Monday, July 15, 2013

The Philosopher's Bomb, Part 1

Magnificent Men and their Atomic Machines

The Philosopher's Bomb: Discovering New Elements with Nuclear Explosions

Part I

With special thanks to Dr. Steve A. Becker and Dr. David W. Dorn

At seven o'clock in the morning, on November 1st, 1952, a man pushed a button on a control console on the USS Estes. Fourteen minutes and 59.4 seconds later, a series of detonators fired on the island of Eniwetok. The explosives compressed a hollow sphere of uranium-plutonium alloy to a fraction of its former size; at the same moment, a burst of neutrons from the initiator in the sphere's center split a handful of atoms. Those fissioning atoms released more neutrons, which split more atoms, releasing more neutrons, building in a fraction of an instant to apocalyptic force.

The superheated plasma began to push out from the boiling heart of the primary – but the X-rays leapt ahead of it, bouncing down the bomb casing to a refrigerated canister of cryogenic deuterium-tritium wrapped in polyethylene. The X-rays evaporated the plastic, the vapor pushing like rocket exhaust, compressing the D-T around a thin rod of uranium, which began to fission itself, releasing still more energy – and the fusion reaction ignited as deuterium + tritium became helium-4 + neutron, a torrent of heat and light that shattered the morning calm with the force of 10.4 million tons of TNT. This was IVY MIKE, the United States' first test of a hydrogen bomb, a weapon tapping nuclear fusion – the same process that powers the stars.[Rh]

Figure 1: IVY MIKE Cloud[CTBTO]
(US Government)

Soon after the blast, Air Force F-86 fighters flew through the drifting remains of the mushroom cloud. Filters mounted on their wings collected samples of the radioactive debris from the explosion. The samples were flown to Kirtland AFB in New Mexico, and from there to Los Alamos Scientific Laboratory, where they were dissolved in fuming nitric and perchloric acid to be separated into their constituent elements for analysis. This was tricky business; the samples were contaminated with vaporized coral, which at times reacted energetically with the acids – some samples literally caught fire.

The results were puzzling: heavy beta radiation was being emitted by the plutonium in the samples, even though plutonium ordinarily emits alpha particles. Further investigation revealed that the beta radiation was due to three previously unknown isotopes of plutonium: plutonium-244, -245, and -246.

The news stunned Glenn Seaborg, head of nuclear chemistry research at the AEC's Berkeley laboratory. The Berkeley group had been trying to produce plutonium-244 by bombarding plutonium-239 with neutrons in the Materials Test Reactor in Idaho. Producing detectable amounts of the isotope in a reactor was expected to take years of irradiation; the intense neutron flux produced by MIKE had done it in an instant.

Acting on a hunch, Seaborg arranged for the Berkeley group to get hold of some of the samples from the explosion. Seaborg had co-discovered plutonium in 1940 and led the team that had found elements 95 through 98 in the late 40s. Some rough calculations suggested that the torrents of neutrons produced in the blast might have produced even heavier atoms than plutonium-246 – atoms that might have decayed to previously unknown elements. After careful chemical separation, the Berkeley team produced a residue emitting alpha particles at energies produced by no other known material. Elements 99 and 100 – later named Einsteinium and Fermium – had been found.[HGS]

Figure 2: Einsteinium Glowing From Radioactive Decay[Wiki]
(US Government)

In the hellstorm of the nuclear blast, some of the neutrons produced by the fusion reaction had collided with atoms in the uranium-238 tamper. Many of the uranium atoms split, adding still more energy to the blast, but some instead absorbed a neutron to form uranium-239. Then, some of the uranium-239 atoms had absorbed a second neutron, and a third, and a fourth. Up to 17 neutrons were absorbed by a single atom of uranium, producing uranium-255, which then immediately began to decay, spitting out beta particles.

