Decode Cosmic Ray, One Muon At a Time
I built a Compact Cosmic Ray Muon Detector for my undergraduate research project. It can “capture” muon particles from cosmic rays and display statistical information to the small OLED screen. It can also perform muon coincidence detection with two different photon sensors.
This blog series aims to document my research journey and share my relatively limited knowledge about muon and particle detection.
Welcome to Part 2 of this series: From Muons to Electrons! A thorough design process is detailed below, with (hopefully) simpler explanations and plenty of diagrams.
Part 1 of the series can be found here: Cosmic Inspiration.
Our Solution to Bridge the Gap — Educational & Research Purposes
Design Criteria
As existing solutions fall short on both educational and research purposes, we bridged the gap by introducing the Compact Cosmic Ray Muon Detector.
We redesigned the muon detector with the following criteria:
· Compact:
With the advancement in silicon multiplier and scintillator technology, small-size components are utilized to significantly reduce the size of the detector.
· Portable:
Operated as a standalone unit, our detector can be taken to field trips and installed in remote areas due to its mobile nature.
· Low-noise:
Advancement in integrated circuits and printed circuit boards can reduce noise and interference.
· Low-power:
Our detector can be powered via small batteries or mobile power bank.
· Flexible:
The system can be configured to perform different tests and experiments. For example, timing and energy resolutions.
· User-friendly:
Out detector is easy to assemble and interface. It has the potential to serve as an educational platform to benefit other college students, professors, and researchers.
Altitude Sensor
The density of muon particles (muon flux) highly depends on the altitude above sea level. Specifically, the lower the altitude is, the fewer muons can be captured, as their energy decays via object penetration (P. Hansen, 2003). Our detector features an additional functionality — an altitude sensor to measure height above sea level.
Design Process — End to End
Overview
The design process can be divided into four stages:
1. Silicon Photomultiplier Characterization.
2. Simulations: Silicon Photomultiplier Configurations and System Bring-Up.
3. Prototyping: Printed Circuit Board Design and Firmware Development.
4. System Testing and Fine-Tuning.
Silicon Photomultiplier Characterization
Summary: It’s vital to understand the operation of SiPM and model SiPM with characteristics parameters.
In the past, Photomultiplier Tube (PMT) was used as sensitive light detectors in the ultraviolet, visible, and near-infrared ranges of the electromagnetic spectrum.
Thanks to solid-state technology, Silicon Photomultiplier (SiPM) has replaced PMT across major applications, providing advantages in size reduction, saving board space, insensitivity to magnetic fields, low-light detection capabilities, and low-voltage operation.
SiPM, a sensitive photon sensor, is a critical component of the Muon detector since it shapes the sensing and timing of the electrical pulse and determines the signal quality fed into the system.
As a result, understanding the characteristics of SiPM is paramount in the design of the Muon detector (SensL, 2017). Plenty of research has been done to model the SiPM as an electrical model.
My attempt to parameterize the SiPM is illustrated in the figure below. All parameters such as Rq (quenching resistor), Cq (parasitic capacitance), Nf (number of fired cells), etc. can be easily modified via proper commands.
LTSpice, a free and popular simulation software, is utilized to examine SiPM’s behavior and performance. I may write an article about the convenience of using LTSpice in the verification of circuit behavior and performance.
Parameter-Driven Simulations
Summary: Simulations of SiPM’s output signals can be achieved for further scrutiny.
With an electrical model of a SiPM device, simulations can be done to extract the analog signal from a SiPM read-out circuit. The operation voltage, or bias voltage, of the SiPM is approximately 35V. A typical 50-ohm load is connected to convert output current from the SiPM to an analog voltage.
Of the simulation result above, the SiPM device has 1000 cells and 2 cells get fired, which generates the output signal — 1mV peak and 100ns wide.
Multiple Configurations — Optimization
Summary: The simulation tool is utilized to optimize the performance of SiPM devices connected in various configurations.
Due to their small size, an array of SiPM devices can be combined and configured to achieve optimal performance. The simulated circuit that connects each device is designed to allow different series and parallel configurations among the SiPM detectors.
The goal of examining multiple configurations is finding the best combination of time resolution and operating power.
Time resolution, also known as timing performance of the SiPM device, is “useful for triggering, event processing, and angular resolution.” (A. Kryemadhi, 2017) In other words, a sharp and thin output pulse (short timing) achieves higher detection accuracy when a muon hits the detector. The operating power of SiPM detector(s) is a critical factor directly related to the Compact and Low-power design criteria. Since the system is expected to be mobile, the power source solely relies on a battery or small power bank. Therefore, obtaining fine time resolution and minimizing input power provides tangible benefits and improvements to our design.
The series configuration achieves better timing performance yet requires a higher bias voltage. The parallel configuration operates with lower bias voltage, but the timing performance is worse due to the circuit’s increased capacitance from the parallel connection.
