Transcript Fundamentals of Radiation

Science Teacher Workshop
“Get a Half- Life”
NC Chapter - Health Physics Society
NCSU – Nuclear Engineering Dept
American Nuclear Society
Basics of Radiation
NOTE


This is a revision of the talk presented
at the NCSTA on 11/12/04.
The threshold value on the Radiation
Bioeffects slide(slide 23) is 100 Rem for
deterministic effects.
Theories and Models


Theory: tentative explanation of observed
phenomena; never proven; represents most
logical explanation based on currently available
evidence; becomes stronger as more
supporting evidence is gathered; provides a
context for predictions
Model: useful way of describing and explaining
interrelationships of ideas; can be mental,
physical, and/or verbal representation of an
idea; represent what we know about an idea or
concept; under constant change as new data
are obtained
[NC Science Standard Course of Study & Grade Level Competencies]
Models of Atoms
Quantum Mechanical
Model of Electron Orbits
Bohr
Model
+
Proton
(nucleus)
1s, 2s
-
Electron
Electron orbits nucleus
~1015 times per second
2p
2d
What Is “Radiation”?
Ionizing
Non-Ionizing
ionizes [strips electrons
from] atoms; includes:
many other modes of
interaction; includes:
Particulate
-alpha
-beta
-neutron
- etc.
Electromagnetic
“Radioactive” vs.
“Radiation”
Radiation
Source
[radioactive
material or X-ray
device]
Irradiated
Material
Radiation
“Nuclide” vs. “Radionuclide”


Nuclide - general term referring to any known
isotope, whether stable (about 290) or unstable
(about 2200), of any chemical element
Radionuclide - a radioactive nuclide
- Shleien (1998), pp. G12 & G17
Nuclide Designation:
A
Z
X
Where:
A = the atomic mass (no. protons + no. neutrons)
Z = the atomic number (number of protons)
X = the symbol for the chemical element
Note: Physical & Chemical properties depend only on Z!
Why Some Atoms Decay:
Nuclear Forces and Stability
Beryllium
Atom [6Be]
Forces acting on
6Be nucleus
nuclear
force
n
p
p
n p n
nucleus
electrons
proton
neutron
electrostatic
repulsion
Z (number protons)
The Curve of
Nuclear Stability
Unstable – too
many protons
N=Z
N ≈ 1.5 Z
Unstable – too
many neutrons
N (number of neutrons)
Z (number protons)
Chart of the
Nuclides
Unstable – too
many protons
Unstable – too
many neutrons
N (number of neutrons)
[Brookhaven National Laboratories - http://www2.bnl.gov/CoN/]
Principal Types of
Ionizing Radiation
PARTICULATE
ELECTROMAGNETIC
Alpha () – helium nucleus
Gamma () - photon
- from heavy nuclei [Z>82] X-ray (X) – photon
Beta () – electron
<250 keV max: "Low Energy
Beta"
>250 keV max: "High
Energy Beta"
Neutron (n) – uncharged
Note: It is customary to categorize radionuclides according to
the type of radiation emitted.
What About X-rays?
BREMSSTRAHLUNG X-RAYS
high speed
electron
target nucleus
[e.g. tungsten]
X-rays
CHARACTERISTIC X-RAYS
“hole” at lower orbital
X-ray
electron falls into
lower orbital
Penetrating Abilities of
Various Radiations

Radiation Source




ALPHA
Paper [or dead layer of skin]
Plastic
Lead or Concrete
 BETA



1
GAMMA & X-RAYS
,
0n
X
NEUTRON
Water
All Together Now:
14C
 14N +  (low energy; 156 keV max)
[T½=5730 y]
32P

32S+
(high E; 1,156 keV max)[T½=14.3 d]
60Co
 60Ni +  (high E; 318 keV max) + 's
(1.17 MeV & 1.33 MeV) [T½ = 5.27 y]
238U
 234Th + α’s (4.15 MeV & 4.2 MeV) +  (50
keV) [T½ = 4.5 x 109 yr]
Radioactive Decay
& Half-Life [T½]
Decay Constant [λ] = 0.693/T½
 Ao
1.0 __
_
A(t)/Ao
½
¼
0
__
_
__
_
__
_
__
_
__
_
__
_
__
_
0
A(t) = Aoe-λt = Aoe-(0.693/T½)t
= Ao(½)t/T½

Negative Exponential Decay Curve


1T½ 2T½ 3T½
time

4T½


5T½
6T½
Specific Activity
[activity per unit mass or volume]

Inversely proportional to Half-Life



Pure Phosphorous 32 (T½ = 15 days)


Long half-life → low specific activity
Short half-life → high specific activity
Sp. Act. = 286,000 Ci/g
Pure Carbon-14 (T½ = 5730 years)

