An experimental study of neutron elastic scattering at forward angles

High-statistics measurements of differential cross sections and recoil polarizations for the reaction $\gamma p \rightarrow K^+ \Sigma^0$ have been obtained using the CLAS detector at Jefferson Lab. We cover center-of-mass energies ($\sqrt{s}$) from 1.69 to 2.84 GeV, with an extensive coverage in the $K^+$ production angle. Independent measurements were made using the $K^{+}p\pi^{-}$($\gamma$) and $K^{+}p$($\pi^-, \gamma$) final-state topologies, and were found to exhibit good agreement. Our differential cross sections show good agreement with earlier CLAS, SAPHIR and LEPS results, while offering better statistical precision and a 300-MeV increase in $\sqrt{s}$ coverage. Above $\sqrt{s} \approx 2.5$ GeV, $t$- and $u$-channel Regge scaling behavior can be seen at forward- and backward-angles, respectively. Our recoil polarization ($P_\Sigma$) measurements represent a substantial increase in kinematic coverage and enhanced precision over previous world data. At forward angles we find that $P_\Sigma$ is of the same magnitude but opposite sign as $P_\Lambda$, in agreement with the static SU(6) quark model prediction of $P_\Sigma \approx -P_\Lambda$. This expectation is violated in some mid- and backward-angle kinematic regimes, where $P_\Sigma$ and $P_\Lambda$ are of similar magnitudes but also have the same signs. In conjunction with several other meson photoproduction results recently published by CLAS, the present data will help constrain the partial wave analyses being performed to search for missing baryon resonances.

The nuclear incoherent π 0 photoproduction cross section from 12C is evaluated at forward angles in the 4.0 to 6.0 GeV energy range using the multicollisional intranuclear cascade model MCMC. The model incorporates some improvements in comparison with previous versions associated with the momentum distribution (MD) for light nuclei - extracted from the available (e,e ′p) data - as well as the evaluation of the shadowing effects during the photo-nucleus interaction. The final results of the single and double differential cross sections at forward angles are very sensitive to the MD parameterizations due to the Pauli principle, which largely suppresses the cross sections for low momentum transfer. The attenuation of the nuclear cross section due to pion - nucleus final state interactions is approximately 40% (without nuclear shadowing), which is in nice agreement with the predictions from the Glauber model. The single and double π 0 differential cross sections are presented for possible applications for the interpretation of the inelastic background in the PrimEx experiment at the Jefferson Laboratory. © 2007 American Institute of Physics.

Total and differential cross sections from literature, on the production of K+ mesons in pA interactions at projectile energies between T=0.8 and 2.9 GeV, covering the transition across the free nucleon-nucleon threshold at 1.58 GeV, have been investigated. From the target-mass dependence of the production cross sections no evidence for the expected change of the dominant reaction mechanism from two-step to direct kaon production was found. At T=1.0 GeV the A dependences of the total cross sections and of the most recent data from COSY-Juelich, differential cross sections measured under forward angles, are strongly different. The invariant K+ production cross sections show an overall exponential scaling behavior with the squared four-momentum transfer between the beam proton and the produced K+ meson for t< -0.05 GeV^2 independent of the beam energy and emission angle. The data from COSY-Juelich reveal a strongly different t dependence in the region of t>0 GeV^2. Further data at forward angles and different beam energies should be taken in order to explore this region of kinematically extreme conditions.

We investigate the photoproduction of ω off a nucleon target by using a Reggeized model where π(135) + σ(500) + f1(1285) + f2(1270) + pomeron exchanges are included in the t-channel for the reaction at forward angles. The reaction mechanism at low energy is featured by the dominance of the π exchange with the absorptive cuts introduced to modulate the pion contribution to both γp → ωp and reactions. The necessity of σ exchange is illustrated in the analysis of the cross-section from natural parity exchanges. Cross-sections for the differential, total, and spin density matrix with parity and beam polarization asymmetries are reproduced and compared with existing data on . Scaled differential cross-sections of Jefferson Lab data on are investigated at the production angle θ = 90° with the nonlinear trajectories for a saturation. Differential and total cross-sections for the reaction are analyzed to compare with recent experimental data at the CBELSA/TAPS Collaboration. An application of the reaction is presented to demonstrate the significance of the π exchange in the total and differential cross-sections.

