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Portable Amplifier for Semiconductor-Based Radiation Detectors by Douglas Nichols, B.S.E.E. A Thesis In Electrical Engineering Submitted to the Graduate Faculty of Texas Tech University in Partial Fulfillment of the Requirements for the Degree of Master of Sciences Approved Dr. Changzhi Li Chair of Committee Dr. Jing Li Co-Chair of Committee Mark Sheridan Dean of the Graduate School December 2016 Copyright 2016, Douglas Nichols Texas Tech University, Douglas Nichols, December 2016 ACKNOWLEDGMENTS This thesis would not have been completed without the guidance and support of Dr. Changzhi Li. I am very thankful for his advice and encouragement. I would like to thank Dr. Jing Li for being on my defense committee. I am also thankful that I could use the Nanophotonics Center's equipment and would like to thank Dr. Hongxing Jiang, Dr. Jingyu Lin, and Dr. Jing Li for allowing me to use their lab. I would also like to thank Avisek Maity who was extremely helpful in helping me set up several tests and providing me with technical advice. Brennan Branch was also very helpful when it came to programming and I would like to thank him as well. My family and friends have also played a big part in giving me support and encouragement throughout my schooling for which I am very thankful. ii Texas Tech University, Douglas Nichols, December 2016 TABLE OF CONTENTS ACKNOWLEDGMENTS ................................................................................... II TABLE OF CONTENTS .................................................................................... III ABSTRACT .......................................................................................................... V LIST OF FIGURES ............................................................................................ VI INTRODUCTION ................................................................................................. 1 Neutron and Gamma Detection Background .................................................... 1 Types of Radiation Detectors ............................................................................ 3 Gas Based Detectors ................................................................................... 3 Scintillation Detectors ................................................................................. 7 Semiconductor-Based Neutron Detectors ................................................... 8 Nanophotonics Device .................................................................................... 11 AMPLIFIER DESIGN ....................................................................................... 13 Amplifier for Detector..................................................................................... 13 Other Circuit Designs for Detector ................................................................. 18 PIN Photodiode ............................................................................................... 21 TEST RESULTS AND DESIGN CHANGES .................................................. 23 Original Design Parameters ............................................................................ 23 Testing Setup and Process......................................................................... 24 Compare Coin vs Booster ......................................................................... 25 Changes for Next Board Design ............................................................... 27 REV 2 .............................................................................................................. 27 Testing Setup and Process......................................................................... 28 Changes for Next Board Design ............................................................... 32 REV 3 .............................................................................................................. 32 Testing Setup and Process......................................................................... 36 AMPLITUDE MEASUREMENT WITH LABVIEW..................................... 40 CONCLUSION AND FUTURE WORK .......................................................... 46 Future Work .................................................................................................... 46 Conclusion ...................................................................................................... 47 BIBLIOGRAPHY ............................................................................................... 48 iii Texas Tech University, Douglas Nichols, December 2016 APPENDICES ..................................................................................................... 51 Rev 2 Amplifier Outputs ................................................................................. 51 Additional Printed Circuit Board .................................................................... 52 iv Texas Tech University, Douglas Nichols, December 2016 ABSTRACT Research was conducted to build a portable radiation detector on printed circuit board (PCB) level. The history of detecting gamma radiation and neutrons is briefly discussed as well as the different types of detectors. The current design for the detector circuit as well as previous designs are included. Each of the three designs were implemented on printed circuit boards. The detector circuit was tested with two different commercial diodes to detect gamma radiation. The test setup as well as the tests results for the detector is discussed. It was discovered that this device worked correctly in detecting gamma rays. Eventually the detector circuit will be used with an array of custom made semiconductor-based neutron detectors to detect neutrons. Future steps works to increase efficiency and functionality are described. v Texas Tech University, Douglas Nichols, December 2016 LIST OF FIGURES Figure 1: Radiation Portal Monitor [7] ................................................................... 2 Figure 2: Gas Detector [10]..................................................................................... 5 Figure 3: Pulse-Height Spectrum for Neutron Detection [15] ................................ 6 Figure 4: Scintillation Detector [19] ....................................................................... 8 Figure 5: Neutron Detector [20]............................................................................ 10 Figure 6: Honeycomb Neutron Detector [22] ....................................................... 11 Figure 7: Gamma Detector REV 1 Design ........................................................... 14 Figure 8: Preamplifier for Portable Detector ........................................................ 17 Figure 9: Preamplifier output and Pulse Shaping Amplifier Output [28] ............. 18 Figure 10: CR-150 Preamplifier Evaluation Board Schematic [29] ..................... 19 Figure 11: CR-160 Gaussian Shaping Amplifier Schematic [30] ......................... 20 Figure 12: Photodiode Electrical Model ............................................................... 22 Figure 13: Rev 1 Design with Coin Battery .......................................................... 24 Figure 14: Output Pulse from the Output .............................................................. 25 Figure 15: Booster Output Without a Diode ......................................................... 26 Figure 16: Rev 2 with shield cover in place .......................................................... 28 Figure 17: Signal at Test Point .............................................................................. 29 Figure 18: Test Setup for Rev 2 Design for Test with Caesium 137 .................... 30 Figure 19: Signal at Test Point with Caesium 137................................................ 31 Figure 20: MCA Measurement ............................................................................. 32 Figure 21: Rev 3 Board with Changes .................................................................. 33 Figure 22: REV 3 Design ...................................................................................... 35 Figure 23: Test Setup for Rev3 Design for Test with Caesium 137 ..................... 36 Figure 24: Output pulses with Caesium 137 ......................................................... 