Search

We report on first experimental tests of a neutron magnetic spin resonator at a very cold neutron beam port of the high flux reactor at the ILL Grenoble. When placed between two supermirror neutron polarizers and operated in a pulsed traveling-wave mode it allows to decouple its time- and wavelength-resolution and can therefore be used simultaneously as electronically tunable monochromator and fast beam chopper. As a first ‘real’ scientific application we intend its implementation in the PERC (p roton and e lectron r adiation c hannel) project related to high-precision experiments in neutron beta decay.

In the second part of our presentations we discuss the results of diffraction experiments from nanodiamond-polymer composite gratings performed with slow neutrons, i.e., at wavelengths longer than a nanometer. One goal of our investigations is to develop flexible, handy, low cost neutron diffractive optical elements (gratings) which can be tailored to serve as mirrors, two-port beamsplitters, multiport beamsplitters, polarizing beamsplitters or even can be assembled to form an interferometer. Basically three adjustable multiplicative parameters are decisive for the performance, i.e., the reflectivity or diffraction efficiency, of such gratings: the wavelength of the neutrons, the thickness and the neutron scattering density modulation of the grating. While the _rst is solely determined by the application one has in mind, the second parameter can be adjusted during the production process of the sample or by tilting the grating about an axis parallel to the grating vector, thus increasing the effective thickness. The third parameter, however, can only be tuned via the production process of the gratings but offers an enormous flexibility due to a variety of nanoparticles and the polymeric host materials at hand. For neutrons the important criteria are to design gratings having high coherent scattering length density modulation while avoiding incoherent scattering and absorption at the same time. For interferometric purposes an ideal grating will have high reflectivity and low angular selectivity. Here, we show first results obtained with nanodiamond-polymer composite gratings and outline potential ways to improvements.

We demonstrate for the first time the use of nanodiamond in constructing holographic nanoparticle-polymer composite transmission gratings with large saturated refractive index modulation amplitudes at both optical and slow-neutron wavelengths, resulting in efficient control of light and slow-neutron beams. This is so because nanodiamond possesses a high refractive index at optical wavelengths and large coherent and small incoherent scattering cross sections with low absorption at slow-neutron wavelengths. In the first part of the work we describe the synthesis of nanodiamond, the preparation of photopolymerizable nanodiamond-polymer composite films, the construction of transmission gratings in nanodiamond-polymer composite films and light optical diffraction experiments. Experiments of slow-neutron diffraction from such gratings will be described in the second part.

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 A wavelength. One-to-one imaging was obtained using 16.7 A 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.

We investigate the applicability of polymer-ionic liquid composites as optical elements for light, as well as for slow neutrons. The gratings are recorded using two-beam mixing and are characterized experimentally based on their diffraction properties. We produced a set of samples differing in their thickness, ranging from 10 m &ndash, 100 m . We demonstrate that it is possible to prepare transmission gratings with a lattice constant of &Lambda, = 480 n m , resulting in thick gratings for light, as well as neutrons. The presented samples show low optical losses in the Vis-UV spectrum and exhibit refractive index modulations of about 10 - 3 at &lambda, = 543 n m . However, further improvements have to be made to obtain efficient neutron optical components.

Ultracold neutrons (UCNs) are formidable probes in precision tests of gravity. With their negligible electric charge, dielectric moment, and polarizability they naturally evade some of the problems plaguing gravity experiments with atomic or macroscopic test bodies. Taking advantage of this fact, the qBounce collaboration has developed a technique - gravity resonance spectroscopy (GRS) - to study bound quantum states of UCN in the gravity field of the Earth. This technique is used as a high-precision tool to search for hypothetical Non-Newtonian gravity on the micrometer scale. In the present article, we describe the recently commissioned Ramsey-type GRS setup, give an unambiguous proof of principle, and discuss possible measurements that will be performed., Comment: accepted by J. Phys.: Conf. Ser. (2019), Proceedings of PPNS 2018

