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Ba(OH)(2) equilibria in the system Ba-O-H-F, with application to the formation of Ba2YCu3O6.5+x from BaF2-precursors
No full textThe ex situ process for fabricating Ba2YCu3O6.5 (+) (x) superconducting tapes from BaF2-based precursors involves a hydration/oxidation reaction at approximate to 730 degrees C to 750 degrees C generally written as: (2 BaF2 + Y + 3 Cu)(amorphous) + (2 H2O + 2.25 O-2)(g) -> Ba2YCu3O6.5 (+) (x)(s) + 4 HF(g). However, microscopic observations of partially processed films suggest the presence of a transient liquid phase during conversion. Alternatively, the conversion reaction can be rewritten as the sum of several intermediate steps, including the formation of a barium hydroxide liquid: (BaF2)(amorphous) + 2 H2O(g) -> Ba(OH)(2)(liq) + 2 HF(g). To evaluate the possibility of a hydroxide liquid conversion step, thermodynamic calculations on the stability of Ba(OH)(2)(liq) have been completed from 500 degrees C to 900 degrees C at 0.1 MPa p(total). Based on currently available data, the calculated phase diagrams suggest that a viable hydroxide reaction path exists in the higher part of this temperature range. The calculations indicate that Ba(OH)(2)(liq) may be stable at log p(H2O) (Pa) values from approximate to 4 to 5, provided log p(HF) (Pa) values can be maintained below 0 to -1. Limited experimental confirmation is provided by results of an experiment on BaF2(s) at 815 degrees C, 0.1 MPa p(H2O), in which essentially all F at the surface was replaced by O. It is therefore possible that processing routes exist for producing Ba2YCu3O6.5 + x based on the presence of a Ba(OH)(2) liquid, which might have an effect on conversion rates and texturing in the superconducting film
High-precision determination of the neutron coherent scattering length
No full textThe neutron coherent scattering length b(c) has been determined interferometrically to an uncertainty of about 5 x 10(-5) by measuring the nondispersive phase. We propose improving the uncertainty to about 10(-6) by optimizing various parameters of the interferometric experiment. Any uncertainty in the bc determination arising from possible variations in the constitution of the ambient air can be eliminated by performing the experiment in vacuum. When such uncertainty is attained, it becomes necessary to account for the neutron beam refraction at the sample-ambient interfaces, to infer the correct b(c) from the observed phase. The formula for the phase used hitherto is approximate and would significantly overestimate b(c). The refractive index for neutrons can thus be determined to a phenomenal uncertainty of about 10(-12)
First tests of Li-6 doped glass scintillators for ultracold neutron detection
No full textWe report the results of test measurements aimed at determining the performances of Li-6 doped glass scintillators for the detection of ultra-cold neutrons. Four types of scintillators, GS1, GS3, GS10 and GS20, which differ by their Li-6 concentrations, have been tested. The signal to background separation is fully acceptable. The relative detection efficiencies have been determined as a function of the neutron velocity. We find that GS10 has a higher efficiency than the others for the detection of neutrons with velocities below 7 m/s. Two pieces of scintillators have been irradiated with a high flux of cold neutrons to test the radiation hardness of the glasses. No reduction in the pulse height has been observed up to an absorbed neutron dose of 1 x 10(13) cm(-3)
On the measurement of the neutron lifetime using ultracold neutrons in a vacuum quadrupole trap
No full textWe present a conceptual design for an experiment to measure the neutron lifetime (similar to 886 s) with an accuracy of 10(-4). The lifetime will be measured by observing the decay rate of a sample of ultracold neutrons (UCN) confined in vacuum in a magnetic trap. The UCN collaboration at Los Alamos National Laboratory has developed a prototype UCN source that is expected to produce a bottled UCN density of more than 100/cm(3) [1]. The availability of such an intense source makes it possible to approach the measurement of the neutron lifetime in a new way. We argue below that it is possible to measure the neutron lifetime to 10(-4) in a vacuum magnetic trap. The measurement involves no new technology beyond the expected UCN density. If even higher densities are available, the experiment can be made better and/or less expensive. We present the design and methodology for the measurement. The slow loss of neutrons that have stable orbits, but are not energetically trapped would produce a systematic uncertainty in the measurement. We discuss a new approach, chaotic cleaning, to the elimination of quasi-neutrons from the trap by breaking the rotational symmetry of the quadrupole trap. The neutron orbits take on a chaotic character and mode mixing causes the neutrons on the quasi-bound orbits to leave the trap
