electrometer is used together with the condenser type chamber.First, the chamber is fully charged in the electrometer, the shadow of thestring is at the fully charged position (zero position).Second, the chamber is removed from the electrometer and exposed toradiation. Ions collected on the electrode reduces the charge.Third, the chamber is placed back in the electrometer, the position of thestring now moves away from the zero position, depending on the reduction incharge on the electrode. conductive fiber 12
  13. 13. 6.5 Practical Thimble Chambers (condenser)Stem leakage caused by ionization produced in the stem,rather than in the air cavity. Its effect depends on the fieldsize and can be corrected. source determination of stem effectcollimator radiation beam radiation protective chamber cap beam point of measurement 13
  14. 14. 6.5 Practical Thimble Chambers (Farmer) graphite airinsulator electrode (Al) Commercially available, provides better (flatter) energy response characteristics. Air cavity volume is typically 0.6 cm3 14
  15. 15. 6.6 Electrometers (other electrometer)The electrometer is connected to the chamber during the entiretime of exposure and readout. (Unlike condenser chamber/stringelectrometer, in which the chamber is attached to theelectrometer during reading, but detached during exposure.)It can be used to measure the charge (integration mode, nC),current (rate mode, nA), or direct exposure-reading (directreading dosimeter mode). 15
  16. 16. 6.7 Special Chambers Build-up Thin slab Air cavity windowParallel-plate chambers:fixed spacing (typically2mm) between 2 parallel-plate electrodes. Suitablefor shallow depths and Collecting To electrometerelectron beams. electrodeExtrapolation chambers: similar to parallel-plate chambers,except that the electrode spacing is variable. By varying thespacing, one can estimate the surface dose by extrapolatingto zero electrode spacing. 16
  17. 17. 6.8 Ion Collection (saturation) Ion current Saturation region Incomplete collection due to ion recombination in the chamber Chamber voltage 17
  18. 18. 6.8 Ion Collection (collection efficiency) number of ions collected collection efficiency = number of ions producedCollection efficiency can be measuredusing the ‘two voltage’ method in which scanningone voltage is set at the operating voltage pulsed(typically V1=300 V), and the second radiationvoltage half of its value (e.g. V2=150 V). pulsed Pion at V1The recombination correction factor, Pion, radiationcan be obtained from the ratio of the continuouscharges (Q1/Q2) collected at the 2 radiationvoltages.The voltage on the chamber should bechosen so that the collection efficiency Q1/Q2(1/Pion) is at least 99%. 18
  19. 19. 6.9 Chamber Polarity EffectsPolarity effect: the charge collected by an ion chamberchanges in magnitude when the polarity of the collectingvoltage is reversed.This effect can be minimized by taking the average of the 2readings with reversed polarity.The polarity effect is more severe for electron beams thanfor photon beams. 19
  20. 20. 6.10 Environmental ConditionsChamber calibration factor is stated under standardconditions (P=760 mm Hg, T=22°C). In normal use,chamber is unsealed and communicated to the outsideatmosphere. The density (and the mass) of air in the aircavity is affected by the temperature and pressure, andtherefore need to be corrected.  760   273 + T CT , P = ×   P   295 P is the pressure in mm of Hg, and T is the temperature indegrees of Celsius (adding 273 converts Celsius to absolutetemperature in degrees of Kelvin). 20
  21. 21. Example: A chamber was calibrated under the standardconditions: T=22°C, P=760 mm Hg. When the chamber isused under the room conditions: T=25°C, P=750 mm Hg,and reads a charge of M=3×10-8 Coulomb, what is thecorrected reading Mcorr? 273 + 25 760 M corr =M× × 273 + 22 750 −8 298 760 = 3 ×10 × × 295 750 −8 = 3.07 ×10 Coulomb 21
  22. 22. 6.11 Measurement of Exposure X = M • CT,P • Cs • Cst • NC X is the exposure in Roentgen. M is the chamber reading. Nc is the chamber exposure calibration factor traceable to NIST. CT,P is the temperature, pressure correction factor. Cs is the correction factor for recombination loss. Cst is correction factor for stem leakage. 22

×