Neutrons are the ideal probes to test gravity at short distances – electrically neutral and only hardly polarizable. Furthermore, very slow, so-called ultracold neutrons form bound quantum states in the gravity potential of the Earth. This allows combining gravity experiments at short distances with powerful resonance spectroscopy techniques, as well as tests of the interplay between gravity and quantum mechanics. In the last decade, the qBounce collaboration has been performing several measurement campaigns at the ultracold and very cold neutron facility PF2 at the Institut Laue-Langevin. A new spectroscopy technique, Gravity Resonance Spectroscopy, was developed. The results were applied to test various Dark Energy and Dark Matter scenarios in the lab, like Axions, Chameleons and Symmetrons. This article reviews Gravity Resonance Spectroscopy, explains its key technology and summarizes the results obtained during the past decade.

Position-sensitive detection of ultracold neutrons (UCNs) is demonstrated using an imaging charge-coupled device (CCD) camera. A spatial resolution less than 15 $\mu$m has been achieved, which is equivalent to an UCN energy resolution below 2 pico-electron-volts through the relation $\delta E = m_0g \delta x$. Here, the symbols $\delta E$, $\delta x$, $m_0$ and $g$ are the energy resolution, the spatial resolution, the neutron rest mass and the gravitational acceleration, respectively. A multilayer surface convertor described previously is used to capture UCNs and then emits visible light for CCD imaging. Particle identification and noise rejection are discussed through the use of light intensity profile analysis. This method allows different types of UCN spectroscopy and other applications., Comment: 12 figures, 28 pages, accepted for publication in NIMA

Fornal and Grinstein recently proposed that the discrepancy between two different methods of neutron lifetime measurements, the beam and bottle methods, can be explained by a previously unobserved dark matter decay mode, $n\ensuremath{\rightarrow}X+\ensuremath{\gamma}$. We perform a search for this decay mode over the allowed range of energies of the monoenergetic $\ensuremath{\gamma}$ ray for $X$ to be dark matter. A Compton-suppressed high-purity germanium detector is used to identify $\ensuremath{\gamma}$ rays from neutron decay in a nickel-phosphorous-coated stainless-steel bottle. A combination of Monte Carlo and radioactive source calibrations is used to determine the absolute efficiency for detecting $\ensuremath{\gamma}$ rays arising from the dark matter decay mode. We exclude the possibility of a sufficiently strong branch to explain the lifetime discrepancy with 97% confidence.

EPJ Web of Conferences, 219, ISSN:2100-014X, ISSN:2101-6275, International Workshop on Particle Physics at Neutron Sources (PPNS 2018), Grenoble, France, May 24–26, 2018, ISBN:978-2-7598-9082-8

We demonstrate that the observation of neutron stars with masses greater than one solar mass places severe demands on any exotic neutron decay mode that could explain the discrepancy between beam and bottle measurements of the neutron lifetime. If the neutron can decay to a stable, feebly-interacting dark fermion, the maximum possible mass of a neutron star is 0.7 solar masses, while all well-measured neutron star masses exceed one solar mass. The survival of $2 M_\odot$ neutron stars therefore indicates that any explanation beyond the Standard Model for the neutron lifetime puzzle requires dark matter to be part of a multi-particle dark sector with highly constrained interactions., 4 pages, 7 figures. v2: relativistic corrections added, results shown for wider range of dark fermion masses, conclusions unchanged

The ultracold neutron (UCN) source at Los Alamos National Laboratory (LANL), which uses solid deuterium as the UCN converter and is driven by accelerator spallation neutrons, has been successfully operated for over 10 years, providing UCN to various experiments, as the first production UCN source based on the superthermal process. It has recently undergone a major upgrade. This paper describes the design and performance of the upgraded LANL UCN source. Measurements of the cold neutron spectrum and UCN density are presented and compared to Monte Carlo predictions. The source is shown to perform as modeled. The UCN density measured at the exit of the biological shield was $184(32)$ UCN/cm$^3$, a four-fold increase from the highest previously reported. The polarized UCN density stored in an external chamber was measured to be $39(7)$ UCN/cm$^3$, which is sufficient to perform an experiment to search for the nonzero neutron electric dipole moment with a one-standard-deviation sensitivity of $\sigma(d_n) = 3\times 10^{-27}$ $e\cdot$cm., Comment: 6 pages, 6 figures

