High Flux Beam Reactor Research Articles

High resolution electron microscopy for structure and mapping.

Electron microscopes are very valuable to the human genome project. Brookhaven’s Scanning Transmission Electron Microscope (STEM) is unique in many respects. It is like the National Synchrotron Light Source (NSLS) or the High Flux Beam Reactor (HFBR) at Brookhaven, but on a much smaller scale. The STEM uses electrons that are focussed through the sample, and detected. It is a scanning microscope because it scans the beam much like a television set. This is in contrast to most electron microscopes that are fixed beam, flood the sample with electrons, and then use a lens after the sample that reimages the material at higher magnification. There are some advantages of scanning, which I will describe.KeywordsFull ScaleInverted RepeatScan Transmission Electron MicroscopeGold ClusterHuman Genome ProjectThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

A new facility for averaged resonance capture studies at the Omega West reactor

A facility has been developed at the Omega West reactor at Los Alamos to provide neutron beams of 2 and 24 keV average neutron energies. The neutron beam currents are comparable with those obtained at the Brookhaven high flux beam reactor. A three-crystal pair spectrometer has been developed to allow acquisition of high resolution primary neutron capture gamma ray spectra. This will be used for nuclear spectroscopy and structure studies.

Neutron diffraction structure refinement of the zeolite gismondine at 15 K

A single crystal neutron diffraction study was carried out at 15 K on gismondine (Ca 3.91Al 7.77Si 8.22O 32 17.6H 2O; a=10.011(3), b=10.614(6), c=9.853(3) A ̊ , β = 93.11(2)°; P2 1/c) . The final agreement factor for the anisotropic refinement is R( F 2)=0.081 ( R w( F 2)=0.099) using 1941 diffraction intensities collected at the Brookhaven National Laboratory High Flux Beam Reactor (HFBR). Refinement of the neutron scattering lengths for tetrahedral atoms and mean tetrahedral TO distances are consistent with an essentially ordered distribution of Si and Al and the possibility of a minor substitution of excess Si in the Al(1) site. The extraframework cation site has full Ca occupancy and a coordination involving two framework oxygens and 4.3 water oxygen atoms in two geometrical configurations. One configuration involves three fully occupied water molecules (OW1, OW2, OW3) and one (OW4), with a statistical occurrence of 70%; the second configuration shows two sites (OW5, OW6) replacing OW4 with an occurrence of 30%. Proton positions were located for all the water molecules; disordered hydrogen sites were found for OW3 and OW4. The hydrogen bonding configuration is related to the hydrogen—hydrogen and hydrogen—calcium electrostatic repulsion.

Hydrogen-deuterium exchange in structural biology

Abstract The large difference in neutron scattering length of hydrogen and deuterium atoms provides a unique tool to study biological macromolecules. These molecules exist in an aqueous environment and have an atomic composition of about 50% hydrogen atoms with the rest being mainly carbon, oxygen and nitrogen. By simply changing the ratio of hydrogen to deuterium the contrast of a sample constituent can be changed without altering the chemical composition. The scattering difference between the hydrogen isotopes has now been used to study structural details of proteins, viruses, nucleic acid protein complexes and membranes. The use of small-angle scattering analysis from proteins in solution is a particularly good example of the power of this new technique. From the first experiments with myoglobin, hemoglobin and TMV solutions in 1968 at the HFBR in Brookhaven it became, however, soon evident that the flux was low and the detection system of a conventional spectrometer was inadequate to measure precisely small differences in the scattering pattern. Since these solution scattering patterns are circularly symmetrical, a development of multidetector systems was the first step to alleviate the need for better data. From an early five detector system [1] the development of linear and two-dimensional position sensitive detectors continues to this day [2–6]. Today, position sensitive area-detectors with an efficiency of 80% and 1.3 mm resolution with an active area of 50 cm × 50 cms are available. These detectors have a counting rate capability of 2 × 10 5 neutrons/s. Other instrumentation developments to facilitate the study of biomolecules include (1) cold moderators, (2) thin film multilayer monochromators [7–9], (3) neutron guides made either of multilayer supermirrors or coated with 58 Ni [10,11], (4) various crystal and mirror focusing devices [12,13]. Some of these features have been incorporated in a small-angle neutron spectrometer at the High Flux Beam Reactor at the Brookhaven National Laboratory. This spectrometer has a unique combination of features such as a high resolution area-detector, adjustable wavelength bandwidth and automated sample changer.

