IN 1899 A GERMAN PHYSICIST named Friedrich O. Giesel built an apparatus to investigate why a magnet alters the conductivity of air electrified by radium. He set up a glass tube so that rays emitted by radium at one end of the tube made a spot of light on a fluorescent screen at the other end. When Giesel put the tube between the poles of a magnet, the spot spread toward one side of the screen or the other, depending on the polarity of the magnet. He concluded that the screen must be excited by electrically charged particles from the radium, because such particles would be deflected by the magnetic field. But in answering this riddle the experiment posed others more difficult to solve. Why does radium emit charged particles in the first place-and does it do so?

These questions still challenge some of the best minds in physics. During the past half-century the particles detected by Giesel have been named beta rays and identified as electrons. Beta rays of opposite charge have been detected and identified as positrons. A sophisticated version of Giesel's tube, called the beta-ray spectrometer, has become one of the most important-tools of modern physics. Nonetheless no completely satisfactory theory of the origin of beta rays has yet been proposed.

All this suggests that the serious study of beta rays lies beyond the reach of the amateur. But some amateurs, notably those of high-school age, like to test their reach. A case in point is a group of boys in Seattle, Wash. They built a beta-ray spectrometer which, with its accessories, weighs more than half a ton. A measure of their success is the fact that the apparatus is now in use in the department of physics at the University of Washington.

The project, which spanned two years, is described by the boys responsible for its completion: John Kulander and Sydney Handlin, who are now students at the University. "As members of the newly organized Garfield High School Physical Science Club," they write, "we wanted to construct an instrument capable of serious research. A telephone call to Fred H. Schmidt of the University of Washington physics department resulted in an invitation to make a tour of the physics laboratories. We accepted in high excitement and examined many research instruments, including a 60-inch cyclotron, a cloud chamber and a beta-ray spectrometer.

"After much discussion, we finally decided to build a beta-ray spectrometer of the magnetic-lens type. Dr. Schmidt agreed to become our adviser. Many conferences were held with him as well as with graduate students at the University, including Richard Maltrud and John Penning.

"After completing the basic design and working drawings for the instrument, we tackled a practical side of scientific work which, as subsequent experience proved, had been dismissed too lightly: the promotion of funds. We needed pumps for the high-vacuum system, a motor-generator set with an output of a kilowatt, electronic components a substantial quantity of magnet wire and other materials, to say nothing of tools and machine-shop facilities. We estimated that $1,000 would get us started, and decided to approach the Seattle Post-Intelligencer with the problem. The resulting story caught the eye of William Q. Hull, an editor of Chemical and Engineering News, and led to another article. We then solicited numerous business men and industrial organizations in the Seattle area. Help came from many. A local theater owner contributed the first $1,000. E. E. Hanselman, the principal of Garfield High School in 1954, gave us part of an old storeroom for a laboratory. During this phase of the project we learned that not only copper and steel but also a large quantity of shoe leather can go into the construction of laboratory apparatus.

"Instruments of the type we subsequently built measure the kinetic energy of beta particles emitted during the decay of radioactive isotopes. Much of what has been learned about the decay process has been inferred from such measurements. The instrument is also capable of measuring the energies of gamma rays. This is accomplished by covering the source of gamma radiation with a material from which the radiation dislodges electrons. The energies and intensities of the electrons are measured as though they were beta rays, and the properties of the gamma radiation are derived from this information.

"One might suppose that all the particles emitted by the decaying atoms of a given radioactive isotope would have the same velocity and the same kinetic energy. This, however, is not the case. Measurements show that great numbers of particles are emitted with random energies: some low, some intermediate and some high. When plotted as a graph, the energy measurements make a broad, smooth curve which may have one or more sharp peaks. The peaks mark energy levels at which exceptionally large numbers of particles with the same energy are observed.

