Nuclear fission is the breaking apart of a large nucleus into two roughly equal nuclei. It turns out that to get a uranium nucleus to split into smaller pieces, one must hit it gently with a slow-moving neutron. Electrons are too light to do much damage, and protons or positively charged alpha particles are deflected by the large positive charge of the uranium nucleus and therefore can’t get close enough to do any harm. Thus, until the discovery of the neutron by Chadwick in 1932, there was not a suitable tool with which to strike the uranium atom.
However, the neutrons released from radioactive decays in Chadwick’s experiment were too energetic. A fast neutron has a large momentum, and through the de Broglie relationship (Chapter 3), the larger the momentum, the smaller the de Broglie wavelength. Finding the nucleus within an atom is always a difficult trick��if the electron’s probability cloud, which denotes the “size” of the atom, were the size of your thumbnail (about one square centimeter) then the nucleus on the atom would be a single cell in the thumbnail. In 1937 Italian physicist Enrico Fermi discovered that passing a beam of neutrons through a length of wax caused the neutrons to slow down as they collided with the large paraffin molecules, but not come to rest, as they did when striking a similar length of lead. The slower the neutron is moving, the lower its momentum, and the larger its de Broglie wavelength. A larger wavelength gives the neutron more of a chance to intersect with the nucleus’s matter-wave, just as you have a greater chance of coming across a bush in a garden at night if you walk with your arms outstretched rather than flat against your sides.
If the neutron strikes the uranium nucleus, then there is a chance that the strong force within the nucleus will capture this neutron (recall that the strong force has a very short range, and the neutron has to be right at the nucleus to feel it), making the uranium nucleus slightly heavier. But the tower of protons and neutrons in the uranium nucleus is already barely stable, and the addition of one more neutron turns out to be too much for the nucleus to support. So it usually tumbles into two smaller nuclei (typically krypton, with thirty-six protons and eighty-nine neutrons, and barium, with fifty-six protons and 144 neutrons, but alternative fracture products are observed), along with releasing either two or three more slowly moving neutrons,37 and energy, in the form of kinetic energy of the smaller nuclei and gamma rays.
Where does the kinetic energy of the nuclear fission by-products come from? Electrostatics. While gravity and electromagnetism can exert a force even when objects are miles and miles apart (though the force gets weaker the greater the distance), the strong force holding the nucleus together disappears for lengths larger than the diameter of a neutron. Consequently, once the two large nuclear fragments break apart in the fissioning uranium, there is no strong force to hold them. But the thirty-six protons in the krypton nucleus and the fifty-six protons in the barium nucleus repel each other, and as they are initially very close, the repulsive force between them is strong. The kinetic energy of the nuclear fission products, which accounts for the horrible destructive capacity of an atomic blast, derives from basic electrostatics. Elements such as uranium or plutonium are easier to break apart than lighter elements, but all matter would violently explode if the strong force could be, even momentarily, turned off, as in Watchmen’s unfortunate Dr. Osterman from Chapter 5.
Many chemical reactions, such as when dynamite undergoes combustion, give off heat as a by-product. By “heat” I mean that the reaction products have a larger kinetic energy than the initial reactants. Nearly all chemical reactions have an energy scale of roughly one electron Volt per molecule, within a factor of ten or so (that is, sometimes the reaction takes a fraction of an electron Volt, while in some other cases, depending on the chemistry, the reaction could involve ten electron Volts or more). In contrast, a single uranium nucleus undergoing fission and splitting into two smaller nuclei releases about two hundred million electron Volts of energy. Consequently, the energy released in fission is much higher, per atom of initial material, than in a chemical reaction. But two hundred million electron Volts, from a single uranium atom, would be less noticeable than a mosquito bite. By gathering together several thousand trillion trillion uranium atoms, the resulting energy released can be devastating, even though these thousand trillion trillion uranium atoms would weigh only a few pounds. It would take more than twenty thousand tons of dynamite to release an equivalent amount of energy.
