This Letter presents neutron diffraction data from a novel, biologically relevant, lyotropic membrane system which is highly alignable (#1.0 ± mosaic) in a magnetic field and gives rise to a number of welldefined Bragg reflections. The system, composed of two different phosphorylcholine lipids, undergoes a rare nematic ! smectic phase transition upon doping the system with paramagnetic ions (e.g., 2.7 wt % Tm 31 ). In addition, the isotropic phase occurs at a lower temperature than the smectic phase, in contrast to other lyotropic systems and in contrast to the phase behavior predicted by the McMillan model [Phys. Rev. A 4, 1238(1971] of smectic ordering. [S0031-9007(96)

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Measurements of the effective absorption coefficient I" have been made, for a range of neutron energies, of large single crystal specimens of quartz, silicon and sapphire, at 300 K and 80 K. The effectiveness of such filters is given by the ratio of I" for (unwanted) epithermal neutrons to that for the desired thermal neutron beam. Our results indicate that sapphire is significantly more effective than either quartz or silicon for a wide range of neutron energies.

Transmission measurements have been made at 80 and 300 K for sapphire and beryllium filters in service on a triple-axis neutron spectrometer. The measurements agree with earlier work and confirm that sapphire is a more effective neutron filter than either quartz or silicon. Curves of transmission as a function of neutron wavelength are presented in a form useful for planning thermal neutron experiments in which higher-order thermal neutrons and fast neutrons must be removed from the incident beam.

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