The standard model of cosmology provides a robust description of the evolution of the universe. Nevertheless, the small magnitude of the vacuum energy is troubling from a theoretical point of view. An appealing resolution to this problem is to introduce additional scalar fields. However, these have so far escaped experimental detection, suggesting some kind of screening mechanism may be at play. Although extensive exclusion regions in parameter space have been established for one screening candidate - chameleon fields - another natural screening mechanism based on spontaneous symmetry breaking has also been proposed, in the form of symmetrons 11. Such fields would change the energy of quantum states of ultra-cold neutrons in the gravitational potential of the earth. Here we demonstrate a spectroscopic approach based on the Rabi resonance method that probes these quantum states with a resolution of E=2 x 10^(-15) eV. This allows us to exclude the symmetron as the origin of Dark Energy for a large volume of the three-dimensional parameter space., Comment: 13 pages, 4 figures

We report on the evaluation of commercial electroless nickel phosphorus (NiP) coatings for ultracold neutron (UCN) transport and storage. The material potential of 50~$\mu$m thick NiP coatings on stainless steel and aluminum substrates was measured to be $V_F = 213(5.2)$~neV using the time-of-flight spectrometer ASTERIX at the Lujan Center. The loss per bounce probability was measured in pinhole bottling experiments carried out at ultracold neutron sources at Los Alamos Neutron Science Center and the Institut Laue-Langevin. For these tests a new guide coupling design was used to minimize gaps between the guide sections. The observed UCN loss in the bottle was interpreted in terms of an energy independent effective loss per bounce, which is the appropriate model when gaps in the system and upscattering are the dominate loss mechanisms, yielding a loss per bounce of $1.3(1) \times 10^{-4}$. We also present a detailed discussion of the pinhole bottling methodology and an energy dependent analysis of the experimental results., Comment: 13 pages, 14 figures

Testing basic principles of particle physics does not necessarily require a high-energy accelerator. As Peter Geltenbort explains, a growing community of neutron scientists is probing similarly fundamental problems at the opposite end of the energy scale.

We propose a basic formula and demonstration for a high-resolution quasi-elastic neutron scattering (QENS) by combining the time-of-flight (TOF) method with Modulation of Intensity by Zero Effort (MIEZE) type neutron spin echo spectroscopy. The MIEZE technique has the potential to develop a unique approach to study on slow dynamics of condensed matter; however, the energy resolution is limited owing to the hypersensitivity of the MIEZE signal contrast to the echo condition, which is strongly affected by the alignment of the instruments and the sample. The narrow allowance of the optimal alignment is a major obstacle to the wide use of this technique. Combining the TOF method with MIEZE (TOF-MIEZE), the hypersensitivity of MIEZE signals is significantly alleviated with a short pulsed beam. This robustness is very useful to optimize experimental alignments and enables accurate measurements of QENS. The experimental results demonstrate the characteristic of the TOF-MIEZE technique and are well described by the formula presented in this study.

We discuss the analysis of neutron rocking curves measured with holographic nanoparticle-polymer composite gratings with SANS instruments and at very-cold neutron beamlines. In particular, the effects of broad incident wavelength-distributions and of nonuniformity of the refractive-index profile along the grating depth are assessed.

The emission probabilities and the energy distributions of tritons, α and 6He particles emitted in the spontaneous ternary fission (zero excitation energy) of 250Cf and 252Cf and in the cold neutron induced fission (excitation energy ≈ 6.5 MeV ) of 249Cf and 251Cf are determined. The particle identification was done with suited Δ E – E telescope detectors, at the IRMM (Geel, Belgium) for the spontaneous fission and at the ILL (Grenoble, France) for the neutron induced fission measurements. Hence particle emission characteristics of the fissioning systems 250Cf and 252Cf are obtained at zero and at about 6.5 MeV excitation energies. While the triton emission probability is hardly influenced by the excitation energy, the 4He and 6He emission probability in spontaneous fission is higher than for neutron induced fission. This can be explained by the strong influence of the cluster preformation probability on the ternary particle emission probability.

