Building a Cosmic Ray Detector Part 2: Assembly and Initial TestingFollow article
CosmicWatch theory of operation, and assembling and testing the main PCB.
CosmicWatch is a project from MIT in the US and Poland’s National Center for Nuclear Research, that enables anyone with basic electronic skills to build a low-cost desktop detector for the muon particles which are created when cosmic rays collide with the earth’s atmosphere.
In Part 1 in this series we took a look at primary vs. secondary cosmic rays, how we would go about detecting the latter, and introduced CosmicWatch. In this post, we will take a look in more detail at the theory of operation for the desktop muon detector, before going on to cover assembling and testing of the main PCB, with some thoughts also on reproducible design.
Theory of operation
The plastic scintillator material we’ll be using is Bicron BC408 and this is specified as having a high light output, with stated applications including muon detection. When secondary cosmic rays pass through the aluminium enclosure of our detector and then the block of scintillator material, this will generate a tiny flash of light, which will then be detected by the coupled photomultiplier.
In the previous post, we discussed how a silicon photomultiplier (SiPM) device is far more convenient than a photomultiplier tube (PMT) since it is a great deal smaller and does not require a power supply of up to thousands of volts. However, a SiPM does require a high voltage by digital standards, of around 30v. The detector is powered via USB and hence has a 5V supply, so a boost circuit based upon the LT3461 step-up DC/DC converteris used.
When a photon strikes the SiPM device it results in an avalanche effect, whereby a single electron turns into a current in the order of millions of electrons. The photomultiplier device is structured as microcells and the current produced is proportional to how many of these were triggered, which corresponds to the incident photon flux and allows us to measure this.
The voltage associated with a single microcell discharge is in the order of a few millivolts and when a muon passes through the scintillator, it’s apparently typical to see a few dozen photons. Hence amplification is required and this is taken care of by an LT1807 precision dual op-ampbased circuit, which provides a gain factor of around 24x.
The amplified signal is next sent to a peak detector, which holds the pulse sufficient time for an Arduino to measure the voltage, before then decaying ready for the next pulse.
The microcontroller ADC samples the waveform at approximately 178kHz. The Arduino also takes care of other tasks, such as converting the measured pulse amplitude to SiPM pulse amplitude, recording event time and dead time between events, controlling the OLED screen, and sending data to a computer attached via USB.
There is also a small board with a Micro SD card socket, which attaches to the mainboard and can be used to record data locally. This may prove particularly useful where the most compact solution is desirable, or where only battery power is available, for example.
We started out assembling the main PCB by soldering the passive components, such as resistors and capacitors. The vast majority of the components are SMD and so an illuminated magnifier or microscope comes in handy.
Next, the ICs were soldered, along with pin headers.
A 2x4 male header on the underside of the board is used to connect the small Micro SD card PCB. The instructions note that the 6-pin header from the Arduino Nano should be used, along with 2x pins from its main header strips. However, we’re using a genuine Arduino — instead of a clone — and this comes with all the headers soldered. This wasn’t an issue, though, as we had some strips of 0.1” pitch header and simply cut two 4-pin lengths of this to use.
Next, the Arduino Nano was soldered and following this, it then became apparent that we did have an issue: its 6-pin header will interfere with the SiPM PCB when this is fitted to the 6-pin female header on the main PCB. De-soldering the Arduino wouldn’t be much fun, since it was reasonably difficult to insert and hence removal after being soldered is likely to be even less fun. So it was decided that we’ll have to simply clip the pins on the Arduino header.
Following the completion of assembling the mainboard, we can carry out some basic testing. First, we apply power via the Arduino Nano USB socket and then measure the SiPM power supply at the 6-pin female header, to confirm the operation of the boost circuit. Above we can see that we did in fact measure the expected 29.5V.
Now on to programming the Arduino.
The firmware makes use of a number of Arduino libraries and most of these come bundled with the IDE, with just a couple to install.
The sketch was then opened in the IDE, compiled and uploaded to the Arduino Nano.
A Seeed Studio OLED modulehad been cabled to the 4-pin socket at the front of the mainboard, which sprung into life after programming. Obviously, at this point any readings displayed will be erroneous, since the SiPM device is not connected.
A number of the items specified in the bill of materials are generic/clone parts with vague identifiers. As such it’s not always going to be easy to procure them and much easier to get something close but not compatible. With this in mind it was decided to try and substitute these for authentic parts — e.g. genuine Arduino Nano — and those from recognisable vendors, since it should make it easier for others to recreate the build. This approach has worked so far with the OLED module, while the genuine Arduino Nano should be fine with the 6-pin header clipped, although de-soldering this prior to fitting the Arduino to the main PCB is likely preferable.
For the Micro SD card slot, a number of parts were tried against the footprint, some of which we already had in the workshop and a couple more that were added to the parts order. However, none of these quite fit and so we’ll have to try again to find one that does.
While making something as low cost as possible is clearly a laudable aim and particularly when it’s intended to be built by schools and colleges, there is also something to be said for reproducibility, with designs utilising parts that can be sourced globally and reliably over time. Of course, one shouldn’t complain too much when a design such as this has been made freely available and one solution to such issues is to create your own version that addresses them!
In the next post, we’ll take cover put together the SiPM plus scintillator assembly and testing the completed detector with a Raspberry Pi connected to its USB port.