Spin-up Neutrons Research Articles

Fermi polaron in low-density spin-polarized neutron matter

We study the properties of a spin-down neutron impurity immersed in a low-density free Fermi gas of spin-up neutrons. In particular, we analyze its energy ($E_\downarrow$), effective mass ($m^*_\downarrow$) and quasiparticle residue ($Z_\downarrow$). Results are compared with those of state-of-the-art quantum Monte Carlo calculations of the attractive Fermi polaron realized in ultracold atomic gases experiments, and with those of previous studies of the neutron polaron. Calculations are performed within the Brueckner--Hartree--Fock approach using the chiral two-body nucleon-nucleon interaction of Entem and Machleidt at N$^3$LO with a 500 MeV cut-off and the Argonne V18 phenomenological potential. Only contributions from the $^1S_0$ partial wave, which is the dominant one in the low-density region considered, are included. Contributions from three-nucleon forces are expected to be irrelevant at these densities and, therefore, are neglected in the calculation. Our results show that for Fermi momenta between $\sim 0.25$ and $\sim 0.45$ fm$^{-1}$ the energy, effective mass and quasiparticle residue of the impurity vary only slightly, respectively, in the ranges $-0.604\,E_F < E_\downarrow < -0.635\,E_F $, $1.300\,m < m^*_\downarrow < 1.085\, m$ and $0.741 <Z_\downarrow< 0.836$ in the case of the chiral interaction, and $-0.621\,E_F < E_\downarrow < -0.643\,E_F $, $1.310\,m < m^*_\downarrow < 1.089\, m$ and $0.739 <Z_\downarrow< 0.832$ when using the Argonne V18 potential. These results are compatible with those derived from ultracold atoms and show that a spin-down neutron impurity in a free Fermi gas of spin-up neutrons with a Fermi momentum in the range $0.25\lesssim k_F \lesssim 0.45$ fm$^{-1}$ exhibits properties very similar to those of an attractive Fermi polaron in the unitary limit.

Low-Density Neutron Matter and the Unitary Limit

We review the properties of neutron matter in the low-density regime. In particular, we revise its ground state energy and the superfluid neutron pairing gap and analyze their evolution from the weak to the strong coupling regime. The calculations of the energy and the pairing gap are performed, respectively, within the Brueckner–Hartree–Fock (BHF) approach of nuclear matter and the Bardeen–Cooper–Schrieffer (BCS) theory using the chiral nucleon-nucleon interaction of Entem and Machleidt at N3LO and the Argonne V18 phenomenological potential. Results for the energy are also shown for a simple Gaussian potential with a strength and range adjusted to reproduce the1S0neutron-neutron scattering length and effective range. Our results are compared with those of quantum Monte Carlo (QMC) calculations for neutron matter and cold atoms. The Tan contact parameter in neutron matter is also calculated, finding a reasonable agreement with experimental data from ultra-cold atoms only at very low densities. We find that low-density neutron matter exhibits a behavior close to that of a Fermi gas at the unitary limit, although, this limit is actually never reached. We also review the properties (energy, effective mass, and quasiparticle residue) of a spin-down neutron impurity immersed in a low-density free Fermi gas of spin-up neutrons already studied by the author in a recent work where it was shown that these properties are very close to those of an attractive Fermi polaron in the unitary limit.

Realizing a (nearly) 100% neutron beam polarization

We report a neutron beam polarization P = [99.9982 ± 0.0024]%, which means an improvement of two orders of magnitude compared to known values. Actually, one can assume this neutron beam to be 100% polarized, because due to the special beam guidance the overlap of spin-up neutrons and spin-down neutrons could be suppressed down to 1.2 × 10−12 [rel. units]. These values are about four orders of magnitude below the detection limit of a monochromatic neutron flux of 108n· cm−2s−1.

