Laser calibration

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Both the prototype and final TPC have aluminium strips on the central cathode that are intended for laser calibration. To that end a plane of UV laser light is sent into the TPC intersecting with the strips. Each laser pulse should then create a bunch of electrons with well defined position and time at each of the strips.

Photoelectron efficiency tests

One worry with the intended geometry is that the laser hits the far strips at a very glancing angle, and it was assumed the photoelectron efficiency (i.e. number of photoelectrons per photon) would go down in favour of reflection. To prevent this two methods of surface treatment were considered, and both the validity of the aforementioned assumption as well as the effectiveness of the treatments were investigated in a test setup at UBC. Details and results are given in this report.

Laser operating procedure

The following describes the standard operating procedure for running the laser setup on the prototype.

Handheld control unit


NEVER have the key inserted in the "ignition" when the optical fiber is disconnected, the black box is open, or the TPC is open (e.g. gas lines disconnected).


Manual shutter: left open, right closed
  1. Check integrity of enclosure, i.e. optics box closed and bolted down, fiber attached at both ends.
  2. Turn on laser system with key on main unit.
  3. Push button "Flash lamp -> Ready" on handheld unit.
  4. Push button "Flash lamp -> Start" on handheld unit.
  5. Set collimator via mvat program on computer.
  6. Open manual shutter on laser head.
  7. Push button "Q-switch -> Start" on handheld unit.


Short pause

Simply push stop/start for Q-switch.

Longer pause

  1. Stop Q-switch.
  2. Stop flash lamp.
  3. Close manual shutter.
  4. After pause restart according to after-key part of start-up procedure.


  1. Stop Q-switch.
  2. Stop flash lamp.
  3. Close manual shutter.
  4. Wait a few minutes for cool-down.
  5. Turn off and remove key.

General best practice

  • Limit laser shots to a minimum, to reduce wear on laser, flash lamp, and fiber.
  • To that end, do non-critical steps like attenuator setting and DAQ start before starting Q-switch.
  • Ideally allow system some time to warm up between turning on flash lamp and Q-switch.


Laser requirements

MEMS Multiplexer

Raspberry Pi setup

Operating System

We're running the most recent version of Raspberry Pi OS as downloaded from

Required packages

Install the following with apt get:

  • emacs
  • libcap-dev (required to access SPI as non-root user)

SPI library

The recommended way to interface with SPI from a raspberry pi is now no longer using the wiringPi library, instead recommends bcm2835. This is a direct hardware C library for pin communication that includes examples for GPIO, SPI, and I2C.

Access SPI as non-root user

Following the instructions from [1] non-root access to the SPI pins requires the following:

sudo apt install libcap-dev
  • add current user to kmem group
sudo adduser $USER kmem
  • give group kmem access to /dev/mem
echo 'SUBSYSTEM=="mem", KERNEL=="mem", GROUP="kmem", MODE="0660"' | sudo tee /etc/udev/rules.d/98-mem.rules
sudo reboot
  • uncomment the BCM2835_HAVE_LIBCAP line in src/bcm2835.h
sed -i 's/\/\/#define BCM2835_HAVE_LIBCAP/#define BCM2835_HAVE_LIBCAP/' src/bcm2835.h
  • compile libbcm2835 as normal
sudo make install # (make check fails)
  • in addition to -lbcm2835, compilation of example or own code requires -lcap, e.g.
gcc -o spi spi.c -l bcm2835 -lcap
  • after compilation, give program permission to access cap
sudo setcap cap_sys_rawio+ep <progname>

C++ classes to simplify the use are provided in the mems code.