Modulating permanent magnet sextupole lens (PMSx) for focusing pulsed cold neutrons is under development. The synchronized modulation of its field gradient suppresses the chromatic aberration which arises from the Time Of Flight method. The strength of the magnetic field, the torque, and the rise of temperature during its operation are studied on a fabricated prototype. Experiments on focusing pulsed very cold neutrons (VCN) at ILL (Institute of Laue Langevin, France) were carried out and VCN with around λ =40 A were focused by the PMSx at a focal length of about 0.5 m. The experimental results are presented in conjunction with the principle of the neutron focusing and the modulating method of the focal strength of permanent magnet lens with the double ring structure.

We have fabricated ultracold neutron detectors that consist of silicon charged particle detectors coupled with thin nickel foils coated with either natural LiF or 10 B implanted into vanadium. The foils convert neutrons into energetic, readily detectable, charged particles which are in turn detected by silicon detectors. The detectors were tested at the Institut Laue-Langevin with a gravitational spectrometer. From a rigorous Monte Carlo simulation of the experiment, the minimum detection cutoff velocities (effective potentials) were determined to be 309 � 17 cm=s (49:8 � 2:7 neV) for LiF and 367 � 39 cm=s (70:3 � 7:5 neV) for 10 B/V. Although the result for LiF is consistent with expectations, the result for 10 B/V is significantly higher. We interpret this discrepancy as due to contamination. We also show that while a thicker foil is more efficient for ultracold neutron detection, a thinner foil is more ideal for determining the cutoff velocity. Published by Elsevier B.V.

The emission probabilities and the energy distributions of tritons and α particles emitted in the spontaneous ternary fission (zero excitation energy) of 244Cm, 246Cm and 248Cm and in the cold neutron induced fission (excitation energy ≈ 6.5 MeV ) of 243Cm, 245Cm and 247Cm are determined. The particle identification was done with suited Δ E − E telescope detectors, at the IRMM (Geel, Belgium) for the spontaneous fission and at the ILL (Grenoble, France) for the neutron induced fission measurements. Hence particle emission characteristics of the fissioning systems 244Cm, 246Cm and 248Cm are obtained at zero excitation energy and at an excitation energy around 6.5 MeV. Whilst the triton emission probability is hardly influenced by the excitation energy, the 4He emission probability in spontaneous fission is about 20% higher than for neutron induced fission. This could be explained by the influence of the cluster preformation probability on the ternary α emission.

The Fermi potential Vf of diamond-like carbon (DLC) coatings produced with laser-controlled vacuum arc deposition and that of diamond, Al, Si, Be, Cu, Fe and Ni was measured using two different methods, (i) transmission of slow neutrons through foils in a time-of-flight experiment and (ii) cold neutron reflectometry (CNR). For diamond-like carbon in transmission we obtain Vf = (249 ± 14) neV. This is approximately the same as for beryllium and consistent with the theoretical expectations for the measured diamond (sp3) content of 45%. For an sp3-content of 67%, we find Vf = (271 ± 13) neV from reflectometry, again in agreement with theory. These findings open new perspectives in using DLC as storage volume and neutron guide coatings for ultracold neutron sources.

We present scattering cross sections $\sigma_\text{scatt}$ of ultracold neutrons (UCN) in liquid deuterium at T = 20.6 K, as recently measured by means of a transmission experiment. The indispensable thorough raw data treatment procedure is explained. A calculation model for coherent and incoherent scattering in liquid deuterium in the hydrodynamic limit based on appropriate physical concepts is provided and shown to fit the data well. The applicability of the incoherent approximation for UCN scattering in liquid deuterium was tested and found to deliver acceptable results., Comment: 10 pages, 7 figures

To complete our study of ultracold neutron (UCN) storage-vessel coatings, we have measured the Fermi potential for neutrons on diamondlike carbon coatings produced by laser induced vacuum arc deposition. A sample with an sp3 content of 0.45, measured using, for the first time, neutron transmission had a Fermi potential of ( 249 ± 14 ) neV . A second sample with an sp3 fraction of 0.67, measured using cold neutron reflectometry, gave ( 271 ± 13 ) neV . These values complete the demonstration that there is a viable alternative to Be in UCN physics.

