Speaker
Description
In ultraperipheral collisions (UPC) of relativistic nuclei without overlap of their nuclear density distributions the collision partners are mutually impacted by Lorentz-contracted Coulomb fields [1]. This leads to a variety of processes induced by equivalent Weizsäcker-Williams photons. In particular, large neutron emission cross sections ($\sim 200$ b) were measured in the ALICE experiment at the LHC for the electromagnetic dissociation (EMD) of $^{208}$Pb nuclei at $\sqrt{s_{\rm NN}}=5.02$ TeV [2], leading to the production of lead isotopes as secondary nuclei. As follows from calculations [3], Tl, Hg, Au, Pt, Ir, Os, Re, W, Ta and Hf are also produced in EMD of $^{208}$Pb at the LHC with the cross sections of 1-30 b exceeding or comparable to the total $^{208}$Pb-$^{208}$Pb hadronic cross section of 7.67 b. The cross sections of neutron and proton emission in the EMD are expected to be smaller for nucleus-nucleus collisions at NICA. However, it is important to calculate these cross sections in order to evaluate a possible contamination of the detected hadronic events by EMD events, which can affect the detector performance.
By means of RELDIS model [1], we have simulated the EMD of $^{209}$Bi and $^{124}$Xe in $^{209}$Bi-$^{209}$Bi and $^{124}$Xe-$^{184}$W collisions in the MPD experiment at NICA in collider and fixed target modes, respectively. The cross sections of the production of given numbers of forward neutrons and protons, their energy and angular distributions have been calculated. Mutual EMD events in $^{209}$Bi-$^{209}$Bi collisions were also modeled, since such events can in principle mimic very peripheral hadronic events. EMD of $^{124}$Xe was also modeled in UPC with CsI target in the BM@N experiment. In such collisions, the emission of a single neutron from electromagnetically excited $^{124}$Xe dominates, providing a unique well-collimated monoenergetic neutron beam that can be used for detector calibration or applied research, e.g. to study unexplored radiobiological effects of high energy neutrons.
References
[1] I.A. Pshenichnov, Electromagnetic excitation and fragmentation of ultrarelativistic nuclei, Phys. Part. Nucl. 42 (2011) 215.
[2] S. Acharya et al. (ALICE Collaboration), Neutron emission in ultraperipheral Pb-Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV, Phys. Rev. C. 107 (2023) 064902.
[3] U.A. Dmitrieva, I.A. Pshenichnov, Production of various elements in ultraperipheral $^{208}$Pb-$^{208}$Pb collisions at the LHC, Phys. Part. Nucl. Lett. 20 (2023) 1228.
| Section | Heavy ion collisions at Intermediate and high energies |
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