Radiation Safety Fundamentals and Radiation Physics
Core concepts
Ionizing radiation removes electrons from atoms. Alpha and beta are particulate; gamma and x-rays are electromagnetic photons. Industrial radiography uses gamma rays from sealed sources - Ir-192 (about 74-day half-life), Co-60 (about 5.3 years), Se-75 (about 120 days) - or x-rays produced when accelerated electrons strike a target (kVp controls penetration/energy, mA controls intensity/output).
Four quantities are tested constantly: activity (Ci, Bq; 1 Ci = 3.7 x 10^10 Bq), exposure (roentgen, defined in air), absorbed dose (rad or gray; 100 rad = 1 Gy), and dose equivalent (rem or sievert; 100 rem = 1 Sv). For gamma, x-ray and beta radiation the quality factor is 1, so 1 R is roughly 1 rad and 1 rem in tissue.
Rules that solve most problems
Decay: activity after n half-lives = A0 / 2^n. Inverse square: I1 x d1^2 = I2 x d2^2, so doubling distance cuts dose rate to one quarter. Attenuation: each half-value layer halves the beam; each tenth-value layer cuts it by 10. Photon interactions to recognize: photoelectric (dominant at low energy/high atomic number), Compton scattering (mid-energy, produces backscatter and film fog), pair production (above 1.022 MeV).
Traps
Activity is not dose rate - a large source far away can deliver less than a small source close by. Do not interchange R, rad and rem. Many questions quote a dose rate at 1 m and ask for another distance while the stated time spans a half-life; check decay. Convert mR to R before dividing.
Worked example
A 40 Ci Ir-192 source (about 0.48 R/h per Ci at 1 m) gives 19.2 R/h at 1 m. At 4 m: 19.2 / 16 = 1.2 R/h. After one 74-day half-life the same point reads 0.6 R/h.