Discrete Final States Research Articles

Interplay between multiple intranuclear scattering and pickup in proton-induced emission of3He into the continuum

The reaction 58 Ni(p, 3 He) 56 Co was investigated in the range from 80 to 120 MeV in order to determine whether the analyzing power of the dir- ect pickup process decreases towards higher incident energies. Such quenching of the analyzing power is suggested by studies of inclusive proton-induced 3 He emission into the continuum, in which the direct pickup is preceded by mul- tiple intranuclear N-N interactions. In pickup to discrete final states, it is now found that, as the incident energy is increased, the signature of large analyzing power moves to smaller scattering angles. This behaviour is consistent with the trend found in earlier continuum studies, thus the present study allows an unambiguous interpretation of the quenching effect.

Effects of nuclear correlations on theO16(e,e′pN)reactions to discrete final states

Calculations of the $^{16}$O$(e,e'pN)$ cross sections to the ground state and first excited levels of the $^{14}$C and $^{14}$N nuclei are presented. The effects of nuclear fragmentation have been obtained in a self-consistent approach and are accounted for in the determination of the two-nucleon removal amplitudes. The Hilbert space is partitioned in order to compute the contribution of both long- and short-range effects in a separate way. Both the two-proton and the proton-neutron emission cross sections have been computed within the same models for the reaction mechanism and the contribution from nuclear structure, with the aim of better comparing the differences between the two physical processes. The $^{16}$O$(e,e'pp)$ reaction is found to be sensitive to short-range correlations, in agreement with previous results. The $^{16}$O$(e,e'pn)$ cross section to $1^+$ final states is dominated by the $\Delta$ current and tensor correlations. For both reactions, the interplay between collective (long-range) effects and short-range and tensor correlations plays an important role. This suggests that the selectivity of $(e,e'pN)$ reactions to the final state can be used to probe correlations also beyond short-range effects.

The 16 O(e,e‘pp)14 C reaction to discrete final states

Differential cross sections for exclusive 16O(e,e'pp)14C processes are computed within a distorted-wave framework which includes central short-range correlations and intermediate \Delta+ excitations. The cross sections are compared to high-resolution data from the MAMI facility at Mainz for a central energy and momentum transfer of \omega=215 MeV and q=316 MeV respectively. A fair agreement between the numerical calculations and data is reached when using spectroscopic information extracted from a 15N(d,3He)14C experiment. The comparison between the calculations and the data provides additional evidence that short-range correlations exclusively affect nucleon pairs with a small center-of-mass momentum residing in a relative S state.

The reaction [formula omitted]Li [formula omitted]B and its implications for [formula omitted

The reaction 7 Li (π +,π −) 7 B has been measured at incident pion energies of 30–90 MeV. 7 Li constitutes the lightest target nucleus, where the pionic charge exchange may proceed as a binary reaction to a discrete final state. Like in the Δ-resonance region the observed cross sections are much smaller than expected from the systematics found for heavier nuclei. In analogy to the neutron halo case of 11 Li this cross section suppression is interpreted as evidence for a proton halo in the particle-unstable nucleus 7 B.

Photon-induced two-nucleon knockout reactions to discrete final states

Cross sections and photon asymmetries of the 16O(γ, pn) 14N and 16O(γ, pp) 14C knockout reactions are calculated for transitions to the low-lying discrete final states of the residual nucleus in the photon-energy range between 100 and 400 MeV. Exclusive reactions may represent a test of reaction mechanisms and a promising tool for investigating the dynamics of nucleon pairs in different states. Cross sections and asymmetries for both (γ,pn) and (γ,pp) turn out to be only slightly affected by short-range correlations and dominated by two-body currents. Therefore, two-nucleon knockout reactions induced by real photons appear well suited to investigate the nuclear current and the selectivity of individual transitions to its different components.

Selectivity of the16O(e,e′pp)reaction to discrete final states

Resolution of discrete final states in the $^{16}$O(e,e$'$pp)$^{14}$C reaction may provide an interesting tool to discriminate between contributions from one- and two-body currents in this reaction. This is based on the observation that the $0^+$ ground state and first $2^+$ state of $^{14}$C are reached predominantly by the removal of a $^1S_0$ pair from $^{16}$O in this reaction, whereas other states mostly arise by the removal of a $^3P$ pair. This theoretical prediction has been supported recently by an analysis of the pair momentum distribution of the experimental data. In this paper we present results of reaction calculations performed in a direct knock-out framework where final-state interaction and one- and two-body currents are included. The two-nucleon overlap integrals are obtained from a calculation of the two-proton spectral function of $^{16}$O and include both long-range and short-range correlations. The kinematics chosen in the calculations is relevant for recent experiments at NIKHEF and Mainz.

