Radioactivity Nature of radioactivity Spontaneous disintegration of atomic

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 -- 2 N/2 P+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 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

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. B 685 Biomedical. Tracers. 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 = I 0/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. ) 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/chapter 2. 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. 1038/121865 a 0 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 (λ 3650) effect in silver bromide was found to be about twice as great as that produced by the blue (λ 4358), thus supporting the original prediction that in very thin layers of the order of 1 -5µ the effect at λ 3650 would rise to nearer ten times that at λ 4358, 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 λ 4600 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 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/20030322/real/sld 007. htm

Detection Methods Liquid Scintillation detection http: //wlap. physics. lsa. umich. edu/umich/ph ys/satmorn/20030322/real/sld 008. htm

Detection Methods Scintillation counting often uses a coincidence counting circuit & is subject to saturation: http: //www. canberra. com/pdf/Literature/Tim ing%20 Coin%20 Counting%20 SF. 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/train 2. 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