[B._Beckhoff,_et_al.]_Handbook_of_Practical_X-Ray_(b-ok.org).pdf

From the oxidizing influence of the o 2 from the air

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from the oxidizing influence of the O 2 from the air. Two alternative approaches exist: the vacuum-compatible chamber, pumped by a primary ( mbar) or secondary ( 10 6 bars) pumping system and the He chamber. The former is quite cumbersome as it requires real o-rings or gaskets and a rigid structure which cannot be miniaturized. Very often, the whole microprobe setup is en- closed in such a chamber of large dimensions, which puts a strict requirement on all the motor drives which must be vacuum-compatible and not heat up. The He chamber, on the other hand can be miniaturized as it does not need a rigid structure. Miniature plastic chambers can be easily adapted to the topmost sample stages and filled with He at atmospheric pressure. The cham- bers feature windows for the incident and transmitted beams made of Kapton or Mylar films and sometimes the detector is also inserted in the chamber through a window. The whole chamber can be made of a polyethylene bag surrounding the sample, the detector snout and the beam in/out windows. Sample Holder Sample holders exist in a very large variety, both commercial and homemade. The most frequent ones are systems from Nonius or Huber, which consist of goniometer heads which can accept cryo-loops or magnetic fixtures for fast lock-in. Alternatively, the samples can be glued or sandwiched between thin ultrapure plastic film such as Kapton or Ultralene of a few microns thickness and fixed on a 35 mm film slide. The slide can then be easily manuipulated and clamped into position on a special clamp which allows easy mounting on the sample topmost stages. For micropositioning, sometimes sample holders are transfered from light microscopes which allow comfortable positioning on indexed remote-controlled stages identical to the ones on the beamline. Once a particular zone has been aligned on the microscope, the whole sample holder can be transfered to the beamline drives and the position of the microscope cross-hairs will be at the center of the X-ray microbeam. Microscope A horizontal conventional microscope of high magnification ( 100) is rou- tinely installed on the beamline, linked to the outside high resolution one. The beamline microscope must have a large working distance, normally 3.5– 10 cm, so as to allow a variety of relatively bulky sample environments (oven, DAC, cryostats, etc.). It must also have a remotely controlled zoom allowing to change magnification on the sample face from outside the experimental hutch and gradually focus on the zone of interest. Commercial systems from Mitutoyo, Olympus or Questar are routinely used. Detector This undoubtedly is the most critical link in the whole chain of items found in a photon microprobe. First of all, to avoid useless saturation of the counting
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480 A. Simionovici and P. Chevallier system, the detection geometry takes into account the large horizontal po- larization of the synchrotron beam. To minimize scattering from the sample, the detector is placed in the orbital plane and perpendicular to the beam direction.
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