Measurement of the neutron F2 structure function via spectator tagging with CLAS

We report on the first measurement of the F(2) structure function of the neutron from the semi-inclusive scattering of electrons from deuterium, with low-momentum protons detected in the backward hemisphere. Restricting the momentum of the spectator protons to ?100 MeV/c and their angles to ?100° relative to the momentum transfer allows an interpretation of the process in terms of scattering from nearly on-shell neutrons. The F(2)(n) data collected cover the nucleon-resonance and deep-inelastic regions over a wide range of Bjorken x for 0.65相似文献   

The excitation function for the 89Y(n, 2n)88Y reaction

Activation cross sections for the ⁸⁹Y(n, 2n)⁸⁸Y reaction at neutron energies between 12.6 and 17.8 MeV have been measured by using the mixed-powder method and γ-ray detection by a Ge(Li) spectrometer. Using the ²⁷Al(n, α)²⁴Na reaction for monitoring the neutron flux, the measured cross-section values for the ⁸⁹Y(n, 2n)⁸⁸Y reaction were found to be 331±32 mb, 603±58 mb, 820±79 mb, 1040±100 mb, 1072±103 mb, 1172±112 mb, 1221±117 mb and 1218±117 mb at the respective incident neutron energies of 12.6±0.1 MeV, 13.3±0.1 MeV, 14.0±0.4 MeV, 14.9±0.3 MeV, 15.1±0.3 MeV, 16.0±0.4 MeV, 16.7±0.5 MeV and 17.8±0.7 MeV. The measured values are compared with the experimental values of others and with the theoretical values obtained from calculations based on the statistical model for the formation of a compound nucleus and its subsequent emission of neutrons.     

Cross-sections for the formation of isomeric pair Ge through (n, 2n), (n, p) and (n, α) reactions measured over 13.73 MeV to 14.77 MeV and calculated from near threshold to 20 MeV neutron energies

The cross-sections for formation of isomeric pair, ⁷⁵Ge^m(σ_m) and ⁷⁵Ge^g(σ_g), through ⁷⁶Ge(n, 2n), ⁷⁵As(n, p) and ⁷⁸Se(n, α) reactions were measured at 13.73 MeV, 14.42 MeV and 14.77 MeV neutrons and also estimated using EMPIRE-II and TALYS codes over neutron energies from near threshold to 20 MeV. For each (n, 2n), (n, p) and (n, α) reaction, the cross-section initially increases with neutron energy, but starts decreasing as the neutron energy exceeds the respective threshold of (n, 3n), (n, pn) and (n, αn) reactions. The higher values of σ_m relative to σ_g reveal that the transitions of the excited ⁷⁵Ge from higher energy levels to metastable state (7⁺/2) are favored as compared to unstable ground state (1⁻/2). The present values of cross sections for formation of ⁷⁵Ge^m,g through (n, 2n) and (n, α) reactions are lower, and that of (n, p) reaction are higher compared to most of the corresponding literature cross-sections.     

Precise 2H(p,2p)n data in the small recoil region

Differential cross sections for the ²H(p,2p)n reaction at 508 MeV have been measured in a coplanar geometry for the proton angles 41.4°–41.4° and 30.1°–53.75°, for neutron recoil momenta up to 55 MeV/c. A missing strength of 6.6% relative to the PWIA is found, with an absolute uncertainty of (1.9–2.7)%. The agreement is better when rescattering and final state interaction are taken into account. The calculation results are also compared to large recoil data previously published.     

Excitation functions of some neutron threshold reactions on isotopes of molybdenum

Cross sections were measured for (n,p) reactions on ^{92, 95, 96, 97, 98}mo, (n, α) reactions on ^{92, 98}Mo, and (n, 2n) reaction on ¹⁰⁰Mo for the first time in the neutron energy range of 5.9 to 9.6 MeV. The quasi-monoenergetic neutrons were produced via the ²H(d, n)³He reaction using a deuterium gas target at a compact cyclotron. Use was made of the activation technique in combination with high-resolution γ-ray spectroscopy. Some systematic trends observed in the excitation functions are discussed. For the various isotopes of molybdenum, with increasing mass of the target nucleus, the thresholds of (n,p) and (n,α) reactions increase and the magnitudes of cross sections near the maxima of the excitation functions appear to decrease. Hauser-Feshbach calculations show that in general the excitation functions of (n, p) and (n, α) reactions are described within a factor of 2 by the statistical model only up to about 8 MeV; the (n,2n) reaction on ¹⁰⁰Mo, however, is reproduced well from threshold up to 15 MeV by this model.     

