The Los Alamos Primer was a printed version of the first five lectures on the principles of nuclear weapons given to new arrivals at the top-secret Los Alamos laboratory during the Manhattan Project. They were originally given by the physicist Robert Serber after being delivered in person on April 5-14, 1943, based on conclusions reached at a conference held in July and September 1942 at the University of California, Berkeley by Robert Oppenheimer. The notes from the lecture which became the Primer were written by Edward Condon.
The Primer contained the basic physical principles of nuclear fission, as they were known at the time, and their implications for nuclear weapon design. It suggested a number of possible ways to assemble a critical mass of uranium-235 or plutonium, the most simple being the shooting of a "cylindrical plug" into a sphere of "active material" with a "tamper"—dense material which would focus neutrons inward and keep the reacting mass together to increase its efficiency (this model, the Primer said, "avoids fancy shapes"). They also explored designs involving spheroids, a primitive form of "implosion" (suggested by Richard C. Tolman), and explored the speculative possibility of "autocatalytic methods" which would increase the efficiency of the bomb as it exploded.
The Primer became designated as the first official Los Alamos technical report (LA-1), and though its information about the physics of fission and weapon design was soon rendered obsolete, it is still considered a fundamental historical document in the history of nuclear weapons. Its contents would be of little use today to someone attempting to build a nuclear weapon, a fact acknowledged by its complete declassification in 1965. It used to be available for download from Los Alamos's website, but after the September 11 attacks, Los Alamos restricted access to all of their technical reports. It can still be found on internet mirrors, though.
In 1992, an edited version of the Primer with many annotations and explanations by Serber was published with an introduction by Richard Rhodes.