Here you can find everything you need to build yourself a detector, or more.
This project utilizes a silicon photomultiplier (short SiPM) which is way smaller and more robust than a traditional photomultiplier tube and does not need a high-voltage supply (traditionally ~1000 V).
The following 6 mm SiPMs are recommended for this project:
- MICROFC-60035-SMT-TR
- MICROFC-60035-SMT-TR1 (the exact same as above, however will be more expensive to purchase as sold in smaller batches to suppliers)
- AFBR-S4N66C013 (a good replacement if the above are unavailable, but already officially EOL'd, so keep that in mind)
Here are some very helpful in-depth datasheets about the particular MicroFC SiPM recommended here:
- C-Series SiPM Sensors datasheet
- Linearity of the Silicon Photomultiplier
- Introduction to the SiliconPhotomultiplier (SiPM)
- Biasing and Readout of ON Semiconductor SiPM Sensors
The hardware consists of the main detector (hardware in this folder) which includes amplification, pulse detection and energy measurement. If you already have a SiPM/crystal assembly compatible with voltages around 30 V, you may use it with the detector board and connecting wires directly to the correct pads. Otherwise, you can use my SiPM carrier board, which holds the SiPM and all the optional decoupling as well as a temperature compensation circuit for the SiPM. There is also a carrier board for the Broadcom SiPM.
The heart of the detector board is the Raspberry Pi Pico 2 which uses its ADC to measure the pulse amplitude (i.e. the energy) immediately after an event occurs starting with an interrupt. I can really recommend you reading the datasheet or maybe also having a look at a deeper analysis of the Pico ADC (only relevant to Pico 1), if you're interested:
- Raspberry Pi Pico 2
Characterizing the Raspberry Pi Pico ADC(only relevant to Pico 1)
Here are some front and back side renders of the detector PCB. Size is about 120 x 50 mm. If you don't need the additional space to mechanically mount the SiPM/scintillator assembly to the rest of the detector board, you can just cut it off at the white line and you're left with an even smaller detector.
On the back side of the PCB there is also a jumper to connect the analog ground to the rest of the ground plane. You can solder that if you want to and you know exactly what you're doing. It's not needed, though, that's why it's open by default.
There are also broken-out pins for the power supply, I2C, SPI and UART connections. These can be used to modify the device, e.g. by adding a display or using a battery charger. You can have a look at the great Raspberry Pi Pico 2 datasheet for more info on this.
Some more info on the peripheral headers can be found in the software section of this project.
You can also put a little jumper from VBUS to VSYS if you only want to use the detector with the USB connection on the Pico. This bridges the diode on the Raspberry Pi Pico 2 and saves you a couple of mW.
As for the SiPM connections, there is a female 2.54mm pin header on the board and also two MCX jacks. The pin headers are just a very easy way to get everything up and running and are fine if you're not really touching them and the scintillator is mounted very close to the header (i.e. on the PCB).
If you want to have a more secure and more importantly shielded way of connecting the SiPM, use MCX cables and the corresponding jacks on the board. There are special MCX breakout cables that have the connectors on one end and then terminate to some stranded copper wire on the other side. You can use this to solder the cable directly to the SiPM breakout board and then have the plug on the other side. However, because you need two cables (one for power and one for signal), be sure to keep them as close together as possible (e.g. twist them together) as to not create a huge ground loop area.
Here is a helpful image about the potentiometer settings for revision 4.x:
As you can see it is pretty simple to set the required voltage and threshold. Just turn any potentiometer clockwise to increase the corresponding parameter, turn anti-clockwise to decrease.
There are two buttons on the device:
- A small one on the Raspberry Pi Pico 2 itself that is only used to reboot the device into bootloader mode. This is handy if you want to change or update the firmware of the device.
- A larger one next to the Pico that is used to toggle between spectrum and Geiger mode by short-pressing (<1 second) and toggling the buzzer by long-pressing (>=1 second) it.
The finished standard MicroFC-, tiny MicroFC- and AFBR- SiPM carrier boards are there to allow for easier packaging with the scintillator as well as to be reusable for different detectors as that's by far the single most expensive part and you'll want to use it as long as possible.
If you are using a larger-diameter scintillator, it's probably better for you to use something like the MicroFC SiPM Array Board, which has a 2x2 matrix of SiPMs resulting in 4x the active area. This will increase the energy resolution for large crystals.
I got most of my scintillators (used NaI(Tl), LYSO, ...) on eBay. Just search for some keywords or specific types, you'll probably find something. Just be sure to look out for signs of wear and tear like scratches on the window or yellowing (!) in NaI crystals as these can deteriorate performance significantly.
You can obviously also buy brand-new scintillators, however, these can be much more expensive. It really depends on the manufacturer there. You can get some cheap 1" x 1" NaI(Tl) scintillator from China for <100 USD.