There are a number of different forms of radioactive decay. Very heavy atoms primarily decay via alpha emission, spontaneous fission, and beta-minus emission. In alpha emission, the nucleus jettisons a helium-4 ion, consisting of two protons and two neutrons. In spontaneous fission, the atom splits apart entirely, producing two daughter atoms of varying mass. In beta-minus emission, one of the neutrons in the nucleus is transformed into a proton, emitting an electron in the process. Nuclides with too many neutrons – such as uranium-255 – tend to decay very rapidly via beta emission. Since the number of protons in the nucleus determine what element it is, and each fleeing beta particle transforms a neutron into a proton, the result is literal alchemy: the transmutation of one element into another. The uranium-255 atoms spit out eight electrons, ending with a nucleus of 100 protons and 155 neutrons: fermium-255. Atoms that absorbed fewer neutrons produced other isotopes; in all, fifteen previously undiscovered isotopes were identified in the MIKE debris.[HGS][Be2]

Figure 3: Producing New Elements in Thermonuclear Explosions

This is a very different process then was used in previous attempts to discover new elements. The usual approach was to slam light nuclei – originally hydrogen or helium, later oxygen, nitrogen, or other elements – into heavy nuclei in particle accelerators. The two nuclei would fuse, with some neutrons flying away like shrapnel; since the compound nucleus would have more protons than either of its progenitors, it would constitute a new element. However, since the ratio of neutrons to protons needed for the nucleus to be stable increases as the atomic number increases, the resulting atom tended to have too few neutrons for stability, and decayed extremely quickly, usually by alpha emission. Alpha emission removes two protons from the nucleus, turning the atom into a lighter, already known element. After the MIKE shot, scientists at Berkeley successfully synthesized elements 101 (Mendelevium) in 1955 and 103 (Lawrencium) in 1961 using this approach.

Figure 4: Producing New Elements Through Ion Fusion

The process that occurred in MIKE went in the opposite direction, adding a dozen or more neutrons to a heavy nucleus, producing a neutron-heavy isotope of a known element. This then decayed by turning neutrons into protons, producing a previously unknown element.

After the discovery of elements 99 and 100 in the MIKE debris, the AEC dredged the coral around Eniwetok for more samples for research. For the 1954 through 1956 testing seasons, a laboratory was established on the island itself, so that samples could be analyzed before short-lived isotopes had time to decay, and the debris clouds of the hydrogen bomb tests combed for new elements beyond 100. But no new elements were found – in fact, the yields of heavy isotopes were lower than in the MIKE shot.[Ho] To discover new elements would require going beyond MIKE, designing devices that would produce neutron intensities well beyond anything the AEC had ever built before.

There had been interest in using nuclear explosives as scientific instruments since the Manhattan Project in the 40s, but little work had been done on the concept. The scientists were simply too preoccupied with developing the hydrogen bomb and building up the nuclear arsenal to pursue less immediate concerns; in addition, secrecy restrictions would inevitably hinder publication of any results from such an experiment.[Co2] But by the mid-1950s, the hydrogen bomb had been successfully tested and the diffusion plants and production reactors were churning out enough fissile material for hundreds, then thousands of bombs. With the most urgent problems of the nuclear weapons program solved and spurred by the results of MIKE, scientists at Los Alamos quietly began to investigate ways to make use of the incredible energy released by the hydrogen bomb.[Co]

In 1957, Los Alamos issued a classified report discussing possible uses of nuclear explosives as sources of energy and particles for scientific experiments. Production of new elements by multiple neutron capture – replicating the results of MIKE in heavier elements – appeared to be one of the most promising avenues of investigation.[Co2]

The scientists of the AEC proposed turning the most powerful weapon ever built by humankind into a tool for peaceful research – for probing the deepest secrets of the atom.

Basic Research at the AEC
Whether produced by adding neutrons or ions, the new elements were themselves radioactive, eventually decaying by spontaneous fission or alpha emission into known elements. The further up the periodic table, the more quickly the elements tend to decay, and the harder they are to produce – some can only be made in quantities measured in tens of atoms making it difficult to find practical applications.

Nonetheless, the Atomic Energy Commission provided substantial support for superheavy element research throughout the 40s, 50s, and 60s. Millions of dollars were invested in new particle accelerators and research reactors, as part of a broad program of investigation into basic nuclear physics of all kinds – a program that grew steadily throughout the 50s and early 60s.

