Polarized Cold Neutrons Research Articles

Polarized Cold-neutron Reflectometry at JRR-3/MINE2 for the Development of Ultracold-neutron Spin Analyzers for a Neutron EDM Experiment at TRIUMF

Polarized Cold-neutron Reflectometry at JRR-3/MINE2 for the Development of Ultracold-neutron Spin Analyzers for a Neutron EDM Experiment at TRIUMF

A measurement of parity-violating asymmetry in polarized cold neutron capture on 3He

The n3He experiment measured the parity-violating directional asymmetry in the proton emission direction relative to the initial neutron polarization in the capture of polarized cold neutrons in the reaction n + 3He → T + p. Data taking was completed at the end of 2015 at the SNS, and two independent analyses of the proton parity asymmetry have since been completed and are in agreement. I will present the methods used to calculate the asymmetry, and the final results of the experiment.

First Precision Measurement of the Parity Violating Asymmetry in Cold Neutron Capture on ^{3}He.

We report the first precision measurement of the parity-violating asymmetry in the direction of proton momentum with respect to the neutron spin, in the reaction ^{3}He(n,p)^{3}H, using the capture of polarized cold neutrons in an unpolarized active ^{3}He target. The asymmetry is a result of the weak interaction between nucleons, which remains one of the least well-understood aspects of electroweak theory. The measurement provides an important benchmark for modern effective field theory and potential model calculations. Measurements like this are necessary to determine the spin-isospin structure of the hadronic weak interaction. Our asymmetry result is A_{PV}=[1.55±0.97(stat)±0.24(sys)]×10^{-8}, which has the smallest uncertainty of any hadronic parity-violating asymmetry measurement so far.

A proposed upgrade for the polarized cold neutron spectrometer, D7

The concept for a proposed upgrade to the D7 diffuse scattering spectrometer at the Institut Laue-Langevin, Grenoble, is presented. The upgrade consists of rebuilding the primary spectrometer, substantially relaxing the incident beam divergence to match the divergence that can be accepted by the secondary spectrometer. Monte-Carlo simulations and acceptance diagram calculations have been used to verify the performance of the proposed instrument relative to the current D7. The proposed upgrade is not the final instrument plan, as modifications and compromises will need to be applied for space compatibility with neighbouring instruments. The concept offers an intensity upgrade for a nominal sample of a factor >20. The relative wavelength spread of the incident beam will be reduced by a factor ≈3. The increased divergence will result in a degraded Q-resolution for small scattering angles from the sample, but a reduced resolution at large scattering angles.

Relation of Experimental Features of P-Even, T-Odd Asymmetries in Ternary Nuclear Fission Induced by Cold Polarized Neutrons to Triple and Quintuple Scalar Correlations

In employing the concept of the isotropy of space, the coefficients D of P-even, T-odd asymmetry in the angular distributions of prescission alpha particles emitted as third particles in the ternary fission of nonoriented target nuclei that is induced by polarized cold neutrons are expressed in the lowest orders of perturbation theory in the neutron polarization vector in terms of two P-even scalar coefficients D3 and D5 associated with, respectively, triple and quintuple correlations depending on the unit vectors kα, kLF, and σn, which determine the features of the coefficients being studied. On the basis of the above representation, the experimental values of the coefficients D3 and D5 are determined by using the experimental values of D and the angular distributions of alpha particles emitted in the analogous reaction induced by unpolarized neutrons. The resulting coefficients D3 and D5 are compared with the analogous coefficients found by means of the classical method of trajectory calculations and by means of quantum-mechanical fission theory relying on the concept that it is the rotational mechanism that is responsible for the appearance of the asymmetries being studied. It turns out that the classical method, which disregards the interference between fission amplitudes for different S-wave neutron resonances, leads to an irremovable contradiction between the calculated coefficient D3 and the analogous experimental coefficient for the 233U target nucleus. In the case of employing the quantum-mechanical approach, it is concluded that three-body calculations of the Coriolis interaction—perturbed amplitude of the angular distributions of alpha particles moving in the Coulomb fields of fission fragments are required for the 233U target nucleus.

