The world’s first detonation of an atomic weapon, codenamed “Trinity,” took place on July 16, 1945, in the remote Jornada del Muerto desert of New Mexico. The test involved a plutonium implosion device, marking the start of the nuclear age. The ground zero site, now part of the White Sands Missile Range, remains radioactive almost eight decades later. The levels are extremely low and pose minimal risk to visitors. This residual radioactivity results directly from the fission process and the half-lives of the materials created during the explosion.
The Initial Radiological Signature
The 18.6 kiloton detonation released an immense burst of energy, instantaneously creating a highly contaminated zone. The blast vaporized the 100-foot steel tower the device was mounted on and initiated nuclear fission reactions. This process created a vast array of highly radioactive, short-lived fission products deposited on the surrounding landscape.
The intense heat melted the desert sand and asphalt beneath the device. This molten material mixed with the fission products and solidified into a glassy, greenish substance called Trinitite. Trinitite represents the primary physical remnant of the blast, trapping some radioactive contaminants within its structure. The immediate fallout was severe, with radiation exposure rates reaching around 15 Roentgen per hour in nearby ranches hours after the detonation.
Current Measured Radiation Levels
Today, the overall radiation levels at the ground zero area of the Trinity Site are notably low. Monitoring confirms that the radiation is only about 10 times greater than the natural background radiation found in the region. Many locations on Earth, such as the Colorado Plateau, naturally exhibit higher levels of radioactivity than the current Trinity Site.
The residual radiation comes primarily from Cesium-137 (Cs-137), with trace amounts of Americium-241 (Am-241) also being detectable. For visitors, a one-hour visit to the fenced ground zero area results in a whole-body exposure of approximately one-half to one millirem (mrem). This minimal dose is a fraction of the average annual exposure an American receives, which is around 620 mrem from natural and medical sources.
The highest localized radiation source remaining is within the Trinitite itself. While the activity is mild, the green glass fragments contain concentrated radioactive elements, causing the slightly elevated readings at ground zero. The U.S. Army continues to monitor the site to ensure the levels remain within acceptable limits for public access periods.
Understanding Isotope Half-Life
The reason the Trinity Site is no longer severely contaminated is the principle of radioactive decay, governed by an isotope’s half-life. The half-life is the time required for half of the radioactive atoms in a sample to decay into a more stable form. The vast majority of the short-lived fission products created in 1945 had half-lives ranging from seconds to a few years, meaning they decayed rapidly in the immediate aftermath of the test.
The current residual radioactivity is dictated by the longer-lived isotopes, with Cesium-137 being the dominant contaminant. Cs-137 has a half-life of approximately 30 years. Since the test occurred in 1945, the site has passed through over two full half-lives for Cs-137. This means the amount originally deposited has been reduced by more than 75 percent through natural decay.
This predictable decay rate explains why the site is measurable but not hazardous. The decay of Cs-137 will continue to be the main factor determining the site’s radioactivity for centuries. Trace Americium-241, a decay product of Plutonium-241, also contributes to the long-term contamination, but its total quantity is very low.
Safety Protocols for Public Access
The Trinity Site is located deep within the active White Sands Missile Range and is only opened to the public twice each year for a few hours. This limited access period is a primary safety protocol, minimizing the cumulative exposure time for visitors. The U.S. Army has determined the radiation exposure risk to be extremely low for a short visit, comparable to the dose received on a transcontinental airplane flight.
Visitors are permitted to walk to the ground zero marker and the surrounding fenced area, but strict guidelines prevent material collection. Official guidance strongly advises against disturbing or collecting any materials, especially Trinitite. Although the dose from a single piece is negligible, collecting the material would concentrate the localized radiation source, which is prohibited.