A given mass of uranium is dangerous, but half this mass is not. Why not? When the uranium nucleus captures a slow-moving neutron and fissions into two lighter nuclei, it also releases two or three slowly moving neutrons. Thus, the decay of one uranium atom provides the means to cause two more uranium nuclei to undergo fission, and each one of those can make two more nuclei decay. Starting with the fission of a single atom, a large number of additional atoms can be induced to decay in a chain reaction—but only if the neutrons emitted from the first uranium atom strike other nuclei. Remember that most of the atom is empty space and that the diameter of the nucleus is only one ten thousandth that of the atom itself. If the decaying uranium atom does not have a sufficient number of other atoms surrounding it, then there will be low-level decays that provide energy (useful for an electrical power plant) but not enough reactions to yield an explosive chain reaction.
The trick to making an atomic bomb is to have two separate pieces of uranium, each less than the “critical mass” (so defined as at this mass a chain reaction is ensured), and bring them together into one volume quickly enough that the reactions do not die out but continue to grow. It’s not the mass itself that is critical for a chain reaction, but the number of uranium atoms, so that the released neutrons have a high probability of striking another nucleus and initiating another fission event. In this case a hundred pounds of uranium is transformed into an atomic bomb that can annihilate several square miles and cause extensive damage at larger distances.
Children in the early 1950s could learn all about radioactivity if their parents shelled out fifty bucks for the Gilbert’s U-238 Atomic Energy Lab. This kit was the nuclear physics version of a chemistry set and came complete with radioactive sources that emitted alpha, beta, and gamma radiation, a Geiger counter, and a mini-cloud chamber for seeing the tracks created by high-speed radioactive particles. The kit included both an instruction manual and an informational comic titled Learn How Dagwood Splits the Atom. This comic featured text that was scientifically thorough and accurate, with an introduction by Joe Considine, an International News Service correspondent who covered the Bikini Atoll nuclear tests and wrote the script for the 1947 docudrama about atomic energy The Beginning or the End (not to be confused the 1957 science fiction film The Beginning of the End, which featured the attack of radioactive giant locusts), and a foreword by Lieutenant General Leslie R. Groves, the head of military operations at the Manhattan Project. In the accompanying comic, Mandrake the Magician shrinks Dagwood Bumstead, his wife, Blondie, and their kids and dogs to subatomic size, so that they, together with Popeye, Olive Oyl, and Wimpy, can observe firsthand the inner workings of nuclear decay and fission. Figure 22 shows a page from this booklet, as Dagwood, unable even with Popeye’s assistance to overcome the strong nuclear force holding a uranium 235 nucleus together, is nevertheless able to initiate a chain reaction of fission decays when he uses a “neutron bazooka” to strike the nucleus just right.
While the world read in their newspapers on August 7, 1945, of the previous day’s successful detonation of an atomic bomb by the U.S. military over Hiroshima, Japan—this was not the first time atomic weapons entered the public consciousness. Figure 23 shows a Buck Rogers newspaper strip published in 1929. When the submarine Buck and his colleagues are on is held fast by a giant octopus, their only hope is to blast themselves free, using the awful destructive potential of an atomic torpedo. A full sixteen years before the Manhattan Project, Phil Nowlan and Dick Calkins, creators of the “Buck Rogers, 2429 A.D.” comic strip were confident that their readers would know that an at
omic torpedo was a more powerful version of the regular underwater missile.
Moreover, according to adventure pulp magazines, Japan knew as well of the ability of atomic weapons to destroy a major city, six years before the U.S. bombing of Hiroshima and Nagasaki. In Secret Service Operator No. 5, issue # 47, published in September 1939, it is the United States that is attacked by the invading troops of the “Yellow Vulture,” a thinly disguised, racist version of the Japanese Empire. In a tale titled “Corpse Cavalry of the Yellow Vulture,” the troops of the Yellow Vulture obliterate Washington, D.C., killing the president, Agent Q-6 (father to Operator no. 5), and most of the Washington establishment by using an atomic bomb.
Figure 22: Page from Learn How Dagwood Splits the Atom in which Mandrake the Magician, having shrunk Dagwood Bumstead and his family to subatomic size, narrates the mechanism of a uranium fission chain reaction, while Dagwood grabs his daughter and tries to quickly exit the nuclear pile.
One of the earliest recorded uses in fiction of “atomic” as a modifier to signify the enhanced lethality of a weapon is in a 1914 science fiction novel by H. G. Wells. In The World Set Free, Wells describes atomic bombs raining down with horrible destructive power and dropped from noiseless, atomic-powered airplanes.
