POWTEX (under construction)

POWTEX scheme

The high-intensity TOF diffractometer POWTEX is designed and built by RWTH Aachen University and Forschungszentrum Jülich. The University of Göttingen cooperates by developing the texture analysis and by providing additional sample environments. Both projects have been granted by the German Federal Ministry of Education and Research (BMBF). POWTEX will be part of the JCNS instrumentation pool.

POWTEX is an acronym for POWder and TEXture because the instrument will fulfil the needs of solid-state chemistry, geoscience, and materials science communities with regard to powder and texture diffraction.

The instrument design combines several new concepts. The neutron guide system consists of two double-elliptic parts with a common focal point of 1×1 cm² at the position of the pulse chopper. Hence, the focal point is an “eye of a needle” in time and space, optimising the time resolution by the counter-rotating disks of the pulse chopper and reducing the background from the source. The second guide focuses on the small sample spot of 1×1 cm². The guide system was optimised using the Monte Carlo simulation package VITESS. Herein, several new developments were implemented:¹ The second guide will correspond to an octagonal cross section to achieve a Gaussian intensity and divergence distribution at the sample. The last part of the second guide is replaceable by an absorbing guide to reduce the beam divergence by a factor of two for the vertical and horizontal dimensions.

In order to produce sharp pulses of 10 µs, the pulse chopper uses two counter-rotating discs at 200 Hz and a short distance of only 0.5 cm. A variation of the chopper settings allows a flexible time structure to change the wavelength band or the pulse frequency (pulse-overlap mode like at POLDI, PSI yields a ten times higher intensity).

Because of the ³He shortage, two new detector developments were initiated (⁶LiF-WSF and ¹⁰B-Jalousie). While both prototypes successfully passed a test beam time, the ¹⁰B-Jalousie detector was chosen as the future POWTEX detector concept. The detector design will cover a huge solid angle (10 sr), and its dimensions will allow large sample environments (e.g. in situ experiments). The high detector coverage is of particular importance for texture measurements because it avoids the need for tilting the sample. Furthermore, it allows texture measurements without sample rotation (without sample environment), and it reduces the number of sample rotations with sample environment. It allows simultaneous strain/stress/texture measurements and recrystallisation analysis. For powder diffraction, the covered solid angle relates directly to the efficiency/measurement time. With the three-dimensional (2θ, TOF, intensity) diffractograms and adapted data treatment, POWTEX will simultaneously benefit from the higher resolution in back-scattering and the high intensity at lower angles.

[1] Houben, A. et al., NIM-A, , 680, 124 (2012).