Bayo Canyon, TA-10
70 years ago this secluded canyon outside Los Alamos, NM served host to a set of experiments that would forever change the course of human history. The deadly radioactive sources, massive explosions, and determined minds replaced with only hiking trails. All that remains here today is a bit of unique radioactive debris and a few warning signs and markers as testament to what lies below.
"Gadget," pictured at right, was the product of the work at Bayo Canyon. The first atomic device ever tested, Gadget was an implosion type nuclear weapon using a core, or "pit," or Plutonium-239.
With the discovery of a new fissile metal, Plutonium, the planners of the Manhattan Project began a second project to produce a bomb with the new element along with existing plans for one utilizing Uranium-235. Scientists at Los Alamos had a high degree of confidence that the gun-barrel design for combining two subcritical masses of 235U would work, but more work was needed on a device utilizing Plutonium.
A problem soon arose when scientists at Los Alamos started to measure the Spontaneous Fission (sf) rates of the Plutonium samples they were receiving from the graphite piles used as production reactors during at the project. In the breeding process, neutrons were being captured by Uranium-238 to produce short lived Uranium-239 which quickly decayed into the required Plutonium-239. In cyclotron experiments, the Plutonium produced by ion bombardment was relatively free of isotopic contaminants. The pile produced produced Plutonium, however, sat in the pile absorbing too many neutrons. Significant quantities of the 239Pu being produced captured an additional neutron to form Plutonium-240. An even-numbered isotope, this 240Pu was a prodigious source of neutrons from Spontaneous Fission reactions.
Bomb designers realized the rates of neutron production would cause a "fizzle" if this new material was utilized in the conventional gun-barrel design in order to achieve supercriticality. The original plan for the "Thin Man" plutonium gun-barrel device was scrapped and a crash program was soon underway at Los Alamos for the much more technically challenging implosion design. In a such a design a sphere of Plutonium metal would be symmetrically crushed into a supercriticality using an array of high explosive lenses. This approach to building a nuclear weapon was incredibly challenging and scientists at the lab were tasked with developing techniques to image these implosions to the refine the design. The "RaLa" experiments in Bayo Canyon became the most successful technique developed, and it was said that "RaLa became the most important single experiment affecting final bomb design" at Los Alamos during the Manhattan Project.
The concept of the RadioLanthanum (RaLa) experiments at Bayo Canyon worked essentially like an inverse x-ray machine. Instead of taking a picture of a process using an external radiation source and detectors, the principle behind traditional radiography, the RaLa experiments used the imaging of an internal radioactive source to understand the dynamics of an implosion. Specially the short lived radioactive isotope Lanthanum-140. 140La was chosen because of its short half-life (1.6781 days), gamma energy (1.6 and 0.5 MeV), and good availability, its parent Barium-140 was an abundant fission product, produced in Uranium slugs at the Graphite Reactor, X-10, in Oak Ridge, Tenn. The Barium-140 was crudely separated and shipped to Los Alamos in lead lined transportation casks where the 140La would be "milked" from the parent Barium-140 in Bayo Canyon. For the RaLa shots, typically the source strength was a few hundred curies (Ci) of 140La, but occasionally sources of more than 2 kCi were used.
The cameras for these experiments were originally pressurized ion chambers which provided the speed and high area necessary to image the changing transmission of 140La gamma rays during the implosions. By the early 1950s these ion chambers were replaced by detectors filled with liquid scintillator which allowed for improved resolution. Combining the high explosive destructive experiments with a flammable solvent lead to a predictably spectacular fireball during these later tests.
The first RaLa shot in Bayo Canyon occurred on September 22, 1944, although initial experiments lacked a high degree of symmetry in the implosion due to the use of Primacord for the detonation circuit. The introduction of exploding bridge-wire detonators in February 1945 provided an implosion that designers were satisfied would produce an atomic explosion. Hydrodynamic experiments at Bayo Canyon continued after the war until 1962 with 254 experiments utilizing the RaLa method. In 1951, the radiological dispersals at Bayo Canyon became of interest to simulate and model the dispersion of fallout from atmospheric atomic testing that started that year at the Atomic Energy Commission's Nevada Proving Ground north of Las Vegas, Nevada.
Bayo canyon Today
I first visited the Bayo Canyon site in 2008, and since then have conducted several investigations of what still remains there. Carl Willis accompanied me on early visits to the site, and has a write up on his site here. Remediation work has been conducted since my first expeditions, primarily in regards to Strontium-90 contaminated soils around the former radiochemistry facilities at the site, TA-10-1. The Barium-140 shipped to the site in the early years for use in the RaLa generators contained substantial quantities of this longer lived Strontium-90, which is also a high-yield fission product chemically similar to Barium (as well as Calcium and other Alkaline Earth Metals). This relationship means that 90Sr poses a biological hazard as a bone-seeker, and also is well incorporated into calcium-rich plants at the site who's roots grow deep to the contamination buried below. Other than small amounts of this radioisotope buried below the ground, nothing remains of the lab site at TA-10-1
The most interesting remnants of the tests occur at ground zero or the "shot point" for the tests, where debris still litters the canyon floor from the experiments. Debris includes a wide manner of materials including Lead, Copper, Steel, and natural and depleted Uranium and some components are still identifiable. Many of these fragments of debris are still radioactive to some degree, primarily from Uranium used as tampers and as surrogates for Plutonium pits. As with the other expeditions on this site, analysis of this debris allows for a reconstruction of the kinds of experiments conducted here. As you can see below, many pieces of debris harbor obvious signs of being through energetic explosions. Much of the debris is from the test packages, with many of the pieces of wiring and cables being part of the instrumentation for the shots.
Much of my research on Bayo Canyon has been compiled from the following reports, including some excellent documentation by John C. Taschner, a Los Alamos health physicist who worked on the RaLa experiments in the late 1950s.