A next generation steady state neutron source

Abstract The Center for Neutron Research, now in the initial planning stage, is a reactor-based neutron facility that will be used for neutron scattering, isotope production, and materials irradiation. It will be a national user facility, open to scientists from universities, industry, and government laboratories. The design goal is to produce a thermal flux of 5 to 10 × 1015 neutrons cm-2 s-1 in a large heavy water reflector. Approximately 30 modern neutron-scattering instruments will be included with a strong emphasis on cold neutron experiments. Criteria and constraints involved in the core design are discussed in this article and the characteristics and performance of one preliminary design are presented. The estimated construction cost is about $300 million, and the annual operating cost will be about $15 million. The scientific productivity should be more than 30 times greater than at the High-Flux Isotope Reactor or the High-Flux Beam Reactor.

Magnetic structure of the heavy-fermion compound U2Zn17.

PHYSICAL REVIE%' VOLUME 33, NUMBER 5 Magnetic structure of the heavy-fermion compound MARCH 1986 U2Znt7 D. E. Cox, G. Shirane, and S. M. Shapiro Physics Department, A Tcf T Brookhaven National Laboratory, Bell Laboratories, Upton, New York G. Aeppli 600 Mountain Avenue, Murray Hill, New Jersey 07974 Z. Fisk and J. L. Smith Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 l. Kjems Risd National Laboratory, Roskilde, Denmark H. R. Ott Laboratorium I'ur I estkorperphysik, Eidgenossische Technische Hochschule-Honggerberg, 8093 Zurich, Switzerland (Received 26 September 1985) The phase transition of U2zni7 at 9. 7 K has been investigated by neutron powder diffraction. The transi- tion corresponds to the onset of antiferromagnetic order where the U moments are oriented antiparallel to their nearest neighbors within the basal planes and the near neighbor along the c axis of this rhombohedral compound. At 5 K, the ordered moments lie within the basal planes and are of magnitude (0. 8+0. 1) p, &, which is substantially below the paramagnetic moment of 2.25', &/U atom given by high-temperature sus- ceptibility data. The list of heavy-fermion systems, which display ex- traordinarily large ( & 400m, ) effective conduction electron masses as deduced from the low-temperature specific heat, and metals remaining now includes both superconductors Recent magnetic paramagnetic to the lowest temperatures. susceptibility and specific-heat measurements suggest that there are also heavy-fermion systems which undergo transi- tions to magnetically ordered ground states. ' In this paper, we present the first neutron-scattering determination of magnetic order in a heavy-fermion system, U2Zni7. The essential results are as follows. First, the ordering, which 10 K, is exceedingly simple, with the mag- sets in at T~ netic unit cell identical to the nuclear unit cell, and the mo- ments associated with the two U atoms in the primitive unit cell oriented in opposite directions. Second, the ordered moment is (0. 8+0. 1)p, s/U atom which is well below the moment of 2. 25@, s /U atom deduced from the high- temperature (bulk) susceptibility. U2Zni7 was made by heating the appropriate amounts of U and Zn to 1050'C in an evacuated BeO crucible. Ap- proximately 20 g of material were crushed into a coarse pounder and loaded into a cylindrical aluminum sample hold- er in an atmosphere of helium. Data were collected at the Brookhaven High Flux Beam Reactor with 2.353-A neu- trons from a pyrolytic-graphite monochromator in the (002) filter suppressed higher-order setting. A pyrolytic-graphite wavelengths. For the structure determination at 15 K ( T~), a pyrolytic-graphite analyzer in the (004) setting was used in order to optimize the resolution at higher scattering angles. The collirnations were 20'-open-40'-20' for in-pile, monochromator-sample, sample-analyzer, and analyzer-detector, respectively. The resulting diffraction pattern showed a number of weak impurity peaks in addi- tion to those characteristic of U2Zni7. The major impurity was identified as Zn, estimated to be about 5'/0 by weight. The remaining peaks were an order of magnitude weaker «1% of the strong U2Zn~7 peaks) and (with intensities could not be identified. The d spacings are listed in the cap- tion to Fig. 1, and do not correspond to those of UZni2 or e-U. At room temperature, U2Zn&7 (Ref. 3) has the Th2Zn~7- type structure, which is one of a series of ordered structures [e. g. , Pu3Zn22, UZnt2 (Ref. 5)] which can be derived from that of CaCu5. In U2Zni7, Zn occupies the Cu sites and U is ordered on two-thirds of the Ca sites, the remaining one- third being replaced by pairs of Zn atoms about 2. 6 A apart. The ordered structure has rhombohedral symmetry, space group R3m, with hexagonal lattice constants v 3a and 3c with respect to those of the parent CaCu5-type cell. Be- cause the structural parameters for U2Zni7 near its 10-K transition were unknown, we carried out a Rietveld analysis of the data sho~n in Fig. 1. Regions around the impurity peaks and Al reflections from the sample holder were ex- cluded from the refinement, and background contributions were estimated by interpolation between values obtained by averaging over regions where no Bragg peaks were present. Table I displays the lattice parameters and atomic coordi- nates given by the profile refinement. In the upper frame of Fig. 1, the solid line represents the calculated profile which best fits the data, while in the lo~er frame, the difference between the calculated and observed profiles is shown. The results are in exce11ent agreement with a room-temperature x-ray scattering determination of the structure. Belo~ 10 K some very weak additional scattering at some of the low-angle nuclear peak positions was observed. To gain more intensity tics to be obtained and thus allow adequate counting statis- in a reasonable period of time, the