XANES investigation of chemical states of nitrogen in polyaniline

X-ray absorption and photoelectron spectra of polyaniline were investigated in order to characterize the chemical bonding of its nitrogen atoms. The identification of neutral and charged nitrogen groups is of special interest. X-ray absorption near-edge structure (XANES) at the nitrogen K edge shows characteristic resonance structures reflecting transitions from the N1s core-level into unoccupied states. For comparison N1s X-ray photoelectron spectra were recorded representing the initial state of the N1s −12p( π ∗ σ ∗ ) transitions. Due to the increasing energy differences between the N1s initial state and the unoccupied π and σ states, discrete final states, originating from the different chemical bonding at the nitrogen atom (N, NH +  and NH), were resolved in the N K XANES.

Exclusive (e,e′pp) knockout reactions

Exclusive cross sections of the 16O(e,e′pp) 14C knockout reaction are calculated for transitions to the low-lying discrete final states of the residual nucleus. Short-range correlations and two-body currents, due to the excitation of a Δ resonance in the intermediate state, are included in the calculations. Final-state interactions are taken into account by means of phenomenological spin-dependent optical potentials. Recoil-momentum distributions for transitions to states with different angular momentum exhibit different shapes, which are basically determined by the cm orbital angular momentum of the initial proton pair.

One-nucleon transfer reactions in the12C+12C system at 344.5 MeV

Angular distributions of cross sections have been measured for single nucleon transfer reactions induced by 344.5 MeV12C ions on a12C target. DWBA analyses have been performed for transitions to discrete final states. Spectroscopic information has been derived and is compared with results of other studies.

Gamow-Teller strength in the (p,n) reaction at 136 MeV on 20Ne, 24Mg, and 28Si.

Gamow-Teller (GT) strength distributions were studied in the (p,n) reaction at 136 MeV on the self-conjugate s-d shell nuclei $^{20}\mathrm{Ne}$,${\mathrm{}}^{24}$Mg, and $^{28}\mathrm{Si}$. The measurements were performed in two separate experiments with the beam-swinger neutron time-of-flight facility at the Indiana University Cyclotron Facility. The flight paths were 91 and 131 m, respectively, for the two experiments. The neutrons were detected in large-volume plastic-scintillation detectors. The overall time resolutions were about 825 ps; this provided energy resolutions from 300 to 400 keV. GT strength was identified as \ensuremath{\Delta}l=0 contributions in transitions to discrete final states and also in the background and continuum. The 0\ifmmode^\circ\else\textdegree\fi{}, \ensuremath{\Delta}l=0 cross sections were converted to B(GT) units using a ``universal'' conversion formula calibrated to (p,n) reactions on other even-even s-d shell nuclei. The resulting B(GT) distributions were compared with full s-d shell-model predictions. The distribution for $^{24}\mathrm{Mg}$ is described well, but the distributions for $^{20}\mathrm{Ne}$ and $^{28}\mathrm{Si}$ are described poorly. The total B(GT) strength observed in discrete states (up to 12 MeV of excitation) for each reaction is 65\ifmmode\pm\else\textpm\fi{}10 % of that predicted. If one considers B(GT) strength observed in the continuum above a calculated quasi-free-scattering background, the strength increases to 70--100 % of that predicted. If one considers B(GT) strength in an analysis of the full continuum (up to \ensuremath{\sim}20 MeV), the entire amount predicted may be observed. These results are consistent with that observed in other light nuclei.

Electroproduction of charged pions from light nuclei

Electroproduction of charged pi mesons from light nuclei to discrete final states is studied in a distorted wave impulse approximation. The Blomqvist and Laget pion photoproduction operator is extended to the case of electroproduction by including virtual photons. Gauge invariance is included in the theory by making the Feynman diagrams for the process individually gauge invariant. Conservation of the electroproduction current and isolation of the electron's kinematical aspects are done by introducing modified virtual photon polarization vectors. Cross section calculations for single nucleon electroproduction performed with the real photoproduction parameters of Blomqvist and Laget and standard forms of electromagnetic form factors agree with data. The single nucleon virtual photoproduction operator is used in a distorted wave impulse approximation to describe electroproduction from a nuclear target leading to a discrete final nuclear state. Nuclear transition densities which enter into elastic electron scattering and electroproduction processes are compared. It is found that because of the change of nuclear isospin, electroproduction provides information on a larger number of transition densities than inelastic electron scattering for a given target nucleus. Methods of integrating over the electron's kinematics are presented for single arm experiments, that is, where only the produced pion is detected. Single arm electroproduction cross sections are calculated for 10B and 12C targets. For the 10B case, good agreement with data is found. For the 12C electroproduction cross sections, no conclusions can be reached because the quality of present data is poor. It is found that in these experiments, contributions to the cross section from longitudinal virtual photon components are negligible because of small values of virtual photon invariant mass.

