Strange Glow
Page 7
Alpha particles have short ranges, high energies, and a double positive charge (2+). They can be attracted toward negative electrodes, and they can be deflected from their paths by magnetic fields. Thus, they are highly amenable to experimental manipulation. Further, they are so large that you can actually follow their paths under the microscope with the use of a tiny fluorescent screen. In short, they make an excellent atomic tool with which to explore the nature of the nucleus. That’s exactly how Rutherford intended to use them.
FIGURE 4.2. TWO COMPETING MODELS OF ATOMIC STRUCTURE. J. J. Thomson’s “plum pudding” model of the atom (top), where electrons are immersed in a “pudding” of positive charge could not be reconciled with Rutherford’s bounce-back experiments using gold foil. Rutherford found that when high-energy alpha particles (arrows) were shot at a very thin foil made of gold, most alpha particles just passed through, but a very small number bounced back. Rutherford’s explanation was that the mass of an atom is concentrated in a central, positively charged nucleus, surrounded by a nearly massless cloud of electrons (bottom). An alpha particle bounces back only when it makes a direct hit on a gold nucleus. The implication of the experiment was that most of the volume of an atom is just empty space.
Rutherford hoped to prove the existence of the atomic nucleus and measure its size using the alpha particles as an atomic probe. Of course, neither the atomic nuclei nor the alpha particles can be seen directly, so making direct measurements was not possible. Nevertheless, since an individual alpha particle’s path can be seen with the help of a fluorescent screen, he had the ability to count them. When he pointed a beam of alpha particles from a radium source at a piece of thin gold foil, most of the particles passed through the empty space of the atoms, pulling the gold’s electrons as they went but otherwise moving unobstructed through the foil.15 But some of them—and just some of them—bounced back toward their source. He used a miniature fluorescent screen and a microscope to detect the alpha particles that bounced back, visible as flashes on the screen. Rutherford and his assistants took turns sitting in a dark room looking through a microscope for many hours, meticulously recording the number of flashes they saw per hour of observation time (i.e., the bounce-back rate). They ended up recording a very small, but significant, number. Rutherford was elated. He was later to say that this bounce-back finding was “quite the most incredible event in my life … as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back to hit you.”16 But exactly why did those alpha particles bounce back?
Rutherford correctly reasoned that an alpha particle bounced back whenever it made a direct hit with a nucleus of the gold atoms. He further correctly reasoned that the probability of hitting a nucleus was dependent upon how large that nucleus was. That is, the bigger the nucleus, the more likely an alpha particle would hit it and bounce back. Since their counts of flashes had shown that the probability of hitting the nucleus was extremely low (i.e., most of the particles did not bounce back), the nucleus of the gold atom must be extremely tiny. Assuming that the nucleus’s size was directly proportional to the probability of a bounce back, he estimated the size of the gold nucleus to be 14 femtometers (0.000000000000014 meters) across. Yes, it was small, very small indeed.
After he had determined its size, Rutherford next focused on determining what the nucleus was made of. He knew that there must be protons in it because of its overall positive charge. But what else was in there? As it turned out, the chemists were able to provide some insight.
Chemists had been able to measure the relative masses of atomic nuclei of different elements by cleverly employing what they already well knew about a fundamental property of gases. Under standard temperature and pressure conditions, equal volumes of gases have equal numbers of atoms.17 So the difference in masses per unit volume must be due to the differences in relative masses of their nuclei.18
Employing the above approach, the chemists were able to show that the masses of the nuclei for all elements they measured seemed to differ by a multiple of the mass of a single proton. For example, hydrogen weighed the same as one proton, helium’s mass equaled four protons, and carbon’s mass equaled twelve protons. This discovery led to the conclusion that atomic nuclei are composed exclusively of protons.
All of this seemed to fit, but it didn’t sit well with Rutherford. He recognized one huge problem that he would struggle with for years. It was that pesky “conservation of charge” issue. Most atoms were known to be electrically neutral; that is, the negative charges of their orbital electrons were balanced by the same number of positive charges in the nucleus. If atoms were composed of all those protons, they would have more protons than electrons and, therefore, have an excess number of positive charges, which they obviously did not have. So how could nuclei be composed of only protons? That simply could not be true. Rutherford’s answer was to describe a new particle that he called the neutron. It was a particle that weighed the same as a proton but had no charge (i.e., it was neutral). It solved the problem by providing the required nuclear mass without adding surplus charge.19
To many, Rutherford’s neutron sounded more like a creative accounting gimmick than a real physical particle, so the burden was on Rutherford to prove the neutron’s existence. This would be no small feat since all atomic particles known up to this time (alpha particles, beta particles, electrons, and protons) were charged and could, therefore, be detected and measured exclusively based on their electrical properties. Detecting a particle that had no charge was a major problem. Unless they could be detected and shown to be real physical objects, neutrons would be considered just a figment of Rutherford’s overactive imagination.