Microanalytical techniques for boron analysis using the 10B(n,α)7Li reaction

In order to predict the efficacy of boronated compounds for neutron capture therapy (NCT), it is mandatory that the boron concentration in tissues be known. Various techniques for measurement of trace amounts of boron (1-100 ppm) are available, including chemical and physical procedures. Experience has shown that, with the polyhedral boranes and carboranes in particular, the usual colorimetric and spark emission spectroscopic methods are not reliable. Although these compounds may be traced with additional radiolabels, direct physical detection of boron by nondestructive methods is clearly preferable. Boron analysis via detection of the prompt-gamma ray from the 10B(n, alpha)7Li reaction has been shown to be a reliable technique. Two prompt-gamma facilities developed at Brookhaven National Laboratory are described. One, at the 60-MW high flux beam reactor, uses sophisticated beam extraction techniques to enhance thermal neutron intensity and reduce fast neutron and gamma contamination. The other was constructed at Brookhaven's 5-MW medical research reactor and uses conventional shielding and electronics to provide an "on-line" boron analysis facility adjacent to beams designed for NCT, thus satisfying one of the requisites for clinical application of this procedure. Technical restrictions attendant upon the synthesis and testing of boronated biomolecules often require the measurement of trace amounts of boron in extremely small (mg) samples. A track-etching technique capable of detecting ng amounts of boron in mg liquid or cell samples is described. Thus it is possible to measure the boron content in small amounts (mg samples) of antibodies, or boron uptake in cells grown in tissue culture.

A neutron diffraction study of [N(PPh3)2]+[B2H7]–·CH2Cl2at 80 K

The structure of the [B2H7]– anion, reported by us earlier in an X-ray structure determination, has been analysed more accurately by neutron diffraction. Data were collected at 80 K on a sample of [N(PPh3)2]+[B2H7]–·CH2Cl2 using the Brookhaven High Flux Beam Reactor. The essential results of the X-ray analysis are confirmed, with more accurate molecular parameters. The [B2H7]– anion has a non-crystallographic Cs symmetry, with a bent geometry [B–H–B 127(2)°] and staggered terminal B–H bonds. Average terminal and bridging B–H bond lengths are 1.18(2) and 1.27(6)Å respectively, and the B ⋯ B distance is 2.27(1)Å. There is a slight asymmetry of the B–H–B bridge [B(1)–H 1.32(2), B(2)–H 1.21(2)Å], which is on the borderline of being significant. [N(PPh3)2]+[B2H7]–·CH2Cl2 crystallizes in the triclinic space group P, with a= 9.633(1), b= 10.513(1), c= 16.938(1)Å, α= 90.599(7), β= 93.142(8), γ= 93.744(8)°, and Z= 2 at 80 K. The final R factor is 0.096 for the 1 479 reflections with Fo2 > 3σ(Fo2).

