In the last couple of posts, we’ve begun our Countdown to the Cold War by talking about the reorganized at Los Alamos in the fall of 1944 to develop a method known as implosion. You can read the last post in the series by clicking HERE.
The next step on the Manhattan Project’s Countdown to the Cold War occurred on September 22, 1944, and was known as the RaLa experiment. Very early in the implosion research program, it became obvious that being able to systematically verify the success or failure of implosion would be a crucial measure for success. But very few experimental measures of implosion existed at the time.
In particular, for a successful atomic weapon, it was imperative that the scientists be able to engineer a symmetric implosion. Early attempts at creating implosion revealed a wide range of asymmetric behaviors that scattered material unevenly. In order to measure the symmetry of implosion, it became necessary to observe implosion events with instruments. One technique that was developed for observing implosion was known as RaLa.
RaLa is a shorthand for the active ingredient in a RaLa test: radiolanthanum. Radiolanthanum (La-140) is a manmade radioactive isotope of lanthanum. According to Critical Assembly (by Hoddesson, et al), Robert Serber first outlined what would become the RaLa method on November 1, 1943. Serber was arguably Robert Oppenheimer’s right-hand man at Los Alamos and someone familiar to folks there for the Los Alamos Primer, the introductory lectures that kicked off the Manhattan Project’s bomb design effort.
The RaLa method depended upon the use of gamma radiation given off by the radiolanthanum isotope. Gamma radiation—or just gamma rays—are a very energetic type of electromagnetic radiation. The EPA.gov website devoted to radiation protection has this to say about gamma rays:
Gamma photons have about 10,000 times as much energy as the photons in the visible range of the electromagnetic spectrum. Gamma photons have no mass and no electrical charge. The are pure electromagnetic energy.
Highly energetic gamma rays travel at the speed of light and easily pass through most materials. It is this set of properties that made them useful in characterizing the implosion necessary for setting off an atomic bomb.
Serber hypothesized that by placing an amount of radiolanthanum in the center of the metal sphere to be compressed by implosion, the strength of the gamma rays emitted during that implosion would vary in such a way that the scientists could use instruments to understand how symmetrical the implosion was. Serber knew that, as an implosion event progressed in a metallic core (uranium or plutonium for the atom bomb), there would be significant changes in the density of the material being compressed. These changes in density would retard the gamma rays in predictable ways. In addition, because the gamma rays would radiate out from the center of the sphere, the scientists would be able to collect information about the implosion in three physical dimensions.
Given that the radiolanthanum material would be at the center of an explosion, there would of course be radioactive debris and dispersal of that debris. Gamma radiation is ionizing—releases electrons—and therefore has biological implications, meaning that it affects human bodies. And because gamma rays penetrate materials, they can be very dangerous. In this way, the RaLa experiments constitute the world’s first production of radioactive fallout, a waft of the Cold War to come. In order to minimize human exposure to the radiation that would be released, the RaLa experiments were held offsite in Bayo Canyon, located about two miles east of Los Alamos—a sort of lab away from lab. Checking the wind direction or measuring fallout, however, weren’t much a priority for these early radioactive test explosions.