Transcript Illinois Institute of Technology

Illinois Institute of Technology
Physics 561
Radiation Biophysics, Lecture 10
Deposited Radionuclides;
Exposures to Radiation
1 July 2014
Andrew Howard
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Class Overview
u
Radionuclides, continued
– Routes of entry
– Physics & chemistry of
nuclides
– Dosimetry and activity
– Tritium, noble gases
– Alkali metals
– Alkaline earths
– Halogens
– Uranium & plutonium
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Exposure from natural and
man-made sources
– Population dosimetry
– Dose equivalent and
equivalent dose
– Radiation weighting
factors
– Natural sources
– Man-made sources
u
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Internally deposited radionuclides
Why radionuclides are studied in
the context of internal deposition
• Exposure works differently from
external exposure: acts over shorter
length scales
• Often involves high-LET forms that
would never have biological effects if
they were external
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How do they get in?
u
u
u
Ingestion - intake in food & water though GI
tract & tracheal clearance
Inhalation - breathed-in radionuclides traveling
through nasopharyringeal passages to the lung
Injection - only intentional
(except in bad Hollywood movies)only relevant in a few therapeutic contexts
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Ingestion
u
u
Intake through digestive system
Various fates:
–
Excretion
u
u
–
–
–
Urine
Feces
Incorporated into blood,
e.g. via glutathione conjugation
Incorporation into lymph
Bile with radionuclides that have
collected into the liver out of the
circulatory system can be secreted
back into the digestive system
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Inhalation
u
Respiratory system:3 compartments:
–
–
–
Deposition (graph sideways from book-fig. 15.2):)
% deposition
u
Nasopharyngeal (NP)
Tracheobronchial (TB)
Deep-lung parenchyma (P)
90
70
50
30
20
10
5
1 0.1
DP
DN-P
DT-B
0.2
0.5
1.0
2.0
5.0
10
Activity mean aerodynamic diameter, µm
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Inhalation: Fate of Radionuclides
u
u
u
u
u
Radionuclides enter respiratory system via
nose & mouth
Travel through trachea
Either travel farther down to bronchi & lungs or
are sent back up to be exhaled or swallowed
Physical fate primarily function of size & shape
Size Matters!
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What happens to nuclides if they get
into the deep lung?
Fate depends on chemistry
If particles are moderately to very water-soluble,
they pass into the bloodstream readily
–
–
There’s a lot of surfactant (detergent) lining the lung
surface that helps to solubilize things
once in the blood, the compounds get metabolized
or cleared or both
If the material is very insoluble it gets gobbled
up by macrophages
–
–
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Particles go to lymph nodes inside macrophage
Ultimately the lymph empties into the blood
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Physics and Chemistry of Inhaled
Radionuclides
Shape matters, too!
Biological response depends substantially
on shape because cells react very
differently to needles as compared to cubes
–
–
–
Asbestos: caused mostly by needleshaped fibers, independent of their
chemical nature
Spheres of the same compounds would
be harmless
Macrophages respond peculiarly to
needle-shaped particles
Surface area to volume ratios influence
biological fate!
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Chemistry of
Radionuclides: General
Chemistry is neutron-independent, i.e. every isotope
behaves identically (exception: 3H) . . . (until decay
occurs)
Nuclides of elements without ordinary biological
function are metabolized approximately like their
nearest vertical neighbors in the periodic table
