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 ${}^{243}\mathrm{Am}{({}^{48}\mathrm{Ca},xn)}^{291-x}\mathrm{Mc}$ reaction, that were separated by mass-to-charge ratio ($A/q$) and identified by the combined $\mathrm{BGS}+\mathrm{FIONA}$ apparatus. One event occurred at $A/q=284$ and was assigned to ${}^{284}\mathrm{Nh}$ ($Z=113$), the $\alpha$-decay daughter of ${}^{288}\mathrm{Mc}$ ($Z=115$), while the second occurred at $A/q=288$ and was assigned to ${}^{288}\mathrm{Mc}$. This experiment represents the first direct measurements of the mass numbers of superheavy elements, confirming previous (indirect) mass-number assignments.

Figure 1

Schematic of $\mathrm{BGS}+\mathrm{FIONA}$ with the labels indicating the positions of 1. BGS, 2. BGS focal plane detector, 3. gas catcher, 4. RFQ, 5. RFQ trap, 6. acceleration region, 7. focusing element, horizontal and vertical steerers, 8. shielding wall, 9. focusing element, horizontal and vertical steerers, 10. C2 detector, 11. trochoid spectrometer, and 12. FIONA focal-plane detector. The purple shaded area or line represents the beam image of ${}^{288}\mathrm{Mc}$ traveling through the BGS and FIONA.

Figure 2

(top) Experimental data from the ${\mathrm{At}}^{1+}$ calibration run showing the $\alpha$-particle energies versus the positions they were observed in the focal plane detector; (middle) experimental data from the ${\mathrm{At}}^{1+}$ calibration runs taken before and after each event was observed and showing the $A/q$ separation; (bottom) expected location of $A/q=283–292$ mass peaks at the FIONA focal-plane detector. The locations of the two observed events are shown by the squares.

Figure 3

Deviation of the observed focal plane position from the expected position (after scaling) as a function of $A/q$. The open circles correspond to the results for known isotopes, ${{}^{254}\mathrm{N}\mathrm{o}}^{2+}$, ${{}^{255}\mathrm{Lr}}^{2+}$, ${{}^{151}\mathrm{Ho}}^{1+}$, ${{}^{200}\mathrm{At}}^{1+}$, ${{}^{208}\mathrm{Fr}}^{1+}$, ${{}^{216}\mathrm{Po}}^{1+}$, ${{}^{245}\mathrm{Fm}}^{1+}$, ${{}^{254}\mathrm{No}}^{1+}$, and ${{}^{255}\mathrm{Lr}}^{1+}$ and the error bars are smaller than the symbols. The red squares correspond to the two events of ${}^{284}\mathrm{Nh}$ and ${}^{288}\mathrm{Mc}$ discussed in the text. The lines represent the expected centroids of masses with $(A\pm 1)/q$ from the scaled mass. From the commissioning measurements, the observed position agrees within $\pm 0.5\mathrm{mm}$ of the predicted (scaled) position, giving a position uncertainty that is small compared to the separation between adjacent mass numbers.

Figure 4

(left) Average of known decay properties assigned to ${}^{288}\mathrm{Mc}$ and its daughters [5, 11, 29]; (right) details of decay chains detected at the FIONA focal plane. Unobserved decays within each decay chain are indicated as “unobserved” and are assumed to have been emitted out of the open end of the detector array. The $x$ position of decays observed in the focal-plane detector is also given.