Mass measurements of neutron-rich nuclei near $N=70$

The astrophysical origin for the chemical elements between the first and second $r$-process peaks is a matter of intense debate, with a number of nucleosynthesis processes at explosive stellar environments possibly contributing to their production. Reliable data on the trends of neutron separation energies of neutron-rich isotopes are required to model neutron-capture processes that would produce these elements. Masses of ${}^{104}\mathrm{Y}$, ${}^{106}\mathrm{Zr}$, ${}^{112}\mathrm{Mo}$, and ${}^{115}\mathrm{Tc}$ have been measured with the time-of-flight-magnetic-rigidity $\left(\mathrm{ToF}\text{−}B\rho \right)$ technique at the National Superconducting Cyclotron Laboratory at Michigan State University. The experiment is the first application of the $\mathrm{ToF}\text{−}B\rho$ technique at the S800 spectrograph that reached the mass region relevant to heavy-element nucleosynthesis. The two-neutron separation energy deduced from the measured masses exhibits a smooth trend consistent with the theoretical predictions within the range of experimental uncertainty, indicating that there is no sudden shape transition in these isotopes as hinted at by previous data.

Figure 1

Schematic layout of ToF-$B\rho$ mass measurement setup at the NSCL.

Figure 2

Position spectrum of MCP detector using ${}^{124}\mathrm{Sn}$ beam and a mask, which gives a position resolution ($\sigma$) of $\approx 0.7\mathrm{mm}$.

Figure 3

$Z$ versus $A/Q$ particle identification plot in this work. The inset shows the zoomed region around the fully stripped ${}^{119}\mathrm{Rh}^{{45}^{+}}$, ${}^{116}\mathrm{Ru}^{{44}^{+}}$ and the hydrogen-like ${}^{119}\mathrm{Pd}^{{45}^{+}}$, ${}^{116}\mathrm{Rh}^{{44}^{+}}$ to demonstrate the high PID resolution.

Figure 4

(a) The contour plot of the log-likelihood of samples from the GMM algorithm. (b) 2-D histogram of GMM probabilities of samples belonging to each ion clustered by GMM. For each event we display the value for the cluster of highest probability. The horizontal red line highlights the probability of 0.5. The absence of events below this line indicates that there is only a bi-cluster mixture (i.e., negligible overlap between more than two clusters).

Figure 5

The local corrections to the ToF for ${}^{110}\mathrm{Mo}^{42+}$. The correlation plots of raw ToF versus $X_{\text{MCP}}$ (a) before and (b) after the $X_{\text{MCP}}$ correction. (c) The correlation plot of $X_{\text{MCP}}$-corrected ToF versus $X$ positions on one CRDC detector. (d) The comparison of distributions of the raw ToF, and the corrected ToFs by the $X_{\text{MCP}}$ and by the $X$ positions on two CRDCs in addition.

Figure 6

(a) The $m/q$ distributions of each ion with the new masses marked by the underlined bold-italic letters. (b) Mass excess difference between the present work and the AME2020 (values estimated from trends from the mass surface were used for unknown masses).

Figure 7

Trends of two-neutron separation energies of (a) yttrium, (b) zirconium, (c) molybdenum, and (d) technetium isotopes as calculated from the AME2020 (filled black squares for known-mass and open for extrapolated-mass isotopes), the masses reported here (filled red circles), the masses from previous experiments [55, 57] (orange stars), as well as by the mass models of $\mathrm{WS}4+\mathrm{RBF}$ (magenta diamond) [23], FRDM2012 (blue triangle) [22], and HFB-31 (cyan plus marker) [58].