Figure 5: AEC Funding for Basic Research[NSF][IC]
Numbers in 2012 US Dollars

The reasons behind this were complex. On a practical level, although the half-lives of known elements shrank rapidly as the atomic number and mass increased, there was no obvious reason why a region of greater stability might not exist further up the periodic table.[Co2] Many known superheavy synthetic elements, such as americium, are fissionable, and heavier isotopes tend to release more neutrons per fission than lighter ones. This means they have a smaller critical mass, so heavy element-fueled reactors or bombs could be smaller and more compact than uranium or plutonium fueled equivalents. The advantages of the known transuranics are too slim to justify the higher cost of the fuel. But if stable superheavy elements could be found further up the periodic table, they might be sufficiently superior to justify their use, at least in specialized applications such as powering spacecraft.[Al][Ka] It was also thought they might also find uses in diagnosing or treating disease.[Al]

An astute reader will note that these are essentially the same applications that existing artificial isotopes and elements were already being put to. Since no one knew the properties of as-yet-undiscovered elements, except that they would probably be radioactive and fissile, all one could really say about their possible uses is that they might be useful for the same things that radioactive and fissile materials were already being used for. In reality, while the AEC might suggest these as possible uses, they were probably primarily motivated by less tangible goals.

First, the AEC needed to attract talented nuclear physicists to work on weapons and other military projects, tasks relatively few scientists found appealing. By funding basic research, the AEC was able to recruit physicists who would be unwilling to work for them if they were limited to military work. Second, the AEC felt itself to be competing with the Soviets in all fields – having the highest-energy particle accelerator, the biggest research reactor, the most powerful supercomputer was a matter of prestige and pride, and many scientists were not shy about invoking the specter of communism to gain support. No AEC employee wanted American schoolchildren someday memorizing “Kurchatovium” or “Sakharovium” from the periodic table.

Third, the AEC saw basic research as an investment for the future. The atomic energy enterprise, after all, was born from theoretical physics – the AEC had been founded in part on the synthetic element plutonium, churned out by the ton by the production piles at Hanford and Savannah River. Basic research restocked the capital that the Manhattan Project had drawn on. Even just learning the precise half-life of new heavy elements in the instants before they boiled away would add to the growing understanding of the atomic nucleus. The General Advisory Committee, the AEC's leading scientific consultants, called basic research “our first line of defense for 10 or 20 years hence.”[W]

But, even more fundamentally – and I admit I cannot prove this – I suspect the primary reason for the AEC's support of basic scientific research was that many leaders of the AEC bureaucracy were, themselves, scientists. Scientists do not need a reason to seek new knowledge; it is a goal in and of itself. The bombs were being set off anyway, and many experiments could be done for minimal cost as add-ons to weapons tests. Why not put them to good use?

The First Steps
The first AEC nuclear test intended to produce superheavy elements was fired at Eniwetok on May 11th 1958. Codenamed BUTTERNUT and yielding 81 kilotons, it was a test of the fission primary of Los Alamos lab's TX-46 hydrogen bomb, and was detonated on a barge in the atoll's lagoon.[Su][DoE] According to George Cowan, the device was “slightly modified to include a region of high neutron flux.”[Co2] The target is uncertain, but according to a later paper a study was made of multiple neutron capture in plutonium as a preliminary to the test, suggesting that it may have been plutonium-240.[Kn] Although the weapons test was successful, Cowan reports that “failure of a vital component spoiled the [superheavy element] experiment.”[Co2]

Cowan would be a major figure in the Los Alamos program to use nuclear explosives in physics experiments. After working on the Manhattan Project and the 1946 CROSSROADS nuclear tests, he had returned to university to get his Ph.D., which he paid for in part by selling a patent on a method to make silver formate to Aerojet-General. Returning to Los Alamos in the late 40s, he had become involved in nuclear forensics – attempting to guess the progress of Soviet weapons development from the fallout of their nuclear tests – and in developing diagnostic methods for the hydrogen bomb. He had been on the Estes for the MIKE test, and couriered the fallout samples that would lead to the discovery of Einsteinium and Fermium back to the U.S. By the late 50s he was head of the Los Alamos radiochemistry group.

Figure 6: George Cowan, c. 1950s[ESVA]
(US Government)

Another test that year, QUAY, also fired by Los Alamos Scientific Laboratory, included a different type of physics experiment.[CD] QUAY, like BUTTERNUT, was primarily a weapons test, possibly of the fission primary of the XW-50 hydrogen bomb, which was later used in the Pershing ballistic missile.[Su] The test attempted to measure differences in how uranium atoms broke apart at different neutron energies: how frequently they split symmetrically versus asymmetrically. The use of a nuclear explosive provided a uniquely intense source of neutrons compared to a research reactor or a particle accelerator.

