Adaption of the MIT cosmic ray detector for Whitworth's balloon system

Research Group: Lester Dery, Kyle Fitch, and Kayley Rich

Launch: Whitworth Spring 2020

''The adaptation of MIT's cosmic ray detector, the muon detector, was done by Whitworth's Near Space lab. Cosmic rays are atom fragments that rain down on the Earth from outside of the solar system. The intention of this project was to be able to send the device into near space, so that the lab could study the frequency and strength of the radiation in these rays. This team utilized a muon detector, adapted from MIT, in an enclosed box where a plastic scintillator would be used for detection, and a silicon photomultiplier would collect data from the light in the interaction. The light is collected as a pulse through the system and stored in the flight computer so that it could later be examined by the team. This made it necessary for coding to be added to the original design, allowing the team to store the data acquired in near space. We determined that we would have to adjust two main aspects of MIT's design; the peak circuit and the amplification circuit. However, this project was not launched in to near space, nor was it finished, due to the COVID-19 pandemic that closed campus 2 months early. ''

Background
The goal of this project was to modify the predesigned muon detector created by MIT students so that our group could send it into near space with different materials than MIT used. By doing this we, hopefully, would have been able to detect the amount of cosmic rays in the area, and how much energy each ray produced. One benefit of using a muon detector is that it allows for us to measure the exact energy that is emitted by these waves. Cosmic radiation is an ionization radiation produced when primary photons and alpha particles from outside the solar system interact with the earth’s atmosphere. Radiation to humans or living things is very harmful and can cause cancer among other things; using the muon detector as a form of researching tool can help make a breakthrough on how to prevent high radiation increases on earth.

Our muon detector consists of a plastic scintillator which could have been used in various forms to achieve better results such as to detect and to measure radioactive contamination and to monitor nuclear materials. Another use of the scintillator is to generate light to take photos by converting  ultraviolet light into visible light. By doing so, quality photos of high altitude or space objects including the Whitworth balloon system can be taken. It can also be used in monitoring weather conditions - this is a project that has been taken on by NASA. It is called the Scintillation Prediction Observation Research Task (SPORT). Aspects of this project can be applied to future cosmic ray projects https://www.nasa.gov/mission_pages/sport/index.html

The common Geiger counter is only able to give a simple yes or no, on or off signal, which does not reach the microscopic degree that a muon detector can. With the completion of our project, it would have created a more compact way to measure the radiation in near space, advancing scientific understanding of the decrease of radiation as it comes closer to the Earth. MIT's muon detector was only able to detect radiation on earth due to its need to be reset by hand, which meant that we would need to modify it to be able to reset itself. In previous years Whitworth students have used Geiger counters to detect cosmic rays; however, this only allowed for counts per minute. Although that is still incredible, a muon detector can detect the values of these radiation particles, giving us more information by combining the idea of MIT with Whitworth's balloon system.

Muon Detector Construction
Due to its small size, the muon detector requires a very compact build that means the circuits will have to sit almost atop one another with a very small amount of space separating each. The main photomultiplier and scintillator material are held together by electrical tape and small screws before being placed inside of the enclosure. They are placed above all other circuits and hooked via several small wires to the peak circuit board. The other circuits are placed below and hooked to the main PCB, which would then have wires running out of it into a flight computer. All of this would sit safely in the Styrofoam box that would be sent into near space. Unfortunately, this team did not make it to the process of placing anything within the enclosure.

A huge piece of this project was the scintillator material. The dimensions for this are 5x5x1cm and it is an extremely delicate material to work with. It is likely that you will receive a rough cut piece of material which is easily fixed by a very small pass, about 1mm, along the edges of the photomultiplier using a milling machine with lots of coolant at a low speed. The scintillator material needs to be drilled at the points of a 3x3cm square, using a 5/16" drill bit, in a pecking motion. Drill at low speed and use lots of coolant or it is likely that the material will melt. When you have finished preparing the scintillator material, you will need to polish the edges, but not the top since it is already clear and finished; Eljen is good for reference. Afterwards, it will be wrapped in tin foil after applying optical gel and attached to the photomultiplier using 5/16" screws; do not tighten against the face or it may crack the Scintillator.

Electrical
The electrical design of this project uses two printed circuit boards (PCBs), and handles the entirety of the process of receiving and relaying the signal from the silicon photomultiplier itself. This includes functions such as receiving the signal, modifying it, amplifying it, converting it to a readable format, and sending it to the flight computer for us to read.

