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Trinity (nuclear test)

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The Trinity explosion, 16 ms after detonation.
Country United States
Test site Trinity Site, New Mexico
Date July 16, 1945
Test type Atmospheric
Device type Fission
Yield 20 kilotons of TNT (84 TJ)
Previous test none
Next test Operation Crossroads
Trinity Site
U.S. National Register of Historic Places
U.S. National Historic Landmark District
N.M. State Register of Cultural Properties
Trinity Site Obelisk
Trinity (nuclear test) is located in New Mexico
Location: White Sands Missile Range
Nearest city: San Antonio, New Mexico
Coordinates: 33°40′38.28″N 106°28′31.44″W Coordinates: 33°40′38.28″N 106°28′31.44″W
Area: 36,480 acres (147.6 km2)
Built: 1945
NRHP Reference#: 66000493
NMSRCP #: 30
Significant dates
Added to NRHP: October 15, 1966
Designated NHLD: December 21, 1965
Designated NMSRCP: December 20, 1968

Trinity was the code name of the first detonation of a nuclear device. This test was conducted by the United States Army on July 16, 1945, in the Jornada del Muerto desert about 35 miles (56 km) southeast of Socorro, New Mexico, at the new White Sands Proving Ground, which incorporated the Alamogordo Bombing and Gunnery Range. (The site is now the White Sands Missile Range.) The date of the test is usually considered to be the beginning of the Atomic Age.

Trinity was a test of an implosion-design plutonium device. The weapon's informal nickname was "The Gadget". Using the same conceptual design, the Fat Man device was detonated over Nagasaki, Japan, on August 9, 1945. The Trinity detonation produced the explosive power of about 20 kilotons of TNT (84  TJ).


The creation of atomic weapons arose out of political and scientific developments of the late 1930s. The rise of fascist governments in Europe, new discoveries about the nature of atoms and the fear that Nazi Germany was working on developing atomic bombs converged in the plans of the United States, the United Kingdom, and Canada to develop powerful weapons using nuclear fission as their primary source of energy. The Manhattan Project, as the American nuclear physics effort was called, culminated in the test explosion of a nuclear device at what is now called the Trinity Site on July 16, 1945, and the atomic bombings of Hiroshima and Nagasaki a few weeks later.

The Manhattan Project

The two types of fission-bomb assembly methods investigated during the Manhattan Project. The gun-type assembly was not tested before it was detonated at Hiroshima. Because of the novel and untried features of the implosion-bomb design, Fat Man, J. Robert Oppenheimer and the other scientists at Los Alamos decided that it was necessary to test this one before attempting to use one as a weapon against the enemy.

U.S. and British researchers were investigating the feasibility of nuclear weapons as early as 1939. Practical development began in earnest in 1942 when these efforts were transferred to the authority of the U.S. Army and became the Manhattan Project. The weapons-development portion of this project was located at the Los Alamos Laboratory in northern New Mexico, though much other development and production work was carried out at the Clinton Engineer Works near Oak Ridge, Tennessee (the separation of uranium-235); the Hanford Engineer Works near Hanford, Washington (the production and separation of plutonium-239); in and near Chicago, Illinois (at the University of Chicago and the Argonne National Laboratories); and at the University of California, Berkeley.

These research, development, and production efforts focused both on the development of the necessary fissile materials to power the nuclear chain reactions in the atomic bombs and on the design, testing, and manufacture of the bombs themselves.

From January 1944 until 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. Multi-pronged 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 that had been created in pilot plants and in physics-laboratory cyclotrons. From these experimental results, it was thought that the creation of a bomb was as simple as forming a critical mass of fissile material.

The production of both uranium-235 and plutonium-239 were enormous undertakings given the technology of the 1940s and accounted for 80% of the total costs of the project. Theoretically, enriching uranium was feasible through pre-existing techniques in physics (e.g., modifying particle accelerator technology), though it proved difficult to scale to industrial levels and was extremely costly.

Plutonium, by contrast, could theoretically be produced most easily in nuclear reactors, but the technology and science involved was wholly new. The first experimental nuclear reactor had been developed and constructed by Enrico Fermi and his team of co-workers by the end of 1942 at the University of Chicago ( Chicago Pile-1), which proved that there were no obvious physical limitations to producing a slow-neutron nuclear chain reaction. Work began on constructing large plutonium-breeding reactors at Hanford, Washington, in October 1943. The first reactor-bred plutonium was produced in the B-Reactor, the first full-scale plutonium-production reactor in the world. The first large batch of plutonium was refined at Hanford in the "221-T plant", using the bismuth phosphate process, from December 26, 1944, to February 2, 1945. This was delivered to Oppenheimer's team at the Los Alamos laboratory on February 5, 1945. In the meantime, the X-10 Graphite Reactor, a scaled-down version of the Hanford reactors, was built in Oak Ridge, Tennessee, and went into operation in November 1943.

