WUNDERKIND
It was the laws of classical physics that had enabled calculation of the average particle energies needed to split an atom, and those calculations had produced discouragingly high estimates of the voltage requirements. Yet, classical physics was no longer the only game in town. A new probability-based approach to interpreting nuclear events, called quantum mechanics, was capturing the imagination of some physicists. Largely the brainchild of Danish physicist Niels Bohr (1885–1962), quantum mechanics was attracting many disciples because it allowed physicists to understand some complicated atomic processes that couldn’t be adequately probed with classical physics.
One of the disciples of Bohr and his quantum mechanical approaches was George Gamow (1904–1968), a young man of tremendous intellectual ability who was soon to become a friend and colleague of Rutherford and the other Cavendish scientists.30 By all accounts Gamow was a child prodigy.31 Born in Odessa, Ukraine, in 1904 (the same year as the plum pudding model’s birth), Gamow’s revolutionary ideas would allow the Cavendish scientists to split the atom before he reached his 29th birthday.
Gamow’s familiarity with quantum mechanics gave him some insight into splitting the atom that had escaped the classical physicists. He realized that it was not impossible for the lower energy particles to split the atom but, rather, it was just extremely unlikely that they would do so. In other words, it had a very low probability of happening. He pointed out, however, that if exceedingly large numbers of lower energy particles were shot at a lot of nuclei, it was likely that at least some of them would split some atoms, which the experimenters should be able to detect as visible particle tracks on their fluorescent screens. Basically, the problem of finding a needle in a haystack could be overcome just by using a lot of needles.
Gamow’s quantum mechanical ideas, known as tunneling theory, and their implications for splitting the atom were not a secret. He had published a series of papers and most classical physicists were aware of them. Nevertheless, the classical physicists had trouble fully accepting their implications. Even Albert Einstein (1879–1955) struggled with the probability theories of quantum mechanics and famously expressed his skepticism of its probabilistic nature in his oft quoted assertion, “God does not play dice!” Nevertheless, Gamow was a respected scientist despite his tender age, and his calculations were reason to be optimistic about the possibility of splitting the atom at lower voltages. According to Gamow’s calculations, the job could be done with just 300,000 volts,32 still a respectable amount of voltage but within grasp. And so, despite the potential challenges posed by voltage restrictions, Rutherford and his team decided to keep trying.
Gamow’s insight had lowered the bar for voltage. But even attaining 300,000 volts was a formidable goal. Also, a series of fitful starts, technical setbacks, and engineering obstacles delayed progress. Many accelerator designs were tried and found to be unworkable, requiring construction of multiple prototypes, which cost the group more time and money, and strained the limits of their laboratory facilities. Yet, they would not be deterred. By 1932, after many failures, Rutherford’s group finally constructed a proton accelerator that they hoped would be up to the task. For good measure they had designed it to accommodate 700,000 volts, which was more than twice what Gamow’s calculations said were needed to do the job.
Apart from the energy insight provided by Gamow, the scientific team also knew that choosing the right atom to split would be critical to achieving success. They correctly anticipated that using lithium-7 as the target element would improve their prospects another twofold. Lithium has a molecular mass of seven (three protons and four neutrons). If another proton is forced into its nucleus at high energy, it should produce a larger, highly energized nucleus (four protons and four neutrons), which would then likely split to form two alpha particles (each with two protons and two neutrons). Thus, for every one proton absorbed by lithium-7, two alpha particles should be emitted (a nuclear reaction termed “p, 2a”); this was a doubling of alpha particle yield!
This insight about how lithium-7’s nucleus would absorb a single high-speed proton and double the alpha particle yield was absolutely spot-on, and this awareness was central to achieving success. As we shall see later, lithium-7 can also, surprisingly, absorb a single high-speed neutron and then emit two neutrons (a nuclear reaction termed “n, 2n”). This other nuclear reaction of lithium-7, which can double the yield of neutrons, was unanticipated by anyone. Unfortunately, it is this unexpected reaction of lithium-7 that would one day take everyone by surprise and contribute to one of the worst nuclear weapon test accidents of the twentieth century. But for now, nuclear weapons were far from anyone’s mind.
