bakgrnd_radiaton

Radiol ogical Prot ecti o n

For practical purpose s of assessing and regul ating the hazards of ionizing radiation to workers and the general population, weighting factors are u s ed.

A radiation weighting factor is an estimate of the effectiveness per unit dose of the given radi ation relative a to low-LET standard.

Gy (joule/kg) can be used for any type of radiation.

Gy does not describe the biological effects of the different radiations.

Weighting factors are dim e nsionless m u ltip licative factors used to convert physical dose (Gy) to equivalent dose (Sv) ; i.e., t o place bi ological effects from exposure to different t ypes of radia tion on a common scale.

A weighting factor is not an RBE.

Weighting factors represent a conser vative judgment of the envelope of experimental RBEs of practical re levance to low-level human exposure.

Radiation Weighting factors


Radiation Type and Energy Range	Radiation Weighting Factor, W R
X and γ r a ys, all ener gies	1
Electrons posit rons a nd m uons, all energies	1
Neutrons:
< 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, (other than recoil protons) and energy > 2 MeV,	2-5
α particles, fission fragm e nts, heavy nuclei	20

[ICRU 60, 1991]

For radiati on types and energies not listed in the Table above, the following relationshi ps are used to cal cul a te a weig hting factor.

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[ICRP, 199 1 ]

Q = 1.0 L < 10 keV/µm

Q = 0.32 L – 2.2 10 ≤ L ≤ 100 keV/µm

Q = 300/(L) 1/2 L ≥ 100 keV/µm L = unrestricted LET in water (keV/ µm )

Radiation Typica l LE T values

1.2 MeV 60 Co gamma 0.3 keV/ µ m

250 kVp x rays 2 keV/ µ m

10 MeV protons 4.7 keV/ µ m

150 MeV protons 0.5 keV/ µ m

14 MeV neutrons 12 keV/ µ m Heavy charged par ticles 100-2000 keV/ µ m

2.5 MeV alpha particles 166 keV/ µ m

2 GeV Fe ions 1,000 keV/ µ m


Tissu e weightin g factors
Tissue	Tissue Weighti ng Factor, W T
Gonads	0.20
Red bone marrow	0.12
C o l o n	0 . 1 2
Lung	0.12
Stom ach	0.12
Bladder	0.05
B r e a s t	0 . 0 5
Liver	0.05
Esophagus	0.05
Thyroid	0.05
S k i n	0 . 0 1
Bone surfaces	0.01
Remainder	0.05

(ICRU 60, 1991; NCRP 116, 1993)

Dosimetric Quantities


Quantity	Definition	New Units	Old Units
Exposure	Charge per unit m a ss of air 1 R = 2.58 x 10 -4 C/kg	- - -	R o e n t g e n (R)
Absorbed dose to tissue T from radiation of type R D T,R	Energy of radiation R absorbed per unit mass of tissue T 1 rad = 100 ergs/g 1 Gy = 1 joule/kg 1 Gy = 100 rads	gray (Gy)	Radiation absorbed dose (rad)
Equivalent dose to tissue T H T	Sum of contributions of dose to T from different radiation types, each m u ltiplied by the radiation weighting factor (w R ) H T = Σ R w R D T,R	Sievert (Sv)	Roentgen equivalent m a n (rem )
Effective Dose E	Sum of equivalent doses to organs and tissues exposed, each m u ltiplied by the appropriate tissue weighting factor (w T ) E = Σ T w T H T	Sievert (Sv)	rem

Committed Equivalent Dose: for radionuclides incorp orated into the body, the integrated dose over time. 50 years fo r occupational exposure, 70 years for mem b ers of the general public.

Committed Effective Dose: effective dose integrated over 50 or 70 years.

Sources of Radiation Exposure

Other 1%: Occupat i onal; Fallout; Nuclear Fuel Cy cle; Miscellaneous

Annual average in U.S.

3.6 mSv

Annual estimated average effective dose equivalent received by a member of the population of the United States .


Source	Average a nnual effective dose
	( µ S v )	( m r e m )
Inhaled (Radon and Decay Products)	2000	200
Other Internally Deposited Radionuclides	3 9 0	3 9
Terrestrial Radiation	280	28
Cosm ic Radiation	270	27
Cosm ogenic Radioactivity	10	1
Rounded total from natural source	3000	300
Rounded total from artificial Sources	600	60
T o t a l	3 6 0 0	3 6 0

Radioactivity in Natu re

Our world is radioactive and has been si nce it was created. Over 60 radionuclides can be found in nature, and they can be placed in three general categories:

Primordial - been around si nce the creation of the Earth Singl y-occurring

Chain or series

Cosmogenic - formed as a result of cosm ic ray interactions

Primordial radionuclides

When the earth was first form ed a relativ ely large num b er of isotopes would have been radioactive.

