The residual motion of the atoms forming the cloud corresponds to temperatures near absolute zero.Īfter the cooling process, the two vertical lasers are used to gently toss the cloud upward (the "fountain" action), and then all the lasers are turned off. The lasers slow down the atoms, and a cold cloud of cesium atoms forms in the intersection of the six laser beams. Six infrared laser beams then are directed at right angles to each other at the center of the chamber. First, a gas of cesium atoms is introduced into the clock's vacuum chamber. NIST-F3 and NIST-F4 are referred to as fountain clocks because they use a fountain-like movement of atoms to calibrate the offset of a microwave frequency from the unperturbed cesium clock transition frequency used to define the SI second. The links to previous Cs fountains can be found in NIST-F1 and NIST-F2. Work is ongoing to evaluate the stability of the apparatus and to characterize the residual frequency biases in NIST-F3. This long-term stability is about a factor of 100 better than can be achieved with a hydrogen maser, and it illustrates the advantages of a cold-atom frequency reference. In the first measurement campaign, NIST-F3’s frequency offset exhibited drift below 10 -17/day over five months. The frequency offset of NIST-F3 has been characterized to within a few parts in 10 -15 in fractional frequency units. Initial evaluations of NIST-F3’s frequency offset and stability were completed recently. It will provide a stable frequency reference that can be used in the NIST time scale and assist with the evaluation of NIST’s primary and secondary frequency standards. Instead, NIST-F3 is intended to be a stable system that operates with high up-time. Unlike the other NIST fountains, NIST-F3 is not intended to realize the definition of the second of the International System of Units (SI) with state-of-the-art accuracy. NIST-F3 is a cesium fountain frequency reference. It is expected to reach an accuracy approaching the 10 -16 level in fractional frequency. Once the evaluation is completed, NIST-F4 will contribute to UTC and calibrate the absolute frequency of next-generation optical atomic clocks. This fountain is an upgrade of the NIST-F1 apparatus. Cesium chloride (CsCl) and cesium nitrate (CsNO 3) are cesium's most common compounds and are primarily used in the production of other chemicals.NIST-F4 is a primary frequency standard that is currently under evaluation. Cesium hydroxide is the strongest base known and will attack glass. Since it is easily ionized and has a high mass, cesium ions may one day be used as a propellant in ion engines on spacecraft.Ĭesium reacts violently with water and ice, forming cesium hydroxide (CsOH). Cesium is also used in atomic clocks, in photoelectric cells and as a catalyst in the hydrogenation of certain organic compounds. Cesium readily combines with oxygen and is used as a getter, a material that combines with and removes trace gases from vacuum tubes. Cesium is recovered from cesium azide by heating it.Ĭesium has the second lowest melting point of all metallic elements, which limits its uses. Metallic cesium is too reactive to easily handle and is usually sold in the form of cesium azide (CsN 3). To obtain pure cesium, cesium and rubidium ores are crushed and heated with sodium metal to 650☌, forming an alloy that can then be separated with a process known as fractional distillation. Obtaining pure cesium is difficult since cesium ores are frequently contaminated with rubidium, an element that is chemically similar to cesium. Today, cesium is primarily obtained from the mineral pollucite (CsAlSi 2O 6). They named cesium after the blue lines they observed in its spectrum. Cesium was discovered by Robert Wilhelm Bunsen and Gustav Robert Kirchhoff, German chemists, in 1860 through the spectroscopic analysis of Durkheim mineral water.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |