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 alpha-decay chains, produced via the Am-243(Ca-48,xn)(291-x)Mc 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 (284)Nh (Z = 113), the alpha-decay daughter of (288)Mc (Z = 115), while the second occurred at A/q = 288 and was assigned to (288)Mc. This experiment represents the first direct measurements of the mass numbers of superheavy elements, confirming previous (indirect) mass-number assignments.