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Recently, the n$^3$He collaboration reported a measurement of the parity-violating (PV) proton directional asymmetry $A_{\mathrm {PV}} = (1.55\pm 0.97~\mathrm {(st\ at)} \pm 0.24~\mathrm {(sys)})\times 10^{-8}$ in the capture reaction of ${}^3$He$(\vec {n},{\mathrm p}){}^3$H at meV incident neutron energies. The result increased the limited inventory of precisely measured and calculable PV observables in few-body systems required to further understand the structure of hadronic weak interaction. In this letter, we report the experimental and theoretical investigation of a parity conserving (PC) asymmetry $A_{\mathrm {PC}}$ in the same reaction (the first ever measured PC observable at meV neutron energies). As a result of S- and P-wave mixing in the reaction, the $A_{\mathrm {PC}}$ is inversely proportional to the neutron wavelength $\lambda$. The experimental value is $(\lambda\times A_{\mathrm {PC}})\equiv\beta= (-1.97 \pm 0.28~\mathrm{(stat)}\pm 0.12~\mathrm{(sys)}) \times 10^{-6}$ Amstrongs. We present results for a theoretical analysis of this reaction by solving the four-body scattering problem within the hyperspherical harmonic method. We find that in the ${}^3$He$(\vec {n},{\mathrm p}){}^3$H reaction, $A_{\mathrm {PC}}$ depends critically on the energy and width of the close $0^-$ resonant state of ${}^4$He, resulting in a large sensitivity to the spin-orbit components of the nucleon-nucleon force and even to the three-nucleon force. The analysis of the accurately measured $A_{\mathrm {PC}}$ and $A_{\mathrm {PV}}$ using the same few-body theoretical models gives essential information needed to interpret the PV asymmetry in the ${}^3$He$(\vec {n}, {\mathrm p}){}^3$H reaction., Comment: 8 pages, 3 figures

We describe the earliest measurements of the DT fusion cross section commissioned by the Manhattan Project, first at Purdue University in 1943 and then at Los Alamos 1945-6 and later, in 1951-2. The Los Alamos measurements led to the realization that a 3/2$^+$ resonance in the DT system enhances the fusion cross section by a factor of one hundred at energies relevant to applications. This was a transformational discovery, making the quest for terrestrial fusion energy possible. The earliest measurements were reasonably accurate given the technology of the time and the scarcity of tritium, and were quickly improved to provide cross section data accurate to just a few percent. We provide a previously-unappreciated insight: that DT fusion was first reported in Ruhlig's 1938 University of Michigan experiment and likely influenced Konopinski in 1942 to suggest its usefulness for thermonuclear technologies. We report on preliminary work to repeat the 1938 measurement, and our simulations of that experiment. We also present some work by Fermi, from his 1945 Los Alamos lectures, showing that he used the S-factor concept about a decade before it was introduced by nuclear astrophysicists., Comment: 10 pages, 8 figures

We report the first precision measurement of the parity-violating asymmetry in the direction of proton emission with respect to the neutron spin, in the reaction $^{3}\mathrm{He}(\mathrm{n},\mathrm{p})^{3}\mathrm{H}$, using the capture of polarized cold neutrons in an unpolarized active $^3\rm{He}$ target. The asymmetry is a result of the weak interaction between nucleons, which remains one of the most poorly understood aspects of electro-weak theory. The measurement provides an important benchmark for modern effective field theory (EFT) calculations. Measurements like this are necessary to determine the spin-isospin structure of the hadronic weak interaction. Our asymmetry result is $A_{PV} = \left( 1.58 \pm 0.97 ~\mathrm{(stat)} \pm 0.24~\mathrm{(sys)}\right)\times10^{-8}$, which has the smallest uncertainty of any parity-violating asymmetry measurement so far.

Significant progress has been made to experimentally determine a complete set of the parity-violating (PV) weak-interaction amplitudes between nucleons. In this paper we describe the design, construction and operation of the n$^3$He experiment that was used to measure the PV asymmetry $A_{\mathrm{PV}}$ in the direction of proton emission in the reaction $\vec{\mathrm{n}} + {^3}\mathrm{He} \rightarrow {^3}\mathrm{H} + \mathrm{p}$, using the capture of polarized cold neutrons in an unpolarized gaseous $^3\mathrm{He}$ target. This asymmetry has was recently calculated \cite{Viviani,Viviani2}, both in the traditional style meson exchange picture, and in effective field theory (EFT), including two-pion exchange. The high precision result (published separately) obtained with the experiment described herein forms an important benchmark for hadronic PV (HPV) theory in few-body systems, where precise calculations are possible. To this day, HPV is still one of the most poorly understood aspects of the electro-weak theory. The calculations estimate the size of the asymmetry to be in the range of $(-9.4 \rightarrow 3.5)\times 10^{-8}$, depending on the framework or model. The small size of the asymmetry and the small overall goal uncertainty of the experiment of $\delta A_{\mathrm{PV}} \simeq 1\times10^{-8}$ places strict requirements on the experiment, especially on the design of the target-detector chamber. In this paper we describe the experimental setup and the measurement methodology as well as the detailed design of the chamber, including results of Garfield++ and Geant4 simulations that form the basis of the chamber design and analysis. We also show data from commissioning and production and define the systematic errors that the chamber contributes to the measured $A_{\mathrm{PV}}$. We give the final uncertainty on the measurement.

