How To Detect An Atomic Bomb
u.s. air force weather agency history office2006_4_46The Soviet Union tested its first nuclear weapon at 7:00 a.m. local time on August 29, 1949. As soon as the explosion was over, Lavrenti Beria, the secret-police head, who also ran the country’s nuclear program, telephoned an observer in another part of the test site who had been present at American weapons tests in 1946. “Did it look like the American one?” he shouted. After verifying that it did, Beria immediately called Joseph Stalin with the good news that the test was successful. He was crestfallen when Stalin, who had placed his own man at the scene, replied, “I know already.”
The Americans knew too, within days. The Soviets had kept the test under tight security, but the U.S. Air Force’s airborne observers showed conclusively that a nuclear explosion had taken place. The technique they used had been developed just in time; even two years earlier it would have been beyond the state of the art. It had come into existence through the leadership of Lewis Strauss, a commissioner of the Atomic Energy Commission (AEC).
Strauss was a wealthy financier with a taste for physics and international politics. He got his start in the latter pursuit during World War I, when he was a traveling salesman for the family’s shoe business and his mother suggested that he go to Washington to work with Herbert Hoover, who was organizing Belgian war relief. He eventually became Hoover’s personal secretary and made several trips abroad with him. In 1919, still only 23, he joined the investment-banking house of Kuhn, Loeb. He married the daughter of one of the partners before becoming a partner himself.
His involvement with physics dated from the mid-1930s, when both his parents died of cancer. “I became aware,” he later wrote, “of the inadequate supply of radium for the treatment of cancer in American hospitals.” He searched for inexpensive ways to produce a radioactive isotope of cobalt as a substitute, and this led him to the physicist Leo Szilard. As the two men grew closer, Szilard made Strauss, who had never gone to college, one of the first nonscientists to learn of the prospect of an atomic bomb.
Strauss had been a naval reservist since 1926, and he spent World War II as an aide to James Forrestal, the Secretary of the Navy. Shortly after the war, having maintained his ties to the physics community, he urged that the Navy “test the ability of ships of present design to withstand the forces generated by the atomic bomb” by detonating such a bomb in the immediate presence of such vessels. This was done in the Operation Crossroads series of tests at Bikini Atoll in the Pacific in mid-1946. The success of that project marked Strauss for a key role in nuclear policymaking, and in 1947 the Senate confirmed his appointment to the AEC.
At the outset Strauss wanted to know what the Army’s Manhattan Engineer District (commonly known as the Manhattan Project), which had built the atomic bomb, was doing to detect Soviet nuclear tests. Fallout from American tests had been observed thousands of miles from the test site, and Strauss thought the same method could be used to detect Soviet activity. Those measurements, of course, had been taken by monitors who knew that a test had just occurred; detecting a test without knowing when to expect it would be much harder. Still, wrote Strauss in April 1947, “continuous monitoring of the radioactivity in the upper atmosphere … would perhaps be the only means that we would have for discovering that the test of an atomic weapon had been made by any other nation.”
Within a month of Strauss’s challenge, the Central Intelligence Group (CIG), predecessor of the Central Intelligence Agency, was looking at the problem. It concluded that there was no way yet to detect a distant nuclear explosion, but it saw three possible methods: seismic, by monitoring the earthquake-like effect produced by a test; acoustic, by listening to its roar; and radiological, by collecting its radioactive debris. In June Adm. Roscoe Hillen-koetter, director of the CIG, estimated that it would take two years to set up a workable system.
That didn’t suit the AEC. Its chairman, David Lilienthal, replied in July, “We cannot regard a two-year period as acceptable or realistic.” Strauss went to Forrestal for help. When Strauss told him that no one was keeping watch on possible Soviet tests, Forrestal was incredulous: “Hell! We must be doing it!” A few phone calls soon convinced him that we weren’t. Strauss told him: “Jim, if neither of the Armed Services takes on this responsibility, the Commission will. If we do it, we’ll have to buy planes and hire pilots. We’ll have to get an appropriation for that. When we ask for the money, that will be the first time Congress will know that monitoring hasn’t been going on all this time.” As Strauss wrote in his memoirs, “Forrestal saw the point immediately.”
In mid-September Strauss met again with Forrestal, who was still Navy Secretary but about to become the nation’s first Secretary of Defense. Forrestal bucked the matter over to Gen. Dwight D. Eisenhower, the Army’s Chief of Staff, who decided that the Air Force, newly established as a separate service, should handle it. The Air Force had three squadrons of B-29 bombers modified for weather observation. They were set to fly out of Alaska—downwind of Central Asia, the remote and desolate region where analysts guessed Soviet weapons tests were likely to take place. One of these weather squadrons, the 375th, took on the assignment of looking for nuclear isotopes in the atmosphere as well.
