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Neutron decay from the giant resonance via the B10(e,e'n) reaction
Abstract
The cross sections and angular correlations for neutron decay into various states in the residual nucleus following the B10(e,e'n) reaction have been measured over the excitation energy range of 18-33 MeV at an effective momentum transfer of 0.56 fm-1. In the giant resonance, neutron emission leads to the population of two higher excited states in addition to the ground-state transition: 6.97 MeV 7/2-(n5) and 11.70 MeV 7/2-+12.06 MeV 3/2-(n6,7). This is the first observation of the neutron population of these states. The angular correlations for n0 show a strong forward-backward asymmetry, which suggests interference from a transition with the opposite parity to E1. The angular correlations for n5 and n6,7 have a peak shift of about 50° at lower excitation energy and recover above about 24 and 25 MeV for n5 and n6,7, respectively. Their patterns are considerably different from that for n0. The angular correlations for each transition were fitted with a Legendre polynomial. The longitudinal-transverse interference coefficient C2/A0 is negligible for all populations. For n0 decay, all Legendre coefficients bi are positive, but b2 and b3 for the n5 and n6,7 decays are negative at lower excitation energy, and the latter causes a shift of the forward peak. The negative values may come from the signs of the phase differences of cosdelta21 and cosdelta20. The B10(e,e'n) cross section measured up to Ex~32 MeV agrees well with that of B10(gamma,n), except for a peak at 23 MeV of the giant resonance. In comparison with shell-model calculations, the partial cross section for n0 is sizable up to higher excitation energy, and predicted large partial cross sections populating the 6.97 MeV 7/2- and 11.70 MeV 7/2-+12.06 MeV 3/2- states in the giant resonance were not observed.
This document is part of the Supplement containing the complete sets of data of Subvolume G 'Supplement to I/25 A-F' of Volume 25 'Excited Nuclear States' of Landolt-Börnstein - Group I 'Elementary Particles, Nuclei and Atoms'. It provides properties of excited nuclear states for the isotope B-10 (Boron)
This document is part of the Supplement containing the complete sets of data of Subvolume G 'Supplement to I/25 A-F' of Volume 25 'Excited Nuclear States' of Landolt-Börnstein - Group I 'Elementary Particles, Nuclei and Atoms'. It provides properties of excited nuclear states for the isotope B-9 (Boron)
This document is part of the Supplement containing the complete sets of data of Subvolume A 'Nuclei with Z = 1 - 29' of Volume 25 'Excited Nuclear States' of Landolt-Börnstein - Group I 'Elementary Particles, Nuclei and Atoms'. It provides properties of excited nuclear states for the isotope B-9 (Boron).
This document is part of the Supplement containing the complete sets of data of Subvolume F 'Supplement to I/25 A-E' of Volume 25 'Excited Nuclear States' of Landolt-Börnstein - Group I 'Elementary Particles, Nuclei and Atoms'. It provides properties of excited nuclear states for the isotope B-10 (Boron)
Predictions of coincidence 12C(e, e' x) reactions at low
momentum transfer are presented in a continuum self-consistent RPA
theory with Skyrme forces. SK3 and SKM interactions are used. We
investigate the role of different form factors and multipolarities in
12C(e, e'p 0) and 12C(e, e'n
0) angular distributions at forward and backward electron
kinematics. The incident electron energy is Ei = 126 MeV and
ω = 22.5 MeV is the nuclear excitation energy. The sensitivity of
12C(e, e'p 0) angular correlations in forward
direction to monopole excitations is especially emphasized.
The absolute total photoneutron cross sections of 10B and 11B have been deduced from bremsstrahlung yield curves for the reactions measured from threshold to 28 MeV. Neutron yields were measured using direct neutron counting and were analysed using the variable-bin Penfold-Leiss method. The integrated cross sections from thresholds to 27.6 MeV were found to be 0.36 ± 0.04 for 10B, and 0.37 ± 0.04 for 11B, both given in units of 60 NZ/A. It is found that shell model calculations which have been carried out for these nuclei are inadequate in their description of the detail seen experimentally, particularly in the case of 10B. A discussion of the possibility of isospin splitting in 11B is also given.
