VPT Visual Inspection and Grid Alignment

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Contents

• Equipment required for visual inspection
• Inspection procedure
• Alignment of anode grid
• Tips

Equipment required

You will need the following equipment to inspect a batch of VPTs.

• A clear desk
• Good lighting
• A notebook (or database) to record observations
• A knife
• A permanent transparency pen
• Equipment for grid alignment - see below
• A bin to dispose of discarded packaging
• A box to keep the VPT passports
• A tray to hold the inspected VPTs, with cardboard inserts to prevent them rattling around.

All of these items should be provided at the visual inspection station in the ECAL Supercrystal Assembly Area at RAL, along with a simple rig for aligning the anode grids.

Procedure for visual inspection of VPTs

Using a knife, carefully open the VPT box by cutting the wrapper.  The tray should then slide easily out of the outer box.  The tray and the foam end-pieces can be discarded.  Slide the VPT out of the foam wrapper, and proceed as follows.   Any anomaly should be noted.

• Remove the paper "passport".
• Check the flying leads:
  1. Pull firmly to ensure that the leads and pins are not loose.
  2. Inspect the wire-wrapping connecting the pins to the leads.  The joint should be strong, with no loose strands of wire.
• Check that the sealing stub of the glass envelope does not protrude through the end of the resin plug.
• Verify that the bar-code and the RIE production number match the passport details.   The passport may now be placed in the passport box.
• Examine the photocathode coverage on the face-plate.  The coating should be uniform and semi-transparent.  Typical defects are
  • Blotchy appearance of photocathode
  • Crescent-shaped area missing photocathode coating - estimate the area (<5%, 5%, 10%, 20% etc)
• Examine the anode grid.  This should be centred on the VPT axis, and stretched tight.  Typical defects are
  • Grid off-centre
  • Wrinkled appearance
  • Grid tilted
• Mark the anode grid alignment on the side of theVPT using the transparency pen.
• Store the VPT face-down in a tray for later testing.

Alignment of VPT grid

It is important to know the alignment of the anode grid before the VPT is tested in the high-field rig.  The visual inspection station has a simple alignment rig consisting of a low-powered laser, a screen, and a VPT holder.  The alignment procedure is as follows: follow the links for a pictorial guide.

• Place the VPT in the holder with the photocathode uppermost, and the yellow wire facing the open side of the holder.
• Switch on the laser and observe the interference pattern on the screen.
• Rotate the VPT holder until the pattern of dots is aligned with the vertical and horizontal lines on the screen.
• Use a transparency pen to mark the grid alignment on the side of the VPT.  The picture should be self-explanatory.  In words, mark the line where the envelope of the VPT intersects the plane defined by the laser beam and the VPT axis.
• If the pattern of dots cannot be seen, put the VPT aside for aligment with a microscope.

Tips

You may find the following suggestions useful.

• Inspect the VPTs in batches for extra efficiency - take all of the VPTs out of their boxes, then inspect them all, then carry out the grid alignment, and finally put them all into the tray.
• In some cases it is hard to see the diffraction pattern when the VPT is placed in the alignment grid.  If so,
  • try moving the laser across the face of the VPT
  • turn the lights down

Bruce Kennedy, 21 October 2002