Measurements of discrete nuclear reactions induced by a radioactiveLi8beam

Energy- and angle-resolved nuclear reactions, primarily one-nucleon stripping, have been observed for targets of CD{sub 2}, Be, and C, using a radioactive {sup 8}Li beam ({ital T}{sub 1/2}=842 ms) at {ital E}=14.3 MeV. The ({sup 8}Li,{sup 7}Li) neutron transfer reaction populating discrete final states was identified and angular distributions were obtained. This direct-reaction channel has large cross sections (10--100 mb/sr) due, in part, to the unique nuclear properties of {sup 8}Li. Implications for astrophysical abundances of Li isotopes may be significant.

12,13C(p

The results of a comparative study of the $^{13}(\mathit{p}\ensuremath{\rightarrow}$,${\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}$) and $^{12}(\mathit{p}\ensuremath{\rightarrow}$,${\ensuremath{\pi}}^{+}$) reactions at ${T}_{p}$=200 MeV and 30\ifmmode^\circ\else\textdegree\fi{}\ensuremath{\le}${\ensuremath{\theta}}_{\mathrm{lab}{}^{\ensuremath{\pi}}\mathrm{\ensuremath{\le}}150\mathrm{\ifmmode^\circ\else\textdegree\fi{}}}$ are reported. The isospin-dependence of the NN\ensuremath{\rightarrow}NN\ensuremath{\pi} processes inside nuclei is investigated by comparing cross sections and analyzing powers for the $^{13}(\mathit{p}$,${\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}$) continuum production to those for corresponding NN\ensuremath{\rightarrow}NN\ensuremath{\pi} processes in the free case. The comparison favors the dominance of a quasifree two-nucleon mechanism in nuclear pion production. Relative populations of discrete final states in the $^{13}(\mathit{p}$,${\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}$) spectra are compared, confirming the dominance of the two-nucleon mechanism and suggesting the identification of some high-spin states in the mirror nuclei $^{14}\mathrm{C}$ and $^{14}\mathrm{O}$. Cross-section and analyzing power angular distributions are presented for discrete final states populated strongly in $^{13}(\mathit{p}$,${\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}$) and $^{12}(\mathit{p}$,${\ensuremath{\pi}}^{+}$) up to 24 MeV excitation energy. Cross-section data for a number of $^{12}$,13C(p,${\ensuremath{\pi}}^{+}$) transitions are compared to (p,${\ensuremath{\pi}}^{+}$) shell-model calculations employing the elementary pp\ensuremath{\rightarrow}d${\ensuremath{\pi}}^{+}$ amplitude along with realistic nuclear wave functions. Isolated anomalous features exhibited by strong $^{12}$,13C(p,${\ensuremath{\pi}}^{+}$) transitions to two highly excited discrete states in $^{13}\mathrm{C}$ and $^{14}\mathrm{C}$ are discussed and alternative possible interpretations for these states are presented.

Single-particle properties of 51V and 90Zr studied with the (e,e'p) reaction

Proton spectral functions for the reactions 51V(e, e'p) and 90Zr(e, e'p) have been obtained with the coincidence setup at NIKHEF-K. The coincidence cross sections up to 21 MeV excitation energy were measured in the missing momentum range −40 < Pm < 280 MeV/ c with an absolute precision of 4%. The achieved 130 keV energy resolution allowed to deduce momentum distributions ρ( P m ) for transitions to many discrete final states. A CDWIA analysis of the measured ρ( P m ) yielded rms radii of the single-particle orbitals and spectroscopic factors for the various transitions. The uncertainties in the analysis due to the treatment of the final-state interaction could be reduced by a combined analysis of the present data and elastic proton-scattering data in the same mass and energy region. The extracted spectroscopic factors are compared to those deduced from the (d 3, He) reaction. The strength in the continuum (up to E x = 21 MeV) has been decomposed into various orbital components by using the dependence of ρ nlj( P m) on the values of n and l. Whereas in general the orbit radii and the binding energies are in fair agreement with the results of mean-field calculations, the total spectroscopic strength for valence orbits is only 40–60% of the single-particle sum-rule estimate. For deep-hole states we find 60–80%. These values are about 15% smaller than the predictions of recent mean-field calculations that include correlations. Possible origins of this difference are discussed.

