This is an optional step if you're using the carrier boards. If you're not and just soldering wires directly to the SiPM or doing it otherwise, skip this part. Note, however, that soldering wires to the SiPM itself is much harder than using the breakout board.

You can choose to add the optional SMD components, to add some more power decoupling. This might help stability especially for longer cable lengths. If you're doing so, bridge the solder jumper and connect wires to VCC, GND and A(node) pads. If you're not using the SMD parts, solder directly to A(node) and C(athode) and the jumper remains open.

Center the SiPM on the scintillator crystal and put some silicon grease or other special coupling material between the two parts to optimize the optical coupling (and minimize reflections). This step is important!

Use black electrical insulation tape or similar non-transparent material to wrap the whole assemby, but watch out for the cables, of course. This will reduce light passing to the SiPM to an absolute minimum, otherwise it won't work properly. You should use multiple layers of tape just to be sure.

Tip: I'm using tightly-wrapped Kapton tape on the outer-most layer to avoid the insulation tape from getting too loose.

Solder all the components of the main detector board. Orient yourself with the schematic and BOM. You can skip this step if you got yourself a fully assembled one, obviously. If you need more info, you can also have a look at the GitHub repo.

With a multimeter, adjust the SiPM voltage by turning the potentiometers. Each full turn adds about 250 mV to the output, so you have plenty of resolution here. The potentiometers are connected in series, so you can use either and/or both of them to get to your voltage. Once you set it, you don't have to change it ever, if you remain in the same temperature region (e.g. indoors).

If you are using the recommended MicroFC-60035 SiPM, the best voltage is at about 29.6 V. As a rule of thumb, you can look up the datasheet for your SiPM and use the breakdown voltage + the maximum recommended overvoltage and subtract 100 mV or so for good measure, just to be safe.

This can be done quickly using just a USB connection and the Arduino IDE or download the ready-to-use firmware files and drag-and-drop it on the microcontroller. If the pico doesn't get recognized for any reason, try holding the "BOOTSEL" button while plugging it into the USB port.

The last step for this to work is to set the peak discriminator to a useful level. This is done using the "threshold" potentiometer on the main detector board. Turn it until you get to a certain point where the ACTivity LED goes from solid on (very bright) to blinking (more dimly), now just turn it a bit in the direction of less blinking to be sure to not get noise in your signal. You can also connect the Pico straight away to Gamma MCA and read your current counts per second from there.

You're aiming somewhere in the neighborhood of 10 - 50 cps for the background radiation, which, again, highly depends on your scintillator material, size and SiPM. However, it definitely shouldn't be close to 0 or in the range of 100s or even 1000s of cps. Using the spectrum can also help since if the threshold is set too low, you'll have a noise peak to the left of your spectrum.

This will shift the lowest registered energy for the spectrometer, i.e. the left end of the spectrum.

This will change depending on your set SiPM voltage, you'll find the best value over time and experimenting. You'll need to reset this if you change either of these parameters.

That's it, welcome to the world of gamma spectroscopy! Happy tinkering! Your end result for the background spectrum should look something like this:

If you didn't get the correct settings right away, don't be sad. Depending on your setup, you need to experiment with the gain and threshold settings a little bit and with every change see how the detector reacts.