Neutron-neutron Final-state Interaction Research Articles

Correlated two-neutron emission in the decay of the unbound nucleus <span class="aps-inline-formula"><math><msup><mrow></mrow><mn>26</mn></msup><mi mathvariant="normal">O</mi></math></span>

We study the two-neutron decay of the unbound ${}^{26}\mathrm{O}$ nucleus with a three-body model assuming an inert ${}^{24}\mathrm{O}$ core and two valence neutrons. We first discuss the importance of the neutron-neutron final state interaction in the observed decay energy spectrum. We then show that the energy and and angular distributions for the two emitted neutrons manifest a clear evidence for the strong neutron-neutron correlation in the three-body resonance state. In particular, we find an enhancement of two-neutron emission in back-to-back directions. This is interpreted as a consequence of dineutron correlation, with which the two neutrons are spatially localized before the emission.

Lifetime and Fragment Correlations for the Two-Neutron Decay ofO26Ground State

The structure and decay of 26O are investigated in a three-body 24O+n+n model suitable for studies of the long-lived (including radioactivity time scale) states. We have found extremely strong effects of the subbarrier configuration mixing on the decay width of true 2n emitters due to core recoil and neutron-neutron final state interaction. This effect far exceeds the analogous effect in the true 2p emitters. Our calculations provide reasonably narrow boundaries for the lifetime vs decay energy dependence for the true 2n emission. An upper limit of ∼1 keV for the decay energy of the unbound 26O is inferred based on the recent experimental lifetime value.

Determination of the 1 S 0 neutron-neutron scattering length in the nd breakup reaction at energies in the range E n = 40−60 MeV

The first results are obtained in a kinematically complete experiment devoted to measuring the n + d → p + n + n reaction yield at energies in the range En = 40−60 MeV and various angles of divergence of two neutrons (Δθ = 4°, 6°, and 8°) in the geometry of neutron-neutron final-state interaction. The 1S0 neutron-neutron scattering length ann is determined by comparing the experimental energy dependence of the reaction yield with the results of a simulation in the Watson-Migdal approximation, which depend on ann. For En = 40 MeV and Δθ = 6° (the best statistics in the experiment), the value ann = −17.9 ± 1.0 fm was obtained. A further improvement of the experimental accuracy will make it possible to remove the existing disagreement of the results from different experiments.

STUDY OF THE nd BREAKUP REACTION AT NEUTRON ENERGY OF 40-60 MeV

We report the first results of a kinematically complete experiment on measurement of nd breakup reaction yield at neutron beam RADEX of Institute for Nuclear Research. In the experiment two neutrons are detected in geometry of neutron-neutron final-state interaction which allow one to determine the 1S0 neutron-neutron scattering length ann. Experimental dependence of the reaction yield on relative energy of two secondary neutrons was compared with predictions of Watson-Migdal theory. For ∆Θ=6° and En=40 MeV the value of neutron-neutron scattering length ann = -17.9 ± 1.0 fm is obtained. The further improving of experimental uncertainty will allow one to remove the existing difference of results obtained in various experiments.

Cross section of the 3H(d, 3He)nn reaction at a deuteron beam energy of 37 MeV

Inclusive spectra and differential cross sections of the 3H(d, 3He)nn reaction, measured at Ed = 36.9 MeV are presented. The shape of 3He spectra was reconstructed by modeling amplitudes of the neutron-neutron final state interaction (Watson-Migdal amplitudes), sequential decay via the 4He* resonance (E* = 21.2 MeV, Γ = 0.7 MeV), and their interferences. The model allowed the determination of the angular dependence of the differential cross section of the 3H(d, 3He)nn reaction accompanied by singlet nn-pair production. The results are compared to the supermultiplet potential model of the lightest nuclei interaction.

An experimental setup for studying neutron-neutron final state interaction on the neutron channel of the Moscow Meson Factory

A setup for measuring the singlet nn-scattering length in the n + d → p + n + n reaction is described. It is composed of a neutron hodoscope with an angular aperture of 12° and a scintillation detector for protons escaping at an angle of 90° with the beam direction. The exit angles and the energies of a proton and both neutrons are measured. The neutron energies are measured using the time-of-flight method. At a time resolution of ∼0.6 ns and a flight base of ∼5.5 m, the accuracy in measuring the neutron energy is ∼1% at an energy of ∼15 MeV. The dependence of the reaction yield on the relative energy of two neutrons is investigated. The neutron-neutron final-state interaction manifests itself as a peak in this distribution.

