A group of dating techniques for certain rocks that depends on the decay of the radioisotopes uranium–238 to lead–206 (half-life 4.5 × 109 years) or the decay of uranium–235 to lead–207 (half-life 7.1×108 years). One form of uranium-lead dating depends on measuring the ratio of the amount of helium trapped in the rock to the amount of uranium present (since the decay 238U → 206Pb releases eight alpha-particles). Another method of calculating the age of the rocks is to measure the ratio of radiogenic lead (206Pb, 207Pb, and 208Pb) present to nonradiogenic lead (204Pb). These methods give reliable results for ages of the order 107–109 years.