Decay neutrons from the [sup 40]Ca([ital e],[ital e][prime][ital n])[sup 39]Ca reaction were studied in the giant resonance region. The cross sections and angular distributions, separated for [ital n][sub 0] and [ital n][sub 1] decays, were obtained for excitation energies between 19 and 27 MeV, at the effective momentum transfer of 0.35 fm[sup [minus]1]. Legendre polynomial coefficients obtained from fitting the data are compared with those from the ([ital e],[ital e][prime][ital p]) reaction. In the energy range 19--21 MeV, the interference coefficients [ital b][sub 1] and [ital b][sub 3] for the ground state transition are in agreement, but the noninterference coefficient [ital b][sub 2] is different. The different behavior of the angular distribution for protons and neutrons may suggest the interference of the decay from a [ital T]=0 quadrupole resonance and the [ital T]=1 giant dipole resonance. A similar tendency was also seen in comparing with the ([ital e],[ital e][prime][ital p][sub 1]) reaction. The Legendre polynomial coefficients for the [ital n][sub 0] decay in the ([ital e],[ital e][prime][ital n]) reaction, transformed to the photon point, agree well with those of the ([gamma],[ital n][sub 0]) reaction. The reduced total cross section is consistent between the ([ital e],[ital e][prime][ital n]) and ([gamma],[ital n]) reactions, but the cross section for ([ital e],[ital e][prime][ital n][sub 0]) is larger than that of ([gamma],[ital n][sub 0]) near the peak of the resonance. The values of the longitudinal-transverse interference term are close to zero in the present region, which is rather small compared with the value near the resonance of the ([ital e],[ital e][prime][ital p]) reaction.
The photoneutron cross section of 10B was measured from threshold to 24.5 MeV with bremsstrahlung from a 30 MeV electron synchrotron using yield scanning techniques. Neutrons were detected by a high efficiency BF3-counter detection system. The cross section shows a split E1-giant resonance region with major maxima at 20.2 and 23.0 MeV and a very minor resonance at 17.0 MeV. Apart from these the cross section is void of structure.
We show by analytical expressions the various solutions for the multipole decomposition of (e,e'a0) angular correlations when longitudinal excitations of three different multipolarities interfere.
In this paper we present experimental details of (e, e' n) measurements and of the performance of an NE213 scintillation detector used in the experiments. The neutron detection system, the n/gamma discrimination techniques, the detection efficiency for low energy neutrons, and typical data obtained for the 12C(e, e' n)11C reaction are described.
The cross sections and angular distributions of (gamma,p) and (gamma,d) reactions on 10B have been measured in the giant resonance region by the (e,p) and (e,d) reactions applying the virtual photon theory. The E2 cross section of the (gamma,p0) reaction derived by a model dependent analysis of the angular distributions shows contributions of 26.5 ± 4.5% to the E2 energy weighted sum rule. The (gamma,p0,2) integrated cross section up to 27 MeV is 6.1 ± 0.2% of the E1 sum rule value which is smaller than 30% for 10B(gamma,n). This suggests that the (gamma,p) reaction leaves high-lying residual states as the prediction of the shell model calculation. The E2 strength deduced from the 10B(gamma,d0) reaction is 0.14 ± 0.05% of the E2 energy weighted sum rule, which is one order of magnitude smaller than the case of neighboring odd-odd nuclei; 6Li and 14N. This may be suppressed by a competition with (gamma,d1).