A systematic investigation of the (α, 2nγ) reaction on medium-heavy nuclei

Exclusive neutron spectra and angular distributions have been measured for 28–35 MeV (α, 2nγ) reactions on various nuclei in the 80 ≦ A ≦ 210 region. Pre-equilibrium processes dominate the 35 MeV (α, 2nγ) reaction mechanism in much of this region. Analysis of systematic variation in the neutron spectrum parameters shows that the reaction mechanism is strongly correlated with the target neutron excess parameter (N?Z/A. Analysis of the γ-decay of the entry states shows that well-defined incident angular momentum windows exist for the pre-etjuilibrium (α, 2nγ) reaction. These features are discussed in terms of various models for the reaction mechanism.     

Cross-section measurement for the ~(93)Nb(n,2n)~(92g)Nb reaction at the neutron energy of 14.6 MeV by the AMS method

The cross-section for the~(93)Nb(n,2n)~(92g)Nb reaction has been measured at the neutron energy of 14.6 Me V using neutron activation and accelerator mass spectrometry(AMS)determination of the long-lived product nuclide~(92g)Nb.The neutron energy was generated from the D+T neutron source at the China Institute of Atomic Energy(CIAE).The neutron flux was monitored by measuring the activity of~(92m)Nb produced in the competing reaction channel of~(93)Nb(n,2n)~(92m)Nb.At the neutron energy of 14.6 MeV,the~(93)Nb(n,2n)~(92g)Nb reaction cross-section of(736±220)mb was obtained for the first time.     

Cross-Section Measurements for Reactions on Platinum Isotopes in the Neutron Energy Range of 13.5 to 14.6MeV

Cross-sections for (n,2n) and (n,p) reactions have been measured on platinum isotopes at the neutron energies of 13.5 to 14.6 MeV using the activation technique. Data were reported for the following reactions: Pt(n,2n)197m+gPt,198Pt(n,2n)197mPt,192Pt(n,2n)191Pt, 194Pt(n,p)194Ir, 195Pt(n, p)195mIr, and 196Pt(n,p)196mIr. At the neutron energies of 13.5, 14.1 and 14.6 MeV, the cross sections in mb are 2038±159, 1919±73 and 1836±68 for 198Pt(n,2n)197m+gPt reaction; 974±37, 1055±39 and 1042±39 for 198Pt(n, 2n)197mPt reaction, respectively. The cross sections are 1680±103, 1810±67 and 2047±97 for 192Pt(n,2n)191Pt reaction, and 1.0±0.2, 1.6±0.2 and 1.8±0.2 for 195Pt(n, p)195mIr reaction, respectively, at energies of 14.1, 14.4 and 14.6MeV. At 14.1 and 14.4 MeV neutron energies, the cross sections are 3.8±0.4 and 5.4±0.5 for 194Pt(n,p)194Ir reaction. While, the data are (1.13±0.07) and (1.18±0.06) mb at energies of 13.5 and 14.4 MeV, respectively, for 196Pt(n,p)196mIr reaction.The neutron fluences were determined using the monitor reaction 93Nb(n, 2n)92mNb or 27Al(n, a)24Na. A comparison was made between the present cross sections and the collected data for the measured reactions.     

