The first of the post-war tests took place at “Crossroads,” in July 1946, less than a year after the war’s end. There, at Bikini atoll in the Marshall Islands, two bombs were exploded, one of them under water. These explosions were followed by three more at “Sandstone” at the Enewetak atoll, also in the Marshall Islands, in April and May 1948.{9} Planners in the United States, with much future testing in mind, were all too aware of the costs of conducting tests many thousands of miles away (more than 10,000 people, most of them military personnel, took part in Sandstone{10}). They began asking themselves the question: Can we find a test site within the continental United States?
A European would smile at such a question about the home continent. But in the United States, there were still large tracts of unoccupied or very sparsely occupied land. I remember seeing Edward Teller, in 1950, poring over a table-top-sized map of the United States looking for likely test locations. He pointed to quite a few. (Why Teller was involved in site selection I don’t know, but he liked to be near the center of things, and he very likely took the position that we can find and will find a continental site when some of his colleagues may have considered it a fruitless search. Teller was also part of the group that pointed to Aiken, South Carolina, at the meeting mentioned near the end of Chapter 9 (page 104). Why I was present at both meetings I don’t know. I guess the curious were admitted.) In fact a site in Nevada some 65 miles northwest of Las Vegas was designated for nuclear weapons testing on January 11, 1951, and, with lightning speed, put to use.{11} Five tests took place there in Operation “Ranger” in the last week of January and first week of February 1951. In October and November of that year, in Operation “Buster Jangle” at the Nevada site, seven more bombs were exploded (one of them a fizzle).{9} It is very likely this Buster Jangle series that Carson Mark was working on in February 1951 when Stan Ulam barged into his office to tell him about a new idea.
Over the years, the Nevada test site became the favored location for weapons tests. Of the 1,054 reported tests in the period 1945–1992, 928 were carried out in Nevada, 828 of them underground and 100 above ground.[61] The remaining tests were carried out mostly in the Pacific (106) with a scattering (20) in other places, including even two in Mississippi.{9}
Just three of those thousand-plus tests come into my story, all three conducted at Enewetak. They were Greenhouse George and Greenhouse Item in May 1951 and Ivy Mike on November 1, 1952 (October 31 in the United States).
By the time I arrived in Los Alamos in mid-1950, the authorities there had decided that thermonuclear burning needed some experiments, not just theorizing. In February of that year the “Family Committee” had come into being, with Edward Teller as its chair, to oversee thermonuclear developments.{14} (The name, I was told, reflected the fact that the committee was to consider a new family of weapons—or perhaps all families of weapons.) Members included leading lab scientists and engineers from various divisions—including those concerned with theory, experiment, chemistry, metallurgy, and testing. I was not a member but did attend some of its meetings, most of which were concerned with planning for the Greenhouse tests scheduled for the following May.
Available minutes of the Family Committee meetings run from its fourth meeting in March 1950 to its twenty-seventh meeting on November 15, 1950.{15} At that meeting, Norris Bradbury the lab director, reminded the committee that its charge was to look beyond Greenhouse at all phases of the thermonuclear program.{16} To what extent it did so I don’t know since if any minutes were recorded for meetings after November 15, they remain shrouded in secrecy. To call the minutes of the earlier meetings “available” is a slight overstatement. They are works of modern art, studies in black and white, containing fully blacked out pages intermingled with partially whited out pages, with a few sentences peeking out here and there. The sentences that do make it into the light show that the committee was largely concerned with experiments planned for the George and Item “shots” at Greenhouse—experiments with names like FLUMEX, GANEX, TENEX, DINEX, and PHONEX. (A saying at the lab at the time was that if a really clean experiment were ever designed, it would be called KLEENEX.)
Part of my job in my first months at Los Alamos was to carry out calculations on the expected performance of some of these experiments. The George shot was the more complicated of the two. It used a large fission bomb, reportedly the largest to that date (225 kilotons), to ignite a small quantity of a liquefied (and frigid) mixture of deuterium and tritium. This was in the spirit of the 1946 invention of von Neumann and Fuchs that I described in the previous chapter—although, as I mentioned there, I don’t recall the von Neumann-Fuchs invention ever being discussed in connection with planning for George. The ignition occurred because radiation from the fission bomb ran out ahead of the expanding matter from the fission explosion. The fission bomb was, according to descriptions now posted, cylindrical rather than spherical—perhaps the only one of its kind ever made.{17} That geometry meant that there was a channel out of which radiation could pour onto the nearby container of liquefied deuterium and tritium. Von Neumann and Fuchs had considered a gun-type weapon to achieve the same end. The questions of interest were: Did the DT mixture burn? If so, what fraction of it was consumed? What temperature was reached in the DT container? (Energy release from the DT burning, even if complete, was inconsequential relative to the energy of the fission trigger.)
It was to answer these and related questions that all the diagnostic …EX’s were devised. It is a tribute to the skill of the experimental physicists and the engineers that so much could be learned in so short a time—much less than a thousandth of a second. The fast (very fast) cameras, the radiation detectors, the particle detectors: All had to send their reams of data, through wires and over the airwaves, in the instant before they breathed their last—true swan songs.
One of the most straightforward ways to verify DT burning is to look for the 14-MeV neutrons that are emitted (see the reaction equation on page 110). They stand out because they are more energetic than any other neutrons coming from other sources, and they hasten to the detectors faster than anything else except light. A neutron of this energy travels at about one-sixth the speed of light. Atoms of metal from the exploding bomb, even at their extreme temperature, are a few hundred times slower than that. Suppose that an array of detectors is located 30 meters (about 100 feet) from the bomb. Light will reach the detectors in a tenth of a microsecond, 14-MeV neutrons in six-tenths of a microsecond, and atoms of iron (from a 15-million-kelvin temperature maelstrom) in about 200 microseconds. The work of the detectors needs to be completed in something like one ten-thousandth of a second.
To everyone’s relief, the unconventional fission bomb exploded as predicted, and the diagnostic experiments worked. The data from Greenhouse George (May 9, 1951) indicated that the DT mixture had been heated past its ignition temperature and had indeed burned—the first thermonuclear reaction to be achieved on Earth. There was even evidence that the DT container had experienced some radiation implosion and/or plasma-induced implosion of the kind von Neumann and Fuchs had visualized, although implosion had not been planned. George had been “put to bed” well before the ideas of radiation implosion and an equilibrium Super had surfaced{18} and seemingly after the work of von Neumann and Fuchs had receded from the hippocampi of the various physicists.
61
Some of the above-ground tests in Nevada produced unacceptably high concentrations of radioactive fallout in locations such as St. George, Utah, where, beginning in the 1950s, elevated rates of cancer were reported.{12} There was also at least one serious incident of fallout from a test in the Pacific, where, in 1954, the crew of the “Lucky Dragon No. 5” fishing boat received high doses of radiation. The captain died seven months later and the lives of other crew members may have been shortened.{13}