Quantities, Units and Definitions

Transcription

1 Quantities, Units and Definitions ICRU 51 - Quantities and Units in Radiation Protection Dosimetry (1993) A. FLUENCE dν Φ da a. dn number of particles incident on a sphere of cross sectional area da b. units : m Differential Quantities dφ a. = fluence rate or flux dt b. dφ dε

2 c. dφ dω B. ENERGY IMPARED, ε ε R in R out + Q a. ε = energy imparted by ionizing radiation to matter in a volume b. R in = radiant energy incident on a volume sum of the energies (excluding rest energy) of those charged and uncharged ionizing particles entering the volume c. R out = radiant energy emerging from a volume sum of the energies (excluding rest energy) of those charged and uncharged particles leaving the volume d. Q = sum of all changes ( + sign; - sign) of rest mass energy of nuclei and elementary particles in any interactions which occur in the volume

3 e. units: J Joules f. ε mean energy imparted = <ε>. C. ABSORBED DOSE, D he fundamental dosimetric quantity 2. D dε dm a. dε = mean energy imparted by ionizing radiation to matter of mass dm b. units: J / kg 1 J / kg = 1Gy (i) Gy = Gray

4 D. ABSORBED DOSE RAE, D D dd dt 1 1 a. units: J kg s or Gy 1 s E. LINEAR ENERGY RANSFER or LINEAR COLLISION SOPPING POWER, L L dε d a. de = mean energy lost by a charged particle, due to collisions with atomic electrons, in traversing a distance d b. units: J 1 m c. common units: ev/m or kev/μm

5 d. his definition is for what is called unrestricted linear energy transfer (LE). If there is a cutoff in energy, we have restricted LE. F. LINEAL ENERGY, y y ε a. ε = energy imparted to the matter in a volume of interest b. = mean chord length in the volume of interest 2. units: J 1 m 3. An energy deposition event consists of statistically correlated depositions of energy such as those by high-energy particles and their secondary electrons

6 4. LE non-stochastic quantity y stochastic quantity G. DISRIBUION OF ABSORBED DOSE IN LE, D L D L dd dl a. dd = absorbed dose contributed by primary-charged particles with LE between L and L + dl 2. he distribution of absorbed dose in y (lineal energy) is also used because it is more readily measurable than D L a. ICRU 40 (1986) discusses relations between LE distributions (calculated quantities) and lineal energy distributions (measured quantities).

7 I. DOSE-EQUIVALEN QUANIIES (ICRU 51) A. QUALIY FACOR, Q Introduced to weight the absorbed dose for the biological effectiveness of the charged particles producing the absorbed dose. 2. Q(L) is specified by ICRP 60 (1991) a. Q( L) 1 for L 10 kev / μm = 0.32L 2.2 for 10 < L < 100 kev / μm L in H 2O 300 / L for L 100 kev / μm 3. Q at a point in tissue is given by 1 Q = Q( L) D L dl D L a. D = absorbed dose at the point in question

8 b. he integration is performed over the distribution D L due to all charged particles, excluding their secondary electrons B. DOSE EQUIVALEN, H H QD 2. Units = J 1 kg 1 J kg =1Sv a. Sv = Sievert 3. Note that H is defined for routine radiation protection practices (prevention of stochastic effects such as cancer) and is not applicable for acute (high-level) exposures, such as occur in radiation accidents 4. he dose equivalent at a point can be written as H = Q( L) D L dl L

9 C. DOSE EQUIVALEN RAE, H H dh dt Units: J kg s or Sv 1 s III. MEAN VALUES OF QUANIIES A. hese are often sufficient for radiation protection purposes and are used for limitation purposes. B. MEAN ABSORBED DOSE IN AN ORGAN (ORGAN DOSE), D D 1 m m D dm a. m = mass of organ tissue 2. D in a specified tissue or organ in the ratio of the energy imparted, ε, to the tissue or organ and m, its mass.

10 3. D depends on the ambient radiation field (in the organ) and on the size and orientation of the body in this field. C. MEAN QUALIY FACOR, Q For a specified tissue or organ 2. Recall Q 1 m D m QD dm QD = H and hen: Q = H = Q( L) D L dl L 1 m D m L Q( L) D L dldm a. Q is based upon type and energy of radiation existing in the organ of interest this is not the same radiation field as the external field outside the body. b. In most cases, Q is not known because the energy spectrum in the organ is not known (measured) but is only inferred (calculated) approximate Q by Q - see ICRU 40

11 IV. QUANIIES USED FOR LIMIAION PURPOSES A. ICRP 60 (1991) EQUIVALEN DOSE, H H R w D R, R a. D,R = mean organ/tissue dose due to radiation of type R b. w R = radiation weighting factor 2. RADIAION WEIGHING FACORS, w R (NCRP 116) a. NCRP A dimensionless factor selected to account for differences in biological effectiveness of different types of radiation, within the range of doses of concern in radiationprotection activities. b. w R specifically relates to the type (proton, neutron, alpha, etc.) of incident radiation or, in the case of internal emitters, the radiation emitted by the source.

12 c. w R is analogous to Q d. w R is independent of the tissue or organ being irradiated ABLE 4.3 FROM NCRP 116

13 3. EFFECIVE DOSE, E Ε w H a. Units: Sv b. is a sum over all irradiated tissues or organs c. Can also use Ε = w wrd, R R d. w = tissue weighting factor the proportionate detriment (stochastic) of tissue when the whole body is irradiated uniformly

14 NCRP 116 able 5.1

15 f. Note: w = 1 g. For uniform, whole body irradiations, H is the same (maybe) for all organs and tissues Ε = w H = H w = 1 V. OHER DEFINIIONS AND QUANIIES OF INERES A. EXPOSURE, X H Χ dq dm a. dq = sum of all electrical charges of one sign on all ions produced in air when all the electrons liberated by photons in an air volume of mass dm are completely stopped b. units : C kg coulombs/kg

16 2. 1 R Roentgen = 2.58x10 4 C kg B. KERMA, K dε K k dm a. de k = sum of the initial kinetic energies of all particles liberated by indirectly ionizing radiations (photons and neutrons) in a volume element of mass dm. 2. Units: J/kg or Gy 3. Kerma is used only with primary radiations that are indirectly ionizing

17 C. SOPPING POWER (Collisional Losses), S c S ( d / dx) c c a. -d = average energy lost by a charged particle in traversing a distance dx b. he subscript c denotes that only energy losses due to collision are included. D. PAHLENGH, s, and RANGE, R s total distance traversed by a particle without relation to direction. a. s is the total distance traveled

18 2. R s a. R is the average path length for many identical monoenergetic particles b. R does not include diffusion once thermal energies are reached. VI. ALARA PRINCIPLE A. ALARA As Low As Reasonably Achievable ALARA application and implementation discussed in NCRP 107 (1990).

19 a. It is the continuation of good radiation protection programs and practices b. In ICRP 55 (1989), the ICRP used Justification to mean ALARA 2. ALARA means keeping exposures as low as reasonably achievable in relation to benefits to be obtained from these exposures a. It does not mean reducing exposures as low as possible b. ALARA means that radiation limits are not design guidelines but are truly upper bounds on radiation exposures. hese bounds or limits are not to be exceeded!