Question: Louis Alexander Slotin, was a Canadian physicist and chemist who worked on the Manhattan Project, Slotin was the second person to die from a criticality accident, following the death of Harry Daghlian, who had been exposed to what?

Context articles: Criticality accident
Cecil Kelley criticality accident
Louis Slotin
Nuclear criticality safety
Harry Daghlian
Demon core
Michael Lista
Prince Louis of Battenberg
Alvin C. Graves
Bernice Weldon Sargent

A criticality accident is an uncontrolled nuclear fission chain reaction.
 It is sometimes referred to as a critical excursion or a critical power excursion or a divergent chain reaction.
A criticality accident occurred on December 30, 1958, at the Los Alamos National Laboratory in Los Alamos, New Mexico, in the United States.
 It is one of only ten such events to occur outside of a nuclear reactor, though it was the third such event to take place in 1958 after events on 16 June at the Y-12 Plant in Oak Ridge, Tennessee, and on 15 October at the Vinča Nuclear Institute in Vinča, Yugoslavia.
 The accident involved plutonium compounds dissolved in liquid chemical reagents, and it killed one man, Cecil Kelley, a chemical operator, by severe radiation poisoning, within 35 hours.
Louis Alexander Slotin (1 December 1910 – 30 May 1946) was a Canadian physicist and chemist who worked on the Manhattan Project.
 During World War II, Slotin conducted research at Los Alamos National Laboratory.
 He performed experiments with uranium and plutonium cores to determine their critical mass values.
 On 21 May 1946, Slotin was conducting a demonstration when he accidentally initiated a fission reaction, which released a burst of hard radiation.
 He received a lethal dose of radiation and died of acute radiation syndrome nine days later.
 Slotin was the second person to die from a criticality accident, following the death of Harry Daghlian, who had been exposed to radiation by the same core that killed Slotin.
 Slotin was publicly hailed as a hero by the United States government for reacting quickly and preventing his accident from killing any colleagues.
 He was later criticized for failing to follow protocol during the experiment.
Nuclear criticality safety is a field of nuclear engineering dedicated to the prevention of nuclear and radiation accidents resulting from an inadvertent, self-sustaining nuclear chain reaction.
 Additionally, nuclear criticality safety is concerned with mitigating the consequences of a nuclear criticality accident.
 A nuclear criticality accident occurs from operations that involve fissile material and results in a sudden and potentially lethal release of radiation.
 Nuclear criticality safety practitioners attempt to prevent nuclear criticality accidents by analyzing normal and credible abnormal conditions in fissile material operations and designing safe arrangements for the processing of fissile materials.
 A common practice is to apply a double contingency analysis to the operation in which two or more independent, concurrent and unlikely changes in process conditions must occur before a nuclear criticality accident can occur.
 For example, the first change in conditions may be complete or partial flooding and the second change a re-arrangement of the fissile material.
 Controls (requirements) on process parameters (e.g., fissile material mass, equipment) result from this analysis.
 These controls, either passive (physical), active (mechanical), or administrative (human), are implemented by inherently safe or fault-tolerant plant designs, or, if such designs are not practicable, by administrative controls such as operating procedures, job instructions and other means to minimize the potential for significant process changes that could lead to a nuclear criticality accident.
Haroutune Krikor "Harry" Daghlian Jr. (May 4, 1921 – September 15, 1945) was a physicist with the Manhattan Project which designed and produced the atomic bombs that were used in World War II.
 He accidentally irradiated himself on August 21, 1945, during a critical mass experiment at the remote Omega Site of the Los Alamos Laboratory in New Mexico, resulting in his death 25 days later.
The demon core was a 6.2 kg subcritical mass of plutonium measuring 89 mm in diameter, roughly the size of a softball, best known for its involvement in two criticality accidents.
 The core was slated for use in a third World War II nuclear bomb, but remained in use for testing after Japan's surrender.
 It was designed with a small safety margin to ensure a successful explosion of the bomb.
 The device briefly went supercritical when it was accidentally placed in supercritical configurations during two separate experiments intended to guarantee the core was indeed close to the critical point.
 The incidents happened at the Los Alamos laboratory in 1945 and 1946, and resulted in the acute radiation poisoning and subsequent deaths of scientists Harry Daghlian and Louis Slotin.
 After these incidents the spherical plutonium core was referred to as the "demon core".
Michael Lista (born September 1, 1983) is a Canadian poet.
 He is the author of "Bloom", a book of poems about Canadian Manhattan Project physicist Louis Slotin.
 He writes a monthly column on poetry for "The National Post" and lives in Toronto.
Admiral of the Fleet Louis Alexander Mountbatten, 1st Marquess of Milford Haven, {'1': ", '2': ", '3': ", '4': "} (24 May 1854 – 11 September 1921), formerly Prince Louis Alexander of Battenberg, was a British naval officer and German prince related to members of the British Royal Family.
Alvin Cushman Graves (November 4, 1909 – July 19, 1965) was an American nuclear physicist who served at the Manhattan Project's Metallurgical Laboratory and the Los Alamos Laboratory during World War II.
 After the war, he became the head of J (Test) Division at Los Alamos, and was director or assistant director of numerous nuclear weapons tests during the 1940s and 1950s.
 Graves was badly injured in the 1946 laboratory criticality accident in Los Alamos that killed Louis Slotin, but recovered.
Bernice Weldon Sargent, {'1': ", '2': ", '3': ", '4': "} (24 September 1906 – 17 December 1993) was a Canadian physicist who worked at the Manhattan Project's Montreal Laboratory during the Second World War as head of its nuclear physics division.
 In his 1932 doctoral thesis, he discovered the relationship between the radioactive disintegration constants of beta particle-emitting radioisotopes and corresponding logarithms of their maximum beta particle energies.
 These plots are known as "Sargent curves".