Those with half-live s of less than about 10 8 years would by now have decayed into stable nuclides.

The progeny or decay products of the l ong-lived radionuclides are also in this heading.

Primordial nuclide examples


Nuclide	Half-life (years)	Natural Activity
Uranium 235	7.04 x 10 8	0.72 % of all natural uranium
Uranium 238	4.47 x 10 9	99.27 % of all natural uran ium ; 0.5 to 4.7 ppm total uranium in the co mm on rock types
Thorium 232	1.41 x 10 10	1.6 to 20 ppm in the comm on rock types with a crustal average of 10.7 ppm
Radium 226	1.60 x 10 3	0.42 pCi/g (16 Bq/kg) in limestone and 1.3 pCi/g (48 Bq/kg) in igneous rock
Radon 222	3.82 days	Noble Gas; annual average ai r concentrations range in the US from 0.016 pCi/L (0.6 Bq/m 3 ) to 0.75 pCi/L (28 Bq/m 3 )
Potassium 40	1.28 x 10 9	Widespread, e.g., soil ~ 1-30 pCi/g (0.037-1.1 Bq/g)

Natural Radioactivity in soil

How m u ch natural radioactivity is found in an area 1 square m i le, by 1 foot deep (total volume ~ 7.9 x 10 5 m 3 )?

Activity levels vary greatly depending on soil type , m i neral make-up and density (~1.58 g/cm 3 ). This table represents calculations using typical num bers.

Natural Radioactivity by the Mile


Nuclide	Activity used in calculation	Mass of Nuclide	Activity
Uranium	0.7 pCi/gm (25 Bq/kg)	2,200 kg	0.8 curies (31 GBq)
Thorium	1.1 pCi/g (40 Bq/kg)	12,000 kg	1.4 curies (52 GBq)
Potassium 40	11 pCi/g (400 Bq/kg)	2000 kg	13 curies (500 GBq)
Radium	1.3 pCi/g (48 Bq/kg)	1.7 g	1.7 curies (63 GBq)
Radon	0.17 pCi/gm (10 kBq/m 3 ) soil	11 µ g	0.2 curies (7.4 GBq)

“ Single” primordial nuclides

 At least 22 naturally occurring single or nor-series prim ordial radionuclides have been identified.

 Most of these have such long hal f-lives, small isotopic and elemental abundances and sm all biological uptake and concentration that they give little environm ental dose.

 The m o st im portant is potassium -40. Potassium is a metal with 3 natural isotopes, 39, 40 and 41. Only 40 K is radioactive and it has a half life of 1.26 x 10 9 year s.

Chain or series-decaying primordial radionuclides

Radioactiv e series r e fers to any of four independent sets of unst a ble heavy atom ic nuclei that decay through a sequence of alpha and beta decays until a stable nucleus is achieved.

Three of t h e sets, th e thorium series, uranium series, and actinium series, called natural or classical series, are h eaded by naturally occurring species of heavy unstable nuclei that have half-lives co mp arable to t h e age of the element s .

Im portant point s about series-decaying radionuclides

 3 main series

 the fourth (neptuni um ) exists only w ith man-m a de isotopes, but probably existed early in the life of the earth

 the 3 m a in series decay schemes all produce radon (but prim ary radon source, the longest half-life, is the uranium series).


Series na me	Begins	T 1/2	E n d s	G a s (T 1/2 )
Thorium	232 Th	1.4 x 10 10 yr	208 Pb	220 Rn (55.6 sec) thoron
Uranium	238 U	4.5 x 10 9 yr	206 Pb	222 Rn (3.8 days) radon
Actinium	235 U	7.1 x 10 8 yr	207 Pb	219 Rn (4.0 sec) actinon

Uranium 238 decay scheme.

 Branching occurs when the radionuclid e is unstabl e to both alpha and beta decay, for example, 21 8 Po.

 Gamma em ission occurs in m o st steps.