37 Figure 25: Output pulse with Caesium 137 .......................................................... 38 Figure 26: MCA Measurement ............................................................................. 39 Figure 27: MSP432 ADC and UART Flow Chart ................................................ 43 Figure 28: LabVIEW Program for Receiving and Plotting Data .......................... 44 Figure 29: LabVIEW Front Panel for Receiving and Plotting Data ..................... 45 Figure 30: Output Of Second Amplifier ............................................................... 51 Figure 31: Output of Third Amplifier ................................................................... 51 vi Texas Tech University, Douglas Nichols, December 2016 Figure 32: Rev 2 Design with Shield Cover Removed ......................................... 52 vii Texas Tech University, Douglas Nichols, December 2016 CHAPTER 1 INTRODUCTION The purpose of the following research was to design a portable neutron detector. The Nanophotonics Center at Texas Tech University is currently working on a semiconductor-based neutron detector. The desire was to make a portable detector that would one day include the neutron detector as well as a circuit to amplify the signal and transmit that signal to a portable device. The work completed and described in this paper focused on designing and testing the circuit that would eventually amplify the signal from the detector. The voltage bias for the Nanophotonics' detector is currently very high, so the portable circuit could not currently provide a high enough bias to test with the semiconductor-based detector. The detector circuit was instead tested with commercial diodes to detect gamma radiation. It was thought that if the portable device would work well with detecting gamma radiation, the portable device could eventually be implemented with the neutron detector. Neutron and Gamma Detection Background Radiation detection is used in several different fields such as astronomy, medical, defense and others. One major use for radiation detection is to detect nuclear material used to build nuclear weapons. These materials are known as Special Nuclear Materials (SNM). Some of the more known SNM are plutonium-239, uranium-233, and uranium-235 [1]. In the last several years, terrorist attacks have increased, and the fear of terrorists using nuclear weapons has grown. Radiation detectors are used at ports of entry as well as at national borders to monitor for smuggling of nuclear 1 Texas Tech University, Douglas Nichols, December 2016 materials [2]. Nuclear materials give off radiation that can be detected; however, not all radiation is easy to detect. Gamma radiation are waves with high energy that can travel large distances as well as penetrate obstacles [3]. Neutrons are nuclear particles that travel at high speeds and are also able to penetrate obstacles [3]. Because gamma and neutron radiation are able penetrate materials such as shipping containers, radiation detectors are usually used to detect neutron or gamma radiation. Radiation Portal Monitors (RPM) are large detectors that are used to scan cargo containers as well as vehicles for SNM, shown in Figure 1. These RPMs are used all over the United States at ports of entry and national borders [4]. The first RPMs that were implemented used polyvinyltoluene (PVT) scintillators which detected gamma radiation [5]. The problem with these detectors was that they were not able to provide information on the energy level, and they could only provide the radiation count. The detectors often set off alarms when materials with non-threatening radiation were detected [6]. Figure 1: Radiation Portal Monitor [7] 2 Texas Tech University, Douglas Nichols, December 2016 Neutron detectors were also implemented often in conjunction with gamma detectors. This reduced the amount of alarms caused by materials that were not considered threatening. A combination of gamma and neutron RPM detectors are often used now. Neutron detection is more efficient at detecting nuclear material than gamma detectors. This is due to the fact that most materials do not give off neutrons, and the ambient amount of neutrons is low. The ambient amount of neutrons is usually around 0.005 to 0.02 πππ’π‘ππππ /(π × ππ2 ) [1]. If the amount of neutrons detected is higher than the ambient, it is due to nuclear materials. Most RPM neutron detectors use Helium-3 (3He) based detectors. Unfortunately, in 2008 there was an extreme shortage of 3He. Since then laboratories and universities have pursued alternative materials that can be used to detect neutrons [8]. Types of Radiation Detectors There are several different types of detectors such as gas based detectors, scintillators, and semiconductor-based detectors. Gas Based Detectors Gas detectors are the oldest type of radiation detectors. These types of detectors work by using certain gasses to interact with gamma rays or neutrons. A metal cylinder filled with a certain gas is used with a single charged wire running through the middle of the cylinder, shown in Figure 2. When the detector is in the presence of a radiation source, radioactive particles will travel out from the source. Some of the gamma rays will travel into the metal cylinder and will transfer energy 3 Texas Tech University, Douglas Nichols, December 2016 directly to electrons in the gas. The amount of energy transferred to the electron will vary depending on the energy of the gamma ray and the atomic number of the gas being used [9]. There are two main types of interaction between a gamma ray and an electron; the first is Compton scattering. Compton scattering occurs when only part of the gamma ray's energy is transferred to an electron. Photoelectric absorption occurs when a gamma ray transfers all its energy to the electron [9]. The voltage difference between the metal cylinder and the charged wire will create an electric field. The electrons will be drawn to the positively charged wire and will produce a charge surge when the electrons reach the wire [10]. The small pulse will correlate to a detection. The electric field can be increased by increasing the voltage of the charged wire; once the field is greater than the threshold, additional ionization will occur. Ionized electrons will then multiply by colliding with atoms and freeing more electrons. The energy of the secondary ionizations is proportional to the energy of the original ionizations. This enables a detector to determine the energy of the original particles; this type of detector is known as proportional detector [11]. However, for gamma detection, proportion counters only work for tens of keV [10]. A Geiger-Mueller (GM) detector is a gas detector where the voltage of the wire is increased enough so the ionization in the gas saturates [10]. A GM detector cannot determine the different energy levels of the original gamma rays. Instead, a GM detector is used for counting the number of gamma rays and would not be able identify SNM. Another drawback of GM detectors is that they suffer from what is known as dead time. Dead time occurs right after an event is detected; during this time the 4 Texas Tech University, Douglas Nichols, December 2016 detector cannot detect any additional events. Due to dead time, GM detectors are not used when accurate counts are needed for large amounts of radiation [12]. Figure 2: Gas Detector [10] For gas based neutron detectors, the process is similar to gas based gamma detectors. However, neutrons do not interact with electrons directly like gamma rays do. Special gases are used to interact with the neutrons that will eventually produce a charge cloud. Boron trifluoride (BF3) and 3He are the most commonly-used gases for neutron detection [13]. Any neutrons that pass through the cylinder will react with a 3 He atom. This will produce 3He and a proton [14]. The proton will then ionize the atoms in the area to produce charges. The charges will then ionize more atoms in the cylinder producing more charges. Once the charge cloud has been produced, it will interact with the tube and the wire the same way a gas based gamma detector would. 