Trinity Test

Tony Chen
December 6, 2021

Submitted as coursework for PH241, Stanford University, Winter 2021


Fig. 1: Trinity test fireball. [3] (Source: Wikimedia Commons)

Trinity, a code name chosen by J. Robert Oppenheimer, at the time director of the Los Alamos Laboratory, for the first detonation of a nuclear device. It was conducted at 5:29am on July 16th, 1945. The test was conducted in the Jomada del Muerto desert about 35 miles southeast of Socorro, New Mexico, 130 miles south of Albuquerque, New Mexico. The nuclear device was nicknamed The Gadget, which shares the same design as the Fat Man that was detonated over Nagasaki, Japan 23 days later. Most of the high level scientists for the Manhattan Project were present and observed the test, which included Robert Oppenheimer, Richard Feynman, Enrico Fermi, John von Neumann and Vannevar Bush.

Due to the technology limitations of the 1940s, production of the fissile isotope uranium-235 was incredibly difficult. The proposed way of producing uranium was through mass spectrometers, and was estimated to take 27,000 years to produce a gram of uranium, while kilograms of uranium was needed for a nuclear bomb. Because of this, the scientific effort was directed at developing a plutonium based implosion-type nuclear weapon, where a fissile core would be surrounded by two different types of explosives, fast and slow burning. These two types of explosives would produce two different shockwaves at different speeds, focusing the shock waves inwards and compressing the plutonium core to supercritical state. [1]

The idea of a real test of this implosion device was brought up early 1944, and was finally approved by Lesile Groves, who was a Lieutenant General who directed the Manhattan Project after much deliberation. The main concern was that it would cost billions of dollars of plutonium for such live testing. Initially, a small scale test that would detonate inside of a containment vessel was brought up, which could lead to potentially recovery of some of the plutonium used. Oppenheimer strongly argued against this idea and insisted that a real scaled test must be performed where the energy release is comparable with that contemplated for final use. [1]

Estimation of Energy

To estimate the energy yield of the Trinity Test was not an easy task, as there are various numbers estimated by different parties and methods. Initially before the blast, the theoretical division at Los Alamos predicted a yield between 5 to 10 kilotons of TNT. [1] The most direct measurement would have come from fifty beryllium-copper diaphragm microphones, which indicated a yield of 9.9 kilotons of TNT. Immediately after the blast, two Sherman tanks were dispatched to the crater created by the detonation, and the analysis of soil samples indicated a total yield of 18.6 kilotons of TNT. [2] Enrico Fermi, wrote this note after the Trinity test:

"About 40 seconds after the explosion the air blast reached me. I tried to estimate its strength by dropping from about six feet small pieces of paper before, during, and after the passage of the blast wave. Since, at the time, there was no wind I could observe very distinctly and actually measure the displacement of the pieces of paper that were in the process of falling while the blast was passing. The shift was about 2 1/2 meters, which, at the time, I estimated to correspond to the blast that would be produced by ten thousand tons of T.N.T." [3]

Sir Geoffrey Ingram Taylor, upon seeing the only publicly released photo such as Fig.1, performed a famous dimensional analysis calculation in his paper to estimate the energy yield to be roughly 17.5 kilotons, which astonished U.S. Army because of how close it is to the actual number. This dimensional analysis calculated is explained in detail in this paper, which is fascinating to read about if interested. [4]


The heaviest fallout contaminated outside of the testing zone happened at 30 miles from the detonation ground zero. The fallout caused local beta burns on a lot of livestock in the local ranches. The U.S. Army bought 88 cattles from this area and some of the most severely affected cattles were kept at Los Alamos for observation. In August of 1945, Kodak started to observe spotting and fogging in their film products. These products were packaged in cardboard containers. After a series of investigations, Dr. J. H. Webb concluded that these spotting and fogging was caused by the fallout contaminated water used by a paper mill in Indiana which manufactured cardboard pulp from local corn husks. This incident led to the U.S. Atomic Energy Commission issuing warning and maps of potential contamination for future atmospheric nuclear tests to the photographic industry. [5]

© Tony Chen. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.


[1] L. Hoddeson et al., Critical Assembly: A Technical History of Los Alamos During the Oppenheimer Years, 1943-1945 (Cambridge University Press, 1993).

[2] W. S. Loring, dsBirthplace of the Atomic Bomb: A Complete History of the Trinity Test Site (McFarland, 2019).

[3] P. L. Cantelon et al., eds., The American Atom: A Documentary History of Nuclear Policies from the Discovery of Fission to the Present (University of Pennsylvania Press, 1992).

[4] M. A. B. Deakin, "G.I. Taylor and the Trinity Test," Int. J. Math. Educ. Sci. Technol. 42, 1069 (2011).

[5] J. H. Webb, "The Fogging of Photographic Film by Radioactive Contaminants in Cardboard Packaging Materials," Phys. Rev. 76, 375 (1949).