Plutonium is a synthetic element not found in nature in appreciable quantities. It also has relatively complicated physics, chemistry, and metallurgy compared to most other elements. The only prior plutonium isolated for the project had been produced in cyclotrons in very minute amounts. In April 1944, Emilio Segrè received the first sample of reactor-bred plutonium from the X-10 reactor and discovered that it was not as pure as cyclotron-produced plutonium by a significant degree. Specifically, the longer the plutonium remained irradiated inside the reactor—which is necessary for high yields of the metal—the greater its content of the isotope plutonium-240. Pu-240 undergoes spontaneous fission at an appreciable rate, and that releases excess neutrons. These extra neutrons implied a high probability that a gun-type bomb with plutonium would detonate too early, before a critical mass was formed, scattering the plutonium and producing a small "fizzle" of a nuclear explosion many times smaller than a full explosion. The practical result was that a simple gun-type atomic bomb (the proposed Thin Man) would not work as had been hoped.

A diagram showing the bottom half of an implosion design similar to the one used in the Trinity gadget. The neutron initiator is at the centre, surrounded by the plutonium core, surrounded by a pusher and tamper, surrounded by the explosive lenses, all within a casing. This diagram is not a literal depiction of the Trinity gadget and contains a few elements not found in the gadget.

The impossibility of solving this problem of a gun-type bomb with plutonium was decided upon in a meeting in Los Alamos on June 17, 1944. This forced a search for a different, more practical design for a plutonium-fueled bomb, and an implosion-type atomic-bomb design (i.e., the Fat Man design) was selected as the most practical one at that time. This required a great deal of research work and experimentation in engineering and hydrodynamics before a practical design could be worked out.

In an implosion bomb, a small spherical core of plutonium would be surrounded by high explosives that burned with different speeds. By alternating the faster and slower burning explosives in a carefully calculated spherical configuration, they would produce a compressive wave upon their simultaneous detonation. This " lensing" effect focused the explosive force inward with enough force to compress the plutonium core to several times its original density. This would rapidly reduce the necessary size of the critical mass of the material, making it supercritical. It would also activate a small neutron source at the centre of the core, which would assure that the chain reaction began in earnest.

The advantage of the implosion method was that it was far more efficient in use of material—only 6.2 kg of plutonium would be needed for a full explosion, compared to the 64 kg of enriched uranium used in the "Little Boy" weapon. The engineering difficulties, though, were daunting. Though explosive lenses had been pursued during the war, the art was still very new, and the tolerances required in terms of timing and symmetry were unprecedented. Should the timing or symmetry be off, the bomb would not detonate fully, and instead just disperse the plutonium into the surrounding area. The entire Los Alamos laboratory was reorganized in 1944 to focus on designing a workable implosion bomb.

Scientists were confident that an implosion device would work, but these new design difficulties were great. It was decided that a full-sized test would be required before any military use, even though it would sacrifice one of a very small number of bombs. In early 1944, plans for a July 1945 test were finalized. The uranium weapon would not be tested; its success could be more or less guaranteed by measurements ahead of time.

The actual Fat Man atomic bomb, which used the same design as in the Trinity test, was exploded over Nagasaki on August 9, 1945, after the Trinity test had proven its operability in an atomic explosion.

The gadget

"The gadget" was the code name given to the first bomb tested. It was so called because it was not a deployable weapon and because revealing words like bomb were not used during the project for fear of espionage. It was an implosion-type plutonium device, similar in design to the Fat Man bomb used three weeks later in the atomic bombing of Nagasaki, Japan.

In the Fat Man design, a subcritical sphere of plutonium was placed in the centre of a hollow sphere of high explosive. Numerous exploding-bridgewire detonators located on the surface of the high explosive were fired simultaneously to produce a powerful inward pressure on the core, squeezing it and increasing its density, resulting in a supercritical condition and a nuclear explosion. The actual eventual Fat Man and the "gadget" devices were not strictly "Fat Man type", as the design was modified into a production design, and both were strictly one-off prototypes.

Schematic cross-section of the gadget. See Fat Man article for details.

The gadget was tested at Trinity Site, New Mexico, near Alamogordo. For the test, the gadget was lifted to the top of a 100-foot (30 m) bomb tower. It was feared by some that the Trinity test might "ignite" the earth's atmosphere, eliminating all life on the planet, although calculations had determined this was unlikely. Less wild estimates thought that New Mexico would be incinerated. Calculations showed that the yield of the device would be between 0 (if it did not work) and 20 kilotons of TNT (84 TJ). In the aftermath of the test, it appeared to have been a blast equivalent to 18 kilotons of TNT (75 TJ).