The experiment was first attempted in May 1958 at Eniwetok, but failed due to a problem with the neutron shielding. The name of the test is not recorded, but it may have been the TOBACCO test of May 30th 1958, an earlier test of the XW-50.[Su] It was tried a second time later that year at the Nevada Test Site. On October 10th, a 0.079-kiloton nuclear weapon exploded at the top of a 100-foot-tall steel tower.[Su][DoE] Neutrons produced by the fission reaction first passed through a hydrogenous material to slow them to between 1,000 ev and 0.282 ev.[CTB] Although the specifics for QUAY are not known, in most ensuing experiments 3 to 5 centimeters of polyethylene between two layers of lead were used.[Di] The slowed neutrons then passed through a 30-meter-long evacuated pipe, and then were filtered into a straight beam by a slit in the 5-foot-thick wall of a reinforced bunker. The neutrons struck a two-foot-diameter rotating wheel revolving precisely 67.68 times per second, with six uranium-235 foil samples mounted on its rim.[CTB]

Figure 7: QUAY Tower Before Detonation[CROH]
(US Government)

Figure 8: QUAY Rotating Wheel Experiment[CTB]
(US Government)

Neutrons traveling at different speeds arrived at the wheel at different times, exposing different areas on the wheel to neutrons of a different, specific energy level. After all of the samples had been irradiated, explosive squibs fired, slamming a cadmium-covered steel shutter over the beam port to keep the samples from being damaged or contaminated by debris or slower neutrons. Then the blast wave hit and the bunker's roof fell in.[CTB]

The roof collapse delayed recovery of the samples, but a week later the Los Alamos scientists dug back into the bunker and retrieved the foils. The foils were then cut apart into slices and analyzed to determine the yield of barium-140, molybdenum-99, and silver-111. The ratio of barium to molybdenum was used to confirm that the individual fission yields were as expected for thermal neutron fission, while the ratio of silver to molybdenum determined the relative amount of symmetric versus asymmetric fission. A fission that produced silver would be close to symmetric, since the other fission fragment would have a mass of 121 or 122, while one that produced molybdenum would be asymmetric, with the other fragment massing 133 or 134. The analysis showed a variation of 56% in the ratio of symmetric to asymmetric fission over the energy range concerned, with noticeable spikes and troughs in symmetry at specific energy levels. Unfortunately, however, the data was contaminated by fissions from neutrons that made it through the cadmium shielding or that were produced by fission within the wheel, raising questions about the validity of the results.[CTB]

But these two or three scientific tests were almost the last. In March of 1958, the Soviet Union announced a unilateral moratorium on nuclear testing; the US and UK followed suit, and the nuclear powers began meeting to negotiate a permanent test ban. This moratorium applied to peaceful nuclear detonations as well. For the moment, the research program was put on hold.

Meanwhile, interest was also growing in other non-military uses of nuclear explosives.
Interest in peaceful applications of nuclear explosions dated back to the earliest days of the Manhattan Project. The Soviet ambassador to the UN, Andrei Vishinsky, even tried to claim the USSR's first nuclear test in 1949 was actually for peaceful purposes:

The Soviet Union did not use atomic energy for the purpose of accumulating stockpiles of atomic bombs,... it was using atomic energy for purposes of its own domestic economy: blowing up mountains, changing the course of rivers, irrigating deserts, charting new paths of life in regions untrodden by human foot.”[No]

But it was not until the Suez crisis in 1956 that the idea began to receive official scrutiny in the US. During a meeting of weapons scientists at Lawrence Livermore lab, one of the physicists, Harold Brown, mused on the possibility of using thermonuclear explosives to cut a new canal through Israel to replace the blockaded Suez. Another scientist at the meeting, Gerald Johnson, later recalled that “we simply pulled out a map, developed a profile from the Gulf of Aquaba to the Mediterranean and rapidly decided in principle it was technically possible.”[Ka][Ki]

Once the idea had been raised, a plethora of other possibilities came to mind. The hydrogen bomb could be used to dig harbors, excavate highway passes, break up rock for mining, produce heat for electricity, and create useful radioactive isotopes. A symposium was held on the concept in February of the next year. Proposals were mostly still very vague; papers at the symposium focused on power production and excavation, and the only scientific application discussed was seismic sounding.[PS58] But the idea – dubbed Project Plowshare – was very appealing to the scientists of the AEC, and in June of 1957 the AEC decided to formally begin a research program into peaceful applications of nuclear explosives.[Ka] The work in scientific applications of nuclear explosives found a natural home within Plowshare.