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Major characteristics of the radionuc lides that comprise the natural decay series for 232 Th, 235 U, and 238 U

Natural 232 Th decay series			Natural 235 U decay s e ries			Natural 238 U decay s e ries

Nu clid e	H a l f - l i f e b	Princi ple m ode of decay c	Nu clid e	H a l f - l i f e b	Princi ple m ode of decay c	Nu clid e	H a l f - l i f e b	Princi ple m ode of decay c

232 T h	1 . 4 E+ 1 0 y		235 U	7 . 0 E+ 0 8 y		238 U	4 . 5 E+ 0 9 y	
228 R a	5 . 75 y		231 T h	1 . 06 d		234 T h	2 4 .1 0 d	
228 A c	6 . 13 h		231 P a	3 . 3 E+ 0 4 y		234 P a	1 . 17 m i n	
228 T h	1 . 91 3 y		227 A c	2 . 2 E+ 0 1 y	 (1.4 %)	234 U	2 . 5 E+ 0 5 y	
					 ( 9 8. 6 % )
224 R a	3 . 66 d		227 T h	1 8 .7 d		230 T h	7 . 5 E+ 0 4 y	
220 Rn	55. 6 s		223 F r	2 1 .8 m i n		226 R a	1 . 6 E+ 0 3 y	
216 P o	1 . 5 E –0 2 s		223 R a	1 1 .4 3 d		222 Rn	3. 8 5 d	
212 P b	1 0 .6 4 h		219 A t	5 6 s		218 P o	3 . 1 m i n	
212 B i	1 . 01 h	 (3 6% )	219 Rn	3. 9 6 s		218 At	1 . 5 s	
		 (64 % )
212 P o	3 . 0 E –0 7 s		215 B i	7 . 6 m i n		214 P b	2 7 m i n	
208 Tl	3 . 05 3 m i n		215 P o	1 . 8 E –0 3 s		214 B i	1 9 .9 m i n	
208 P b	( s t a b l e )	( s t a b l e )	215 At	1 . 0 E –0 7 s		214 P o	1 . 6 E –0 4 s	
			211 P b	3 6 .1 m i n		210 Tl	1 . 30 m i n	
			211 P o	2 5 . 2 s		210 P b	2 2 . 6 y	
			211 B i	2 . 14 m i n		210 B i	5 . 01 d	
			207 Tl	4 . 77 m i n		210 P o	1 3 8. 4 d	
			207 P b	( s t a b l e )	( s t a b l e )	206 H g	8 . 2 m i n	
						206 Tl	4 . 20 m i n	
						206 P b	( s t a b l e )	( s t a b l e )


b	y–years; d–days; h–hours; m i n–m i nutes; and s–seconds.
c	 –alpha decay;  –negative beta decay; EC–el ectron capture; and IT–isom e ric transition (radioactive transition from one nuc lear isom er to another of lower energy).

Cosm ogenic Rad i ation

Cosmogenic Nuclid es


Nuclide	Half-life	Source	Specific Activity
C-14	5730 yr	Cosm ic-ray interactions, 14 N(n,p) 14 C	~15 Bq/g
Tritium	12.3 yr	Cosm ic-ray interactions with N and O; spallation from cosm ic-ray s , 6 Li(n,alpha) 3 H	1.2 x 10 -3 Bq/kg
Be-7	53.28 days	Cosm ic-ray interactions with N and O	0.01 Bq/kg

Som e other cosm ogenic radionuclides are 10 Be, 26 Al, 36 Cl, 80 Kr , 14 C, 32 Si, 39 Ar, 22 Na, 35 S, 37 Ar, 33 P, 32 P, 38 Mg, 24 Na, 38 S, 31 Si, 18 F, 39 Cl, 38 Cl, 34m Cl.

Track structure of a cosm ic ray collision in a nuclear em ulsion

Variations in cosmic ray intensity at the earth’s surface are due to:

 Tim e : sunspot cycles

 Latitude: magnetic field lines

 Altitude: attenuation in the upper atm o sp here

Dose at the surface from cosmic rays

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Internal Radi ati o n

What makes a radionuclide biologically important?

 Abundance (both elemental and isotopic)

 Half-life

 Decay scheme (emission type and energy)

 Chemi cal state

 Chem ical behavior i n the body

 Does it co ncentrate?

 Ultimate location

 Rat e of ex cretion

How do the series radionuclid es contribute to our dose? Inhalation

Isotopes of radon (inert gas, but m a y decay in the lung)

Dust; e.g., our m a in source of uranium is due to resuspension of dust particles from the earth. Uranium is ubiquitous, a natu ral constituent of all rocks and soil.

