effective dose calculation

7/28/2019 Effective Dose Calculation

1/29

Assessment of effective dose in

various external exposureconditions

By

Khalil-Ur-Rahman

AE, CNS,PNRA.


2/29

April 14, 2008 Radiation Protection Course 2

Exposure Conditions

Gamma Ray and X-ray exposureconditions

Neutron Exposure conditions


3/29


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


4/29


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


5/29


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.


6/29


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.


7/29April 14, 2008 Radiation Protection Course 7

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



Large Acute Doses: Late Effects

Cancer

Mutation

Cataracts

Fertility



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



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 %.


11/29


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 ?


12/29


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.


13/29


Cases of leukemia cancer


14/29


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


15/29


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


16/29


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


17/29


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


18/29


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


19/29


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


20/29


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 )


21/29



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


22/29



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)

.


23/29


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

..

. .


24/29


Dose assessment for Gamma Rays


25/29


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.


26/29


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


27/29


Dose from Neutron-Simulation


28/29



29/29

April 14 2008 Radiation Protection Course 29

effective dose calculation

Documents