Edward Teller became a particular sponsor of the Plowshare program. Teller was a brilliant scientist and an irrepressible font of ideas, although notoriously unable to distinguish good ones from bad. After working on the Manhattan Project, he had left Los Alamos because he believed the lab was dragging its feet developing the hydrogen bomb, and helped to found Lawrence Radiation Laboratory at Livermore (LRLL) as a second weapons lab. A healthy rivalry has existed between Livermore and Los Alamos ever since; Isidor Rabi, a physicist and member of the AEC's powerful General Advisory Committee, described the relationship as like that between two brothers, both friends and competitors.[Be][LAS] Plowshare became one of Teller's many enthusiasms; he did not work on the Plowshare technology personally, but he reviewed proposed device designs and he aggressively promoted it among politicians and the public. And he encouraged Livermore scientists to involve themselves in the superheavy element program, previously Los Alamos' domain.[Ka][Co][Do]

Figure 9: Edward Teller, on Right, at GNOME Test Site[LLNL]
(US Government)

Lawrence Livermore held a second Plowshare symposium in 1959, this time in San Francisco, from May 13th to May 15th, 1959. Interest in the program had grown rapidly: where the unclassified component of the first symposium included only 13 papers, the second symposium counted 50, including discussions of using hydrogen bombs to create new elements. Los Alamos held its own symposium on July 6th-8th, 1959, chaired by George Cowan, specifically devoted to scientific applications of nuclear explosions: dispersing radioactive tracers to map air currents, experiments on the interplanetary medium, RADAR imaging of distant planetary bodies, and using bombs as neutron sources, again including to produce new elements.

Despite the testing moratorim, the labs also planned a number of nuclear tests. The proposed shots included Project OILSAND, which would use nuclear explosions to extract oil from oil shale; Project CHARIOT, a multi-explosion project in northwestern Alaska to excavate a new harbor; and project GNOME, a test in a salt formation near Carlsbad, New Mexico, that would serve a number of purposes, such as testing whether industrial heat and useful radioisotopes could be produced using nuclear explosions. Livermore was the main center of Plowshare work and the GNOME test was sponsored by them, but they agreed to include add-on experiments by other labs, including a neutron physics experiment developed by Cowan and other scientists at Los Alamos.[Ka] But none of these tests could proceed while the moratorium continued to be in effect.

Entering the 60s
Plowshare remained in limbo through 1959 and 1960. But, in 1961, two events took place that would have major ramifications for the program.

The first event was the appointment of Glenn Seaborg to head the Atomic Energy Commission by newly elected President John F. Kennedy. By 1961, Seaborg had codiscovered no fewer than eight synthetic elements. As a scientist himself, Seaborg naturally regarded basic research as a major priority of the commission, and authorized substantial increases in funding.

Figure 10: Glenn Seaborg as Chairman of the AEC, c. 1964[NA]
(US Government)

Even before becoming chairman of the AEC, Seaborg had been instrumental in arguing for construction of the High Flux Isotope Reactor at Oak Ridge, one of whose purposes was to produce known synthetic heavy elements. As chairman he supported a National Transplutonium Production Program to oversee production and distribution of curium, berkelium, einsteinium, and fermium for physics experiments. Seaborg was a strong supporter of the Plowshare program in general, and admitted he had a “particular interest” in the superheavy element program.[Se][JCAE65B]

Although he may also have been motivated by more personal concerns – one anonymous AEC staffer said that “subconsciously, at least, someone [Seaborg] would like to have an element named Seaborgium”[Wa] – he explicitly justified the superheavy element program to Congress as a search for basic knowledge without anticipated practical applications:

Chairman PASTORE: What would be the essentiality of these isotopes?

Dr. SEABORG: Some of them produced in this manner might prove to have practical applications. It might be that we could produce some of the already known isotopes more economically in quantity this way. Perhaps the main use, however, would be in increasing our knowledge of atomic structure and nuclear structure.

Chairman PASTORE: It would be basic knowledge?