Externally - gamma emission occurs in most decay steps.

Internally -Consum ption in food and drinking water

Natural Radioactivity in the body


Nuclide	Total Mass of Nuclide	Total Activity of Nuclide	Daily Intake of Nuclides
Uranium	90 µ g	30 pCi (1.1 Bq)	1.9 µ g
Thorium	30 µ g	3 pCi (0.11 Bq)	3 µ g
Potassium 40	17 m g	120 nCi (4.4 kBq)	0.39 m g
Radium	31 pg	30 pCi (1.1 Bq)	2.3 pg
Carbon 14	95 µ g	0 . 4 µ Ci (15 kBq)	1.8 µ g
Tritium	0.06 pg	0.6 nCi (23 Bq)	0.003 pg
Polonium	0.2 pg	1 nCi (37 Bq)	~0.6 µ g

It would be reasonable to assum e that all of the radionuclides found in your environm ent would exist in the body in some sm all am ount. The inte rnally deposited radion uclides contribute about 11% of the total annual dose.

Uranium

 Present in all rocks and soil, and t hus in both our food and in dust.

 High concentrations in phospha te rocks (and thus in commercial fertilizers).

 Absorbed by the skeleton which receives roughly 3 µ Sv/year from uranium .

Radium

 Also present in all rocks and soils.

 Food is a m o re im portant sourc e of intake

 226 Ra and its daughter products (beginni ng with 222 Rn) contribute the major dose com ponents from naturally occurring internal em itters.

 Dissolves readily, ch em ically sim i lar to calcium.

 Absorbed from the soil by plants and passed up the food chain to hum ans

 variations in Ra levels in soil lead to variations in Ra levels in food

 80% of the total body Ra is in bone (~7mrem / year).

 The rest is uniform ly distributed in soft tissue.

Thorium

 Lots in dust but little is incorporated in food

 Thorium is present in the highest c oncentrations in pulm onary lym ph nodes and lung, indicating t h at the principle source of exposure is due to inhalation of suspended soil particles.

 Ultimately a bone seeker with a long residence time

 Since it is very slowly rem oved from bone, concen tration increases with age.

Lead

 Also a bone seeker, half-life in bone is ~ 10 4 days.

Polonium

 Unlike other naturally occurring  -em itters, 210 Po deposits in soft tissue not bone.

 Two groups exist for which the dose from 210 Po is apt to be exceptionally high.

o Cigarette sm okers

o Residents of the nort h who subsist on caribou and reindeer.

 Reindeer eat lichens t h at absorb tr ace el ements in the at mosphere ( 210 Po and 210 Pb). The 210 Po content of Lapps living in nort h ern Finland is ~12 times higher tha n the residents of southern Finla nd.

 Liver dose in the Laplanders is 170 m r em /year com p ared to 15 m r em /year for those in the south.

Doses from Medical Applications

Commercial Air Travel

Calculated cosmic ray doses to a p erson flying in subsonic and supersonic aircraft under normal sola r conditions


Route	Subsonic flight at 36,000 ft (11 km)			Supersonic flight at 62,000 (19 km )
	Flight duration (hrs)	Dose per round trip		Flight duration (hrs)	Dose per round trip
	Flight duration (hrs)	( m r a d )	( µ G y )	Flight duration (hrs)	( m r a d )	( µ G y )
Los Angeles- Paris	1 1 . 1	4 . 8	4 8	3 . 8	3 . 7	3 7
Chicago-Paris	8 . 3	3 . 6	3 6	2 . 8	2 . 6	2 6
Ne w York -Paris	7 . 4	3 . 1	3 1	2 . 6	2 . 4	2 4
Ne w York - London	7 . 0	2 . 9	2 9	2 . 4	2 . 2	2 2
Los Angeles-New York	5 . 2	1 . 9	1 9	1 . 9	1 . 3	1 3
Sydney-Acapulco	1 7 . 4	4 . 4	4 4	6 . 2	2 . 1	2 1

Issues:

 Shoul d airline people be considered ge neral public? or radiation workers?

 What about corporate aviation? (altit udes alm o st as high as supersonic Concorde but travel is sub-soni c and thus time in air is high)

 Business travelers: frequent fliers have no restriction of # hours per year in flight.

 What about pregnant wom e n?

 Shoul d the traveling public be alerted to sunspot activity?

 Is legal action possibl e?