Neutron powder diffraction in Nuclear physics institute of ASCR

 

Přemysl Beran

 

Nuclear Physics Institute of ASCR and Research Centre Řež, Ltd., 25068 Řež, Czech Republic

pberan@ujf.cas.cz

 

Research team of the Nuclear physics institute has long time experience with the neutron diffraction. The diffraction instruments used up to now were dedicated for studying mainly the strains and stresses in the bulk materials but classical powder diffraction was missing. This free space was filled by the new powder diffractometer that was build and made operational in the second half of 2008.

Nuclear physics institute acquired multidetector bank from the recently closed research reactor in Studtsvik in Sweden. Around this multidetector as a base part new instrument was build that would like to open the neutron powder diffraction technique to large number of new researchers and teams that want to discover advantages of neutron powder diffraction or researches using international neutrons facilities to make first reasonable experiments near home laboratory with easier access.

This new instrument is called MEREDIT (Medium resolution diffractometer) and it is incorporated in to the international NMI3 (Integrated Infrastructure Initiative for Neutron Scattering and Muon Spectroscopy) project. Within this project collaboration and experience exchange with other laboratories and institutions is possible.

Text Box: Figure 1. MEREDIT instrument: the detector bank of 35 3He counters is moved on air pads. The central HUBER goniometer provides enough space for placing the extensions. The beam size is adjustable by movable slits (middle-right).
The MEREDIT instrument is placed at the horizontal channel number 6 in the reactor hall of LWR15 (Light water reactor) in Řež. The default reactor operational power is 10MW and its operation/standby schedule is 3/1 week. The diffractometer consists of two large HUBER goniometer circles. The smaller one provides enough space for placing the different sample environments such as a cryostat, furnace, Euler goniometer, deformation rig, etc. The detector bank is mounted in a moulded neutron shielding made from boron carbide powder in epoxy resin. The bank consists of 35 3He counters with corresponding 10' Soller collimators. They are all individually adjustable and set at angular intervals of 4.00 degrees. The detector bank moves on air pads, which provides together with the stepping motor positioning accuracy down to 0.01 degree in 2q. Diffraction pattern can be collected in the angular range from 2 to 148 degrees in 2q.

Three different wavelengths can be selected using two different automatically exchangeable monochromators. The details about the monochromators and corresponding secondary beam parameters are given in the Table 1.

Table 1. Monochromators and secondary beam details

Monochromator

Reflexion

Wavelength (Å)

Minimum Dd/d (x10-3)

Neutron flux (n.cm-2.s-1)

Beam size

3 bent Si

monocrystals

(422)

1.271

3.9 (at 56° 2q)

~8.8x105

2x4 mm2

(311)

1.877

4.4 (at 59° 2q)

~8.6x105

2x4 mm2

3 mosaic Cu crystals

(220)

1.460

4.9 (at 71° 2q)

~3.6x106

4x4 mm2

 

All movable parts are driven by stepping motors and controlled by PC. The data from 35 counters are collected using two 24-channel Tedia cards. All communication and data collection is made with help of the special software ReMeSys (Regulation and Measurement System). This software is under continuous development to provide the best interface for out diffraction instruments and can easily adopt new extensions and experimental needs. The experiment flow is controlled by scripts written in Pascal like language what provide high flexibility needed for automation of complex experiments.

Text Box: Figure 2. In-situ deformation measurement with deformation rig on the MEREDIT instrument.
There are several sample environments to enhance the measurement ability of the instrument: vacuum furnace able to cover the measurement from the room temperature up to ~1000°C; close cycle cryostat operating from room temperature down to ~10K. For textured samples, it is also possibility to mount up the Euler goniometer. A special deformation rig permits in-situ measurements under uniaxial stress/pressure or fatigue cycles (see Figure 2.).

Collaborations with external researchers were established and the first interesting results of diffraction studies of inter-phase strain in the ZrO2/TiCN ceramics (Catholic University of Leuven, Belgium, see Figure 3.) or hydrogen storage materials ScAl0.8Mg0.2 (University of Uppsala, Sweden) are ready to be published. By using the deformation rig we studied in collaboration with the Institute of Physic of Materials of ASCR the deformation of TRIP or chromium-aluminium steels. Occasionally the instruments is also used as a testing facility for orientation the single crystals used for development of the new monochromators.

Text Box: Figure 3. Measured and calculated neutron diffraction pattern of sintered ZrO2/TiCN ceramic sample. Pattern was measured with step of 0.05° and delay of 200 seconds per one step (total experiment time 4.5 hour).
In collaboration with the Faculty of Mathematics and Physics of the Charles University we plane to use this neutron powder diffractometer for study of the magnetic structure of intermetallic compounds.

The MEREDIT instrument has the power to be universal neutron powder diffractometer which provide easy access to the advantages of neutron diffraction technique and the researchers are welcome to try and taste the neutron science in practice.

Acknowledgement

Author would like to thank for financial support from Research Centre Rez (MSM2672244501).