Constant Flux and Time-of-Flight Instrumentation

Abstract

Use of small-angle and ultra-small angle scattering instruments in a tandem provides a unique opportunity of exploring sample structure on the length scales between Angstroms and microns in a single experiment. This chapter considers major types and components of small-angle neutron and x-ray scattering instruments. Constant flux instrumentation is discussed based on the General Purpose SANS diffractometer (HFIR, ORNL) and the double-crystal USANS instrument (NCNR, NIST). Operational principles and major components of time-of-flight SAS instrumentation are considered based on the Extended Q SANS and the Time-Of-Flight USANS instruments (SNS, ORNL). X-ray SAS instrumentation is discussed based on SAXS and USAXS diffractometers available at APS, ANL. Advantages and disadvantages of the constant flux and time-of-flight instruments for various types of experiments and different systems are discussed and illustrative examples of the scattering data obtained from various samples using SANS/ USANS and SAXS/USAXS instruments are presented. General Purpose SANS Instrument at HFIR, ORNLIn the early 2000s, a series of upgrades was undertaken at HFIR, including replacement of the beryllium reflector, installation of a supercritical hydrogen cold source (T~20 K) in the horizontal beam 4 (HB-4) tube, and construction of a guide hall that hosts a suite of cold source instruments including a GP-SANS instrument on cold guide 2 (CG2) and a biological SANS instrument (Bio-SANS) on cold guide 3 (CG3) that is optimized for the study of biological systems. In addition to the SANS facilities, a suite of other instrumentation has been installed (see the current guide hall layout in Fig. 3.1 The GP-SANS instrument [1] can be divided into four major components: the delivery system that brings neutrons to the sample, the sample area, the post sample flight path/detection system, and the instrument control and data handling electronics.The neutron delivery system consists of the cold source, main guides, velocity selector, and removable guide sections. The energy spectrum of neutrons in thermal equilibrium with the water moderator kept at a constant temperature T is described by the Maxwellian distributionwhere λ and m are the wavelength and the mass of the neutron, respectively, and k B is the Boltzmann constant. The temperature of the water moderator is usually close to room temperature. Hence, the corresponding neutrons, with a maximum peak flux around λ~1 Å, are called thermal neutrons ( Fig. 3.2). SANS experiments require neutrons with a considerably longer wavelength, and the Maxwellian energy spectrum can be shifted towards lower energies (longer wavelengths) by passing them through an additional moderator (i.e., cold source) near the reactor core that is maintained at a much lower temperature. The cold source used in HB-4 is situated close (<40 cm) to the center of the HFIR core. It represents an aluminum vessel filled with 505 mL of supercritical hydrogen that is refrigerated to~20 K at a pressure of 14.5 bar (Fig. 3.3). The cold ...