Light leak testing for MilliQan bars

Tue Feb 8, 2022 16:04

The following describes light leak testing procedures for the MilliQan bars.

First coarse test

First, it is good to check for large light leaks by measuring the pulse rate with and without room lights on. To avoid damage to the PMT, it is best to do the first test at a relatively low voltage and then ramp up the voltage in steps. For example, running at +500 V the rate of pulses above 30 mV should be less than 100 Hz and due mostly to cosmic ray backgrounds (including gammas from nearby cosmic ray muons). Then at +1000 V, the rate should be less than or about 1 kHz, and there should be no significant difference between the rate measured with room lights on and off. Finally, you can run at the nominal voltage of +1400 V and the rate of pulses above 30 mV should be a few kHz, and maybe up to 30 kHz. Anything higher than that indicates that there is likely a large light leak somewhere.

If a large light leak is located with any of the tests described above, it is best to locate and fix that large light leak before proceeding to find smaller ones. You can locate a large leak by running with the room lights off and moving a flashlight (i.e. a phone light) along the length of the bar to see where the rate jumps up.

Typical sources of problems are the corners of the square end of the bar, the edges near the PMT end, and particularly the place where the LED leads come out of the PMT mount.

You can fix these with local application of an extra layer of black tape. However, beware the HV risk near the place where the base connects to the PMT. A small gap between them can leave the PMT pins exposed, with HV on them.

It is also common for there to be "general" light leaks along the bar if the tape is not completely opaque. Wrapping another layer of tape is faster than trying to find every small hole.

Test for small leaks

To find small leaks, the approach of watching the trigger rate change with flashlight position is arduous. For more direct feedback, you can use the "Light leak tester" board to scan over the bar. It connects to the output of the PMT amp board and has a comparator that flashes a small red LED every time a pulse is detected above an adjustable threshold. The bottom of the board has a blue LED that you scan over the bar; when that hits a light leak, the red LED will brighten. This provides finer resolution scanning and rapid, visual feedback for the light leak location. This works best with the HV set at maximum, i.e., +1400 V, so every photon detected will cause a brief flash of the red LED.

Photos of the light leak tester are below.

Photos of the light leak tester. On the left with an arrow indicating the red indicator LED. On the right scanning along bars looking for locations where the red LED brightness increases noticeably.

The ribbon cable is the same pinout as the one used to power the PMT amp boards. Alternatively you can just power it by two jumper wires with +5V and ground connected as shown below.

Documentation on the light leak tester board

The tested board was laid out in EasyEDA. The schematic and board files are shown below (click for PDF).

The source files are: Schematic and PCB