For the first time, ultracold neutrons have been stored in a volume formed of foils coated with diamond-like carbon. The coefficients for loss, η, and depolarization, β, per wall collision were determined simultaneously. The results for different samples, η ∼ 10 −4 and β ∼ 10 −5 – 10 −6 , are among the best values measured for high Fermi potential materials without elaborate surface pre-treatment. Thin foil storage material will be widely applicable, e.g., in neutron β-decay studies.

Solid deuterium (sD 2 ) is a favourite converter material for the production of ultra-cold neutrons (UCN) in the next generation of UCN sources. In order to investigate sD 2 properties relevant for this purpose, an apparatus for the preparation and characterization of ortho-D 2 crystals has been built. It will be used for measurements of neutron transmission and scattering cross-sections as well as irradiation experiments with simultaneous optical control of the target conditions. The present paper describes the main components of the apparatus, i.e. the cryogenic optical D 2 cell, the gas system, the optical photography system and the Raman spectroscopy system.

Charged particles emitted in ternary fission appear to have a Gaussian-shaped energy distribution, except for the α particles, for which mostly an important non-Gaussian low-energy tailing is reported. We realised good experimental conditions for studying the 235 U(n th , f) ternary α energy distribution at the reactor of the ILL in Grenoble. Thanks to an intense and clean neutron beam, a small, thin sample of highly enriched U could be used, with an activity of only 1.6 Bq, so the measurements could be done without absorber in between the sample and the Δ E – E detector. With the resulting low detection limit of 6 MeV, a clearly asymmetric energy distribution was obtained, deviating by 6% from a Gaussian shape. This energy distribution has been deconvoluted into components corresponding to “true” ternary α 's and α 's originating from the decay of 5 He particles. Ternary α emission yield data have been re-evaluated accordingly.

Ultra-cold neutrons (UCN), neutrons with energies low enough to be confined by the Fermi potential in material bottles, are playing an increasing role in measurements of fundamental properties of the neutron. The ability to manipulate UCN with material guides and bottles, magnetic fields, and gravity can lead to experiments with lower systematic errors than have been obtained in experiments with cold neutron beams. The UCN densities provided by existing reactor sources limit these experiments. The promise of much higher densities from solid deuterium sources has led to proposed facilities coupled to both reactor and spallation neutron sources. In this Letter we report on the performance of a prototype spallation neutron-driven solid deuterium source. This source produced bottled UCN densities of 145±7 UCN/cm 3 , about three times greater than the largest bottled UCN densities previously reported. These results indicate that a production UCN source with substantially higher densities should be possible.

The 39 Ar ( n th , α ) 36 S reaction has been studied for the first time. A sample containing 2.85×10 14 39 Ar atoms was produced at the ISOLDE facility at CERN. The number of 39 Ar atoms in the layer was determined by measuring the 39 Ar β -activity using a primary standardisation method. Subsequently, the sample was irradiated with thermal neutrons at the High Flux Reactor of the Institut Laue–Langevin. An upper limit of 0.29 b was obtained for the 39 Ar ( n th , α ) 36 S reaction cross-section.