( pi,p) reactions at low excitation energy.

Results of the first experimental search for the excitation of discrete final states following exclusive (..pi../sup -/,p) reactions are reported. The measurements include differential cross section at theta/sub lab/ = 25/sup 0/ for (..pi../sup +- /,p) reactions on /sup 24/Mg, /sup 27/Al, /sup 40/Ca, and /sup 58/Ni at T/sub ..pi../ = 120 MeV and (..pi../sup -/,p) on /sup 12/C at T/sub ..pi../ = 145 MeV. The (..pi../sup -/,p) reactions yield peaks with dsigma/d..cap omega..less than or equal to1 ..mu..b/sr compared to 7--22 ..mu..b/sr for peaks from (..pi../sup +/,p) reactions. The shape of the continuum in an excitation energy range of 10--40 MeV was found to be independent of pion charge and target. The ratios of (..pi../sup +/,p)/(..pi../sup -/,p) averaged over excitation energy and the ratios of the same reaction on different nuclei are presented. The magnitude of the proton yield in the low excitation continuum is more than 20 times larger for ..pi../sup +/ than for ..pi../sup -/, which supports a two-nucleon absorption model including pion charge exchange.

Photoproduction of charged pions to discrete nuclear states

We study the nuclear photoproduction of charged pions to discrete final states at intermediate energies. The model for the nonresonant production amplitude uses the experimental single nucleon multipoles. The resonant production mechanism includes the medium effects of delta-propagation in the framework of the Δ-hole model. This goes beyond the standard DWIA approach and we discuss the importance of these medium modifications. Applications are presented for p-shell nuclei and the results discussed in detail.

18O(p

Angular distributions of the differential cross sections and analyzing powers of the $^{18}\mathrm{O}$(p,${\ensuremath{\pi}}^{\mathrm{\ensuremath{-}}}$) and $^{26}\mathrm{Mg}$(p,${\ensuremath{\pi}}^{\mathrm{\ensuremath{-}}}$) reactions leading to both discrete final states and the continuum have been measured at ${T}_{\mathrm{p}=201}$ MeV. The two reactions exhibit quite similar selectivities and angular distributions. For the $^{18}\mathrm{O}$(p,${\ensuremath{\pi}}^{\mathrm{\ensuremath{-}}}$${)}^{19}$Ne reaction, the strongest transitions are to known high spin states, which are favored by the high momentum transfer of the reaction. These states are presumed to have predominantly two-particle--one-hole configurations with respect to the target nucleus because of the two-nucleon nature of the reaction mechanism. It is conjectured that the states most strongly populated in the $^{26}\mathrm{Mg}$(p,${\ensuremath{\pi}}^{\mathrm{\ensuremath{-}}}$${)}^{27}$Si reaction have a similar structure.

Photo- and electroproduction of pions at intermediate energies to discrete nuclear states

We discuss the electromagnetic production of neutral and charged pions to discrete final states at intermediate energies. Both production and distortion of the outgoing pion are dominated by Δ-excitation. It is shown how pion production can be used to study the effects of the nuclear medium on Δ-propagation.

Photoproduction of neutral pions to discrete nuclear states

We study the photoproduction of neutral pions on nuclei to discrete final states at intermediate energies. Both photoproduction and distortion of the π 0 are dominated by Δ-excitation, which requires a careful treatment of the Δ-nucleus dynamics. We use the Δ-hole approach, which has been used to describe a variety of pion- and photon-induced nuclear reactions. It is shown that π 0 photoproduction is very sensitive to medium effects of Δ-propagation and can therefore be used to investigate the Δ-nucleus interaction. The validity of the distorted-wave impulse approximation for this reaction is examined. Results for low-lying excitations in 12C are compared with the available data.

Electro-pion production on 16O to bou nd states of the residual nuclei

Differential cross sections for the reactions 16 O(γ, π − π + −) 16 F 16 F to the sum of the four lowest lying states in 16F and 16N have been measured as a function of angle for pions with a kinetic energy of 30 MeV. The extracted ratios R = σ(γ, π −) σ(γ, π +) , the first ones to discrete final states as a function of angle, are in fair agreement with results obtained for the nucleon. For positive pions the energy dependence of the cross section has been measured at the angles of 45° and 90°. Distorted wave impulse approximation calculations fail to describe the energy dependence.