Dr. SEABORG: Basic knowledge but of a very important kind, because the nuclear properties of these elements are unique. They have unique structures, unique methods of decay found nowhere else in the periodic table. The study of these radioactive properties leads to knowledge about nuclear structure that you can't get any other way.”[JCAE65]

The second major event of 1961 was the breakdown of negotiations over a test ban. In August the Soviet Union announced they would resume nuclear testing. The ensuing test series included Tsar Bomba, the largest nuclear device ever detonated.

The US quickly responded by resuming its own nuclear tests. Plowshare was in business – and its first test would be GNOME, on December 10th, 1961.[Ka]

GNOME was intended to serve a number of different functions for the Plowshare program. It would provide information on the physics of a nuclear blast in salt – important since, at the time, the AEC had never conducted an underground detonation except in the alluvium and volcanic tuff of the Nevada Test Site. It would test whether the heat of the blast would be retained in the molten salt, allowing it to be extracted later for power, and the feasibility of retrieving the valuable radioisotopes produced by the blast. It would be used to test seismic sensing techniques to detect underground nuclear explosions. And it would include several neutron physics experiments. GNOME was not intended to generate heavy elements, but it would provide neutrons for other physics experiments, and the residue would be used to develop chemical analysis techniques for separating new elements in future tests.

Detonating the device in salt would make recovery of material containing these radioactive isotopes – including, eventually, new elements – much easier. An underground nuclear blast initially creates a roughly spherical cavity filled with vaporized rock. As heat leaks out through the cavity walls, the rock vapor condenses into a liquid, pooling at the bottom of the chamber; most of the radioactive material – including any newborn heavy elements – would collect there as well. In ordinary stone, the cavity ceiling soon collapses, diluting the molten rock, laden with radioisotopes, with solid, relatively uncontaminated rock. But it was expected that, in salt, the cavity would remain intact, with the molten salt solidifying to form a plug at the bottom of the chamber. Not only would the radioactive material be more concentrated, the salt would also be easier to process chemically than the hard rock of the Nevada Test Site.[Ra] After waiting a year to allow the radioactivity to decay, 10,000 to 35,000 tons of salt would be mined from the bottom of the bomb cavity and processed to recover a milligram or so of superheavy elements, hopefully including previously undiscovered types.[CTD64]

The idea had actually been born as the Megaton Ice-Contained Explosion (MICE) project in the early 1950s, a brainstorm of Ted Taylor, a Los Alamos weapons designer. At the time, the AEC was having difficulty producing enough plutonium and tritium to meet the military's insatiable demand for nuclear weapons. Plutonium-239 is produced by uranium-238 absorbing a neutron, while tritium is produced by lithium being split by neutrons; manufacturing either one is thus essentially a matter of finding ways to make neutrons efficiently. The AEC used giant nuclear reactors at Hanford and Savannah River as neutron sources, but Taylor proposed, instead, using high-yield hydrogen bombs, buried in arctic ice and surrounded by uranium or lithium seed material. The blast would transmute the seed material and mix the product with molten ice, which could be easily pumped to the surface for processing. The lack of convenient ice sheets put a halt to that version of the plan, but MICE instead refocused on using rock salt, which was more readily available.[Mc][D]

The idea caught the attention of John von Neumann, an AEC commissioner and one of the greatest mathematicians of the 20th century. Research into the idea included an underground nuclear test at the Nevada Test Site on October 8th 1958, code named TAMALPAIS.

TAMALPAIS was a 0.072-kiloton underground weapons test, testing the XW-48 155mm atomic artillery shell. Experiments for MICE and in support of the anticipated GNOME test were included as add-ons. The device was surrounded with 2 feet of rock salt before the detonation, to determine if plutonium or tritium could be extracted from the salt. A gas sampling system developed by Lawrence Livermore and Oak Ridge National Laboratory was also installed, in which an evacuated pipe to leading an underground chamber about 125 feet away drew off samples of the gases and dust produced by the blast. Bottles of tritium gas were also placed in the shot room to “spike” the explosion gases.