The role of the 14N(n,p)14C reaction in neutron irradiation of soft tissues

The cell-killing potential of the ¹⁴N(n,p)¹⁴C reaction was considered with regard to neutron absorption in human nuclear DNA and respiratory phosphates for: (A) 10¹² thermal neutrons in 1 kg of soft tissue, (B) a mono-energetic beam of 2 MeV neutrons incident in 1 kg of soft tissue such that the total collision kerma was 10 J/kg, and (C) an evenly distributed 0–66 MeV neutron beam, also incident in 1 kg such that the total collision kerma was 20 J/kg. For case (A) 0.0017 ¹⁴N(n,p)¹⁴C reactions could be expected per DNA double strand, case (B) 0.053, and case (C) 0.0039. The probabilities that a proton emitted outside the nucleus would cross nuclear DNA were estimated from ¹⁴N tissue content for adult skeletal muscle, liver, and kidney tissues, for (1) nuclear DNA being concentrated in a sphere of 1.8 μm diameter, and (2) nuclear DNA being evenly distributed in a spherical nucleus 5 μm in diameter. It was concluded that even in a nitrogen-rich tissue exposed to a collision kerma of 20 J/kg by a 0–66 MeV fast neutron beam, the ¹⁴N(n,p)¹⁴C reaction directly kills at most 10 cells in every 1000, 4 of these by DNA nitrogen absorption and 6 by the ¹⁴N(n,p)¹⁴C protons emitted elsewhere in the cell. However, the dose due to the ¹⁴N(n,p)¹⁴C reaction should be measured where exposure to thermal neutron fluxes is significant. For therapeutic neutron doses the number of respiratory phosphate molecules in which the ¹⁴N(n,p)¹⁴C reaction occurs is insignificant, and doses from ¹⁴C-decay after neutron therapy are also negligible.     

Measurements of the Cross-sections of Produced Short-Lived Nuclei Induced by Neutrons around 14 MeV on Isotopes of Tungsten

New experimental cross-section data for the180 W(n,2 n)179 mW,186 W(n,2 n)185 mW and186 W(n,p)186 Ta reactions at the neutron energies of 13.5 and 14.4 MeV are obtained by the activation technique. The neutron beams are produced by means of the3 H(d,n)4 He reaction. The gamma activities of the product nuclei are measured by a high-resolution gamma-ray spectrometer with a coaxial high-purity germanium detector. The neutron fluence is determined using the monitor reaction93 Nb(n,2 n)92 mNb. The results in the current work are discussed and compared with the measurement results found in the literature. It is shown that these higher accuracy experimental cross-section data around the neutron energy of 14 MeV agree with some previous experimental values from the literature within experimental uncertainties.     

Cross sections and reaction mechanisms of (p,pxn) reactions on 208Pb in the 24–52 MeV range

The cross sections for the reactions ²⁰⁸Pb(p, pn)^207mPb, ²⁰⁸Pb(p, p2n)^206mPb have been measured for 24, 28, 36, 44 and 52 MeV incident protons. The experimental results are shown to be consistent with the clean knockout mechanism for the (p, pn) reaction and with knockout of one neutron followed by evaporation of another for the (p, p2n) reaction. We deduce a theoretical formula for the cross section for these reactions.     

The Mo(p, n)Tc reaction cross section and its use as a normalization for shuttle-system measurements

The ¹⁰⁰Mo(p, n)¹⁰⁰Tc reaction cross section was determined from 2 to 11 MeV (c.m.), by counting the delayed γ-rays from the ¹⁰⁰Tc decays (half-life = 15.8 s). These yields were measured with Ge(Li) and NaI(Tl) detectors and thick natural Mo targets. The use of this reaction as a normalizing reaction for cross-section measurements with a rapid target-transfer system is discussed. This technique was applied to the measurement of the astrophysically interesting ²³Na(p, n)²³Mg cross section (²³Mg HALF-LIFE = 11.57s). The results agree well with an independent ²³Na(p, n)²³Mg cross-section measurement.     

