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Neutron diffraction experiments on metallic europium have been conducted over a temperature range from room temperature to 4°K using a sample in the form of europium filings. The data show that the metal undergoes a transition to an antiferromagnetic state at a Neel temperature TN of 91 °K and that this ordering process continues until saturation develops at about 20°K. A model which conforms to the data consists of a helical spin structure with the magnetic moments lying parallel to a cube face and with the rotation axis directed perpendicular to the moments or along the [100] direction. The pitch of the helix was observed to change very slightly with temperature; the period was 3.5a at T/TN=1 and increased to 3.6a at T/TN=0.05 where a is the lattice spacing. On the basis of the above model and the intensities of the magnetic reflections, the calculated ordered moment at each atom site is 5.9±0.4 /J,B per atom. This measured moment is somewhat less than the maximum theoretical value of 7 [XB expected from divalent europium characterized by a spin only ground state of 8 57/2. Values of magnetic moment times magnetic form factor, /*/, as calculated from Eu coherent magnetic reflections are in agreement with similar values of /if derived from the compound EuO. The intensities of the magnetic diffraction peaks deviate from a Brillouin behavior and instead follow a temperature dependence proportional to (TN-T) 112 over a considerable region below the Neel temperature. The Debye temperature, as derived from the nuclear peak intensities, exhibited a variation from 70 to 120°K over a corresponding sample temperature range of 100 to 293°K.

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Neutron diffraction studies of several of the D2O ice polymorphs were made with helium pressures to 3.5 kbar. Volume compressions (ΔV / V) were obtained for ices Ih, Ic, and IX at 2.1 to 2.8 kbar. The equilibrium phase boundaries between ices I–II and I–III in the presence of helium are shifted toward higher pressure values. The crystal structures of ices Ih, Ic, II, and IX at approximately 2.5 kbar are the same as the structures found for these ice forms at atmospheric pressure. A tetragonal structure is indicated for ice III with lattice parameters at − 23°C and 2.7 kbar of a = 6.674 ± 0.015 Å and c = 6.97 ± 0.03 Å. The c / a ratio for ice IX has been found to be 1.000 ± 0.003 both at 1 bar and at 2.8 kbar.

Neutron diffraction data from a powder sample of ErAg show two antiferromagnetic transitions at temperatures of 18 ± 1 K and 9.5 ± 0.5 K. The magnetic superlattice reflections for the two structures can be indexed on a tetragonal magnetic cell which has basal plane edges twice that of the chemical cell (am = 2a0) and the same dimension in the unique c-axis direction (cm = c0). Between 18 and 9.5 K the magnetic ordering is similar to HoAg and conforms to a sinusoidally modulated magnetization wave propagating in the 〈100〉 direction with the moments tilted from the c axis at angles ranging from 40° (at 18 K) to 80° (at 10 K). The modulation wavelength of 53 ± 2 Å (7.4am) is incommensurate with the unit chemical cell and is temperature independent. In this incommensurate ordering region the ordered moment increases with decreasing temperature to a maximum value of 6.8μB at T = 10 K. Below 9.5 K, the antiferromagnetic ordering is a commensurate (+ − + −) layer-type structure similar to that of DyAg and TbAg, in which adjacent (110) ferromagnetic planes have oppositely directed moments. The tilt of the moments from the c axis is 85° at T = 9 K and decreases to 75° at T = 4 K. The ordered moment on the Er atom sites is 7.2 ± 0.2μB at T = 4 K.

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The magnetic properties of two dialuminides, DyAl2 and NdAl2, have been investigated by neutron diffraction and susceptibility measurements. Both compounds show Curie-Weiss behavior in the paramagnetic region and are basically ferromagnetic below Curie temperatures of 62°K and 65°K, respectively. At low temperatures, the compound NdAl2 is a pure ferromagnet, whereas DyAl2 is almost completely ferromagnetic, but has the additional property of a weak antiferromagnetic ordering. The susceptibility measurements show an effective moment per molecule of 9.7±0.1 and 3.1±0.05 μB for DyAl2 and NdAl2, respectively. The neutron diffraction data show a rare-earth-ordered saturation moment of 9.1±0.4 and 2.5±0.1 μB on the Dy and Nd atom sites, respectively. The Al atom sites exhibit no measureable moment within the limits of experimental detection (0.5 μB). Both compounds show a temperature-dependent magnetic ordering process which obeys a Brillouin function. The experimental magnetic form factor for both the Dy3+ and Nd3+ ions diminishes more rapidly with increasing (sinθ)/λ than present theory postulates.

The neutron spectrum associated with the fission of U 235 induced by slow neutrons has been remeasured from 0.18 to 12 Mev. Two different experimental techniques were employed in this measurement: (1) the time-of-flight method covering the energy range from 0.18 to 2.7 Mev, and (2) the photographic plate method encompassing the energy range from 0.35 to 12 Mev. The combined results of these measurements fit the relation, N{E)^Ke^E' QMh sinh[(2.29£)»], where N(E) is the neutron flux, E is the neutron energy in Mev and K is a constant. The simpler expression, N(E) =KEh~°-17bE , also fits the data well for neutron energies below 9 Mev.f This work was performed under the auspices of the U. S.