Projectile fragmentation of ${}^{86}\mathrm{Kr}$ at 64 MeV/nucleon

We measured fragmentation cross sections produced using the primary beam of ${}^{86}\mathrm{Kr}$ at 64 MeV/nucleon on ${}^9\mathrm{Be}$ and ${}^{181}\mathrm{Ta}$ targets. The cross sections were obtained by integrating the momentum distributions of isotopes with $25⩽Z⩽36$ measured using the RIPS fragment separator at RIKEN. The cross-section ratios obtained with the ${}^{181}\mathrm{Ta}$ and ${}^9\mathrm{Be}$ targets depend on the fragment masses, contrary to the simple geometrical models. We compared the extracted cross sections to EPAX; an empirical parametrization of fragmentation cross sections. Predictions from current EPAX parametrization severely overestimate the production cross sections of very neutron-rich isotopes. Attempts to obtain another set of EPAX parameters specific to the reaction studied here to extrapolate the neutron-rich nuclei more accurately have not been very successful, suggesting that accurate predictions of production cross sections of nuclei far from the valley of stability require information of nuclear properties that are not present in EPAX.

Figure 1

Layout of the experimental facility at RIKEN. The LINAC injector and the K540 cyclotron are shown along with the experimental areas E1–E6. The RIPS fragment separator is located in experimental areas D and E6 [6].

Figure 2

RIPS fragment separator consisting of two dipoles (D1 and D2) and 12 quadrupoles (Q1–Q12). The momentum acceptance was determined by the momentum slit placed at F1. The beam intensity monitor (MOMOTA) is shown in the top left oval below the target position. The particle identification setup was located at the F2 and F3 focal planes.

Figure 3

Primary beam charge state distributions for ${}^{86}\mathrm{Kr}+{}^9\mathrm{Be}$ (closed circles) and ${}^{86}\mathrm{Kr}+{}^{181}\mathrm{Ta}$ (closed squares) plotted as a function of number of unstripped electrons, $Z-Q$. Solid and dashed curves show calculation by GLOBAL code [8] as implemented in LISE++ [9] for ${}^9\mathrm{Be}$ and ${}^{181}\mathrm{Ta}$ targets, respectively.

Figure 4

Particle identification spectrum for the ${}^{86}\mathrm{Kr}+{}^9\mathrm{Be}$ reaction measured at a 2.07 Tm magnetic rigidity setting. Left panel shows the PID with three gates around elements with $Z=28$, 31, 34. Right panel shows the corresponding projections to charge state, $Q$, versus $A/Q$ ratio plane for the three elements from bottom to top, respectively.

Figure 5

Dependence of the transmission correction factor, ɛ, on fragment mass number, $A$, for the ${}^{86}\mathrm{Kr}+{}^9\mathrm{Be}$ reactions.

Figure 6

Momentum distributions for ${}^{64}{\mathrm{Zn}}^{30+}$ produced in fragmentation of ${}^{86}\mathrm{Kr}$ on the ${}^9\mathrm{Be}$ target. The solid curve represents a fit with Eq. (3) and the dotted curve is a Gaussian fit, to the right side of the momentum distribution, to show the asymmetry of the experimental distribution.

Figure 7

Measured cross sections for 180 fragments produced in the ${}^{86}\mathrm{Kr}+{}^9\mathrm{Be}$ reactions.

Figure 8

Measured cross sections presented as isotope distributions for $25⩽Z⩽36$ elements detected in the ${}^{86}\mathrm{Kr}+{}^{181}\mathrm{Ta}$ reactions (filled circles) and in the ${}^{86}\mathrm{Kr}+{}^9\mathrm{Be}$ reactions (open squares) at 64 MeV/nucleon. EPAX calculations are shown as dashed (${}^{86}\mathrm{Kr}+{}^9\mathrm{Be}$) and solid (${}^{86}\mathrm{Kr}+{}^{181}\mathrm{Ta}$) curves.

Figure 9

Measured cross sections presented as isotope distributions for $25⩽Z⩽36$ elements detected in the ${}^{86}\mathrm{Kr}+{}^9\mathrm{Be}$ reactions at 64 MeV/nucleon. Experimental fragmentation data are shown as filled squares. EPAX predictions are shown as solid curves. For comparison, open triangles show the published data of ${}^{86}\mathrm{Kr}+{}^9\mathrm{Be}$ at 500 MeV/nucleon [20].

Figure 10

Ratios of the fragmentation cross sections on Ta and Be targets, ${\sigma }_{\mathrm{Ta}}(A,Z)/{\sigma }_{\mathrm{Be}}(A,Z)$, for fragments with $25⩽Z⩽36$ for the ${}^{86}\mathrm{Kr}$ beam. Only ratios with relative errors smaller than 25% are shown. Open and solid symbols represent odd and even elements starting with $Z=25$. The horizontal dashed and dotted lines indicate the ratio calculated by the EPAX formula and Eq. (4), respectively.

Figure 11

Ratio of the experimental cross sections and predicted cross sections from EPAX (top panel) and modified ${\mathrm{EPAX}}_{\mathrm{Kr}}$ formula (bottom panel). For clarity, isotopes from each element are joined by the dashed lines. Same symbol conventions used in Fig. 10 are adopted here; open and solid symbols represent odd and even elements from $Z=25$ (open circles) to $Z=36$ (closed triangles). The bold dashed curve joining the $N=50$ proton-removed isotopes (${}^{84}\mathrm{Se}$, ${}^{83}\mathrm{As}$, ${}^{82}\mathrm{Ge}$, ${}^{81}\mathrm{Ga}$, ${}^{80}\mathrm{Zn}$, and ${}^{79}\mathrm{Cu}$) is obtained from a fit. The curve allows extrapolation of the production estimates of very neutron-rich nucleus such as ${}^{78}\mathrm{Ni}$.

Figure 12

Ratios of ${}^{86}\mathrm{Kr}+{}^9\mathrm{Be}$ fragment experimental cross sections to EPAX [4] (open symbols) and to our modified ${\mathrm{EPAX}}_{\mathrm{Kr}}$ (solid symbols) predictions plotted as a function of nuclear charge, $Z$, for $42⩽N⩽50$ isotones.