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-110m) 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-110m 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.
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.
The spectrum obtained from the same streak of metal after re-exposure to neutrons for 100 hours and a delay of approximately 2 months before counting. Activation products from gold and copper have decayed away and the gamma-ray spectrum of silver-110m is now observed. In this case the sample is closer to the detector than for the earlier measurement and the measurement takes 100 minutes.
Two months later, the scientist repeats the procedure of counting the samples and standards, except that this time the plastic capsules are closer to the detector, each count is for 100 minutes, and the sample changer operates for about a week. A typical spectrum looks like that in the [figure on page 39].
The scientist can now compute ratios for the three elements in each sample and compare them with the standard, but he decides that a computer could do it faster and with fewer errors. The data collected during the two series of counts are therefore sent to a data processing center where, in a matter of minutes, a computer does the following for each of 50 samples:
1. Finds the 0.411-MeV gamma-ray peak for gold-198.
2. Determines the total counts in the peak.
3. Repeats the process for the corresponding wire standard.
4. Corrects the total count for the wire for the small amount of radioactive decay that occurred in the few minutes between the sample count and the standard count.
5. Computes the ratio: [total count for sample/total count for standard (corrected)]
6. Repeats all the above for the 0.511-MeV gamma ray for copper-64 and (in the longer counts) for the 0.658-MeV gamma ray for silver-110.
7. Computes the ratios: [sample to standard (for copper)/sample to standard (for gold)] and [sample to standard (for silver)/sample to standard (for gold)].
8. Tabulates and prints the ratios found in Step 7.
Radioactivity ratios for 50 “gold” coins. Above are the silver to gold ratios. There are two groups of genuine coins. Five known forgeries show considerably higher ratios than the genuine coins. Two of the suspect coins also show high ratios but the third, suspect A, shows a ratio that falls into one of the genuine groups. Below are the copper to gold ratios. Again there are two groups of genuine coins. (The same coins make up the two groups here as above.) The five known forgeries again show higher ratios than the genuine ones and again the same two suspects appear to be forgeries. Suspect A, however, shows a ratio similar to one group of the genuine specimens. One therefore concludes that suspect A is genuine and that B and C are not.