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Trinity - 1945
J. R. Oppenheimer - photo by Alfred Eisenstaedt
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Trinity was the code name of the first nuclear weapons test of an atomic bomb. This test was conducted by the United States Army on July 16, 1945, at a location about 56 km southeast of Socorro, New Mexico, at the White Sands Proving Ground, now the White Sands Missile Range.
The Trinity test was the culmination of three years of intensive work by the Manhattan Project at a cost of $2 billion ($33 billion USD as of 2025). Formally designated as the Manhattan Engineer District (MED), the project was control by the U.S. Army Corps of Engineers headed by General Leslie R. Groves. The scientific research was directed by American physicist J. Robert Oppenheimer. The origin of the name Trinity is thought to have been provided by Oppenheimer with reference to the poetry of John Donne
The Manhattan Project had identified two fissile isotopes for potential use in bombs: uranium-235 and plutonium-239. The U-235 isotope had to be isolated painstakingly using vast facilities, slowly and at enormous expense. Pu-239 was a synthetic element that could be 'bred' in nuclear reactors by bombarding uranium with neutrons.
The gun design
A gun-type weapon using plutonium had been developed called 'Thin Man'. In this design, one sub-crirical mass is fired down a barrel into another sub-critical mass forming a super-critical mass and an uncontrolled chain reaction. Plutonium is intrinsically unsuited to a gun-type system, as it has a very high rate of spontaneous fission, emitting neutrons and leading to pre-detonation before the super critical mass can be fully assembled. Additionally the plutonium being produced in reactors of the time contained significant amounts of the isotope Pu-240 which had a spontaneous fission rate 1000 times higher than u-239.
Issues with plutonium predetonation had been attempted to be overcome by using larger, longer and heavier barrels to achieve ever higher muzzle velocities and barrel pressures. Aerodynamically the drop case was also virtually useless. The design was unworkable, and was abandoned in July 1944.
Plutonium was substituted with enriched uranium making this design viable. The gun-type system using uranium was so certain to work, that no test was required. At it's most fundamental level the design, the design involved smashing to spicey rocks together as fast as possible.
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At this juncture, a billion dollars had been invested in synthesizing plutonium, which was suddenly unusable unless a viable weapon system could be developed to utilise it.
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The implosion design
The second device under development used an implosion system. This used explosive 'lenses' to produce a perfectly spherical inward explosive wave with sufficient force and uniformity to rapidly compress the solid plutonium core to several times its original density. The increase in density caused the core – previously subcritical – to become supercritical. At the same time, the shock wave activated an 'initiator' at the centre of the core which flooded the assembled supercritical mass with neutrons.
Code named 'Urchin', this initiator consisted of pellet with alternating layers of beryllium polonium-210 weighing about 7 grams mounted inside the pit. Hydrodynamic forces act on the grooved shell utilizing the Munroe effect, mixing the beryllium and polonium, causing the beryllium to emit large amounts of neutrons. An initiator is not required for an explosive chain reaction, but by kickstarting the chain reaction at the precisely optimal super critical state it increases the efficiency and the yield.
Plutonium is a highly problematic material to work with. It emits alpha radiation, and as it oxidises produces highly toxic dust which can spontaneously ignite. Plutonium also exists in six different allotropes or phases, all of which have significantly different physical properties depending on pressure and density. At room temperature it is in it's α (alpha) phase, brittle and hard to work with. The Los Alamos scientists alloyed it with gallium allowing it to be in it's δ (delta) phase, which workable at room temperature. The plutonium pits were also coated in galvanic silver to prevent hazardous oxidation.
Creating an implosion system presented extreme challenges in physics, engineering and hydrodynamics. The explosive lenses used two different explosives, one fast and one slow to shape the compressive force into a perfect sphere. Any inconsistencies and sufficient compression would not be achieved.
Much of the design work was achieved mathematically (in a era before computers), but actually testing the designs presented significant challenges. How could the implosion be evaluated when the device was sealed, and utterly destroyed in the test? The solution was to introduce a source of gamma radiation into the test metal cores, and use external detectors to visualise and evaluate the symmetry of explosive compression. This series of tests known as the RaLa Experiments, were crucial to the success of the implosion design.
Additionally the explosives lenses had to be fired simultaneously with microsecond tolerances. This problem was solved by using exploding-bridgewire detonators, which use high voltages to vaporize a piece of fine wire in a few microseconds. The resulting shock and heat initiate the high explosive.
The implosion system in the trinity design required a heavy uranium tamper. The tamper serves both reflect neutrons back into the core, and to by way of its mass hold the exploding critical mass together a few nanoseconds longer increasing efficiency.
