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).

Radioactivity

Å nm μm mm

Atoms

Polymers, organelles, membranes

10’s-100’s nm

Electron microscope resolution

~1 nm

Proteins1-20 nm

Light microscope resolution~100 nm

Cells1-100 μm

Visual resolution~0.1 mm

The Scale of Matter

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

electron orbitals

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(sec-1)

E(joules)

E(eV)

10-3 3x1011 20x10-23 0.00125

10-6 3x1014 20x10-20 1.25

10-9 3x1017 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.

B685BiomedicalTracers.htm

Hyperlink

A Webpage on the Campbell Website with links to sites on radioactivity, radiation monitoring, and radiation safety among others.

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/Dx

2. 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.)

http://www.mega.nu:8080/nbcmans/8-9-html/part_i/chapter2.htm

A discussion of the processes involved is found in section 216-224 of the following US Army document:

Ion chamber dischargeFilm exposure (latent image formation)Thermoluminometer or storage phosphorGeiger-Mueller detectionFlow countersScintillation 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:10.1038/121865a0The Mechanism of Formation of the Latent Photographic Image

AbstractIn 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/phys/satmorn/2003/20030322/real/sld007.htm

Detection Methods

Liquid Scintillation detection

Detection Methods

http://wlap.physics.lsa.umich.edu/umich/phys/satmorn/2003/20030322/real/sld008.htm

http://www.canberra.com/pdf/Literature/Timing%20Coin%20Counting%20SF.pdf

Scintillation counting often uses a coincidence counting circuit & is subject to saturation:

Detection Methods

Ion pair formation

Photoelectric effect

Bond breakage

Thermal damage

Free radical formation & reaction

Cell lysis

Inadequate cellular repair --> mutation or apoptosis

Chemical toxicity

Modes of Biological Danger

TDS

Minimize time of exposure

Maximize distance from source

Optimize shielding from source

Radiation Protection

Radiation Protection

Examples of training programs:http://www.osha.gov/SLTC/radiationionizing/introtoionizing/ionizinghandout.html

http://www.ehso.emory.edu/radiation/RSO/Training/train2.htm

General radiation safetyhttp://www.uiowa.edu/~hpo/radiation/rpg.pdf

Medical radiation safetyhttp://www.uiowa.edu/~hpo/manuals/mrpg/MRPG.pdf

Laser safetyhttp://www.uiowa.edu/~hpo/manuals/laserman/lasermanual.pdf