Neutron diffraction study of the zeolite thomsonite

A neutron study of a single crystal of thomsonite (Na(Ca,Sr) 2Al 5Si 5O 20.6H 2O at 293K; a 13.088(2) b 13.052(2) c 13.229(2) Å; Pncn) yielded a final agreement factor of R( F 2) = 0.048 for conventional anisotropic refinement using 4626 diffraction intensities from the Brookhaven National Laboratory high-flux beam reactor. The SiO and AlO distances correlate inversely with SiOAl angle as in scolecite and edingtonite, and the scattering lengths for the tetrahedral nodes are consistent with alternation of Si and Al. Both sites for extra-framework cations are associated with elliptical 8-rings. The Na,Ca site contains 0.5 Na and 0.5 Ca, and is surrounded by a square antiprism containing 4 water-oxygens in one square (2.45, 2.50, 2.59, 2.60 Å). Each square of water-oxygens is shared with an adjacent antiprism and one edge between framework-oxygens is shared with a second antiprism to give an infinite chain parallel to c. There is no evidence for long-range alternations of Na and Ca atoms. The Ca site is split into two positions displaced 0.28 Å from the centroid of 6 framework-oxygens and 2 water-oxygens, and is coordinated to two framework-oxygens (2.44, 2.46 Å) and two water-oxygens (2.34, 2.35 Å) in a square and to two framework-oxygens (2.61, 2.65 Å) on one side of the square. Some Sr may substitute in the Ca site. There is no evidence for ordered occupancy of Ca sites. All protons are in positions corresponding to electrostatic equilibrium. Those associated with Na,Ca lie close to the shared square, while those associated with Ca are almost coplanar with the cation and the water-oxygen. The distances from the proton to the water- and framework-oxygens are negatively correlated for edingtonite and thomsonite.

Special Issue on Neutron Scattering

The articles in this issue review the current status of neutron-scattering facilities in the US and abroad, discuss in more depth some recent contributions of neutron scattering to fundamental studies of condensed matter—including biophysics—and suggest future scientific opportunities in these fields. The photo at right shows one of the sources that makes all these studies possible: the core of a reactor. Neutrons from fission reactions here are transported out of the reactor, collimated, and sent to scatter off a sample. This particular reactor is the High Flux Beam Reactor at Brookhaven National Laboratory.

A neutron diffraction study of the bonding of zeolitic water in scolecite at 20 K *

Abstract A neutron diffraction study at 20 K of a single crystal of scolecite (CaAl2Si3O10 · 3H2O, a = 6.516(2), b = 18.948(3), c = 9.761(1) Å, β = 108.98(1)°; Cc, Z = 4,Dc = 2.28 g/cm3) utilized 2109 diffraction intensities from the Brookhaven National Laboratory High Flux Beam Reactor (λ = 1.0505(1) Å, R(F) = 0.016). There is no indication of substitutional disorder in the structure. The Ca atoms coordinate four framework oxygens and three water oxygens in a distorted pentagonal bipyramid. The hydrogen bonds to the framework oxygens vary in length [O … H: 1.746(1) – 2.119(1) Å] and linearity [O – H … O:130.9(1)° – 177.1(1)°]. The O – H distances in water fall in the range 0.979 – 0.993 Å, and the water angles H – O – H are 106.7(1)°, 107.3(2)° and 111.1(1)°. A neutron scattering length for Ca of 0.465(3) × 10−14 m is suggested.

