Uranium–uranium dating
U, with a half-life of about 4.5 billion years, decays to U through emission of an alpha particle to thorium-234 (Th), which is comparatively unstable with a half-life of just 24 days. Th then decays through beta particle emission to protactinium-234 (Pa)). This decays with a half-life of 6.7 hours, again through emission of a beta particle, to U. This isotope has a half-life of about 245,000 years. The next decay product, thorium-230 (Th), has a half-life of about 75,000 years and is used in the uranium-thorium technique. Although analytically simpler, in practice U/U requires knowledge of the ratio at the time the material under study was formed and is generally used only for samples older than the ca. 450,000 year upper limit of the Th/U technique. For those materials (principally marine carbonates) for which these conditions apply, it remains a superior technique.
Astrological transit
Uranus takes about 84 years to complete a full return. However, one of its most important influences is felt when it reaches the halfway stage in its journey, at around age 42. The Uranus half-return is the origin of what for many people is the 'mid-life crisis'. Women, especially those with children growing up and leaving home, may feel that life has passed them by. A lot of women also begin the pre-menopause stage at this time, and many men experience the 'male menopause'. Permanent and successful long-term relationships may be unsettled or disrupted by casual flirtations and affairs. The Uranus half-return can be stressful and tense, but also energising. It is the ideal time for people to break out of routine lifestyles that they have been stuck in for years, and develop new interests.