–
–
Not entirely successful substitutions
Sometimes: Very small discrimination ratio
Alkali metals: Li, Na, K, Rb, Cs, Fr
Elaborate mechanisms for handling K; none for Rb
so Rb tends to behave like K (but not like Na).
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Chemistry and Metabolism
Periodic Table of the Elements
Lanthanide Series
Actinide Series
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Dose to Specific Organs
Distribution over time can be complex
It takes some time for each organ to receive
its dose, and as things clear (both physically
and biologically) the concentrations will
diminish
Note in fig. 15.3 that the 131I dose to the
thyroid is not predominant over the others
because it’s such a small organ
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Fig.15.3, my version
Distribution of 131I
% of administered activity
100
10
Stomach
Lungs
Thyroid
Extracellular
Whole Body
1
0
10
20
30
40
50
60
70
80
0.1
Time, hrs
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Complex Case: 99mTc
Three emissions with different activities and
energies
Effective D value depends on tabulating
individual contributions
Absorbed dose depends on applying this D value
in a Monte Carlo analysis of deposition in various
organs: see tables 15.2 and 15.3 in the text
Further variability (beyond limitations of these
models) come from the fact that real people
aren’t identical to the “standard man”
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Dosimetry of specific nuclides
We describe equivalent dose as
HT,R = wRDT,R
where DT,R is the absorbed dose in tissue T from
radiation type R, wR is the radiation weighting
factor for this radiation type, and HT,R is the
equivalent dose actually experienced by the tissue
This is similar to the concept of RBE except that it
emphasizes that tissues respond differently to
different types of radiation.
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Tritium
TP=12.3y
Mostly in the form of water
Turnover: TB = 10 days, like ordinary water
Low-energy beta and it’s cleared quickly so
the hazard is pretty low.
Some 3H can get incorporated into
macromolecules—that could be more severe.
Hydrogens in different organic molecules
have different exchange rates; T~10-10s for
hydrogens attached to N or O; T ~ minutes or
forever for C-H hydrogens.
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Example from Amino Acids
Serine has C-H, N-H, and O-H
bonds in it.
Hydrogens attached to carbons are
essentially non-exchanging
Hydrogens attached to N and O
exchange in microseconds or faster
Exchange lives of some NH and OH
are longer because they’re solventprotected.
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Exchangeable Hydrogens in DNA
The hydrogens on
the ribose ring of
DNA are not
exchangeable: in
fact, there are
very few
exchangeable H’s
in DNA!
(e.g. amine H’s)
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Noble Gases: Krypton and Radon
85Kr
–
–
common in nuclear power
Not incorporated in the body much
because it’s not very reactive
Therefore not a serious biological hazard
Radon is important:
we’ll talk about it in chapter 16.
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Alkali Metals
Sodium isotopes are used in diagnostics
40K is an important background irradiator: see
chapter 16
Cesium (and rubidium) are produced in fission.
–
–
–
–
–
137Cs
ended up in the atmosphere as a
component of fallout
Behave like potassium
TB ~ 50-150 days
TP ~ 30 y so biological clearance dominates
excretion has 2-component model
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Two-Component Model
Fraction Remaining
Model looks like
A(t) =
A0[p1exp(-l1t) +
p2exp(-l2t)]
With
p1 + p2 = 1
Two-compartment behavior
1
0
1
2
3
4
5
6
0.1
0.01
0.001
A1
A2
0.0001
Time, sec
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7
Alkaline Earths
These are typically divalent (2+)
Be, Mg, Ca not very important except as research
subjects
Sr has practical significance
90Sr is major component of fallout from nuclear
–
weapons testing.
90Y90Zr) so it’s dangerous
– Tp = 28 y ( to
89Sr is important too (T = 50.5 d, - to 89Y)
–
p
– Sr is a Ca analog and tends to concentrate in
tissues where Ca2+ is supposed to concentrate
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Alkaline earths, concluded
(Tp =12.8d, - to 140La140Ce)
common in fallout and reactor output; but it
has a short half-life
226Ra (T =1600Y,  to
p
222Rn218Po214Pb214Bi214Po210Pb
210Bi206Tl206Pb) is important too
140Ba
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Sr and Ra retention and effects
Ra retained somewhat less than Sr,
but a reasonable amount stays around for years
Did childhood leukemias increase in the US because
of fallout in the 1950’s? Unclear; it’s hard to get
unambiguous evidence of environmental effects on
human health for anything except smoking.
Radium dial painters got sarcomas of bone and
carcinomas of the sinus epithelium
224Ra was used in treatments after WWII
It can be used to quantitate Pu carcinogenicity
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Halogens: Iodine
Iodine is concentrated in the thyroid
131I is an important fission product
– Short physical half-life (8 days,  to
131Xe)
–
Moves quickly through the food
chain via milk
125I
used in imaging & brachytherapy:
Td=59.4 d, EC to 125Te
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Iodine,
concluded
Major releases of 131I:
–
–
–
Chernobyl (1986)
Windscale nuclear
plant in England (1957)
Flawed episode of West
Wing, season 7
(“Duck & Cover”)
Radioactive iodine can be
competed away with iodine
in table salt
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Uranium
Naturally occuring even though all its isotopes
are radioactive
Precursor of other high-Z elements
238U is common— long half-life
235U is 0.7% of natural mixture
–
–
–
percentage can be reduced (“depleted”)
Or enhanced (“enriched”)
Undergoes fission when bombarded with slow
(“thermal”) neutrons
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Uranium, continued
Plentiful in reactors and weapons
Toxic to kidney, independent of radioactivity
Decay modes:
–
–
 emitter to 231Th  … 207Pb, T1/2 = 7*108 y
238U : much more common (99.3%)
 emitter to 234Th  … 206Pb, T1/2 = 4.5*109 y
235U:
In one year, 1g of 238UO2 produces 5.70*10-13
moles of 234ThO2 = 0.295 ng, generating 3.44*1011
depositions along the way = 1090 dps = 0.295
µCi.
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Plutonium
Two common isotopes:
238Pu: T
1/2 = 86.4y
and 239Pu : T1/2 = 24890 y
Inhaled Pu in lung:
cancer, some lymphatic-system
damage
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Is Pu the most toxic substance?
Is it the most toxic substance in the world?
–
–
Available exposure routes limited
I wouldn’t want to eat it, but there are worse
toxicants even among metals; and biohazardous
substances (e.g. aflatoxin) are much nastier on a
per-g or per-mole basis
Exposure through fallout: 400 megacuries
worldwide.
But it still has to get inside us to do damage.
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How nasty is Pu, really?
It’s probably about as carcinogenic as
radium if it’s deposited in the skeleton
Lung tumors are likely with inhalation of
large quantities;
possibility of lavage exists as a mitigation
Depositions of Pu elsewhere might be
cancerous if they stay around long enough
Kidney toxicant, like Uranium? Probably:
the actinides have chemistries that are
similar one to the other
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Important nuclides, Z ≤ 80
Nuclide
mode product final
3H
3He
3He
40K
40Ar
40Ar
EC
85Kr
85Rb
85Rb
87Rb
87Sr
87Sr
89Sr
89Y
89Y
90Sr
90Y
90Zr
99m Tc
99Tc
99Ru

