The Amazing Story of Quantum Mechanics (17 page)

Knowing that the positive charges in the atom were in the nucleus answered the question of the structure of the atom but raised several more. It was known from chemistry that the number of positive charges in an atom (balanced by an equal number of negatively charged electrons) determined its chemical nature. Hydro gen has one proton in its nucleus, helium has two, carbon has six protons, while gold has seventy-nine. The electron’s mass is nearly two thousand times smaller than a proton’s, so nearly all of the mass of the atom derives from its nucleus. But the weight of an atom does not correspond to the number of net positive charges it has. Hydrogen has a mass equivalent to a single proton, but helium’s mass is equal to that of four protons, carbon’s is twelve, and gold’s mass would suggest that it has 197 protons in its nucleus.

How can helium have a nucleus with only two positive charges, but a mass four times larger than that of hydrogen? For a while, physicists thought that the nucleus contained both protons and electrons. That is, a helium nucleus would consist of four protons and two electrons. That way, it would have a mass four times larger than hydrogen’s single proton, as observed, but a net charge of +4-2 = +2, which also agreed with the experiments. As the electron has a much smaller mass than the proton, measurements at the time were not precise enough to rule this possibility out.

Experiments on the nuclear magnetic field (remember that protons have small magnetic fields, as discussed in Chapter 4) and how it influenced the manner by which the electrons in the atom absorbed light (more on this when we discuss magnetic resonance imaging) led scientists to conclude that a helium nucleus, for example, could not have four protons and two electrons. Instead there must be two protons in a helium nucleus, and two other particles that weigh as much as a proton but have no electrical charge. In 1932, James Chadwick bombarded beryllium with alpha particles and detected a new part of the atom: the neutron. Thus one mystery about the nucleus was solved—the atom consisted of electrons orbiting a nucleus that contained protons and neutrons.

But this left another, more challenging mystery. As it is well known that like positive charges repel one another (this was, after all, the basis by which Rutherford had discovered the nucleus—by observing it repel positively charged alpha particles), then why do the positively charged protons in the nucleus not fly away from one another? The answer is—they do! Protons “feel” electrical forces inside the nucleus just the same as outside the nucleus. The fact that they stay inside the small nuclear volume implies that they feel a second, stronger force that prevents them from leaving the nucleus. A clue about this force is found by considering the heavier siblings of each element, termed “isotopes.” Two atoms are isotopes if their nuclei have the same number of protons (thus making them identical chemically) but differing numbers of neutrons (thus giving them different masses). There are versions of hydrogen that have one proton and zero, one, or two neutrons,
³⁶
but there are no isotopes of helium or any other element that have two or more protons and no neutrons. This indicates that the neutrons in the nucleus play a crucial role in providing the “strong force” that holds the nucleus together (the same strong force we encountered in Chapter 5).

How much stronger is this force than electromagnetism? If this additional force were ten times greater than the electrical repulsion, then it would be hard to make heavy elements such as silicon, with fourteen protons, or titanium, with twenty-two protons. If the force were a thousand times stronger, then we might expect to see elements with several hundred protons in the nucleus, and we do not. The fact that the heaviest natural element found on Earth is uranium, with ninety-two protons, indicates that this strong attractive force holding the nucleus together is roughly one hundred times greater than the electrical repulsion between the protons.

But even uranium is not stable, and if you wait long enough, all of your uranium will undergo transmutations to smaller elements by a process known as radioactive decay. Lead, with fifty-six protons and 126 neutrons, is the largest element that does not decay and is therefore stable. You can construct heavier nuclei, but when the “tower of blocks” of protons and neutrons becomes too tall (for each additional proton means more neutrons have to be present to keep it together), eventually the slightest perturbation will cause the tower to collapse. When it does, it loses energy by emitting radiation in the form of high-energy photons (gamma rays) or high-speed subatomic particles, such as electrons, neutrons, or alpha particles.

In fact, some of the larger nuclei are so unstable that all you have to do is give them a tap, and they fly apart. Uranium, so valuable in the middle of the 1950s that it would tempt Mickey Rooney out into an atomic testing site, is one such element. A dictionary from the end of the nineteenth century described uranium as “a heavy, practically worthless metal.” But this was before Otto Hahn and Fritz Strassmann split a uranium nucleus apart in 1938.

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 atomic 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.