3 He detectors are proportional counters, so a spectrum of the neutron energy can be measured. The electric pulses created from the detector can then be amplified and measured with equipment like a multichannel analyzer (MCA). The amplitudes of the pulses will correlate with the energies of the neutrons. In a pulse spectrum, the number 5 Texas Tech University, Douglas Nichols, December 2016 of counts per amplitude (channel) are plotted. Figure 3 shows a spectrum for neutron detection using 3He. Figure 3: Pulse-Height Spectrum for Neutron Detection [15] One major concern with neutron detectors of all kinds is their sensitivity to gamma radiation. Nuclear materials tend to emit approximately ten times more gamma rays than neutrons [16]. Gamma rays can interact with the electrons in neutron detectors and cause the detector to output extra pulses. When analyzing the spectrum later, it is often possible to determine the difference between pulses caused by neutrons and pulses caused by gamma rays. However, it is usually desirable to have neutron detectors that are not sensitive to gamma rays. Thermal neutrons can be absorbed by materials that are not as sensitive to gamma rays [16]. However, detectors used to detect fast neutrons are more sensitive to gamma radiation, shown in Table 1. 3 He detectors and BF3 are both sensitive to detecting thermal neutrons and are not very 6 Texas Tech University, Douglas Nichols, December 2016 sensitive to gamma rays. This also means the efficiency for detecting fast neutrons is not very high. In order to capture fast neutrons but still use 3He or BF3, a moderator is often used. A moderator is a material that is used to slow down fast neutrons so they more easily be detected [17]. Moderators are usually used to coat the outside of the detector. Table 1: Interaction Probability of Neutrons and Gamma Rays [16] Scintillation Detectors A scintillation detector uses materials that emit light when they are struck by radiation [18]. The emitted light can then be captured by a photo sensor, and the output from the sensor can be analyzed to determine the pulse spectrum. There are two main types of scintillators: organic and inorganic. Scintillators are frequently used to detect neutrons due to their fast response time and their low cost [16]. Neutron detectors that use scintillation are fast neutron detectors. One problem with organic scintillators is their high sensitivity to gamma radiation. It is difficult to discern when 7 Texas Tech University, Douglas Nichols, December 2016 neutrons or gamma radiation are being detected due the fact that the pulse-height spectra overlap [16]. Figure 4 shows a setup for a scintillation detector. Figure 4: Scintillation Detector [19] Semiconductor-Based Neutron Detectors Semiconductor-based detectors are much more compact, and therefore more portable, than gas detectors. The small size also has a downside, since bigger surface areas will be able to absorb more gamma rays or neutrons. Therefore, special care is taken to make semiconductor-based detectors more efficient in capturing neutrons or gamma rays. The structure for some semiconductor detectors is the same as a diode. A PIN photodiode can be used to capture gamma radiation. A PIN diode has three regions: it has a p-type and a n-type region with an intrinsic layer between. PIN diodes are similar to standard p-n junction diodes except with PIN diodes the depletion region is fixed and is not dependent on bias voltage. For gamma detection, the energy of the gamma radiation can be transferred to the semiconductor to create electron-hole pairs. The number of electron-hole pairs produced depends on the energy transferred from 8 Texas Tech University, Douglas Nichols, December 2016 the gamma ray. As mentioned earlier, Compton scattering occurs regularly, meaning the full energy would not transferred to the detector. As mentioned before, neutrons do not interact directly with electrons. Like gas based neutron detectors, semiconductor-based neutron detectors detect thermal neutrons. Thermal neutrons have a low energy of .026 eV [20]. One of the simpler designs for semiconductor-based neutron detection is to use a GaAs p-n junction diode with a neutron reactive layer on top [20]. A diagram can be seen in Figure 5. The neutron interacts with the reactive layer releasing charged particles that can travel into the p-n junction. The charged particles will then ionize atoms in their paths [16]. The process is similar to gas detectors in that the detector will be biased so the electron and hole pairs will then travel to the two contacts and create an electrical charge that can be measured. One example of this would be a neutron detector with a Boron-10 (10B) reactive layer. A 10B layer will absorb a thermal neutron (.0259 eV) and will eventually produce a 480 keV gamma ray and a 1.47 MeV alpha particle [21]. The charged particles will then interact with the electrons in the p-n junction, as mentioned earlier, which will produce a signal. 9 Texas Tech University, Douglas Nichols, December 2016 Figure 5: Neutron Detector [20] The neutron reactive layer is crucial for neutron detectors. There are a couple materials that are usually used for the reactive layer. 10B and Lithum-6 (6Li) are two common materials used for the reactive layer. 6Li is corrosive and difficult to handle, so the more popular form is 6LiF which is more stable [21]. These two materials are used because of their availability and their production of charged particles when they absorb a neutron. Another important factor for semiconductor-based detectors is the design of the device. The planar neutron detectors such as the one shown above had low efficiencies since the reactive layer was usually on top. In order to successfully absorb neutrons, the reactive layers were often thick. This meant the charged particles produced from the absorption did not always reach the semiconductor. Modern semiconductor neutron detectors are now three-dimensional. Three-dimensional detectors have trenches, or holes, in the semiconductor material that are filled material that will react with neutrons [22]. This allows for the absorption of neutrons while having the semiconductor all around to catch the emitted charged particles. Most of 10 Texas Tech University, Douglas Nichols, December 2016 the semiconductor-based detectors today also implement different microstructures such as micro pillars, honeycomb patterns, and deep trenches, shown in Figure 6 [22]. Figure 6: Honeycomb Neutron Detector [22] Nanophotonics Device The purpose of the portable amplifier is to one day merge with a custom-built semiconductor neutron detector. The device is being created at Texas Tech University by Dr. Jiang and Dr. Lin. The device currently requires a bias of 500V, but the amplifier circuit currently only provides a bias of 48V. Eventually the device will be able to function at a lower bias. The detector will eventually consist of an array of several smaller semiconductor detectors; this will allow the detector to be more 11 Texas Tech University, Douglas Nichols, December 2016 efficient. The array of detectors would also have a much higher capacitance than just one device, which will cause the device to have a larger noise gain. The device uses Hexagonal Boron Nitride. The detection efficiency for thin films of Hexagonal Boron Nitride was found to be approximately 80% [23]. 12 Texas Tech University, Douglas Nichols, December 2016 CHAPTER 2 AMPLIFIER DESIGN Amplifier for Detector A semiconductor detector can be used to absorb the energy from neutrons or gamma radiation. The charge produced from these radiation detectors is very small. In order to measure the output of the detector the signal is usually amplified by 100-1000 V/V. In this case, however it is not as simple as just amplifying the circuit. Since the output from the semiconductor device is very small it will be hard to differentiate the signal from the noise floor. The front end of the circuit must have a low noise figure. In order to detect the output from the device a charge sensitive preamplifier with a low noise input can be used to convert the charge to a voltage [24]. A series of amplifiers can then be used to amplify the signal further. Another concern for this circuit is that it must be portable. Several implementations of the design were created before settling on a final design. The original design was adapted from a gamma detector designed by Maxim Integrated [25]. The first adapted circuit can be seen below in Figure 7. 