X-ray and neutron diffraction studies of the polyhydrido complex FeH6Mg4Br3.5Cl0.5(C4H8O)8: discussion of binary transition-metal hydride anions of the type [MHx]n-

The molecular structure of the polyhydrido complex FeH6Mg4X4(THF)8 (X4 = Br/sub 3.5/Cl/sub 0.5/) has been determined by single-crystal X-ray and neutron diffraction methods. The X-ray structure determination was carried out at room temperature, revealing an octahedral (FeH6)U core with (MgBr(THF)2) units situated over four of the eight faces of the octahedron. The (FeH6)U core exhibits crystallographic twofold symmetry, with an average Fe-H distance (as determined in the X-ray study) of 1.69 (9) A. Neutron diffraction analysis of the title compound (carried out at 80 K at the Brookhaven High-Flux Beam Reactor on a crystal of volume 12 mmT) fully confirmed this basic molecular geometry, yielding much more precise H positions. The Fe-H (1.609 (2) A) and Mg-H (2.045 (18) A) distances obtained from the neutron study support the idea that the Fe-H bond is largely covalent in nature while the Mg-H interaction is largely ionic. A detailed discussion of available structural information for binary transition-metal hydride anions, (MH/sub x/)/sub n-/, is given in the text. Crystallographic details for the X-ray analysis of the title compound C32H70Br/sub 3.5/Cl/sub 0.5/FeMg4O8 is included. Details for the neutron analysis at 80 Z are also reported. 52 references, 2 figures, 4 tables.

An experimental study of the critical fluctuations in a two-dimensional Ising model

Neutron scattering measurements have been made of the critical fluctuations in K2CoF4 at the Brookhaven High Flux Beam reactor. The critical fluctuations were studied for both temperatures above and below Tc. Above Tc the scattering is well described by the Ornstein-Zernike form and the exponents and amplitude of kappa (the inverse correlation length) are in excellent accordance with theory. Below Tc the Ornstein-Zernike form gives a reasonable description of the results, but the resulting kappa are very different from those expected theoretically. The approximate form developed by Tarko and Fisher (1975) gives a better description of the results and the resulting parameters are in accordance with theoretical expectations. The authors conclude that the critical fluctuations in the d=2 Ising model are not described by the Ornstein-Zernike form. The amplitude ratios have been measured for the correlation length, the susceptibility, and the ratio connecting the Bragg scattering to the susceptibility by using two-scale-factor universality. The results in each case agree with theory to within the experimental error which varies between 10 and 15%.

The level structure of 114Cd from (n, γ) and (d, p) studies

Gamma rays and conversion electrons have been measured following thermal neutron capture in 113Cd using the crystal spectrometers GAMS and the β-spectrometer BILL at the High Flux Reactor of the ILL at Grenoble. Primary γ-rays following thermal and average resonance neutron capture at E n = 2 keV and 24 keV were recorded at the High Flux Beam Reactor at the Brookhaven National Laboratory. The 113Cd(d, p) 114Cd reaction was studied with the Q3D spectrograph at the Munich tandem accelerator. Combining all these experimental results an almost complete level scheme of 114Cd was constructed up to 3.3 MeV including 48 excited levels with spin and parity information. The level scheme is discussed in terms of particle-hole excitations across the Z = 50 closed shell coupled to collective states, as well as in an interacting boson configuration mixing scheme.

A Memory Intensive Functional Architecture for Distributed Computer Control Systems

A memory-intensive functional architecture for distributed data-acquisition, monitoring, and control systems with large numbers of nodes has been conceptually developed and applied in several large-scale and some smaller systems. This discussion concentrates on (a) the basic architecture; (b) recent expansions of the architecture which now become feasible in view of the rapidly developing component technologies in microprocessors and functional large-scale integration circuits; (c) implementation of some key hardware and software structures and one system implementation which is a system for performing control and data acquisition of a neutron spectrometer at the Brookhaven High Flux Beam Reactor. The spectrometer is equipped with a large-area position-sensitive neutron detector.