"The broad, smooth part of the curve is referred to as the continuous or primary energy-spectrum of the substance under measurement. The peaks are called line spectra. It has been found that the continuous spectrum is caused by particles ejected from nuclei, while the line spectra are due to the ejection of particles from one or more of the inner shells of the system of electrons which surround the nucleus. The curve of one radioisotope of cesium made by our instrument is shown in the accompanying illustration [Figure 2]. The height of the curve is proportional to the number of electrons ejected at each level of energy. Energy increases are plotted from left to right. The peaks at K and L represent emission from the "K" and "L" shells of the cesium atom.

"Three types of beta decay are known. One involves the emission of high-energy electrons, observed by Giesel. In this form of decay a neutron emits an electron and is transformed into a proton. The atom has thereby gained a proton and is transformed into the element immediately above it in the periodic table. Cesium 137 undergoes such decay, and becomes barium 187. The barium is left in an excited state after the decay and returns to the "ground" state by emitting a gamma ray. Occasionally the gamma ray ejects an electron from the K or the L shell. In another kind of beta decay a proton emits a positron and becomes a neutron. Here the daughter element is one step down the periodic table. In the third type of beta decay a proton interacts with an electron in the innermost electron shell (the K shell) and also becomes a neutron. This transformation is called K-electron capture.

"How is it that the identical atoms of a beta-ray-emitting isotope can emit beta rays of various energies? What happens to the energy apparently left behind when a low-energy beta ray is ejected? This energy cannot simply vanish; that would violate the well-known law of nature which states that energy cannot be destroyed, but only transformed. A solution for this riddle was proposed in 1927 by the physicist Wolfgang Pauli. He invented a fictitious particle and assigned to it the responsibility of carrying precisely enough energy away from the decaying atom to balance the books. Called the neutrino, the particle had to be much lighter than the electron. It also had to carry no electric charge A particle which met these specifications would be difficult to detect. The existence of the neutrino was demonstrated, however, in 1956 [see "The Neutrino," by Philip Morrison; SCIENTIFIC AMERICAN, January, 1956]. The beta-ray spectrometer cannot detect neutrinos directly. But the amount of energy the neutrinos carry away from the decaying isotope can be established by measuring the energy of the beta particles.

"In making such energy measurements the beta-ray spectrometer takes advantage of the fact that it is more difficult to change the course of fast-moving particles than that of slow ones. A charged particle is deflected by a magnetic field when its path makes an angle with the lines of force in the field. For a given angle and kinetic energy, particles of low energy are deflected more than those of high energy. Conversely, particles of any energy can be made to follow a prescribed trajectory by making appropriate adjustments in the strength of the magnetic field. The trajectory of a beta ray in a magnetic field is therefore determined by the speed and mass of the particle, the angle made by the heading of the particle with respect to the lines of magnetic force, and the strength of the field [see illustration above left]. Particles diverging from a source located on the axis of a short magnetic field follow a corkscrew trajectory; those of like heading and energy will come to focus on the axis at the opposite end of the field [see illustration above]. Particles of more or less energy can be brought to the same focus by adjusting the strength of the field. This assumes, of course, that no obstacles such as molecules of gas hamper the flight of the particles. The number of particles arriving at the focus can be detected by several methods, including Giesel's fluorescent screen. We used a Geiger-Muller counter to record the relative intensity of the particles.

"To provide a clear flight path for the beta particles, the radioactive substance under measurement is enclosed in an evacuated chamber. Some gas molecules remain to bounce around at random inside the chamber, however, because a perfect vacuum cannot be produced. These collide with and divert occasional particles. Some high-energy beta rays also dislodge electrons from molecules of gas, and even from the walls of the vacuum chamber. As a consequence some particles which should be counted do not reach the focus, and others which should not are deflected into the counter. The Geiger tube responds to gamma rays; these must also be excluded from the count. Error from these sources is reduced by equipping the chamber with a system of baffles [see illustration below]. Depending upon the strength of the magnetic field, rays ejected from the source above a certain critical angle strike a ring baffle at the source end of the vacuum chamber. Those below this angle pass through the center opening and proceed to a baffle with a ring-shaped opening, located in the middle of the chamber. Here rays above a certain critical energy level collide with the center stop. Those of lower energy pass through the ring-shaped opening. They proceed through a similar opening in another baffle and enter the counter. A cylinder of lead located on the axis between the latter two baffles shields the counter from gamma rays, which, because they are not deflected by a magnetic field, would travel in a straight line from the radioactive source.

