Researchers from Lawrence Berkeley National Laboratory developed a new measurement technique that directly determines the masses of two super-heavy isotopes
The method for synthesizing super-heavy elements includes smashing together their nuclei. However, identifying the mass of the resulting short-lived isotopes is a challenging task and often depends on indirect methods. Now, a team of researchers led by Jacklyn Gates from Lawrence Berkeley National Laboratory, California directly measured the masses of two super-heavy isotopes. These two firm data points offer confirmation that previous mass measurements of neighboring isotopes in the periodic table were correct. The research was published in the journal Physical Review Letters on November 28, 2018.
The mass of a single nucleus is normally determined by tracking its decay to some well-known daughter nucleus. The approach sometimes requires several steps. For instance, a nucleus decays by emitting four alpha particles, with each alpha particles containing two protons and two neutrons. These alpha particles later reach at the identifiable daughter nucleus nobelium-255 (atomic number Z=102). A reverse approach can help researchers to determine that the original nucleus was darmstadtium-271 (Z=110). However, for super-heavy elements with atomic numbers above Z=113, the daughter nuclei are largely unknown. This led the researchers to adopt other, less certain, techniques for identifying the decay channels of the isotopes in this mass range.
In the current research, the team irradiated an americium-243 (Z=95) target with a beam of calcium-48 (Z=20) ions to generate super-heavy nuclei with a variety of masses. These nuclei later travel through a series of devices and are spatially separated on the basis of their mass-to-charge ratio. The filtering in followed by a silicon detector that records alpha emissions. According to the researchers, any detected alpha locates the position and the mass of an embedded nucleus. The team used this method to detect isotopes of moscovium (Z=115) and nihonium (Z=113) and determined their masses to be 288 and 284, respectively. Moreover, the team measured decay times and energies to demonstrate that their results are consistent with previous super-heavy mass studies.