In 1942, Slotin was invited to participate in the Manhattan Project, and subsequently performed experiments with uranium and plutonium cores to determine their critical mass values.

With the assistance of one of his mentors, he obtained a fellowship to study at King's College London under the supervision of Arthur John Allmand,[2] the chair of the chemistry department, who specialized in the field of applied electrochemistry and photochemistry.

"[5] During an interview years later, Sam stated that his brother had gone "on a walking tour in Spain" and he "did not take part in the war" as previously thought.

Afterwards, he spent six months working as a special investigator for Dublin's Great Southern Railways, testing the Drumm nickel-zinc rechargeable batteries used on the Dublin–Bray line.

[8] Slotin might have been present at the start-up of Enrico Fermi's "Chicago Pile-1", the first nuclear reactor, on 2 December 1942; the accounts of the event do not agree on this point.

His expertise on the subject garnered the attention of the United States government, and as a result he was invited to join the Manhattan Project, an effort to develop an atomic bomb.

He moved to the Los Alamos National Laboratory in New Mexico in December 1944 to work in the bomb physics group of Robert Bacher.

[2] At Los Alamos, Slotin's duties consisted of dangerous criticality testing, first with uranium in Otto Robert Frisch's experiments, and later with plutonium cores.

[2] In the winter of 1945–1946, Slotin shocked some of his colleagues with a bold action by repairing an instrument 6 feet (1.8 m) under water inside the Clinton Pile while it was operating, rather than wait an extra day for the reactor to be shut down.

Later estimates suggested that this dose might not have been fatal on its own, but he then received additional delayed gamma radiation and beta burns while disassembling his experiment.

[2] On 21 May 1946, with seven colleagues watching, Slotin performed an experiment that involved the creation of one of the first steps of a fission reaction by placing two half-spheres of beryllium (a neutron reflector) around a 3.5-inch-diameter (89 mm) plutonium core.

[2][13] At 3:20 p.m., the screwdriver slipped and the upper beryllium hemisphere fell, causing a "prompt critical" reaction and a burst of hard radiation.

[17] This peculiar response was of no value for determining the actual doses received by the men in the room and put Schreiber at "great personal risk" of additional exposure.

[2] The six other men present at the time of the reaction included Alvin Cushman Graves, Samuel Allan Kline, Marion Edward Cieslicki, Dwight Smith Young, Theodore P. Perlman, and Pvt.

The United States Army physician responsible for the Los Alamos base hospital, Captain Paul Hageman, said that Slotin's, Graves', Kline's and Young's "immediate condition is satisfactory.

Louis Hempelman, M.D., a consultant to the Los Alamos Scientific Laboratory, believed that it is not possible to establish a causal relation between the incident and the specifics of the death from such a small sample.

[19] He stated this opinion in a publicly available report that included personal medical information which he had previously declined to release to Cieslicki during the final stages of his illness.

In the absence of personal dosimetry badges, the study authors relied on measurements of sodium activation in the victims' blood and urine samples as their primary source of data.

In this model, the equivalent X-ray doses were much higher, but would be concentrated in the tissues facing the source, whereas the gamma component penetrated the whole body.

Subsequent criticality testing of fissile cores was done with remotely controlled machines, such as the "Godiva" series, with the operator located a safe distance away to prevent harm in case of accidents.

[34] On 14 June 1946, the associate editor of the Los Alamos Times, Thomas P. Ashlock, penned a poem entitled "Slotin – A Tribute": May God receive you, great-souled scientist!

While you were with us, even strangers knew The breadth and lofty stature of your mind Twas only in the crucible of death We saw at last your noble heart revealed.

[2] The official story released at the time was that Slotin, by quickly removing the upper hemisphere, was a hero for ending the reaction and protecting seven other observers in the room: "Dr. Slotin's quick reaction at the immediate risk of his own life prevented a more serious development of the experiment which would certainly have resulted in the death of the seven men working with him, as well as serious injury to others in the general vicinity.

[22] Graves, like Slotin, had previously displayed a low concern for nuclear safety and later alleged that fallout risks were "concocted in the minds of weak malingerers.

"[22] Schreiber, another witness to the accident, spoke out publicly decades later, arguing that Slotin was using improper and unsafe procedures, endangering the others in the lab along with himself.

[2] Slotin's accident is echoed in the 1947 film The Beginning or the End, in which a scientist assembling the bomb destined for Hiroshima dies after coming into contact with radioactive material.

[35] It was recounted in Dexter Masters' 1955 novel The Accident, a fictional account of the last few days of the life of a nuclear scientist suffering from radiation poisoning.

[40] According to Weinberg and Wigner,[41] Slotin was the first to propose the name dollar for the interval of reactivity between delayed and prompt criticality; 0 is the point of self-sustaining chain reaction, a dollar is the point at which slowly released, delayed neutrons are no longer required to support chain reaction, and enters the domain called "prompt critical".