• Computer chips and other solid-state devices rely on carefully controlled distributions of electrons and holes in semiconductor materials. • Production of ions by ionizing radiation can cause catastrophic failure of electronic devices. • The single event effect results when a single ionizing particle can produce large numbers of ions. • Electronics in satellites are packaged in “hardened” materials to protect against cosmic rays.
56 Methods of Detecting Radiation • To assess radiation doses, the type and amount of radiation must be measured. • The first measurements used a zinc sulfide phosphor, which produced tiny flashes of light when struck by radiation and the light flashes were counted manually. • A scintillation counter uses a fluorescent screen to detect radiation, but the resulting photon strikes a phosphor that releases an electron instead of light flashes. • A photomultiplier tube amplifies the electronic signal, producing a current pulse registered electronically.
57 Methods of Detecting Radiation • In a Geiger-Mueller tube, radiation passes through a thin window into a gas-filled tube, producing ions in the gas. The resulting ions are attracted to oppositely charged electrodes, producing a pulse of electric current.
58 Methods of Detecting Radiation • A film-badge dosimeter monitors the radiation exposure for people who work with radioactive isotopes. • Radiation darkens photographic plates. • The darkened badge and a record of exposure provides a warning mechanism if safety levels are exceeded. • All measurement methods must take background radiation into account when making measurements. • Cosmic rays and natural radioactive isotopes in soil, air, and water are sources of background radiation. • Background radiation must be subtracted from measurements of radioactive sources.
59 Measuring Radiation Dose • The interplay between ionizing power and penetrating power results in a number of different ways to express radiation dose. • The quality factor , Q , is used to calculate the equivalent dose and is also known as the relative biological effectiveness ( RBE ). • The value of Q varies from a value of one for high-energy photons to about 20 for alpha particles. • Various scales of exposure and dose
60 Modern Medical Imaging Methods • Modern imaging methods include the use of radioisotopes to obtain images of specific organs and elaborate techniques such as positron emission tomography ( PET ). • During an X-ray, X-ray radiation passes through the body and a photographic image is produced based on the amount of radiation absorbed. • Bone absorbs X-rays more strongly than organs or other tissues, and is an excellent orthopedic diagnostic tool.