A neutron diffraction study of the zeolite edingtonite

A neutron diffraction study at 294 K of a single crystal of edingtonite (Ba2Al4Si6O20 ⋅ 7H2O; a 9.537(3) b 9.651(2) c 6.509(2) Å; P21212) utilized 1876 diffraction intensities from the Brookhaven National Laboratory high-flux beam reactor. The agreement factor R(F2)=0.055 for conventional anisotropic refinement was reduced to 0.045 for a Gram–Charlier expansion up to fourth order for the thermal factors of the water atoms. The Si–O and Al–O distances correlate inversely with the Si–O–Al angle as in scolecite. There is no indication of substitutional disorder. The barium atom is coordinated to three pairs of framework oxygens (2.89, 2.96, and 3.04 Å) and two pairs of water oxygens (2.79 and 2.79 Å). Two framework oxygens have weak hydrogen bonds to both water molecules [O(4)–OW(1) 2.87, −OW(2) 2.96; O(5) −OW(1) 3.02, −OW(2) 3.02 Å] and the other three framework oxygens are each bonded to a Ba atom. The OW–H ⋅⋅⋅ O angles (163.5°, 165.1°, 173.9°, and 178.0°) are fairly close to 180°, the H ⋅⋅⋅ O distances are long (1.91, 2.02, 2.09, and 2.10 Å) and the observed uncorrected OW–H distances range from 0.928(6) to 0.959(4) Å. Only seven out of the eight water positions are occupied [W(1) 84% occupancy; W(2) 91%]. The average rms displacement of each hydrogen (0.32, 0.29, 0.27, and 0.24 Å) correlates approximately with the hydrogen bond length (2.09, 2.10, 2.02, and 1.91 Å). Third- and fourth-order tensor components in the displacements of the water molecules may result from anharmonic or curvilinear vibrations; however, the effect of the static displacements of the center-of-motion from interaction with unoccupied water sites may also be important.

Structural temperature dependence in α-lithium iodate: Neutron and x-ray study between 20 and 500 K

The crystal structure of hexagonal α-LiIO3 has been determined by neutron and x-ray diffraction at ten temperatures from 20 to 500 K. Four sets (1248 to 1622 reflections per set) of neutron data at λ=1.0467 Å were measured at the Brookhaven High Flux Beam Reactor at 20, 100, 200, and 295 K. The integrated intensities of a further 13 data sets (1706 to 2963 reflections per set) were measured using MoKα radiation on a CAD-4 x-ray diffractometer. The structure at each temperature was refined by least squares in space group P63. The final agreement factors fell between 0.023 and 0.050. The unit cell dimensions at 295 K are a=5.481 27(6), c=5.171 65(4) Å, with nonlinear thermal expansions. The principal change in structure with temperature is in the atomic positions of Li and O relative to I: The positions are given by O(z)T=−0.8435(3) Å+62(4)×10−9 T2 Å K−2 and Li(z)T=0.3808(10) Å+167 (18)×10−9 T2 Å K−2. The iodate ion has a constant I–O bond length of 1.8081(4) Å, but the O–I–O angle increases from 99.97(2)° at 20 K by 0.5° over the temperature range studied. The LiO6 octahedron has Li–O distances of 2.070 and 2.140(1) Å at 20 K, increasing to 2.106 and 2.151(9) Å at 460 K. The mean-square amplitude of thermal vibration varies nonlinearly with temperature for all atoms, with zero-point rms displacement of 0.040 Å for I, 0.067 Å for O, and 0.094 Å for Li. The Debye temperature is estimated as 350 K on the basis of the 295 K amplitudes of vibration.

Neutron diffraction structural study of pyroelectric Ba(NO2)2⋅H2O at 298, 102, and 20 K

Neutron diffraction studies of barium nitrite monohydrate [Ba(NO2)2⋅H2O] have been made at 20, 102, and 298 K on the same crystal. Intensities of 954 (20 K), 2179 (102 K), and 2393 (298 K) reflections were measured at the Brookhaven National Laboratory high flux beam reactor. The structures were refined on the basis of the hexagonal space group P65 to yield agreement factors R(F2) = 0.028 (20 K), 0.034 (102 K), and 0.061 (298 K). The thermal expansion is nonlinear along both a and c axes, with α1 = 3.8×10−6 K−1, β1 = 2.8×10−8 K−2, α3 = 21.3×10−6 K−1, β3 = 10.9×10−8 K−2. The cell dimensions are a = 7.0524(24) Å, c = 17.6372(80) Å at 20 K, a = 7.0559(23) Å, c = 17.6810(77) Å at 102 K, and a = 7.074 90(3) Å, c = 17.890 87(12) Å at 298 K. The mean-square amplitude of thermal motion of Ba is proportional to temperature in the range 298 to 20 K, with small departures from linearity for the O and N amplitudes and larger departures for the H nuclear amplitudes. The Ba2+ ion is coordinated by nine oxygen atoms and one nitrogen atom at an average distance of 2.881 Å at 20 K, 2.884 Å at 102 K, and 2.902 Å at 298 K. The increase in length corresponds completely to the lattice expansion. A maximum difference within a nitrite ion of 0.015 Å in N–O bond lengths, observed at 298 K, is reduced to 0.008 Å at 20 K for a mean N–O distance of 1.254 Å. One O–H bond of 0.958 Å, which forms a bifurcated hydrogen bond with two oxygen atoms, is identical in length to that in the free water molecule; the other is slightly elongated by formation of a nearly linear hydrogen bond to a nitrogen atom. The largest nuclear displacement between 298 and 20 K in this pyroelectric crystal is only 0.038 Å along the polar axis.