The sampling system was apparently mostly successful with 8 out of 10 tanks recovered containing useful material, despite an accident during reentry when hydrogen produced by the radiation from the bomb exploded. (Fortunately, no one was seriously hurt).[Fa][La][Su][Cl]

Figure 11: TAMALPAIS Gas Sampling System[La2]
(US Government)

MICE was shut down in 1958 after the death of von Neumann, its primary sponsor, without having produced any plutonium. But the idea of using nuclear explosions to make useful radioisotopes – not necessarily just plutonium and tritium – continued in the GNOME experiment.[Mc][BC][GJM][Le]

The 3.1 kT GNOME device was buried at the end of a long L-shaped tunnel, 1,184 ft deep and 987 ft long. The tunnel was excavated with a buttonhook at the end, so that it would collapse before the blastwave could reach the experiment. Neutrons from the burst would pass through 2 inches of lead followed by 1.5 inches of polyethylene to slow them to a few tens to a few thousands of electron-volts. They would then enter a vacuum tube leading through the emplacement tunnel to the bottom of the access shaft.

Figure 12: GNOME Layout[Ra]
(US Government)

The initial beam would be split into three separate separate beams: two would strike neutron detectors, while the third would be split again and then hit a pair of rotating wheels. The top, smaller wheel, contributed by George Cowan's team at Los Alamos, held samples of uranium-235, while the bottom wheel, from Lawrence Livermore, had samples of uranium-238, thorium-232, gold-197, and hafnium-180.[FNER]

Figure 13: GNOME Rotating Wheel Experiment[GHH]
(US Government)

The Los Alamos wheel would be a replication of the QUAY test over a wider energy range and with more precision: the greater yield of the GNOME device meant more neutrons, while the longer pipe between the bomb and the wheels meant the neutrons would be better separated into different energy levels than in the earlier experiment. It would also test whether the sample wheel could be recovered without damage prior to using the technique on more-dangerous plutonium-239 in a later experiment.[CBP] The Lawrence Livermore wheel would measure the neutron capture cross-section of various materials at different energy levels to greater precision than had previously been possible. The capture cross-section is the probability that a neutron will be absorbed by an atom of the material as it passes through a sample of given thickness; this can vary substantially between different neutron energy levels, making precise data very valuable. This would be of particular interest for uranium-238 and thorium-232, which are both nuclear fuels, but which must be “bred” into plutonium-239 and uranium-233 by neutron absorption before they can fission; knowing the precise graph of the neutron capture cross-sections would make designing nuclear reactors easier.[Ra][Co3]

Besides the main tunnel, two pipes would be drilled from the surface towards the bomb before the blast, one from directly above the device, the other offset by 30 to 75 ft, both ending 150 ft above the bomb. If they didn't collapse from the shock of the blast, these pre-drilled holes would offer quicker access to the blast zone to collect samples.[V]

Like TAMALPAIS, the GNOME shot also included a test of a gas sampling system, once again developed by Oak Ridge and Lawrence Livermore. At the time, Oak Ridge was also researching the use of nuclear explosions to provide energy to drive industrial chemical reactions, and had a natural interest in recovering the product more quickly than could be achieved by drilling back into the blast cavity. A gas sampling system would also be useful in future heavy elements tests, since any new elements produced might have very short half-lives, short enough they would decay away before the experimenters could drill back into the cavity.[CTD64]

A pipe was drilled from the surface to the blast chamber, and samples of material placed at the bottom, including aluminum, thorium, and mixtures of nitrogen with hydrogen, oxygen, or carbon. The heat of the bomb would vaporize the material and drive it up the tube before they could be destroyed by the blast, into catcher chambers in a shed on the surface, made of reinforced concrete to contain any explosive energy that leaked through the pipe.[La] A zinc amalgam coating was also applied to the bottom of the tube as a tracer, to see if the vaporized segment of the pipe would be jetted up the drill hole when it was vaporized by the gamma rays, in the moments before the blast wave destroyed it.[CTD62]

Figure 14: GNOME Prompt Sampling System[La]
(US Government)

The Atomic Energy Commission invited guests from news groups, the UN, and ten foreign governments – including the Soviet Union, who declined – to attend the blast, the first attempt to use the power of the atomic bomb for peaceful purposes. At a public meeting at Carlsbad High School the night before the test, Edward Teller called the test the “miracle of the decade.” Nearby potash mines and Carlsbad Caverns were temporarily evacuated in case of more severe than anticipated ground shock, with the AEC picking up the bill.[Sz]

