A clock in the clay
The ancient city of Beikthano, one of the Pyu city-states in central Myanmar, is a site of low brick mounds and fortifications. For archaeologists, determining the precise timeline of its construction and occupation posed a challenge. Traditional methods offered a broad picture, but the city's long history remained blurry. The answer came from the bricks themselves, through a technique called thermoluminescence (TL) dating. This method measures the accumulated radiation dose in a mineral since it was last heated to a high temperature.
The science works like this: crystalline minerals, such as quartz and feldspar found in clay, are constantly exposed to natural radiation from the soil and cosmic rays. This environmental radiation knocks electrons out of their normal positions within the crystal lattice. Most electrons fall back, but some become trapped in imperfections in the crystal structure. Over centuries, more and more electrons get trapped, storing energy. When a sample of this brick is taken to a laboratory and heated to over 500°C, the trapped electrons are freed. They release their stored energy as a faint pulse of light, or thermoluminescence. The intensity of this light is directly proportional to the total radiation dose absorbed since the brick was first fired. By measuring the light and calculating the annual radiation dose the brick received from its burial environment, scientists can determine how much time has passed since its "clock" was last reset by the heat of the kiln.