A painting in the Guggenheim collection initially attributed to French modern artist Fernand Léger has languished out of view for decades after it was suspected to be a fake.Now scientists have confirmed that the artwork is a indeed forgery; in a first, they detected faint signatures of Cold War-era nuclear bombs in the canvas that reveal the painting was created after Léger's death.Carbon 14 is a radioactive variation of carbon, and because plants pick up both types through photosynthesis, all living organisms — cotton plants included — have the same ratio of carbon 14 to stable carbon as the atmosphere.But a series of nuclear bomb tests in the 1950s and 1960s spiked this normally consistent ratio.

The team used a particle accelerator to measure the concentration of carbon 14 (an isotope of carbon that has more neutrons than normal carbon 12) in the fabric, which would in turn allow them to determine when the canvas was produced, or more specifically, when the cotton was cut to make the canvas.

Radiometric dating is the method for establishing the age of objects by measuring the levels of radioisotopes in the sample. It decays to nitrogen 14 with a half life of 5730 years.

Carbon 14 is created by cosmic rays in the upper atmosphere.

"After 1955 the level of radiocarbon in the atmosphere, and thus in living organisms, almost doubled in about 10 years," Pier Andrea Mandò, head of the Florence division of the INFN, explained in a statement.

"It is due to this rapid change that works from those years can be dated extremely accurately," Mandò added.