The Making of the Atomic Bomb

The atomic number of a given element is the total number of protons in its nucleus. Hydrogen has one such proton; its atomic number is one. Helium has two protons for an atomic number of two; oxygen has eight protons and an atomic number of eight. Uranium contains 92 protons and, at 92, has the highest atomic number of the common natural elements; atoms above 92 occur almost always in laboratories. 

The atomic weight of a given element is the total number of protons and neutrons in its nucleus. Some elements have multiple atomic weights due to varying numbers of neutrons in their nuclei; these are the isotopes of a given element. 

Electrons expelled from atoms undergoing radioactive decay are called beta particles, in contrast with “alpha” and “gamma” particles from the same process. 

When small atoms bind together to become larger atoms, as in the thermonuclear processes inside a star, they give up a tiny portion of their mass as binding energy. Very small and very large atoms are loosely packed and need to give up a higher “packing fraction” of their mass to stay together. Atoms in the middle—nickel, iron, and tin, for example—are tightly stable and require a low packing fraction. Small atoms give off energy when fused together into larger ones, as when hydrogen fuses into helium in the center of the sun or in a thermonuclear bomb; very large atoms, such as uranium or plutonium, give off their binding energy when split apart. Atoms in the middle don’t have enough of a packing fraction to split or fuse for nuclear energy purposes. 

Bohr solved the problem of Rutherford’s atom—a nucleus surrounded by a cloud of electrons that somehow manage not to crash into the nucleus—by demonstrating that the electrons maintain stable orbits, moving out of them only when photons deliver discrete quanta of energy to the electrons, causing them to jump to higher-energy orbits. The electrons then radiate photons as quanta and drop back down to lower orbits. The theory ties together much of the early version of quantum mechanics. 

A chain reaction occurs when free neutrons strike a heavy material, such as uranium or plutonium, and cause those atoms to fission, releasing more neutrons that, in turn, cause more fissioning in a self-sustaining progression. A chain reaction requires a critical mass of material to sustain itself; below that mass, the reaction will peter out. 

All his life, Bohr was both intrigued and plagued by his idea of complementarity: firstly, that the self has two often-contradictory sides; secondly, that subatomic objects simultaneously are particles and waves; and thirdly, that nuclear weapons present both a threat of worldwide annihilation and a cure for war. In science and international relations, these ideas have become established principles. 

When fissionable materials are close enough together to sustain a chain reaction, this is called critical mass. Criticality can be achieved in a controlled manner, as in a nuclear power plant through careful use of dampening control rods, or suddenly through the sudden coming together of nuclear materials in a nuclear bomb. 

Physicists refer to the odds of an event happening within a given area as its cross section. Thus, if the odds of a baseball breaking a one-square-foot glass window are one in ten, that window has a “‘disintegration cross-section’ of 1/10 square foot and an ‘elastic cross-section’ of 9/10 square foot” (282). Cross sections in nuclear physics refer to microscopically tiny events, such as the small likelihood of a slow neutron striking an atom of U238 and causing the atom to fission and the much larger likelihood of a neutron striking an atom of U235 and causing fission. 

A cyclotron, first envisioned by Szilard, is a machine that uses magnets to speed up atoms to extremely high speeds and smash them into each other so the parts can be studied. Early cyclotrons were smaller than rooms; the biggest is the Large Hadron Collider in Switzerland, a large tunnel that runs in an underground circle 17 miles long. 

One of two atomic bombs detonated over Japan at the end of World War II, Fat Man was an implosion device—a plutonium core surrounded by uranium and crushed to criticality by a shell of exploding TNT—detonated over the Japanese city of Nagasaki on August 9, 1945. It was the third nuclear weapon ever discharged, after the New Mexico Trinity test and the Little Boy gun-type device that exploded over Hiroshima on August 6. Fat Man destroyed most of Nagasaki and killed tens of thousands of Japanese. 

Very large atoms such as uranium can be bombarded by neutrons and broken in two, a process called fission. The heaviest atoms have so many protons that their mutual repulsion nearly overcomes the binding power of the strong force, so that a single neutron, striking the nucleus, can cleave it in two, the positively charged daughters pushing each other away at three percent of the speed of light. Uranium, when fissioned, gives off stray neutrons that, in turn, cause nearby uranium atoms to fission. One gram of uranium contains 2.5 sextillion atoms; if the U235 isotope is separated out and pushed together densely, the material undergoes a rapid chain reaction that releases energy in an explosion, as in an atom bomb. 

During World War II, Edward Teller and others conducted preliminary theoretical studies on a bomb that used the intense pressure of an imploding atomic bomb to fuse hydrogen into helium, releasing a thousand times as much energy as an atomic bomb. Such thermonuclear weapons, also known as hydrogen bombs or H-bombs, became reality when the US detonated the first one in 1951. 

A gamma particle is an extremely high-energy photon, also called a gamma ray, emitted during the radioactive decay of an atom. Often accompanying gamma rays during decay are “alpha” and “beta” particles. 

Elements are arranged by their atomic number—their number of protons—and weighted by the total of their protons and neutrons. Some elements have varieties, or isotopes, with slightly different numbers of neutrons. Some of these isotopes are stable, while others break down and radiate particles to become a more stable isotope or a different element altogether. Radioactive isotopes are used in science, medicine, atomic power, and nuclear weapons. 

Little Boy was a gun-type nuclear weapon—a shell of U235 was fired through a large tube several feet long into a core of U235, causing it to go massively critical and explode. Little Boy was dropped from the B-29 bomber “Enola Gay” over Hiroshima on August 6, 1945 and detonated above the city, releasing energy equal to 12,500 tons of TNT. Much of the city was reduced instantly to rubble, and half of the city’s residents were killed by the blast or its radiation. Little Boy, the first of two atomic bombs detonated over Japan at the end of World War II, was the second nuclear device ever exploded, after the New Mexico Trinity test and prior to the August 9 detonation of Fat Man over Nagasaki. 