A homogeneous deposit of (2.58±0.12)×10 17 26 Al atoms was produced applying the method of electrodeposition. With this target, the 26 Al(n th ,p i ) 26 Mg and the 26 Al(n th , α i ) 23 Na reactions were studied at the high flux reactor of the ILL in Grenoble. Thermal cross section values for five transitions were obtained. The value for the 26 Al(n th , p 0 ) 26 Mg cross section, (8.1±0.8) mb, is a factor 3 lower than reported in the literature. For the 26 Al(n th , p 1 ) 26 Mg cross section, a value of (1942±95) mb was found, which is in reasonable agreement with the literature values. The result obtained for the 26 Al(n th , p 2 ) 26 Mg cross section, (17±4) mb, lies more than a factor 10 below the upper limit reported. A cross section value for the 26 Al(n th , α 0 ) 23 Na reaction of (261±13) mb was found, which is more than a factor 3 lower than reported in the literature. The 26 Al(n th , α 1 ) 23 Na reaction finally was observed for the first time resulting in an accurate cross section value of (74±4) mb.

A first experimental study of the neutron induced charged particle reactions on $^{37}$Ar and $^{39}$Ar as a function of the neutron energy has been made. Therefore, complementary measurements were performed with thermal neutrons at the High Flux Reactor of the ILL (Grenoble, France) and with resonance neutrons at the GELINA neutron spectrometer of the IRMM (Geel, Belgium). The stellar cross sections, which were calculated from the obtained data, show important discrepancies with the theoretical values used in recent network calculations.

It is generally accepted that the main cause of ultracold neutron (UCN) losses in storage traps is upscattering to the thermal energy range by hydrogen adsorbed on the surface of the trap walls. However, the data on which this conclusion is based are poor and contradictory. Here we report a measurement, performed at the Los Alamos National Laboratory UCN source, of the average energy of the flux of upscattered neutrons after the interaction of UCN with hydrogen bound in the semicrystalline polymer PMP (trade name TPX), [C_6H_(12)]_n. Our analysis, performed with the mcnp code which applies the neutron-scattering law to UCN upscattered by bound hydrogen in semicrystalline polyethylene, [C_2H_4]_n, leads us to a flux average energy value of 26±3 meV, in contradiction to previously reported experimental values of 10 to 13 meV and in agreement with the theoretical models of neutron heating implemented in MCNP.

The 236 U(n th ,f) cross section was measured for the first time at the high-flux reactor of the InstitutLaue-Langevin in Grenoble, yielding a value of (0.3 ± 1.0) mb. This means that σ f ≤ 1.3 mb, which is about two orders of magnitude lower than previously adopted. This result was obtained by combining a highly enriched 236 U sample with a very clean neutron beam and assuming that the Westcott factor g f = 1 for 236 U(n,f) with cold neutrons. The new value is compatible with the latest fission resonance data and with the subthreshold character of the thermal neutron induced fission of 236 U.

Building on the experience that Mampe gained with his ultra-cold neutron (UCN) storage bottle MAMBO I (Phys Rev. Lett. 63 (1989) 593) the successor experiment MAMBO II has been designed and set up. The basic method of this experiment relies on the storage of UCN in a rectangular glass vessel of variable size with walls covered by Fomblin oil. A preliminary value for the lifetime of 881.0±3 s has been measured.

A prototype of a solid deuterium (SD 2 ) source of Ultra-Cold Neutrons (UCN) is currently being tested at LANSCE. The source is contained within an assembly consisting of a 4 K polyethylene moderator surrounded by a 77 K beryllium flux trap in which is embedded a spallation target. Time-of-flight measurements have been made of the cold neutron spectrum emerging directly from the flux trap assembly. A comparison is presented of these measurements with results of Monte Carlo (LAHET/MCNP) calculations of the cold neutron fluxes produced in the prototype assembly by a beam of 800 MeV protons incident on the tungsten target. A UCN detector was coupled to the assembly through a guide system with a critical velocity of 8 m/s ( 58 Ni). The rates and time-of-flight data from this detector are compared with calculated values. Measurements of UCN production as a function of SD 2 volume (thickness) are compared with predicted values. The dependence of UCN production on SD 2 temperature and proton beam intensity are also presented.

Ultra-cold neutrons (UCN) can be stored in a trap if their energy is lower than the trap wall potential. It is well known that the neutron density in a trap decreases due to neutron beta-decay, upscattering and absorption on surfaces but we have identified a complementary escape channel. This arises from a small increase in the energy of UCN during their interaction with a surface. Higher-energy neutrons can then escape into the bulk material or penetrate through the trap wall if it is thin enough.

