Nature of radioactivity:

1. 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).

2. The Scale of Matter Electron microscope resolution ~1 nm Light microscope resolution ~100 nm Visual resolution ~0.1 mm Å nm μm mm Atoms Cells 1-100 μm Proteins 1-20 nm Polymers, organelles, membranes 10’s-100’s nm

3. The Scale of Atoms electron orbitals 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

4. 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.

5. Energy Scales in Radioactive Decay & Medical Imaging

6. 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.

7. 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, 1/2. 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.

8. 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 > 1/2 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.

9. Hyperlink A Webpage on the Campbell Website with links to sites on radioactivity, radiation monitoring, and radiation safety among others. B685BiomedicalTracers.html

10. The information retrieval engine (Decay.exe) 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.

11. 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.

12. Energy Transfer (cont.) A discussion of the processes involved is found in section 216-224 of the following US Army document: http://www.mega.nu:8080/nbcmans/8-9-html/part_i/chapter2.htm

13. Detection Methods Ion chamber discharge Film exposure (latent image formation) Thermoluminometer or storage phosphor Geiger-Mueller detection Flow counters Scintillation detection

14. Detection Methods Film exposure (latent image formation) http://www.kodak.com/global/en/service/pubs/kpro/radiography/W37TOC.shtml

15. Detection Methods Geiger-Mueller detection http://wlap.physics.lsa.umich.edu/umich/phys/satmorn/2003/20030322/real/sld007.htm

16. Detection Methods http://wlap.physics.lsa.umich.edu/umich/phys/satmorn/2003/20030322/real/sld008.htm Liquid Scintillation detection

17. Detection Methods Scintillation counting often uses a coincidence counting circuit & is subject to saturation: http://www.canberra.com/literature/934.asp

18. 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

19. Radiation Protection TDS Minimize time of exposure Maximize distance from source Optimize shielding from source

20. Radiation Protection Examples of training programs: http://www.ehs.neu.edu/train0(a.htm http://www.osha.gov/SLTC/radiationionizing/introtoionizing/ionizinghandout.html http://www.ehso.emory.edu/radiation/RSO/Training/train2.htm http://www.uiowa.edu/~hpo/training/sealedsource/sld001.htm