An atomic fountain is a cloud of atoms that is tossed upwards in the Earth's gravitational field by lasers. If it were visible, it would resemble the water in a fountain. While weightless in the toss, the atoms are measured to set the frequency of an atomic clock.https://www.nist.gov/public_affairs/releases/n99-22.cfm How the NIST-F1 Caesium Fountain Clock Works The primary motivation behind the development of the atomic fountain derives from the Ramsey method of measuring the frequency of atomic transitions. In broad strokes, the Ramsey method involves exposing a cloud of atoms to a brief radiofrequency (rf) electromagnetic field; waiting a time ''T''; briefly exposing the cloud to the rf field again; and then measuring what fraction of the atoms in the cloud have transitioned. If the frequency of the rf field is identical to the atomic transition frequency, 100% of the atoms will have transitioned; if the frequency of the field differs slightly from the transition frequency, some of the atoms will not have transitioned. By repeatedly sending clouds of atoms through such an apparatus, the frequency of the field can be adjusted to match the atomic transition frequency. The precision of the Ramsey method can be increased by increasing the wait time ''T'' of the cloud. The use of an atomic fountain with a cooled atomic cloud allows for wait times on the order of one second, which is vastly greater than what can be achieved by performing the Ramsey method on a hot

atomic beam Atomic beam is special case of particle beam; it is the collimated flux (beam) of neutral atoms. The imaging systems using the slow atomic beams can use the Fresnel zone plate A zone plate is a device used to Focus (optics), focus light or other ...

. This is one reason why NIST-F1, a caesium fountain clock, can keep time more precisely than NIST-7, a caesium beam clock.

History

The idea of the atomic fountain was first proposed in the 1950s by

Jerrold Zacharias Jerrold Reinach Zacharias (January 23, 1905 – July 16, 1986) was an American physicist and Institute Professor at the Massachusetts Institute of Technology, as well as an education reformer. His scientific work was in the area of nuclear physi ...

. Zacharias attempted to implement an atomic fountain using a thermal beam of atoms, under the assumption that the atoms at the low-velocity end of the

Maxwell–Boltzmann distribution In physics (in particular in statistical mechanics), the Maxwell–Boltzmann distribution, or Maxwell(ian) distribution, is a particular probability distribution named after James Clerk Maxwell and Ludwig Boltzmann. It was first defined and used ...

would be of sufficiently low energy to execute a reasonably sized parabolic trajectory. However, the attempt was not successful because fast atoms in a thermal beam struck the low-velocity atoms and scattered them.

References

Atoms Atomic physics Atomic clocks Nuclear technology {{AMO-physics-stub