The scientists had finally reached the moment of truth. It was time to attempt to split the atom. They had their proton machine gun and their lithium-7 target. Next to the target they placed a fluorescent screen, their trusted tool for visually detecting alpha particles. The thought was that, if the proton beam were successful in splitting the lithium atoms, there should be alpha particles released as nuclear fragments that would be visible on the neighboring screen.
The big day was Thursday, April 14, 1932. The accelerator was fired up to the required voltage and the protons were injected. All eyes were on the screen. It glowed! Microscopic inspection of the screen confirmed their supposition: the glow was being produced by plainly visible alpha-particle tracks. When they interrupted the proton flow into the accelerator, the alpha particles stopped, showing that protons were responsible for the particle production. Other controls further validated their greatest hope; the protons were splitting the lithium atoms! History had been made, and Cockcroft and Walton had earned the Cavendish yet another Nobel Prize.33
FLYING BY THE SEAT OF THEIR PANTS: RADIATION SAFETY AT THE CAVENDISH
Despite all his work with radioactivity, it is not apparent that Rutherford ever suffered any adverse health effects from radiation. The same cannot be said of his laboratory manager at the Cavendish, George Crowe. The only radiation safety rule at the Cavendish was simply that those handling radioactivity should wear rubber gloves to protect their skin from radiation burns, since by this time the dermal effects of radioactivity were well established. Nevertheless, Crowe typically disregarded the rule because he found the gloves a nuisance when conducting delicate work with radioactive sources. Consequently, he lost some feeling in his fingertips and developed skin sores that refused to heal. He ended up having multiple skin grafts and, ultimately, one finger amputated.34
Besides the gloves, little else was done to protect the scientists at the Cavendish from radiation. It wasn’t so much that the scientists were cavalier about their own safety. It was, rather, that radiation’s protracted risks appeared small to them compared to what they considered their major health threat—immediate death by electrocution. This, as we learned earlier, was the same concern that surrounded Edison’s electric light bulb. It was also the concern of Marconi and his team while transmitting radio waves. Marconi’s workers routinely wore very large, electrically insulating gloves whenever operating a live telegraph key.
Of particular concern were corona effects, in which electrons tended to accumulate on the sharp edges or points of an electrified apparatus and then jump like a small lightning bolt to something nearby. Unlike radiation, which couldn’t be seen during an experiment, coronas could sometimes be observed forming on the surface of glass tubes, as twitching blue lights, or heard as a hissing sound, like a snake about to strike.
Perhaps inspired by Edison’s animal electrocutions, a demonstration experiment was ordered by Rutherford to impress on his charges the dangers inherent in their work. Under the supervision of a medical professor, a corona bolt blew a half-inch hole through the skull of a laboratory rat, thereby establishing a healthy respect for electricity among all the scientists who witnessed it.35 Compared to sudden death by electrocution, the delayed threats posed by radiation seemed quite trivial.
Because of the electrocutio
n hazard, the Cavendish scientists took precautions to remain far away from live electrical apparatuses whenever possible. Still, sometimes they needed to be nearby in order to observe the experiment. Cockcroft and Walton therefore made a wooden hut that they could stay in during their experimental runs. They took pains to electrically insulate the hut so that they were completely safe from electrocution while inside. For good measure, they decided to make the hut radiation safe as well. To deal with the radiation hazard, they lined the hut with lead foil and then hung a fluorescent screen inside. If the screen seemed to be glowing a little too much during an experiment, they simply added another layer of lead foil before the next run.36 This was radiation protection at its crudest, but it was better than nothing. In any event, no one seemed to be harmed from the radiation and, more importantly to the scientists, nobody in the Cavendish laboratory was ever electrocuted.
