Quantum Physics

29.4 The Decay Processes 953they were wrapped established that they were about 1950 years old. The scrolls arenow stored at the Israel museum in Jerusalem.Smoke Detectors Smoke detectors are frequently used in homes and industry forfire protection. Most of the common ones are the ionization-type that use radioactivematerials. (See Fig. 29.9.) A smoke detector consists of an ionization chamber,a sensitive current detector, and an alarm. A weak radioactive source ionizes theair in the chamber of the detector, which creates charged particles. A voltage ismaintained between the plates inside the chamber, setting up a small butdetectable current in the external circuit. As long as the current is maintained, thealarm is deactivated. However, if smoke drifts into the chamber, the ions becomeattached to the smoke particles. These heavier particles do not drift as readily asdo the lighter ions, which causes a decrease in the detector current. The externalcircuit senses this decrease in current and sets off the alarm.Radon Detection Radioactivity can also affect our daily lives in harmful ways. Soonafter the discovery of radium by the Curies, it was found that the air in contact withradium compounds becomes radioactive. It was then shown that this radioactivitycame from the radium itself, and the product was therefore called “radium emanation.”Rutherford and Soddy succeeded in condensing this “emanation,” confirmingthat it was a real substance: the inert, gaseous element now called radon (Rn).Later, it was discovered that the air in uranium mines is radioactive because of thepresence of radon gas. The mines must therefore be well ventilated to help protectthe miners. Finally, the fear of radon pollution has moved from uranium minesinto our own homes. (See Example 29.4.) Because certain types of rock, soil, brick,and concrete contain small quantities of radium, some of the resulting radon gasfinds its way into our homes and other buildings. The most serious problems arisefrom leakage of radon from the ground into the structure. One practical remedy isto exhaust the air through a pipe just above the underlying soil or gravel directly tothe outdoors by means of a small fan or blower.APPLICATIONSmoke DetectorsCurrentdetector+ –AlarmRadioactivesourceIonsFigure 29.9 An ionization-typesmoke detector. Smoke enteringthe chamber reduces the detectedcurrent, causing the alarm to sound.APPLICATIONRadon PollutionApplying Physics 29.3In 1991, a German tourist discovered the well-preservedremains of a man trapped in a glacier in the ItalianAlps. (See Fig. 29.10.) Radioactive dating of a sampleof bone from this hunter – gatherer, dubbed the“Iceman,” revealed an age of 5 300 years. Whydid scientists date the sample using the isotope 14 C,rather than 11 C, a beta emitter with a half-life of20.4 min?Explanation 14 C has a long half-life of 5 730 years, sothe fraction of 14 C nuclei remaining after one half-lifeis high enough to accurately measure changes in thesample’s activity. The 11 C isotope, which has a veryshort half-life, is not useful, because its activitydecreases to a vanishingly small value over the ageof the sample, making it impossible to detect.If a sample to be dated is not very old—say, about50 years—then you should select the isotope of someother element with half-life comparable to the age ofthe sample. For example, if the sample containedhydrogen, you could measure the activity of 3 H(tritium), a beta emitter of half-life 12.3 years. As ageneral rule, the expected age of the sampleshould be long enough to measure a change inRadioactive Dating of the IcemanHanny Paul/Gamma Liaisonactivity, but not so long that its activity can’t bedetected.Figure 29.10 (Applying Physics 29.3) The body of an ancient man(dubbed the Iceman) was exposed by a melting glacier in the Alps.