First Direct Measurements of Superheavy-Element Mass Numbers

An experiment was performed at Lawrence Berkeley National Laboratory's 88-in. Cyclotron to determine the mass number of a superheavy element. The measurement resulted in the observation of two α-decay chains, produced via the ²⁴³Am(⁴⁸Ca,xn)²⁹¹⁻ˣMc reaction, that were separated by mass-to-charge ratio (A/q) and identified by the combined BGS+FIONA apparatus. One event occurred at A/q=284 and was assigned to ²⁸⁴Nh (Z=113), the α-decay daughter of ²⁸⁸Mc (Z=115), while the second occurred at A/q=288 and was assigned to ²⁸⁸Mc. This experiment represents the first direct measurements of the mass numbers of superheavy elements, confirming previous (indirect) mass-number assignments.