A small-angle neutron scattering (SANS) diffractometer has been recently installed at the guide hall of the Dhruva reactor at Trombay. The diffractometer uses a BeO filter as the monochromator. The mean wavelength of the incident neutron beam is 5.2 Â, with a wavelength resolution of approx mately 15%. The angular divergence of the incident beam is±0.5 and the beam size at the sampl position is 1.5 cm x 1.0 cm. The scattered neutrons are detected in an angular range of 1-15 using a linear He3 position-sensitive gas detector. The Q range of the diffractometer is 0.018 0.32 A~'. The diffractometer is well suited for the study of a wide variety of systems having charac teristic dimensions between 10 and 150 A. This paper gives the design features and the resolutio function of SANS diffractometer.

Thermal neutrons (energy~ 25 meV) from nuclear reac-An indigenously developed SANS diffractometer has tors or accelerator-based neutron sources are now rou-been operating at CIRUS reactor, Trombay for the last te tinely used for studying the microscopic structure and years or so3, 4. This diffractometer has been used by sci dynamics of the condensed matter. Neutron scattering, in tists at the Bhabha Atomic Research Centre, Mumbai a fact, consists of a family of techniques and different tech-university researchers5" 17. With the availability of guides niques explore the different aspects of structure and dyna-at the Dhruva reactor, the above SANS diffractometer has mics of the materials. The technique of small-angle been suitably modified and installed in the guide labora neutron scattering (SANS) is used for studying the struc-tory of the reactor. The neutron flux at the Dhruva reactor ture of a material on length scale of 10 to 1000 À (refs (~ 1.8 x 1014 n/cm2/s) is more than that at the CIRU 1, 2). In particular, SANS is used to study the shapes and reactor (~ 6 x 1013 n/cm2/s). Moreover, the guide labo sizes of the particles dispersed in homogeneous medium. tory provides an environment where the general neutr The particle could be a macromolecule (biological mole-background is quite low. In view of these two facts an cule, polymer, micelle, etc.) in a solvent, a precipitate of the modifications in the instrument, the SANS diffrac material A in a matrix of another material B, a microvoid meter at Dhruva provides data which are of much bett in a metal or a magnetic inhomogeneity in a nonmagnetic quality than those obtained earlier at CIRUS. This pap matrix. In suitable cases, SANS also provides information gives the details of the SANS diffractometer, which i about interparticle interactions. now operating at the Dhruva reactor. The effect of instru SANS is a diffraction experiment. It involves scattering mental resolution function as obtained using Monte Carlo of a monochromatic beam of neutrons from the sample simulations has also been discussed. The plan of the paper and measuring the scattered neutron intensity as a tunc-is as follows. First, we give a general discussion on the tion of the scattering angle. It is different from conven-design of a SANS diffractometer in the next section. The tional diffraction experiments, where the structure of the design features of the SANS diffractometer at Dhruva are material is examined at atomic resolution (~ 1 Â). The then given, followed by details of the instrumental resolu wave vector transfer Q (= An sin 0/A, where 20 is the scat-tion function. Next, the calibration procedures and the tering angle and A is the neutron wavelength) in SANS results of the typical measurements are discussed. In the experiments is small, typically in the range 0.001 to end, a summary is given. 1.0 A"'. To obtain low Q values, SANS instrument uses …