THE ATOMIC MAGICIAN: THE CONSERVATION OF MASS TRICK
Besides the dilemma of conservation of charge, there was another annoying problem with the nucleus that needed to be dealt with—the conservation of mass. As the chemists well knew, matter could neither be created nor destroyed. That is, the products of a chemical reaction had to weigh the same as the reactants. Mass must be conserved.37 But for nuclear reactions, this fundamental scientific law did not seem to hold true. Had God made an exception for nuclear reactions? Not likely.
Let’s imagine the old magician’s trick of pulling a rabbit out of a hat but with a slight twist. As usual, the magician has secreted a rabbit in a hidden compartment in the hat, and shows the audience an apparently empty hat. But before going further, this magician weighs the hat in front of the audience and shows everyone that the hat weighs 2 kilograms. Then he pulls a rabbit out of the hat and weighs the rabbit: 1 kilogram. Then he weighs the now empty hat. The audience expects the hat to weigh 1 kilogram since everyone knows that 2 minus 1 equals 1. But the scale inexplicably reads: 0.5 kilograms! What’s this? How can 2 minus 1 equal 0.5? This is true magic! But where did that extra mass go?
Now, imagine that the rabbit coming out of a hat is equivalent to an alpha particle coming out of an atomic nucleus. That is, when the nucleus releases its alpha particle during radioactive decay, the mass of the alpha particle plus the remaining nucleus weighs less than the original nucleus containing the alpha particle. This is exactly the situation that exists when a radioactive atom decays. There seems to be a loss of mass, and the result is what is called a mass deficit. Where did that mass go? Nuclear magic!
It was Albert Einstein who revealed the secret behind this magic. Einstein understood that the missing mass of the hat had changed into the hop of the rabbit. That is, he recognized that mass could be changed into energy. His theory of relativity suggested it was possible for mass to become energy and, conversely, for energy to become mass. Not only that, he proposed a simple equation that showed the relationship between energy and mass: E = mc2. In words, energy (E) is equal to mass (m) times the speed of light (c) multiplied by itself. This relationship suggests that mass and energy are actually different forms of the same thing. They are, therefore, interchangeable! What this means is that, when an atom decays, it releases energy equivalent to the mass that was lost.
Don’t try to wrap your mind around this; even Einstein struggled with it. The bottom line is that it works. If you convert the mass deficit to energy, the resulting quantity is exactly equal to the energy of the radiation emitted during the radioactive decay. In effect, a portion of the mass of the nucleus was converted into energy and escaped from the atom as radiation. Interesting you may say, but what’s the big deal?
You’ll see how big a deal it is if you take the next logical step and ask the following: What would be the energy released if just one gram of matter (e.g., about one sugar cube) were entirely converted into energy? You can answer this for yourself using Einstein’s equation.
Given that the speed of light, c, is a constant (i.e., it doesn’t change) and equals 299,792,458 meters per second, you can calculate for yourself the total amount of nuclear energy in one gram (i.e., 0.001 kilograms) of sugar:
E = mc2
E = mass in kilograms × (speed of light in meters per second)2
E = 0.001 kilograms × (299,792,458 meters per second)2
E = 90,000,000,000,000 joules (90 trillion joules)
Exactly how much energy is 90 trillion joules?38 Consider this: it’s the energy of 10,000 lightning bolts, or the energy needed to heat 1,000 homes for one year, or the energy needed to put 10 space shuttles in orbit, or the energy released by one atomic bomb. As you can plainly see, it’s a lot of energy. And it’s also one hell of a magic trick!
So the energy released from atoms as radiation is simply the mass gone missing.
