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RADIATION QUANTITIES & UNITS

RADIATION QUANTITIES & UNITS. RADIATION QUANTITIES & UNITS. Activity Absorbed Dose Equivalent Dose Effective Dose Collective Effective Dose Exposure. RADIATION QUANTITIES & UNITS. Activity

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RADIATION QUANTITIES & UNITS

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  1. RADIATION QUANTITIES & UNITS

  2. RADIATION QUANTITIES & UNITS • Activity • Absorbed Dose • Equivalent Dose • Effective Dose • Collective Effective Dose • Exposure

  3. RADIATION QUANTITIES & UNITS

  4. Activity • The rate of decay, or the number of nuclei of a radionuclide decaying per second is known as its activity. • A= dN / dt • Units of Activity: • The SI unit of activity is Becquerel (Bq) • 1 Bq = 1 dps(disintegration per second) • The old classical unit is Curie(Ci). 1 Ci is the activity of 1 g of Ra equilibrium with its daughters • 1 Ci = 3.7  1010 Bq = 3.7  1010 dps

  5. Activity: • What is the activity in both Bq and Ci for a source emits 7.4 ×109 photons in 3 minutes. • A= dN / dt • = 7.4 ×109/(3×60) • = 41.1 GBq • = 41.1×106/3.7×1010 • = 1.11 mCi

  6. Half-Life and the Mean Life • Half-life T1/2 and λ are related by T1/2 = 0.693/λ • Average or mean life Ta = 1/λ = 1.44 T1/2

  7. After n half lives the activity will be A = A0(1/2)n = A0 / 2n

  8. Rolf Sievert(1896-1966) Harold Gray1905-1965

  9. Absorbed Dose:(Any medium, any type of radiation) The ionizing radiation energy absorbed per unit mass of the matter is known as absorbed dose D = dE / dm Units of absorbed dose The SI unit of absorbed dose is J/kgand the special name is Gray (Gy) 1 Gy = 1J/kg The old classical unit is Rad, 1 Rad = 100 ergs/g

  10. Relation between Gy and Rad 1 Gy = 1 J/Kg 1J =107 ergs = 107 ergs/1000g =104 ergs/g = 100 (100 ergs/g) = 100 Rad 1 Gy = 100 Rad

  11. Equivalent Dose Equivalent Dose is a quantity which takes into effect 'radiation quality', which relates to the degree to which a type of Ionizing Radiation will produce biological damage. Equivalent Dose is obtained by multiplying the Absorbed Dose(D)by a Radiation Weighting Factor(WR). The resulting quantity can then be expressed numerically in Sieverts(Sv) or in the old units of Rem . The quantity is independent of the absorbing material (i.e. tissue).

  12. Equivalent Dose HT:Equivelent dose in Sievert (Sv) DT,R: Absorbed dose in Gray (Gy) wR: Radiation weighting factor If we have a field composed of different types of radiation; (i.e. different wR) the equivalent dose is:

  13. Equivalent Dose

  14. Equivalent Dose Example1: A person has absorbed a fast neutron dose of 0.2 mGy; calculate the equivalent dose? Solution: H=D×wR =0.2 ×20=4 mSv

  15. Equivalent Dose Example 2: A person has received dose from the followings: (0.4mGy slow neutrons, 6.0mGy gamma rays and 0.1mGy alpha particles); calculate the equivalent dose?

  16. Effective Dose The Effective Dose is obtained by taking the Equivalent Dose(H)and multiplying by a Tissue Weighting Factor (WT) which relates to the organs / tissues under consideration. The quantity can be used to express detriment to the whole body as a summation of several different doses of radiation with varying radiation weighting factors ‘radiation type’ and targets.

