The construction of the instrument MEREDIT (Medium Resolution Neutron Powder Diffractometer) was finished at the end of the year 2008. It is placed on the horizontal channel number 6 in the reactor hall of LWR15 (light water reactor) in Řež near Prague. The default reactor operational power is 10MW and its operation/standby schedule is 3/1 week. The essential part of the diffractometer - a multi-detector bank - was achieved from the closed research reactor in Studtsvik in Sweden.
The diffractometer consists of two large HUBER goniometer circles. The smaller one provides enough space for placing the different sample environments (see below). 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°. The detector bank moves on air pads, which provides together with the stepping motor positioning accuracy down to 0.02° in 2θ. Diffraction pattern can be collected in the angular range from 2 to 148 degrees in 2θ.
Three individual wavelengths of the secondary beam can be selected using two different automatically exchangeable monochromators. The details about the monochromators and corresponding secondary beam parameters can be found here.
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 our 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.
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; sample changer - carousel - for up to six samples measured at room temperature. A special deformation rig permits in-situ measurements under uni-axial stress/pressure or fatigue cycles. For textured samples, it is also possibility to mount up the Euler goniometer.
Investigation of atomic and magnetic structure with all advantages and also disadvantages of the neutron powder diffraction. Using the different sample environments is possible to make temperature dependency of structural/magnetic parameters. With deformation rig in-situ strain and/or stress control diffraction experiment able to see multiple reflections of studied material.
The diffractometer has been incorporated in the international NMI3 (Integrated Infrastructure Initiative for Neutron Scattering and Muon Spectroscopy) project. Within this project collaboration with international institutions and researchers will be carried out and can be financially supported. For more information consult access page and the links therein.
Neutron powder diffraction
Here is example of the advantages for using the neutron diffraction technique:
- deep sample penetration - study of the changes taking place in the volume of the sample not just on the surface; possible to use closed sample environments
- see the light elements - neutrons can "see" the light elements as a hydrogen (deuterium) or oxygen; distinguishing elements standing beside in the periodic table
- direct interaction with magnetic structure - observation of the magnetic reflections -> study of the magnetic structure
There are also disadvantages when you work with the neutrons. The main disadvantages are listed below:
- large sample needed - one of the biggest disadvantage; for the good experiment we need the sample volume of few (3-4) cc
- small neutron flux - the reactor provide small neutron flux so the collecting time can be long