13 Texas Tech University, Douglas Nichols, December 2016 Figure 7: Gamma Detector REV 1 Design 14 Texas Tech University, Douglas Nichols, December 2016 For the portable circuit a commercial PIN photodiode is used to capture the gamma radiation. Since, both semiconductor-based gamma detectors and neutron detectors provide similar charge pulses at the output, the circuitry used to amplify and determine the spectrum can be used for both. A bias of 12V is used to reverse bias the diode so that when gamma rays strike the diode a small charge will be produced which will create a small charge pulse. The chare sensitive preamplifier will convert the quick charge pulse to a voltage signal. The voltage signal will then be amplified and shaped by three active band pass filters. Finally, a comparator is used to compare the signal to a voltage level of 3.1V. When a gamma ray strikes the diode, this will cause a small negative pulse at the front end of the circuit. There are four inverting amplifiers which means that there will be a negative pulse at the front end of the comparator. The first four operational amplifiers had a non-inverting input of 3.1V. A voltage divider was used to supply the non-inverting input of the comparator with 2.9V. The comparator will output a positive pulse when the inverting input of the comparator drops below 2.9V. The voltage divider was used to ensure that the comparator would not produce pulses due to noise. The 10kβ¦ resistor (R4) of the voltage divider can be increased to increase the threshold of the comparator. A variable resistor could have been used to change the input of the comparator, but due to the noise of potentiometers a non-variable resistor was used. One of the most important concerns for the amplifier circuit is the front end of the circuit. The front end of the circuit must have a very low noise input and the 15 Texas Tech University, Douglas Nichols, December 2016 preamplifier needs to have a low input capacitance. The circuit need to have low noise levels because the detectors produce small currents. If the noise level of the front end is large it would increase the signal to noise ratio (SNR) which would decrease the efficiency of the system. The circuit for the front end of the system can be seen in Figure 8. Two large resistors, R1 and R2, in conjunction with C1 reduce any noise from the 12V source. Notice that R3 is also large in order to reduce resistor current noise. The sensitivity of the charge amplifier is also important. C3 controls the charge sensitivity as well as the temperature stability for the amplifier [24]. The charge sensitivity of an amplifier can be determined by the equation: π = π πΆπ π₯ Π [26]. Where e is the charge of an electron and Cf is the feedback capacitor C3, and Π is the amount of energy needed to create a hole-pair. In silicon, the amount of energy lost to create an electron-hole pair is 3.62eV [26]. The charge sensitivity for the charge amplifier was 9.4mV/MeV. The preamplifier is used to convert the charge pulse to a voltage signal. The output of the preamplifier has a quick ramp up time but a long decay equal to π 3 × πΆ3. In this case the decay is 47µs. 16 Texas Tech University, Douglas Nichols, December 2016 Figure 8: Preamplifier for Portable Detector The output of the preamplifier would look similar to the top image seen in Figure 9. Only the amplitude of the peak correlates to the energy level of the gamma ray, so the long decay can be filtered out. The second stage of the detector circuit is the shaping amplifier. Shaping amplifiers are used after preamplifiers to shape the output and force the signal to return to baseline faster [27]. In the circuit used for the portable detector, three operational amplifiers were used to create a shaping amplifier. The first amplifier in the shaping amplifier is a bandpass filter with a cutoff from 160kHz to 160kHz, this will capture the short ramp time and pulse of the output signal from the preamplifier. It will also filter out most of the decay of the output signal. The bottom image seen in Figure 9 shows what the output from the first stage of the shaping amplifier should look like. The next two amplifiers are used to amplify the signal further, while still filtering out any noise. The next two amplifiers both have cutoffs at 16kHz and 160kHz. The Gain Bandwidth of the MAX 4477 is 10MHZ, which means with a bandwidth of 144 the maximum gain would be 69V/V. Therefore, 17 Texas Tech University, Douglas Nichols, December 2016 two amplifiers were used. The max 4477 were used for every stage due to their low noise and low distortion. Figure 9: Preamplifier output and Pulse Shaping Amplifier Output [28] Other Circuit Designs for Detector The commercial circuitry that can be used to shape and amplify the signals from radiation detectors will be discussed here. The CR-150 evaluation board is the front end of the circuit; the schematic can be seen in Figure 10. The board uses a CR11x which is a charge sensitive preamplifier used to amplify the small signal from the external detector. The board also includes a power supply regulation circuit, which takes up most of the board. The board can be powered with a positive and negative voltage between 8V and 15V. A single supply of 24V can also be used to power the board, the voltage will then be split into a positive and a negative rail [29]. One big difference to note is that the portable detector device mentioned earlier uses a single supply and only a positive rail is used in the circuit. The preamplifier board can be 18 Texas Tech University, Douglas Nichols, December 2016 used with a detector with a bias of up to 2000V. This was also another big advantage of this board because the Nanophotonics group needs a large bias for their device. Figure 10: CR-150 Preamplifier Evaluation Board Schematic [29] The output of the preamplifier on the CR-150 circuit can then be connected to a shaping amplifier. A CR-160-R7 "Gaussian shaping amplifier evaluation board" can be used in conjunction with the CR-150 to amplify the signal and filter out unwanted noise. The first three Op Amps in the CR-160 schematic, Figure 11, is an adjustable wide-band amplifier with low noise [30]. The gain can be adjusted from 0 to 100. The 19 Texas Tech University, Douglas Nichols, December 2016 CR-200 chip is a shaping amplifier which allows the output pulse to return to baseline much faster than the signal from the preamplifier. This is necessary for applications with high count rates. The shaping amplifier also filters out any unwanted noise and has a gain of 10. The overall gain of the board can be adjusted from 0 to 1000. The portable detector circuit has a gain of 1000 but it is not adjustable. Overall the CR-160 and CR-150 are one of the best options for amplifying and shaping the output from a neutron or gamma detector. However, both boards would be needed for testing a neutron or gamma detector. Whereas the portable detector device has both the preamplifier and the shaping amplifier on one board. So, this setup is not small enough for handheld use like the portable detector. However, some of the Cremat ICβs, like the shaping amplifier, could replace some of the circuitry on the portable device to improve performance. Figure 11: CR-160 Gaussian Shaping Amplifier Schematic [30] 20 Texas Tech University, Douglas Nichols, December 2016 PIN Photodiode For gamma detection one of the most important components is the PIN photodiode. The area of the diode as well as the reverse bias on the diode are the key parameters for increasing the sensitivity. A larger diode allows for more gamma radiation to be captured in the depletion area of the diode. A larger diode will also have higher capacitance which will increase the noise gain [25]. A larger reverse bias will increase the electric field between the two junctions which will increase the amount of ionization. In turn this will increase the amount of photocurrent produced. However, the dark current will also increase when the reverse bias is increased [31]. The dark current or leakage current will add noise to signal. The diodes used for the portable detector had capacitances ranging from 15pF to 75 pF. The area of the diodes used ranged from 2.7 ππ2 to 6.6 ππ2 . A equivalent circuit for a PIN diode can be seen in Figure 12. πΌπ is the current produced from the absorption of gamma rays, and πΌπ is the leakage current. π π is the junction resistance and is