GERMANY AND FRANCE WEIGH IN
While Rutherford and his Cavendish colleague James Chadwick (1891–1974) were pondering how they might detect neutrons, an intriguing new report got their attention. German physicist Walther Bothe (1891–1957) and his student Herbert Becker had been able to show that when high-energy alpha particles from polonium hit a target made of beryllium, some of the alpha particles were absorbed by the beryllium atoms, which in turn emitted extremely powerful rays of some kind. The rays had no charge, so Bothe and Becker assumed that they must have been of the electromagnetic rather than the particulate type. They suggested that excess energy from the absorption of the alpha particle into the beryllium was being emitted as a powerful gamma ray.20
French scientists Irène Curie—the daughter of Marie and Pierre Curie—and her husband, Frédéric Joliot, were able to replicate the experiment and, thereby, validate the findings. They also showed that if these same gamma rays then went through a layer of paraffin wax, the rays were further changed into very high-energy protons. Since paraffin has a high concentration of hydrogen atoms, the interpretation was that the gamma rays were knocking out the hydrogen nuclei in the form of positively charged proton particles.21 What’s more, they calculated the energy needed for a gamma ray to expel the hydrogen nuclei at the observed proton energies as being 55 million electron volts!22
These experimental findings from two reputable laboratories were not in question, but their interpretations of what was happening in the nucleus were too fantastic for Rutherford and Chadwick to accept. Extremely high-energy gamma rays like this had never been described before and their existence was not thought possible.
Still, Chadwick’s imagination was set into motion by these tantalizing clues. He soon developed another interpretation of what was going on, which he was ultimately able to demonstrate experimentally. But it would not come easily. Joliot and Curie had a major advantage over him in conducting their experiments: they were using polonium that they had purified themselves. Polonium was a rare and precious radioisotope that the Cavendish scientists could not purify or purchase. And polonium was vastly superior to radium as a source of alpha particles because it emitted high-energy alpha particles, while radium emitted lower-energy alpha particles. It also released about 5,000 times more alpha particles than the sam
e quantity of radium.23 The Cavendish scientists had no polonium at their disposal. Help was on the way, however, and it was coming from the United States.
AMERICA SENDS REINFORCEMENTS
Norman Feather (1904–1978), a former Cambridge student who had done a research stint at the Cavendish, had developed a strong interest in nuclear architecture and radioactivity. In fact, he had worked briefly with Chadwick on a project to measure the penetration ability of alpha particles. They developed a mutual appreciation for each other’s scientific talents, and remained in close contact. Capitalizing on a growing interest in the field of radioactivity among American academic physicists,24 Feather came to the United States in 1929 with a one-year appointment to conduct radioactivity research at the Johns Hopkins University in Baltimore.25
Searching for radioactive sources to work with, he was happy to find that nearby Kelly Hospital had an abundance of the radioactive gas radon, encapsulated in small sealed glass ampules. The ampules were no longer of value to the hospital because the radon they contained had decayed to the point that it could no longer be used for radiation therapy (a subject that we will explore in chapter 6).26 So the hospital gave him a couple of spent radon ampules for his radioactivity research.27 Even partially decayed radon was very rare and expensive because the purified radium needed to generate the radon was in very limited supply. At the time, the Kelly Hospital had the largest supply of pure radium in the world—5 grams (about one teaspoon full)—which they used to produce radon gas for encapsulation in therapeutic ampules. The gift of the old radon ampules was a research windfall for Feather.
After working at Johns Hopkins for the year, Feather was preparing to return to the Cavendish when he stopped by the Kelly Hospital to see if they could spare a few more spent ampules. They gave him 300! He packed them in his luggage and returned to the Cavendish, riches in hand.
At some point Feather realized that he had more than just a stash of partially decayed radon in his possession. Radon-222, he knew, ultimately decayed into polonium-210, the very radioisotope that Marie Curie had discovered and that Chadwick so badly needed. He knew that the old radon ampules should have accumulated within them appreciable amounts of polonium. It wouldn’t be long before Chadwick would have his precious polonium.
Over a period of a few months Chadwick purified the polonium from the radon ampules and encased it at the end of a short metal barrel, effectively producing an alpha particle shotgun. He would use this gun to reveal the elusive neutron and prove that it was more than the just a nuclear accounting gimmick.
RUTHERFORD’S NEUTRON PREDICTION IS VINDICATED
Using the polonium alpha particles and various target elements, Chadwick was able to show a pattern of emissions that were not at all consistent with a high-energy gamma ray but could easily be explained as the result of a proton lacking charge, which was precisely what a neutron was defined to be! The mysterious neutron predicted by Rutherford had been found, its existence inferred despite its lack of charge, simply by its interactions with other particles that were charged, and, therefore, measurable. In other words, a ghost particle had been revealed by its shadow.
Now, in 1932—12 years after the neutron had first been predicted by his mentor, Rutherford—Chadwick had finally confirmed the existence of this mysterious particle. With his discovery, all the component parts of the atom were in place. The equally massed protons and neutrons together constituted the nucleus’s mass, and much smaller electrons traveled in orbits around it. There were no electrons in the nucleus, and there was a lot of empty space. For his achievement, Chadwick was awarded the Nobel Prize in Physics in 1935. Plum pudding was dead.
