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.

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.)

The two spectra are virtually identical except that the suspect sample has one obviously different peak in channel 157 and a somewhat smaller peak in channel 183. Referring to an energy calibration curve for the pulse height analyzer, you find that these channels correspond to 0.559 and 0.657 MeV respectively. A search of a table of nuclides, arranged by gamma-ray energy, reveals that this combination is emitted by arsenic-76, which would be the activation product for arsenic. Other data also indicate that for arsenic there should be a number of smaller peaks, including some corresponding to energies of 1.216, 1.228, 0.624, and 1.441 MeV. A closer look at the spectrum of the suspect sample reveals that these are also present.

Finally, noting that the half-life of arsenic-76 is approximately 27 hours, you wait a day and count the sample again in the same position as the previous count. A decrease in the heights of the 0.559 and 0.657 MeV peaks, by a little less than half in 24 hours, confirms that arsenic is the unusual element in this sample. It may not be the only impurity causing the peculiar behavior of this semiconductor, but it does seem a likely candidate.

The gamma-ray spectrum obtained after activation of a sample of “pure” silicon having “ordinary” properties of this type of semiconductor. Only very small quantities of various trace impurities are indicated.

The gamma-ray spectrum obtained after activation of a sample of silicon having “unusual” electrical properties. While most of the spectrum is identical with that obtained from the ordinary material, there is an interesting difference.

Using the equation given on page 12, the approximate known values for half-life, sample weight, neutron flux, and periods of irradiation and decay after irradiation, and an estimated value for the number of arsenic-76 atoms measured by the gamma-ray spectrometer, you calculate that the arsenic content of the sample is approximately 44 parts per million (ppm). (See appendix.)

With this information as a starting point, you are now ready to proceed with further research on the properties of your semiconductor, e.g., if you double the concentration of arsenic, how will that affect its properties?

In a Hospital

The Problem

You are a physician treating a patient who, because of a severe calcium deficiency, has been suffering from osteoporosis (a softening of the bones). You think you are on the right track with your treatment, but you would like to be sure in order to know whether you should continue the treatment or try something else. You would have your answer if you knew that the calcium content of his skeleton had stopped decreasing. How can you determine the amount of calcium in a living human being?

The Solution

You know that the usual techniques for determining calcium in the bones are not very useful. They are either too inaccurate to show that your patient’s calcium loss has been stopped or can only be used to measure the calcium content of the bones in his extremities. The latter is not satisfactory because these few bones may not be representative of the rest of his skeleton.

Recently, however, there have been reports of neutron activation analysis of whole persons, in which the calcium content of their bones has been measured with unusually good reliability. This has been accomplished by scientists and doctors working at the University of Washington School of Medicine in Seattle.

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