Now that it had the planes, all the Air Force needed was a way to collect and interpret the data. It soon learned that this would be no small order. Fallout from a ground test might be detectable, but no less an expert than J. Robert Oppenheimer, the wartime director of the Manhattan Project, believed that debris from a shot at high altitude would consist of too few particles, and too small ones, to detect. In effect, the fallout would simply vanish into thin air. A shot at altitude would also fail to produce an earthquake, and its noise might be discerned only within a few hundred miles, a minuscule distance in the vastness of Central Asia. In December 1947 an advisory panel chaired by Oppenheimer expressed “grave doubts that the techniques and the instruments for detection and evaluation of remote detonations of an atomic bomb are available either potentially or actually.”
But the Air Force was not about to give up so easily. Preparations were well under way for a set of American bomb tests called Operation Sandstone, to be held at Eniwetok Atoll in the Pacific, and the Army’s Signal Corps laboratory at Fort Monmouth, New Jersey, was purchasing the necessary instruments to detect and measure radioactivity. Soon after, through happenstance, the start-up firm of Tracerlab, Inc., became involved.
Tracerlab was small but aggressive, a product of the huge increase in interest in nuclear science that had come out of World War II. The company, which included many Manhattan Project veterans, had begun in March 1946 by manufacturing instruments for detecting radioactivity. It also obtained radiochemicals from the AEC and reprocessed them for medical, university, and industrial laboratories. In December 1947 Fortune magazine had called the company “the first real business to be built out of by-products of the atomic bomb.”
The outfit had its main office in Boston. Late in January 1948 one of its engineers visited Fort Monmouth to repair some equipment. In chatting with the staff, he found out that the Signal Corps was about to purchase a large quantity of radiological instruments. The purpose was classified. The engineer hurried back to Boston and told his company’s sales manager, Dana Atchley. Atchley passed the word on to Tracerlab’s chief scientist, Dr. Frederick Henriques, and the two men made a beeline to Fort Monmouth. They had security clearances, which gave them access to key people, but they lacked a “need to know” about the reasons for this specific procurement. Army officials therefore could not speak freely to them, but that didn’t stop the visitors from asking questions.
u.s. air force weather agency history office2006_4_46aAs Henriques recalled in 1981 for the anthropologist and physicist Charles A. Ziegler, “We had to winkle the information out of them. We go down there and these people are desperate. We started talking to them. I’m a scientist, they were scientists … and just in chatting it became obvious that what these people were trying to do was to use our own test procedures to mask setting up a means of detecting Russian bombs. The Pacific tests would also be a calibration of these procedures. Dana was busy counting up all the scalers, cutie pies (survey meters), and so forth, that they would need. Who was going to man this stuff? They said they had nobody to run it. So I said I’d collect the staff to run the stuff. It was our equipment … and who is more competent to keep it running than our people? This was the sales pitch.”
Henriques was also on hand early in March 1948 when the Air Force hosted a meeting that included Air Force officers and scientists as well as top people from the AEC. These included Edward Teller, one of the AEC’s leading theorists. The participants agreed that of the three methods for detecting tests, only the seismic approach had promise, and then only for ground tests. Oppenheimer had said: “This program is nonsense. Seismic data is sound, but the possibility of collecting fission fragments from a thousand miles away is very remote.”
At the end of the day, Henriques and some AEC scientists returned to his hotel room and continued to talk over the problem. He stayed up most of the night doing calculations and eventually concluded that “this stuff has got to condense of itself and form particles.” Oppenheimer’s skepticism had been based on the belief that the particles of radioisotopes would be too small to trap or detect. But Henriques found that the agglomeration of very hot and atomized bomb debris might produce particles large enough to collect, even in a high-altitude airburst. The next day he showed the calculations to Teller and to Dr. Roderick Spence, a division director at the Los Alamos nuclear laboratory, who agreed that they looked promising.
The Sandstone shots, which would be used to test Henriques’s ideas, were only weeks away, and the Air Force hurried to get ready. Henriques recalled how fast it happened: “Money began to flow; no negotiations, nothing. We didn’t even estimate an overhead rate.” The weapons tests had the primary purpose of evaluating new design concepts. The second of them, Sandstone Yoke, yielded 49 kilotons, almost four times that of the Hiroshima weapon. The Air Force evaluated the seismic and acoustic methods, with disappointing results (though seismic detection would later prove useful for underground tests and enormous explosions such as hydrogen bombs). But the radiological method worked well.
Just as Henriques had predicted, analyzable quantities of fission products could be obtained thousands of miles from ground zero—including, as examination with a microscope revealed, some tiny but perfect metallic spheres. These had formed even though the Sandstone shots had taken place near ground level and their residue was contaminated with a great deal of vaporized soil. The spheres would stand out even more clearly in a shot made at high altitude.