How did the general population know about “atomic weapons” years before the Manhattan Project? It was thanks in part to the writings of Frederick Soddy, Ernest Rutherford’s colleague in earlier studies of nuclear radioactivity. Soddy penned a series of popular science books, the best known of which, The Interpretation of Radium: Being the Substance of Six Free Popular Experimental Lectures Delivered at the University of Glasgow, was a best seller when published in 1909. It made quite an impression on Herbert George Wells, who incorporated the concept of atomic-based weapons weighing only a few pounds and releasing tremendous energy and lingering radiation damage into his novel The World Set Free. In Wells’s novel, an atomic war between the nations of Europe and the United States leads to the formation of a proto-United Nations, where the surviving world leaders decide to form a new world order and establish a one-world government based upon the principles of socialism, rejecting capitalism, which was to blame for leading the nations into a nuclear confrontation.
Figure 23: Buck Rogers, in his daily syndicated newspaper strip in 1929, employs an “atomic torpedo” to devastating effect.
This novel made a strong impression on one particular reader in 1932. Both Wells’s vision of a one-world government run by socialistic principles and, equally important, his descriptions of horrific atomic weapons galvanized Hungarian physicist Leo Szilard. This fan of Wells was no ordinary reader—Szilard would, in 1933, be the first to conceive of a possible nuclear chain reaction (patenting the idea in 1934—four years before Hahn and Strassmann first split a uranium nucleus!). In 1939, Szilard wrote a letter to President Franklin Roosevelt, signed by Albert Einstein, urging the development of a nuclear weapons program, which became the Manhattan Project. Thus a popular science book by Soddy, written for a general audience, inspired an H. G. Wells science fiction novel suggesting the possibility of atomic weapons, and this novel in turn was directly responsible for the creation of actual atomic bombs. When publisher Hugo Gernsback launched his science fiction pulp magazine Amazing Stories in 1926, with a reprint of a story by Wells, it is doubtful that he realized how prophetic would be his magazine’s motto: “Extravagant Fiction Today . . . Cold Fact Tomorrow.”
CHAPTER TEN
Radioactive Man
The fates of Mickey Rooney and Tor Johnson in The Atomic Kid and The Beast of Yucca Flats, respectively, are of course ridiculous, unrealistic portrayals of the effects of exposure to radiation. By the mid-1950s, Doris Day’s lighthearted song about the wonders of a Geiger counter would give way to darker implications regarding the effects of nuclear weapon testing.
Ten years after the use of atomic bombs at the end of World War II, science fiction films would clearly and unambiguously establish that the real risk of exposure to radioactive fallout is unchecked gigantism. James Whitmore and James Arness battled ants mutated to the size of helicopters by lingering radioactivity in the New Mexico desert in the 1954 Warner Bros. film Them! Exposure to an atomic testing site would similarly transform Lieutenant Colonel Glenn Manning into The Amazing Colossal Man (1955), who would return to wage the War of the Colossal Beast (1958); feasting on fruits containing radioactive isotopes would create giant locusts, signaling The Beginning of the End (1957); a diet of radioactively contaminated fish similarly causes an octopus to grow to fantastic size in It Came from Beneath the Sea (1955); and radiation in a swamp would provoke The Attack of the Giant Leeches (1959). Occasionally, radioactive exposure would instead lead to miniaturization, as reflected in the strange case of The Incredible Shrinking Man (1957) and the experiments of Dr. Cyclops (1940), whose shrinking beam was powered by atomic rays five years before the Manhattan Project.
“Radioactivity” is an umbrella term for particle or light emissions from nuclei. As discussed in the previous section, when electrons in an atom move from one quantized energy level to another, they do so via the emission or absorption of light,38 which can span a wide range of wavelengths, from the microwave and infrared, to visible light, to ultraviolet and X-rays. Application of the rules of quantum mechanics to the protons and neutrons inside the atomic nucleus find that similarly, only certain quantized energy levels are possible. The energy spacing between these quantized levels is much larger than in the atom, thanks to the Heisenberg uncertainty principle. As the spatial extent of the nucleus is much smaller than that of the atom itself, the uncertainty in the location of the protons and neutrons is reduced. Consequently the uncertainty in the value of their momentum is increased, and the larger the momentum (mass times velocity), the greater the kinetic energy (momentum squared divided by twice the mass). While typical electronic transitions in an atom involve energies of about a few electron Volts, and occasionally one can observe X-ray emission, which has an energy of a thousand electron Volts, nuclear energy transitions involving electromagnetic radiation consist of gamma rays with energies of several million electron Volts.