With the elusive neutron now in hand, all the major particulate radiations had also been identified. We now know that the smaller particulate radiations represent free-moving protons, neutrons, and electrons (beta particles). In contrast, the larger alpha particles, which Rutherford first identified as simply fast-moving helium nuclei, consist of exactly two protons and two neutrons (just like the nuclei of helium gas). All of these different types of particles are flying through space with great energy and dissipating that energy by interacting with various molecules they encounter along their paths. The future would reveal fission products and cosmic radiations as other forms of particulate radiation, but by 1932, the major particulate radiations were all defined in terms of their subatomic counterparts. These particles are very different from each other in their sizes, energies, and charges, and we will see that these differences can have varying consequences on health.
By this time, too, the higher-energy electromagnetic radiations that we’re familiar with today were all known. X-rays emanate from the electron shells of the atom, while relatively higher-energy gamma rays emanate from the nucleus. X-rays can be produced artificially by using a high-voltage electric current as Roentgen did, while gamma rays are a result of natural radioactive decay and derive specifically from the decaying atom’s nucleus. Apart from their origins in different locations within the atom, x-rays and gamma rays are fundamentally the same thing—electromagnetic waves with very short wavelengths. As might be expected, since they have wavelengths of similar size, they produce similar effects on health. We shall soon learn more about this.
SPLITTING ATOMS
While Chadwick was busy discovering the neutron with his little alpha particle shotgun, two other Cavendish scientists, John Cockcroft (1897–1967) and Ernest Walton (1903–1995), were busy supersizing things. Not content with just ejecting protons from nuclei, they had a larger goal. They wanted to split the atom.28
Cockcroft and Walton had decided that the best way to look inside the nucleus would be to split the nucleus apart and see what the fragments were made of. During Rutherford’s earlier bounce-back experiments, they had witnessed, in addition to heavy fluorescent tracks from the alpha particles, fainter but distinct fluorescent tracks from some other type of particle. They correctly postulated that the other particle tracks represented individual protons that had been chipped away from the nucleus by the alpha particle bombardment. They calculated the yields of these small particles and found them to be very low. Then they calculated the amount of polonium that they would need to significantly increase alpha particle bombardment in order to chip more of the atom away; it was depressingly high. In addition, they didn’t simply want to chip away at the nucleus; they wanted to blow it apart. Alpha particles, even from polonium, were just not going to do the job.
What the Rutherford team really needed was some sort of particle machine gun; it would help to have an instrument that could rapidly fire hundreds of millions of high-energy particles at nuclei and blast them apart. The radium and polonium shotguns that they had been using were just not up to the task. Cockcroft and Walton began to work on making this machine gun.
The first question was what to use for bullets. That seemed to be straightforward. The element hydrogen is composed of a single proton and a single orbiting electron, making it electrically neutral. If you strip off the electron, a positively charged proton would be left, which would make an excellent bullet. But how could that electron be removed?
As it turns out, the hydrogen nucleus with its single proton has a weak grip on its single orbiting electron. By moving an electrical current through a chamber of hydrogen gas, the hydrogen atoms could be easily scrambled into their constituents—positively charged protons and negatively charged electrons—and the protons could be drawn off by attracting them toward a negatively charged cathode. So a continuous source of protons turned out not to be a problem; all that was needed was a good supply of hydrogen gas and a little electricity to run through it.
The next issue was how to shoot the protons at a target, and that was a trickier problem. Again, since the protons were positively charged, it was theoretically possible to propel (accelerate) them toward a target by placing the target close to the negative electrode (i.e., cathode) within a strong electrical field.29 All that would be needed would be to feed a s
tream of protons into a particle acceleration tube. The more particles fed in, the more bullets would shoot out; and the higher the electrical voltage across the tube, the faster (and more energetic) the bullets would be when they hit the target. But how much particle energy would be required to split an atom? Or, more importantly, how high would the voltage across the tube have to be in order to achieve the required particle energies? One thing was certain, the voltage requirements would be substantial. And that was the rub. Where to get the voltage?
So the issue really wasn’t whether or not the atom could be split. Most scientists accepted the notion that the atom’s nucleus could be split. What they questioned was how much energy it would take to split it. Nearly everyone thought that prohibitively high voltages were needed to break the atomic nucleus apart. In fact, several physicists had calculated the voltages required, using different sets of assumptions. All came up with electrical requirements in excess of one million volts! In Rutherford’s day, this was a staggering amount of voltage. So Rutherford’s goal of splitting the atom was not viewed as a crazy idea, but rather just impractical, given the technology of the day.
High voltages had been commercially achieved, however, on an industrial scale using massive transformers. These voltages, nevertheless, were attained only momentarily and were highly unstable. Furthermore, the cost and size of these commercial transformers put them well out of reach for experimentation in physics laboratories. So the Cavendish scientists were either going to have to find a whole lot of money, or they were going to have to come up with another idea. They chose the latter.