Neutron diffraction structural study of pyroelectric Sr(NO2)2⋅H2O at 294, 100, and 20 K

Strontium nitrite monohydrate [Sr(NO2)2⋅H2O] has been studied at 294, 100, and 20 K by neutron diffraction. Integrated intensities of 2958 (294 K), 4239 (100 K), and 2154 (20 K) reflections were measured at the Brookhaven National Laboratory high flux beam reactor, resulting in 2068 (294 K), 2030 (100 K), and 1934 (20 K) independent structure factors. Structural refinement in monoclinic space group P21 led to R(F)=0.029 (294 K), 0.019 (100 K), and 0.034 (20 K). Lattice constants at 295 K are a=12.5831(4), b=8.94941(15), c=4.4860(2) Å, β=99.111(4)°; at 100 K, a=12.539(2), b=8.909(1), c=4.475(1) Å, β=99.086(20)°; and at 20 K, a=12.542(2), b=8.901(1), c=4.472(1) Å, β=99.030(20)°. The expansivity of all four lattice parameters is nonlinear, with α1=32.8×10−6 K−1, β1=76×10−9 K−2; α2=31.6×10−6 K−1, β2=44×10−9 K−2; α3=15.6×10−6 K−1, β3=15×10−9 K−2; α4=−2.8×10−6 K−1, β4=−21×10−9 K−2 for αTi =α295i [1+αiΔT+βi(ΔT)2], ΔT=T−295 K. The mean-square amplitude of Sr varies linearly with temperature between 20 and 294 K, and extrapolates to a zero-point value of 0.0010(2) Å2. A survey shows that atoms with masses greater than about 80 have similar zero-point amplitudes which increase toward the maximum exhibited by H, with its zero-point amplitude of about 0.023 Å2, as the atomic mass decreases. The coordination (9 and 10) of the two independent Sr2+ ions remains unchanged with temperature: the coordination shell radius decreases on average by 0.012 Å from 294 to 20 K. Comparison with previous x-ray structural results on Sr(NO2)2⋅H2O shows the latter are influenced by scattering from the nitrogen atom lone-electron pair. Small but significant differences in N–O distances among the four independent NO−2 ions are detected at 294 K (0.020 Å) and at 20 K (0.017 Å). The mean N–O distance at 20 K for all four ions is identical at 1.253–1.254 Å. The maximum atomic displacement between 294 and 20 K is that by a nitrite ion oxygen, of 0.045 Å.

A Memory Intensive Functional Architecture for Distributed Computer Control Systems

A memory-intensive functional architecture suitable for distributed data acquisition, monitoring and control systems with large numbers of microprocessor based nodes has been conceptually developed and applied in several large scale and some smaller systems. This discussion concentrates on (a) the basic architecture; (b) expansions as they become feasible in view of continuing rapid developments in the microprocessor technology and in functional large-scale integration circuits; (c) the implementation of some key hardware and software structures; and (d) two system implementations, which are (1) a system for monitoring and managing of animal inhalation chambers and restricted access rooms as well as for experimental data acquisition and data analysis at an Inhalation Toxicology Research Facility and (2) a data acquisition and control system for a spectrometer with a 20 by 20 cm position-sensitive detector used in neutron scattering experiments at the Brookhaven High Flux Beam Reactor.