[en] The study of the Hadronic Weak Interaction, and particularly of the stangeness- conserving HWI between nucleons, is of interest for several reasons: The fact that the QCD symmetries cannot explain some properties observed in the strangeness-changing HWI opens the possibility for a non-trivial QCD dynamical process that either is related to the presence of the strange quark, or is a more general process that affects light quarks as well; measurement of weak amplitudes in the ∆S = 0 sector could discriminate between the two possibilities. The ∆S = 0 HWI is also perhaps the only via to study weak neutral currents at low energies, and constitutes a probe for quark-quark correlations in nucleons. At low energies, using an Effective Field Theory (EFT) approach, the nucleon-nucleon (NN) HWI can be parameterized in terms of five weak transition amplitudes involving S and P waves. There is a program to determine these weak amplitudes through the measurement of Parity Violating (PV) observables in few-nucleon systems in experiments that make use of low-energy neutron beams at the ORNL Spallation Neutron Source (SNS) and the NIST Center for Neutron Research (NCNR). Some of the experiments of this program will be presented, with emphasis on the NPDGamma experiment, currently measuring at the SNS the PV asymmetry in the spatial distribution of the gamma rays emitted in the ~n + p ! d + γ nuclear reaction (Aϒ ), which occurs through the I = 1 3S1 - 3 P1 transition, dominated by long-range pion exchange. The goal of the experiment is to measure Aϒ to a precision of 10^-8. Preliminary results will be discussed

[en] In order to study the ¹²C+¹²C reaction at low energies, a combination of an intense heavy ion beam on a thick target and the detection of secondary gamma rays on a well-shielded germanium detector has been used. Preliminary cross-section results are reported for E_c.m.=2.63-3.45 MeV. (orig.)

[en] Angular distributions were measured for neutron elastically scattered by ^natPb at 2.9, 3.0, 3.1, 3.2 and 3.4MeV at very forward angles. The mono-energetic neutron flux was produced at the tandem accelerator facility of the National Institute of Nuclear Research (ININ), Mexico, using the Associated Particle Technique (APT) in the D(d, n)³He reaction. Optical model predictions with best potential systematic show a fairly good agreement with our data, and also the need for a better understanding of the neutron scattering process at very small angles from the theoretical point of view. (orig.)

[en] This work presents a review of fast neutron detection, with an emphasis on the use of plastic scintillating materials. We developed neutron detectors both at the Instituto de Fisica, UNAM and at the Tandem Accelerator Laboratory, ININ. We will describe in detail our detectors and their characteristics. We will present some results on the development of a large position-sensitive neutron detector in two dimensions. In particular, we present a phenomenological formula that describes the complex dependence of the amount of light reaching a detector as a function of the position of the light source within the scintillating material. First measurements with a very small number of detectors are presented and show encouraging results.

[en] A sensitive measurement of parity-violating (PV) observables in few-nucleon systems can shed light on our current understanding of the hadronic weak interaction at low momentum transfers. Theoretical models describe the nucleon-nucleon weak interaction at low energies with 6 parameters that need, in principle, to be determined in the same number of independent experiments. In this context, a series of experiments with cold neutrons are being proposed and developed. Particularly, experiments that aim to measure the parity-violating asymmetry in the distribution of the gamma-rays emitted in the capture of polarized neutrons by protons and deuterium, will be discussed in this paper.

[en] A new procedure to obtain an online measurement of the beam intensity for charged particle accelerators, as well as the total charge deposited on a target, using a commercial beam profile monitor (BPM) is described in this paper. This procedure can be an alternative to the use of Faraday cups in experiments where their implementation is not feasible

[en] High precision fundamental neutron physics experiments have been proposed for the intense pulsed spallation neutron beams at JSNS, LANSCE, and SNS to test the standard model and search for new physics. Certain systematic effects in some of these experiments have to be controlled at the few ppb level. The NPD Gamma experiment, a search for the small parity-violating γ-ray asymmetry A_Y in polarized cold neutron capture on parahydrogen, is one example. For the NPD Gamma experiment we developed a radio-frequency resonant spin rotator to reverse the neutron polarization in a 9.5 cm x 9.5 cm pulsed cold neutron beam with high efficiency over a broad cold neutron energy range. The effect of the spin reversal by the rotator on the neutron beam phase space is compared qualitatively to rf neutron spin flippers based on adiabatic fast passage. We discuss the design of the spin rotator and describe two types of transmission-based neutron spin-flip efficiency measurements where the neutron beam was both polarized and analyzed by optically polarized ³He neutron spin filters. The efficiency of the spin rotator was measured at LANSCE to be 98.8 ± 0.5% for neutron energies from 3 to 20 meV over the full phase space of the beam. Systematic effects that the rf spin rotator introduces to the NPD Gamma experiment are considered

[en] The abBA experiment will measure, in the same apparatus, four correlation parameters in the free neutron β-decay: the electron-antineutrino angular correlation (a), the Fierz interference term (6), and the asymmetries, with respect to the neutron spin direction, of the electron (A)and antineutrino (B).The precise determination of these parameters, together with the neutron lifetime, will provide important information about the Standard Model (SM) and will establish constraints for new physics. In this paper we describe the experimental methodology of abBA as well as some of the advances that have been done so far.

[en] We have observed depolarization effects when high intensity cold neutron beams are incident on alkali-metal spin-exchange-polarized ³He cells used as neutron spin filters. This was first observed as a reduction of the maximum attainable ³He polarization and was attributed to a decrease of alkali-metal polarization, which led us to directly measure alkali-metal polarization and spin relaxation over a range of neutron fluxes at Los Alamos Neutron Science Center and Institute Laue-Langevin. The data reveal a new alkali-metal spin-relaxation mechanism that approximately scales as √(φ_n), where φ_n is the neutron capture-flux density incident on the cell. This is consistent with an effect proportional to the concentration of electron-ion pairs but is much larger than expected from earlier work

[en] The NPDGamma collaboration reports results from the first phase of a measurement of the parity violating up-down asymmetry A_γ with respect to the neutron spin direction of γ rays emitted in the reaction n-vector+p→d+γ using the capture of polarized cold neutrons on the protons in a liquid parahydrogen target. One expects parity-odd effects in the hadronic weak interaction between nucleons to be induced by the weak interaction between quarks. A_γ in n-vector+p→d+γ is dominated by a ΔI=1, ³S₁-³P₁ parity-odd transition amplitude in the n-p system. The first phase of the measurement was completed at the Los Alamos Neutron Science Center spallation source (LANSCE), with the result A_γ=[-1.2±2.1 (stat.)±0.2 (sys.)]x10^-7. We also report the first measurement of an upper limit for the parity-allowed left-right asymmetry in this reaction, with the result A_γ,LR=[-1.8±1.9 (stat.)±0.2 (sys.)]x10^-7. In this paper we give a detailed report on the theoretical background, experimental setup, measurements, extraction of parity-odd and parity-allowed asymmetries, analysis of potential systematic effects, and LANSCE results. The asymmetry has an estimated size of 5x10^-8 and the aim of the NPDGamma collaboration is to measure it to 1x10^-8. The second phase of the measurement will be performed at the Spallation Neutron Source at Oak Ridge National Laboratory.