Ironically, the very scientists who revealed that matter and energy were interchangeable were slow to realize the practical implications of their work. After Einstein published his famous equation, E = mc2, he received a letter from a layman who had made some calculations similar to the one we just did that shows the huge amount of energy in a gram of matter. The man asked Einstein whether he realized that he had provided the world with the strategy for making a bomb of enormous potential. Einstein had not. In fact, he thought the concept was foolish.39
Rutherford was also once queried about the potential for producing electrical power by splitting atoms, but he didn’t think it possible. Yes, he said, he fully realized that splitting a single atom with a proton accelerated by 125,000 volts produces atomic fragments with energies equivalent to accelerations by 16,000,000 volts. Nevertheless, he pointed out, only one proton out of ten million was actually entering a target nucleus (à la Gamow’s calculations). All of the rest of the protons along with their energies were being wasted just to split that one atom. So in the end, he said, splitting atoms, rather than being a power source, was an energy-losing proposition!40
It is amazing that these scientists, who had so readily discarded the scientific dogma of their day and overcome tremendous technical obstacles in achieving their own scientific visions, would so easily dismiss as misguided fantasy the readily apparent implications of their own work. But their eyes were very soon opened. On August 12, 1933, just one year after those few atoms of lithium were split for the first time at the cost of tremendous high voltage energy input, a young Hungarian scientist, Leó Szilárd (1898–1964), proposed a way that large uranium atoms might be split with virtually no energy input at all. He suggested that splitting large nuclei (i.e., fission) might be readily achieved by means of a nuclear chain reaction.
THE DOMINO EFFECT: FISSION
When it comes to nuclei, being big is not good. We mentioned earlier that atomic nuclei tend toward having a similar number of protons and neutrons, and atoms that have an excess of one or the other are likely to be unstable and, therefore, radioactive. But there is another condition essential to the stability of nuclei—small size. Remember the idea of the nucleus as a baseball? While baseballs are happy and stable, it turns out that larger balls are not. When atoms start having protons in excess of 100, moving from baseball to basketball size, the density of positive charge becomes so great that even the neutrons can’t dilute the nuclear charge enough to keep the nucleus stable. In this situation, the large nuclei tend to decay in a special way. They spontaneously split apart into smaller nuclei. This process is called nuclear fission, and it is a major mode of radioactive decay for very heavy nuclei, like uranium-235. When uranium-235 splits, it produces various fission products, including multiple atoms with smaller nuclei, and it also typically ejects two or three neutrons as particulate radiation.
Just as with other forms of radioactive decay, the sum of the masses of the fission fragments is again less than the whole nucleus before fission, and the amount of energy released is determined by the mass deficit, as we’ve already discussed. This process is similar to other types of radioactive decay with one huge exception: the emitted neutrons can go on to be absorbed by other large
nuclei nearby and induce them to split by fission, thereby releasing more neutrons. Those neutrons then go on to interact with other nuclei, and on and on, ad infinitum. In this way, a chain reaction can be born, and the cumulative mass deficit can translate into a huge release of energy.
Under normal situations, however, a chain reaction doesn’t usually occur. That is because neutrons are highly penetrating and more likely to escape from the volume of radioactive material than they are to react with neighboring heavy nuclei. But as the volume (and thus the mass) of fissionable material increases, and the number of fissionable nuclei becomes more concentrated (i.e., enriched), the probability of inducing a chain reaction increases. The point at which a self-sustaining chain fission reaction occurs spontaneously is called criticality, and the mass of material required to produce criticality is called the critical mass.
THE CHICAGO PILE
The first man-made criticality was achieved by Italian physicist Enrico Fermi (1901–1954). Fermi was a gifted theoretical and experimental physicist, who was awarded the Nobel Prize in 1938 for his discovery of the transuranic elements (i.e., elements that have more protons in their nuclei than uranium does). Unfortunately, in that same year, he was forced to leave Italy with his Jewish wife, because new Italian racial laws were making her a target for persecution. He immigrated to the United States, and was ultimately recruited to work on the Manhattan Project, a secret program to make an atomic bomb.41
Fermi was tasked with the responsibility of experimentally demonstrating that a nuclear chain reaction was more than just a theoretical possibility. He started work immediately and built himself a small nuclear reactor under the abandoned west stands of the University of Chicago’s Stagg Field Stadium. The reactor was called a pile because it amounted to literally a pile of uranium and graphite blocks. The graphite (i.e., carbon) was used to slow the fast neutrons down so they would be more readily absorbed by the uranium atoms. The reactor had no radiation shielding, nor any type of cooling system, since Fermi claimed to have 100% confidence in the accuracy of his experimental calculations, which predicted that none would be necessary.42
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