Time-of-Flight Diffractometer Developed for Biological Small-Angle Scattering at Argonne’s Intense Pulsed Neutron Source, IPNS-I

Abstract

A low resolution time-of-flight (TOF) small-angle diffractometer has been installed and tested at the IPNS-I pulsed spallation neutron source at Argonne National Laboratory. The instrument views at room temperature a polyethylene moderator, 10 × 10 × 5 cm, surrounded by graphite with a beryllium reflector and is decoupled with cadmium. This instrument currently employs a single-beam focusing collimation system over an 11-m flight path with a 3-m sample to detector distance. A two-dimensional position sensitive detector, 17 × 17 cm2 in area, is utilized in conjunction with an on-line microprocessor to detect and process scattering events. The time-averaged neutron flux on the sample has been measured as 2.24 × 104 neutrons/cm2/sec over the wavelength range of 0.5 to 5.0 A, without correction for detector efficiency. The time-averaged background flux just outside the penumbra of the beam has been reduced to 10−6 the main-beam intensity. Small-angle scattering has been observed for several biological specimens which have been inserted by use of an automated temperature controlled sample changer. Normal hemoglobin in aqueous saline solution, measured over a concentration range of 50 to 250 mg/ml, has a radius of gyration (Rg) of 26.5 A. Cytochrome c in 99.8% D20 at a concentration of 50 mg/ml has yielded an Rg value of 12.4 A. Scattering from canine high density lipoprotein at a concentration of 40 mg/ml has been observed at different H2O/D2O solvent ratios and will be used to determine the solution structure of the protein core and to study the incorporation of exogenous lipid into the particle. (This work is supported by the U.S. Department of Energy under contract No. W-31-109-ENG-38.)