The 375th Weather Reconnaissance Squadron now had what it needed. In April 1949 it began flying two daily missions out of Eielson Air Force Base, in Fairbanks, Alaska: One went to the North Pole and back, the other was a one-way to Yokuta, Japan, with a return flight the next day. Other routes could also be flown when necessary. Each plane carried 9-by-22-inch paper filters that picked up airborne particles and were checked on landing for radioactivity. If this radioactivity exceeded the natural background level, further measurements were taken to establish the cause.
When still more analysis was necessary, the filters were quickly flown to a new Tracerlab center in Berkeley, California. There the scientists dissolved them and carefully separated out certain fission products, including radioactive isotopes of barium, cerium, lead, molybdenum, and zirconium. They then calculated “radioactive birthdays” by measuring the rates of decay and counting backward to establish when each isotope had been created. Only if all the birthdays were identical could the isotopes have been created in an atomic explosion.
Unusually high levels of radioactivity were recorded 111 times in the months before the first Soviet nuclear test, and each time they proved to be from natural causes, such as volcanoes. Still, Tracerlab’s repeated exercises gave it the opportunity to perfect its technique. Then, on September 3, 1949, a WB-29 landed at Eielson with a filter sample obtained while flying east of Russia’s Kamchatka Peninsula. It exhibited radioactivity four times as great as the level that Tracerlab had set as an alert. Standard tests quickly showed that this reading had no natural explanation, so off the filter went to Berkeley, to be supplemented by numerous additional samples from subsequent flights. One sample taken two days later showed a radioactive decay rate 20 times the alert level.
“It was terrifically exciting once we … found out these were fresh fission products,” said Lloyd Zumwalt, the head of the lab, to Ziegler decades later. “… We worked night and day for a period of two weeks. I didn’t sleep more than four hours a day. Our little group was working around the clock. At the end of two weeks we went to Washington to report what we had found.”
Zumwalt and Henriques presented their results to a panel that included Oppenheimer and was chaired by Vannevar Bush, the wartime head of the Office of Scientific Research and Development. The panel also heard reports from Los Alamos, the Navy (which had been measuring the radioactivity of rainwater collected from the roof of its headquarters in Washington), and the British. All agreed that there had been a nuclear explosion in Soviet territory. Tracerlab, however, had the precision that the others lacked. The best that Los Alamos and the Navy could do was to place the date of the detonation within a few weeks. By contrast, Tracerlab placed it at “29 August, at 0000 Greenwich Mean Time,” which missed the actual time by only an hour.
An extensive community of scientists had begun studying the debris once it became clear that it might have resulted from a weapons test. John Manley, a senior AEC scientist and associate director of Los Alamos, later noted that “by September 14, 95 percent of the experts analyzing the data were convinced” that the samples had resulted from a bomb and not, for example, a reactor accident.
The quantity of debris, combined with a knowledge of weather patterns, provided a decent estimate of the bomb’s yield, around 20 kilotons. Its composition gave a good idea of how the bomb had been built. There were only a few possible ways to make an atomic bomb, and each one had its characteristic mix of isotopes. Tracerlab deduced that the Soviet device was probably a plutonium bomb with a uranium tamper, or shell. Otherwise there was no way to account for the presence of both plutonium and uranium 238. In sum, it was a virtual carbon copy of the weapon dropped on Nagasaki in 1945.
These findings helped explain how the Soviets had gotten the bomb so quickly. Since late 1946 the CIG (and then, later, the CIA) had been predicting the first Soviet test around mid-1953. But the evident similarity of this bomb to the one dropped on Nagasaki, and the detailed understanding of implosion processes that it required, showed that the Soviets had conducted espionage on a massive scale. On September 20 President Harry S. Truman was told the news. Three days later he issued a statement to the public: “We have evidence that within recent weeks an atomic explosion occurred in the U.S.S.R.”
Few laboratory analyses have ever had such an impact, because during the fall of 1949 the communists were on a winning streak. The forces of Mao Zedong had overthrown those of Chiang Kai-shek, and Mao was on the verge of formally proclaiming the People’s Republic of China. With the Soviets holding the bomb, it took little imagination to foresee a unified command of Russians and Chinese armed with the might of Soviet industry, deploying the limitless reserves of China’s manpower, and wielding atomic weapons.
Truman reacted by escalating America’s nuclear program. For several years Edward Teller had been calling for the United States to move toward weapons of much greater destructiveness by building a hydrogen bomb. Powerful voices opposed him, partly for fear of what such a weapon could do and partly because it was not at all clear, with the science of the time, that one could be made. But early in 1950, fearing a nuclear-armed communist alliance, the President announced his decision: He was directing “the Atomic Energy Commission to continue its work on all forms of atomic weapons, including the so-called hydrogen or super bomb.”
