Radioactivity
Nature of radioactivity:
Spontaneous disintegration of atomic
nuclei, usually in nuclei that deviate
from a balance of protons & neutrons.
Radiation involves release of energy either
as kinetic energy of ejected particles
(electrons -- β particles, positrons, or orbital
electrons; α particles -- 2N/2P+2, a He
nucleus; neutrons) or as electromagnetic
radiation (X- rays from intranuclear
transitions; γ- rays from orbital shifts of
electrons).
The Scale of Matter
Electron
Light
microscope microscope
resolution resolution
~1 nm
~100 nm
Å nm
μm
Atoms
Visual
resolution
~0.1 mm
mm
Cells
1-100 μm
Proteins
1-20 nm Polymers,
organelles,
membranes
10’s-100’s nm
electron orbitals
The Scale of Atoms
Diameters of atoms ~ 10 - 1 nm, 1 Å
Diameters of nuclei ~10 - 6 nm
Most of atomic volume is empty!
Nuclear “strong force” is intense
but acts only over short distances.
nucleus
Tracer Behavior
Properties of bulk matter, e.g.,
classical mechanical behavior, is the
result of statistical averaging of the
behavior of atoms. In cases where
detection looks at behavior of very few
atoms, e.g., radiation, fluorescence,
MRI, & some spectral techniques,
properties may derive from quantum
behavior of individual atoms, or
Poisson statistical behavior of small
numbers of atoms or molecules.
Energy Scales in Radioactive
Decay & Medical Imaging
Photon Energetics
λ
(m)
ν
E
(sec-1) (joules)
10-3
3x1011 20x10-23 0.00125
10
-6
10
-9
14
3x10
17
3x10
E
(eV)
20x10
-20
1.25
20x10
-17
1250
ν = c/λ = 3x108 m/sec; E = hν
Atomic isotopes
that deviate most
from P=N (Z=A-Z)
tend to undergo
radioactive decay;
the larger P+N (A),
the more likely α
emission or
fission will occur.
Atomic Half-life & Related Quantities
Each radioisotope undergoes
spontaneous, stochastic, decay at a
characteristic rate not affected by
environmental factors. The time needed
for half a given mass of isotope to
radioactively decay is a half-life, τ1/2.
The time needed for 1/2 a given mass of
chemical to undergo chemical
degradation (that may be secondary to
radioactive decay) is a chemical half-life.
Half-life & Related Quantities (cont.)
Loss, clearance, of 1/2 the mass of an
atom or molecule from a biological
system into which it is introduced is a
biological half-life; this may be < or >
τ1/2 or chemical half-life.
Metabolic half-life is a chemical half-life
dependent on biochemical processes.
Circulatory half-life is loss of 1/2 the
mass of an atom or molecule from the
circulatory compartment of a biological
system, regardless of disposition due
to movement, metabolism, degradation,
chemical or radioactive decay.
Hyperlink
A Webpage on the Campbell
Website with links to sites on
radioactivity, radiation monitoring,
and radiation safety among others.
B685BiomedicalTracers.htm
The information retrieval engine
(Decay) is freeware that describes
the types & energies of radiation
generated by most radioisotopes.
The half-life of the isotopes &
other basic atomic information are
also given.
Energy Transfer to Surroundings
Energy delivery is governed by the
inverse square law which describes the
intensity of radiation at distance Dx
beyond the source, Ix = I0/Dx2. Only
radiation that fails to interact with its
transmitting medium defies this rule.
Interactions with surroundings occurs
by elastic & inelastic collisions with
electronic shells or nuclei, ion-pair
formation, electron-positron formation
or annihilation, electronic excitation, or
particle path bending near nuclei.
Energy Transfer (cont.)
A discussion of the processes
involved is found in section 216224 of the following US Army
document:
http://www.mega.nu:8080/nbcmans/
8-9-html/part_i/chapter2.htm
Detection Methods
Ion chamber discharge
Film exposure (latent image formation)
Thermoluminometer or storage phosphor
Geiger-Mueller detection
Flow counters
Scintillation detection
Detection Methods
Film exposure (latent image formation)
http://www.e-radiography.net/radtech/l/latent_image.htm
F. C. TOY, Letters to Editor, Nature 121, 865-865 (02 June 1928) | doi:10.1038/121865a0
The Mechanism of Formation of the Latent Photographic Image
Abstract
In a communication to NATURE of Sept. 24, 1927 (vol. 120, p. 441), the preliminary results were
described of experiments made in an attempt to correlate the mechanism of the latent image
formation with that responsible for producing changes of conductivity on illumination. It was shown
that the apparent absence of the photo-conductivity effect in the ultraviolet was due to two things:
(1) the small penetration of that light, and (2) the use of thick layers of the silver halide. With thinner
layers, of the order of 70µ, the ultra-violet (λ3650) effect in silver bromide was found to be about
twice as great as that produced by the blue (λ4358), thus supporting the original prediction that in
very thin layers of the order of 1-5µ the effect at λ3650 would rise to nearer ten times that at λ4358,
which is the ratio of photographic effects in very thin layers of slow, pure silver bromide emulsions.
It was further predicted that in very thin layers the ‘hump’ of maximum sensitivity at λ4600 in the
photo-conductivity-wave-length curve would disappear. How completely these conclusions have
now been verified can be seen from the accompanying graph (Fig. 1). The inference is that in very
thin layers of silver bromide the three curves representing (1) the relative photo-conductivity effects,
(2) the relative photographic effects, and (3) the relative light absorptions, each plotted against the
wave-length for equal incident intensity, are closely the same, indicating that in all probability the
primary stage of the photographic mechanism is intimately connected with that which produces
conductivity changes on illumination.
Detection Methods
Geiger-Mueller detection
http://wlap.physics.lsa.umich.edu/umich/ph
ys/satmorn/2003/20030322/real/sld007.htm
Detection Methods
Liquid Scintillation detection
http://wlap.physics.lsa.umich.edu/umich/ph
ys/satmorn/2003/20030322/real/sld008.htm
Detection Methods
Scintillation counting often uses a
coincidence counting circuit & is
subject to saturation:
http://www.canberra.com/pdf/Literature/Tim
ing%20Coin%20Counting%20SF.pdf
Modes of Biological Danger
Ion pair formation
Photoelectric effect
Bond breakage
Thermal damage
Free radical formation & reaction
Cell lysis
Inadequate cellular repair --> mutation
or apoptosis
Chemical toxicity
Radiation Protection
TDS
Minimize time of exposure
Maximize distance from source
Optimize shielding from source
Radiation Protection
Examples of training programs:
http://www.osha.gov/SLTC/radiationionizing/intro
toionizing/ionizinghandout.html
http://www.ehso.emory.edu/radiation/RSO/Trainin
g/train2.htm
General radiation safety
http://www.uiowa.edu/~hpo/radiation/rpg.pdf
Medical radiation safety
http://www.uiowa.edu/~hpo/manuals/mrpg/MRPG.
pdf
Laser safety
http://www.uiowa.edu/~hpo/manuals/laserman/las
ermanual.pdf