Speaker
Description
The Science case of a macromolecular single crystal neutron diffractometer is based on two pillars: The first one is given by the fact, that neutrons are scattered from the nuclei. In particular, hydrogen has a scattering cross section in the same order of magnitude than carbon, nitrogen or oxygen. This renders neutron scattering complementary to x-ray or electron scattering or transmission electron microscopy techniques (cryo-EM) [1,2]. This first pillar has been challenged recently by the cryo-EM technique [5]. The second pillar is the much smaller radiation damage caused by the neutron beam as compared to other techniques. This pillar will not be challenged in the near future by other techniques. It even allows to collect data at room temperature or human body temperature, where biological macromolecules are active and perform for example enzymatic functions. The disadvantage of single crystal neutron scattering is the need for a crystal size which typically measures between 0.1 and 1 mm³, which is a challenge to grow. But the small sample size makes the technique of neutron single crystal diffraction profit from small but intense neutron sources. Due to Liouville's theorem, instruments with a small sample cross section profit from neutron sources with small radiating surfaces more than for example small angle scattering instruments which usually work with a larger sample cross section of 1 cm by 1 cm. In this study, we want to motivate the construction of a single crystal diffractometer for so called High Brilliance Compact Accelerator-driven Neutron Sources (HiCANS). As an example, we want to assess the feasibility of an instrument at the Jülich High Brilliant Neutron Source (HBS) [3] and explore its capabilities in terms of resolving large unit cell axes exceeding 150 Å, which are common in protein crystallography and present significant challenges for neutron beamlines. Modern time resolved detectors will allow a shutterless operation based on neutron events sorted into Bragg reflexes or background respectively. We will discuss the role of shifting the wavelength band as an optimization between the scattering power of the crystal and the resolution needs for answering the requested scientific questions. This feature and the length requirement for a sufficient resolution in reciprocal space might render an elliptic or ballistic neutron guide concept more feasible as the previously discussed Selene concept [4].