NC Chapter - Health Physics SocietyNCSU – Nuclear Engineering DeptAmerican Nuclear Society

Science Teacher Workshop “Get a Half- Life”

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

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

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