Nuclear scientists continue to expand the periodic table as they detect new elements. These novel elements, such as the recently discovered 118, pose a challenge to researchers because they are so fleeting.
When a heavy element disintegrates, or decays, it gives off a radiation signature that can be used to prove that it existed. The three main types of radiation emitted are alpha particles (which are essentially helium nuclei), beta particles (including electrons and the electron anti-particles called positrons) and gamma rays (which are high-energy photons). Radiation given off by an element's decay is unique and some of the decays produce alpha particles of distinct energy. As a rule of thumb, the heavier the atom and the higher the atomic number--the number of protons in the nucleus--the more energetic the alpha particle given off by its decay will be. For example, the isotope radium 226 (one of the lightest alpha emitters, with atomic number 88) emits an alpha particle of 4.78 million electron volts (MeV), whereas element-110 emits an alpha particle with about 11 MeV.
Scientists create heavy elements by bombarding two lighter elements that together add up to the mass of the desired new element. One of the elements is stationary and thus called the target. The other, called the projectile, is accelerated in a cyclotron or other type of particle accelerator and effectively shot into the first element. A tiny fraction of the time the two elements stick together and form the new element, which then quickly decays. Sometimes it takes millions of collisions and several weeks of bombardment to create one atom of the new element. Specially positioned particle detectors interfaced to elaborate computer systems record the decays for later analysis.
In addition to using the unique energies of the alpha particles emitted to identify new elements, heavy-element hunters also use a cascade of alpha emissions to confirm their existence. Each alpha decay lowers the atomic number by two. For example, evidence for element-114 consisted of a chain of alpha particles that were all detected at the same location in the detector within a given time range. They signaled the decay of element-114 289 (the isotope with 114 protons and 175 neutrons, giving it a mass number of 289) first to element-112 (mass number 285), then element-110 (mass number 281) and finally element-108 with a mass number of 277. Putting all of this information together is tricky business, but it can serve as convincing evidence that an element that no longer exists was, in fact, created.