The deuteron breakup reaction induced by protons of 65, 85 and 100 MeV

Coplanar energy sharing spectra for p + d breakup at 65, 85 and 100 MeV proton bombarding energies were measured using the University of Maryland sectored isochronous cyclotron, by measuring the energies of either two protons or one proton and one neutron in coincidence. The detector angles were chosen to enhance either the p-p or p-n quasifree scattering, or the p-n final state interaction. The energy dependence of the peak cross section at equal symmetric quasifree scattering angle pairs was extracted for the ²H(p, 2p)n and ²H(p, pn)p reactions. Quasifree angular distributions were obtained for the reaction ²H(p, 2p)n at 65 MeV and for the reaction ²H(p, pn)p at 65, 85 and 100 MeV. The plane wave impulse approximation theory can only qualitatively reproduce the shape of the quasifree scattering peak in the energy sharing spectra and the shape of the p-p quasifree angular distribution. The discrepancies observed between the plane wave impulse approximation theory and the experimental data imply that the presence of the spectator particle (i.e., the multiple scattering effects) has a strong influence on the magnitude and the shape of the experimental results. Multiple scattering calculations were carried out in the three-body model of Aaron, Amado and Yam except that the S-wave separable two-body amplitudes were modified to fit two-nucleon elastic scattering data at higher energies. Comparisons of the results of these multiple scattering calculations to the experimental data show excellent quantitative agreement throughout the energy range and the angular region of this experiment, except for a few cases in which this model is inherently insufficient; namely, regions in which the Coulomb interaction is important, or regions for which a Hulthén wave function is inaccurate and the off-shell effects are not properly taken into account.     

n-p short-range correlations from (p,2p+n) measurements

We studied the 12C(p,2p+n) reaction at beam momenta of 5.9, 8.0, and 9.0 GeV/c. For quasielastic (p,2p) events p(f), the momentum of the knocked-out proton before the reaction, was compared (event by event) with p(n), the coincident neutron momentum. For |p(n)|>k(F)=0.220 GeV/c (the Fermi momentum) a strong back-to-back directional correlation between p(f) and p(n) was observed, indicative of short-range n-p correlations. From p(n) and p(f) we constructed the distributions of c.m. and relative motion in the longitudinal direction for correlated pairs. We also determined that 49+/-13% of events with |p(f)|>k(F) had directionally correlated neutrons with |p(n)|>k(F).     

Neutron-fission competition near threshold: The influence of shells and pairing on the decay of the 241Cm compound nucleus

The excitation functions of the reactions ²³⁸Pu(α, 2n) and ²⁴¹Am(p, 2n), both leading to ²⁴⁰Cm under fission competition, are measured between threshold and 8 MeV excess energy. The two identical excitation functions show a bump-like structure 5 MeV above threshold, which is impossible to explain by the use of statistical reaction theories with standard level density expressions. A compound reaction calculation based on shell dependent level densities for the neutron and fission channels, respectively, with the inclusion of pairing leads to a detailed understanding of the observed structure.     

Electron-induced neutron knockout from 4He

The differential cross section for electron-induced neutron knockout in the reaction 4He(e,e(')n)(3)He has been measured for the first time with a statistical accuracy of 11%. The experiment was performed in quasielastic kinematics at a momentum transfer of 300 MeV/c and in the missing-momentum range of 25-70 MeV/c. The comparison of the data with theoretical calculations shows an impressive increase of the cross section resulting from final state interaction effects. Specifically, the p-n charge-exchange process dominates the cross section in this kinematical regime.     

Proton- and neutron H11/2 excitations in 63 141 Eu78

An (α,p6n) in-beam γ-ray experiment has established the high-spin states of the N=78 nucleus¹⁴¹Eu up to I=35/2^? at 4.4 MeV excitation. The levels above 3 MeV involve the couplings of the h_11/2 valence proton to either two aligned h_11/2 proton particles or two aligned h_11/2 neutron holes. The observed spin sequences are in accord with the shell model for these couplings.     

Quasielastic 3He(e,e'p)2H reaction at Q2 = 1.5 GeV2 for recoil momenta up to 1 GeV/c

We have studied the quasielastic 3He(e,e(')p)2H reaction in perpendicular coplanar kinematics, with the energy and the momentum transferred by the electron fixed at 840 MeV and 1502 MeV/c, respectively. The 3He(e,e(')p)2H cross section was measured for missing momenta up to 1000 MeV/c, while the A(TL) asymmetry was extracted for missing momenta up to 660 MeV/c. For missing momenta up to 150 MeV/c, the cross section is described by variational calculations using modern 3He wave functions. For missing momenta from 150 to 750 MeV/c, strong final-state interaction effects are observed. Near 1000 MeV/c, the experimental cross section is more than an order of magnitude larger than predicted by available theories. The A(TL) asymmetry displays characteristic features of broken factorization with a structure that is similar to that generated by available models.