Non-equilibrium emission of complex fragments fromp+Aucollisions at 2.5 GeV proton beam energy

Abstract

Energy and angular dependence of double differential cross sections d 2 σ/dΩdE was measured for reactions induced by 2.5 GeV protons on Au target with isotopic identification of light products (H, He, Li, Be, and B) and with elemental identification of heavier intermediate mass fragments (C, N, O, F, Ne, Na, Mg, and Al). It was found that two different reaction mechanisms give comparable contributions to the cross sections. The intranuclear cascade of nucleon-nucleon collisions followed by evaporation from an equilibrated residuum describes low energy part of the energy distributions whereas another reaction mechanism is responsible for high energy part of the spectra of composite particles. Phenomenological model description of the differential cross sections by isotropic emission from two moving sources led to a very good description of all measured data. Values of the extracted parameters of the emitting sources are compatible with the hypothesis claiming that the high energy particles emerge from pre-equilibrium processes consisting in a breakup of the target into three groups of nucleons; small, fast and hot fireball of ∼ 8 nucleons, and two larger, excited prefragments, which emits the light charged particles and intermediate mass fragments. The smaller of them contains ∼ 20 nucleons and moves with velocity larger than the CM velocity of the proton projectile and the target. The heavier prefragment behaves similarly as the heavy residuum of the intranuclear cascade of nucleon-nucleon collisions.

Non-equilibrium processes in p + Ag collisions at GeV energies

“…Because of this it was possible to analyze the present data separately for individual isotopes and in the form of double differential cross sections d 2 σ/d dE. The double differential cross sections d 2 σ/d dE measured in the present experiment for a silver target at three proton beam energies (1.2, 1.9, and 2.5 GeV) indicate very similar behavior to that observed in our previous studies for other targets: Al [5], Ni [3,4], and Au [1,2,4]. First, they increase systematically with beam energy but the angular dependence of the energy spectra as well as the shape of spectra practically do not change.…”

“…The shape of the experimental spectra as well as their angular dependence vary smoothly with the beam energy in similar way to our previous results obtained at this beam energy range for heavier (Au [1,2]) and lighter (Ni [3,4] as well as Al [5]) targets. The low-energy part of the energy spectra of all ejectiles is almost angle independent, whereas the high-energy part is anisotropic: the forward direction of emission of the high-energy particles is clearly favored.…”

“…To ensure the interpolation between individual emission angles as well as extrapolation to the lowest and highest energies of emitted 7 Be particles, the present data were parametrized by a simple formula representing the isotropic emission from two sources moving forward along the beam direction (see Appendix of Ref. [1] for details of the parametrization). A very good reproduction of the data by this formula led to a statistical uncertainty of the absolute value of the cross sections not larger than 9%; however, the uncertainty of the literature value of the production cross section σ ( 7 Be) which is believed to be smaller than 10% [20], should be also taken into account.…”

“…However, high-energy products of the intranuclear cascade, i.e., nucleons and pions, should manifest their presence by non-isotropic (forward peaked) angular distributions and high-energy tails of their energy spectra. It is, however, frequently observed that angular distributions of composite LCPs as well as those of IMFs are forward peaked and the corresponding energy spectra extend to larger energies than predicted by emission of particles from the equilibrated residual nuclei [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. Such an effect was reported both at proton energies of several GeV (4.9-5.5 GeV) [12,14,15] and at low energies (0.18-0.48 GeV) [3,6,10] for light (e.g., nickel [3]) as well as for heavy (e.g., uranium [15]) nuclei.…”

Ranking and validation of the spallation models for description of intermediate mass fragment emission from p + Ag collisions at 480 MeV incident proton beam energy

“…It is well known for proton-induced reactions that the qualitative properties of the spectra and angular distributions of the spallation products indicate the presence of two different processes [1][2][3][4][5][6][7][8][9][10][11][12]. One of them contributes to low-energy part of the spectra (energy range smaller than 30-50 MeV), has evaporative-like character and therefore produces isotropic angular distributions.…”