Measurement of cross sections for charge pickup reaction by ⁵⁶Fe on Al, C and CH₂ targets at 500 MeV/n

Abstract

The nuclear charge pickup cross sections of ⁵⁶Fe on polyethylene (CH₂), carbon and aluminum targets at an energy of 497 MeV/n were investigated using CR-39 nuclear track detector. The dependence of charge pickup cross section on target mass was studied.

Introduction

Nuclear charge pickup reactions, being presumably very peripheral interactions by their nature, have drawn much attention in heavy-ion induced interactions at various energies. With a magnetic spectrometer at the Lawrence Berkeley Laboratory Bevalac energy, Olson et al. [1] measured very small cross sections ($<0.1\text{mb}$) for 1.05 and 2.10 GeV/n ¹²C going to ¹²N, i.e. (¹²C, ¹²N) reaction, for H, Be, C, Al and Ag targets, and Olson et al. [2] measured larger (0.5 to 1.0 mb) cross sections for 1.7 GeV/n ¹⁸O going to ¹⁸F, i.e. (¹⁸O, ¹⁸F) reaction, for Be and Th targets. Using a magnetic spectrometer and a time-of-flight (TOF) system at Saturne, Bachelier et al. [3] measured the energy transfer in the charge transfer reaction of 0.95 GeV/n ²⁰Ne going to ²⁰Na, i.e. (²⁰Ne, ²⁰Na) reaction, for C, Y, and Pb targets. Two peaks, one at a low excitation energy due to $np\to pn$ and one at $\sim 300\text{MeV}$ due to excitation of a target nucleon to a delta resonance, were detected. Roy-Stephan [4], [5] reported the charge pickup cross sections for 900 MeV/n ²⁰Ne in H, C, and Pb targets and for 900 and 1100 MeV/n ¹²C in H, C, and Pb targets. Gerbier et al. [6] measured the charge pickup cross sections for 0.6 to 0.8 GeV/n ¹⁹⁷Au going to any isotope of Hg, i.e. (¹⁹⁷Au, Hg) reaction, a large cross section, $\sim 35\text{mb}$, and accompanied by a surprisingly large momentum downshift was reported. Ren Guoxiao et al. [7] measured the charge pickup cross sections for 1.7 GeV/n ⁵⁶Fe, 1.46 GeV/n ⁸⁴Kr, 1.28 GeV/n ¹³⁹La, and 0.8 GeV/n ¹⁹⁷Au on a CR-39 target, combining their results with data on charge pickup by ¹²C, ¹⁸O, and ²⁰Ne projectiles, they found that the cross section for charge pickup reactions by ∼ GeV/n projectiles was generally given to within a factor of 2 by the expression ${\sigma }_{\mathrm{\Delta }Z=+1}=1.7\times {10}^{-4}{\gamma }_{PT}{A}_P^2$ (in mb), where ${\gamma }_{PT}=A_P^{1/3}+A_T^{1/3}-1.0$ implied peripheral collisions. This steeper projectile mass dependence was not well understood physically for a nuclear process. Since these charge pickup interactions were presumably very peripheral collisions by their nature, the cross sections may be expected to depend on the impact parameter ${\gamma }_{PT}=A_P^{1/3}+A_T^{1/3}-1.0$, and being presumably a surface interaction, may also be expected to depend on the cross sectional or surface area, proportional to $A_P^{2/3}$. Then nuclear charge pickup reactions of heavy ions on various targets were investigated at BNL [8], [9], [10], [11], [12], [13], [14], SPS [15], [16], [17], [18], [19], GSI [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], and LBL [31], [32] energies. The dependence of nuclear charge pickup cross section on the beam energy, target size, and projectile size were studied based on available data, but conclusions were not consistent to each other. Generally, the nuclear charge pickup cross section decreases with the increase of the beam energy, and increases with projectile size and target size.

At low and intermediate energies, but below the Fermi energy, the mechanism responsible for charge pickup was transfer reaction where the final states were populated by sequential proton-pickup neutron-stripping processes (or vice versa). At these energies, the projectile velocity was smaller than the Fermi velocity of nucleons inside the nucleus, and because of the overlapping of Fermi spheres at the moment of contact, the nucleons move from one Fermi sea into another. At relativistic energies, however, the Fermi spheres of projectile and target were totally non-overlapping, preventing any transfer of, e.g., a target proton to the projectile. Instead, we can assume Δ-resonance formation and decay in nucleon-nucleon $(NN)$ collisions to be the most likely elementary processes in which a projectile neutron can be converted into a proton, e.g., by $n\to {\mathrm{\Delta }}^{\circ }\to p+{\pi }^{-}$ with subsequent absorption of the proton in the projectile and emission of the ${\pi }^{-}$ [27] and, possibly, neutrons. At intermediate and high energies, two mechanism, e.g., proton transfer through the nuclear overlap zone and Δ-resonance formation and decay in nucleon-nucleon $(NN)$ collisions, may make a contribution simultaneously.

In this paper, the nuclear charge pickup cross sections of ⁵⁶Fe on Al, C and CH₂ targets at an energy of 497 MeV/n were investigated using CR-39 nuclear track detector.