JOURNAL OF MAGNETIC SHIELD CASE STUDIES

Atomic Clocks - Aren't They the Ones that Glow in the Dark?
Case #11349

The US Naval Observatory maintains the Master Clock for the United States. The observatory decided to build a clock with improved precision, undertaking a "Cesium Fountain Atomic Clock" based on laser-cooled neutral cesium atoms.

Atomic clocks are the most precise timekeeping devices on earth. Their uses include measuring the rotation of the earth, which may vary by 4 to 5 milliseconds per day, and aiding navigational systems such as the global positioning system in computing distances. Atomic clocks are tuned to the frequency of the electromagnetic waves that are emitted or absorbed when certain atoms or molecules make the transition between two closely spaced, or hyperfine, energy states. Because the frequency of these waves is unaffected by external forces, the corresponding period of the waves can be used as a standard to define time intervals. In this type of clock, cesium-133 atoms in one hyperfine energy state are subjected to microwave radiation that is near the resonant frequency of the transition to another hyperfine energy state. The microwave frequency is adjusted, and when the correct frequency is reached, many atoms make the transition to the new energy state.The frequency of the microwave radiation is then used to determine the period of the microwave, or the time interval between wave crests.The second is defined as the duration of 9,192,631,770 periods of radiation. The most stable cesium-atom clocks have an error of about plus or minus one second in one million years.

For an atomic clock to achieve the utmost accuracy, external disturbances, including magnetic fields, must be carefully controlled. To accomplish the critical magnetic shielding - function, the Naval Observatory contacted Magnetic Shield Corporation.

After reviewing the The US Naval Observatory's exacting specifications for their required shield, we outlined our plan for implementation and the necessary budget. When we were awarded the order, we immediately went to work, commencing fabrication using alloy sheet from our substantial inventory. A three-layer shield set was delivered and installed to protect the clock.