Reconstructing the first atomic bomb test from a chunk of scorched earth

If the CSI family of television shows has blunted your appetite for impossibly omniscient crime scene analysis, consider the real, and very serious, science of nuclear forensics. If someone flouts the ban on nuclear weapons testing, we want to know as much about it as possible. And the resources backing that effort are substantial.

Seismic waves betray the occurrence of underground tests, and air samples grabbed soon afterward can contain the radioactive proof. But both are transient, and even radioactivity at the site of the explosion can fade too quickly to be of much use. A group of researchers at Los Alamos National Laboratory have demonstrated a new technique than can reveal the potency of the bomb from the debris—even decades after the fact.

To test the technique, they tried it out on the famous 1945 Trinity test site in New Mexico, where the very first atomic bomb was detonated less than a month before nuclear bombs were dropped on Hiroshima and Nagasaki. The heat of the blast fused the sandy surface into glassy rock that took on the name “trinitite.” Immediately after the explosion, that trinitite would have been loaded with short-lived radioactive isotopes that could tell you about how the bomb functioned, but the most important indicators dissipate within months.

Analyzing several pieces of trinitite, the researchers sought to measure isotopes produced by the atomic chain reaction that were not radioactive, and therefore still present 70 years on. This requires laboratory precision far beyond what was possible in the 1940s.

In the fission chain reaction of the Trinity bomb, radioactive plutonium atoms split into predictable pairs of elements. The isotopes that are also highly radioactive will split again, continuing along a cascade of well-defined pathways until they end up at isotopes that are more stable. Zirconium-95 and zirconium-97 are part of that cascade and, critically, none of their nuclear “ancestors” are gases, so their abundance in the glassy trinitite is tied to the strength of the chain reaction in a straight-forward way.

Unfortunately, they have radioactive half-lives of about 17 hours and 64 days, respectively. Since those isotopes have had plenty of time to decay away since 1945, we would now need to analyze the things they turned into, instead. As it happens, they produce non-radioactive molybdenum-95 and molybdenum-97. So there’s your forensic clue—measure the higher-than-normal amount of those molybdenum isotopes in the glassy trinitite today, and you can basically figure out how many atoms of plutonium spit apart in the 1945 blast.

With careful lab work, several isotopes of molybdenum were measured and used to calculate how much radioactive zirconium produced by the bomb was originally in each gram of the trinitite. There is also some plutonium-239 in the trinitite, which is the portion of the bomb that failed to join in the chain reaction. By measuring this remnant plutonium, and by knowing how much of the plutonium in the chain reaction cascade ends up as zirconium, you can now work back to both the percentage of plutonium atoms in the bomb that were split and the percentage that failed to contribute to the blast.

In this case, the calculated “efficiency” of the Trinity device was just over 20 percent—meaning only 20 percent of the plutonium in the bomb split in the fission chain reaction. Because we do know the actual mass of plutonium that was inside the Trinity device, that efficiency can tell you the explosive yield of the blast. Estimates of the Trinity bomb’s yield have ranged from 8 to 61 kilotons, with the official estimate coming in at 21 kilotons. This new estimate based on molybdenum isotopes is actually pretty close to that at just a shade over 22 kilotons.

In this case, the interest is historical, but this adds another tool to the kit for monitoring nuclear weapons tests—whenever they may have occurred.

PNAS, 2016. DOI: 10.1073/pnas.1602792113  (About DOIs).