Production of new neutron-rich isotopes near the $N=60$ isotones ${}^{92}\mathrm{Ge}$ and ${}^{93}\mathrm{As}$ by in-flight fission of a 345 MeV/nucleon ${}^{238}\mathrm{U}$ beam

A search for new neutron-rich isotopes near the $N=60$ isotones ${}^{92}\mathrm{Ge}$ and ${}^{93}\mathrm{As}$ has been performed using a 345 MeV/nucleon ${}^{238}\mathrm{U}$ beam at the RIKEN Nishina Center RI Beam Factory. Fission fragments produced were analyzed and identified in flight using the large-acceptance two-stage separator BigRIPS. We have observed a total of 15 new neutron-rich isotopes: ${}^{84}\mathrm{Cu}, {}^{86,87}\mathrm{Zn}, {}^{88,89}\mathrm{Ga}, {}^{91,92}\mathrm{Ge}, {}^{93,94,95}\mathrm{As}, {}^{96,97}\mathrm{Se}, {}^{99,100}\mathrm{Br}$, and ${}^{103}\mathrm{Kr}$.

Figure 1

Schematic layout of the BigRIPS separator. The labels STQn, Dn, and Fn indicate the positions of superconducting quadrupole triplets, room-temperature dipoles, and foci, respectively. The energy degraders and detectors are also indicated. See text for more details.

Figure 2

$Z$ versus $Z_{\mathrm{F}5}$ correlation plot for fission fragments produced in the ${}^{238}\mathrm{U}+\mathrm{Be}$ reaction at 345 MeV/nucleon. Here, $Z$ and $Z_{\mathrm{F}5}$ respectively represent the atomic numbers derived from the Δ$E$ measurement using the MUSIC detector and from the calculated energy loss in the F5 degrader. The red circles indicate the elliptical $Z$ gates defined as $|Z-Z_0|<3{\sigma }_Z$ and $|Z_{\mathrm{F}5}-Z_0|<3{\sigma }_{Z\mathrm{F}5}$, where $Z_0, {\sigma }_Z$, and ${\sigma }_{Z\mathrm{F}5}$ represent the peak centroid, the absolute $Z$ resolution, and the absolute $Z_{\mathrm{F}5}$ resolution, respectively. The resolutions are those in terms of standard deviation σ. The elliptical gates are actually circles with the same 3σ radius, because the resolutions are measured to be ${\sigma }_Z={\sigma }_{Z\mathrm{F}5}=0.23$ for $Z$ blobs strongly seen in the figure. See text for more details.

Figure 3

$Z$ versus $A/Q$ particle identification plots for fission fragments produced in the ${}^{238}\mathrm{U}+\mathrm{Be}$ reaction at 345 MeV/nucleon. The arrows in the top panel (a) indicate that the upper-right and right-hand sides of an isotope with mass number $A$ correspond to those with mass numbers $A+3$ and $A+1$, respectively. The bottom panel (b) shows the plot enlarged near the region of new isotopes, where the red solid lines indicate the limit of known isotopes.

Figure 4

$A/Q$ spectra of the Cu, Zn, Ga, Ge, As, Se, Br, and Kr isotopes. The peaks labeled with mass numbers correspond to the new isotopes identified in this work. The solid and dotted lines indicate the expected location of the new isotope and the $\pm 2{{\sigma }_A}_{/Q}$ range, respectively, where ${{\sigma }_A}_{/Q}$ represents the absolute $A/Q$ resolution in terms of standard deviation. Note that ${}^{101}\mathrm{Br}$ was already observed in our previous work [19]. See text for more details.

Figure 5

Measured production cross sections of neutron-rich isotopes from the ${}^{238}\mathrm{U}+\mathrm{Be}$ reaction at 345 MeV/nucleon shown as a function of mass number along with theoretical predictions. The errors shown are statistical only. The open and filled circles indicate those measured in this work for known and new isotopes, respectively. The solid and dashed lines show those predicted from the $\mathrm{LIS}{\mathrm{E}}^{++}$ abrasion fission (AF) calculations, using the three excitation-energy region (3EER) model and the initial fissile nuclei (IFN) analyzer model, respectively. The squares and triangles represent those from our previous measurements in Ref. [17] and Ref. [19], respectively. See text for more details.

Figure 6

Section of nuclear chart showing the new isotopes observed in this work. The black squares indicate stable isotopes. The known and new isotopes are labeled in yellow and light blue, respectively. The squares with a small dot indicate isotopes whose production cross sections were measured in this work. Note that ${}^{101}\mathrm{Br}$ has been previously observed [19].