The Silicon Photomultiplier (SiPM) PCB
This PCB is designed to attach to the plastic scintillator, and carries the silicon photomultiplier, which is an extremely sensitive light detector. A silicon photomultiplier (SiPM) is a photodetector that produces an analog output signal. The SiPM itself has two connections. The first connection is to allow power to reach the SiPM, and the second is to let the SiPM send a signal out. This is the most important part of this circuit board. There are two other important parts of this board. The first is the six pin connector that interfaces with the processing PCB and allows power to come from the processing board, as well as allowing the signal from the SiPM to be sent to the processing board. The other important part is the series of resistors and capacitors designed to refine the power coming into the SiPM. In a perfect world, the power would be a constant value, but in the real world there's plenty of random variances and fluctuations. These resistors and capacitors take care of that by forming something called a low-pass filter. Any frequencies above a certain level are reduced down, while any frequencies below that level are allowed through unchanged. This is important because if the SiPM gets too high-frequency of power, it will sometimes alter the readings to show fluctuations in the amount of light even when there is none.



The Processing PCB
This PCB handles the main functions of the project, and is much more complex. Unfortunately, due to COVID-19, we were unable to build it. The first part of this board is the two connections. First, there is a simple six-pin connector that connects directly to the six-pin connector on the SiPM PCB, and to ground. The other connection is a screw terminal with five connections that interfaces with the flight computer, and thus with the microcontroller. The first connection is to ground, to provide a reference point that aligns with the rest of the electronics. The second one, PeakOut, takes the signal voltage from the SiPM and holds it until the flight computer says to reset it. The third, PulseDetect, is a circuit with a simple yes/no trigger. It sends the flight computer the 'yes' signal when the SiPM is sending a signal, which tells it to read the PeakOut signal. The fourth connection is called TrigLevel, and carries a voltage from the flight computer to give the processing board a way to know how big of a pulse is needed to be considered an SiPM signal. The fifth and final connection is PeakDetectReset, which, when a high voltage is received from the flight computer, tells the peak detect-and-hold circuit to reset itself.

In terms of power, there are multiple parts on this board that need 5 volts. The trouble is, the battery runs between 6.5 and 8.4 volts, which would overload parts of this circuit and is too inconsistent to fix with a resistor network. To fix that, there is a regulator that takes the input voltage from the battery, whatever it may be, and reduces it to 5 volts before outputting it to the circuit. There is also another part called a Zener diode that works to protect the regulator in case the battery shuts down. While there are several parts that need 5 volts, the SiPM itself needs 30 volts, which is clearly more than even the unregulated battery can provide. To deal with that situation, there is a DC-DC booster included in the circuit, between the regulator and the output terminal for the SiPM board. The capacitors on the diagram work to 'smooth' the incoming voltage from the regulator and the outgoing voltage from the booster itself, while the resistors tell the circuit how much the booster is actually boosting the voltage. This is very similar to what the low-pass filter on the SiPM PCB does. The final areas of interest on the processing board are the actual processing parts. This is handled by the operational amplifiers, or op amps. Op amps do many things, mostly involving performing complex mathematical operations on voltages. The first one for this board simply takes the signal from the SiPM and amplifies it, and then converts it to a form we can read. This form is called a non-inverting amplifier. Actually, the op amp we use (LT1807) is technically two op amps combined, so on a circuit diagram it's represented by a box instead of the traditional op amp symbol, the triangle. The left side of this op amp system is used to amplify the SiPM signal. The basic purpose of all of the resistors and capacitors shown is to (much like with the capacitors from the DC-DC booster diagram) smooth the signal so it doesn't show fluctuations that shouldn't be present in the readings (since without this system any fluctuation in the power supply would create fluctuations in the readings). The amplified and stabilized signal is then sent through the SiPM-Amp junction. The right side is more complex. First, the signal enters through the SiPM-Amp junction, and is directed by the diode D2 into the capacitor. The op amp then reads the voltage from the capacitor and sends it via OUTB to the PeakOut junction. Finally, after the Interrupt Generator (next topic) tells the flight computer to send a reset signal, the capacitor is drained through the MOSFET transistor to make room in the capacitor for the next SiPM reading. The other op amp in this board (not shown) is in a different form, called a comparator configuration. The role of this op amp is simply to tell the flight computer whether a signal has been received, and thus whether to transmit a reset signal to drain the capacitor in the Peak Detection system.

Software
The flight computer connections plays a major role in the software aspect of this project. Most of the appliances are attached to the flight computer connections including the microcontroller. It is primarily used in conjunction with the microcontroller so that we can tell how much data is recorded per time. The flight computer connections allows the microcontroller to collect data from the muon detector.