125I
125Te
125Te
EC
131I
131Xe
131Xe
137Cs
137Ba
137Ba
140Ba
140La
140Ce
* Misstated in Alpen as 128d
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T1/2
12.33y
1.28*109y
10.8 y
4.75*1010y
50.5d
28.8y
6h
59.4d
8.0d
30.1y
12.8d*
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comment
reactor byproduct
naturally occurring
reactor byproduct
naturally occurring
fallout
significant in fallout
medical imaging
imaging reagent
fallout: found in milk
weapon byproduct
weapons, reactors
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Important radionuclides, Z > 80
Nuclide
mode product final
219Rn
215Po
207Pb

222Rn
218Po
206Pb

223Ra
219Rn
207Pb

224Ra
220Rn
208Pb

226Ra
222Rn
206Pb

232Th
228Ra
208Pb

233U
229Th
209Bi

235U
207Pb
,SF 231Th
238U
234Th
206Pb

238Pu
234U
206Pb

239Pu
207Pb
, SF 235U
SF= spontaneous fission
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T1/2
4.0s
3.8d
11.4d
3.66d
1600y
1.4*1010y
1.59*105y
7.04*108y
4.47*109y
87.7y
24110y
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comment
gas; see next chapter
gas; see next chapter
on 235U chain
on 232Th chain
on 238U chain
pseudo-stable!
detectable
detectable; fissionable
predominant U isotope
fallout; space vehicles
fallout, reactors
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Human exposures to ionizing radiation
This is the last chapter in Alpen, but clearly it’s not the
final part of what we will discuss in this course
We want to offer you a fuller understanding of some
special topics, including hormesis, as well as a brief
introduction to general biochemistry.
Those additional lectures are intended to provide you
with a better context for the material that Alpen offers.
Anyway …
the point of the remainder of this lecture is to consider
human exposures to ionizing radiation, whether they’re
natural or anthropogenic.
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What can and can’t we control?
Significant sources of risk from exposure to
ionizing radiation to the population as a whole
–
–
Natural background
Diagnostic applications of ionizing radiation
Anything else?
–
–
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Therapeutic X-rays and isotopes:
few people so population dose is tiny
Consumer applications (e.g. Cathode Ray
Tube TV receivers): tiny per-person dose
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The Meta-question:
Are low doses worse than zero doses?
Clearly most people don’t get exposed to
distinctly high doses of ionizing radiation
We’re going to spend some time thinking
about how to quantitate and how to assess
risk from various sources, particularly as part
of natural and man-made background
But:
Do we really know that these background
levels pose a net risk; or is the hormesis
concept operating here?
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Population Dosimetry
Biologically effective dose from n different sources:
We can describe the dose equivalent as H = Si=1n DiQiNi
–
–
–
Di is dose from source or radiation type i
Qi corrects for LET-dependent biological effectiveness
Ni corrects for nonuniformities in distribution and anything else
This is an old way of doing things...
Reference source of exposure:
Whole-body exposure to  rays
Adjustments to dose depend on
–
–
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Type of radiation
Portion of person irradiated
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Why Consider Equivalent Dose?
It enables us to compare radiation types & exposure
modalities on a quasi-equal footing
Unit of equivalent dose: Sievert corresponds to the
Gray for dose. Rem corresponds to rad.
1 Sv = 1 Gy if using reference exposure
Equivalent dose HT = SRwRDT,R
(Might underestimate health risks of the reference
source!)
High LET sources have high radiation weight factors,
up to a point (see last week’s lectures)!
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Radiation Weight Factors, wR
Radiation type
wR
Xrays,  rays
1
e-, e+, µ
1
Neutrons
– <10keV
5
– 10-100keV
10
– 100-2000 keV 20
– 2-20MeV
10
– > 20 MeV
5
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Protons (non-recoil),
> 2MeV
2
 particles,
fission fragments
20
Relativistic heavy ions 20
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Effective Dose
Effective dose E is defined so that the
probability of cancer and genetic effects is the
same no matter where and how uniformly the
deposition occurs:
E = STwTHT = STRwTwRDT,R
Implicit here is the concept that wR is
independent of wT, which isn’t completely true;
but it’s close enough given how vague the
values are!
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Tissue Weight Factors, wT
wT=0.01
wT=0.05
wT=0.12
wT=0.20
Bone surface
Skin
Bladder
Breast
Liver
Esophagus
Thyroid
Remainder
Bone Marrow
Colon
Lung
Stomach
Gonads
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What are we trying to do here?
These tissue weighting factors are taking into
account the portion of the radiation that ends up
in the relevant tissue
That, in turn, depends on the tendency to
concentrate certain atoms in particular organs
But it also depends on the actual size (fraction
of total body weight) associated with that organ
