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Assessment of effective dose in
various external exposureconditions
By
Khalil-Ur-Rahman
AE, CNS,PNRA.
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Exposure Conditions
Gamma Ray and X-ray exposureconditions
Neutron Exposure conditions
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Basic Terms
Exposure Ability of radiations to Ionize (charge) the air is called exposure
(coulombs/kg)
X = q/m
Roentgen1 R = 2.58 E-4 Coulombs/Kg
Production of one electrostatic unit (1 esu = 3.33 E-10 coul.) ofcharge of one sign from the interaction of gamma rays in 0.001293 gof air (1 cm3 of air at STP), is defined as I Roentgen.
SI unit is exposure unit (X unit), is the production of 1 coulomb ofcharge in 1 kg of air.
1 X unit = 3876 R
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Basic Terms
Imparted Energy
Energy deposition available for producing biological
effects, Q is nuclear reaction energy
ED
= Ein
Eout
+ Q
Absorbed Dose
Imparted Energy per unit mass
Kerma
Kerma is defined as sum of all the kinetic energies of allcharged, ionizing particles released by indirectly ionizing
radiations (mostly neutrons) per unit mass
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Acute Exposure
The exposures/ doses for short time
The effects of acute exposure mayhave
Late effects
Early Effects
The effects which are evident with in 60 daysare called Early Effects. Early effects are nonstochastic in nature.
Late Effects are evident after 60 days andcan be stochastic and non stochastic.
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Large Acute Doses: Early Effects
Acute Dose (rems) Probable Observed Effects (Clinically observed Early Effects)
5 to 75 Chromosomal Aberration and temporary depression of white blood
cell level in some individuals. No other observable effects.
75 to 200 Vomiting in 5 to 50 % of exposed individuals with in a few hours,
with fatigue and loss of appetite. Moderate blood changes. Recovery
with in few weeks for most symtoms.
200 to 600 Doses > 300 rem, all exposed individuals will exhibit vomiting with
in two hours. Sever blood changes, with hemorrhage and increased
susceptibility to infection
Loss of hair after 2 weeks, only 20% survive at the upper end of the
range
600 to 1000 Vomiting with in 1 hour, sever Blood changes, hemorrhage,
infection and loss of hair
From 80- 100% of exposed individual will succumb within 2
months; Surviving ones will be convalescent over a long period.
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Large Acute Dose: Late Effects
There are considerable
data showing late
Radiation effects in
persons who receive a
large acute whole bodydose at least once in
their life.
Latent Period: Period of
zero Prob. Of cancer. Plateau: Constant Prob.
Risk coefficient
Simplified Model of Radiation Induced
Cancer
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Large Acute Doses: Late Effects
Cancer
Mutation
Cataracts
Fertility
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Risk Calculation Examples (Acute Exposure)
In a radiation accident, a 30 year old maleworker receives an acute whole body doseof 25 rem. Prob. Of death
Solution: Assumtions : death from Bone cancer
Avg age in US = 77 years,
Latent period is 10 year and risk coefficient from 40 year on ward is 0.2
per year per 10
6
rem (WASH-1400).
Prob. Of death = Dose * pleatu period * Risk Coefficient
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Example (Acute Exposure)
= 25 * 30 * 0.2 *10-6
= 1.5 x 10-4 (chance of 1.5 in 10,000)
Similarly probability due to other cancers can be
computed.So, total prob. = 4.7 x 10-3
The prob. of dying from cancer at the age of 30years or older is approximately 0.25.
Radiation increases the prob. by (4.7 x 10-3 /0.25= 0.019) 2 %.
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Acute Exposure: Large Population
To determine the long term effects of an
acute exposure, prob. of each cohort in
population should be computed.
Problem:
In hypothetical radiation accident, one
million person were exposed each receiving 1
rem exactly. How many leukemia can beexpected to result in the population ?
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Acute Exposure: Large Population-2
We may Need:
Population Distribution and cohorts (wewill use US data)
Risk coefficient, latent period and pleatuperiod for each cohort/group
No. of persons, which will be falling inleukemia cancer are, 28.4.
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Cases of leukemia cancer
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Chronic Low Doses
Doses of a few millirems per day, which accumulate up to
a few rems per year, are of considerable concern in the
development of Nuclear Power.
This is the dose level permitted under current standards
of radiation protection
Due to lack of adequate human data, these effects are
estimated, or postulated, by various dose-response models.
For simplicity and conservatism, linear hypothesis
model is used. Linear model entirely ignores biologicalrepairable mechanism
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Chronic Exposure
The exposures in which doses are received over a timespan
Late effects of chronic low doses is especially a tedious,if based on linear quadratic
F(D)= D + D2
the and are the linear coefficients, can range 1E-1to 5E-1 Gy-1 and Quadratic coefficient 1E-1 and 5E-2Gy-1
With such a function, it makes a difference weatherannual dose of 20 mSv (5 rem) is received in one day or14 mrem per day through out the year
With the linear model, fractionation of dose is irrelevant So only the total dose over a given interval need to be
consider
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Example
A radiation worker takes a job at age of 18 year. Hecontinues to job for 50 year and retires at age of 68. Hereceive avg occupational dose of 5 rem through out hiscareer. Find prob. of death due to bone cancer.
