[Illustration: Nuclear reactor] This is called a “swimming pool” reactor because the nuclear fuel, built into metal rods, is held in a framework at the bottom of a deep pool of water. The water serves as a shield to protect workers from the radiation and also helps the reactor “go” by slowing down neutrons to make them more likely to interact with the target atoms. “Swimming pool” reactors are frequently used for neutron activation analysis and typically provide neutron fluxes of over 10¹³ (10 million million) neutrons per square centimeter per second. [Illustration: Quartz capsules] These sealed quartz capsules contain samples to be irradiated in a nuclear reactor. They are about to be placed in the aluminum can, which will be sealed and positioned at the end of an aluminum pole, close to the core of a “swimming pool” reactor. Often samples are placed in plastic tubes and are carried in and out of a reactor by air pressure in a pneumatic tube system. You carefully scrape off a small amount of material, weigh it on a sensitive balance, and put it into a short piece of pure quartz tubing. You do the same with an ordinary piece of silicon for comparison and then seal both tubes with an oxygen-gas torch. Although the tubes are both ¼ inch in diameter and about 1 inch long, the first tube is just slightly longer so you will be able to determine which is which after the irradiation. Off it goes to the reactor in a carefully wrapped package along with instructions to irradiate the tubes for 12 hours in a neutron flux of about 10¹³ neutrons per square centimeter per second and to return them as quickly as possible after they are removed from the reactor. The following week, the samples are delivered about 4 hours after they were removed from the reactor. Working quickly but carefully, you note that they are radioactive but easily handled by ordinary laboratory techniques. You break the quartz tubes one at a time and attach each of the two pieces of silicon to a card with self-sticking tape. Then you place each card, in turn, on a holder close to the gamma-ray detector for a period of 10 minutes. A spectrum, which is a graph of the quantity of radiation recorded in each increment of energy over the range observed for each of the samples, is plotted automatically at the end of the counting period and you may now compare the compositions of the two samples. (See the figure on the next two pages.)