A Thorium Nuclear Clock

[John Clark]

Although not yet published the journal Physical Review Letters has accepted a new paper on nuclear thorium clocks which sounds very exciting, you can read the abstract here:

Laser excitation of the Th-229 nucleus

This opens up the possibility of making a clock accurate to one second in 300 billion years. Because of time dilation, with such a clock you could detect extremely tiny variations in gravity helping geologists and archaeologists see what's going on underground. If GPS satellites had a clock like this you could find your position within a tiny fraction of an inch, allowing automatic systems to land on aircraft carriers even in high seas. It would provide a new way to detect gravitational waves and  a new way to search for dark matter. You could even use it to see if some of the fundamental constants of nature are really constant or are changing with time. Best of all a thorium nuclear clock, unlike present day atomic clocks, could be entirely solid state and would have no need for elaborate vacuum chambers or cryogenic cooling, so they could be small, cheap and portable.

The researchers found they could use a tabletop ultraviolet laser to excite the nucleus of a thorium-229 atom into a higher energy state called a "nuclear isomer". When the thorium is in that excited state there is a 50% chance it will drop down to a lower energy state by emitting a photon of energy in 630 seconds. Lots of elements have isomers but most of them have transitions that are in the hard X-ray or gamma ray region, however the thorium-229 transition is in the ultraviolet region that is much lower and much more accessible. To be precise it emits a photon with 8.35574 electron volts of energy and a wavelength of 148.3821 nanometers with an error of plus or minus 5 in the last digit. By way of comparison, that is ultraviolet but the wavelength is about 10 times longer (and thus far easier to produce) than the ultraviolet light ASML uses to make the most advanced generation of chips.  

    John K Clark    See what's on my new list at  Extropolis

tc!