Consider iodine in the thyroid and calcium in
bone; but most other nuclides are more
promiscuous in their distribution.
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Committed Equivalent Dose
ICRP idea looking at time-integral over time 
of equivalent dose rate in a specific tissue T
following intake
Thus for a single nuclide absorbed at time t0,
HT() = ∫t0t0+ (dHT(t)/dt)dt
Usually  is taken as 50 years for occupational
exposures and 70 years for general public
Smaller  values apply to already-aged
populations
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Committed Effective Dose
Add up the committed equivalent doses
over all the tissues irradiated
E() = ST wTHT() =
ST wT∫t0t0+ (dH(t)/dt) dt
Units for all of these are Sieverts
wR is like Q in the dose equivalent definition
wT is like the N value in the dose equivalent
definition
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Collective Dose
Collective Dose S: the aggregate dose associated
with an exposure received by a population of N
individuals
Thus if we know <E> and N then
(collective dose) S = <E>N
is proportional to expected number of cases of
disease (assuming linear response and 0 threshold).
Multiple populations (subgroups i = 1, . . . N):
S = SiHiPi
Thus if at collective dose level E, the probability
P(cancer) = 10-5, then in a population N = 106 we
expect ~10 cancer cases
This notion works well with stochastic endpoints
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Risk Factors
Weighting factor for any organ is the ratio of the
risk for that organ to the total risk
These estimates involve
–
–
–
Fatal cancers
Genetic risk
Life shortening
Purpose:
risk-weighted dose estimate for a mixture of types
of radiation or for radiation of parts of the body
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Total Stochastic Detriment, part I
Lifetime risk coefficients, 10-2 Sv-1
Organ
Popu- wT
Rad
wT
lation
Workers
Bladder
0.29 0.040
0.23
0.042
Bone Marrow
1.04 0.143
0.83
0.150
Bone Surface
0.07 0.010
0.06
0.011
Breast
0.36 0.050
0.29
0.052
Colon
1.03 0.142
0.82
0.148
Esophagus
0.24 0.033
0.19
0.034
Liver
0.80 0.110
0.64
0.116
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Total Stochastic Detriment, Concluded
Organ
Popu- wT
lation
Rad
wT
Workers
Ovary
Skin
Stomach
Thyroid
Remainder
Gonads (genetic)
Grand Total
0.15
0.04
1.00
0.15
0.59
1.33
7.25
0.12
0.03
0.80
0.12
0.47
0.80
5.53
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0.021
0.006
0.138
0.021
0.081
0.183
1
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0.022
0.005
0.145
0.022
0.085
0.145
1
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So let’s look at specifics
Up to now we’ve been arming you with a
few analytical tools.
Now we’ll look at actual sources of
background
–
–
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Natural
Anthropogenic
Medicinal
Occupational
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External Natural Sources
A few natural sources matter:
Soil-borne radionuclides
Airborne radionuclides
Cosmic rays from deep space
Total Exposure from these sources:
around 0.7 - 3 millisieverts per year
Dominated by U-Th series (mostly Rn),
40K, 87Rb, Cosmic rays
Varies significantly with altitude
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Why 40K and 87Rb?
Very long half-lives
40K:
–
–
–
–
T1/2 is 1.3*109y, i.e. about 0.25* age of the earth
K common in earth’s crust so there’s a lot of 40K
40K is only 0.01% of total K, though
2 decay modes: EC or + +  to 40Ar or - to 40Ca
87Rb
–
–
–
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(- to 87Sr)
even longer T1/2: 5.0*1010y (3* age of universe)
27.2% of the natural abundance
But there isn’t much Rb in the soil
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Radon as a natural source
Substantial concern in recent years about
radon such that wR = 20 for Rn alphas and
decay products.
n.b.: should we actually consider indoor
radon a natural source?
Pay attention to fig. 16.1!
– Nuclear industry, consumer products, air travel are
trivial for the general population
– Most of the natural background is Rn daughters
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Distribution of Doses
Natural background and
medical dose dominate
This is a redrawing of Alpen’s
fig. 16.1, corrected.
Annual dose, mSv
2.4
1
Natural
Background
Medical
Fallout
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Nuclear Consumer
Industry Products Air Travel
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What are the natural sources?
Series Primordial radionuclides starting
mostly from 238U, 232Th, and 235U;
especially 222Rn from 226Ra
Nonseries primordial radionuclides: 40K, 87Rb
Cosmogenic radionuclides:
Elements in earth’s crust or in atmosphere
interact with cosmic ray: mostly 14C, 3H, 22Na
(and 7Be)
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Outdoor, Extracorporeal Sources
Mostly uranium-thorium series
but also some 40K
Varies widely from place to place:
150-1400 µGy per year,
depending on where you are
What matters is gamma emitters here: wR=1
because nothing else gets into the skin
Typical 222Rn exposure is 232µGy per year;
with wR=20, that’s almost 5 millisieverts—high