POLANO: Wide angle spin analysis using polarizing supermirrors m = 5.5

We report on a wide angle neutron spin analyzer for neutron energies up to 40 meV (λ = 1.4 Å) that was recently realized in collaboration with Tohoku University and KEK, Japan (Ohoyama et al., 2014; Yokoo et al., 2014). Curved polarizing Fe/Si supermirrors with m = 5.5/1.5 (Schanzer et al., 2016) are arranged radially around the sample covering an angular range of 40° if all channels are equipped with supermirrors. The mirrors are saturated by a vertical permanent magnetic field B≃40 mT generated by NdFeB magnets. It was optimized using finite element analysis. The polarization and transmission of the spin filter have been determined at the beamline BOA at the Swiss spallation neutron source (SINQ) to be P=(0.975±0.01) and Tup=T=(0.55±0.017) for spin-up neutrons at λ=3 Å (E=9 meV), respectively. The transmission is excellent considering that approximately one third of the neutrons hit the front faces of the polarizing blades. The experimental results are in good agreement with Monte Carlo simulations using the software package McStas.

Wide-angle transmission analyzer for polarized neutrons using equiangular spirals

Much progress in neutron spectroscopy has been achieved with the invention of direct geometry time-of-flight (TOF) spectrometers, which allow an efficient determination of the scattering function S(Q,E) over large ranges in momentum Q and energy E. For separating (i) magnetic from nuclear and (ii) incoherent from coherent scattering, polarization analysis of the scattered neutrons is often required. Typically, the polarization is analyzed by means of 3He spin filters or by curved magnetized mirrors, which are arranged radially around the sample reflecting the spin-up neutrons while the spin-down neutrons are absorbed. In the following we describe the design of a wide-angle polarization analyzer that uses polarizing mirrors with an equiangular profile in transmission, i.e. the spin-down neutrons are transmitted to the detector while the spin-up neutrons are reflected and finally absorbed. Because of the almost constant angle of reflection the bandwidth is large, i.e. it is given by the ratio QP=mmax∕mmin, where mmax and mmin≃0.68 are the maximum and minimum useful m-values of the polarizing supermirror. For small samples and mmax=5, QP=7.4 is achieved. The proposed design eliminates the significant losses of neutrons due to absorption by the front faces of the polarizing blades of wide-angle analyzers in reflection. Moreover, in transmission, the phase space of the neutrons is barely affected by the mirrors and the polarization is rather constant.

Focusing and imaging of cold neutrons with a permanent magnetic lens.

This paper reports imaging of objects with slow neutrons, specifically very cold neutrons and cold neutrons, at Institut Laue Langevin, using novel, permanent magnet (Nd2Fe14B) compound refractive lenses (MCRL) with a large 2.5 cm bore diameter. The MCRL focuses and images spin-up neutrons and defocuses spin-down neutrons via a large, radial magnetic field gradient. A single lens neutron microscope, composed of an MCRL objective lens with 2-fold magnification, was tested using very cold (slow) neutrons at 45 Å wavelength. One-to-one imaging was obtained using 16.7 Å polarized neutrons. The magnetic field gradient of the MCRL was measured by raster-scanned pencil beams on D33. Finally, a compound neutron microscope was realized using an MCRL condenser lens, which provided increased illumination of objects, and an MCRL as objective lens to produce 3.5-fold magnification.

New compact neutron supermirror transmission polarizer

A new compact neutron supermirror transmission polarizer is suggested. The polarizer consists of a set of plates transparent to neutrons placed in the magnet gap. There are no air gaps between the plates. Polarizing supermirror coating without absorbing underlayer is deposited on the polished surfaces of the plates. Magnetic and nonmagnetic layers of the supermirror coating as well as the material of the plates have nearly equal neutron-optical potentials for spin-down neutrons. There is a considerable difference between neutron-optical potentials of layers in the supermirror structure for spin-up neutrons. As a result, spin-up neutrons reflect from the supermirror coating and deviate from their initial trajectories whereas spin-down neutrons do not practically reflect from the coating and, consequently, do not deviate from their initial trajectories. Thus, spin-down neutrons dominate near the axis of distribution of intensity on the angle for the beam transmitted through this polarizer, i.e., the beam is substantially polarized. Application is discussed of this polarizer in a research facility for small angle scattering of monochromatic neutrons with wavelengths λ = 4.5÷20 Å. The polarizing cross section of the beam of this facility is 30×30 mm2. Calculated parameters are presented of a polarizer on silicon plates with supermirror CoFe/TiZr (m = 2) coating. The suggested polarizer is compared with solid state bender, S-bender and widely known transmission neutron polarizer V- cavity in the same spectral range. Two polarizers are used to cover the wavelength range λ = 4.5 ÷20 Å: the first one whose length is 50 мм covers the range λ = 4.5 ÷10 Å and the second one whose length is 21.2 мм covers the range λ = 10 ÷20 Å. The length of each of these polarizers is more than 30 times smaller than that of V-cavity! On the other hand, basic parameters of the proposed polarizer, polarization of the beam falling on the sample P and transmission coefficient T-of the main spin component, exceed those of V-cavity. T- = 0.8 − 0.9 for both polarizers and for each wavelength range. Polarization P is very high. P is better than -0.99 for wavelength range λ = 12.5 ÷ 20 Å at the beam divergence of 24 mrad.