Test planning

In March 1944, planning for the test was assigned to Kenneth Bainbridge, a professor of physics at Harvard University, working under explosives expert George Kistiakowsky. A site had to be located that would guarantee secrecy of the project's goals even as a nuclear weapon of unknown strength was detonated. Proper scientific equipment had to be assembled for retrieving data from the test itself, and safety guidelines had to be developed to protect personnel from the unknown results of a highly dangerous experiment. Official test photographer Berlyn Brixner set up dozens of cameras to capture the event on film.

Test site

Trinity Site (red arrow) near Carrizozo Malpais

The heads of the project considered eight candidate sites, including San Nicolas Island (California), Padre Island (Texas), San Luis Valley, El Malpais National Monument, and other parts of New Mexico. A Mojave Desert Army base near Rice, California was considered the best location, but was opted against because General Leslie Groves, military head of the project, did not wish to have any dealings with Gen. George S. Patton, commander of the base, whom he disliked. The site finally chosen was at the northern end of the White Sands Proving Ground, in Socorro County between the towns of Carrizozo and San Antonio, in the Jornada del Muerto in the southwestern United States ( 33.6773°N 106.4754°W / 33.6773; -106.4754). In late 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 December 30, 1944. This unit set up initial security checkpoints around the area, with plans to use horses for patrols. The distances around the site proved too great, so they resorted to using jeeps and trucks for transportation.

Throughout 1945, other personnel arrived at Trinity Site to help prepare for the bomb test. As the soldiers at Trinity Site settled in, they became familiar with Socorro County. They tried to use water out of the ranch wells, but found the water so alkaline they could not drink it. They were forced to use U.S. Navy saltwater soap and hauled drinking water in from the firehouse in Socorro. Gasoline and diesel fuel were purchased from the Standard Oil bulk plant in Socorro.

Three bunkers were set up to observe the test. Oppenheimer and Brig. Gen. Thomas Farrell watched from the South bunker 5.7 miles (9.2 km) from the detonation, while Gen. Leslie Groves watched from the base camp 10 miles (16 km) away.


The exact origin of the name is unknown, but it is often attributed to laboratory leader J. Robert Oppenheimer as a reference to the poetry of John Donne. In 1962, General Groves wrote to Oppenheimer about the origin of the name, asking if he had chosen it because it was a name common to rivers and peaks in the West and would not attract attention, and elicited this reply:

I did suggest it, but not on that ground... Why I chose the name is not clear, but I know what thoughts were in my mind. There is a poem of John Donne, written just before his death, which I know and love. From it a quotation: "As West and East / In all flatt Maps—and I am one—are one, / So death doth touch the Resurrection." That still does not make a Trinity, but in another, better known devotional poem Donne opens, "Batter my heart, three person'd God;—."

Test predictions

The explosives of the gadget were raised up to the top of the tower for the final assembly.

The observers set up a betting pool on the results of the test, with predictions ranged from zero (a complete dud) to 45 kilotons of TNT (190 TJ). Physicist I. I. Rabi later won the pool with a prediction of 18 kilotons of TNT (75 TJ).

In addition Fermi personally offered to take wagers among the top physicists and military present on whether the atmosphere would ignite, and if so whether it would destroy just the state, or incinerate the entire planet. This last result had been previously calculated to be almost impossible, although for a while it had caused some of the scientists some anxiety. Rhodes speculates that Fermi may have been making a point about a "new force being loosed on the Earth", and how little really was known about it. Bainbridge was furious with Fermi for scaring the guards who, unlike the physicists, did not have the advantage of their knowledge about the scientific possibilities.

Test preparation

Men stack crates of high explosives for the 100 ton test

A pre-test calibration explosion of 100 tons of TNT (420 GJ), spiked with 1,000 curies (37  TBq) of fission products from the Hanford reactor, was detonated on a wooden platform 800 yards (730 m) to the south-east of Trinity ground zero on May 7. The fireball of the conventional explosion was visible 60 miles (97 km) away. The smaller explosion proved an invaluable experiment that helped scientists prepare instruments in time to study the atomic bomb explosion two months later.

For the actual test, the plutonium-core nuclear device, referred to as "the gadget," was hoisted to the top of a 100-foot (30 m) 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, fully assembled and ready to test

The gadget was assembled at the nearby McDonald Ranch House on July 13, the components having arrived on July 12. After assembly, it was carefully winched up the tower the following day.

General Groves had ordered the construction of a 214-short-ton (194 t) steel canister code-named "Jumbo" to recover valuable plutonium if the 5 short tons (4.5 t) 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, although its tower did not.

