You manage to obtain an appointment for your patient and you accompany him to the hospital for the analysis. There he is placed on a rotating platform with his head encircled by a plastic helmet and his arms and legs submerged in a water-filled plastic container. See the photograph on the next page. The platform is located in a beam of neutrons emanating from a beryllium target 15 feet away, which is being bombarded by deuterons from a 22-MeV cyclotron. The purpose of the water is to surround the bones in that part of the subject’s skeleton with a neutron moderator equivalent to the body tissue surrounding the rest of his skeleton. (A neutron moderator slows down the neutrons and thus makes them more likely to activate the calcium in the bones.) On each side of the patient, there are two plastic containers permanently filled with a solution containing a known quantity of calcium. These serve as standards for the analysis. The beam of neutrons is turned on for 35 to 40 seconds. It is then interrupted while platform and patient are rotated 180 degrees. The irradiation is resumed so that a uniform dose of neutrons bombards the patient from both front and back. During the irradiation your patient receives a dose of radiation equivalent to approximately 10 ordinary chest X rays and one of the calcium isotopes in his bones (calcium-48) is activated to calcium-49. The latter has a half-life of only 8.8 minutes and so counting must begin soon after the irradiation. [Illustration: Patient] A patient in position for whole body irradiation with neutrons generated by an accelerator. His arms and legs are surrounded by plexiglas containers filled with water and his head is encased in a plexiglas helmet. On either side of him are containers, which serve as standards, filled with an aqueous solution of a calcium salt. The patient is standing on a turntable that is rotated 180 degrees after half the irradiation is completed so that the dose of neutrons is uniformly distributed to the front and the back of the patient. [Illustration: Patient] A patient in position for whole-body gamma-ray spectrometry. The detectors are scintillation crystals that produce pulses of light proportional in intensity to the energy of the gamma ray absorbed in the crystal. The patient is scanned from head to foot in approximately 12½ minutes at a rate that is varied to compensate for the gradual decay of the calcium-49 radioactivity during this period. Near the patient’s head are two calcium standard solutions in plexiglas containers.