Periodic magnetic field as a polarized and focusing thermal neutron spectrometer and monochromator

A novel periodic magnetic field (PMF) optic is shown to act as a prism, lens, and polarizer for neutrons and particles with a magnetic dipole moment. The PMF has a two-dimensional field in the axial direction of neutron propagation. The PMF alternating magnetic field polarity provides strong gradients that cause separation of neutrons by wavelength axially and by spin state transversely. The spin-up neutrons exit the PMF with their magnetic spins aligned parallel to the PMF magnetic field, and are deflected upward and line focus at a fixed vertical height, proportional to the PMF period, at a downstream focal distance that increases with neutron energy. The PMF has no attenuation by absorption or scatter, as with material prisms or crystal monochromators. Embodiments of the PMF include neutron spectrometer or monochromator, and applications include neutron small angle scattering, crystallography, residual stress analysis, cross section measurements, and reflectometry. Presented are theory, experimental results, computer simulation, applications of the PMF, and comparison of its performance to Stern-Gerlach gradient devices and compound material and magnetic refractive prisms.

Polarized nuclear matter using a modified density dependent Seyler-Blanchard potential

The binding energy of polarized nuclear matter with excess of neutrons, spin-up neutrons, and spin-up protons contains three symmetry energies: the spin symmetry energy, the isospin symmetry energy, and the spin-isospin symmetry energy. The potential used here for polarized nuclear matter is a modified density-dependent Seyler-Blanchard potential with an explicit dependence on the density of protons with spin-up and -down ${\ensuremath{\rho}}_{p\ensuremath{\uparrow}}$ and ${\ensuremath{\rho}}_{p\ensuremath{\downarrow}},$ also on the neutron density with spin-up and -down ${\ensuremath{\rho}}_{n\ensuremath{\uparrow}}$ and ${\ensuremath{\rho}}_{n\ensuremath{\downarrow}}.$ It is found that the binding energy per particle, pressure, velocity of sound, and entropy agree very well with previous theoretical estimates.

New compact neutron polarizer

A new type of a neutron polarizing bender was developed in co-operation with BENSC and ANSTO. It is based upon bent thin silicon wafers coated on one side with SiFeCo polarizing supermirrors and on the other side with Gd. Initial tests at BENSC in a 300 Oe magnetic field yielded a transmission of spin-up neutrons of about 55% over an angle range of 0.75° and flipping ratios > 30. Subsequent tests at ANSTO at 1200 Oe yielded a transmission of 48% with a flipping ratio > 45.

Inversely polarizing CoCu neutron supermirrors

A new type of a neutron polarizing supermirror was developed which reflects spin-down neutrons, contrary to all hitherto produced polarizing supermirrors which reflect spin-up neutrons. Reflectivities of 80–90% and polarizations up to 96% were achieved in a scattering vector interval from 0.176 to 0.35 nm −1.

Nonstatic Spin-Isospin Order in Light Nuclei

A semimicroscopic model of the nucleus is presented in which spin-isospin order is realized as zero-point one-dimensional oscillations of spin-up protons and spin-down neutrons against spin-down protons and spin-up neutrons. This phase is favored by the one-pion-exchange potential in light deformed nuclei. The model is characterized by the lowering of the energy of the first excited state and enhancement of the $B(M2)$.

M3Suppression in Hartree-Fock Theory

With a zero-range interaction a valence spin-up neutron will not interact with core spin-up neutrons and hence will not deform them; it will deform spin-down neutrons. This introduces a spin-quadrupole correlation. An ideal operator for probing this correlation is the $M3$ operator, since this operator is crudely $E2\ifmmode\times\else\texttimes\fi{}M1$. The suppression of the $M3$ moment in $^{17}\mathrm{O}$ due to this effect, as well as due to the spin dependence of the interaction, is considered.