How good are superheavy elementZandAassignments?

Abstract

Proton number, Z, and mass number,A, assignments for newly discovered heavy element nuclides have historically been made by observing decay to a daughter with well-established Z and A, and then observing the well-know decay of that daughter. For all of the new superheavy element isotopes observed in 48 Ca irradiations of actinide targets, this correlation technique has not been possible, because the -decay chains end in spontaneous fission of previously unknown isotopes. Consequently, Z and A assignments have been made by less-direct means. The superheavy element Z and A assignment methods are summarized, and possibilities for how they may be incorrect are explored. While it is highly likely that most of the superheavy element Z and A assignments are correct, there is a real need for a direct proof. How SHE Z and A assignments have been madeFor the SHE isotopes produced in 48 Ca irradiation of actinide targets, and all of the daughter isotopes resulting from successive -decays, experimenters have resorted to lessdirect means of Z and A assignment. These less-direct means are excitation functions, cross bombardments, and decay systematics. a

Colloquium : Superheavy elements: Oganesson and beyond

“…Another short-term goal is, therefore, to connect these two superheavy regions to provide a direct mass/charge identification. This can be achieved through, e.g., X-ray spectroscopy (Ackermann, 2015;Rudolph et al, 2013) and direct mass measurements (Gates, 2016;Gregorich, 2016). Recently, the first direct measurements of the mass numbers of 288 Mc and 284 Nh were carried out at Berkeley (Gates et al, 2018), confirming the previous (indirect) mass-number assignments.…”

First Direct Measurements of Superheavy-Element Mass Numbers

“…confirming and extending the JINR SHE claims. In previous discoveries up to Nh [proton number (Z) 113], mass number (A) and Z assignments were made by observing the α-decay chain of a new element isotope until it decayed into an isotope with well-established Z, A, and decay properties [13].…”

“…For these SHE isotopes, presumed Z and A assignments have been made based on (i) decay systematics, (ii) production rate as a function of bombarding energy (excitation functions), (iii) producing the same isotopes using different target materials [cross bombardments, e.g. 287 114 produced in the 244 Pu( 48 Ca, 5n) and 242 Pu( 48 Ca, 3n) reactions] and (iv) the assumption that deexcitation of the excited compound system through charged particle emission does not occur [2,3,13]. However, these four techniques are indirect and, ultimately, depend on the accuracy of nuclear mass models [14,15].…”

On the search for elements beyondZ=118. An outlook based on lessons from the heaviest known elements

“…While the current assignment appears correct based on indirect arguments, K.E. Gregorich argues in his contribution to these proceedings that the assignment appears "good, but not good enough" [12]. Still, IUPAC approved these data as originating from element 115 [1], and especially highlighted the cross-bombardment aspect in its assessment, which formed the basis for their approval.…”