We present a technical approach to determine whether a nuclear weapon experiment was a supercritical test—tests with a self-sustaining, growing fission chain reaction. It is based on on-site gamma spectroscopy measurements taken of test debris weeks or months later. Analyzing simulated spectra, we can define thresholds above which intensity ratios of select fission products and plutonium-239 emission lines reliably indicate supercriticality. Within ranges that include most practical scenarios, our method is independent of test parameters and sensitive information, including age, shape, and mass of the tested fissile material, initial fissions at experiment’s start and time passed until inspection. Our method can reliably detect any supercritical tests with an explosive yield greater than 38 g TNT equivalent. This method could be applied as a confidence-building measure among nuclear-armed states willing to prohibit supercritical tests.

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• Fission
• Nuclear explosions
• Physics & society
• Direct numerical simulations
• Gamma decay
• Gamma ray spectroscopy
• Monte Carlo methods
• Numerical techniques