Project of neutron beta-decay a-asymmetry measurement with relative accuracy of (1-2) x 10(-3)
No full textWe are going to use a polarized cold neutron beam and an axial magnetic field in the shape of a bottle formed by a superconducting magnetic system. Such a configuration of magnetic fields allows us to extract the decay electrons inside a well-defined solid angle with high accuracy. An electrostatic cylinder with a potential of 25 kV defines the detected region of neutron decays. The protons, which come from this region will be accelerated and registered by a proton detector. The use of coincidences between electron and proton signals will allow us to considerably suppress the background. The final accuracy of the A-asymmetry will be determined by the uncertainty of the neutron beam polarization measurement which is at the level of (1-2)x10(-3), as shown in previous studies
(Audio Part 4 of 4) Oral history interview of Thomas Gary, November 4, 2005 / with David Lide, Hans Oser, Harriet Hassler, Sam Kramer, Joyce Brown, and Mike Stogsdill
No full textOral history interview of Thomas Gary, November 4, 2005.
Thomas Gary came to the National Bureau of Standards (NBS) in 1962 and began his career at the Laboratory General Mechanic in the Microscopy and Diffraction Section of the Metallurgy Division. He discusses the jobs he had at the National Institute of Standards and Technology (NIST) as a scientist, engineer, and administrato
Two coils resonant Ramsey's method for the measurement of time reversal invariance violation in neutron transmission
No full textIt is proposed within the framework of Ramsey's method to register two-dimensional spectra, depending on the neutron phase and neutron energy, for measuring parity (P) and time (T) violating amplitudes of the interaction of polarized neutrons with polarized La-139 nuclei in region of the p-wave resonance. The form of the phase spectrum and corresponding expressions for the asymmetries are obtained on the basis of a formalism of a spin density matrix. It is shown that the ratio of the P,T,-violating to P-violating imaginary amplitudes can be obtained from the measurements of the neutron phase spectrum with polarized and unpolarized La-139 target
On the way to experimental test of the time reversal invariance in the nuclear reactions
No full textTime (T) violation can be related with charge-parity (CP) violation through the CPT theorem. The CP violation was discovered experimentally in the K-0-meson decays about 35 years ago. The T violating interaction related with the CP violation violates parity as well. However, an extension of the theory beyond the locality of the interactions might violate the CPT theorem. The result of the CPLEAR experiment [1], which has given direct evidence of T violation in the elementary-particle phenomena, could be considered under assumption of the CPT invariance
Superconducting UCN polarizer for a new EDM spectrometer
No full textA test experiment has shown that the number of ultracold neutrons (UCN) of one polarization state, transmitted through a 100 micro-m 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
Direct nn-scattering measurement with the pulsed reactor YAGUAR
No full textAlthough crucial for resolving the issue of charge symmetry in the nuclear force, direct measurement of nn-scattering by colliding free neutrons has never been performed. At present the Russian pulsed reactor YAGUAR is the best neutron source for performing such a measurement. It has a through channel where the neutron moderator is installed. The neutrons are counted by a neutron detector located 12 m from the reactor. In preliminary experiments an instantaneous value of 1.1 x 10(18)/ cm(2)s was obtained for the thermal neutron flux density. The experiment will be performed by the DIANNA Collaboration as International Science & Technology Center (ISTC) project No. 2286