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ID29 - attenuators

last modified 17-09-2008 09:39

ID29 Attenuators


Two different sets of attenuators exist on ID29: white beam attenuators and monochromatic beam attentuators. The white beam attenuators are located just before the monochromator, while the monochromatic attenuators are located in the experimental hutch before the slit box.


White Beam Attenuators (return to Quick Index)

There are 3 attenuator axes each with 4 different attenuating filters which can be inserted into the white (un-monochromated) X-ray beam. The beam "sees" axis 1 first (known for command purposes as "attf" standing for attenuator-front), axis 2 second ("attm", attenuator middle) and axis 3 third ("attb", attenuator back). Due to the power in the unattenuated white beam, it is necessary that attenuators be inserted in sequence i.e. if you wish to insert an attenuator from axis 2 (attm), then you must have previously inserted an attenuator from axis 1 (attf).

To move an attenuator into the beam you require to "send" it to one of its 5 different pre-set positions (4 different filters plus a position where the attenuator is completely removed from the beam). This is done through the CONTROL PANEL GUI or directly from the OPTICS control window:

OH> wm attf attm attb   [returns the positions of the front, middle & back attenuators]
OH> mv attf 1   [moves axis 1, the front attenuator, to position 1 i.e. out of the beam]
OH> mv attf 5   [moves axis 1, the front attenuator, to position 5 i.e. 10mm of pyro-carbon in beam]
OH> mv attm 2   [moves axis 2, the middle attenuator, to position 2 i.e. 0.93 mm of Al in beam]
OH> mv attb 2   [moves axis 3, the back attenuator, to position 2 i.e. 2.9 mm of Al in beam]

One of the most likely causes of "low or no" beam is when attenuators have been introduced and not removed.

The following white beam attenuators are available. The plots of the calculated transmission vs energy for each attenuator can be obtained by following its link.

Table of White Beam Attenuators (return to Quick Index)

position 1 2 3 4 5
attf (axis 1) empty 3mm pyro-carbon 5mm carbon ATJ 5mm pyro-carbon 10mm pyro-carbon
attm (axis 2) empty 0.93mm Al 1.84mm Al 2.43mm Al 2.9mm Al
attb (axis 3) empty 2.9mm Al 2mm pyro-carbon 1mm pyro-carbon 0.5 mm pyro-carbon

Monochromatic Beam Attenuators (return to Quick Index)

    The monochromatic attenuators are a series of independently controlled foils of varying thicknesses so that different foil combinations can give the desired beam intensity.

    To attenuate the beam with the monochromatic attenuators use the mattset and mattstatus commands. NOTE: These commands identical under both EXP and OPTICS SPEC sessions. E.g.:

      EXP> mattstatus [returns the positions of all monochromatic attenuators]

      EXP> mattset 8 [inserts attenuator "8" = 1mm of pyro C]

      EXP> mattset 0 [removes all attenuators]

    The monochromatic attenuators are assigned values which follow a binary scale (1,2,4,8,etc..) with increasing attenuation factor. Different combinations of attenuators can be used by entering a mattset value between 0 (all out) - 255 (all in), e.g.,

      EXP> mattset 35 [inserts attenuators 32, 2 & 1]

      EXP> mattset 17 [inserts attenuators 16 & 1]

      EXP> mattset 2049 [inserts attenuators 1 & 2048]

    Table of Monochromatic Beam Attenuators (return to Quick Index)

    mattset 1 2 4 8 16 32 64 128 256 512 1024 2048
    thickness 0.1mm 0.25mm 0.5mm 1mm 2mm 4mm 0.1mm 0.25mm 0.5mm 1mm 2mm 4mm
    material pyro C pyro C pyro C pyro C pyro C pyro C Al foil Al foil Al foil Al foil Al foil Al foil

    Table of Monochromatic Beam Attenuator Transmissions (return to Quick Index)

    mattset 1 2 4 8 16 32 64 128 256 512 1024 2048
    Fe K-edge
    transmission    		 ? 71.4% 50.6% 27.3% 11.4% ? ? 1.39% 0.712% 0.538% 0.260% ?
    Cu K-edge
    transmission    		 ? 84.6% 71.2% 50.4% 26.9% ? ? 8.46% 0.711% 0.055% 0.039% ?
    Se K-edge
    transmission    		 ? ? ? ? 59.0% ? ? 35.0% 13.4% 5.20% 1.61% ?

 
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