usually around 25 Mβ¦ or larger. Because this resistor is large and in parallel with the circuit it will not affect the circuit. π π is the contact resistance of the diode and is usually very small, around 1β¦ and is typically ignored. The junction capacitance, πΆπ, is due the capacitance of the depletion region. The junction capacitance is one of the concerning parameters of semiconductor devices used for radiation detection. The junction capacitance directly impacts the output noise of the diode. So the junction capacitance needs to be very small, diodes with capacitance around 50 pF or less are desired. The smaller the 21 Texas Tech University, Douglas Nichols, December 2016 capacitance the better, since the photocurrent is very small, the output noise also needs to be extremely small or the Signal to Noise Ratio (SNR) will be very low. This also applies to neutron detection; the neutron detector needs to have a very low capacitance to have a larger SNR. Figure 12: Photodiode Electrical Model 22 Texas Tech University, Douglas Nichols, December 2016 CHAPTER 3 TEST RESULTS AND DESIGN CHANGES Original Design Parameters The original circuit required a voltage supply of 12V. In order to provide a bias of 12V for a portable device, two different approaches were originally implemented. The first approach was to use a Linear Technology 8570 booster to boost the voltage to 12V from a 9V battery. The second approach was to use just batteries, a 9V and a 3V 2450 coin cell battery. In addition, a regulator needed to be used to supply 5V to the Op Amps for both implementations. A Linear Technology LT1129-5 5V regulator was used to provide an output of 5V based on the 12V input. The regulator is a Micro Power Low Dropout Regulator; the device was picked because it has a very low quiescent power which is desired for a small battery powered portable device. The LT8570-1 Linear Technology is a DC/DC Boost Converter that can be configured to produce a range of voltages. The converter was set up to output 12V with a 5V input. One concern is that the switching speed of the DC/DC converter may cause noise that might interfere with the detector portion of the circuit. The converter was configured to switch at a frequency of 100MHz which would be filtered out by the four band pass filters. The frequency is also high enough that it should not affect the low frequency signal of the detector. A picture of the original board design with an oscilloscope probe at the output can be seen in Figure 13. 23 Texas Tech University, Douglas Nichols, December 2016 Figure 13: Rev 1 Design with Coin Battery Testing Setup and Process The first board was tested using a Infinium MSO9254A oscilloscope. In order to test the boards functionality a source of gamma radiation would be needed. Due to hazards of radioactive materials, the first tests were conducted with fluorescent lights. These tests were used to confirm if the detector was able to detect any photons and properly amplifying the signal. The first board design did not have proper shielding and the diode was exposed. Figure 14 shows the output of the comparator when tested with the lights on. As expected, the photodiode detects all the photons and is constantly producing a current. Since the shaping amplifier produces a positive and a negative pulse with a width of 6µs the output from the comparator has a period of 12µs and is centered at 2.9V. Only the number of counts could be obtained from the output of the comparator. The pulses before the comparator would could be measured 24 Texas Tech University, Douglas Nichols, December 2016 to create a spectrum of the energy. When the detector was tested with the lights off the detector still picked up noise from other sources. Lab equipment such as oscilloscopes and power supplies also gave off noise. The noise was sometimes large enough to cause the comparator to output positive pulses. Figure 14: Output Pulse from the Output Compare Coin vs Booster One of the concerns with the first board layout was that the board as well as the diode were picking up external noise. If this was the case, the entire board would need to be shielded to prevent noise from creating false positives at the output. A test was performed in order to determine if both the board and the diode were picking up external noise or just the diode. The diode was removed for both the coin board and the booster board. Both boards were then tested in the lab with fluorescent lights on to 25 Texas Tech University, Douglas Nichols, December 2016 determine if the boards themselves were picking up any noise. An oscilloscope was used to monitor the output of both boards. It was discovered that the board with the coin battery did not produce any pulses at the output. However, the board with the booster had several pulses at the output. These pulses would be false positives and would diminish the accuracy of the detector. The noise seen at the output may be due to the booster since the design with the coin battery did not have any noise. It is possible that the switching frequency of the booster may causing interference with the rest of the board. The false positives due to the noise at the output of the booster board can be seen below. Figure 15: Booster Output Without a Diode 26 Texas Tech University, Douglas Nichols, December 2016 Changes for Next Board Design In order to properly test whether the device can detect gamma radiation the detector will need to be properly shielded before being tested with a source of gamma radiation. The device picked up too much noise from external devices such as fluorescent lights as well as other equipment. The next implementation would need shielding for the detector as well as the front stage of the circuit. The board also needed to be more compact. REV 2 In order to eliminate external noise, a PCB EMI shield from Wurth Electronics was implemented in the second design. The shield covered the diode as well as the first stage of the circuit. The shield blocked the diode so that it would not detect any light, and also protected the diode and the first stage from electromagnetic interference. The layout was altered in order to reduce the amount of noise from longer traces. More vias were used to connect the ground planes on both the front and back side of the board. Figure 16 shows the second board design with an oscilloscope probe at the output of the board. The test point is the middle test peg on the right side of the board in Figure 16. 27 Texas Tech University, Douglas Nichols, December 2016 Figure 16: Rev 2 with shield cover in place Testing Setup and Process To compare the first and second board design, the same test was used with the lights. An oscilloscope was used to measure the output of the board in a lab with fluorescent lights. The purpose of this test was to determine if the PCB shielding eliminated the noise from the fluorescent lights. To determine if the signal from the diode was amplified the test point before the comparator was tested with the fluorescent lights on and without the shield. Figure 17 displays the signal at the test point, which is the signal right before the comparator. The comparator would then output a positive pulse when the signal at the test point is negative. The signal at the output for each amplifier can be seen in Appendix A. The signal in Figure 17 had a frequency of 80 kHz. The period of the signal is 12µs, the negative pulses should correlate to the amplitude of the charge pulse from the preamplifier. However, there is 28 Texas Tech University, Douglas Nichols, December 2016 also a positive pulse of 6µs. This means that the shaping amplifier is amplifying part of the decay of the output from the preamplifier. Figure 17: Signal at Test Point The output of the first board is the same as the output of the second board so the functionality without the shield is the same. The test was then repeated with the lights turned off. The second board produce fewer pulses from noise. This was probably due to a better layout and strategically placed vias to ground. Once it had been determined that the second board seemed to be working correctly the board was then tested with the shield. Even with the fluorescent lights on the oscilloscope did not measure any pulses at the output of the board. This was the desired outcome. Since, the shield eliminated most of the unwanted interference the next set of tests could be 29 Texas Tech University, Douglas Nichols, December 2016 completed. The next step was to test the board with a source of gamma radiation. The radioactive element that was used to test the board was Caesium 137. The test set up for Rev 2 and the source of Caesium 137 can be seen below in Figure 18. Figure 18: Test Setup for Rev 2 Design for Test with Caesium 137 Caesium 137 is an element that produces gamma rays with an energy of 662 keV. The boards were tested at a distance of 6 inches from the source of Caesium. At the time of the tests the source of Caesium had a radioactivity of 39.15 mCi. At a 1 distance of 6 inches the flux of the Caesium was 4.97×105 ππ2 ×π ππ. The detector circuit was tested first without any exposure to the Caesium, to determine the background noise. The detector circuit was then exposed to the Caesium. Figure 19 shows the pulse seen at the test point of the detector circuit. Remember the inverted pulse is the pulse that correlates to the energy level of the gamma radiation. 