Quantum Physics

29.8 Radiation Detectors 963Figure 29.15 Computer-enhancedMRI images of (a) a normal humanbrain and (b) a human brain with aglioma tumor.a, SBHA/Getty Images; b, Scott Camazine/ScienceSource/Photo Researchers, Inc.(a)(b)the two spin states. These transitions result in a net absorption of energy by thespin system, which can be detected electronically.In MRI, image reconstruction is obtained using spatially varying magnetic fieldsand a procedure for encoding each point in the sample being imaged. Some MRIimages taken on a human head are shown in Figure 29.15. In practice, a computer-controlledpulse-sequencing technique is used to produce signals that arecaptured by a suitable processing device. The signals are then subjected to appropriatemathematical manipulations to provide data for the final image. The mainadvantage of MRI over other imaging techniques in medical diagnostics is that itcauses minimal damage to cellular structures. Photons associated with the rf signalsused in MRI have energies of only about 10 7 eV. Because molecular bondstrengths are much larger (on the order of 1 eV), the rf photons cause little cellulardamage. In comparison, x-rays or -rays have energies ranging from 10 4 to 10 6 eVand can cause considerable cellular damage.29.8 RADIATION DETECTORSAlthough most medical applications of radiation require instruments to makequantitative measurements of radioactive intensity, we have not yet explained howsuch instruments operate. Various devices have been developed to detect the energeticparticles emitted when a radioactive nucleus decays. The Geiger counter(Fig. 29.16) is perhaps the most common device used to detect radioactivity. It canbe considered the prototype of all counters that use the ionization of a medium asthe basic detection process. A Geiger counter consists of a thin wire electrodealigned along the central axis of a cylindrical metallic tube filled with a gas at lowpressure. The wire is maintained at a high positive voltage of about 1 000 V relativeto the tube. When an energetic charged particle or gamma-ray photon entersthe tube through a thin window at one end, some of the gas atoms are ionized.The electrons removed from these atoms are attracted toward the wire electrode,Wire electrodeGasThinwindowTo amplifierandcounter+(a)–Metallictube© Hank Morgan/Photo Researchers(b)Figure 29.16 (a) Diagram of aGeiger counter. The voltage betweenthe wire electrode and the metallictube is usually about 1 000 V. (b) AGeiger counter.