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Absolute measurement of the critical scattering cross section in cobalt

Description: Small-angle neutron scattering techniques have been used to study the angular distribution of the critical scattering from cobalt above T/sub c/. These measurements have been put on an absolute scale by calibrating the critical scattering directly against the nuclear incoherent scattering from cobalt. In this way the interaction range r$sub 1$, which appears in the classical and modified Ornstein--Zernike expressions for the asymptotic form of the spin pair correlation function and is related to the strength of the spin correlations, has been determined. We obtain r$sub 1$/a = 0.46 +- 0.03 for the ratio of the interaction range to the nearest-neighbor distance in cobalt. This result is in good agreement with theoretical predictions. Lack of agreement among previous determinations of the ratio r$sub 1$/a made in iron failed to provide a definitive comparison with theory. (auth)
Date: January 1, 1975
Creator: Glinka, C. J.; Minkiewicz, V. J. & Passell, L.
Partner: UNT Libraries Government Documents Department

Polarizing multilayer spectrometer for neutrons

Description: Polarizing neutron monochromators were prepared by sputtering thin-film multilayers with d-spacings from 40 to 85A on large float-glass substrates. Peak reflectivities as great as 90% and polarizing efficiencies of 98% were measured. Increased angular acceptances were obtained by fabricating multilayers with multiple d- spacings. A planned polarized beam spectrometer which incorporates the multilayers and which has a variable energy resolution independent of angular beam divergence is described.
Date: January 1, 1981
Creator: Majkrzak, C.F.; Passell, L. & Saxena, A.M.
Partner: UNT Libraries Government Documents Department

Neutron scattering experiments on well-staged graphite - FeCl/sub 3/

Description: This paper reports results of a neutron diffraction study on both the magnetic and the lattice dynamic structure of graphite-FeCl/sub 3/. The FeCl/sub 3/ system was selected for this study because of the interesting magnetic properties previously reported for this system. Of particular interest is the low temperature magnetic phase transition, which has been shown to be stage dependent. Special emphasis was given to the study of the low temperature magnetic state of the Fe/sup 3 +/ spins and to the longitudinal phonon modes for k/sub z/ axis acoustic branches. All experiments reported here were carried out on a stage 2 sample of graphite-FeCl/sub 3/. The sample was prepared by a two-zone vapor growth technique and was characterized by (00l) x-ray and neutron diffraction patterns to be > 95% stage 2, with only minor inclusions of other stages.
Date: January 1, 1981
Creator: Axe, J.D.; Majkrzak, C.F.; Passell, L.; Satija, S.K.; Dresselhaus, G. & Mazurek, H.
Partner: UNT Libraries Government Documents Department

Martensitic transformation of CsFeS/sub 2/ driven by the singlet to Neel state transition

Description: CsFeS/sub 2/ is regarded as an example of the quasi one-dimensional alternating antiferromagnetic Heisenberg chain above about 70K. In accordance with this picture, an energy gap of 10MeV was observed for the singlet to triplet excitation at a zone center near the transition temperature by our neutron measurements. Large excitation width implies a strong coupling of excitons to phonons, and LA phonons along <001> direction become ill-defined for q greater than 0.3. At about 70K, the first order transition occurs, whereby the singlet ground state changes to a Neel state and the simultaneous structural transformation takes place, which is a martensitic transformation. Mechanism for such martensitic transformation is discussed based on the similarity of the magnetic excitation and phonon behaviors between the present compound and the ..gamma.. Mn alloys with Fe and Cu, which are itinerant electron magnetic systems. 10 refs., 3 figs.
Date: January 1, 1985
Creator: Ito, Y.; Nishi, M.; Passell, L.; Majkrzak, C.F. & Shirane, G.
Partner: UNT Libraries Government Documents Department

Rebuilding the Brookhaven high flux beam reactor: A feasibility study

Description: After nearly thirty years of operation, Brookhaven`s High Flux Beam Reactor (HFBR) is still one of the world`s premier steady-state neutron sources. A major center for condensed matter studies, it currently supports fifteen separate beamlines conducting research in fields as diverse as crystallography, solid-state, nuclear and surface physics, polymer physics and structural biology and will very likely be able to do so for perhaps another decade. But beyond that point the HFBR will be running on borrowed time. Unless appropriate remedial action is taken, progressive radiation-induced embrittlement problems will eventually shut it down. Recognizing the HFBR`s value as a national scientific resource, members of the Laboratory`s scientific and reactor operations staffs began earlier this year to consider what could be done both to extend its useful life and to assure that it continues to provide state-of-the-art research facilities for the scientific community. This report summarizes the findings of that study. It addresses two basic issues: (i) identification and replacement of lifetime-limiting components and (ii) modifications and additions that could expand and enhance the reactor`s research capabilities.
Date: January 1, 1995
Creator: Brynda, W. J.; Passell, L. & Rorer, D. C.
Partner: UNT Libraries Government Documents Department

Preliminary analysis of gain measurements at the HFBR liquid hydrogen moderator

Description: The High Flux Beam Reactor (HFBR) is a 60 MW steady state neutron source. As part of the facility a cold neutron source is included in one of the beam tubes (H-9). The arrangement of this source is shown in Figure 1, which shows the reactor core, beam tube H-9, and the cold source with its attached helium cooling lines and hydrogen feed lines. The liquid hydrogen chamber is in the shape of an oblate spheroid and has a volume of 1.466 liters, and an aspect ratio of 1:3. Aluminum is used as the material of construction. The wall thickness of the chamber varies, with the thinnest value being on the flatter parts of the oblate spheroid. This design minimizes the amount of metal in the direction of the neutron beam of interest. Gain is defined as the ratio of the flux at a specific wave length leaking from the front face of the cold source, with and without the liquid hydrogen present. Measurements of the gain were made at several wavelength for the HFBR cold source. The change in the neutron spectrum at a particular wave length is a strong function of the scattering kernel of the moderator. Thus, these measurements can be used as integral data to validate calculational models and scattering kernel data for liquid hydrogen, and shed light on the actual mixture of ortho/para hydrogen in the cold source. Two scattering kernels for each of the states of liquid hydrogen (ortho and para) were available at the beginning of the study. The total scattering cross section for each of these are shown. The two ortho kernels are seen to be quite similar. However, in the case of para-hydrogen there is seen to be a significant difference between the two cross sections at lower energies or longer ...
Date: May 1, 1998
Creator: Ludewig, H.; Aronson, A.; Todosow, M.; Passell, L. & Wildgruber, U.
Partner: UNT Libraries Government Documents Department