Angular correlations have been measured for the 16O(e,e'n0) and 16O(e,e'n3) reactions in the energy region omega=20-31 MeV at a momentum transfer of 0.33 fm-1. They indicate dominance of the giant dipole resonance. The data are compared with a random-phase approximation calculation. A predicted backward-peaked angular correlation in the neutron decay, due to interference between the T=0 and T=1 resonances, was not observed. Legendre coefficients extracted from the angular correlation data suggest the existence of a broad E2 resonance over the giant dipole resonance region. The opposite signs of the correlation parameters b1 and b3 for the 16O(e,e'n0) and 16O(e,e'p0) reactions suggest that the E2 resonance below about 22 MeV is isoscalar. Above about 22 MeV, large positive values for b1 and b2 in 16O(e,e'p0) compared to small ones in (e,e'n0) reflect the contribution resulting from the direct-knockout process in (e,e'p0). The angular correlations for 16O(e,e'n3) and 12C(e,e'n0) were confirmed to be similar as expected from the similarity of their particle-hole configurations. The good agreement including the fine structure between the form factors for 16O(e,e'n0) and 16O(gamma,n0) confirmed the dominance of the giant dipole resonance in this energy region.
A model for the description of particle emission after electro-excitation of a nucleus is developed. The coupling between the individual decay channels is treated within the framework of the continuum random-phase approximation. The interference terms between charge and current operators as well as the interference term between the two different transverse components of the current operator are included in the calculation. The model is applied to the proton decay of 16O. The sensitivity of the angular distribution of the emitted particle to the presence of weak multipole strengths is analysed and signatures for monopole strength are discussed. Coincidence experiments are shown to be a selective tool to excite bound states embedded in the continuum. Quasi-free knock-out is found to start immediately above the giant-dipole-resonance region.
Absolute (gamma, xn) cross sections for 10B and 11B were measured up to 35 MeV using a quasi-monoenergetic photon beam from positron annihilation in flight. The experimental cross sections were compared with many-particle shell-model calculations and data from previous bremsstrahlung experiments. The 11B photoneutron cross section is discussed with respect to possible isospin splitting. A systematic review of the photoneutron cross sections of the p-shell nuclei integrated up to about 30 MeV seems to show a strong influence of the alpha-cluster structure of the target nuclei.
The high frequency resonances recently observed for (γ,n)
reactions as well as photo-fission are interpreted in analogy with the
"reststrahl frequencies" of polar crystals. The estimated frequencies
are in good agreement with the experimental results. An interesting
consequence of this interpretation is the conclusion that strong
resonance scattering of γ-rays should take place at a frequency
characteristic of the scattering nucleus.
A general expression for the electron-scattering coincidence cross section for the reaction A1(e, e′ X) A2 with a nuclear target is derived in the one-photon exchange approximation. The result is exact to lowest order in α, the fine-structure constant. It is expressed in terms of four kinematic factors involving the electron scattering variables in the laboratory frame, and four combinations of transition matrix elements of the nuclear current operator expressed in the center-of-momentum (COM) frame. The nuclear matrix elements are decomposed into transition amplitudes of definite angular momentum using a helicity analysis. General expressions for the angular distribution of particle X in the COM frame are then derived. The analysis is independent of the detailed structure of the nucleus and particle X and depends only on general symmetry considerations and the existence of a local electromagnetic current operator for the hadronic target. A unitary transformation from the helicity basis for the final particle X and A2 to an LS coupling basis is relevant if X is massive and a finite number of total angular momenta J are involved in the reaction. Tables of angular correlation coefficients are given for the case where the initial nucleus A1 has J1π = 0⁺. They constitute one of the most useful results of this paper.
The neutron emission cross sections have been measured from targets of Li6, Li7, B10, B11 and O16 when they were irradiated with X-rays in the energy range from 7 to almost 30 MeV. The integrated cross sections are respectively 0.40±0.03, 0.49±04, 0.44±0.32, 0.42±0.03 and 0.26±0.02 in units of the classical dipole sum. The average cross sections for the lithium and boron targets appear to be essentially constant in the energy range 25–30 MeV. The data also suggest that the boron cross sections contain a great deal of structure up to the highest energies studied.