Depolarization of ultra-cold neutrons (UCN) stored in material traps was "rst observed. The probability of UCN spin #ip per re#ection depends on the trap material and varies from 7]10~6 (beryllium) to 10~4 (glass). ( 2000 Elsevier Science B.V. All rights reserved.

The ${}^{17}\mathrm{O}{(n}_{\mathrm{th}},\ensuremath{\alpha}{)}^{14}\mathrm{C}$ reaction cross section was determined at the high flux reactor of the ILL in Grenoble relative to the known ${}^{14}\mathrm{N}{(n}_{\mathrm{th}}{,p)}^{14}\mathrm{C}$ cross section. For the flux calibration measurements, ${}^{14}{\mathrm{N}}_{2}$ from the air was used. The ${}^{17}\mathrm{O}(n,\ensuremath{\alpha})$ measurements were performed with several highly enriched oxygen gas samples. This resulted in a precise value of $(244\ifmmode\pm\else\textpm\fi{}7)$ mb for the ${}^{17}\mathrm{O}{(n}_{\mathrm{th}},\ensuremath{\alpha}{)}^{14}\mathrm{C}$ cross section.

The ${}^{41}\mathrm{Ca}{(n,p)}^{41}\mathrm{K}$ and ${}^{41}\mathrm{Ca}(n,\ensuremath{\alpha}{)}^{38}\mathrm{Ar}$ reaction cross sections were determined with thermal neutrons at the high flux reactor of the ILL in Grenoble. For the ${}^{41}\mathrm{Ca}{(n}_{\mathrm{th}}{,p)}^{41}\mathrm{K}$ reaction cross section, a value of $(7\ifmmode\pm\else\textpm\fi{}2) \mathrm{mb}$ was obtained. In the case of the ${}^{41}\mathrm{Ca}{(n}_{\mathrm{th}},\ensuremath{\alpha}{)}^{38}\mathrm{Ar}$ reaction, the transition to the ground state in ${}^{38}\mathrm{Ar}$ has a cross section $\ensuremath{\sigma}{(n}_{\mathrm{th}},{\ensuremath{\alpha}}_{0}) = (42\ifmmode\pm\else\textpm\fi{}6) \mathrm{mb}$, the most prominent decay going to the first excited state in ${}^{38}\mathrm{Ar}$ with a cross section $\ensuremath{\sigma}{(n}_{\mathrm{th}},{\ensuremath{\alpha}}_{1}) = (130\ifmmode\pm\else\textpm\fi{}25) \mathrm{mb}$. This can be explained by the presence of a nearby bound level with ${J}^{\ensuremath{\pi}}{=4}^{\ensuremath{-}}$. Also ${}^{41}\mathrm{Ca}{(n}_{\mathrm{th}},\ensuremath{\gamma}\ensuremath{\alpha}{)}^{38}\mathrm{Ar}$ transitions have been observed with a cross section of $(10\ifmmode\pm\else\textpm\fi{}2) \mathrm{mb}$. The primary $\ensuremath{\gamma}$ transitions are shown to have a $M1$ multipolarity.

Ar-37 atoms were produced via the Cl-37(p, n)Al-37 reaction using the 30 MeV proton beam of the Cyclone 30 cyclotron of the UCL and a NaCl target. After ionization, Ar-37(2+) ions were implanted at 20 keV into 20 mu m thick Al foils. The number of Ar-37 atoms in the layers was determined by measurement of the 2.62 keV KX-rays (emitted during the EC-decay. of Ar-37) with a gas-flow proportional counter. Under vacuum and under intense thermal neutron irradiation, the implantation was found to be stable. Furthermore, the layers were found to be suited for (n, p) and (n, alpha) measurements.

A test experiment has shown that the number of ultracold neutrons (UCN) of one polarization state, transmitted through a 100 μm Al foil when placed in a 5 T magnetic field, is greater by 3.8 times. The increased transmission is due to the higher velocity of the UCN passing through the foil.

