We've just got some new analogue circuit boards. This post is about how we're testing them.
Three of our prototype new analogue amplifier boards
You can find the design files for them here.
How to test a new hardware module? It's a good question. Here are the stages:
1) Visual inspection of the assembled PCB's (these were manufactured and assembled by Seeed studio using their Fusion service - you can find it here:
One SiPM pulse measured on the oscilloscope. Note that the timebase is wrong, it should be 200ns
5) Test with a scintillator. Next we inserted the SiPM's into our scintillator slab and reduced the bias voltage to lower the dark count back to a background figure. Now we looked at pulses from the two analogue outputs on the oscilloscope and check for coincidences visually, and also noise.
Two peaks from two SiPMs inserted into a single scintillator slab. It might even be a muon!
Once we have a scintillator connected we can also profile the accuracy of our amplifier, in terms of our ability to resolve single Photo-Electrons (PE's), these are the nominal minimum unit for detection using an SiPM or MPPC device.
[Etam demonstrates PE levels using persistence on the oscilloscope]
6) Test it with a known good scintillator detector. We used a pair of reference scintillators and PMT's set up with a NIM crate in the lab, together with a coincidence unit to filter for muons.
Stacking the slabs vertically to produce simultaneous coincidences in both sets of slabs.
The output from two slabs (one SiPM embedded in each), was then aligned (physically above) the reference slabs so that the overall area was identical. The output from the SiPM analogue channels was connected to a discriminator and coincidence unit, and then connected to measure coincidence with the reference scintillators. For both the Cosmic Pi and the reference detector there are two variables to be set for each channel; the bias voltage applied to the sensor (SiPM or PMT) and the threshold for an event on that channel. The bias voltages are set according to the temperature and noise figure of each device. The thresholds for all detectors were set to meet the following criteria:
A) Rate of detected muons = 0 when the two slabs are not overlapping
B) Rate of detected muons when the two slabs are overlapping is maximised, but does not exceed the background muon rate of approximately 1 muon/cm2/minute.
Two peaks from two SiPMs connected to the Cosmic Pi Analogue amplifier board, occuring at the same time as a coincidence on the reference scintillators. This is very likely to be a muon!
7) Perform a statistical analysis of the results - this is the stage that we're at right now! Stay tuned for more progress...
A simple counter collects events on the reference detector, the Cosmic Pi analogue circuit with a discriminator and coincidence unit, and finally the coincidence of the Cosmic Pi with the reference detector.
Here we plot the timing distribution of signals captured by the Cosmic Pi and the reference detector. Both are the type of distribution we would expect, however we still have more work to do.
Here we plot the timing distribution of signals captured by the Cosmic Pi and the reference detector. Both are the type of distribution we would expect, however we still have more work to do.