As the protons and neutrons inside the nucleus settle from a higher energy level to a lower level (referred to as the “ground state”), there are other ways for them to shed energy aside from emitting gamma-ray photons. There are some nuclei that can lower their energy by emitting an alpha particle (consisting of two protons and two neutrons). The two protons and two neutrons that comprise a helium nucleus are very tightly bound to each other, so if the large, excited nucleus is going to emit any of its protons or neutrons, it is energetically favorable to do so in packets of alpha particles, rather than expending energy breaking the alpha apart. In this way the number of protons inside the larger nucleus decreases by two, so the electronic repulsion between the protons is reduced as well. The alphas come out with a considerable amount of kinetic energy (several million electron Volts, typically). This made them convenient probes for Rutherford when studying the structure of the atom—investigations that led to the discovery of the nucleus.
Even though the nucleus can lower its energy by ejecting an alpha particle, the particles within the alpha are still subject to the strong force, which acts like a barrier holding the subatomic particles together within the nucleus. This barrier is high enough that ordinarily one would not expect any alpha particles to be able to leave the confines of the nucleus. Since alpha particles have been observed exiting the nucleus, there must be a mechanism by which they are able to leak out through this barrier. Here the bizarre phenomenon of quantum mechanical tunneling comes into play. The strong force is so effective at holding the nucleus together that the alpha particle has only one chance in one hundred trillion trillion trillion of escaping. However, its small spatial uncertainty within the nucleus leads to a large momentum uncertainty, and it “rattles around” inside the nucleus, striking the strong-force barrier a billion trillion times a second. Consequently, if
one waits several billion years, one will see an alpha quantum mechanically tunnel outside of a nucleus. Once beyond the range of the strong force, the alpha particle is propelled at a high velocity by the same electrostatic repulsion that imparted energy to the fragments of a fissioning uranium nucleus.
Several billion years is a long time—so how are we able to see alphas emitted by radioactive isotopes without waiting so long? The answer to this question leads to an understanding of the concept of a radioactive half-life and in turn elucidates how we know the age of the Earth.
First a basic point about probability: In a lottery involving the random drawing of three digits from 000 to 999, there are one thousand possible outcomes. The lottery office draws the three digits at random, so one day the winning number may be 275 and the next it may be 130 or 477, and so on. If I purchase a ticket with one particular combination, say 927, there is thus one chance in a thousand that I will win the jackpot. Assume that I always play this same number, 927. I could win on the very first day. It’s possible, though there is only one chance in a thousand that I will. It is conceivable that I may have to wait extremely long, much longer than a thousand draws, before my one ticket matches the three numbers. Certain combinations may appear as winning numbers many times before my particular ticket pays off.39 I therefore may need to play the game for a long time before my ticket matches that day’s winning numbers.
One important similarity between the lottery scenario and the decay of unstable nuclei is that for both, the chance of an “event” occurring (either matching your ticket’s numbers with that day’s drawings, or having the nucleus undergo a transition to a more stable configuration, with the release of radiation) is the same on any given day. In a real, standard lottery run by most states, there is no restriction on whether a given set of numbers (from the predetermined pool of possible numbers) can be repeated before all other possible combinations are drawn. On any given day, one particular combination of numbers is as likely as any other. Similarly, as the quantum mechanical transition to a lower energy configuration is a probabilistic occurrence, the nucleus is as likely to decay on the first day, the one hundredth, or the millionth. There is no upper limit on how long the nucleus can exist in the excited state before radiating back to a lower energy state. If the nucleus is able to remain in the excited state for a long time, it is not “due” or “expected” to undergo radioactive decay but is as likely to relax to the ground state on the millionth day as on the first. If one plays the lottery long enough, eventually every number that can occur will be drawn. Similarly, if one waits long enough, every unstable nucleus will decay to a lower energy state.
The Amazing Story of Quantum Mechanics Page 12
