I’ll Cover You

Today, we took the system down to begin a substantial task:  putting channel covers on the analog boards.

First, a bit of detector physics background. The NCT detectors are germanium “cross-strip detectors.”  They are wafers of germanium 1.5 cm thick x 7.8 cm tall x 7.8 cm wide.  On each of the two faces of the detector, there are 37 aluminum strips, each 2 mm wide.  The strips are horizontal on one face and vertical on the other, so when a gamma-ray photon interacts  and deposits energy inside the crystal, we see a “hit” on two crossed strips, one on each face.  Thus, we have an X and Y position.  By using the time difference between the signals on the two faces, we can infer the depth (Z) of the interaction, and hence the 3-D position.  If the photon scatters several times through the detector, we can use the Compton scatter formula to reconstruct the direction of the initial photon to an annulus on the sky.  This is Compton imaging, and NCT–the Nuclear Compton Telescope–is thus a Compton telescope.  Compton imaging allows us to image with a very wide field of view (roughly half the sky) while efficiently rejecting background.

One NCT germanium detector, showing the horizontal and vertical strips.

One NCT germanium detector, showing the horizontal and vertical strips.

A schematic of Compton imaging with the NCT detector array.

A schematic of Compton imaging with the NCT detector array.

The detector channels are read out by analog electronic boards stored in “card cages,” one card cage per board.  To decrease noise, we need to cover each channel with a metal cover–that’s the task we began today.  Each detector has 37 strips per side, plus a few extras used for guard rings, for a total of 80 channels per detector (broken into eight boards of ten channels each).  With ten detectors, this means we have 800 channel covers to install!  Each board takes about 20 minutes to cover.  Thankfully, we have six grad students…  As Mark said, today we spent our day at the various “stations of the card cage.”

Supplies:   One analog board, a stack of fastened covers, and a screwdriver.

Supplies: One analog board, a stack of fastened covers, and a screwdriver.

Remove all the screws.  Don't lose any!

Remove all the screws. Don't lose any!

Begin fastening the covers onto the board.

Begin fastening the covers onto the board.

After fifteen minutes, one board down!  Twelve more to go...

After fifteen minutes, one board down! Twelve more to go...

While we’re covering the channels, we’re also trying to improve the thermal properties of the card cages by spreading heatsinking compound (or “thermal gunk”) on their contact surfaces.  This chalky paste helps heat move from the electronic boards into the card cage crates and dissipate into the electronics bay.

Mark applies heatsinking compound to a board.

Mark applies heatsinking compound to a board.

The boards are held (and heat transfered) through thin rails on the sides.

The boards are held (and heat transfered) through thin rails on the sides.

Despite the repetitive nature of the job, we made very good progress on the covering today and kept our good humor.  I’m certainly grateful that I have new music to listen to on my Ipod, though…

One more event of note:  last night, Jane cooked a lovely Passover seder for all of us at the grad student house, and we celebrated Zhong-Kai’s birthday.

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