It worked! The faulty plastic contains 100 times as much copper as the good plastic, specifically 100 parts per million. (If 0.1 milligrams of pure copper gave 1,000,000 counts, then the 0.1 grams of faulty plastic contains (100,000/1,000,000) · 0.1 milligrams or 0.01 milligrams of copper. This is one ten thousandth of the weight of the plastic or 0.01% or 100 ppm.) You relay the information to the plant superintendent almost before he finishes his lunch. He now knows what to do and the crisis is over. In a Museum _The Problem_ You are a curator working with the ancient coin collection of a large museum. A donor has just given the museum a group of 50 gold coins presumably about 1500 years old. After months of careful study, you have satisfied yourself that most of those coins are genuine specimens of that period. Judging from your experience, you decide that a small group of five are definite forgeries. However, there are three others that you suspect are also fakes, but you are not quite certain. You know that both genuine minters and forgers often tried to save money by diluting their gold with less expensive metals such as silver and copper. Since the chances are slim that the forger’s product has the same concentration of gold, silver, and copper as the genuine coins, you realize that a chemical analysis would help you decide if the doubtful pieces were real or fake. An accurate chemical analysis would require a sample of such size that the coin would be ruined as a museum specimen. You need an analytical method that can be applied to an infinitesimal sample. _The Solution_ You are not a scientist but you’ve heard about neutron activation analysis. Therefore, you contact a radiochemist at a local university who is an expert in this field. He decides to use a sampling technique developed by scientists at Brookhaven National Laboratory for sampling metal objects of archaeological interest. You obtain from him a set of 50 quartz plates that have been ground on one side. Following his instructions, you carefully scrape away a small area on the edge of each coin. You then rub each freshly cleaned area across the ground surface of one plate leaving a minute streak of metal similar to a pencil mark. At the scientist’s laboratory, each plate is carefully placed inside a quartz tube. No attempt is made to weigh the tiny streak of metal since you wish only to compare the ratios of the metal concentrations. However, because the samples make a rather bulky package, the scientist is concerned with the uniformity of the neutron flux that each sample will “see”. He therefore also places in each tube an exactly equal weight of a gold—silver—copper alloy wire (of known proportions) to act as a standard neutron-flux monitor. The tubes are then sealed and taken to a reactor to be irradiated for 12 hours.