[Illustration: _This type of nuclear reactor is used for neutron activation analysis._] “So that’s what a nuclear reactor looks like!” said Bill. “Yes,” said Dad. “Where you see the blue glow you can also see rows of fuel elements. Each one contains slugs of uranium encased in aluminum. This is one of a number of different types of reactors. But every nuclear reactor is arranged so that the uranium atoms divide (or fission) many, many times each second. “When this happens, heat is produced that is carried away by the water, and also many, many free neutrons are produced. Those samples, placed down next to the reactor in the bottom of the pool are being bombarded by the neutrons, and some of the elements in the samples absorb the neutrons and become radioactive.” After a while the samples were removed and carried back to the laboratory in a lead box. A short while later, the radiochemist opened the aluminum can, broke open the quartz capsules, and removed the samples for analysis. The boys watched the chemist mount each sample on a card and take it to a room where there was equipment for measuring radioactivity. [Illustration: _Gamma-ray spectrometer. The sample to be measured is placed on a stand over a gamma-ray detector. The pulse-height analyzer is a device that sorts electrical impulses from the detector according to the energy of the gamma rays causing the impulses. The screen displays the gamma-ray spectrum and the electric typewriter automatically types out the data collected when the measurement is complete._] One by one the samples were placed inside a shield consisting of a big pile of lead bricks. When the heavy door was opened, the boys could see a metal can inside the shield, which housed a detector (called a lithium-drifted germanium detector) that measured the gamma rays emitted by the sample. As each sample was placed near the detector the chemist turned on a gamma-ray spectrometer to which the detector was connected. A tiny sample of lead white {sample} is sealed in a quartz vial {vial} which is bombarded with neutrons in a reactor. [Illustration: uncaptioned] Many of the atoms become radioactive, emitting gamma rays. [Illustration: uncaptioned] The sample is placed in a gamma-ray spectrometer and the gamma rays are separated according to their energy. [Illustration: uncaptioned] Gamma-ray spectrum Copper Zinc Antimony Lead Silver Height Antimony The location (energy) of each peak indicates what is present and the height indicates how much! [Illustration: _A gamma-ray spectrum as it appears on the screen of a pulse-height analyzer. The gamma-ray peaks are marked with the name of the element whose radioactive isotope emits the gamma ray; two for cobalt and zinc and one for cesium._]