RaLa Experiment

The RaLa Experiment, or RaLa, was a series of tests during and after the Manhattan Project designed to study the behavior of converging shock waves to achieve the spherical implosion necessary for compression of the plutonium pit of the nuclear weapon. The experiment used significant amounts of a short-lived radioisotopelanthanum-140, a potent source of gamma radiation; the RaLa is a contraction of Radioactive Lanthanum. The method was proposed by Robert Serber and developed by a team led by the Italian experimental physicist Bruno Rossi.

Test to study nuclear shock waves

The tests were performed with 1/8 inch (3.2 mm) spheres of radioactive lanthanum, equal to about 100 curies (3.7 TBq) and later 1,000 Ci (37 TBq),[1] located in the center of a simulated nuclear device. The explosive lenses were designed primarily using this series of tests. Some 254 tests were conducted between September 1944 and March 1962.[2] In his history of the Los Alamos project, David Hawkins wrote: “RaLa became the most important single experiment affecting the final bomb design”.[3]

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Experimental setup for RaLa shot 78 on May 13, 1947, at Bayo Canyon. Each rectangular box contains eight cylindrical fast ionization chambers.

The experiment was suggested on 1 November 1943 by Robert Serber.[1] The idea was to measure the spatial and temporal symmetry of explosive compression of a metal sphere. The test measured changes of absorption of gamma rays in the metal of the sphere as it underwent compression. The gamma ray source was located in the center of a metal sphere. The increase of thickness (of hollow shells) and density (of solid spheres) as the compression progressed was detected as a decrease of intensity of gamma rays outside of the sphere; the lower density explosives did not absorb gamma radiation enough to interfere with the experiment. The gamma rays had to be intense and of the right energy. Too low energy, and they would be fully absorbed in the surrounding metal; too high energy and the difference of attenuation during the implosion would be too low to be practical. The detectors had to provide high speed and large area; fast ionization chambers, then under development, were the only devices then available satisfying the requirements.[4]

Lanthanum-140 was chosen because it emits gamma rays in the desired energy range (1.60 megaelectronvolts (MeV), with fraction of 0.49 MeV), and has very high specific activity, thus providing sufficient radiation intensity to produce usable signals from the ionization chambers. After a test, dispersed La-140 rapidly decays into stable cerium-140, reducing the radiation hazard for the operators after several half-lives. It was also potentially available in larger quantities because its parent nuclide barium-140 is an abundant fission product of uranium. As a consequence, lanthanum-140 samples contained traces of barium-140, caesium-140, and especially strontium-90, which still presents a radioactive contamination problem in the area of the tests.[5] Lanthanum-140 has a specific activity of 5.57×105 Ci/g (20.6 PBq/g); a 1,000 Ci (37 TBq) La-140 source therefore equals about 1.8 mg of lanthanum.[1]

A radiolanthanum sample, precipitated in a tip of a small cone, followed by a plug, was lowered into the center of the metal sphere of the experimental assembly with a device resembling a fishing rod. The cone and the plug were mated to the metal center of the assembly, together forming a metal sphere. A section of the explosive lensing was then returned to its place above the sphere. Several, typically four, ionization chambers were located around the experimental setup. Immediately after the detonation they generated signals that were displayed on oscilloscopes in a blast-proof shelter or a mobile laboratory in a tank, 150 feet (46 m) away, and the oscilloscope traces recorded on cameras. A calibration measurement was performed before and after each test. The ionization chambers and their preamplifiers were destroyed during the explosion, but their simple design allowed their production in sufficient quantities.[6]

The ionization chambers were cylindrical, 2 inches (51 mm) in diameter, 30 inches (760 mm) long, with a wire along the longitudinal axis. They were filled with a mixture of argon and carbon dioxide at 4.5 standard atmospheres (460 kPa). Eight chambers were arranged in a tray and connected in parallel; four trays were located in a tetrahedron around the experimental assembly, recording the gamma radiation around the sphere, sufficiently close to give a signal and sufficiently far away to not be destroyed by the blast before they could record the required information.[6] The initiation of the explosives was initially performed by a multipoint Primacord system. The results were erratic, as the detonations weren’t sufficiently synchronized. Much better results were obtained after February 1945, when exploding-bridgewire detonators, developed by Luis Alvarez‘s G-7 group, became available.[1]

As plutonium was not available, it was substituted with material with similar mechanical properties. Depleted uranium was used but was not optimal because of its opacity for radiation; iron, copper, or cadmium were other choices. Cadmium was the choice for most of the tests. The first shot was performed with an iron mockup of the plutonium pit.[6]

The resulting signal was a fast dip, corresponding to the compression of the cadmium sphere, followed by slower increase, corresponding to the decompression and following dispersal of the sphere and the lanthanum. The differences between the four traces on the oscilloscope display, each indicating the average compression in the direction of the detector, allowed the assessment of the required synchronization accuracy for the detonators.[4]

The RaLa sources were highly radioactive. They had to be lowered to the test apparatus by a 10 feet (3.0 m) long rod.[7] The tests were initially observed from a sealed M4 Sherman tank; the mobile laboratory consisted of two tanks. Each experiment was expected to contaminate an area of about 3,000 square meters (32,000 sq ft) for about half a year. When radiobarium was removed from the radiolanthanum, the short-term contamination levels turned out to be insignificant.[6] Tanks were then replaced with fixed shelters. One of the tanks was later lead-plated, sealed, equipped with self-contained air supply, and used for sampling of fission products in the post-blast debris after the Trinity test.[8] The sources posed a considerable radiation exposure risk; the exposure rate of a 1,000 Ci (37 TBq) source at 1 meter (3 ft 3 in) was 1,130 R/h and 11,000 R/h at 1 foot (0.30 m). Sources with activities up to 2,300 Ci (85 TBq) were used in some tests.[4]

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