  17. Effective Dose E : Effective dose in Sievert (Sv) HT : Equivalent dose in Sievert (Sv) wT : Tissue weighting factor If we have a field irradiate different types of tissues; (i.e. different wT) the effective dose is:

  18. Effective Dose

  19. Effective Dose Example: During the year, a worker recived 5 mGy from internally deposited alpha particles, in the lung, 140 mGy from beta particles in the thyroid and 1.2 mGy externally from uniform whole-body proton irradiation. Calculate the effective dose for this worker? Solution: HLung= 5×20=100 mSv HThyroid= 140×1=140 mSv HWhole-Body= 1.2×5=6 mSv ΣE= (100×0.12)L+ (140×0.05)T + (6×1)W = 25 mSv

  20. Collective Effective Dose This quantity is derived from summing the individual effective doses within an exposed population (or workforce). One type of unit to express this quantity is the man Sv. Ē: The mean effective dose to group. Ni: The number of individuals in this group. S: Expressed in man- Sievert.

  21. Collective Effective Dose Example: The following number of personnel exposed for10years for different types of radiation and received the following equivalent doses per year: Calculate the number of expected cancer cases if the cancer ratio is 6.5×10-2 per Sv?

  22. Collective Effective Dose Solution: Collective effective dose / year = (20×20)+(28×18)+(30×10)+(25×8)+(19×5) = 1.5man-Sv/y Collective effective dose in 10 y = (1.5man-Sv/y×10 y) =15 man-Sv Expected cancer cases = 15 man-Sv×6.5×10-2Sv-1 = 0.98 man ≈ 1 man ( i.e. one person out of the 122 possible to get cancer).

  23. Exposure • The quantity exposure describes an x-ray or gammaray field. It is a measure of the amount of ionization produced in air by the x-rays or gamma rays in air. • Exposure is defined as the amount of charge of either sign produced byXorgamma rays in 1 Kg of airwhen all the charges (e+ and e-) are stopped in the air. • X = dQ / dm

  24. Units of Exposure The SI unit of exposure is 1 C/kg The old classical unit is Roentgen (R) 1 R = 2.58 10-4 C/kg It can also be defined as: 1 R = 3.34 10-10 C/0.001293 g of air Where density of air = 0.001293g/cm3 at STP STP : Standard Temperature and Pressure STP corresponds to 273 K (0° Celsius) and 1 atmpressure. STP is often used for measuring gasdensityandvolume.

  25. Ion Pair Energy The energy required to produce one ion pair in air is 34 eV (1eV=1.610-19 J) 1.6  10-19 C = Charge on 1 electron 1 C = 1/1.6 10-19 electrons 1 C = 6.25 1018 electrons

  26. Relation between Rand Rad 1 R = 2.58 10-4 C/kg 1 R = [2.58 10-4 C 6.25 1018 e/c34eV/e 1.6 10- 12 ergs/eV]/kg 1 R = 87 ergs/g = 0.87 (100 ergs/g) 1 R = 0.87 Rad

  27. Relation between Rand Rem 1 R = 87 ergs/g 1 R = 0.87 Rad Multiply by wR 1 R  wR = 0.87 Rad  wR 1R  1 = 0.87 Rem 1 R = 0.87 Rem 1 R  1 Rem (in practical applications)

  28. Exposure Calculation • The exposure (X) is linear proportional with the tube voltage kVp2.07 ≈ kVp2.00 - when kVp increases the exposure also increase to the power 2. • The exposure (X) is linear proportional with mA and mAs- If mAs is constant, the exposure is constant whether (mA) or the time (s) are changed. • The exposure (X) is linear proportional with the inverse square value of the distance from the x-ray source (1/r2).

  29. Exposure Calculation Ex 1: Calculate the exposure for the following machine parameters: kVp = 80; mAs = 3, r = 50 cm Ex 2: Calculate the exposure for the following machine parameters: kVp = 120; mAs = 5, r = 80 cm

  30. Exposure Calculation Ex 3: If the exposure of unknown kVp X-ray machine is 15mR at 2 meter distance and mAs = 5; find the kVp of this machine.

  31. Summary – Radiation Quantities & Units

  32. HW1 Question 3 A worker receives: 10 μGy of α radiation to his bone surfaces. 20 μGy of β radiation to his thyroid. 8 μGy of x-rays radiation to his colon. 2 μGy of slow neutrons to his entire body. Calculate: The equivalent doses to the mentioned organs and everything else? What is his effective dose?

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