The engineering challenges were overcome, but the complexity of an implosion-style weapon was such, that despite the waste of the vastly expensive and rare fissile material, an initial test would be required.
The device was referred to as the 'gadget' reflecting its experimental nature. The laboratory's weapon physics division being dubbed 'G Division'. At that time it did not refer specifically to the Trinity Test device as that had yet to be developed, but once it was, it became the laboratory code name. The Trinity bomb was officially a Y-1561 device, as was the Fat Man used later in the bombing of Nagasaki. The two were very similar, though the Trinity bomb being a test device lacked fusing and an external ballistic casing.
The Test Site
After rejecting other parts of New Mexico, an unused Mojave Desert Army base near Rice, California, was considered the best test location. General Groves selected the Alamogordo Bombing Range because George S. Patton controlled the California base. In the fall of 1944, soldiers started arriving at Trinity Site to prepare for the test. Sgt. Marvin Davis and his military police unit arrived at the site from Los Alamos on 30 December 1944. This unit set up initial security checkpoints around the area, and throughout 1945, other personnel arrived at Trinity Site to help prepare for the bomb test. Two bunkers were set up to observe the test. Oppenheimer and Brig. Gen. Thomas Farrell watched from a bunker 16 km from the detonation, while Gen. Leslie Groves watched at a bunker 27 km away.
Test Predictions
The observers set up betting pools on the results of the test. Predictions ranged from zero (known as a fizzle) to 18 kilotons of TNT (predicted by physicist I. I. Rabi, who won the bet), to destruction of the state of New Mexico, to ignition of the atmosphere and incineration of the entire planet. This last result had been calculated to be almost impossible, although for a while it caused some of the scientists some anxiety.
Test preparation
Fully assembled Gadget
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There was a pretest explosion of 108 tons of TNT, spiked with 1000 curies of fission products from the Hanford reactor, on May 7th to calibrate the instruments. For the actual test, the plutonium-core nuclear device, nicknamed the gadget, was hoisted to the top of a 30 meter tall steel tower for detonation. The height would give a better indication of how the weapon would behave when dropped from an airplane, as detonation in the air would maximize the amount of energy applied directly to the target, and minimalize nuclear fallout.
The plutonium pit was driven to Trinity site on July 11th in the back of Plymouth sedan. The high-explosive implosion shell, approximately 150 cm in diameter followed, lashed to the bed of an Army truck. The device was assembled by Rudolf Peierls at the nearby McDonald Ranch House on July 13th. After assembly, it was precariously winched up the tower the following day. Robert Oppenheimer can be seen at 00:02, Norris Bradbury at 02:02, Louis Slotin 00:11 far left in glasses and Harry Daghlian center back at 10:11. Both Slotin and Daghlian later died in criticality experiments at Los Alamos.
General Groves had ordered the construction of a massive steel canister shaped like a thermos flask code-named 'Jumbo''. The gadget was to be placed inside so that if the test failed, the enormously valuable plutonium could be recovered. The container had walls 38cm thick and weighed 214 tons. Oppenheimer decided to abandon it since he was confident of success and did not want to corrupt measurements of the explosion. This was a wise decision since the blast would have vaporized jumbo throwing an additional 214 tons of radioactive steel particles into the air. Instead, it was placed in a steel tower 730m from the gadget as a rough measure of how powerful the explosion would be. In the end, Jumbo survived, though its tower did not.
Photography
Approximately fifty different cameras had been set up, taking motion and still photographs. Special Fastax cameras taking 10,000 frames per second would record the minute details of the explosion, with each frame lasting only 100 microseconds. Spectrograph cameras would record the wavelengths of light from the explosion, and pinhole cameras to record gamma rays. Some observers brought their own cameras despite the security. Observer Jack Aeby smuggled in his 35 mm Perfex 44, who took the only known well-exposed colour photograph of the detonation.
A number of rolls of 35mm Pan-X black & white film were exposed known as 'Newsreel rolls'. These 35mm films were run in trusty Mitchell cameras, running at a range of speeds with various lens's and aperture sizes. What would be required to capture the event was mostly guess work, as this was the birth of an entirely new branch of photography.
The detonation was initially planned for 4:00 am but was postponed because of rain and lightning from early that morning. It was feared that the danger from radiation and fallout would be greatly increased by rain, and lightning had the scientists concerned about accidental detonation.