Neutron-deuteron breakup experiment atEn=13MeV: Determination of the1S0neutron-neutron scattering lengthann

We report on results of a kinematically complete neutron-deuteron breakup experiment performed at Triangle Universities Nuclear Laboratory using an ${E}_{n}=13$ MeV incident neutron beam. The ${}^{1}{S}_{0}$ neutron-neutron scattering length ${a}_{\mathit{nn}}$ has been determined for four production angles of the neutron-neutron final-state interaction configuration. The absolute cross-section data were analyzed with rigorous three-nucleon calculations. Our average value of ${a}_{\mathit{nn}}=\ensuremath{-}18.7\ifmmode\pm\else\textpm\fi{}0.7$ fm is in excellent agreement with ${a}_{\mathit{nn}}=\ensuremath{-}18.6\ifmmode\pm\else\textpm\fi{}0.4$ fm obtained from capture experiments of negative pions on deuterons. We also performed a shape analysis of the final-state interaction cross-section enhancements by allowing the normalization of the data to float. From these relative data, we obtained an average value of ${a}_{\mathit{nn}}=\ensuremath{-}18.8\ifmmode\pm\else\textpm\fi{}0.5$ fm, in agreement with the result obtained from the absolute cross-section measurements. Our result deviates from the world average of ${a}_{\mathit{nn}}=\ensuremath{-}16.7\ifmmode\pm\else\textpm\fi{}0.5$ fm determined from previous kinematically complete neutron-deuteron breakup experiments, including the most recent one carried out at Bonn. However, this low value for ${a}_{\mathit{nn}}$ is at variance with theoretical expectation and other experimental information about the sign of charge-symmetry breaking of the nucleon-nucleon interaction. In agreement with theoretical predictions, no evidence was found of significant three-nucleon force effects on the neutron-neutron final-state interaction cross sections.

Measurement of theH2(d,He2)2nreaction atEd=171 MeV and implications for the neutron-neutron scattering length

A measurement of the H-2(d,He-2)(2)n charge-exchange reaction at E-d=171 MeV has been performed in order to study the interaction of the di-neutron system. Spectroscopy of the neutron-neutron final-state interaction with a resolution of about 115 keV (FWHM) was achieved by measuring the momentum of He-2 at forward scattering angles. In the experiment the two protons of the unbound He-2 are momentum analyzed with a magnetic spectrometer and detected with the same detector. A simple reaction theory is employed in order to obtain information about the neutron-neutron interaction from the measured cross section data. The validity of the impulse approximation for the d ->He-2 transition at intermediate energies is discussed and compared to the Watson-Migdal theory of the proton-proton final-state interaction. Predictions of the impulse approximation for the analog transition d ->(2)n are confronted with experimental data. A procedure to extract the neutron-neutron scattering length a(nn) from this comparison is presented. The result is compatible with the recommended value of a(nn)=-18.6(+/- 0.4) fm.

New investigation of the neutron-neutron and neutron-proton final-state interaction in then-dbreakup reaction

The neutron-neutron final-state interaction (FSI) has been investigated in the ${}^{2}\mathrm{H}(n,np)n$ reaction at 25.3 and 16.6 MeV, detecting neutrons and protons in coincidence in a geometry which should enable a practically model-independent determination of the ${}^{1}{S}_{0}$ neutron-neutron scattering length ${a}_{\mathrm{nn}}.$ The analysis was performed by means of detailed Monte Carlo simulations based on rigorous three-body calculations with realistic nucleon-nucleon potentials. At 25.3 MeV, the value of ${a}_{\mathrm{nn}}$ deduced from the absolute cross section in the FSI peak is $\ensuremath{-}16.3\ifmmode\pm\else\textpm\fi{}0.4\mathrm{fm}$ while the relative cross section, normalized in the region of neutron-proton quasifree scattering, gives $\ensuremath{-}16.1\ifmmode\pm\else\textpm\fi{}0.4\mathrm{fm}.$ The relative data obtained at 16.6 MeV yielded ${a}_{\mathrm{nn}}=\ensuremath{-}16.2\ifmmode\pm\else\textpm\fi{}0.3\mathrm{fm}.$ In addition, the ${}^{1}{S}_{0}$ neutron-proton scattering length was measured at 25.2 MeV in the same configuration for comparison. While our results for ${a}_{\mathrm{nn}}$ are incompatible with those of a similar investigation performed recently at 13 MeV where the two neutrons were detected, both results for ${a}_{\mathrm{np}}$ are in good agreement with the accurately known value from free n-p scattering.