Decisive Role of Interference Effects in Describing T-Odd Asymmetries in Reactions Induced by Polarized Cold Neutrons

A detailed analysis of T-odd asymmetries in the reactions of true ternary fission of nuclei that involves the emission of prescission alpha particles is performed within quantum-mechanical fission theory. This analysis is based on taking into account the effect of quantum rotation of a fissile compound nucleus formed upon the capture of a polarized cold neutron by an nonoriented target nucleus on angular distributions of both alpha particles and fission fragments. It is shown that the asymmetries in question cannot be described within the approach relying on the use of the classic method of trajectory calculations without taking into account the interference between the fission amplitudes for different neutron resonance states of a fissile compound nucleus that are formed at the initial stage of the reactions being studied.

First Observation of P-odd γ Asymmetry in Polarized Neutron Capture on Hydrogen.

We report the first observation of the parity-violating gamma-ray asymmetry A_{γ}^{np} in neutron-proton capture using polarized cold neutrons incident on a liquid parahydrogen target at the Spallation Neutron Source at Oak Ridge National Laboratory. A_{γ}^{np} isolates the ΔI=1, ^{3}S_{1}→^{3}P_{1} component of the weak nucleon-nucleon interaction, which is dominated by pion exchange and can be directly related to a single coupling constant in either the DDH meson exchange model or pionless effective field theory. We measured A_{γ}^{np}=[-3.0±1.4(stat)±0.2(syst)]×10^{-8}, which implies a DDH weak πNN coupling of h_{π}^{1}=[2.6±1.2(stat)±0.2(syst)]×10^{-7} and a pionless EFT constant of C^{^{3}S_{1}→^{3}P_{1}}/C_{0}=[-7.4±3.5(stat)±0.5(syst)]×10^{-11} MeV^{-1}. We describe the experiment, data analysis, systematic uncertainties, and implications of the result.

On the Relative Signs of “ROT-Effects” in Ternary and Binary Fission of 233U and 235U Nuclei Induced by Polarized Cold Neutrons

Signs of the ROT-effects in ternary fission of 233U and 235U experimentally defined by PNPI group are the same, whereas in binary fission defined by ITEP group are opposite. This contradiction cannot be explained by the errors in the experiments of both groups, since such instrumental effects would be too large not to be noticed. Therefore, it is necessary to find the answer to this problem in the differences of the ternary and binary fission mechanisms.

On the Possibility of Semiclassical Description of Angular Asymmetries in the Polarized-Neutron-Induced Ternary Fission of Nuclei

The possibility of semiclassical description of ROT and TRI effects in the asymmetry of the angular distribution of products originating from the ternary fission of nuclei that is induced by polarized cold neutrons is considered. It is shown that, in the semiclassical approximation, respective experimental results can be explained qualitatively by taking into account Coriolis effects in the emission of a light third particle from a rotating nucleus that undergoes fission.

First observation of P-odd asymmetry of α-particle emission in the [formula omitted] nuclear reaction

We present measurements of P-odd asymmetry of emission of α-particles in the B10(n,α)Li7 nuclear reaction, which are carried out using beams of polarized cold neutrons at Petersburg Nuclear Physics Institute (PNPI, Gatchina, Russia) and Institut Max von Laue – Paul Langevin (ILL, Grenoble, France) nuclear reactors. The α-particle detector is an ionization chamber with insensitive gaseous layer.We measured the P-odd asymmetry coefficient to be equal to αP-oddB10,α=−(11.2±3.4)⋅10−8.

Magnetic field devices for neutron spin transport and manipulation in precise neutron spin rotation measurements

The neutron spin is a critical degree of freedom for many precision measurements using low-energy neutrons. Fundamental symmetries and interactions can be studied using polarized neutrons. Parity-violation (PV) in the hadronic weak interaction and the search for exotic forces that depend on the relative spin and velocity, are two questions of fundamental physics that can be studied via the neutron spin rotations that arise from the interaction of polarized cold neutrons and unpolarized matter. The Neutron Spin Rotation (NSR) collaboration developed a neutron polarimeter, capable of determining neutron spin rotations of the order of 10−7 rad per meter of traversed material. This paper describes two key components of the NSR apparatus, responsible for the transport and manipulation of the spin of the neutrons before and after the target region, which is surrounded by magnetic shielding and where residual magnetic fields need to be below 100μG. These magnetic field devices, called input and output coils, provide the magnetic field for adiabatic transport of the neutron spin in the regions outside the magnetic shielding while producing a sharp nonadiabatic transition of the neutron spin when entering/exiting the low-magnetic-field region. In addition, the coils are self contained, forcing the return magnetic flux into a compact region of space to minimize fringe fields outside. The design of the input and output coils is based on the magnetic scalar potential method.