Now the AEC had to figure out how to create what Truman wanted. The best that bomb designers had come up with so far was a hybrid design in which the fuel for a potential hydrogen bomb would instead boost the yield of a conventional atomic weapon. Teller had already invented a spherical design for a weapon of this type, called the Alarm Clock. A trial of the booster principle was on the agenda for the next set of atomic tests, which were to take place in 1951. But the unlimited yield of a true H-bomb remained out of reach.
Then, early in 1951, Teller found what he needed, with help from the mathematician Stanislaw Ulam. The key, known as the Teller-Ulam principle, used the intense flood of X rays from an atomic explosion to compress the core of a bomb with far greater force than could be accomplished with the usual method of high explosives. In 1952 this breakthrough led to Ivy Mike, a true fusion device that exploded with a yield of 10.4 megatons. This was nearly a thousand times the yield of Hiroshima; it was so powerful that no one was allowed closer than 30 miles from the test site.
The Soviets, meanwhile, were continuing to develop and test atomic bombs, and the U.S. Air Force was keeping watch. The first Soviet test had been given the name Joe 1, after Stalin. In September 1951 came Joe 2. This time the core proved to be made of uranium 235 rather than plutonium, producing twice the yield of the 1949 shot. The next month Joe 3, which had a composite core, was air-dropped. Those two 1951 tests were used to decide the design of the Soviet Union’s first production atomic bomb, which was issued to the military in 1953.
Everyone expected that the Soviets would eventually detonate an H-bomb, so there was only modest surprise in August 1953 when Georgi Malenkov, chairman of the Council of Ministers, told the Supreme Soviet that “the United States has no monopoly in the production of the hydrogen bomb.” The Soviet H-bomb shot, called Joe 4, went off four days later, on a clear and nearly cloudless day. It took place at ground level, raising enormous, billowing clouds of dust that rolled across the desert while the mushroom rose in an awesome symmetry. The 400-kiloton yield, however, was far from impressive. Even some American atomic bombs were already delivering higher yields.
By now the Los Alamos lab had taken on the analysis of weapons debris as a specialty, eclipsing Tracerlab. Carson Mark, who had led the theoretical work on the Ivy Mike device, told the historian Richard Rhodes: “I remember our being very intensely involved in trying to reconstruct Joe 4, to figure out what they had done on the basis of the debris evidence we had. That was quite an intensive effort involving [Hans] Bethe, [Enrico] Fermi, both over a considerable period, and a bunch of the rest of us sitting around a table. We managed to speak of an object physically quite similar to what Joe 4 must have been. It didn’t lead us to want to emulate it.”
George Cowan, a leader in the analysis of bomb debris, thought that the device must have been something the Soviets had put together in a hurry as a response to Ivy Mike. Bethe told Rhodes that the analysis revealed that Joe 4 “was compressed by high explosives. It was alternating layers of uranium and lithium deuteride, like our Alarm Clock design. All that we figured out just from seeing the debris. We also figured out from the debris that it was a single-stage device.” By contrast, devices such as Ivy Mike were two-stage affairs, in which an atomic bomb acting as the first stage set off a second stage with thermonuclear fuel, which had far more power. A single-stage device amounted to no more than a boosted A-bomb, and American physicists regarded it as a dead end.
How could they tell? The independent nuclear specialist Howard Morland suggests that since Joe 4 relied on high explosives for compression, instead of the much more effective Teller-Ulam radiation compression used in Ivy Mike, the debris from Joe 4 would have included markedly fewer products of nuclear reactions. Another nuclear specialist, Carey Sublette, notes that the radiation implosion of Ivy Mike had produced an extraordinarily high neutron density that gave rise to isotopes as heavy as einsteinium 255 and fermium 255. No one had ever seen such heavy elements before; Ivy Mike was their first known appearance on Earth. The absence of these isotopes in Joe 4 showed that the Soviets had not yet learned to use radiation implosion.
But even here America’s supremacy would be brief. The Soviet physicists Andrei Sakharov and Yakov Zeldovich discovered the Teller-Ulam principle for themselves during 1954, and the two-stage bomb that resulted, with a designed yield of three megatons, was first tested in November 1955.
The day of airborne detection ended when tests moved underground after the United States and the Soviet Union ratified the Limited Test Ban Treaty of 1963. It was an important step toward peace that the Soviets didn’t mind taking, because in the era of atmospheric tests they had lost too many of their military secrets to the physicists of Tracerlab and Los Alamos. The test-ban treaty remains in effect to this day and continues to win plaudits for eliminating radioactive fallout. But it also stands as a memorial to those physicists who contributed to the technical methods of nuclear intelligence and helped the American nuclear community learn just what the Soviets were doing—as well as not doing, just yet.