Inclusive neutron spectra were measured at 0\ifmmode^\circ\else\textdegree\fi{}, 4\ifmmode^\circ\else\textdegree\fi{}, 8\ifmmode^\circ\else\textdegree\fi{}, 15\ifmmode^\circ\else\textdegree\fi{}, 30\ifmmode^\circ\else\textdegree\fi{}, and 42\ifmmode^\circ\else\textdegree\fi{} from Nb-Nb and Au-Au collisions at 800 MeV/nucleon. A peak that originates from neutron evaporation from the projectile appears in the spectra at angles out to 8\ifmmode^\circ\else\textdegree\fi{}. The shapes and magnitudes of the spectra are compared with those calculated from models of nucleus-nucleus collisions. The differential cross sections for Au-Au collisions are about four times those for Nb-Nb collisions. The predictions of the Vlasov-Uehling-Uhlenbeck (VUU) and QMD theories agree with the angular distributions of the differential cross sections except at small angles; the VUU prediction overestimates the angular distributions from a few degrees to about 20\ifmmode^\circ\else\textdegree\fi{}, whereas the QMD prediction underestimates the angular distributions below 8\ifmmode^\circ\else\textdegree\fi{}. The Firestreak model overestimates the angular distribution for Nb-Nb collisions and underestimates it for Au-Au collisions. Also, the VUU and QMD models agree with the measured double-differential cross sections in more angular and energy regions than the Firestreak and intranuclear cascade models; however, none of the models can account for the peaks at small angles (\ensuremath{\theta}\ensuremath{\le}15\ifmmode^\circ\else\textdegree\fi{}).

We present measurements of differential cross sections and decay asymmetries of incoherent φ‐meson photo‐production from the deuteron at forward angles using linearly polarized photons beam in energy range from the production threshold to E = 2.4 GeV. The experiment was performed at the SPring‐8/LEPS facility. The differential cross section of incoherent production shows a large suppression compared with the free proton case. The reduction for the deuteron cannot be clearly explained in terms of simple isospin asymmetry. The decay asymmetry of incoherent φ photo‐production shows that the eta exchange process is small. Since the exclusive K+K−p cross section shows the same reduction with incoherent, both the proton and neutron cross section reduce similarly. Consequently, the π‐η interference effect is small too. The present results suggest that our understanding of φ photo‐production within a nucleus should be rearranged.

Differential scattering cross sections for excitation of helium by electron impact from its ground state to its $2^{1}P$ excited state have been measured at four incident electron energies in the range 26-55.5 eV for scattering angles between 10\ifmmode^\circ\else\textdegree\fi{} and 70\ifmmode^\circ\else\textdegree\fi{}, and at 81.6 eV for scattering angles between 10\ifmmode^\circ\else\textdegree\fi{} and 80\ifmmode^\circ\else\textdegree\fi{}. These differential cross sections have been placed on an absolute scale by normalizing them to the experimental absolute integral cross sections of Jobe and St. John. These experimental differential and integral cross sections have been compared with the results predicted by the Born approximation, and by several other first-order approximations in which direct excitation is calculated in the Born approximation and exchange scattering by various Ochkurlike approximations. The calculations provide reliable tests of these scattering theories since they are made using the accurate generalized oscillator strengths of Kim and Inokuti. As expected, these first-order theories are poor near threshold and exchange is important at high scattering angles for all energies. Further, the absolute magnitude of the calculated integral and small-angle differential cross sections is too large and is within 50% of experiment only at energies greater than 80 eV. These first-order models are in qualitative agreement with the experimental angular dependence at 34-81.6 eV for scattering angles between 10\ifmmode^\circ\else\textdegree\fi{} and 40\ifmmode^\circ\else\textdegree\fi{}. At higher scattering angles (corresponding to momentum transfers greater than about 1.6 a.u.), the calculated differential cross sections fall well below the experimental ones. The phase between the direct and exchange scattering amplitudes was found to be important at all energies, and is apparently not predicted correctly by any of the first-order models examined here. Some approximations for the exchange (e.g., Ochkur approximation and the post interaction form of the Ochkur-Rudge approximation) were found to be better for integral cross sections and some (e.g., prior Ochkur-Rudge approximation) were better for differential cross sections. The use of good analytic self-consistent-field (SCF) wave functions for both the ground and excited states was tested by computing generalized oscillator strengths from them and comparing these results with the calculations using the accurate generalized oscillator strengths. The SCF functions yield differential cross sections in quantitative disagreement (20%) with the accurate results, although the energy and angle dependence of the cross sections is predicted qualitatively correctly.

