Light and heavy transfer products in ${}^{136}\mathrm{Xe}+{}^{238}\mathrm{U}$ multinucleon transfer reactions

Background: Multinucleon transfer reactions (MNT) are a competitive tool to populate exotic neutron-rich nuclei in a wide region of nuclei, where other production methods have severe limitations or cannot be used at all.

Purpose: Experimental information on the yields of MNT reactions in comparison with theoretical calculations are necessary to make predictions for the production of neutron-rich heavy nuclei. It is crucial to determine the fraction of MNT reaction products which are surviving neutron emission or fission at the high excitation energy after the nucleon exchange.

Method: Multinucleon transfer reactions in ${}^{136}\text{Xe}+{}^{238}\text{U}$ have been measured in a high-resolution $\gamma$-ray/particle coincidence experiment. The large solid-angle magnetic spectrometer PRISMA coupled to the high-resolution Advanced Gamma Tracking Array (AGATA) has been employed. Beamlike reaction products after multinucleon transfer in the Xe region were identified and selected with the PRISMA spectrometer. Coincident particles were tagged by multichannel plate detectors placed at the grazing angle of the targetlike recoils inside the scattering chamber.

Results: Mass yields have been extracted and compared with calculations based on the grazing model for MNT reactions. Kinematic coincidences between the binary reaction products, i.e., beamlike and targetlike nuclei, were exploited to obtain population yields for nuclei in the actinide region and compared to x-ray yields measured by AGATA.

Conclusions: No sizable yield of actinide nuclei beyond $Z=93$ is found to perform nuclear structure investigations. In-beam $\gamma$-ray spectroscopy is feasible for few-neutron transfer channels in U and the $-2\mathrm{p}$ channel populating Th isotopes.

Figure 1

The experimental setup comprised the $\gamma$-ray spectrometer AGATA, the ejectile-detecting heavy-ion mass spectrometer PRISMA, and the particle detector DANTE (not to scale). The DANTE MCP on a ${58}^{\circ }$ ring [27] in the scattering chamber covers the grazing angles of the binary reaction products.

Figure 2

(a) IC energy loss matrix of the energy deposited in the first two layers versus the total deposited energy. (b) Yield distribution of nuclear charges from Ga $\left(Z=31\right)$ to La $\left(Z=57\right)$. The peak around $Z=42$ results from asymmetric actinide fission residues. Corresponding fission partners in the Xe region overlap with the multinucleon transfer products in the vicinity of ${}_{54}^{136}\text{Xe}$.

Figure 3

Results of the software-based aberration correction of nonlinear distortions of $A/q$ as a function of the position information of PRISMA start and stop detectors.

Figure 4

Mass spectra for beamlike particles in the range from Te to Ba identified with PRISMA. The resolution in the Ba channel accounts to $A/\mathrm{\Delta }A=298\pm 1$. Along the Xe channel, neutron pickup is not favored, whereas the $\pm x\mathrm{p}$ channels are more evenly distributed. Dashed lines mark mass $A=136$ to guide the eye.

Figure 5

(a) Time-of-flight $\mathrm{\Delta }\mathrm{ToF}$ difference spectrum between PRISMA and DANTE plotted against the nuclear charge $Z$. In the region near Xe the left maximum of the distribution is caused by multinucleon transfer products; these are marked by MNT. The left part of the distribution is caused by fission products and is clearly separated from transfer products. The fission fragments cover a broad $Z$ range and show for lower $Z$ values below $Z=50$ one distinct $\mathrm{\Delta }\mathrm{ToF}$ peak. Selected $\mathrm{\Delta }\mathrm{ToF}$ projections for (b) Ba, (c) Xe, (d) Te, and (e) Sn are shown at the right side.

Figure 6

Recoil Doppler-corrected spectrum of ${}^{238}\text{U}$ with transfer and fissionlike $\mathrm{\Delta }\mathrm{ToF}$ gates. A gate on the right fissionlike $\mathrm{\Delta }\mathrm{ToF}$ peak yields a flat $\gamma$-ray background spectrum (red). The spectrum gated by the left part of the $\mathrm{\Delta }\mathrm{ToF}$ distribution shows distinct known peaks from the ${}^{238}\text{U}$ rotational band up to spin $22\hslash$.

Figure 7

Area of the left (transferlike) and right (fissionlike) peak in the $\mathrm{\Delta }\mathrm{ToF}$ spectrum between DANTE and PRISMA divided by the absolute number of the different identified Xe isotopes. A dashed line marks mass $A=136$ to guide the eye.

Figure 8

Matrix of the time difference between PRISMA and DANTE plotted against the total kinetic energy loss for Xe events. Transfer, fission, and elastically scattered ${}^{136}\text{Xe}$ particles with $\mathrm{TKEL}\approx 0$ are distinguishable. The multinucleon transfer displays a tail towards large TKEL. For the fission channel, the computed TKEL is only qualitative since it is constructed assuming a binary reaction.

Figure 9

Cross sections of a grazing calculation (points) and experimental yields (histogram bars) normalized to the calculated cross section of the $+1\mathrm{n}$ channel ${}^{137}\text{Xe}$. The mass yields are corrected for Xe contamination in the I and Cs channels. Fission events are excluded by a cut in the time-difference spectra between PRISMA and DANTE. Gray-shaded bars indicate the efficiency correction by applying the PRISMA response function to the experimental data.

Figure 10

Tracked singles $\gamma$-ray spectra with a gate on the transferlike $\mathrm{\Delta }\mathrm{ToF}$ peak, Doppler-corrected for different actinide recoils ranging from Ac $\left(Z=89\right)$ to Pu $\left(Z=94\right)$ showing the x-ray energy region. ${}_{92}\text{U}$ KL x rays are indicated with dashed lines. The Pa (corresponding to $Z=55$ Cs ejectiles) and Np (corresponding to $Z=53$ I) channels are corrected for U contamination. The Pu channel corresponding to ${}_{52}\text{Te}$ ejectiles is only weakly populated.

Figure 11

Mass-integrated yields of actinide nuclei $\mathcal{Y}$ from to their x-ray yields in the recoil Doppler corrected $\gamma$-ray spectra (black points) and from the PRISMA-DANTE $\mathrm{\Delta }\mathrm{ToF}$ yield scaled to the x-ray distribution (red triangles) together with theoretical cross sections calculated by grazing (blue rhomboids). All of the distributions are normalized to the Pu channel.

Figure 12

$\gamma$-ray intensities of neutron-induced background radiation with gates on the transfer- and fissionlike $\mathrm{\Delta }\mathrm{ToF}$ peak. In the channels in which neutrons were stripped from the ejectile nucleus, more neutrons react with the surrounding HPGe detectors, hindering the production of actinide binary partners.

Figure 13

Tracked singles $\gamma$-ray spectra of identified ${}^{134}\text{Xe}$ fragments in PRISMA with a gate on the transferlike $\mathrm{\Delta }\mathrm{ToF}$ peak, Doppler-corrected for the actinide binary partner ${}^{240}\text{U}$. (Top) Neutron evaporation to ${}^{239}\text{U}$ and ${}^{238}\text{U}$ dominate the spectrum. (Bottom) A cut on $\mathrm{TKEL}>-65$ MeV yields a neutron-evaporation suppressed spectrum revealing the rotational band of ${}^{240}\text{U}$.

Figure 14

Tracked singles $\gamma$-ray spectra of identified ${}^{138}\text{Xe}$ fragments, Doppler corrected for the binary actinide partner ${}^{236}\text{U}$, no TKEL cut is applied. All observed transitions can be attributed to ${}^{236}\text{U}$, and neutron evaporation channels are suppressed.