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Biological Effects of Radiation Radiation Units

Biological Effects of Radiation Radiation Units. Notes taken from “Introduction to Nuclear Engineering”, 3 rd Edition; J. R. Lamarsh, A. J. Baratta Chapter 9, Sections 9.2. Exposure, Definition and Units. Exposure, X, refers to a g -ray field incident on a body at any point

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Biological Effects of Radiation Radiation Units

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  1. Biological Effects of RadiationRadiation Units Notes taken from “Introduction to Nuclear Engineering”, 3rd Edition; J. R. Lamarsh, A. J. Baratta Chapter 9, Sections 9.2

  2. Thomas Edison State College Exposure, Definition and Units • Exposure, X, refers to a g-ray field incident on a body at any point • Exposure is defined as (9.1) • Here Dq is the sum of the electrical charges on all the ions of one sign produced in air when all the electrons, liberated by photons in a volume of air whose mass is Dm, are completely stopped in air • Exposure has units of Coulombs/(kilogram air)

  3. Thomas Edison State College Conventional Unit, Roentgen • The conventional unit of exposure is the roentgen, R • 1 R=2.58 x 10-4 coul/kg • 1/1000R=1 mR

  4. Thomas Edison State College Exposure Rate • Exposure rate is the exposure per unit time • R/sec or mR/hr • Exposure and exposure rate only pertain to g-rays or x-rays in air at a point outside a body • Exposure and exposure rate do not pertain to neutrons or charged particles • Exposure and exposure rate do not pertain to interactions taking place within a body

  5. Thomas Edison State College Imparted Energy • The biological effect of radiation is a function of how much energy is deposited in a body • An energy balance on a volume/mass of a substance yields the imparted energy, DED: • (9.3) • Here Ein and Eout are the sum of the kinetic energies of all particles incident upon and emerging from the volume • Here Q is the sum of the Q-values any nuclear reactions that take place in the volume

  6. Thomas Edison State College Absorbed Dose, Definition and Units • Absorbed dose refers to the energy imparted, DED, to a mass Dm • Absorbed dose is defined as (9.4) • The conventional unit of absorbed dose is the radiation absorbed dose, with the acronym of rad: 1 rad=.01 J/kg=100 ergs/g • 1/1000 rad=1 mrad • The SI unit of absorbed dose is the gray, Gy • 1 Gy=1 J/kg=100 rads; 1/1000 Gy=1 mGy

  7. Thomas Edison State College Absorbed Dose Rate, • Absorbed dose rate is denoted as • Conventional units are rad/sec and mrad/hr • SI units are Gy/sec and mGy/hr

  8. Thomas Edison State College The Kerma, K • The kerma, K quantifies the sum of the initial kinetic energies of all charged ionizing particles released by indirectly ionizing radiation per unit mass of substance • The energy absorbed in a material due to the attenuation of neutrons is often specified as the kerma or kerma dose • The conventional unit of kerma is rads • The SI unit of kerma is grays

  9. Thomas Edison State College Relative Biological Effectiveness • Biological effects depend upon both the total energy deposited per gram or per unit volume and the way in which the energy is distributed along the path of the radiation • The biological effect of any radiation increases with the linear energy transfer (LET) of the radiation • This is referred to as relative biological effectiveness (RBE)

  10. Thomas Edison State College Quality and Weighting Factors • To quantify RBE, quality factors, Q, are defined as a function of LET (Table 9.1) • For an LET of 3.5 or less, Q=1, for an LET of 23, Q=5, and for an LET of 175 and above, Q=20 • Q is dimensionless and the LET has units of keV/micron • A radiation weighting factor, WR relates the type and energy of incident radiation to Q (Table 9.2) • For 500 keV neutrons Q=100 and WR=20

  11. Thomas Edison State College Equivalent Dose, {Equivalent Dose Rate} • The equivalent dose, H, also called the biological dose, accounts for the type of incident radiation • H=D*WR (9.8) { = *WR (9.10)} • This means that equal equivalent doses delivered to a specific location in the body produce relatively the same effect • This does not mean that equivalent doses delivered to different locations in the body produce relatively the same effect

  12. Thomas Edison State College Dose Equivalent and the Rem • The dose equivalent is defined as the absorbed dose at a point in tissue weighted by a quality factor determined from the LET of the radiation at that point • The NRC uses the same symbol, H, for dose equivalent, as the NCRP does for equivalent dose • The NRC measures dose equivalent in units of rem (roentgen equivalent man)

  13. Thomas Edison State College The Rem • For Q=1, 1 rad=1 rem • 1 Sv=100 rem • Equivalent dose is usually just referred to as dose

  14. Thomas Edison State College Population Dose • Population dose is used to specify the total dose-equivalent to a given group of people • Population dose is used to quantify the dose to a population, but also to quantify the relative effect of a radiation release • (9.11) • Example: In a town of population 2,000, ½ receive a dose of 2 rems, and ½ receive a dose of 1 rem: Hpop=3000 man-rem (see page 475)

  15. The End Thomas Edison State College

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