A search for the bound trineutron from 7Li + n reactions

The 40Ar(n, γ)41Ar neutron capture cross section has been measured between 0.4 and 14.8 MeV neutron energy using the activation technique. The data are important for estimating backgrounds in argon-based neutrino and dark-matter detectors and in the neutrino-less double-beta decay search GERDA, which uses liquid argon as cooling and shielding medium. For the first time the 40Ar(n, γ)41Ar cross section has been measured for neutron energies above 1 MeV. Our results are compared to the evaluation ENDF/B-VII.1 and the calculated prediction TENDL-2013. The latter agrees very well with the present results.

The 13C(γ,π−)13N reaction at Eγ≈ 184 MeV (Tπ ≈ 41 MeV)

Fast neutron capture cross section facility

Neutron-production double-differential cross sections for 870-MeV π+ and π - and 2.1-GeV π+ mesons incident on iron and lead targets were measured with NE213 liquid scintillators by time-of-flight technique. NE213 liquid scintillators 12.7cm in diameter and 12.7cm thick were placed in directions of 15°, 30°, 60°, 90°, 120° and 150°. The typical flight path length was 1.5 m. Neutron detection efficiencies were derived from the calculation results of SCINFUL and CECIL codes. The experimental results were compared with the cascade-evaporation calculation code NUCLEUS. The calculation results are higher typically by a factor of two than the experimental data at neutron energies below about 30MeV. NUCLEUS overestimates π+-incident neutron-production cross sections in forward angles at neutron energies of 100 to 500 MeV.

Neutron-production double-differential cross sections for 870-MeV π+ and π - and 2.1-GeV π+ mesons incident on iron and lead targets were measured with NE213 liquid scintillators by time-of-flight technique. NE213 liquid scintillators 12.7cm in diameter and 12.7cm thick were placed in directions of 15°, 30°, 60°, 90°, 120° and 150°. The typical flight path length was 1.5 m. Neutron detection efficiencies were derived from the calculation results of SCINFUL and CECIL codes. The experimental results were compared with the cascade-evaporation calculation code NUCLEUS. The calculation results are higher typically by a factor of two than the experimental data at neutron energies below about 30MeV. NUCLEUS overestimates π+-incident neutron-production cross sections in forward angles at neutron energies of 100 to 500 MeV.

Activation technique was used to measure 52Cr(n,2n)51Cr, 55Mn(n,2n)54Mn, 75As(n,2n)74As, 93Nb(n,2n)92mNb, and 124Sn(n,2n)123mSn reactions at around 14.0 MeV neutron energy. These elements are being used as structural materials in reactor technology. The correction due to the self scattering and self absorption in the samples and pile up effects in the detector was measured experimentally by simulation technique. This technique provides an exact magnitude of corrections experimentally. The experimental cross sections are compared with latest literature data taken from EXFOR database and also with theoretical predictions accounting for nucleon evaporation and preequilibrium emission using hybrid model (ALICE/90) and TALYS-1.4 code over neutron energies from 12 to 16 MeV.

We have measured differential (P,xin) cross sections, d/sup 2/sigma/d..cap omega..dE/sub n/, from thin targets and absolute neutron yields from stopping-length targets at angles of 7.5/degree/, 30/degree/, 60/degree/, and 150/degree/, for the 113--MeV proton bombardment of elemental beryllium, carbon, aluminum, iron, and depleted uranium. Additional cross-section measurements are reported for oxygen, tungsten, and lead. We used time-of-flight techniques to identify and discriminate against backgrounds and to determine the neutron energy spectrum. Comparison of the experimental data with intranuclear-cascade evaporation-model calculations with the code HETC showed discrepancies as high as a factor of 7 in the differential cross sections. These discrepancies in the differential cross sections make it possible to identify some of the good agreement seen in the stopping-length yield comparisons as fortuitous cancellation of incorrect production estimates in different energy regimes. 13 refs., 20 figs., 4 tabs.