Np Collisions Research Articles

The Glauber model and heavy ion reaction and elastic scattering cross sections

We revisit the Glauber model to study the heavy ion reaction cross sections and elastic scattering angular distributions at low and intermediate energies. The Glauber model takes nucleon–nucleon cross sections and nuclear densities as inputs and has no free parameter and thus can predict the cross sections for unknown systems. The Glauber model works at low energies down to Coulomb barrier with very simple modifications. We present new parametrization of measured total cross sections as well as ratio of real to imaginary parts of the scattering amplitudes for pp and np collisions as a function of nucleon kinetic energy. The nuclear (charge) densities obtained by electron scattering form factors measured in large momentum transfer range are used in the calculations. The heavy ion reaction cross sections are calculated for light and heavy systems and are compared with available data measured over large energy range. The model gives excellent description of the data. The elastic scattering angular distributions are calculated for various systems at different energies. The model gives good description of the data at small momentum transfer but the calculations deviate from the data at large momentum transfer.

Study of the quasi-free [formula omitted] reaction with a deuterium beam at 1.25 GeV/nucleon

The tagged quasi-free np→npπ+π− reaction has been studied experimentally with the High Acceptance Di-Electron Spectrometer (HADES) at GSI at a deuteron incident beam energy of 1.25 GeV/nucleon (s∼2.42 GeV/c for the quasi-free collision). For the first time, differential distributions of solid statistics for π+π− production in np collisions have been collected in the region corresponding to the large transverse momenta of the secondary particles. The invariant mass and angular distributions for the np→npπ+π− reaction are compared with different models. This comparison confirms the dominance of the t-channel with ΔΔ contribution. It also validates the changes previously introduced in the Valencia model to describe two-pion production data in other isospin channels, although some deviations are observed, especially for the π+π− invariant mass spectrum. The extracted total cross section is also in much better agreement with this model. Our new measurement puts useful constraints for the existence of the conjectured dibaryon resonance at mass M∼2.38 GeV and with width Γ∼70 MeV.

Electrochemical Monitoring of Single Nanoparticle Collisions at Mercury-Modified Platinum Ultramicroelectrodes

Here, we report a potentiometric method for detecting single platinum nanoparticles (Pt NPs) by measuring a change in open-circuit potential (OCP) instead of the current during single Pt NP collisions with the mercury-modified Pt ultramicroelectrode (Hg/Pt UME). Similar to the current-time (i-t) response reported previously at Hg/Pt UMEs, the OCP-time (v-t) response consists of repeated potential transient signals that return to the background level. This is because Hg poisons the Pt NP after collision with the Hg/Pt UME due to amalgamation and results in deactivation of the redox reaction. For individual Pt NP collisions the amplitude of the OCP signal reaches a maximum and decays to the background level at a slower rate compared to the comparable i-t response. Due to this, OCP events are broader and more symmetrical in shape compared to i-t "spikes." The collision frequency of Pt NPs derived from v-t plots (0.007 to 0.020 pM(-1) s(-1)) is in good agreement with the value derived from i-t plots recorded at Hg/Pt UMEs (0.016 to 0.024 pM(-1) s(-1)) under similar conditions and was found to scale linearly with Pt NP concentration. Similar to the current response, the amplitude of the OCP response increased with the NP's size. However, unlike the change in current in a i-t response, the change in OCP in a v-t response observed during single Pt NP collisions with Hg/Pt UME is larger than the estimated change in OCP based on the theory. Therefore, the Pt NP sizes derived from the v-t response did not correlate with the TEM-derived Pt NP sizes. In spite of these results the potentiometric method has great value for electroanalysis because of its significant advantages over the amperometric method such as a simpler apparatus and higher sensitivity.

Measurement of the quasi freenp→npπ+π−andnp→ppπ−π0reactions at 1.25 GeV With HADES

We present the results of two-pion production in tagged quasi-free np collisions at a deutron incident beam energy of 1.25 GeV/c measured with the High-Acceptance Di-Electron Spectrometer (HADES) installed at GSI. The specific acceptance of HADES allowed for the first time to obtain high-precision data on π+π− and π−π0 production in np collisions in a region corresponding to large transverse momenta of the secondary particles. The obtained differential cross section data provide strong constraints on the production mechanisms and on the various baryon resonance contributions (∆∆, N(1440), N(1520), ∆(1600)). The invariant mass and angular distributions from the np → npπ+π −and np → ppπ−π0 reactions are compared with different theoretical model predictions.

PION PRODUCTION IN np COLLISIONS AT 1.25 GeV WITH HADES

Preliminary results on charged pion production in np collisions at an incident beam energy of 1.25 GeV obtained with HADES are presented. The np reactions were studied in dp collisions at 1.25 GeV/u using a Forward Wall hodoscope, aimed at registering spectator protons. The separation of np → ppπ-, np → npπ+π- and np → dπ+π- channels is demonstrated.

Scalar-meson production in nucleon-nucleon collisions near threshold

We establish the model-independent spin structure of the matrix elements for the near-threshold scalar-meson production in pp and np collisions, when the final particles are emitted in S-state. Polarization phenomena are derived in a general form. The properties of the t-channel dynamics, which is based on different meson exchanges, are studied in terms of the s-channel parametrization of the matrix element. The predictions of a model, based on $\pi + \sigma$ exchanges are also presented.

