# Intro; EM Radiation; Radioactivity p. of 63 Illinois Institute of Technology

## Intro; EM Radiation; Radioactivity p. of 63 Illinois Institute of Technology

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Intro; EM Radiation; Radioactivity p. of 63 Fluences in addition to Flux Densities Let DN = particles entering a sphere with cross sectional area Da (total area a = 4pr2) Particles enter during time interval Dt Then Particle fluence = F = DN / Da Particle flux density = f = DF / Dt Intro; EM Radiation; Radioactivity p. of 63 Fluence in addition to Flux Visuals Area through which particles enter = Da Total Surface area a = 4pr2 DN particles enter in time Dt Particle fluence F = DN/Da Flux Density = f = DF/Dt Da DN Intro; EM Radiation; Radioactivity p. of 63 Energy Fluence, Flux Density Let DEf = sum of energy (exclusive of rest energy) of all particles entering sphere of cross-sectional area Da Energy fluence: Y = DEf /Da Energy flux density: y = DY/Dt

Intro; EM Radiation; Radioactivity p. of 63 Linear Energy Transfer (LET) LET defined as dEL/dl, where dEL is the energy locally imparted to the medium over the length interval dl. Dimensions: Energy / length; units: J/m restricted range stopping power: dont look as long as energy deposited far from path. Intro; EM Radiation; Radioactivity p. of 63 What does LET depend on Nature of radiation Alpha particles can be stopped by paper Betas can be stopped by aluminum Photons can get through almost anything Nature of medium (density, chemistry) Energy of radiation Intro; EM Radiation; Radioactivity p. of 63 LETs dependence on energy Dependence on energy manifests itself often in subtle ways: e.g. more absorption near absorption edges.

Intro; EM Radiation; Radioactivity p. of 63 Charged Particle Equilibrium CPE exists at a point p centered in a volume V if each charged particle carrying a certain energy out of V is replaced by another identical charged particle carrying the same energy into V. If CPE exists, then dose = kerma. Intro; EM Radiation; Radioactivity p. of 63 Radioactivity Measurements Let dP be the probability that a specific nucleus will undergo decay during time dt. Decay constant of a nuclide in a particular energy state is l = dP/dt. Half-time or half-life: time required as long as half of starting particles to undergone transitions. T1/2= (ln 2) / l (not ln (2/ l), as the book claims) Intro; EM Radiation; Radioactivity p. of 63 Activity Let dN = expectation value (most likely number) of nuclear transitions in time dt. Then activity A = dN/dt = -lN (note that the minus sign is just keeping track of disappearance rather than appearance) If you dont underst in addition to that, you will fail the Health Physics Comprehensive Exam! Dimensions: time-1 Units: 1 becquerel = 1 disintegration /sec Old unit: Curie: 3.7 1010 s-1

Intro; EM Radiation; Radioactivity p. of 63 Charting Decay Schemes We can sometimes find multiple pathways, each with multiple steps, as with 74As here (this is fig. 3.4, p. 37, in Alpen)

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