"We began to build the instrument in the spring of 1955 by machining the vacuum chamber. This work was done in the University of Washington physics shop. The chamber consisted of a heavy brass tube with an inner diameter of six inches. An outside flange was welded to the source end of the tube and an inside flange to the counter end. These were grooved to take rubber rings and provided with a set of end-plates. During final assembly the rubber rings were coated with silicone grease just before the end-plates were bolted in place. The center of the plate at the counter end was provided with a window of aluminum foil a thousandth of an inch thick. The foil was attached by a special vacuum-sealing compound. Work proceeded simultaneously on the electrical circuits, which included the counter and controls as well as a regulated power supply for the magnet. Some of the electrical parts were gifts from local schools and industrial organizations, but most had to be purchased. By the summer of 1955 we had run out of money.

"This meant more fund-raising. Eventually we obtained oil-diffusion pumps from two large organizations. One pump was connected directly to the vacuum chamber at the source end [see Figure 1]. A Welch single-stage mechanical pump brought the vacuum down to five hundredths of a millimeter of mercury. The oil-diffusion equipment went into operation at this point, and reduced the pressure to about 10 microns of mercury.

"We employed three gauges for measuring air pressure in the system. A conventional high-pressure gauge calibrated from 0 to 760 millimeters of mercury was connected between the mechanical and diffusion pumps. Two low-pressure gauges were connected directly to the vacuum chamber. One, a National Research Corporation thermocouple gauge, could measure down to one micron. The other, a VG-1A ionization manometer made by the Consolidated Electrodynamics Corporation, extended the reading to a hundredth of a micron.

"The inner walls of the chamber were sanded and cleaned frequently with acetone to remove substances which release gas after the pumps are started. Under ideal conditions we could reduce the pressure to about one micron.

"The magnetic field was provided by a short coil placed symmetrically around the axis of the vacuum chamber. Wire for the magnet was contributed by A. S. Sheldon, president of the Kennecott Wire and Cable Company. Fortunately it arrived late in 1955, just as we had run out of money again. By this time the instrument was complete except for the magnet and the final assembly of the vacuum system, so we set to work on the coil immediately. It was a job we did not anticipate with relish, because the coil required some 380 pounds of wire. For a time we thought we would have to wind it by hand. Through the cooperation of our principal, however, we were given access to a lathe at a local trade school. Even so the job required 120 man-hours to complete. Each layer of wire had to be placed neatly atop the one beneath it so that the coil would not spread out at the top. Any spreading would distort the magnetic field and decrease the efficiency of the instrument. In all we wound some 18,500 feet of No. 12 magnet wire on a form which consisted of an aluminum tube six inches long and eight inches in diameter. The tube was made into a spool by welding circular plates to its ends. There were 70 layers, each of 78 turns-making a total of 5,460 turns. The coil could take a continuous current of about five amperes without overheating. It generated a field of more than 1,000 gauss at the center of the chamber. This enabled us to focus beta particles with energies of more than two million electron volts.

"The coil functions as a lens. Hence we took extreme care to make sure that it was centered accurately between the source and the focal point and that the axis of the magnetic field coincided precisely with that of the vacuum chamber. The mounting which determined the position of the coil consisted of two concave rollers which rode on the upper surface of the vacuum chamber. The rollers were supported by bearings held in position by a system of cams. The horizontal position of the coil with respect to the source and the focus could be altered merely by pushing the coil along the vacuum chamber. Vertical and lateral adjustments were easily made by changing the settings of the cams.

"The momentum of the focused particles varies in proportion to the current passing through the coil. Fluctuations in the current are therefore reflected in the performance of the lens. A major problem consisted in providing the coil with a source of ripple-free direct current. A motor-generator set was finally located, but we discovered that our power line was not large enough to carry the load. Our principal again came to the rescue and 13rovided the necessary wiring at the expense of the school. Jack Orth of the cyclotron staff at the University helped us with the design of a voltage regulator. This unit maintained the output of the generator constant to one part in a thousand.

