Trinity Shot was part of the secret Manhattan Project - the atomic bomb project charged with designing, developing, testing, and firing a weaponization of the newly discovered phenomenon, fission, the splitting of nuclei of heavy particles to release energy. Nicknamed "Gadget," the device was exploded at the Trinity Site in southeastern New Mexico, near Alamogordo as the first proof test of the concept of implosion. This involved taking a critical mass of plutonium and using detonators to set off high explosives to cause the mass of radioactive material to squeeze in upon itself until it reached a super critical mass. This implosion, then, resulted in the splitting of the nuclei in the plutonium atoms, which produced heat, blast, and radiation.
The blast that lit up the entire New Mexico sky at 5:29:45 a.m. on July 16, 1945 was "brighter than 20 suns and the most spectacular sunrise ever seen." That's how Ben Benjamin, a retired Sandia weaponeer and member of the Manhattan project, described the first test of an atomic bomb at the Trinity Site. In 1945 Ben was a 22-year-old Army sergeant assigned as a technician to the photo-optical division in the Manhattan Project. Selected for the job because of his earlier work in industry making lenses and prisms, he was one of a handful of people in the division responsible for photographing the blast using both still and motion picture cameras.
The McDonald Ranch was where the core of the bomb was assembled; "Ground Zero," the location where the bomb -- always referred as "the Gadget" -- was exploded; and the concrete bunker six miles west of Ground Zero where some of the optics division's cameras were placed to record the blast. He pointed to two other monitoring stations, one six miles south that served as the control station and another six miles north, a photo station.
The photo-optical group -- set up many 35-millimeter motion picture cameras, a spectrograph, and several still cameras at the western monitoring station built in a concrete bunker. The experiments were designed to capture the explosion on film for future scientific study. They knew there would be a ball of fire that would rise and we arranged the cameras to follow it.
The day of the actual test started out with an early morning thunderstorm. The storm delayed the test from its originally scheduled time of 4 a.m. until nearly 5:30 a.m. Five seconds before the blast, an electronic pulse went out to a switchbox in the optics bunker. It was the signal to turn on the cameras. After switching on the cameras, observers put a piece of welder's glass up to his eyes to protect them from the bright light that was expected to come. Evidence of the power in the blast could be seen in some of the moving and still pictures the optics team took. Many of the pictures turned out perfect and provided significant scientific information that allowed scientists to fully analyze the blast.
Immediately after the Trinity test, the first estimate of the device’s yield was about 18 kilotons (the equivalent of 18 kilotons of trinitrotoluene, or TNT), with an estimated 20 percent uncertainty. In the years following World War II, the yield of the device was recalculated to be 21 kilotons. However, after examining archival calculations and recent measurements of radionuclides in rock samples originally taken from near the city of Alamogordo—located just outside the blast zone — Hugh Selby (of the Lab’s Nuclear and Radiochemistry group) and others found that Trinity’s yield was higher still, approximately 24.8 kilotons.
“The new value comes from the powerful combination of advanced inorganic separations chemistry with high precision mass spectrometry—an analytical tool useful for measuring the mass-to-charge ratio of molecules,” Selby explains. “The former purifies the element containing fission fragments of interest from the sea of chemical interferences present in debris. The latter quantifies the minute amounts of bomb-produced isotopes of the element present in the purified sample, relative to the natural background. The level of precision necessary to make such measurements and to reanalyze 75-year-old data was made possible by major advances in both chemistry and mass spectrometry.”
Today, a lava rock obelisk that commemorates the Trinity Site as a National Historic Site marks Ground Zero.
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