PRINCIPLES AND TECHNIQUES OF RADIOISOTOPE APPLICATIONS 133 of from 9000 to 20,000 miles per second but are slowed down rapidly in their passage through matter. In air they may travel up to one to three inches but none can pass through an ordinary sheet of paper. The beta ray (particle) is an electron ejected from a radioactive nucleus. They have much higher speeds than alpha particles in fact, some of them approach the velocity of light. They are more penetrating than alpha particles and can traverse a considerable amount of matter before they slow down. Some can penetrate about 1 cm. of tissue all are stopped by 2 mm. of brass and 1 ram. of lead. The gamma rays are electromagnetic radiation of the same character as x-rays but with wavelengths generally shorter so that their penetrating power is considerably greater. They travel with the velocity of light. These types of radiation have in common the ability to ionize, that is, they knock off electrons from a target material thus ionizing it. Ionization in tissue is of fundamental importance in the production of radiation effects while ionization of air is used in the measurement of radiation dosages. If a gas is exposed to this radiation, it becomes ionized and the amount of ionization produced serves as a measure of the intensity of the rays. Detectors of nuclear particles are in general nothing but ionization devices. The ions formed are detected by various means, such as conduction of an electric current, neutralization of an electric charge or formation of fog around the ion. The ionization chamber, the Geiger counter and the Wilson cloud chamber work in this way. The applications of radioisotopes can be divided into three general cate- gories. First and predominantly, they are used as tracers. This involves the tagging of an element in a chemical or biological process and following its behavior in the system. The tracer acts like a radio station with a Geiger counter as the receiver. A great variety of radioisotopes have been used in such studies. A second class utilizes the penetrating properties of the radiation. Source radiography and thickness measurement devices fall into this cate- gory. The third type utilizes the ionizing ability of the radiation. Food sterilization and medical radiation therapy are good examples. Dwelling on applications for a while, let us consider various problems and see how they can be solved with radioisotopes. To detect impurities in a process, we can add a tracer to suspected con- taminant and spot minute amounts of impurities remaining at any point in the process. Controlling density of process solutions, radioactivity density gauges can be used for measurement of control. These may even work on solutions in closed tanks. For controlling a mixing operation, a tracer is added to one of the corn-

134 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS ponents being mixed. Samples of the mixture are exposed to Geiger counters which give an extremely precise measure of homogeneity and dilution. For example, to control the strength of chemicals in electro- plating, an isotope is added to the bath to measure ion concentration. A measurement of radioactivity in the mixture will tell the strength of the solution. For gauging and controlling thickness of continuous sheet materials, such as paper, rubber, plastic, a beta gauge is used. A beta gauge measures the transmission of radiation through an absorber material. It will give an accurate measure of the product thickness without touching said material. Measuring and controlling the thickness of a coating on products made in continuous sheets can also be accomplished with the beta gauge. A tissue- thin coating of silver, for example, can be measured with accuracy to a millionth of an inch. If a problem involves pollution measurement in streams, add a small quantity of radioactive material to what you are dumping. A sample of water taken from suspected areas of pollution can be checked. No radio- activity generally means no pollution. Radioactive tracer elements are used to provide automatic switching in process lines. The radioactive material activates a Geiger counter which in turn activates valves and pumps. To meter fluids, add a radioactive tracer to your pipeline and mea.sure time elapsed in passing between two Geiger counters. To map underground pipelines or locate leaks in pipelines, a radioactive solution is incorporated therein and monitored with a Geiger counter. Radioactive materials that emit alpha or beta rays will bleed off large quantities of static electricity from paper, machines or any place where static can cause a production headache. There are literally hundreds of applications now in use and surely hun- dreds of potential applications still to be tried. Radioactivity can act as the source of many bits of heretofore hidden information. Health physics considerations in planning for the use of radioisotopes in either research or production is of prime importance. The major health problems result from inhalation and/or ingestion of radioactive materials. Another is radiation from external sources. From the standpoint of ex- ternal sources, protection is a simple matter with low energy radiation, but causes considerable inconvenience at high levels of activity. In general, the techniques used are distance, time and shielding. Actually under most conditions, it is possible to operate at levels well below the so-called tol- erance dose with proper protection measures. You may ask, "How much radiation is safe?" The tolerance dose is 0.3 R/wk. A Roentgen is a measure of x-ray or gamma radiation in a particular region. One Roentgen produces one electrostatic unit of charge as a result of ionization in i cc.