This is a report on a research internship at the University of Oxford in the summer of 2014, which describes the final experimental steps in preparing a prototype of a solid state radiation detector based on a PIN diode. This device has multiple possible applications, e.g. it can be used to monitor cosmic radiation and space weather, which is a topic of great interest for space agencies, satellite networks, and insurance companies. The project was a success and the prototype was ready for a test flight in the end.
A prototype of a solid state radiation detector based on a PIN diode has been developed to the point where it was ready for a test flight on a weather balloon. This device is intended to be used to monitor secondary cosmic radiation and may have the potential to be produced commercially since space weather is an issue of growing importance concerning the economy and technology of modern civilisation. Some of the most crucial experimental work in preparation for the test flight is discussed in this report. One important question was what reverse bias should be applied to the PIN diode. As a result, it was found that the detector’s sensitivity increases with increasing reverse bias until it reaches a saturation value at ∼25V. Another concern was that the prototype may not be robust against temperature differences. This concern could be ruled out to a certain extent.
Table of Contents
1 Introduction
2 Motivation
3 Small Radiation Detector set-up
4 Laboratory testing
4.1 Finding the optimal reverse bias voltage
4.1.1 Introduction
4.1.2 Experiment
4.1.3 Results and data analysis
4.1.4 Interpretation and conclusion
4.2 Temperature dependence of parcel sensitivity
4.2.1 Introduction
4.2.2 Experimental set-up
4.2.3 Result and conclusion
5 Conclusion
Objectives and Topics
This report documents the development and laboratory testing of a solid state radiation detector prototype designed for atmospheric ionisation measurements, specifically targeting secondary cosmic radiation via weather balloon test flights.
- Design and configuration of a PIN photodiode-based radiation detector.
- Determination of the optimal reverse bias voltage for maximum sensor sensitivity.
- Analysis of the detector's performance stability across varying temperature ranges.
- Evaluation of detector performance during an initial field test flight on a weather balloon.
Excerpt from the Book
3 Small Radiation Detector set-up
Figure 1 shows the basic set-up of a PIN photodetector. Basically, it is a PIN diode with a voltage applied such that the negative terminal is connected to the p-doped material and the positive terminal is connected to the n-doped material. The result of this so-called reverse bias is that holes (positive charge carriers) in the p-type region and electrons (negative charge carriers) in the n-type region get pulled away from the depletion layer. This means that the depletion layer widens and in a static situation there is practically no current flowing, apart from a small residual, ”dark” current IS. Furthermore, the I-layer between the p- and the n-layer is an intrinsic semiconductor, which also increases the size of the depletion area by a multitude. This is important in a photodetector because only electron-hole pairs created by incident photons in (or near) the depletion area will get sweeped out of the region by the reverse bias which then creates a current. In conclusion, a large depletion region is needed for maximum efficiency of the photodetector. As one can see from figure 1, only photons (or other kinds of ionising radiation) of sufficient energy, i.e. energy higher than the band gap in the intrinsic semiconductor, will create electron-hole pairs in the active area of the PIN diode.
Summary of Chapters
1 Introduction: Provides an overview of the ongoing project at the University of Oxford regarding the development of a solid state atmospheric ionisation detector.
2 Motivation: Discusses the scientific and commercial importance of monitoring Galactic cosmic radiation (GCR) and compares current technology such as ionisation chambers and Geiger counters.
3 Small Radiation Detector set-up: Details the technical design of the PIN photodiode detector, including circuit shielding and data output capabilities.
4 Laboratory testing: Covers the experimental procedures used to calibrate the device and assess its environmental robustness.
4.1 Finding the optimal reverse bias voltage: Describes the methodology and results of determining the best bias voltage for signal sensitivity, establishing an optimal value of approximately 25V.
4.2 Temperature dependence of parcel sensitivity: Examines the impact of cryogenic temperatures on detector sensitivity, concluding that there is virtually no significant effect.
5 Conclusion: Summarizes the prototype's readiness for flight and acknowledges the subsequent mechanical complications encountered during the test flight.
Keywords
PIN diode, solid state radiation detector, cosmic radiation, reverse bias, ionisation measurements, GCR, laboratory testing, photodetector, sensitivity, atmospheric research, weather balloon, sensor circuit, muons, pulse height, depletion layer.
Frequently Asked Questions
What is the primary purpose of the reported research?
The report aims to detail the development and laboratory testing of a solid state radiation detector prototype capable of monitoring secondary cosmic radiation for use on weather balloons.
Which specific detector technology is utilized?
The project employs a PIN photodiode, which offers benefits such as lower voltage requirements compared to traditional Geiger counters and the ability to perform energy discrimination.
What is the primary research question regarding the detector's electrical configuration?
The research sought to determine the optimal reverse bias voltage required to maximize the detector's sensitivity, identifying ~25V as the ideal setting.
What scientific method was applied to test the sensor?
Controlled experiments were conducted using radioactive sources to calibrate the detector response and measure background interference under varying voltage and temperature conditions.
What findings were made regarding the temperature stability?
Testing showed that the detector's sensitivity remains stable even at temperatures as low as -10°C, suggesting it is robust enough for stratospheric conditions.
What are the key technical specifications of the prototype?
The prototype runs on a 16V supply, weighs approximately 30g, and utilizes RS232/TTL to USB output for data logging of total counts and pulse height.
How does the PIN diode depletion layer affect detector performance?
A larger depletion layer—achieved through optimal reverse bias—increases the active area of the photodiode, thereby improving the efficiency of electron-hole pair collection.
Why was copper foil used in the design?
Copper foil was used to wrap the sensor circuit to shield it from lower energy radiation and visible light, which would otherwise introduce unnecessary noise into the measurements.
What was the outcome of the atmospheric test flight?
While the detector performed as expected in the lower troposphere, it experienced complications and suffered a complete failure at higher altitudes, requiring further investigation.
How is the radioactive decay uncertainty handled?
The report calculates uncertainty in the measured signal rates by assuming a Poisson distribution of events, which is standard practice for radioactive decay studies.
- Quote paper
- Gunther Klobe (Author), 2014, Reverse bias and PIN diode sensitivity. Building a solid state radiation detector, Munich, GRIN Verlag, https://www.grin.com/document/335305