"The three baffles were made of aluminum plate 1/4 inch thick. The hole in the baffle nearest the source is four inches in diameter. The ring-shaped opening in the center baffle is 1/4 inch wide. The third baffle, like the first, is perforated with a four-inch hole. The inner edge of the ring-shaped opening of the third baffle is formed by the lead cylinder of the gamma-ray shield [see illustration in Figure 5 ]. The components of the baffle system were mounted rigidly on three longitudinal rods of 3/16-inch aluminum.

"The instrument was completed and ready for calibration in January, 1956. For a calibration source we used thorium B, a naturally radioactive isotope. This material has a half-life of 10.6 hours and the F line has been established at 1,385 gauss-centimeters, the unit in which the momentum of beta particles is measured. The thorium sample, provided by the University, was applied to a disk of aluminum and anchored in place by a thin coating of Zapon lacquer. The disk was then inserted in the source holder and screwed into the source endplate. After the vacuum chamber had been pumped down, current was applied to the coil and increased in equal amounts from zero to five amperes, as measured by a standard potentiometer circuit. Simultaneous readings of the beta-ray activity were taken and subsequently plotted as shown in the accompanying illustration. This same plot provided data for computing the resolution of the instrument: its capacity to bring beta rays of the same energy to the same focus, and to discriminate against those in other energy bands. The resolution was found to be 6 per cent-not quite as good as our design calculations had led us to expect but satisfactory for an instrument of this type and size.

"In the months remaining before graduation we made studies of various radioactive isotopes as a further check on the instrument's performance. The isotopes included cesium 137, cesium 134 and iodine l3l. The specimens were mounted on disks of aluminum, nylon or rubber. When aluminum foil was used, as in most instances, the source was applied in liquid form and dried on the foil. The resulting crystals were then covered with Zapon to keep them from spilling.

"Our project brought us many unexpected and pleasant experiences. One day in the spring of 1956, for example, we received a call from the principal's office ordering us to report immediately. We could only wonder on the way down what we had done this time. The principal met us with a smile and explained that some engineering officials of the Boeing Airplane Company had invited us to lunch the following day. There we met Gil Hollingsworth, associate director of research, and a group of his fellow-executives. One consequence of this visit was summer jobs with the physical research staff of Boeing's Seattle Division-not only that summer but every summer since.

"At the completion of our senior year we were faced with the final problem of disposing of the spectrometer. No one was left at the high school to carry on the project, so we gave the instrument to the University. There it is now in constant use.

"We feel that we learned a lot from the project, not only in learning how to solve technical problems but also in the art of working with other people. We hope this account of our experience will stimulate others to embark on a similar adventure."

"May I take this opportunity," Howard writes, "to congratulate you on the article appearing in 'The Amateur Scientist' for March. This was certainly a much-needed introduction to an area of biology often overlooked by the amateur. When I had occasion a few years back to introduce a layman to the field of microbiology as a hobby, I chose a series of experiments much like those described in your article.

"There was one statement, however, which somewhat marred an otherwise very fine presentation. This statement, far from attracting interest, might easily serve to repel a person from undertaking bacteriology as a pastime. I am referring to the passage in which you warn the amateur not to expose a culture medium to airborne organisms and incubate the result. You state that this is dangerous because the amateur's likely to capture and cultivate deadly disease organisms.' This is in error, for it is almost impossible that 'deadly disease organisms' would be found in the immediate environment of an amateur experimenter. If he never intentionally introduces them into his laboratory, there is little likelihood of their ever getting there by chance to be cultured. Moreover, the medium recommended in the article probably would not support the growth of most highly pathogenic microbes. If there were any dangerous pathogens, then the experimenter would have just as much opportunity to breathe them in or swallow them as he would to culture them by mistake on his nonenriched nutrient agar."

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