An R-matrix model for three-body final states is presented and applied to a recent measurement of the neutron energy spectrum from the T+T->2n+alpha reaction. The calculation includes the n-alpha and n-n interactions in the final state, angular momentum conservation, antisymmetrization, and the interference between different channels. A good fit to the measured spectrum is obtained, where clear evidence for the 5He ground state is observed. The model is also used to predict the alpha-particle spectrum from T+T as well as particle spectra from 3He+3He. The R-matrix approach presented here is very general, and can be adapted to a wide variety of problems with three-body final states., Comment: 31 pages, 17 figures

We describe a new method to treat low-energy scattering problems in few-nucleon systems, and we apply it to the five-body case of neutron-alpha scattering. The method allows precise calculations of low-lying resonances and their widths. We find that a good three-nucleon interaction is crucial to obtain an accurate description of neutron-alpha scattering., Comment: 4 pages, 2 figures, submitted to Physical Review Letters

We report on the spin.dependent neutron scattering length on 3He from a microscopic calculation of p-3H, n-3He, and d-2H scattering employing the Argonne v18 nucleon-nucleon potential with and without additional three-nucleon force. The results and that of a comprehensive R-matrix analysis are compared to a recent measurement. The overall agreement for the scattering lengths is quite good. The imaginary parts of the scattering lengths are very sensitive to the inclusion of three-nucleon forces, whereas the real parts are almost insensitive., Comment: 9 pages, 1 figure

We report on a microscopic calculation of n-3H and p-3He scattering employing the Argonne v_{18} and v_8' nucleon-nucleon potentials with and without additional three-nucleon force. An R-matrix analysis of the p-3He and n-3H scattering data is presented. Comparisons are made for the phase shifts and a selection of measurements in both scattering systems. Differences between our calculation and the R-matrix results or the experimental data can be attributed to only two partial waves (3P0 and 3P2). We find the effect of the Urbana IX and the Texas-Los Alamos three-nucleon forces on the phase shifts to be negligible., Comment: submitted to Phys. Rev. C

The d+N systems are studied in a three-body model, using phenomenological N-N interactions. The scattering matrices are calculated by using the Kohn-Hulthen variational method. Then, they are analytically continued to complex energies and their singularities are localized. We find a virtual state at E=-1.66 MeV in H-3 and a pair of states at E=(-0.42+-i0.52) MeV in He-3 relative to the d+N thresholds, respectively. In addition, we discuss some general aspects and problems of virtual states which may be useful also in the study of other systems, like Li-10., Comment: 4 pages with 1 figure. The postscript file and more information are available at http://hercules.elte.hu/~csoto

Measurements of the transverse polarization coefficient Kyy' for the reaction 3H(p,n)3He are reported for outgoing neutron energies of 1.94, 5.21, and 5.81 MeV. This reaction is important both as a source of polarized neutrons for nuclear physics experiments, and as a test of theoretical descriptions of the nuclear four-body system. Comparison is made to previous measurements, confirming the 3H(p,n)3He reaction can be used as a polarized neutron source with the polarization known to an accuracy of approximately 5%. Comparison to R-matrix theory suggests that the sign of the 3F3 phase-shift parameter is incorrect. Changing the sign of this parameter dramatically improves the agreement between theory and experiment., Comment: 12 pages, RevTeX, 5 eps figures, submitted to Phys. Rev. C

Resonances of certain light nuclei are explored by studying the complex pole structures of the scattering matrices. Among other results we predict the existence of three-neutron and three-proton resonances, a small spin-orbit splitting in the low-lying He-5 and Li-5 states and the nonexistence of the soft dipole resonance in He-6., Comment: Talk given at the XVth International Conference on Few-Body Problems in Physics (22-26 July, 1997, Groningen, The Netherlands). To be published in the conference proceedings in Nucl. Phys. A. 5 pages with 2 figures. The postscript file and more information are available at http://qmc.lanl.gov/~csoto

We study the first excited state of He4 in a microscopic {H3+p,He3+n} cluster model, including H3 and He3 distortions. The phenomenological 1S0 H3+p scattering phase shift is well reproduced. We localize a complex pole of the S-matrix between the H3+p and He3+n thresholds. The corresponding resonance parameters are E_r=93 keV position relative to H3+p, and Gamma=390 keV width. A pole search is also performed in an extended R-matrix method, and a resonance is found with parameters E_r=114 keV and Gamma=392 keV. The R-matrix approach gives several additional poles, some of which may be connected with an enhanced threshold effect., Comment: 13 pages, 2 figures

We report on a consistent, microscopic calculation of the bound and scattering states in the 4He system employing a realistic nucleon-nucleon potential in the framework of the resonating group model (RGM). We present for comparison with these microscopic RGM calculations the results from a charge-independent, Coulomb-corrected R-matrix analysis of all types of data for reactions in the A=4 system. Comparisons are made between the phase shifts, and with a selection of measurements from each reaction, as well as between the resonance spectra obtained from both calculations. In general, the comparisons are favorable, but distinct differences are observed between the RGM calculations and some of the polarisation data. The partial-wave decomposition of the experimental data produced by the R-matrix analysis shows that these differences can be attributed to just a few S-matrix elements, for which inadequate tensor-force strength in the N-N interaction used appears to be responsible., Comment: 53 pages, PS document can also be found at ftp://theorie3.physik.uni-erlangen.de/pub/publications/he4.ps.gz

We study the low-energy 3/2- and 1/2- states of He-5 and Li-5 in a microscopic cluster model. The scattering phase shifts of Bond (alpha+n) and of Schwandt (alpha+p), respectively, are reproduced well. We determine the resonance parameters by localizing the poles of the analytically continued S-matrix at complex energies. Our results differ from conventional R-matrix resonance parameters, which were extracted from experimental data using the definition of a resonance based on the positions and widths of reaction cross section peaks. However, they nicely agree with the results of an extended R-matrix method which works at complex energies., Comment: 9 pages, 1 figure. Final version. Slightly rewritten, results and conclusions unchanged