From January 1944 to July 1945, large-scale production plants were set in operation, and the fissile material thus produced was then used to determine the features of the weapons. Multipronged research was undertaken to pursue several possibilities for bomb design. Early decisions about weapon design had been based on minute quantities of uranium-235 and plutonium created in pilot plants and laboratory cyclotrons. From these results it was thought that the creation of a bomb was as simple as shooting one piece of fissile material into another to form a critical mass.
The production of uranium-235 proved to be quite difficult with existing technology, but the production of plutonium was easier, as it was a by-product of specially constructed nuclear reactors, the first of which was developed by Enrico Fermi only in 1942. The Plutonium was produced on the Hanford Site in the B-Reactor, the first full-scale plutonium production reactor in the world. This first batch of plutonium was refined in the 221-T plant, using the Bismuth Phosphate process, from December 26, 1944, to February 2, 1945, and delivered to the Los Alamos National Laboratory in New Mexico on February 5, 1945. This reactor-grade plutonium was considerably less pure than cyclotron-produced plutonium however, and the presence of another isotope of plutonium in the resulting product meant that the simple "gun type" model of bomb design would not work; the presence of extra neutrons meant that the weapon would pre-detonate with a greatly reduced yield. This problem was discovered in 1944 and led to a redesign of the bomb as an "implosion" device in which a spherical core of plutonium would be compressed using conventional explosives, which would increase the plutonium's density and thus create a critical mass.
The plutonium sphere would need to be compressed on all sides exactly equally—any error would result in a "fizzle" which would simply eject the valuable plutonium and not result in a large explosion. Because of the difficulties in creating the explosive lenses for perfect compression, Manhattan Project military leader General Leslie Groves and scientific director J. Robert Oppenheimer decided that a test of the concept would have to be undertaken before a weapon could be confidently used in war. The Fat Man bomb, which used the same design, was exploded over Nagasaki on August 9, 1945, after the Trinity test proved its feasibility.
The site was part of the Alamogordo Bombing Range, now the White Sands Missile Range. The test site was at the northern end of the range, between the towns of Carrizozo and Socorro, New Mexico, in the Jornada del Muerto in the southwestern United States (). Two bunkers were set up to observe the test. Oppenheimer and Gen. Thomas Farrell watched from a bunker ten miles from the detonation, while Gen. Groves watched at a bunker seventeen miles away.
The observers set up betting pools on the results of the test. Predictions ranged from zero, a complete dud, 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.
There was a pretest explosion of 108 tons of TNT on May 7 to calibrate the instruments. (Nuclear detonation yields have always been measured by the equivalent in tons of TNT.) For the actual test, the plutonium-core nuclear device, nicknamed the gadget, was hoisted to the top of a 20-meter 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 (as it expanded in a spherical shape), and would generate less nuclear fallout.
The "gadget" was assembled at the nearby McDonald Ranch House, the components arriving on July 12. It was assembled on July 13 and precariously winched up the tower the following day. General Groves ordered the construction of a 240 ton steel canister code-named "Jumbo" to recover valuable plutonium if the 5 tons of conventional explosives failed to compress it into a chain reaction. The container was constructed at great expense in Pittsburgh, Pennsylvania and brought to the test site by rail but by the time it arrived the confidence of the scientists was high enough that they decided not to use it. Instead it was hoisted up in a steel tower 800 yards (730 m) from the "gadget" as a rough measure of how powerful the explosion would be. In the end, "Jumbo" survived, though its tower did not.
The detonation was initially planned for 4:00 a.m. 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.
At 4:45 a.m. a crucial weather report came in favorably, and at 5:10 a.m. the twenty-minute countdown began. Most top-level scientists and military officers were observing from a base camp ten miles (16 km) southwest of the test tower. Many other observers were around twenty miles (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), (11:29:45 UTC) the device exploded with an energy equivalent to around 20 kilotons of TNT (90 TJ). It left a crater of radioactive glass in the desert 10 feet (3 meters) deep and 1,100 ft (330 meters) 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 100 miles (160 km) away, and the mushroom cloud reached 7.5 miles (12 km) in height. As Los Alamos director J. Robert Oppenheimer watched the demonstration, he later said that a line from the Hindu scripture the Bhagavad Gita came to mind:
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...
News reports quoted a forest ranger 150 miles (240 km) west of the site as saying he saw "a flash of fire followed by an explosion and black smoke." A New Mexican 150 miles (240 km) north said, "The explosion lighted up the sky like the sun." Other reports remarked that windows were rattled and the sound of the explosion could be heard up to 200 miles (320 km) away.
John R. Lugo was flying a U.S. Navy transport at 10,000 feet, 30 miles east of Albuquerque, en route to the West Coast. "My first impression was, like, the sun was coming up in the south. What a ball of fire! It was so bright it lit up the cockpit of the plane." Lugo radioed Albuquerque. He got no explanation for the blast, but was told, "Don't fly south.
The Alamogordo Air Base issued a 50-word press release in response to what it described as "several inquiries" that had been received concerning an explosion. The release explained that "a remotely located ammunitions magazine containing a considerable amount of high explosives and pyrotechnics exploded," but that "there was no loss of life or limb to anyone. A newspaper article published the same day stated that "the blast was seen and felt throughout an area extending from El Paso to Silver City, Gallup, Socorro, and Albuquerque. The actual cause was not publicly acknowledged until after the August 6 attack on Hiroshima.
The Manhattan Project's official journalist, William L. Laurence, had put multiple press releases on file with his office at The New York Times to be released in case of an emergency, ranging from an account of a successful test (the one which was used) to more macabre scenarios explaining why all of the scientists had perished in a single freak accident.
Around 260 personnel were present, none closer than 5.6 miles (9 km). At the next test series, Operation Crossroads in 1946, over 40,000 people were present.
Information about the Trinity test was made public shortly after the bombing of Hiroshima. The Smyth Report, released on August 12, 1945, gave some information on the blast, and the hardbound edition released by Princeton University Press a few weeks later contained the famous pictures of a "bulbous" Trinity fireball.
Oppenheimer and Groves posed for reporters near the remains of the mangled test tower shortly after the war. In the years after the test, the pictures have become a potent symbol of the beginning of the so-called "Atomic Age", and the test has often been featured in popular culture.
In 1952, the site of the explosion was bulldozed and the remaining trinitite was disposed of. On December 21, 1965, Trinity Site was declared a National Historic Landmark district, and was listed on the National Register of Historic Places on October 15, 1966.
More than sixty years after the test, residual radiation at the site measured about ten times higher than normal. The amount of exposure received during a one-hour visit to the site is about half of what a U.S. adult receives on an average day from natural and medical sources. The Trinity monument, a rough-sided, lava rock obelisk around 12 feet (3.65 m) high, marks the explosion's hypocenter, and "Jumbo" is still kept nearby.
The site is still a popular destination for those interested in atomic tourism, though it is only open to the public twice a year, on the first Saturdays of April and October. On July 16, 2005, a special tour of the site was conducted to mark the 60th anniversary of the Trinity test, and hundreds (some news sources reported thousands) of visitors arrived to commemorate the occasion.