# Tickling the dragons tail

Skrevet av Susanne Araya

KJM3900 prosjektoppgave våren 2017

Did you know?

After the nuclear bombs exploded over Nagasaki and Hiroshima in 1946, people were becoming more and more aware of the power of atomic weapons.

In the early 1940s a majority of the research performed by scientists served military interests. In this period the U.S. government invested millions of dollars into research projects to give them an advantage in winning the war. During this period there were big advances in transportation, communication, weapons and intelligence gathering.[1]

Louis Slotin and the deadly experiment

Louis Slotin was a Canadian physicist who worked on the Manhattan project during the Second World War. This was a research project that developed the first nuclear weapons and was led by the United States with support from United Kingdom and Canada.[2]

In the 1940s Slotin performed experiments which consisted of risky criticality testing. These experiments later went under the title “tickling the dragons tail”. The title described Slotins flirtation with the possibility of a nuclear reaction. The name was based on a remark from the physicist Richard Feynman who compared these risky experiments with “tickling the tail of a sleeping dragon”3

The most risky experiment was performed during May of 1946. It consisted of screwing two hollow half-spheres of beryllium around a core of fissionable material (see figure 2). The core consisted of a subcritical mass of plutonium, later nicknamed “the demon core» 4 The spheres where moved closer and further from the core to monitor the rate at which neutrons multiplied in the core. A Geiger counter would record the radioactivity as the spheres were brought together. The aim of the experiment was to calculate the critical mass, which was needed to detonate an atomic weapon.

Geiger counter: Instrument used for measuring ionizing radiation.

Slotin was considered a “daredevil” due to his risky behavior. Whenever he performed the experiment, he would remove the shims used to keep the spheres apart and separate them by using just the blade of a simple screwdriver.  A despaired Enrico Fermi said to Slotin: “he would be dead within a year if he continued to float the safety protocols”[3].

Gamma rays: Electromagnetic radiation of very high energies. They are photons with very short wavelengths. These high energy photons are very penetrating matter and can therefore cause major damage on biological tissue.

Beryllium is a chemical element with symbol Be. It is a relatively rare element in the universe, most commonly it occurs as a product of the spallation (process for producing neutrons by means of a particle accelerator and a heavy metal target) of larger atomic nuclei that have collided with cosmic rays. Cosmic rays are high-energy radiation, mainly from the outside of the Solar System.

Figure 2 Configuration of beryllium reflector shells prior to the accident 21 May 1946[4]

Unfortunately, he was right, on May 21 the year 1946 Slotins “flirtation” with death ended in tragedy. Slotin suffered a fatal dose of radiation. His only safety measure, the screwdriver that was used to keep the spheres apart, slipped, causing them to come together, the core became supercritical and flashed a blue glow. A massive wave of neutron radiation moved through the room. Slotin heroically used his body to protect his colleagues from the radiation as much as possible and separated the spheres quickly to stop the chain reaction. Immediately after the exposure, Slotin reported having a sour taste in his mouth and a burning sensation in his left arm. His arms were used to knock the spheres apart. Only 9 days after the incident, after a “totally disintegration of bodily functions”, Slotin died[5].

The critical mass is the smallest mass of fissile material needed to sustain a nuclear chain reaction.
Criticality accident: Is an uncontrolled nuclear chain reaction due to the unintentionally assemble of critical mass.
Criticality testing: Bringing masses of fissile materials to near critical values to determine their critical masses.

Figure 3 positions of the scientists at the time of Slotins accident[6]

Plutonium-239 is naturally radioactive, which means that it emits particles from its nuclei naturally. Typically when plutonium undergoes fission the nucleus splits into two smaller nuclei along with some neutrons. The reaction releases energy in form of heat and gamma rays.  Below you can see the equations for Pu-239, the first one is an absorption reaction which results in fission, this occurs in 73 % of the cases. The second equation is an absorption reaction resulting in radiative capture of incoming neutrons.

$${73\%}: ^1_0n +^{239}Pu \rightarrow ^{240}Pu\rightarrow Fission$$

$$27\%:^1_0n+^{239}Pu\rightarrow ^{240}Pu\rightarrow ^{240}Pu + \gamma$$

Units of gray: Is defined as the absorption of 1 joule of radiation energy per kilogram of matter.

When nuclear reactions are initiated, they “throw out” particles with enough energy to rip electrons off of atoms/molecules. The affected bond make up ion pairs which are extremely chemically reactive, and this is what ionizing radiation consists of8. A radiation  dose (or energy deposited in mass) is measured in units of Grays (Gy).

If the human body is exposed to a large enough dose it can lead to acute radiation syndrome (ARS), where the symptoms depend on the level of exposure. If exposed to a low dose (0.35 Gy) one could expect symptoms such as nausea, vomiting, headaches, fatigue and fever. If the dose is 1-4 Gy blood cells begin to die but one can still recover. However if the dose is between 4-8 Gy it becomes fatal and symptoms can vary from vomiting, diarrhea, dizziness and fever [7].

Slotin was exposed to approximately 10 Gy of gamma and x-ray radiation, he wouldn’t have survived even today with modern treatments available. If the human body is exposed to 8-30 Gy one will experience nausea and severe diarrhea within an hour and die between 2 -14 days after exposure. If the absorbed dose is greater than 30 Gy, death within 48 hours is inevitable8.

Fortunately today, the safety protocols are much stricter and monitored, and the technology has advanced so much, that such an accident like Slotins, is confined to the past.

Fissionable material: Is a material that can sustain a nuclear chain reaction, by definition it can sustain this reaction with neutrons of any energy.

[1] Encyclopedia.com,2016 [Internet]. Available from:http://www.encyclopedia.com/social-sciences/culture-magazines/1940s-science-and-technology-overview [accessed 20 march 2017]

[2]The New Yorker,2016 [Internet]. Available from:http://www.newyorker.com/tech/elements/demon-core-the-strange-death-of-louis-slotin[accessed 20 march 2017]

[3]  The Demon core,2012,[Internet]. Available from: http://www.strangerdimensions.com/2012/07/30/the-demon-core/[accessed 20 march 2017]

[4] A review of criticality accidents,May 2000 [internet] . Available from: https://cns-snc.ca/media/history/pioneers/slotin/slotin.html[accessed 20 march 2017]

[5]APS news,2017 [internet] Available from:https://www.aps.org/publications/apsnews/201405/physicshistory.cfm [accessed 20 march 2017]

[6]   Nerdist,2014[internet] Available from: http://www.nerdist.com/wp content/uploads/2014/12/DemonCore_Diagram.jpg

[accessed 20 march 2017]

[7]GIZMODO,2012 [internet] Available from: http://gizmodo.com/5928171/what-nuclear-radiation-does-to-your-body [accessed 20 march 2017]

Av Susanne Araya
Publisert 27. apr. 2017 12:12 - Sist endret 28. apr. 2017 16:10