Experimental and theoretical investigation of the 2H(n, nnp) reaction and of the neutron-neutron scattering length

An experimental and theoretical investigation of the neutron-deuteron break-up reaction is given. The kinematically overdetermined experiment was performed at an incident-neutron laboratory energy of 10.3 MeV at the multidetector neutron facility of the Erlangen Tandem Laboratory. Calculations based on AGS equations using a charge-dependent modification of the Paris potential were done within the W-matrix formalism. For some configurations, experimental data were corrected by folding the point-geometry calculations with the experimental resolutions. On the whole, the comparison between experiment and calculation shows good agreement, however, some discrepancies persist. From an analysis of the neutron-neutron final-state interaction peak at 13 MeV, a value of a nn = (− 17.0 ± 1.0) fm is extracted for the neutron-neutron scattering length.

Effect of neutron-neutron final state interaction in pion photoproduction by deuterons near threshold energies

We analyze the role of the n-n final state interaction in ..gamma..d..--> pi../sup +/ nn at threshold energies by taking the n-n amplitude which includes the hard core term. The effect of the two-body off-c.m. correction to the amplitude in the final three particle phase space is studied. A simple analytic expression for the cross section, which depicts the correct energy dependence, is obtained.

Measurement of the proton spectrum from theH2(n,p)2nreaction at 13.98 MeV and the neutron-neutron scattering length

The spectrum of protons from the $^{2}\mathrm{H}(n,p)2n$ reaction at ${E}_{n}=13.98$ MeV has been measured at ${\ensuremath{\theta}}_{p}={1.6}^{\ensuremath{\circ}}$ with a magnetic-quadrupole-doublet charged-particle spectrometer. The experimental resolution of 200 keV is more than a factor of 2 improvement over that of previous measurements. The high energy peak in the proton spectrum due to the neutron-neutron final state interaction has been analyzed to yield a value of the neutron-neutron scattering length ${a}_{\mathrm{nn}}$. Comparison of these data with an impulse approximation calculation and with exact three-body calculations indicates a value of ${a}_{\mathrm{nn}}$ which is significantly more negative than the accepted value of ${a}_{\mathrm{nn}}=\ensuremath{-}16.6\ifmmode\pm\else\textpm\fi{}0.5$ fm.[NUCLEAR REACTIONS $^{2}\mathrm{H}(n,p)2n$, $E=13.98$ MeV, ${\overline{\ensuremath{\theta}}}_{p}={1.6}^{\ensuremath{\circ}}$; measured $\ensuremath{\sigma}({E}_{p})$; deduced $n\ensuremath{-}n$ scattering length.]

Neutron-neutron final-state interaction in the reaction 2H(n, 2n)p and the 1S 0 scattering length

Using a double time-of-flight spectrometer, a correlation spectrum of the two neutrons emitted at θ 1 = 17.8° and θ 2 = 11.0° has been measured recording also the energy of the recoil proton. The results of this kinematically complete experiment have been compared by means of the Monte Carlo technique to theoretical predictions of exact three-body calculations (Amado model (AM)) and the Watson-Migdal (WM) approximation. The best fit resulted in a neutron-neutron scattering length a nn(AM) = −16.0 ± 1.2 fm. The WM result a nn(WM) = −16.8 ± 1.3 fm is essential for confirming the estimate of the theoretical uncertainty. The absolute peak-area ratio expt./AM was 1.07 ± 20%.

Final-state-interaction effects in neutron pair emission from pion capture on 12C

Energy spectra of neutron pairs from the 12 C(π −, 2 n) 10 B reaction have been measured. The results disagree with calculations which include the neutron-neutron final state interaction, and are in better agreement with calculations which include no final-state interactions at all.

The influence of the neutron-neutron final-state interaction on the capture of muons by deuterons

The influence of the neutron-neutron final-state interaction on the μ capture in deuterium is calculated. It is shown that this process allows from the theoretical point of view a determination of the neutron-neutron scattering length with an accuracy of about 5%.