Making of the Atomic Clock and the Downsides of Cesium


By Ron B. Davis Jr.Georgetown University

In the fields of chemistry and physics, high-precision time measurements are often necessary. They allow us to measure extremely fast chemical reaction rates and even observe the effects of relativity. So scientists in need of such measurements need a timepiece that ticks much, much faster than once per second! Enter rubidium and cesium.

Cesium metal capsule
Cesium’s properties made it a great candidate to be used for building the first atomic clock. (Image: LuYago/Shutterstock)

Tick… Tock…

As early as the 1930s, physicists like Isidor Rabi had suggested that the fundamental movements, rearrangements, and magnetic characteristics of subatomic particles might make certain atoms like rubidium and cesium useful for measuring the passage of miniscule amounts of time.

Rubidium and cesium are special because their outermost electron is at a relatively great distance from the nucleus, isolated all alone in the valence shell. While this usually makes rubidium and cesium all too willing to give up that electron, if they can be confined in systems that prevent the electron from being given away, some of the properties of that lone electron can be observed.

Rubidium and cesium were identified using their atomic line spectra, allowing scientists to isolate them using updated techniques. And in an interesting twist, the two most reactive alkali metals available have made their greatest impact on science as neutral atoms of rubidium and cesium in atomic clocks.

Cesium in Atomic Clocks 

In 1955, British researchers Louis Essen and Jack Parry brought Rabi’s vision to reality, using cesium-133, its only stable isotope, to make the first atomic clock. In their clock, cesium’s unpaired electron acts like a small magnet, interacting with the nucleus in a special way.

This very special interaction is known as hyperfine splitting, and in this case, it causes the 6s energy level to split into two levels of ever-so-slightly different energy. That 6s electron can transition back and forth between them, releasing a photon in the process.

Louis Essen and Jack Parry next to their atomic clock
Jack Parry (left) and Louis Essen were responsible for building the first atomic clock with cesium-133. (Image: National Physical Laboratory/Public domain)

Cesium’s 6s electron’s hyperfine transition is special for two reasons. First, the frequency of that photon released during each transition is highly consistent. Second, that radiation has a frequency of 9,192,631,770 cycles in one second. 

If we think of each cycle of the radiation’s wave as the ‘tick’ of a clock, a cesium atomic clock ‘ticks’ about ten times a nanosecond! With a clock that ticks that rapidly and uniformly, lengths of time as short as a fraction of a nanosecond can be measured accurately.

This article comes directly from content in the video series Understanding the Periodic TableWatch it now, on Wondrium.

The Downsides of Cesium

Cesium has been put to very good use in measuring time, but cesium also has its dark side, due largely to one of its radioactive isotopes: cesium-137, which tends to form during man-made nuclear reactions. When these reactions release cesium-137 into the environment the way nuclear bomb tests and reactor accidents can, some very dangerous conditions result.

Although cesium-133 is a stable nucleus, cesium-137’s four extra neutrons push it out of the band of nuclear stability, making it radioactive. Cesium-137 tries to compensate for its excess of neutrons with the emission of a beta particle, decaying with a half-life of about 30 years. This is slow enough to ensure that once it is created and released into the environment, it will be around in gradually shrinking amounts, for centuries before it’s entirely gone.

Measuring radioactivity of a plant
Cesium is soluble in water, and if its radioactive isotope, cesium-137, is released into the environment, it can easily get into the tissue of plants and animals. (Image: Wellphoto/Shutterstock)

And to make matters worse, accompanying that beta decay is an unusually high energy gamma emission, caused as the nucleus of the new barium-137 atom rearranges itself into a more stable configuration. That means that radiation from cesium-137 is harmful to us, whether the exposure happens inside or outside of our bodies.

Because cesium is an alkali metal, it shares certain similarities with relatively harmless sodium and potassium. For example, cesium tends to be soluble in water, giving it a natural vehicle to distribute itself through the environment, and into the tissues of animals or plants. All of this adds up to a situation that requires careful monitoring of environments and agricultural products for generations after a nuclear accident that releases cesium-137.

Common Questions about Making of the Atomic Clock and the Downsides of Cesium

Q: Why do scientists need a timepiece that ticks much faster than once a second?

In chemistry and physics, scientists need the ability for high-precision time measurements. Chemical reactions can be extremely fast. Even studying relativity deems high-precision time measurements necessary; measurements which are now possible with atomic clocks.

Q: How was the first atomic clock built?

Louis Essen and Jack Parry happened to build the first atomic clock in 1955. They used cesium-133 which is its only stable isotope. A special interaction, known as hyperfine splitting, causes the energy level of 6s to split into two different energy levels, and the electron transitions back and forth while releasing a photon each time. Since the frequency of the photon releasing is consistent and there are 9,192,631,770 cycles in one second, it is used for time measurement.

Q: Is cesium dangerous?

The first atomic clock used cesium-133, which is a stable isotope. But cesium-137 is a radioactive isotope that is produced after a manmade nuclear reaction. If it’s released into the environment because of a nuclear bomb test gone wrong or a reactor accident, it can be very dangerous. It lasts centuries and its radiation is harmful to us whether inside or outside the body. And since cesium is soluble in water, it has a natural vehicle to enter the tissue of plants and animals.

Keep Reading
Breakdown of Elements and Nuclear Stability
Nuclear Radiation and Its Different Types
Nuclear Fission: The Nucleus Ability to Split into Two Different Nuclei