Threshold J/ψ production in nucleon-nucleoncollisions

We analyse J/ψ-production in nucleon-nucleon collisionsnear threshold in the framework of a general model-independentformalism, which can be applied to any reaction N + N→N + N + V0, where V0 = ω, ϕ or J/ψ. Suchreactions show large isotopic effects: a large difference forpp and pn collisions, which is due to the different spinstructure of the corresponding matrix elements. The analysis ofthe spin structure and of the polarization observables is basedon symmetry properties of the strong interaction. Using existingexperimental data on the different decays of the J/ψ meson, wesuggest a model for N + N→N + N + J/ψ, based on t channelη + π exchanges. We predict polarization phenomena for then + p→n + p + J/ψ reaction and the ratio of cross sections fornp and pp collisions. For the processes η(π) + N→N + J/ψ we apply two different approaches: vector mesonexchange and local four-particle interaction. In both cases wefind larger J/ψ production in np collisions than in pp collisions.

Search for effects of nuclear polarized strangeness in nucleon-nucleon interaction at Nuclotron “NIS project”

The experimental program aimed for searching the effects of nucleon polarized strangeness in the reactions of ϕ and ω mesons production in pp and np collisions is discussed. The physical motivation, kinematical properties outline of the experiment are presented. The experiment is planing to be performed on the accelerator complex of the Laboratory of High Energy of JINR, Dubna.

Photon production in heavy-ion collisions close to the pion threshold

We report on a measurement of hard photons ( E γ >30 MeV) in the reaction Ar+Ca at 180 A MeV at an energy in which photons from the decay of π° mesons are dominating. Simultaneous measurement with the TAPS spectrometer of the photon spectrum and photon-photon coincidences used for the identification of π° enabled the subtraction of π° contribution. The resulting photon spectrum exhibits an exponential shape with an inverse slope of E 0=(53±2 (stat) −5 +8 (syst) ) MeV. The photon multiplicity equal to (1.21±0.03 (stat) +0.3 −0.2 (syst) )·10 −2 is roughly one order of magnitude larger than the value extrapolated from existing systematics. This enhancement of the hard photon production is attributed to a strong increase in the contribution of secondary np collisions to the total photon yield. We conclude that, on average, the number of np collisions which contribute to the hard photon production is 7 times larger than the number of first chance np collisions in the reaction Ar+Ca at 180A MeV.

Hard photons and energetic protons as probes of the nuclear dynamics at intermediate energy

The emission of hard photons and pre-equilibrium protons has been investigated in exclusive experiments performed with MEDEA detector. The observation of the γ-proton anticorrelation indicates that the dominant production mechanism is the first-chance neutron-proton (np) collisions. The anisotropic angular distributions point out the presence of screening effects and are reminiscent of the free np scattering thus giving information of the space-time origin of the energetic protons. Protons, with energy more than twice the kinematical limit for nucleon-nucleon (NN) collisions, have been observed in several reactions around the Fermi energy.

Comment on “Photon spectrum of the neutron-proton Bremsstrahlung at 170 MeV”: Contributions of np → dγ to inclusive photon energy spectra from np and pd collisions near 200 MeV

Several years ago, a measured photon energy spectrum from np collisions was presented as a measurement of free np Bremsstrahlung, and subsequently theoretical groups compared the data to their model calculations of the process np → npγ [F. Malek, A. Nifenecker, J.A. Pinston, F. Schussler, S. Drissi, J. Julien, Phys. Lett. B 266 (1991) 391]. We point out that in np collisions, np capture ( np → dγ) is important and cannot be ignored. It contributes approximately 30% of the cross section for photons above 40 MeV. Capture also makes a significant contribution to the photon spectra produced in p- d collisions, and possibly has a role in p-nucleus collisions. In the former case, better agreement between theory and experiment is obtained when (quasifree) capture is included.

Reply to: Comment on “Photon spectrum of the neutron-proton Bremsstrahlung at 170 MeV”: Contributions of np → dγ to inclusive photon energy spectra from np and pd collisions near 200 MeV [Phys. Lett. B 349 (1995) 272

Reply to: Comment on “Photon spectrum of the neutron-proton Bremsstrahlung at 170 MeV”: Contributions of np → dγ to inclusive photon energy spectra from np and pd collisions near 200 MeV [Phys. Lett. B 349 (1995) 272

High energy photon production in nuclear collisions.

High energy photons may be produced in nuclear reactions by several mechanisms, including bremsstrahlung from the potential field or from nucleon-nucleon collisions. Using infinite and semi-infinite nuclear matter approximations, we obtain simple expressions for the rates. Nucleon-nucleon collisions provide the more important mechanism for both proton-induced and heavy-ion induced reactions, except for the highest energy photons. The photon cross section calculated with a zero range np interaction agrees well with measurements on proton-induced reactions. For heavy-ion reactions, the initial np collisions only account for a third of the measured cross section. The disagreement may be due to an inadequacy of the infinite matter or the zero range approximations, or it may be that multiple collisions are important.