30 Texas Tech University, Douglas Nichols, December 2016 Figure 19: Signal at Test Point with Caesium 137 To get an overall picture of what the spectrum would look like, an Amptek 8000D MCA was used to plot the pulse spectrum based on the signal from the test point of the detector circuit. Figure 20 is the pulse spectrum from the MCA. The red spectrum is the background noise, and was obtained when the detector was not exposed to the Caesium. And the blue spectrum was acquired when the detector was exposed to the Caesium. Since, the signal at the test point has an offset of around 3.1V and is negatively inverted, the gamma counts are the peaks that are left of channel 220, on the blue spectrum. Notice that on the blue spectrum there also a lot more counts left of channel 300 than there were with just the background noise. However, only the gamma peaks that occurred where there was no noise previously are counted as actual gamma counts. For better results with the MCA the offset should be removed and the pulses should not be inverted. 31 Texas Tech University, Douglas Nichols, December 2016 Figure 20: MCA Measurement Changes for Next Board Design In order to get an accurate reading of the number of gamma rays a MCA was used. However, to get an accurate reading from the MCA the output of the detector must have an offset of 0V. The output also needs to be setup so that the positive pulses represent the magnitude of the gamma strike. This meant that the comparator at the end of the circuit needed to be removed. The signal at the output of the shaping amplifier also needed to be inverted. REV 3 The third board design did not have a comparator. Instead an inverting amplifier with a voltage gain of 1V/V was used to invert the signal from the output of the shaping amplifier. The 3V offset was also removed with a passive high pass filter. To eventually merge the amplifier circuit with the neutron detector from the Nano 32 Texas Tech University, Douglas Nichols, December 2016 Photonics Lab, the bias needed to be increased. The bias was boosted to 48V. However, the diode used for detecting gamma radiation had a maximum reverse bias of 20V. A voltage divider was used to decrease the bias to 19V. In the future, the simple voltage divider can be removed by removing the two resistors to use the full voltage bias of 48V. Figure 21 shows the final board. Figure 21: Rev 3 Board with Changes In order to boost the voltage to 48V the LT8570-1 DC to DC converter was used. The booster was configured to boost the voltage from an input range of 9V-16V to an output of 48V with a max current of 50mA. The current draw from the entire system was measured to be 23mA. If a 9V battery was used the voltage would have dropped below 9V over time and the booster would have stopped working. Instead three, 3.7V lithium batteries were used to power the portable detector. The capacity of 33 Texas Tech University, Douglas Nichols, December 2016 the batteries are 4000 mA-Hours. These batteries not only provide an input voltage of 11.1V but will also last significantly longer than a 9V which only has 600mA-Hours. For future improvements, the reverse bias may one day be increased even more. This would require more current draw to power a DC to DC booster with a larger output. The 3.7V lithium batteries would allow for a DC to DC booster with a larger current draw. The implemented changes to the schematic can be seen in Figure 22. The third board design also used a diode from Hamamatsu that had a capacitance around 15pF and an active area of 6.6mm2 [32]. Based on the area of the diode and the flux of the Caesium source the diode would see 32,802 counts per second if 100% of the energy from the gamma radiation was absorbed. 34 Texas Tech University, Douglas Nichols, December 2016 Figure 22: REV 3 Design 35 Texas Tech University, Douglas Nichols, December 2016 Testing Setup and Process A simple test was first completed for rev 3 with the fluorescent lights. The setup was the same as it was for the first two boards. The output and the test point before the last amplifier were tested with and without a shield. In addition, every test was completed with the lights on and again with lights out. The signal at the test point was similar to the other two boards. The device was then tested with the Caesium 137 source. The device was placed 6 inches away from the source. The test results were fist measured with an oscilloscope to determine that the pulse was properly inverted and that there was no longer a DC offset. All the tests were done with the lights off in the lab to eliminate external noise. The test setup can be seen in Figure 23. Figure 23: Test Setup for Rev3 Design for Test with Caesium 137 36 Texas Tech University, Douglas Nichols, December 2016 The output from the portable detector can be seen in Figure 24. The oscilloscope displays a window of 5ms, which shows the pulses caused by gamma radiation and the noise floor. Figure 25 shows a zoomed in image of a pulse with an amplitude of 205mV. The positive pulse has a width of about 6.5 µs which is expected due the shaping amplifier. The negative pulse is the amplified decay from the original pulse of the preamplifier. Ideally this negative pulse would not be present. There was some noise on the signal waveform, seen in Figure 25. This was most likely due to the setup and the long cable used. Since no one could be inside the room when the Caesium 137 was present, a long cable was used to connect the detector to an oscilloscope outside the room. The small pulses due to the noise have a period of .8 µs. The coaxial cable connected with the alligator clips seems to acts a monopole antenna with a resonant frequency of 1.25 MHz. Figure 24: Output pulses with Caesium 137 37 Texas Tech University, Douglas Nichols, December 2016 Figure 25: Output pulse with Caesium 137 The detector was tested with the Amptek 8000D MCA to obtain a pulse spectrum. The detector was first tested without the gamma source and without the lights on to obtain a spectrum of the background noise. The background noise can be seen in red in Figure 26. The detector was then tested with Caesium 137 source, the results for this test are in blue. The detector was able to detect the presence of gamma radiation but was not able to find a peak that would correlate to the 662 keV peak that should be seen for Caesium 137. So, the pulses that were caused by gamma radiation were caused by Compton scattering where the gamma rays only gave up part of their energy to the electrons in the diode. The silicon diode was not able to absorb the full energy from the gamma radiation. 38 Texas Tech University, Douglas Nichols, December 2016 Figure 26: MCA Measurement The gamma count was discovered by summing all the gamma counts after a chosen cutoff. The cutoff is usually chosen to be the largest channel of the background noise counts. The cutoff here was chosen for counts where the gamma counts were 99% greater than the background noise counts. This cutoff was found at channel 35. A gamma count measured over 5 minutes was discovered to be 109494, this would correlate to 364 counts per second, which is 1.1% of what should be seen with a detector with 100% efficiency. 