Depolarization Of Neutrons Research Articles

Determination of the Spatial Resolution in the Case of Imaging Magnetic Fields by Polarized Neutrons

One of the most important parameters characterizing imaging systems (neutrons, X-rays, etc.) is their spatial resolution. In magnetic field imaging, the spatial resolution depends on the (magnetic) resolution of the depolarization of spin-polarized neutrons. This should be realized by different methods, but they all have in common that a spin-polarizing and spin-analyzing system is part of the resolution function. First a simple and useful method for determining the spatial resolution for unpolarized neutrons is presented, and then methods in the case of imaging with polarized neutrons. Spatial resolution in the case of polarized neutron imaging is fundamentally different from ‘classical’ spatial resolution. Because of π-periodicity, the shortest path along which a spin-flip can occur is a measure of ‘magnetic’ spatial resolution. Conversely, the largest detectable magnetic field (B-field) within a given path length is also a measure of magnetic spatial resolution. This refers to the spatial resolution in the flight direction of the neutrons (Δy). The Δx and Δz refers to the spatial resolution in x- or z-direction; however, in these cases a different method must be used. Therefore, two independent methods are used to distinguish longitudinal and lateral spatial resolution, one method to determine the modulation transfer function (MTF) by recording the frequency-dependent fringe contrast of magnetic field images of a coil (longitudinal spatial resolution), and the second method, to observe the fringe displacement at the detector as a function of magnetic motion, provided that the accuracy of the motion is much better than the pixel size (at least half the pixel size) of the detector (lateral spatial resolution). The second method is a convolution of the fringe pattern with the pixel array of the detector.

Magnetic-field uniformity in neutron electric-dipole-moment experiments

Magnetic field uniformity is of the utmost importance in experiments to measure the electric dipole moment of the neutron. A general parametrization of the magnetic field in terms of harmonic polynomial modes is proposed, going beyond the linear-gradients approximation. We review the main undesirable effects of non-uniformities: depolarization of ultracold neutrons, and Larmor frequency shifts of neutrons and mercury atoms. The theoretical predictions for these effects were verified by dedicated measurements with the single-chamber nEDM apparatus installed at the Paul Scherrer Institute.

Losses and depolarization of ultracold neutrons on neutron guide and storage materials

At Institut Laue-Langevin (ILL) and Paul Scherrer Institute (PSI), we have measured the losses and depolarization probabilities of ultracold neutrons on various materials: (i) nickel-molybdenum alloys with weight percentages of 82/18, 85/15, 88/12, 91/9, and 94/6 and natural nickel Ni100, (ii) nickel-vanadium NiV93/7, (iii) copper, and (iv) deuterated polystyrene (dPS). For the different samples, storage-time constants up to $\ensuremath{\sim}460\phantom{\rule{0.16em}{0ex}}\mathrm{s}$ were obtained at room temperature. The corresponding loss parameters for ultracold neutrons, $\ensuremath{\eta}$, varied between $1.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ and $2.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$. All $\ensuremath{\eta}$ values are in agreement with theory except for dPS, where anomalous losses at room temperature were established with four standard deviations. The depolarization probabilities per wall collision $\ensuremath{\beta}$ measured with unprecedented sensitivity varied between $0.7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$ and $9.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$. Our depolarization result for copper differs from other experiments by 4.4 and 15.8 standard deviations. The $\ensuremath{\beta}$ values of the paramagnetic NiMo alloys over molybdenum content show an increase of $\ensuremath{\beta}$ with increasing Mo content. This is in disagreement with expectations from literature. Finally, ferromagnetic behavior of NiMo alloys at room temperature was found for molybdenum contents of 6.5 at.% or less and paramagnetic behavior for more than 8.7 at.%. This may contribute to solving an ambiguity in literature.