Sp. Act. = 4.5 Ci/g
Radioactivity Units
Activity – Amount of radioactive material
curie (Ci): 3.7x1010 disintegration/second
18671934
• 1 Ci = a lot of of activity [based on 1 g radium]
• adult human has ~0.1 microcurie (µCi) 14C
18591906
becquerel (Bq): 1 disintegration/second
• 1 Bq = tiny amount of activity [SI unit]
1852-1908
• adult human has ~3,700 Bq
14C
1 µCi = 37 kBq = 2.22x106 dpm [disintegration/minute]
Interactions of
Radiation with Matter



Ionization: ejection of orbiting electrons from
the atom [Gollnick (1994), p. 51]
Excitation: raising of orbital electrons to
higher energy levels within the atom [Gollnick
(1994), p. 51]
Activation: the process of making a material
radioactive by bombardment with neutrons,
protons, or other nuclear radiation [Shleien
(1998), G-1]
Units of Radiation Dose
Quantity
Unit
Applicability
Exposure:
ionization per unit
mass air; only for
gamma & X-ray
Old: roentgen [R] =
2.58x10-4 C/kgair
(C = coulomb)
SI: no SI unit
Obsolete but still on
many direct reading
instruments;
Absorbed Dose:
energy absorbed
per unit mass
Old: rad=100 erg/g
SI: gray [Gy] 1 J/kg
1 Gy = 100 rad
Short term dose
effects; generally if
Dose Equivalent:
absorbed dose 
weighting factor
based on rad. type
Old: roentgen equiv.
man (mammal)
[rem]=radQ
SI: sievert
[Sv]=GywR
Used for latent
effects (e.g. cancer,
genetic effects); if
dose<100 rad(1Gy)
1 R  1 rad
dose>100rad(1 Gy)
U.S NRC Quality Factors (Q)
Type of radiation
Q
X-, gamma, or beta
Alpha particles, multiple-charged
particles, fission fragments & heavy
particles of unknown charge
1
20
Neutrons of unknown energy
10
High-energy protons
10
- from 10 CFR §20.1004; weighting factors from other
organizations ( e.g. ICRP, NCRP, ICRU) may differ
Radiation Interaction: Main
Chemical Effects in Tissue

Primary reactions [within ~10-10 seconds of passage
of ionizing radiation] - Water molecule dissociates
into free radicals:
H2O  H + OH

Secondary reactions [subsequent 10-5 seconds]
H + H  H2 (gas)
H + OH  H2O (water)
OH + OH  H2O2 (hydrogen peroxide)
from Gollnick (1994)
Radiation Bioeffects
DETERMINISTIC




Chronic vs Acute
Severity increases with
radiation dose
Threshold ~ 100 Rem
Dose & dose rate
dependent
Examples:




Cataract induction
Epilation
Erythema
Blood changes
STOCHASTIC


Probability of occurrence
increases with radiation
dose
Threshold ~10 rem, but
regulatory models
assume no threshold
[ALARA!]
Examples:



Cancer Induction
Genetic Mutations
Developmental
Abnormalities
- NCRP 138 (2001), p. 28; HPS (1995) “Risk Assessment”
Deterministic
Radiation Effects
Health Effect
Blood cell depression
Reversible skin effects
Permanent sterility
Vomiting
Temporary hair loss
Permanent sterility
Skin erythema
Organ
Bone Marrow
Skin
Ovaries
Dose (rad)
50
200
250 – 600
GI
Skin
Testis
300
300 – 500
350
Skin
500 – 600
- [Acute, low LET dose] NCRP 138 (2001)
Stochastic Effects


Cancer: incidence begins to increase in populations
acutely exposed to >10 rem [0.1 Sv], continues to
increase with increasing dose. –BEIR V, 1990
Genetic Effects:  100 rad of low-dose rate, low LET
radiation needed to double the incidence of genetic
defects in humans. -BEIR V, 1990; no human hereditary
effects seen at gonadal doses <0.5 Gy (50 rad) –NCRP
138 (2001)


In Utero Irradiation: developmental & other effects
begin to increase at ~10 rem - NCRP 138 (2001)
Conclusion: “…assessments of radiological risk [should]
be limited to dose estimates near and above 10 rem.” –
HPS Position Statement (1995)
Annual Dose to Member of the
U.S. Population [NCRP 93 (1987)]
Consumer
Products
Artificial 18% Nuclear 3% Other
< 1%
Medicine
4%
Medical X-rays
11%
Cosmic
(Outer Space)
8%
Terrrestrial
(Rocks & Soil)
8%
Internal
(Inside Human
Body)
11%
Other Includes:
Occupational 0.3%
Fallout
< 0.3%
Nuclear Pow er 0.1%
Miscellaneous 0.1%
Natural
(mrem)
Radon
Cosmic
200
27
Terrestrial:
-external
-internal
28
39
Artificial (mrem)
Radon
55%
Natural 82%
-Diag. X-rays
39
-Nuc. Med.
14
-Consumer Pro. 10
-Other
~1
TOTAL
~360
Fundamentals Radiation Protection

TIME -- Limit time near source
Dose = (dose rate)x(time)

DISTANCE -- Stay away
Inverse square law: D2=D1(d1)²/(d2)²

SHIELDING – block radiation

CONTAMINATION CONTROL Universal
Precautions & monitoring