Trinity test explosion, July 16, 1945.
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 as possible about it. And the resources supporting that effort are significant.
Seismic waves betray the occurrence of underground tests, and air samples taken shortly after may contain the radioactive evidence. 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 has demonstrated a new technique that can reveal the bomb’s potency from the rubble — even decades later.
To test the technique, they tried it out at New Mexico’s famous 1945 Trinity test site, where the first-ever atomic bomb was detonated less than a month before atomic bombs were dropped on Hiroshima and Nagasaki. The heat of the blast melted the sandy surface into glassy rock, which was named ‘trinitite’. Immediately after the explosion, that trinitite would have been loaded with short-lived radioactive isotopes that could tell you how the bomb functioned, but the key indicators disappear within months.
By analyzing different bits of trinitite, the researchers tried to measure isotopes produced by the atomic chain reaction that was not radioactive and therefore still present 70 years later. 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, which are also highly radioactive, split again and continue 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 speaking, none of their nuclear “ancestors” are gases, so their abundance in the glassy trinitite is tied to the power of the chain reaction in an uncomplicated way.
Unfortunately, they have radioactive half-lives of about 17 hours and 64 days, respectively. Since those isotopes have had ample time to decay since 1945, we should now analyze the things they have changed into. 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 spewed apart in the 1945 blast.
With careful lab work, various isotopes of molybdenum were measured and used to calculate how much radioactive zirconium produced by the bomb was originally contained in each gram of the trinitite. There is Also some plutonium-239 in the trinitite, the part of the bomb that didn’t participate in the chain reaction. By measuring this leftover plutonium, and knowing how much of the plutonium in the chain reaction cascade ends up as zirconium, you can now calculate back to both the percentage of plutonium atoms in the bomb that split and the percentage that did not contribute to the detonation.
In this case, the calculated “efficiency” of the Trinity device was just over 20 percent, meaning that only 20 percent of the plutonium in the bomb was split in the fission chain reaction. Because we know the actual mass of plutonium that was in the Trinity device, that efficiency can tell you the explosive yield of the blast. Estimates of the yield of the Trinity bomb ranged from 8 to 61 kilotons, with an official estimate of 21 kilotons. This new estimate based on molybdenum isotopes comes pretty close at just 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 have taken place.
PNAS2016. DOI: 10.1073/pnas.1602792113 (About DOIs).