Gravitational depolarization of ultracold neutrons: Comparison with data

We compare the expected effects of so-called gravitationally enhanced depolarization of ultracold neutrons to measurements carried out in a spin-precession chamber exposed to a variety of vertical magnetic-field gradients. In particular, we have investigated the dependence upon these field gradients of spin depolarization rates and also of shifts in the measured neutron Larmor precession frequency. We find excellent qualitative agreement, with gravitationally enhanced depolarization accounting for several previously unexplained features in the data.

Gravitationally enhanced depolarization of ultracold neutrons in magnetic-field gradients

Trapped ultracold neutrons (UCN) have for many years been the mainstay of experiments to search for the electric dipole moment (EDM) of the neutron, a critical parameter in constraining scenarios of new physics beyond the Standard Model. Because their energies are so low, UCN preferentially populate the lower region of their physical enclosure, and do not sample uniformly the ambient magnetic field throughout the storage volume. This leads to a substantial increase in the rate of depolarization, as well as to shifts in the measured frequency of the stored neutrons. Consequences for EDM measurements are discussed.

Ultra-Sensitive Depolarization Study of Polarizing CoTi Supermirrors with the Opaque Test Bench

Abstract We have investigated the depolarization of neutrons in the reflection on polarizing supermirrors, using the opaque test bench. It consists of two opaque 3He cells with in-situ adiabatic fast passage flipping of the helium spin. In a beam initially polarized to AP = 99.98%, depolarization of the order of 10−2 is evidenced after a single reflection on a polarizing supermirror. Depolarization could be reduced though not completely suppressed when working at high magnetizing fields (0.82 T). The preliminary data analysis also suggests a correlation between the m-value of the supermirror and depolarization.

Ultracold neutron depolarization in magnetic bottles

We analyze the depolarization of ultracold neutrons confined in a magnetic field configuration similar to those used in existing or proposed magneto-gravitational storage experiments aiming at a precise measurement of the neutron lifetime. We use an extension of the semi-classical Majorana approach as well as an approximate quantum mechanical analysis, both pioneered by Walstrom et al. [Nucl. Instr. Meth. Phys. Res. A 599, 82 (2009)]. In contrast with this previous work we do not restrict the analysis to purely vertical modes of neutron motion. The lateral motion is shown to cause the predominant depolarization loss in a magnetic storage trap. The system studied also allowed us to estimate the depolarization loss suffered by ultracold neutrons totally reflected on a non-magnetic mirror immersed in a magnetic field. This problem is of preeminent importance in polarized neutron decay studies such as the measurement of the asymmetry parameter A using ultracold neutrons, and it may limit the efficiency of ultracold neutron polarizers based on passage through a high magnetic field.

New constraints for CP-violating forces between nucleons in the range 1 micrometer to 1 centimeter

The range of nucleon interaction 10−4 cm–1 cm is interesting because it corresponds to the mass range of an intermediate particle within the so-called “axion window” that is not closed yet by experiment. Depolarization of ultracold neutrons (UCN) during their storage in material traps can be caused by CP-violating pseudo-magnetic precession of the neutron spin in the vicinity of an unpolarized matter surface. Using the experimental limits for UCN depolarization new constraints were set for the product gSgP of the dimensionless scalar and pseudo-scalar constants, and the parameter λPS=ℏ/mPSc, determining the range of the Yukawa-type interaction of the nucleon via a new pseudo-scalar boson (axion-like particle) with mass mPS:gSgPλPS2⩽2.96×10−21[cm2]for 10−3 cm<λPS<1 cm;gSgPλPS2⩽3.9×10−22[cm2]for 10−4 cm<λPS<10−3 cm. The previous limit on gSgP is improved by 4 to 5 orders of magnitude for λPS ranging from 0.1 cm to 1 cm. Prospects of increasing the accuracy in searching for CP-violating pseudo-magnetic precession are considered. Estimates of possible effects of pseudo-magnetic precession in the framework of theoretical models with CP-violation are discussed.

High-pressure neutron scattering over the ages**In memory of Professor J M Besson and Professor J Rossat-Mignod.

