Large Beamstop for Detector Protection

Detector Overload Protection

It is your responsibility to make sure that you do not over expose the detector!

The image to the left displays severely overloaded pixels (green) which could permanently damage the mar345 image plate detector. Severely overloaded pixels may occur using short exposure times for very strongly diffracting crystals (including ice) or long exposure times, especially during ultra-high resolution measurements. More than 25 scan-erasure cycles were required to completely remove the residual signal from this single exposure. If many images were to be collected using the same crystal and exposure time, it would require many more scan-erase cycles to clear the plate or the plate may even become permanently damaged!

Overexposed image Normal image

Heavy overload of low resolution data may be eliminated by the use of the 'large diameter beamstop' provided at the beamline. The effect is shown in the picture to the left. Not only are the overloads eliminated but the reflections of reduced intensity can be used for auto-indexing and parameter refinement.

Installation of the Large Diameter Beamstop

Do Not Remove the standard beamstop shown below! If the standard beamstop is inadvertently adjusted, bumped, or removed, the support staff must be contacted before an exposure is taken.

Beamstop Stand Shown Before Installation 

Beamstop at BL 9-1, 9-2, 11-1

  

The large diameter beamstop may be quickly installed behind the standard beamstop.

  • Move the detector position back to at least 400 mm.
  • Place the high resolution beamstop on the beamstop base and then tighten the thumb screw as shown below. Do Not move any of the other adjustment screws!
At BL9-1, 9-2 & 11-1
1. 2.
 
3. 4.
  • The large diameter beamstop consists of an Al disc secured to a tungsten holder by a small screw.

  • To assemble the beamstop, choose a disc of appropriate size and attach it to the holder using the screw (below, left)

  • In cases where a single disc does not sufficiently attenuate reflections in center of the detector, a longer screw may used to attach two discs (above, right).

Determination of a Safe Exposure Time

It is your responsibility to make sure that you do not over expose and damage the detector!

  • The first exposure should be no longer than 5 seconds.
  • As a rule of thumb,  images containing more than 1500 overloaded pixels should not be collected.
  • The number of overloaded pixels can be determined by looking at the image in blu-ice.  Overloaded pixels are depicted in green.
  • If there are no overloaded pixels you may try doubling the exposure time.
  • Before starting data collection be sure to check the amount of overloaded pixels obtained with the crystal in different orientations.
  • The large diameter beamstop will allow you to increase your exposure time to obtain higher resolution data while still not damaging the detector.