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QUANTITIES AND UNITS USED IN RADIATION PROTECTIONH.L ANIL RANJITHHEAD, DIVISION OF RADIATION PROTECTION ATOMIC ENERGY AUTHORITY
QUANTITIES: ARE MEASURABLE CHARACTETISTICSEg LENGTH, DOSE ETC. UNITS
ARE USED TO DESCRIBE THE QUANTITYEg METERS, GRAYS ETC.
RADIATION UNITS TO MEASURE RADIOACTIVITYTO EXPRESS ENERGY OF RADIATION EMITEDTO EXPRESS AMOUNT OF ENERGY DEPOSITED IN THE BODYTO QUANTIFY BIOLOGICAL DAMAGES TO IRIDIATED TISSUES
NO.OF DESINTEGRATIONS PRE SECOND
1Bq = 1 dis/s
ORDER OF MAGNITUDE
THE CURIE 1 Curie (Ci) = Activity of 1g of 226Ra 1g of 226Ra disintegrate 3.7x1010 atoms per second 1 Ci=3.7 x 1010 dis/s 1 Ci=3.7 x 1010 Bq 1 Ci=37 GBq
ENERGY OF RADIATION ELECTRON VOLTS
1eV = 1.6 X 10 19 J COBALT- 60 RADIOACTIVE MATERIAL EMITS TWO GAMMA RADIATIONS OF ENERGIES 1.17 MeV AND 1.32 MeV.
DOSEUSES AS A GENERIC TERM THAT CAN APPLY TO ANY OF THE RELEVANT DOSIMETRIC QUANTIESEXPOSURE
IN A GENERIC SENSE TO MEAN THE PROCESS OF BEING EXPOSED TO RADIATION
Exposure UnitIs a measure of ionization produced in airIs used only for X and radiation Is valid for quantum energy less than 3 MeV
1 X unit=1 C/kg air
One exposure unit is defined as that quantity of x or gamma radiation that produces in air, ions carrying 1 coulomb of change( of either sign) per kg air.
ExposureExposure is measured under conditions of electronic equilibriumFor photon energies above about 3 MeV, the ranges of secondary electrons become a significant fraction of the photon attenuation lengths and the departure from equilibrium may be significantThus, exposure is not defined above photon energies of 3 MeV
*Electronic equilibrium is discussed in more detail in another session, but basically, when the same number of electrons are set in motion in a given volume by the primary radiation as come to rest in that same volume, we say that electronic equilibrium has been attained. For electronic equilibrium to exist, the attenuation of the primary radiation beam must be negligible in a distance equal to the mean range of the electrons.
Roentgens (2/3)Is symbolized by R was used as the exposure unit before SI system was adopted is still being used.
RoentgenIs defined as the quantity of x or gamma radiation that produces ions carrying one statcoulomb of charge of either sign per cubic centimeter of air at STP. Charge of the electron=1.6x10-19C =4.8x10-10sC 1C =3x109 sC
*KERMAKERMA (Kinetic Energy Released in a Material):Is the sum of the initial kinetic energies of all charged ionizing particles liberated by uncharged ionizing particles in a material of unit massFor medical imaging use, KERMA is usually expressed in air
SI unit = joule per kilogram (J/kg) or gray (Gy) 1 J/kg = 1 Gy
*Mean absorbed dose in a tissue or organ The mean absorbed dose in a tissue or organ DT is the energy deposited in the organ divided by the mass of that organ.
ABSORBED DOSE(1/2)MEASURES THE ENERGY DELIVERED TO ANY MATERIAL
IN RADIATION PROTECTION
THE MATERIAL CONCERNED IS THE TISSUE OR ORGAN OF THE HUMAN BODY
DEFINED AS THE
ENERGY ABSORBED PER UNIT MASS OF ANY MATERIAL
GRAY OR RADS
1GRAY (Gy) = 1J/kg1RADS = 100 ergs/g
100 RADS = 1 Gy
EQUIVALENT DOSE(1/2) QUANTIFY THE BIOLOGICAL DAMAGE TO THE ORGAN OR TISSUE IRRIDIATEDThe same dose levels of different radiations (ie photons and neutrons) do not have the same level of biological effectRadiation weighting factor, wR(related to radiation quality)
BIOLOGICAL EFFECTS OF AN EXPOSURE ON A ORGAN OR TISSUE DEPEND ON:
ENERGY TRANSMITTED TO THE ORGAN OR TISSUE BY RADIATION
HAMFULNESS OF THE TYPE OF RADIATION INVOLVED (DEGREE OF POWER OF IONIZATION)
Radiation weighting factors, wR1
Type and energy rangesRadiationweightingfactor, wR11510 20105 5Photons, all energiesElectrons and muons, all energiesNeutrons, energy < 10 keV10 keV to 100 keV100 keV to 2 MeV> 2 MeV to 20 MeV> 20 MeV Protons, other than recoil protons, energy > 2 MeVAlpha particles, fission fragments, heavy nuclei201)All values relate to the radiation incident on the body, or, for internal sources, emitted from the source.