Trends in neutron studies at intermediate pressure are considered. They are mostlyassociated with the broadening of the range of methods (elastic, inelastic, small-anglescattering and depolarization of neutrons) and with the creation of necessary equipment. Itis shown that in the studies of pressure-induced and stress-induced transitions, dampingof tunnel modes due to elastic stress, and similar problems, a combination ofquasi-hydrostatic and purely hydrostatic experimental conditions is essential. Tthepossibilities and perspectives for new trends of studies at intermediate pressure areassessed.

Depolarization of ultracold neutrons during their storage in material bottles

The depolarization of ultracold neutrons (UCN) during their storage in traps has been investigated. The neutron spin-flip probability for the materials studied amounts to ∼(1–2)×10 −5 per collision and does not depend on the temperature. The possible connection between the phenomenon of UCN depolarization and that of anomalous losses is discussed.

On the depolarization of ultracold neutrons in traps

Mechanisms of the depolarization of ultracold neutrons in traps reflecting from the trap wall are considered. One is due to neutron spin-flip elastic or quasielastic incoherent scattering from protons of surface hydrogen contaminations. According to the second one, significant depolarization may take place because of a sudden change in the neutron trajectory on reflection from the wall when the neutron moves even at large adiabaticity parameters in nonuniform magnetic field.

Study of matter at high pressure using small angle scattering and depolarization of neutrons

The method of studying matter under high pressure using the measurement of the transmitted neutron beam is proposed. Method is based on the very small angle scattering and depolarization of neutrons combined with the anvil pressure technique. Test experiments have been carried out demonstrating feasibility of the method for studying phase transitions accompanied with the change of density or magnetization and equations of state. The obtainable pressure range of the method is discussed.

Depolarization of UCN stored in material traps

Depolarization of ultra-cold neutrons (UCN) stored in material traps was first observed. The probability of UCN spin flip per reflection depends on the trap material and varies from 7×10 −6 (beryllium) to 10 −4 (glass).

Energy and orientation dependence of neutron depolarization in a large single crystal of ferromagnetic holmium

We report measurements of the depolarization of epithermal neutrons (1.7–59 eV) in magnetic domains of a 2.0-cm-diam cylindrical single crystal of ferromagnetic holmium. The neutrons were polarized by a dynamically polarized proton target and polarization analyzed using a statically polarized rare-earth spin filter. Based on the dependence of the depolarization on neutron energy and crystal orientation, we determined the domains to be laminar, or needlelike, with the long axis slightly deflected from the c crystalline axis and having an average width of 59 μm.

Neutron depolarization in aligned holmium and tests of time-reversal invariance.

The general formalism of quantum mechanics for the depolarization of neutrons in a magnetically anistropic media is reviewed. This formalism is applied to the analysis of recent results on holmium single crystals. Such crystals have been prepared for testing time-reversal invariance in resonance neutron transmission experiments.

Bulk magnetic structure studies using polarized epithermal neutrons

Parity violating effects in the nucleus are normally very small, but in certain compound nuclear resonances such effects are enhanced to several percent, as in the 0.734 eV p-wave resonance in 139La. We have used this enhanced parity violation to measure the depolarization of neutrons traversing a thick single crystal of ferromagnetic holmium. The depolarization was dependent on the relative orientation of the neutron polarization and the holmium c crystalline axis. The effect was explained in terms of neutron precession in an array of highly correlated domains magnetized in the c axis direction. The results show the high degree of correlation of magnetic domains in the single crystal and demonstrate the usefulness of epithermal neutrons in measuring magnetic properties of thick crystals. The use of other parity violating resonances should allow polarization analysis of neutrons over a range of neutron energies.

Small angle scattering of polarized neutrons from an anisotropic superconductor in the mixed state

The spin flip scattering of polarized neutrons by a vortex lattice in a uniaxial superconductor in the intermediate applied field region H c1⩽ H a⪡ H c2 is predicted. We carried out the calculations for different high- T c superconductors to show how this phenomenon can be used to study the uniaxial superconductors. The depolarization of neutrons passing through a superconductors is considered as well.

