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NC Chapter - Health Physics SocietyNCSU – Nuclear Engineering DeptAmerican Nuclear Society
Science Teacher Workshop “Get a Half- Life”
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
Bohr Model
Quantum Mechanical Model of Electron Orbits
Proton (nucleus)
Electron
+ -
1s, 2s
2p
2d
Electron orbits nucleus ~1015 times per second
What Is “Radiation”?Ionizing
ionizes [strips electrons from] atoms; includes:
Non-Ionizingmany other modes of interaction; includes:
Particulate-alpha-beta-neutron- etc.
Electromagnetic
“Radioactive” vs. “Radiation”
Radiation Source
[radioactive material or X-ray
device]Radiation
Irradiated Material
“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:
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!
Z
A X
Why Some Atoms Decay:Nuclear Forces and Stability
pproton
p
p
neutron
Beryllium Atom [6Be]
Forces acting on 6Be nucleus
n n
n
electrostatic repulsion
nuclear force
nucleus electrons
The Curve of Nuclear Stability
Z (number protons)
N (number of neutrons)
N = Z
N ≈ 1.5 Z
Unstable – too many protons
Unstable – too many neutrons
Chart of the Nuclides
[Brookhaven National Laboratories - http://www2.bnl.gov/CoN/]
Z (number protons)
N (number of neutrons)
Unstable – too many protons
Unstable – too many neutrons
Principal Types of Ionizing Radiation
PARTICULATE ELECTROMAGNETIC
Alpha () – helium nucleus - from heavy nuclei [Z>82]Beta () – electron<250 keV max: "Low Energy Beta">250 keV max: "High Energy Beta"Neutron (n) – uncharged
Gamma () - photon X-ray (X) – photon
Note: It is customary to categorize radionuclides according to the type of radiation emitted.
What About X-rays?
high speed electron
X-rays
target nucleus[e.g. tungsten]
BREMSSTRAHLUNG X-RAYS
X-ray
CHARACTERISTIC X-RAYS“hole” at lower orbital
electron falls into lower orbital
Penetrating Abilities of Various Radiations
ALPHA
X
BETA
GAMMA & X-RAYS
NEUTRON
Paper [or dead layer of skin]Plastic
Lead or Concrete
10n
Waterhigh E β
low E β
Radiation S
ource
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]
A(t)/Ao
¼
0___
___
___
___
___
___
___
___
Negative Exponential Decay Curve
Ao Decay Constant [λ] = 0.693/T½
A(t) = Aoe-λt = Aoe-(0.693/T½)t
= Ao(½)t/T½
1.0
½
1T½ 2T½ 3T½ 4T½ 5T½ 6T½
time0
Specific Activity[activity per unit mass or volume]
Inversely proportional to Half-Life Long half-life → low specific activity Short half-life → high specific activity
Pure Phosphorous 32 (T½ = 15 days) Sp. Act. = 286,000 Ci/g
Pure Carbon-14 (T½ = 5730 years)
Sp. Act. = 4.5 Ci/g
Radioactivity UnitsActivity – Amount of radioactive material
curie (Ci): 3.7x1010 disintegration/second
• 1 Ci = a lot of of activity [based on 1 g radium]
• adult human has ~0.1 microcurie (µCi) 14C
1867-1934
1859-1906
1852-1908
becquerel (Bq): 1 disintegration/second
1 µCi = 37 kBq = 2.22x106 dpm [disintegration/minute]
• 1 Bq = tiny amount of activity [SI unit]
• adult human has ~3,700 Bq 14C
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 DoseQuantity 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;
1 R 1 rad Absorbed Dose: energy absorbed per unit mass
Old: rad=100 erg/gSI: gray [Gy] 1 J/kg 1 Gy = 100 rad
Short term dose effects; generally if dose>100rad(1
Gy)
Dose Equivalent: absorbed dose weighting factor based on rad. type
Old: roentgen equiv. man (mammal) [rem]=radQ SI: sievert [Sv]=GywR
Used for latent effects (e.g. cancer, genetic effects); if dose<100 rad(1Gy)
U.S NRC Quality Factors (Q)
Type of radiation Q
X-, gamma, or beta 1
Alpha particles, multiple-charged particles, fission fragments & heavy particles of unknown charge
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 BioeffectsDETERMINISTIC
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 Organ Dose (rad)
Blood cell depression
Bone Marrow 50
Reversible skin effects
Skin 200
Permanent sterility Ovaries 250 – 600
Vomiting GI 300
Temporary hair loss Skin 300 – 500
Permanent sterility Testis 350
Skin erythema 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)]
Natural (mrem)Radon 200Cosmic 27
Terrestrial: -external 28-internal 39
Artificial (mrem)-Diag. X-rays 39-Nuc. Med. 14-Consumer Pro. 10-Other ~1
TOTAL ~360