39 Texas Tech University, Douglas Nichols, December 2016 CHAPTER 4 AMPLITUDE MEASUREMENT WITH LabVIEW In order to eventually display the detector results on a wireless device, a portable method for logging the results was needed. A MCA is usually used for measuring the pulse amplitudes of radiation detectors. The Amptek 8000D uses and analog to digital converter (ADC) with a sample rate of 100 million samples per second to measure the amplitude of the pulses. To replace the MCA a microcontroller with a 14-bit ADC was used to measure the pulses and then transmit those values to LabVIEW. The ADC pins of the MSP432 were attached to the portable detector using jumper wires. The data was sent over serial communication using Universal Asynchronous Receiver/Transmitter (UART). LabVIEW read in the values and plotted all the amplitudes in a histogram similar to the plots from the MCA. The microcontroller used was a MSP_EXP432P401R evaluation module made by Texas Instruments. The evaluation board was used for these tests but eventually the MSP432P401R chip itself could be placed on the PCB with the portable detector. The signal from the portable detector had a very quick output pulse which was 6µs. To measure all of the peaks of these pulses, the MSP432's ADC must sample faster than the pulses. This particular microcontroller was chosen because the ADC can sample at a rate of 1 million samples per second (1Msamp/s). This sample rate will be quick enough to sample the pulse and also determine the peak value. The only part of the pulse needed is the peak; however, to find the peak the ADC polls every 1µs and then compares the value to the previous value. If the present value is higher 40 Texas Tech University, Douglas Nichols, December 2016 than the previous value, the present value will be stored as the peak value. The MSP432 will continue to look for a greater value than the stored peak. If it does not find a peak and the pulse goes back down to the threshold the peak value will be stored in an array that will be sent to LabVIEW via UART serial communication. Two problems with this setup are noise and the transmit time. Since the ADC will constantly be polling every 1µs it will pick up a lot of noise and will likely send most of the noise peaks to LabVIEW. This means it could be sending up to 1 million samples per second. This number would not be 1 million since the pulses caused by the gamma radiation are 6µs which means that several of the ADC values would be thrown out. However, the max pulse count that could be seen for a perfect silicon detector for gamma detection, was about 32802 pulses per second. This means that even with throwing out approximately 9 values per pulse, the MSP432 would still send around 704800 values per second. Each value is 14 bits, so even with the maximum baud rate of 115200 bits per second, the transmission rate would be 8229 peak values per second. If the test was run for 5 minutes like the previous tests it would take 428 minutes to transmit all the data to LabVIEW. Unfortunately, with the loop setup in LabVIEW a baud rate of 115200 would mean that LabVIEW would not be reading the values from the buffer fast enough. This means the histogram on LabVIEW would be lagging significantly far behind. The data in the received buffer could also be overwritten That is why the baud was set to 9600 bits per second and a different scheme of sending the data was used. 41 Texas Tech University, Douglas Nichols, December 2016 To reduce the amount of data that had to be transmitted, the 14-bit ADC value was changed to an 8-bit value. The 4 least most significant bits (LSB) were thrown out as well as the 2 most significant bits (MSB). This reduced the data to one byte of data which significantly cut down on the transmit time. By removing the 4 LSB the values now have a resolution of 920µV rather than the 61µV resolution of the 14-bit value. This 920µV is close to the resolution of the MCA, so the histograms of the two should match. Each column of the histogram varied by a little less than 1mV. Removing the 2 MSB reduced the maximum peak value to only 300mV. The data that was collected from the Rev 3 design showed that in 5 minutes only 5 peaks were collected at 250mV. The 250mV peaks were the maximum peaks detected, so based on this data no peak values should be lost. Since the baud rate was set to 9600 in order for LabVIEW not to miss any data, the transmit time for 8-bit values was 1200 peaks per second. This was better than the 686 peaks per second for 14-bits but was still too slow. Since the baud rate could not be changed the amount of data needed to be reduced further. Since, the goal was to plot the amplitudes of the pulses caused by radiation, part of the noise could be removed. To accomplish this, only the ADC values above a certain threshold were stored and then transmitted. To determine the threshold, the data from the third board design was analyzed. The threshold was chosen based on a voltage value where only 5% of the noise remained. The voltage level chosen was 30mV. This reduced the amount of data that needed to be transmitted but still allowed for most of the pulses caused by radiation to be stored and transmitted. A flow chart of the microcontroller can be seen below in Figure 27. 42 Texas Tech University, Douglas Nichols, December 2016 Figure 27: MSP432 ADC and UART Flow Chart After the MSP432 sent the peak values, LabVIEW then received the data and plotted the peaks in a histogram. In LabVIEW, VISA is used to setup a serial port to read in the data. The default baud rate for this ports is set to 9600 which is the same as the MSP432 baud rate. The data received was a string type and was one byte. The data was converted from an unsigned character to an unsigned integer. The integer was only 8 bits; in order to convert the data back into mV, the data was multiplied by 16 since the 4 LSB were removed. The equation for a voltage based on a measured ADC values is as follows: π = π΄π·πΆ ππππ’π π₯ π΄π·πΆ π ππππππππ π π΄π·πΆ πππ πππ’π‘πππ The reference was 1.2 V so the ADC value was measured by 1200mV. Finally, the value was divided by 2^14 which will give a voltage in mV. The voltages were then plotted in a histogram to give 43 Texas Tech University, Douglas Nichols, December 2016 a spectrum of the gamma energy. The LabVIEW program can be seen below in Figure 28. Figure 28: LabVIEW Program for Receiving and Plotting Data The front panel is what a user would see and is shown in Figure 29. It can be seen in Figure 29 that LabVIEW was able to receive and plot the data from the MSP432. A test was first completed without the source to determine the background noise. Because the MSP432 does not send any pulses with a value less than 30mV, LabVIEW did not receive any values other than 0. However, when tested with the source of Caesium 137, LabVIEW plotted the histogram below, shown in Figure 29. All the pulses in the histogram are from the gamma radiation and not noise. The data is similar to what was seen with the MCA other than the fact that there is no data from 0 to 30, since the MSP432 does not send any data below 30mV. 44 Texas Tech University, Douglas Nichols, December 2016 Figure 29: LabVIEW Front Panel for Receiving and Plotting Data The MSP432 and LabVIEW were able to successfully measure and plot the pulses from the output of the portable detector. The reading from the detector was similar to data obtained from tests with a MCA, which verifies that the MSP432 and LabVIEW are able to give a rough spectrum of the detector. One issue with LabVIEW was that it lagged by a couple seconds so the data that was being plotted was data that had been measured several seconds before. To improve this setup a different microcontroller with more memory could be used. A different user interface that would read the data faster could also be used. These two changes could allow us to display the noise below 30mV and reduce the lag time. 45 Texas Tech University, Douglas Nichols, December 2016 CHAPTER 5 CONCLUSION AND FUTURE WORK Future Work In the future, the entire detector board needs to be completely shielded, so that any external noise does not affect the other stages. The shaping amplifier could be improved by using amplifiers with a higher gain bandwidth. This would mean that only two amplifiers would be needed for the shaping amplifier rather than three. For testing purposes the output connection should be changed to a BNC. This would improve the