The 100-foot-tall tower constructed for the test

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.


Video of Trinity test
Colour photograph of the Trinity explosion by Jack Aeby

At 04:45, a crucial weather report came in favorably, and, at 05:10, the twenty-minute countdown began. Most top-level scientists and military officers were observing from a base camp 10 miles (16 km) southwest of the test tower. Many other observers were around 20 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 05:29:21 (plus or minus 2 seconds) local time ( Mountain War Time), the device exploded with an energy equivalent to around 20 kilotons of TNT (84 TJ). It left a crater of radioactive glass in the desert 10 feet (3.0 m) deep and 1,100 feet (340 m) 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 (12.1 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 Bhagavad Gita, a Hindu scripture:

Now I am become Death, the destroyer of worlds.
Ground zero 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..."

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 (3,000 m), 30 miles (48 km) 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."

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.

An aerial photograph of the Trinity crater shortly after the test. The small crater in the southeast corner was from the earlier test explosion of 108 tons of TNT (450 GJ).

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." An Associated Press article quoted a blind woman 150 miles (240 km) away who asked "What's that brilliant light?" Such articles appeared in New Mexico, but East coast newspapers ignored them.

The air base press release was written by the Manhattan Project's official historian, William L. Laurence of The New York Times. He had prepared four releases for a variety of outcomes, ranging from an account of a successful test (the one which was used) to more catastrophic scenarios involving serious damage to surrounding communities, evacuation of nearby residents, and a placeholder for the names of those killed in the explosion. As Laurence was a witness to the test he knew that the last release, if used, would be his obituary.

Around 260 personnel were present, none closer than 5.6 miles (9.0 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.

Test results

Maj. Gen. Leslie R. Groves and Robert Oppenheimer at the Trinity shot tower remains a few weeks later.

The results of the test were conveyed to President Harry S. Truman, who was eagerly awaiting them at the Potsdam Conference; the coded message ("Operated this morning. Diagnosis not complete but results seem satisfactory and already exceed expectations ... Dr. Groves pleased.") arrived at 7:30 p.m. on July 16 and was at once taken to the president and Secretary of State James F. Byrnes at the "Little White House" in the Berlin suburb of Babelsberg by Secretary of War Henry L. Stimson. 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.

First deployment

Following the success of the Trinity test, two bombs were prepared for use against Japan during World War II. The first, dropped on Hiroshima, Japan, on August 6, was code-named " Little Boy", and used uranium-235 as its fission source. It was an untested design but was considered very likely to work and was considerably simpler than the implosion model. It could not be tested, because there was only enough uranium-235 for one bomb. The second bomb, dropped on Nagasaki, Japan, on August 9, was code-named "Fat Man" and was a plutonium bomb of the type tested at Trinity. The atomic bombings of Hiroshima and Nagasaki killed at least 148,000 people and many more over time. By 1950, the death toll was over 340,000. They were followed days later by the surrender of Japan. Debate over the justification of the use of nuclear weapons against Japan persists to this day, both in scholarly and popular circles.

Site today

Trinity Site Historical Marker

In 1952, the site of the explosion was bulldozed, and the remaining trinitite was disposed of. On December 21, 1965, the 51,500-acre (20,800 ha) area Trinity Site was declared a National Historic Landmark district and, on October 15, 1966, was listed on the National Register of Historic Places.

The landmark includes the base camp, where the scientists and support group lived; ground zero, where the bomb was placed for the explosion; and the Schmidt/McDonald ranch house, where the plutonium core to the bomb was assembled. Visitors to a Trinity Site open house are allowed to see the ground zero and ranch house areas. In addition, one of the old instrumentation bunkers is visible beside the road just west of ground zero.

Remnants of Jumbo

In September 1953, about 650 people attended the first Trinity Site open house. In 1967, the inner oblong fence was added. In 1972, the corridor barbed wire fence that connects the outer fence to the inner one was completed. Jumbo was moved to the parking lot in 1979; it is missing its ends from an attempt to destroy it in 1946 using eight 500 pounds (230 kg) bombs.

More than sixty years after the test, residual radiation at the site measured about ten times higher than normal. The amount of radioactive exposure received during a one-hour visit to the site is about half of the total radiation exposure which 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 (3.7 m) high, marks the explosion's hypocenter, and Jumbo is still kept nearby.

On July 16, 1995, a special tour of the site was conducted to mark the 50th anniversary of the Trinity test, and about 5,000 visitors arrived to commemorate the occasion, the largest crowd for any open house. Since that large crowd, the open houses usually average two to three thousand visitors. The site is still a popular destination for those interested in atomic tourism, though it is only open to the public twice a year during open houses, on the first Saturdays of April and October.

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