compared to Alpen’s published totals
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How variable is the background?
An extreme case:
Ramsar, Iran:
260 mGy (not mSv!) natural
background
Mostly 226Ra in hot springs
Some U, Th in minerals
No evidence that the local
population is suffering from
this exposure
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Indoor, Extracorporeal Sources
Depend slightly on the method of construction
–
–
–
What is the source of the building material?
How leaky is the building (especially for 222Rn);
Well-insulated buildings deliver a high body
burden because we’re trying not to have to heat
them so much--so the gas stays in the house
Therefore, as building practices improve, the Rn
exposure increases
Almost all uranium-thorium series stuff
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Inhaled Radionuclides
222Rn
is the inhalation culprit
Study in New York gave some big numbers
for total body burdens received:
1.9-3 mGy/year, I.e. around 38-60
millisieverts!
This is actually equal to the occupational
limit, so something is wrong: either we need
to get radon out of our houses or we need
to revise the occupational limits upward.
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Nonseries Radionuclides
40K
and 87Rb get into the body through
ordinary metabolism. Beta emitters, wR=1.
Typical doses for 40K:
–
–
–
180 µSieverts/year to gonads
60 µSieverts/year to bone
270 µSv per year to bone marrow
Typical doses for 87Rb:
–
–
–
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10 µSieverts/year to gonads
<10 µSieverts/year to bone
10 µSv per year to bone marrow
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Cosmic Rays and Cosmogenic
Radionuclides
Varies a lot by location
–
–
–
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The higher in altitude you are,
the more you get
Also, the portion contributed by neutrons
goes up as you go higher in altitude
Some variation by latitude and longitude
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Cosmic Radiation and Altitude
Dose and equivalent dose rate
go up with altitude
2
2
Mt.
Whitney
1.0
1
0.5
Chicago
Albuquerque
0.5
Sea
0.25Level
0
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0.25
Altitude, Kilometers
4 km
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World Summary (cf. fig. 16.4)
All but 5% of UTh series is
222Rn daughters
1
Cosmogenic
nuclides
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Cosmic rays
40K
+ 87Rb
U-Th
internal
Rad Bio: Radionuclides
U-Th
external
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Human exposures, worldwide
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Artificial sources
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Another perspective
Human exposures, mSv/yr
Medical
X-Ray
0.38
Terrestrial
0.25
Courtesy Emory
University Radiation
Protection program
Consumer
Products
0.1
Internal
Radon
0.29
2
Nuclear
Medicine
0.14
0.27
Cosmic
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Effective Dose, mSv Yr-1, US+Canada
from natural sources
Source
Lung
WT value
0.12
Cosmic rad
0.03
Cosmogenic
0.001
radionuclides
Terrestrial:
External
0.03
Inhaled
2.00
Nuclides
0.04
In the body
Totals
2.1
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Gonads
0.25
0.07
0.002
Bone
0.03
0.008
—
Marrow Other
0.03
0.03
0.03
0.13
0.004
0.003
Total
1.0
0.27
0.01
0.07
—
0.09
0.008
—
0.03
0.03
—
0.06
0.14
—
0.17
0.28
2.00
0.40
0.23
0.05
0.12
0.44
2.96
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Man-Made Sources
Major contributors to population dose are:
–
–
Medical diagnostic procedures
Smoking (!) 210Pb, 210Po
Individual burdens:
–
–
–
Diagnostic
Smoking
Therapeutic use of x-rays, 
Recall discussion of
additive vs. multiplicative risk
I encourage you to read the details about
anthropogenic sources, but I won’t test you in detail
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Diagnostic X-rays
US bone marrow dose ~ 1mGy = 1 mSv
US genetically significant dose ~ 0.3 mGy
Methods getting better but more procedures are done
Weighted annual effective dose ~ 0.36 mSv
Age and gender-specific adjustments bring that down
to about 0.23 mSv.
Values in 3rd-world countries are lower in spite of the
patients’ experience of higher doses per treatment
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Nuclear medicine
US value ~ 140 µSv; lower elsewhere
Includes nuclides for cardiac function tests, PET
Effective dose is about 0.6 * diagnostic X-ray value
Effective Dose Equivalent, HE, mSv
Genetically significant dose, mGy
Nuclear
medicine
Diagnostic
X-Ray
Bone marrow dose, mGy
0
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0.2
Dose,
mGy or mSv
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Rad Bio: Radionuclides
0.4
0.6
0.8
Nuclear Power
Fuel cycle: mining, milling, refining, UF6,
enriched UF6 (sublimes at 56.5ºC)
Fuel fabrication, power generation, reprocessing,
waste disposal, fuel storage, transportation
These are different from medical background in that
they’re regional rather than global
A sub-population is particularly at risk: namely, miners
and neighbors of the facilities that handle these
compounds
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