A hybrid configuration (a combination of series and parallel) is explored to obtain the best trade-off between time resolution and power.
System Simulations
Summary: The entire electronic system is defined and simulated to visualize signals of each processing stage.
Now, what can these blocks do?
· Voltage Booster: This block contains a boost regulator to increase the +5V USB input voltage to around +29.5V in order to power the SiPM device.
· SiPM Circuit: The SiPM device is placed in this block, along with noise filtering circuits and load resistor. This block directly interacts with the scintillating material and outputs analog pulses which represent muon incidence.
· Amplifier Circuit: An operational amplifier circuit is included in this block, enhancing relatively weak analog pulses from the SiPM Circuit by 34 times for signal processing in later stages.
· Peak & Hold Circuit: It is to detect the peak of the amplified signal and hold the voltage at that level for a sufficient time so that the Arduino Nano (the microcontroller unit of the system) can measure it. The held signal then decays over time, waiting for the subsequent pulse from the next event.
The top panel shows the raw pulse when there is a muon event. An amplified signal, output from the Amplifier Circuit, is shown in the middle panel. The bottom panel simulates the prolonged signal after the Peak & Hold Circuit, ready for the microcontroller’s reading process. All signals are simulated based on minimal noise assumption.
Printed Circuit Board Design
Summary: Printed Circuit Boards are designed with modular approaches for flexibility and easy configurations.
After the simulation stage, I proceed with the Printed Circuit Board (PCB) design. The system is divided into 3 modular boards for flexible integration and experiments: the SiPM Board, the Main Board, and the Process Board.
The SiPM device is soldered on the SiPM board where the scintillating material is mounted on top.
The Main Board houses all electronic components for power and analog signal processing: Voltage Booster, Amplifier Circuit, and Peak & Hold Detector. The SiPM Board is connected to the Main Board via connecting headers.
The Process Board features an altitude sensor, a microcontroller unit (Arduino Nano), MicroSD circuitry to save data, an OLED screen to display useful information, and several LEDs.
Firmware development
Summary: Two different firmware versions were developed to test the functions of the muon detector: Pulse Detection and Coincidence Detection.
Pulse Detection
With a single SiPM device in the muon detector system, the microcontroller is able to measure the analog pulse via the 10-bit analog to digital converter (ADC). If the ADC result is larger than the pre-determined voltage threshold which corresponds to a muon’s event (achieved via experiments and calibration), the muon counter increases by one step and LEDs light up for visual display. The OLED screen is updated every second with total muon count, uptime, and altitude value.
Coincidence Detection
Coincidence detection can be achieved with two SiPM devices arranged vertically or horizontally, and one microcontroller [picture]. With such setup, the microcontroller can determine if there is any muon particle that hits two SiPM devices in a very short time (around one millisecond). The counter increases by one step when coincidence detection occurs.
The microcontroller can sample voltage (SiPM pulses) from two different analog pins tied to two different SiPM devices. If there is a muon event from one device (the ADC result is larger than the threshold), within one millisecond, the microcontroller will sample the other analog pulse. Coincidence happens when both detected pulses are higher than the muon’s threshold during the one-millisecond coincidence time window. The OLED screen is updated every second with total coincidence count, uptime, and altitude value.
Testing
The scope shows live signals when there are muons hitting the detector. The yellow signal is the raw analog pulse from the SiPM Board. The red signal is the output from the operational amplifier. The blue signal shows the result of the peak-and-hold circuitry which prolongs the peak of the amplified signal.
Functionalities, including Pulse Detection and Coincidence Detection, successfully produced expected results and performance. However, we look forward to improving the detector in many aspects.
Please stay tuned for Part 3: Muons and Beyond.
I’d like to receive your feedback, comments, and inquiries. Please drop me a message on Linkedin, with a 100% response rate.
Acknowledgment
I would like to express my very great appreciation to Dr. Abaz Kryemadhi for his innovative ideas, patience guidance, and valuable mentorship. My gratitude is also extended to the Messiah College Smith Scholar Committee and Messiah College Physics Department for funding this research project.
References
A. Kryemadhi, L. B. (2017, February 3). iopscience. Retrieved July 13, 2020, from iopscience: https://iopscience.iop.org/article/10.1088/1748-0221/12/02/C02013
P. Hansen, P. C. (2003, July 8). Flux of atmospheric muons. Retrieved July 1, 2020, from https://arxiv.org/pdf/hep-ph/0307199.pdf
SensL. (2017, February N/A). sensL. Retrieved July 13, 2020, from sensL: https://www.sensl.com/downloads/ds/TN%20-%20Intro%20to%20SPM%20Tech.pdf
Figure 3 Source: https://www.hamamatsu.com/jp/en/product/optical-sensors/pmt/pmt_tube-alone/index.html
Figure 4 Source: https://www.sensl.com/downloads/ds/TN%20-%20Intro%20to%20SPM%20Tech.pdf