Code
1.The code must notifier the user when it reaches a new stage of the program. 2.The code must have a time range which is less than the required value so that it doesn’t over read. The SD card collects the data required. 3.To average and standard deviation, the code must have a float in the while loop saying that every Minutes average the counts and find the standard deviation. This code was not fully tested because of the COVID-19 pandemic.

4.The code for the no button peak detector was created so that there is no need to push a button anytime data needed to be collected. The code will tell the user if the pulse from the peak detector was received, how tall the pulse was or how much voltage was created and finally a code that will be sent to the BS170 to reset without pushing a button.

In general, what the code does is to collect data every minute  in a table form of how many counts in a minute. Because there will be a lot of data collected we decided to write up a code that averages then finds the standard deviation at every minute. This minimizes the amount of data received so that it is easier to understand and use .The code for the photomultiplier was fused with that of the flight connections code to make things easier. This is then uploaded into the SD card and placed in the pod hence saving space.

Raw Code
The flight connections code has been tested by reading the voltage .  Code was still under testing before the COVID-19 Pandemic quarantine

Amplifying circuit
An amplifier is a circuit that produces and increases input signals. It amplifiers positive signal from the SiPM or silicon photo-multiplier. We used resistors for calibration to ensure that our signal does not exceed 3.3 volts. We wanted a positive voltage so we built a non-inverting amplifier to predict the right resistors to be used and to test for 3.3 V. To get 3.3 volts we used larger resistors because that gave us a closer range to 3.3 V. Using smaller resistors gave us a further range from 3.3 V.Smaller resistors need more current but larger resistors need less current. With respect to mathematical calculation we made sure to divide the bigger resistor by the smaller resistor.

Gain= vout/vin. or gain = 1+ R2/R1 vout=( R1/(R1+R2) we should be able to predict the right resistors to use . Use this formula to calculate the right resistors to use to get 3.3volts.

Peak Detector Circuit
A peak detector circuit is used to measure the pulse created by a radioactive particle. The original model for the cosmic ray from MIT had a RC circuit put in place that allowed the voltage to decay after a pulse was detected. However, they would then have to work backwards from the data points they collected to guess the original peak. This would not work for our near space project, so we used a BS170 as our reset; this allowed for a feasible code that paused while it identified the pulse and was then able to reset itself. However, we began with only one MBED which did not allow us enough accuracy to read at lower volts such as 0.8V. We also lowered the count of our capacitor so that it would fill faster allowing for a faster reset. Our circuit consisted of 2 MBEDs, one MOSFET BS170, 100 pF capacitor,100 kΩ resistor and a signal diode. To test the circuit, we used a Packard Pulse Generator that allowed us to see the pulse and an analog diligent that pushed pulses through the circuit. Radiation is traditionally measured in becquerel(Bq), electron-volt (eV) and joule(J). However, we would be measuring it in volts, which requires a simple conversion; 1.0eV= 1.0E18 V.  

Data and Analysis
Due to the disruption of the semester caused by the Covid-19 pandemic of spring 2020, this experiment was never completed or sent to the stratosphere.

Conclusion
In conclusion, charge particles that are going to be detected by the  muon detector. These are decay products of the particles that form when  high- energy  cosmic rays collide with the molecules in the earth's atmosphere. Our project will not only detect but tell us the exact number of cosmic rays, molecules or charged particles involved. If the charged particle passes through the scintillator it produces light that is collected by the photomultiplier(the use of the GoPro Hero HD sensor can be used to improve this feature). To improve this project it is advisable to use some of the features or applications such as the GoPro Hero HD sensor from the CMOS project-Detecting cosmic rays using CMOS sensors in consumer devices in the AHAC( Academic High-Altitude Conference) cosmic rays review section on the main page. Also the listed applications of the scintillator can be adapted into future projects(Background Section). Unfortunately due to the COVID-19 Pandemic we were unable to acquire much data or launch this project.

References/Works Cited
Spenceraxani. “Spenceraxani/CosmicWatch-Desktop-Muon-Detector-v2.” GitHub, M.I.T., 7 Jan. 2020, github.com/spenceraxani/CosmicWatch-Desktop-Muon-Detector-v2. Close, David, and Lisa Ledwidge. “Measuring Radiation: Terminology and Units.” Institute for Energy and Environmental Research, ieer.org/resource/classroom/measuring-radiation-terminology/.