Two sources of new NaI(Tl) scintillators are:
| Brand | Location | Comment |
|---|---|---|
| OST - Photonics | China | Can purchase directly from website |
| Epic-Crystal | China | Also sold by GammaSpectacular |
Since this is a fixed-gain device, I can only highly recommend you to get a scintillator that fits on the PCB and is of a comparable volume as the scintillators I tested. I am using an 18 x 30 mm crystal for most of the spectra that you can see here and it works great. Also, due to the small size of the SiPM in comparison to the scintillator opening, try to use narrower crystals than wider ones if you can. That way, there aren't many light losses to the SiPM. In general, just be sure the volume/size is not orders of magnitue larger or smaller. If that's not possible, consider using the array board. The array board works great with a 1" x 1" NaI(Tl) with this detector for example. There is also an optional trim pot for the gain on the Rev. 4 boards that you can solder and use to change the preamp gain.
If the scintillator is too small, the output voltage will be very low and you might get restricted by the ADC resolution. In this case the energy range will also be much higher than you would ever need (can easily get up to 10 MeV).
If the scintillator is too large, the output voltages will be pretty high and thus decrease your energy range dramatically. The device is limited by the 3.0 V reference voltage in this case, so you might see clipping. Your mileage my vary.
As a general rule of thumb, try to use as much of the space available on the PCB as possible to mount the scintillator and you will (in most cases) be absolutely fine.
You should apply some optical coupling compound between the SiPM and the crystal window to reduce reflections as good as possible (this way the best photon detection efficiency is achieved). There a multitude of options for optical couplants, and depending on your region some may be easier to aquire than others. As a general rule of thumb, silicone oil/grease can also be used and works fine. If it contains polydimethylsiloxane, you're on the right track. Some options:
| Description | Brand | Reflective Index | Viscosity (cPs) | Cost (USD) | Comment |
|---|---|---|---|---|---|
| Index Matching Gel, 3 cc | Thorlabs | 1.43 | 1,060,000 | $47.46 | High viscosity, works well for fibre optic coupling. |
| Index-Matching Fluid | Newport | 1.52 | 100 | $45.60 | Low viscosity may make setting the SiPM and crystal window difficult. However, the RI is the closer to the window properties. Use if you're experienced with assembly. |
| XIAMETER™ PMX-200 Silicone Fluid | Dow Corning | 1.40 | 60,000 - 100,000 | Depends on source | Low cost and used in research due to performance / price. PMX-200 can come in various viscosities, try to get close to 100,000 cPs. |
| Molykote Industrial 4 Electrical Insulating Compound | Dow Corning | N/A | Grease | ~$20 | Used by Ludlum for their survey meter scintillation detectors. |
After you applied some, you press both parts together and wrap everything with light-tight tape, again, I'm just using some black electrical tape here. That's essentially it, now you can solder some wires to the pads on the board to connect them together and secure it in place in the free space on the board.
It's very important to get the SiPM/scintillator assembly light-tight. Otherwise you'll either run into problems with lower energies where noise dominates or outright not measure anything at all, because the sensor is saturating (clipping signals).
==> More assembly instructions can be found on the Hackaday.io project page!
There is an additional LED on the board called ACT for "activity". It's directly connected to the comparator that handles the pulse thresholds and will light up for every impulse that's larger than the set threshold voltage for the duration of said impulse. That means generally you won't see much, because the count rates are pretty low and the pulses are really fast (couple of microseconds).
However, it's helpful when setting the correct threshold voltage, because as soon as you decrease it too low and completely get into the noise floor, the LED will visibly light up.
On top of that, since it's not connected to the microcontroller in any way, it's a nice backup for when the device crashes and/or the count rates get so high that it's completely saturating otherwise. The detector will display ZERO counts per second when it saturates at very high count rates: the count rate will actually gradually go down to zero the higher and higher your ACTUAL (real) count rate is. So if the device displays 0.0 cps, but the LED is lit up brightly, you either have a light leak or you should run for your own safety.
You can get a 3D-printable case for the Open Gamma Detector with different styles of covers.
All the STL files to print the two parts (main body and cover) can be found in /enclosure, as well as some more info on the necessary screws and the USB extension cable.
(This is an old image from Revision 2.0, but the footprint is exactly the same, you get the gist.)
Due to the detector measuring small voltages, energy resolution being limited by noise and a small 470 pF capacitor being on board, it can sometimes be pretty sensitive to EMI. For the absolute best performance, you should put your detector into some kind of metal enclosure. It doesn't need to be a thick metal case, a tin can will most likely suffice.
The four screw holes on the PCB's actual detector part are connected to the circuit GND.
As some level of protection against EMI, the peak and hold capacitor is periodically discharged as to avoid it being charged ever so slightly by interference and the feedback path. This takes place every millisecond by default and cannot be switched on or off via a serial command. This adds less than 1% of additional dead time (~2.5 µs per reset every ms), so should be completely negligible.
Since the Pico 2 W has functionally the exact same size and pinout, it can be a trivial replacement for the standard Pico 2. This can be a great place to start experimenting with connecting to the Open Gamma Detector via Bluetooth or WiFi, there are endless possibilities what you can do with that.
If you switch to the Pico 2 W, though, you will need to modify the stock firmware slightly since internal pins like the input voltage pin are changed.
There is also no firmware build for the Pico 2 W and I don't plan to make one in the near future, since I'm busy enough with the stock firmware, hardware and Gamma MCA, to start building yet another Bluetooth- or WiFi-connected app or whatever.
I'll happily accept PRs if you make something like that, though, so let me know! ;)