2. Exposure (X)
Defined by International Commission on Radiation
Units and Measurements (ICRU) in 1962.
X = q / m
Where q is the sum of the electrical charges
on all the ions of one sign produced in air
when all the electrons (+ve or –ve), liberated
by photons in a volume of air whose mass is
m, are completely stopped in air.
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3. Exposure (X) …..
Exposure has the dimensions of C/kg
One exposure unit (X) is defined as that
quantity of X- or -rays that produces in air,
ions carrying 1C of charge (of either sign) per
kg of air.
1 ‘X’ unit = 1C/kgair
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4. Exposure (X) …..
Roentgen (R):
Unit of exposure, introduced in 1928.
The roentgen shall be the quantity of X- or -
radiation such that the associated corpuscular
emission per 0.001293 grams of air produces,
in air, ions carrying one electro-static unit of
electricity of either sign.
1 R = 1esu / 0.001293gair
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5. Exposure (X) …..
Röentgen is only used for X- or -rays.
Röentgen is not used as a unit of X- or -ray
exposure for quantum energies more than
3 MeV. For energies more than 3 MeV,
exposure is expressed in watt-s/cm2.
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6. Exposure (X) …..
1 R = 87.7 ergs/gair
1 R 96 ergs/gsoft tissue
1 R = 2.58 x 10-4 C/kg
1 ‘X’ unit = 34 J/kgair
1 ‘X’ unit = 37 J/kgsoft tissue
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7. Absorbed Dose (D)
This is the energy absorbed per unit mass. Its
unit is J/kg, Which is given the special name of
gray (Gy).
1 Gy = 1 J/kg
Conventional unit of absorbed dose is rad
(radiation absorbed dose).
1 rad = 100 ergs/g
1 Gy = 100 rad
(1 J = 107 ergs and 1 kg = 1000g)
The gray is universally applicable to all types of
ionizing radiation.
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8. Kerma (K)
It is the sum of the initial kinetic energies of all
the charged, ionizing particles released by
indirectly ionizing radiation per unit mass of
the substance. Its unit is J/kg.
K = dEtr/dm
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9. Equivalent Dose (HT)
It is the absorbed dose averaged over a tissue or
organ and weighted for the radiation quality (WR)
HT = WR . DT.R
DT.R = absorbed dose averaged over the tissue or
organ ‘T’due to radiation ‘R’.
WR = radiation weighting factor selected for type
and energy of radiation.
The unit of equivalent dose is J/kg with the
special name of sievert (Sv). Conventional unit is
rem (röentgen equivalent man).
1 Sv = 100 rem
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10. Radiation Weighting Factor (WR)
The probability of stochastic effects depends
on:
absorbed dose
type and energy of radiation causing the dose
This is taken into account by weighting the
absorbed dose by a factor related to the
quality of the radiation.
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11. Radiation Weighting Factor (WR)
In the past weighting factor has been applied to
the absorbed dose at a point and called the
quality factor ‘Q’. The weighted absorbed dose
was called the dose equivalent ‘H’.
The value of WR has been selected to be
representative of the value of RBE (relative
biological effectiveness).
The values of WR are broadly compatible with the
values of Q, which are related to the linear energy
transfer (LET).
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12. Linear Energy Transfer (LET)
It is the rate of energy transfer per unit path
length.
High LET radiation: , p, n etc.
Low LET radiation: , , X –rays etc.
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13. Radiation Weighting Factors (WR)
Based on ICRP-60 (old Values)
Type and Energy of Radiation WR
Photons, all energies 1
Electrons and muons, all energies 1
Neutrons, energy 10 keV 5
10 keV to 100 keV 10
100 keV to 2 MeV 20
2 MeV to 20 MeV 10
20 MeV 5
Protons 5
-particles, fission fragments, heavy nuclei 20
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14. Recommended radiation weighting factors
Based on ICRP-103 (New Values)
Radiation Type Radiation weighting factor, WR
Photons 1
Electrons and muons 1
Protons and charged pions 2
Alpha particles, fission
fragments, heavy ions
20
Neutrons 2 to 20
A continuous function of neutron energy.
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15. Effective Dose (E)
The weighted equivalent dose (doubly weighted
absorbed dose) has previously been called
effective dose equivalents now called effective
dose.
The effective dose is the sum of the weighted
equivalent doses in all the tissues and organs of
the body.
E = WT . HT
HT = equivalent dose in the tissue or organ ‘T’
WT = weighting factor for the tissue ‘T’
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16. Tissue Weighting Factor (WT)
The relationship between probability of stochastic
effect and equivalent dose also depends on the
type of tissue or organ. The factor by which the
equivalent dose in tissue or organ ‘T’ is weighted
is called the tissue weighting factor WT.
WT represents the relative contribution of that
organ or tissue to the total detriment due to these
effects resulting from the uniform irradiation of
whole body.
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17. The consequences following an absorbed dose
depends on:
1. magnitude of the dose
2. type and energy of radiation (dealt with by
radiation weighting factor)
3. distribution of dose within the body (dealt with
by tissue weighting factor)
4. distribution of dose in time (dose rate and
protraction of exposure)
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18. Tissue Weighting Factors (WT)
Based on ICRP-60 (old values)
Tissue or organ WT
Gonads 0.20
Bone marrow (red) 0.12
Colon 0.12
Lung 0.12
Stomach 0.12
Bladder 0.05
Breast 0.05
Liver 0.05
Oesophagus 0.05
Thyroid 0.05
Skin 0.01
Bone surface 0.01
Remainder 0.05
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19. Recommended tissue weighting factors
Based on ICRP-103 (New values)
Tissue WT ∑ WT
Bone marrow (red), Colon, Lung,
Stomach, Breast, Remainder tissues
0.12 0.72
Gonads 0.08 0.08
Bladder, Oesophagus, Liver, Thyroid 0.04 0.16
Bone surface, Brain, Salivary glands,
Skin
0.01 0.04
Total 1.00
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20. Committed Equivalent Dose
[HT()
Following an intake to the body of a radioactive
material, there is a period during which the
material gives rise to equivalent doses in the
tissues of the body at varying rates. The time
integral of equivalent dose rate is called
committed equivalent dose [HT()]. is the
integration time (in years) following the intake.
When the period of integration’’ is not given, a
period of 50 years is implied for adults or a period
of 70 years for children.
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21. Committed Effective Dose [E()]
If the committed organ or tissue equivalent doses
resulting from an intake are multiplied by the
appropriate weighting factors, WT, and then
summed, the result will be the committed effective
dose.
E() = WT . HT()
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