Overview

The MuonPi Project utilizes an inexpensive Raspberry Pi B+ plug-on module for particle and nuclear physics detectors aimed for measuring muons from atmospheric showers triggered by high energetic cosmic particles. The goal is to set up a distributed network of these detector stations in order to determine the energy, timing, and eventually the origin of the primary particles. To do so, the detected events are timestamped utilizing an on-board GNSS receiver with a precision in the nanosecond range to search for off-line event coincidences between other detectors in the network.

The circuit comprises two inputs for typical input signals (PMTs, SiPMs etc.) with positive or negative pulses of up to few hundreds of mV amplitude and several tens of nanoseconds length. Each channel exhibits switchable inverters, adjustable threshold discriminators (TLV3502 followed by 74LV221 monostable gate) for fast timing and the option for amplitude processing by plugging in a piggy-back peak-detector module (channel 1 only) for amplitude measurement with the on-board slow sampling ADC ADS1015/1115. Timestamping of the single channels or a logic coincidence thereof (XOR or AND) is achieved with the interrupt input of a U-Blox NEO-M8N GNSS module with few tens of nanoseconds precision.

This version of the circuit utilizes two options for bias voltage generation: a cheap commercial DC/DC upconverter (XL6009) as plug-in module (available eg. from eBay) or mounting of the components foreseen for an adjustable on-board DC/DC converter around an LM2733.

Status

• July 2020: V3.0 design is complete. Design review in progress. PCBs ordered at JLCPCB
• August 2020: PCB mounted and tested. Following issues found: a) shtdown pin (pin 4) of U502 connected to GND in design. Bend up this pin and connect to VIN (pin 5) b) transmission line for signal EVT_XOR runs unshielded underneath the comparator U2. This causes cross-talk from ch2 to ch1, when two channels are used. Disconnect pin 5 of U301 from PCB (bend up) in this case. Single-channel-only configuration should work fine on ch1, no workaround is needed in this case.
• Design update in progress for V3.1 (master branch, not finalized yet): swapped inner VCC and GND layers from layers 3 and 4 to 2 and 5 respectively. This assures proper shielding between signal layers (top, inner 3, inner 4, bottom) and should prevent the aggressive cross talk observed in V3.0. Moved misplaced L501 to correct position behind the smoothing capacitor C501. Added large inductor for supply of DC/DC plug-on module in schematic. This should be mounted, when the plug-on voltage module is employed in order to limit the inrush current. Added bypass zero-ohms from input A to output of U102 and U202. These should be mounted only when the polarity switches are omitted.
• Sept/Oct 2020: Design of V3.1 was finalized and PCBs ordered at JLCPCB. Tests revealed, that all bugfixes and improvements (see above) were successfull.

Gallery

3d view of the MuonPi Raspberry HAT: