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 <mml:mmultiscripts>Am 243 </mml:mmultiscripts>(<mml:mmultiscripts>Ca^{48 </mml:mmultiscripts>,x n ) 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 <mml:mmultiscripts>Nh 284 </mml:mmultiscripts> (Z =113 ), the α -decay daughter of <mml:mmultiscripts>Mc 288 </mml:mmultiscripts> (Z =115 ), while the second occurred at A /q =288 and was assigned to <mml:mmultiscripts>Mc 288 </mml:mmultiscripts> . This experiment represents the first direct measurements of the mass numbers of superheavy elements, confirming previous (indirect) mass-number assignments.