After the samples are removed from the reactor, the scientist carefully breaks open each of the quartz tubes and places the sample and the standard piece of wire in separate numbered plastic capsules with lids. For an accurate comparison, each capsule is prepared in the same manner. About 4 hours after the samples are removed from the reactor, he begins the radioactivity measurements. The sample capsules are loaded into an automatic sample-changing mechanism that places each one into an identical position above a lithium-drifted germanium detector. (See the chapter beginning on page 19.) Gamma-ray spectra are collected all day, first from a sample, then from its accompanying standard. Each count takes 2 minutes, and 3 minutes are required between counts for data printout and sample changing. A typical gamma-ray spectrum looks like the one in the figure on the next page. Notice that only gold (gold-198) and copper (copper-64) show up in this short counting time. Later on, radioactivity from silver (silver-110_m_) can be measured using a longer counting time. This can be done because while the activation products from copper and gold have relatively short half-lives (12.8 hours and 2.7 days, respectively), that from silver has a half-life of 270 days. To increase the sensitivity of the analysis for silver, the scientist repackages and re-irradiates the samples and wires for 100 hours. Silver-110_m_ is one of _two_ radioactive isotopes of silver that have the _same_ mass. In this case, one has a higher energy than the other and decays in a different way. This is known as an isomeric state and it occurs for many other elements as well as for silver. [Illustration: Graph: “Counts in one minute per 3.1 KeV channel” _vs_ “Channel”] Element Counts Channel number 0.158 MsV Au-199 600 40 0.412 MeV Au-198 800 120 0.511 MeV Cu-64 50 150 0.676 MeV Au-198 40 205 (Values estimated from graph) The spectrum obtained from a streak of metal on a quartz plate after a 3-hour exposure to neutrons in a reactor and a 6-hour delay before counting. The activation products of gold and copper are obviously present and are easily measured in only 1 minute. [Illustration: Graph: “Counts per 100 minutes per 3.2 KeV channel” _vs_ “Channel number”]