The Explosion
Trinity 16th July 1945
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At 4:45 am a crucial weather report came in favorably, and at 5:10 am the twenty-minute countdown began. Most top-level scientists and military officers were observing from a base camp 16 km southwest of the test tower. Many other observers were around 32 km away, and some others were scattered at different distances, some in more informal situations. Physicist Richard Feynman claimed to be the only person to see the explosion without the dark glasses provided, relying on a truck windshield to screen out harmful ultraviolet wavelengths. The final countdown was read by physicist Samuel K. Allison.
At 05:29:45 local time (Mountain War Time), the device exploded with an energy equivalent to around 20 kilotons of TNT. It left a crater of radioactive glass in the desert 3m deep and 330m wide. At the time of detonation, the surrounding mountains were illuminated "brighter than daytime" for one to two seconds, and the heat was reported as "being as hot as an oven" at the base camp. The observed colors of the illumination ranged from purple to green and eventually to white.
The roar of the shock wave took 40 seconds to reach the observers. The shock wave was felt over 160 km away, and the mushroom cloud reached 12 km in height. After the initial euphoria of witnessing the explosion had passed, test director Kenneth Bainbridge commented to Los Alamos director J. Robert Oppenheimer, "Now we are all sons of bitches." Oppenheimer later stated that while watching the test he was reminded of a line from the Hindu scripture the Bhagavad Gita: Now I am become Death, the destroyer of worlds..
Post Test measurements
The T (Theoretical) Division at Los Alamos had predicted a yield of between 5 and 10 kilotons. Immediately after the blast, two lead-lined M4 Sherman tanks made their way to the crater. Radiochemical analysis of soil samples indicated that the total yield had been around 18.6 kilotons.
Oppenheimer and Groves inspect the tower remains
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The energy of the blast wave was measured by a large number of sensors using a variety of physical principles. The piezoelectric blast gauges were thrown off scale and no records were obtained. Enrico Fermi conducted his own experiment to measure the energy that was released as blast that involved dropping scraps of paper and seeing how far they were displaced.
nuclear explosion and was used for many years after.
The official estimate for the total yield of Trinity including the blast component together with the the explosion's light output and ionizing radiation, is 21 kilotons, of which about 15 kilotons was contributed by fission of the plutonium core, and about 6 kilotons from fission of the natural uranium tamper.
As a result of the data gathered on the size of the blast, the detonation height for the bombing of Hiroshima was set at 575 meters to take advantage of the Mach stem blast reinforcing effect. The final Nagasaki burst height was 500 meters so the Mach stem started sooner.
Knowing that implosion worked, Oppenheimer recommended that the u-235 to be used in a Little Boy could be more economically used in a Fat Man implosion-type weapon. The gun-type design was known to be highly inefficient, but it was already in production. When Little boy detonated over Hiroshima three weeks later just how inefficient it was became apparent. Of the 64kg of HEU used, only 585 grams (0.9%) actually fissioned before the bomb blew itself apart, the un-fissioned uranium becoming part of the fallout. The weapon still produced a yield of 15 kilotons.
The Fat man contained 6.2kg of plutonium, of which 1kg underwent fission giving it an efficiency of about 16%. Later post war improvements of implosion systems raised this efficiency significantly. Fission only weapons are limited in both their efficiency and yield by the number of fission generations that can be practically produced before the physics package tears itself apart.
In the crater, the desert sand, which is largely made of silica, melted and became a mildly radioactive light green glass which was named Trinitite. The crater was filled in soon after the test.
In the official report on the test, General Farrell wrote - "The lighting effects beggared description. The whole country was lighted by a searing light with the intensity many times that of the midday sun. It was golden, purple, violet, gray, and blue. It lighted every peak, crevasse and ridge of the nearby mountain range with a clarity and beauty that cannot be described but must be seen to be imagined..."
Light from the test was seen in Amarillo, Texas, 450 km away. The Second Air Force issued a press release with a cover story that Groves had prepared weeks before, which described the explosion as the accidental destruction of a ammunition magazine on the base. The actual cause was not publicly acknowledged until after the bombing of Hiroshima.
Around 260 personnel were present, none closer than 9 km. At the next test series, Operation Crossroads in 1946, over 40,000 people were present.
The official technical report (LA-6300-H) on the history of the Trinity test was not released until May 1976.
Deployment
Following the success of the Trinity test, two bombs were prepared for use against Japan during World War II during Project Alberta. The first, dropped on Hiroshima, Japan, on August 6, was code-named 'Little Boy', considered a conservative design and highly likely to work. The second bomb, dropped on Nagasaki, Japan, on August 9, was code-named 'Fat Man' used the plutonium implosion system tested at Trinity.
| - DNA 6028F - Defense Nuclear Agency report - Trinity
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