The study of neutron cross sections for elements used as efficient ``absorbers'' of ultracold neutrons (UCN) is crucial for many precision experiments in nuclear and particle physics, cosmology and gravity. In this context, ``absorption'' includes both the capture and upscattering of neutrons to the energies above the UCN energy region. The available data, especially for hydrogen, do not agree between themselves or with the theory. In this report we describe measurements performed at the Los Alamos National Laboratory UCN facility of the UCN upscattering cross sections for vanadium and for hydrogen in CH$_2$ using simultaneous measurements of the radiative capture cross sections for these elements. We measured $\sigma_{up}=1972\pm130$ b for hydrogen in CH$_2$, which is below theoretical expectations, and $\sigma_{up} < 25\pm9$ b for vanadium, in agreement with the expectation for the neutron heating by thermal excitations in solids., Comment: 6 pages 2 figures

The cross sections of the {sup 36}Cl({ital n}{sub th},{ital p}){sup 36}S and the {sup 36}Cl({ital n}{sub th},{alpha}){sup 33}P reactions were measured with thermal neutrons at the high flux reactor of the ILL in Grenoble. For the {sup 36}Cl({ital n}{sub th},{ital p}){sup 36}S reaction cross section a precise value of (47{plus_minus}2)mb was obtained and for the {sup 36}Cl({ital n}{sub th},{alpha}){sup 33}P reaction a cross section value of (0.59{plus_minus}0.07)mb was determined. {copyright} {ital 1996 The American Physical Society.}

The mass, charge and energy distributions and their correlations have been studied for the fragments from thermal neutron-induced fission of 241 Pu, using the fission-fragment spectrometer Cosi-Fan-Tutte. The mass and charge distributions obtained agree with the corresponding radiochemical data. A proton even-odd effect of (10 ± 1.5)% on the nuclear-charge yield and 0.4 ± 0.2 MeV on the total kinetic energy has been determined. A fission-mode analysis on the nuclear-charge distribution has been performed.

The 35 Cl(n, p) 35 S reaction cross section has been measured with thermal neutrons at the High Flux Reactor of the ILL (Grenoble) and with resonance neutrons at the linear accelerator GELINA of the IRMM (Geel). For the thermal cross section, a precise value of 440 ± 10 mb was obtained. In the region up to about 100 keV neutron energy, several resonances were determined with high resolution. These data were used to calculate the maxwellian averaged cross section at various temperatures. Their impact on stellar-nucleo-synthesis calculations is discussed.

Diffraction experiments with holographic gratings recorded in SiO$_2$ nanoparticle-polymer composites have been carried out with slow neutrons. The influence of parameters such as nanoparticle concentration, grating thickness and grating spacing on the neutron-optical properties of such materials has been tested. Decay of the grating structure along the sample depth due to disturbance of the recording process becomes an issue at grating thicknesses of about 100 microns and larger. This limits the achievable diffraction efficiency for neutrons. As a solution to this problem, the Pendell\"{o}sung interference effect in holographic gratings has been exploited to reach a diffraction efficiency of 83% for very cold neutrons., Comment: 7 pages, 3 figures, submitted to Phys. Rev. A

The emission probabilities and the energy distributions of tritons, α and ^6He particles emitted in the spontaneous ternary fission (zero excitation energy) of ^250Cf and ^252Cf and in the cold neutron induced fission (excitation energy ≈ 6.5 MeV) of ^249Cf and 251Cf are determined. The particle identification was done with suited ΔE-E telescope detectors, at the IRMM (Geel, Belgium) for the spontaneous fission and at the ILL (Grenoble, France) for the neutron induced fission measurements. Hence particle emission characteristics of the fissioning systems ^250Cf and ^252Cf are obtained at zero and at about 6.5 MeV excitation energies. While the triton emission probability is hardly influenced by the excitation energy, the ^4He and ^6He emission probability in spontaneous fission is higher than for neutron induced fission. This can be explained by the strong influence of the cluster preformation probability on the ternary particle emission probability.