connection and reduce the noise caused by using alligator clips. A microcontroller chip should be added to the board as well as a Wi-Fi or Bluetooth IC. This will allow the detector board to detect, measure, and transmit the radiation data all from one printed circuit board. Either Wi-Fi or Bluetooth could be used to transmit the data from the detector to a wireless device. The transmit speed for Wi-Fi is faster than Bluetooth. The bit rate for Bluetooth is around 2.1Mbps and the bit rate for Wi-Fi is 600Mbps [33]. For the current setup with the MSP432 and LabVIEW Bluetooth could be used since the MSP432 does not include the noise and is a 8-bit value. If the noise was included and the full 14 bits of the ADC were used, the transmit speed would need to be 14Mbps since the MSP432 samples at 1 million samples per second.. So, in order to transmit both the noise and the peaks Wi-Fi would need to be used. The microcontroller would need to use SPI to communicate with the wireless transmitter, since UART was too slow. 46 Texas Tech University, Douglas Nichols, December 2016 Conclusion The detector circuit was successful in detecting the presence of gamma radiation. A pulse spectrum was plotted with a microcontroller and LabVIEW based on the output of the portable system with a photodiode. The spectrum plotted in LabVIEW was similar to the spectrum plotted with a commercial MCA. The spectrum changed significantly when the detector was in the presence of a gamma source, however there were not any significant peaks on the spectrum. Without distinct peaks on the spectrum it would be impossible to determine the source of the radiation. This was due to sensitivity of the diode used for gamma detection. The detector was able to measure the pulses caused by Compton scattering but was unable to find the photo peak, where the gamma radiation transferred its full energy to device. In the future, a photodiode of different material that absorbs gamma radiation better than silicon could be used for gamma detection. Even though the detectors themselves are different for detecting neutron and gamma radiation, the circuitry used to convert the charge to a Gaussian signal is the same. This is because semiconductor detectors for both neutron and gamma radiation rely on the buildup of charge in the detector. So, the output signal of the two will be similar. The present portable detector system may be used for neutron detection if the neutron detector used didn't require a bias above 48V and had a capacitance similar to the capacitance of the photodiodes used for testing. 47 Texas Tech University, Douglas Nichols, December 2016 BIBLIOGRAPHY [1] S. Lam, "Helium-3 Proportional Counters and Alternatives for Neutron Detection," Stanford, 19 March 2012. [Online]. Available: large.stanford.edu/courses/2012/ph241/lam1/. [Accessed 30 August 2016]. [2] Proportional Technol. Inc., "High sensitivity portable neutron detector for fissile materials detection," in Nuclear Science Symposium Conference Record, 2005 IEEE, 2005. [3] "Radiation Basics," United States Nuclear Regulatory Commission, 17 2014 October. [Online]. Available: http://www.nrc.gov/about-nrc/radiation/healtheffects/radiation-basics.html. [Accessed 15 September 2016]. [4] R. Kouzes, "Detecting Illicit Nuclear Materials," American Scientist, October 2005. [Online]. Available: http://www.americanscientist.org/issues/feature/detecting-illicit-nuclearmaterials/99999. [Accessed 8 Septermber 2016]. [5] J. E. R. K. B. M. J. S. B. S. E.R. Siciliano, "Comparison of PVT and NaI(Tl) scintillators for vehicle portal monitor applications," in Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2005, pp. 647-674. [6] R. K. J. S. E. S. D. S. D. W. James Ely, "The use of energy windowing to discriminate SNM from NORM in radiation portal monitors," in Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2006, pp. 373-387. [7] Aspect Service, "Complex radiation control," Aspect Service, 2016. [Online]. 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[Accessed 4 September 2016]. 48 Texas Tech University, Douglas Nichols, December 2016 [13] Massachusetts Institute of Technology, "Helium-3 Neutron Proportional Counters," [Online]. Available: http://web.mit.edu/8.13/www/JLExperiments/38/tgm-neutron-detectors.pdf. [Accessed 10 October 2016]. [14] S. N. Ahmed, Physics & Engineering of Radiation Detection, San Diego: Academic Press, 2007, p. 191. [15] International Atomic Energy Agency, "Neutron Dosimetry and Monitoring," [Online]. Available: http://www.slideshare.net/leishare/active-methods-ofneutron-detection. [Accessed 10 October 2016]. [16] M. P. B. a. T. W. Crane, "CH13 Neutron Detectors," in Passive Nondestructive Assay of Nuclear Materials, Washington DC, U.S.Government Printing Office, 1991, pp. 379-404. [17] G. F. Knoll, "Radiation Detection and Measurement," John Wiley & Sons, Inc., 2000, p. 538. [18] W. R. Leo, Techniques for Nuclear and Particle Physics Experiments, SpringerVerlag, 1994. [19] H.-C. Schultz-Coulon, "Kirchhoff-Instituie for Physics," [Online]. Available: http://www.kip.uniheidelberg.de/~coulon/Lectures/Detectors/Free_PDFs/Lecture4.pdf. [Accessed 15 September 2016]. [20] S. Harrison, "Semiconductor-Based Thermal Neutron Detectors," 19 February 2013. [Online]. Available: http://large.stanford.edu/courses/2013/ph241/harrison1/. [Accessed 27 September 2016]. [21] R. K. H. G. J. S. D. S. McGregor, "Designs for thin-film-coated semiconductor thermal neutron detectors," in Nuclear Science Symposium Conference Record, 2001 IEEE, 2001. [22] K.-C. H. A. W. J.-Q. J. L. Y. D. I. B. B. Rajendra Dahal, "Development of a large area micro-structured solid-state neutron detector at low cost," Institute for Electrical and Electronics Engineering, 2014. [23] Texas Tech University, "Optoelectronic UV LED and Neutron Detection Using Hexagonal Boron Nitride," Texas Tech University, [Online]. Available: http://ip.innovatetexastech.com/technologies/d-0812_optoelectronic-uv-led-andneutron-detection-using-hexagonal-boron-nitride. [Accessed 20 October 2016]. [24] ORTEC, "Preamplifier Introduction," [Online]. Available: www.orteconline.com/download/Preamplifier-Introduction.pdf. [Accessed 13 September 2016]. [25] Maxim Integrated, "Gamma-Photon Radiation Detector," Maxim Integrated, 2003 September 2003. [Online]. Available: https://www.maximintegrated.com/en/app-notes/index.mvp/id/2236. [Accessed 21 October 2016]. 49 Texas Tech University, Douglas Nichols, December 2016 [26] Hamamatsu, "Characteristics and use of Charge Amplifier," Oct. 2001. [Online]. Available: https://www.hamamatsu.com/resources/pdf/ssd/charge_amp_kacc9001e.pdf. [Accessed 17 October 2016]. [27] Cremat, "CR-200-X shaping amplifier modules," Cremat, 2015. [Online]. Available: http://www.cremat.com/CR-200.htm. [Accessed 20 October 2016]. [28] radiologykey, "Electronic Instrumentation for Radiation Detection Systems," radiologykey, 26 February 2016. [Online]. Available: http://radiologykey.com/electronic-instrumentation-for-radiation-detectionsystems/. [Accessed 3 November 2016]. [29] "CR-150-R5 Evaluation Board," May 2014. [Online]. Available: http://www.cremat.com/CR-150-R5.pdf. [Accessed 20 October 2016]. [30] "CR-160-R7 Gaussian shaping amplifier evaluation board: application guide," Cremat, May 2014. [Online]. Available: http://www.cremat.com/CR-160R7.pdf. [Accessed 20 October 2016]. [31] B. G. S. S. K. Banerjee, in Solid State Electronic Devices Seventh Edition, Pearson Education Inc., 2015, p. 419. [32] Hamamatsu, "Si PIN photodiode S8729-10," Hamamatsu, [Online]. Available: https://www.hamamatsu.com/us/en/product/category/3100/4001/4103/S872910/index.html. [Accessed 1 June 2016]. [33] Diffen, "Bluetooth vs. Wi-Fi," Diffen, [Online]. Available: http://www.diffen.com/difference/Bluetooth_vs_Wifi. [Accessed 1 November 2016]. [34] I. Rittersdorf, "GammaRaySpectroscopy," 20 March 2007. [Online]. Available: http://www-personal.umich.edu/~ianrit/gammaspec.pdf. [35] M. Sibley, "Photodiodes," in Optical Communications, MacMillian New Electronics, 1990, p. 77. 50 Texas Tech University, Douglas Nichols, December 2016 APPENDICES Appendix A Rev 2 Amplifier Outputs Figure 30: Output Of Second Amplifier Figure 31: Output of Third Amplifier 51 Texas Tech University, Douglas Nichols, December 2016 Appendix B Additional Printed Circuit Board Figure 32: Rev 2 Design with Shield Cover Removed 52