Status of the NPDGamma experiment

The NPDGamma experiment measures the parity-violating (PV) gamma asymmetry from polarized cold neutrons captured on protons at the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory (ORNL). The (PV) neutron spin asymmetry A γ of photons from polarized cold neutron capture on protons is proportional to the ΔI=1 long range weak meson coupling $h_{\pi }^{1}$ between nucleons in the hadronic weak interaction (HWI). Liquid para-hydrogen production data taking concluded in April 2014 and once the background aluminum asymmetry measurements are complete, the PV asymmetry A γ can be extracted. Preliminary results of the analysis of A γ are presented.

Magnetic systems for wide-aperture neutron polarizers and analyzers

Requirements on the field uniformity in neutron polarizers are analyzed in view of the fact that neutron polarizing coatings have been improved during the past decade. The design of magnetic systems that meet new requirements is optimized by numerical simulations. Magnetic systems for wide-aperture multichannel polarizers and analyzers are represented, including (a) the polarizer to be built at channel 4-4′ of the reactor PIK (Gatchina, Russia) for high-flux experiments with a 100×150mm2 beam of polarized cold neutrons; (b) the fan analyzer covering a 150×100mm2 window of the detector at the Magnetism Reflectometer (SNS, ORNL, USA); (c) the polarizer and (d) the fan analyzer covering a 220×110mm2 window of the detector at the reflectometer NERO, which is transferred to PNPI (Russia) from HZG (Germany). Deviations of the field from the vertical did not exceed 2°. The polarizing efficiency of the analyzer at the Magnetism Reflectometer reached 99%, a record level for wide-aperture supermirror analyzers.

Measurement of the weak nucleon-nucleon interaction by polarized cold neutron capture on protons

The NPDGamma Experiment at the Spallation Neutron Source at Oak Ridge National Laboratory is measuring the parity-odd correlation between the neutron spin and the direction of the emitted photon in the capture of polarized cold neutrons on protons. A parity violating asymmetry from this process is directly related to the strength of the hadronic weak interaction between nucleons. The experiment was run first with heavier nuclear targets to check systematic e ects, false asymmetries, and backgrounds. Since early 2012 the experiment has been collecting data with a 16-liter liquid parahydrogen target. Data taking will continue through 2013 until statistics for a 10 8 asymmetry mea- surement are expected. The experiment performance will be discussed as well as the status of the asymmetry measurements.

The nEDM experiment at the SNS

The nEDM collaboration proposes to measure the neutron electric dipole moment at the Spallation Neutron Source (Oak Ridge National Laboratory). The nEDM is a clear signature of CP violation. According to the Standard Model the nEDM is very small (∼10−31 e cm), but many theories predict much higher values. In the proposed experiment polarized cold neutrons from the SNS would be trapped in liquid helium at a temperature of about 400 mK. The neutron spin would precess in a very uniform magnetic field (H ∼ 30 mG), and the experiment would measure the change in the precession frequency when a very strong electric field (E ∼ 50 kV/cm) is applied. Polarized 3He atoms serve as a co-magnetometer. The goal of the experiment is to measure the nEDM with an accuracy of ∼9 × 10−28 e cm, which is more than an order of magnitude better than existing results.

P-odd, P-even, and T-odd asymmetries in true quaternary fission of nuclei

The coefficients of P-odd, P-even, and T -odd asymmetries for a third and a fourth prescission particle emitted in the true quaternary fission of nuclei that was induced by polarized cold neutrons were studied on the basis of quantum-mechanical fission theory. By using non-evaporation (nonadiabatic) mechanisms of light-particle emission, these coefficients were compared with the analogous coefficients for prescission third particles emitted in the ternary fission of nuclei.