The blast was scheduled for 8:00 AM on December 10th, 1961, but was delayed repeatedly as the wind refused to cooperate.[Sz] George Cowan was present at the test as part of the Los Alamos contingent. Inspecting the bomb room before the shot, he had noticed faults in the supposedly pure salt where it had been contaminated by water, producing weak points. Waiting in the control room during the countdown, Cowan turned to Teller and offered him a bet. “'It's going to vent. Bet one dollar.' [Teller] replied 'I think not. One dollar. All right.'”[Co]

Finally, at noon the wind finally turned to the right direction, and, with cameras rolling to record the reactions of the crowd, the shot was fired. The earth shook as, more than eleven hundred feet underground, a handful of atoms split, and the soil above the blast jumped four feet in the air. A cache of explosives intended to calibrate the seismic instruments accidentally detonated at the same time, producing a burst of flame and smoke.[Sz]

Figure 15: Crowd Watching GNOME Shot[PS]
(US Government)

Then, two or three minutes later, while the crowd still watched, gray smoke and steam began billowing from the emplacement shaft. Cowan won his dollar.[Co]

It was later determined that an unnoticed fault had allowed vapor from the blast to penetrate into the emplacement tunnel. Extremely high radiation levels – about 100,000 mSv per hour – were reported at the head of the emplacement shaft, and AEC personnel had to wash down cars on a nearby highway to remove clinging fallout. Venting continued for half an hour after the shot, with a trickle still visible the next day.

From the available information, it appears that the fallout died off to essentially safe levels before it left the test site. The highest level of radiation that reached humans was a brief reading of 14 mSv per hour at the highway, and this dissipated in a few minutes. This level of radiation is only dangerous if someone is exposed to it continuously for hours. Still, it was an inauspicious start to the AEC's new program to retool the atomic bomb for peaceful purposes.[Sz][AEC61]

The venting left the interior of the entry shaft dangerously radioactive, and it was six days until levels died low enough to allow reentry. The radiation contaminated recording film used to record neutron fluxes, and most of the short-lived isotopes produced in the cross-sections experiments had died off by the time the wheel was recovered, although some of the data was still recoverable.[FNER] And the Los Alamos fission symmetry experiment was judged a success.[CBP] And the prompt sampling system had failed to collect significant amounts of material – the pipe had become blocked, either due to the shockwave outrunning the escaping products and crimping the pipe shut, or due to the lower pipe inlet vaporizing, the cloud of iron gas then blocking the vaporized samples from escaping.[La]

Five months later, workers tunneled into the cavity created by the blast, revealing beautiful streaks of blue, yellow, and black formed in the salt by chemical reactions produced by the incredible heat of the bomb blast. Even five months later the lingering heat in the cavern was intense enough to prevent long stays, at 100 degrees Fahrenheit with 60% humidity.[Sz] Unfortunately, while the cavity from the blast did survive, the walls and ceiling had partially caved in, mixing 13,000 tons of salt into the molten salt puddle, diluting it heavily.[PS]

Figure 16: GNOME Cavity[DoE2]
(US Government)

Samples of the molten and resolidified salt, laden with radioactive isotopes, were taken from the cavern floor, to be used by Oak Ridge to develop chemical separation techniques to extract useful radioisotopes.[CTD64]

GNOME was intended to be a prelude to a second test in the Carlsbad salt, COACH. The precise genesis of COACH is unclear, but it entered the public record in 1962 after the GNOME detonation, initially scheduled for 1963.[PWA63] COACH would be Plowshare's first heavy element experiment, using a 5 to 10 kT device designed to maximize neutron production, and detonated in salt adjacent to the GNOME site. And COACH, unlike BUTTERNUT and QUAY, would be designed and developed by Lawrence Livermore.

But, before COACH could be fired, the device would first have to be designed, and that was proved to be more difficult than expected. In 1963 it was decided to postpone the COACH shot until fiscal year 1964 while further development was done.[PWA64][Ka] In the meantime, development tests would be carried out at the Nevada Test Site – with the first one scheduled for late 1962.[Ka]

Before the heavy element tests resumed, however, there would first be another neutron physics test – and this one would not be underground, but in the furthest reaches of the sky.

With Special Thanks to my Pre-Readers, Grey Wolf (author of Götterdämmerung) and B. Bennett

Citations can be found here.

1 comment:

  1. I'm really enjoying this piece of Atomic History and the other articles on your site. Excellent work in compiling all this fascinating history.. Bravo Zulu!

    Bob Given