Solution:
Using Linear hypothesis, we may require the lifeexpectancy data from 18 to 68
Risk coefficient for bone cancer is 0.2 per yearper 106 rem
Latent period is 10 years
Plateau Period is 30 years
Using the Additive model, the prob. wascomputed as 1.38 x 10-3
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Calculation of Prob. of cancer for Chronic Exposures
Age Life ExpectancyPlateau Period or
Life expectancy Risk Coe Dose rem Prob. Of cancer
18 59 30 0.4 5 0.00006
19 58 30 0.4 5 0.00006
20 57 30 0.4 5 0.00006
21 56 30 0.2 5 0.00003
22 55 30 0.2 5 0.00003
23 54 30 0.2 5 0.00003
24 53 30 0.2 5 0.00003
25 52 30 0.2 5 0.00003
26 51 30 0.2 5 0.00003
27 50 30 0.2 5 0.00003
28 49 30 0.2 5 0.00003
29 48 30 0.2 5 0.00003
30 47 30 0.2 5 0.00003
31 46 30 0.2 5 0.00003
32 45 30 0.2 5 0.00003
33 44 30 0.2 5 0.00003
34 43 30 0.2 5 0.00003
35 42 30 0.2 5 0.00003
36 41 30 0.2 5 0.00003
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Calculation of Prob. of cancer for ChronicExposures (contd.)
37 40 30 0.2 5 0.00003
38 39 30 0.2 5 0.00003
39 38 30 0.2 5 0.00003
40 37 30 0.2 5 0.00003
41 36 30 0.2 5 0.00003
42 35 30 0.2 5 0.00003
43 34 30 0.2 5 0.00003
44 33 30 0.2 5 0.00003
45 32 30 0.2 5 0.00003
46 31 30 0.2 5 0.00003
47 30 30 0.2 5 0.00003
48 29 29 0.2 5 0.000029
49 28 28 0.2 5 0.000028
50 27 27 0.2 5 0.000027
51 26 26 0.2 5 0.000026
52 25 25 0.2 5 0.000025
53 24 24 0.2 5 0.000024
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Calculation of Prob. of cancer for ChronicExposures (contd.)
54 23 23 0.2 5 0.000023
55 22 22 0.2 5 0.000022
56 21 21 0.2 5 0.000021
57 20 20 0.2 5 0.00002
58 19 19 0.2 5 0.000019
59 18 18 0.2 5 0.000018
60 17 17 0.2 5 0.000017
61 16 16 0.2 5 0.000016
62 15 15 0.2 5 0.000015
63 14 14 0.2 5 0.000014
64 13 13 0.2 5 0.000013
65 12 12 0.2 5 0.000012
66 11 11 0.2 5 0.000011
67 10 10 0.2 5 0.00001
Total 0.00138
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Computation of Exposures and Dose
External Exposure to Gamma Ray Roentgen can be related to energy
deposition in air for charge liberation
To produce 2.58 x10-4 coulomb charge(2.58 x10-4 /1.6 x 10-19= 1.61 x 1015 ionpairs). 34 eV (32 to 36 eV) must bedeposited by Gamma ray in air for theproduction of one ion pair.
So, for 1.61 x 1015 ion pairs in 1 kg ofair, energy deposition is (34 x 1.61 x1015 = 5.47 x 1016 eV )
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Computation of Exposures and Dose
1R = 5.47 x 1016 eV/kg
= 5.47 x 1010 eV/kg
= 87.5 ergs/g
Energy deposition per unit mass is:I.E.(a/)
air
I- gamma ray intensity
E- Energy of gamma Ray(a/)
air-Mass absorption coefficient ofair at energy E
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Computation of Exposures and Dose
X=I.E.(a/)air/ 5.47 x 107
= 1.83 x 10-8 I.E.(a/)air (R/sec)
= 0.0659 IE(a/)air (mR/hr)
Dose calculation from gamma rays
1 rad = 100 ergs per gram of tissue
= 6.25 x 107 MeV/g
.
D=I.E.(a/)tis/ 6.25 x 107= 1.6 x 10-8 I.E.(a/)
tissue (rad/sec)= 0.0576 IE(a/)
tissue (mrad/hr)
.
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Dose assessment due to gamma exposure
To obtain dose rate in tissue that issubjected to an exposure rate X :
D/X = 1.6 x 10-8 I.E.(a/)tissue/1.83 x 10-8
I.E.(a/)air
D = 0.874 ( a/)tissue/ I.E.(a/)air) X
..
. .
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Dose assessment for Gamma Rays
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Dose from Neutrons-external exposure
Dose computations for neutrons issome what more difficult than gammarays due to complex interaction withmatter
Neutron may under go elastic orinelastic, radioactive capture andvarious other reactions
Elastic scatteringstuck nucleolusmay recoil with sufficient energy-causing ionization, charged particle.
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Dose from Neutrons-external exposure
In inelastic scattering and radioactivecapture, gamma rays may also be emittedand in reaction ionization may be produced.
Due to this complexity, energy deposition iscalculated numerically using Monte Carlomethod.
Actual history of neutron and secondaryradiations is traced in simulation
Curves are drawn for different energyneutrons
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Dose from Neutron-Simulation
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