T-odd asymmetries in the angular distributions of evaporative neutrons emitted in the fission reactions of cold polarized neutrons

The structure of T-odd correlations for the evaporation of neutrons in the fission reactions of non-oriented target nuclei by polarized cold neutrons is studied in detail. The rotational angle of the asymptotic direction for the emission of fission fragments in the direction of their flight at the time of the rupture of a fissionable nucleus, associated with the rotation of the axis of symmetry of a polarized fissionable system in the laboratory system of coordinates, is calculated. The coefficients of T-odd asymmetry for evaporative neutrons are calculated.

Determination of the parahydrogen fraction in a liquid hydrogen target using energy-dependent slow neutron transmission

The NPDGamma collaboration is performing a measurement of the very small parity-violating asymmetry in the angular distribution of the 2.2 MeV γ-rays from the capture of polarized cold neutrons on protons (Aγ). The estimated size of Aγ is 5×10−8, and the measured asymmetry is proportional to the neutron polarization upon capture. Since the interaction of polarized neutrons with one of the two hydrogen molecular states (orthohydrogen) can lead to neutron spin-flip scattering, it is essential that the hydrogen in the target is mostly in the molecular state that will not depolarize the neutrons (≥99.8% parahydrogen). For that purpose, in the first stage of the NPDGamma experiment at the Los Alamos Neutron Science Center (LANSCE), we operated a 16-l liquid hydrogen target, which was filled in two different occasions. The parahydrogen fraction in the target was accurately determined in situ by relative neutron transmission measurements. The result of these measurements indicate that the fraction of parahydrogen in equilibrium was 0.9998±0.0002 in the first data taking run and 0.9956±0.0002 in the second. We describe the parahydrogen monitor system, relevant aspects of the hydrogen target, and the procedure to determine the fraction of parahydrogen in the target. Also assuming thermal equilibrium of the target, we extract the scattering cross-section for neutrons on parahydrogen.

T-odd angular correlations in the emission of prompt gamma rays and neutrons in nuclear fission induced by polarized neutrons

Study of the T-odd three-vector correlation in the emission of prompt neutrons from 235U fission by polarized cold neutrons has been continued at the facility MEPHISTO of the FRM II reactor (Technical University of Munich). The sought correlation was not found within experimental error of 2.3 × 10−5. The upper limit for the asymmetry coefficient has been set to |Dn| < 6 × 10−5 at 99% confidence level, whereas for ternary fission correlation coefficient Dα = (170±20) × 10−5. This limit casts doubt on a model that explains the three-vector correlation in ternary fission by the Coriolis mechanism. At the same time, five-vector correlation in the emission of prompt fission neutrons has been measured, which describes the rotation of the fissioning nucleus at the moment it breaks (ROT effect). At the angle 22.5° to the fission axis, the correlation coefficient was found to be (1.57 ± 0.20) × 10−4, while at the angle of 67.5° it is zero within the experimental uncertainty. The existence of ROT effect in the emission of prompt fission neutrons can be explained by the anisotropy of neutron emission in the rest frame of the fragment (fission fragments are aligned with respect to the axis of deformation of the fissioning nucleus), similar to the mechanism of ROT effect in the emission of prompt γ-rays.

Parity-violating asymmetry in the reactions 6Li $(n,\alpha)$ 3H and 10B $(n,\alpha)^{7}$ Li* $\rightarrow$ 7Li $+$ $\gamma$

We present two experiments measuring the parity-violating asymmetry (P-odd) of tritons emission in the reaction 6Li(n,α)3H and γ-quanta emission in the reaction 10B(n,α)7Li* → 7Li + γ with polarized cold neutrons, aiming at estimation of the weak meson-nucleon coupling constants. These experiments started in the PNPI, Gatchina and continued in the ILL, Grenoble. We describe an integral method to measure P-odd asymmetry and the two experiments. The results of our measurements are: \(\alpha{}_{P-odd}^{6Li}=(-8.8\pm2.1)\cdot10^{-8}\) and \(\alpha{}_{P-odd}^{10B}=(0.8\pm3.9)\cdot10^{-8}\).