Depolarization of neutrons in ferromagnetic holmium by means of enhanced nuclear parity violation in 139La.

The depolarization of epithermal neutrons in a thick single crystal of ferromagnetic holmium has been measured and analyzed with a model of neutron precession through a highly correlated array of magnetic domains. The large parity violation in the 0.734-eV p-wave resonance in $^{139}\mathrm{La}$ was used to analyze the neutron polarization, and represents an application of parity violation in nuclear resonances to a measurement in condensed-matter physics.

Polarized neutron scattering of D2O-based magnetic fluids (depolarization)

Magnetite colloidal fluid with a carrier fluid of deuterium oxide (D2O) was prepared to eliminate the incoherent scattering by hydrogen nucleons and the depolarization of cold neutrons was measured. The magnetic fluid was cooled down to 17 K in an external magnetic field of 10 kOe. The sample exhibited a finite residual magnetization Mr, which decreases with increasing temperature from 17 K. With elevating the temperature from 17 to 300 K, the depolarization was measured in three cases in which the directions of the beam, Mr, and polarization are mutually changed. The polarized neutrons transmitting through the sample were depolarized by the magnetic moments in the colloidal particles. The polarizations as a function of the product of Mr and neutron wavelength λ at different temperatures reduce to the same functional form. The model which shows that no interactions exist among the ferrous colloidal particles succeeds partly in explaining the experimental results.

Neutron-spin depolarization in Y9Co7 and Y9(Co0.97Ni0.03)7 (abstract)

We have made new measurements of the spin depolarization of neutrons transmitted through the magnetic superconductor Y9Co7 [B. V. B. Sarkissian, J. Appl. Phys. 53, 8070 (1982) and references therein] and the weak magnet Y9(Co0.97Ni0.03)7 [B. V. B. Sarkissian and J. L. Tholence, J. Appl. Phys. 55, 2025 (1984)]. These measurements were carried out on the IN11 spectrometer at Institut Laue-Langevin (I. L. L.), Grenoble, France. These studies were carried out as a function of temperature and magnetic field using polarized neutrons of wavelength 6.4 Å. The result clearly confirms the existence of sharply differing T-dependence depolarization characteristics for a given field. At the lowest fields (∼2 Oe) the data reveal unexpected behavior in both specimens. No depolarization effect or any prominent feature was observed at either transition of Y9Co7, although some depolarization seems to occur below ∼6.5 K in the Ni-doped specimen in a field of ∼2 Oe. This suggests a behavior similar to that which would occur in systems in which the local magnetization is strongly influenced by the temporal fluctuations of a finite lifetime (analogous to quasilocalized magnetic state). This we associate with the ‘‘hopping’’ process of the c-axis Co atoms along the chains which prohibits magnetic ordering [B. V. B. Sarkissian (these proceedings)]. On the other hand, application of higher fields (&amp;gt;2 Oe) seems to produce stronger effects in both samples. A gradual increase in depolarization is apparent with cooling below ∼6 K in Y9Co7 and ∼6.5 K in the Ni-doped specimen. The depolarization becomes more pronounced in both samples with increasing fields. It is notable however, that in these experiments the depolarization is small and never observed to be complete. We note also that at a given field the depolarization in the Ni-doped specimen exceeds that of Y9Co7 by a sizable factor. The features pertaining the depolarization curves in the presence of small external field (&amp;gt;2 Oe) could be associated to the creation of a large local field which can be ascribed to the stiffening of the spin fluctuations by the applied fields. The stiffening also seems to be enlarged by Ni impurities. There is also a very striking reduction in the depolarization below the superconducting transition temperature Ts (∼2.5 K) of the Y9Co7. This effect weakens with increasing the field. In general this result is consistent with the previously reported depolarization data [B. V. B. Sarkissian, J. Appl. Phys. 53, 8070 (1982)] which have also shown the reduction in the neutron depolarization below Ts in a different specimen and under different conditions. This is an intrinsic effect and could be explained by suppression of the spin density.