Originally run from Manhattan, the project to develop a US atomic bomb moved to the University of Chicago, then to Los Alamos in New Mexico, where the bombs’ construction was finalized and a test in the desert proved the concept in an atomic explosion. The Project then constructed the first and only atom bombs used in warfare, “Little Boy” over Hiroshima and “Fat Man” over Nagasaki, Japan in August 1945. The Manhattan Project gave the US a monopoly over nuclear weapons during World War II and for a few years thereafter. 

A secret committee of British scientists assigned to determine the feasibility of a nuclear weapon, MAUD—a nonsense name meant to mislead the overly inquisitive—in 1941 issued a report urging the rapid development of a bomb made either of U235 or plutonium. An unofficial copy of the report was sent to the US, but the recipient, Advisory Committee on Uranium chairman Lyman Briggs, doubted the report and cached it for months in his office safe. The report finally reached the NDRC’s Vannevar Bush, who reported it to Roosevelt; the president approved, and the American nuclear weapons effort began. 

The Metallurgical Laboratory at the University of Chicago, or Met Lab, was established in February 1942 as part of the US government’s Manhattan Project to develop an atomic bomb. There, scientists extracted plutonium from uranium and conducted the first controlled nuclear chain reaction, a prerequisite to bombs and nuclear power plants. 

Established in 1940, the NDRC oversaw US research into the weaponization of nuclear power. Somewhat similar to England’s secret MAUD committee, it was superseded in December 1941 by the Office of Scientific Research and Development (OSRD). 

Nuclear physics is the study of the nucleus of the atom. The field was founded by Rutherford; a number of the great minds in physics, including Bohr, Fermi, Meitner, and Werner Heisenberg, did work in this field. Many of the issues in nuclear weapons research—beta decay, cross sections, fission, fusion—are problems in nuclear physics. Atomic physics, on the other hand, is the study of the entire atom, including the electrons which orbit the nucleus. These two definitions and their activities tend to overlap. 

(See “Reactor”)

Essentially the successor to the NDRC, the OSRD took the reins of the American nuclear weaponization project as things moved from the theoretical to the developmental stage early in World War II. 

Originally called a quantum in Max Planck’s work, a photon is a unit of energy emitted or absorbed by electrons. Low-energy photons power radio waves; medium-energy photons can be seen as light; high-energy photons make up X-rays and gamma rays. A photon of the right frequency will be absorbed by an electron, causing it to jump from a lower atomic orbit to a higher one; the electron later will emit the same amount of energy as a photon and drop back down to the lower-energy orbit. 

Element 94, plutonium, can be bred from uranium and used in atomic weapons. Plutonium, like U235, fissions efficiently, its fission rate 1.7 times that of the uranium isotope. Plutonium was used in the implosion bomb, Fat Man, over Nagasaki on August 9, 1945. 

This short fiction book, written in the early 1900s by an anti-Semitic Russian, became a bestseller in Germany in the 1920s and purported to be the annals of a secret Jewish conspiracy to conquer the world on behalf of the Devil. Hitler adored the book and, as chancellor of Germany, based much of his domestic policy on the book as he persecuted the German Jewish population. 

Quantum mechanics is the study of the atom and its particles and how they interact. Energy exchanges at the atomic level are delivered in discrete units called quanta, which are delivered by photons absorbed or emitted by atoms. Subatomic particles also can be considered as waves, and their precise location and momentum cannot both be known at the same time, making them unpredictable; instead, all possible behaviors are collected into a mathematical concept called a wave function that calculates a particle’s likely behavior across multiple states at once. 

Some atoms break down into smaller atoms; in doing so, they give off subatomic particles. This is called radioactive decay and is made up mostly of alpha particles—essentially helium atoms liberated from unstable atomic nuclei—beta particles—electrons liberated during atomic interactions—and gamma rays, or high-energy photons. Sometimes an atom accepts an incoming neutron which then emits an electron and becomes a proton; in this way the atom “decays” upward to the next higher element. 

An atomic reactor, or nuclear “pile,” is a large grouping of materials whose atoms are struck by free neutrons and thereby caused to fission, releasing energy and more neutrons that keep the process going. Control rods dampen the process and prevent the resulting chain reaction from spiraling out of control. The first reactor was built by Fermi’s team at the University of Chicago in December 1942; their success showed that nuclear energy could be released in large amounts from atoms properly managed; this proved that atomic weapons were possible. Modern nuclear power plants use reactors, usually sunk in large pools of water, to generate steam that runs electric turbines. 

Science is a formal system for acquiring knowledge that can be verified by others and that makes accurate predictions about phenomena in the natural world. It requires openness, rigorous honesty, and skepticism. There is no orthodoxy in science, only theories that have stood the test of time and that always are subject to further refinement. 

Deep inside stars, pressures and temperatures rise so high that hydrogen atoms smash into each other with such force that they overcome their electrical barriers and become bound together, via the strong force, into larger atoms, making helium and releasing their binding energy. This energy finds its way out of stars and radiates into space, heating any nearby planets. The process of fusing hydrogen into helium is called thermonuclear because it involves intense heat on the order of millions of degrees. A bomb that fuses hydrogen into helium is called a hydrogen bomb, H-bomb, or thermonuclear bomb. 

"Tube Alloys” was the British code name for their atomic weapons program, begun in 1941 under the MAUD group. 

The largest atom normally occurring in nature, uranium comes in two major isotopes, U235 and U238. U235 fissions easily and is thus a good candidate for atomic energy and weapons. U238 can be coaxed into converting into plutonium, a weapons-grade material even more powerful than U235.