*The second major reason that physical quantities are not used directly is that the same quantity (absorbed dose) of different types of radiation may have significantly different degrees of radiation damage a factor of 10 or more. The term given to this quantity is relative biological effect (RBE). This is due to the differences in microdosimetric distribution of energy deposition. The difference is characterized by the quantity linear energy transfer (LET). In the past, the quality factor was used to compensate for RBE differences. However, the ICRP felt that quality factor implied a level of precision that was not justified, and it was replaced with radiation weighting factor, wR.
Neutron radiation weighting factors30wRNeutron energy - MeV
ICRP RecommendationICRP Approximation
*The main impact of radiation weighting factor is on neutron dosimetry since fast neutron interactions are characterized by high LET values. However, as defined by the ICRP, wR for neutron is an energy dependent step function. Those who perform neutron dosimetry calculations, particularly fluence to dose conversion coefficients find such step functions disagreeable because of the discontinuities that result. Therefore, a smooth approximation has been developed that eliminates these discontinuities. It is accepted by the ICRP as long as it is understood that it is an approximation and that wR is defined by the values in the table.
Different body tissues have different biological sensitivities to the same radiation type and doseTissue weighting factor, wT
EFFECTIVE DOSEMEASURES THE RISK OF BIOLOGICAL DAMAGE TO WHOLE BODY TAKING THE RADIOSENSITIVITIES OF TISSUE IRRIDIATED IN TO ACCOUNT
MEASURES THE RISK REGARDLESS OF EXPOSURE INVOLVED.( INTERNAL, EXTERNAL, PARTIAL OR TOTAL)
MEASURES IN THE UNIT OF.
SIEVERT( Sv )
Roentgen (3/3)1R=0.0087 J/kg of airIR=0.0087 Gy = .87 RadIR=0.0096 J/kg in TissueIR=0.0096 Gy in TissueIR=.96 Rad in Tissue1 R=1 Radfor x and rays IR=1 rem = .01 Sv
Multipliers of the equivalent dose to an organ or tissue to account for the different sensitivities to the induction of stochastic effects of radiation.
Tissue or organ wTTissue or organwT
Gonads0.20Bone marrow (red)0.12 Colon0.12Lung 0.12 Stomach0.12Bladder 0.05 Breast 0.05Liver 0.05 Oesophagus0.05Thyroid 0.05 Skin 0.01Bone surface 0.01 Remainder0.05TOTAL1.00Tissue weighting factors
*One of the primary reasons that the physical quantities are not used directly for radiation protection is that different body tissues have different levels of radiation sensitivity and different degrees of susceptibility to radiation induced stochastic effects such as cancer. This table was developed by the ICRP to reflect the relative tissue sensitivity to radiation induction of these effects.
Committed DoseIs a useful subsidiary dosimetric quality to express dose to body during certain time following an intake of radioactive material to the body.
Note :The dose delivery to the body during the above period is at varying rates.
Committed Equivalent DoseDefined as the time integral of the equivalent dose rate and denoted by HT( ) =integration time in years following the intake. If t is not specified Integration time is taken as50 years for adults70 years for children
Committed equivalent dose:
The quantity H(), defined as;
where to is the time of intake, HT(t) is the equivalent dose rate at time t in an organ or tissue T and is the time elapsed after an intake of radioactive substances.When is not specified it will be taken to be 50 years for adults and to age 70 years for intakes by children.
Committed effective dose:
The quantity E(), defined as ;
where HT() is the committed equivalent dose to tissue T over the integration time and WT is the tissue weighting factor for tissue T. When is not specified it will be taken to be 50 years for adults and to age 70 years for intakes by children.
Collective Dose(1/2)Is used to express dose to a group or a population.
Takes account of the no of people exposed to a source and the average dose to the individual.
COLLECTIVE DOSE(2/2) DEFINED AS,
THE PRODUCT OF THE NUMBER OF INDIVIDUAL EXPOSED TO A SOURCE AND THEIR AVERAGE DOSE
UNIT: MAN SIEVERT (man Sv)
Is there RADIATION in this room?
Background RadiationNatural Background