Download Portable Amplifier for Semiconductor

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.
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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
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APPENDICES ..................................................................................................... 51
Rev 2 Amplifier Outputs ................................................................................. 51
Additional Printed Circuit Board .................................................................... 52
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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.
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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
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Figure 32: Rev 2 Design with Shield Cover Removed ......................................... 52
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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
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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]
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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 𝑛𝑒𝑢𝑡𝑟𝑜𝑛𝑠/(𝑠 × 𝑐𝑚2 ) [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
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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
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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
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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
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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
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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
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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.
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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
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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
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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].
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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.
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Figure 7: Gamma Detector REV 1 Design
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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 10kΩ 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
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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: 𝑆 =
𝑒
𝐶𝑓 𝑥 Є
[26]. Where
e is the charge of an electron and Cf is the feedback capacitor C3, and Є 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 𝑅3 × 𝐶3. In this case the decay is 47µs.
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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,
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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
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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
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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 IC’s, 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]
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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 𝑚𝑚2 to 6.6 𝑚𝑚2 . A equivalent circuit for a PIN diode can be
seen in Figure 12. 𝐼𝑠 is the current produced from the absorption of gamma rays, and
𝐼𝑑 is the leakage current. 𝑅𝑗 is the junction resistance and is usually around 25 MΩ or
larger. Because this resistor is large and in parallel with the circuit it will not affect the
circuit. 𝑅𝑠 is the contact resistance of the diode and is usually very small, around 1Ω
and is typically ignored. The junction capacitance, 𝐶𝑗, 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
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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
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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.
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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
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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
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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
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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.
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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
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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
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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 𝑐𝑚2 ×𝑠𝑒𝑐. 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.
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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.
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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
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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
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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.
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Texas Tech University, Douglas Nichols, December 2016
Figure 22: REV 3 Design
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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
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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
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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.
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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.
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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
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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.
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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.
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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: 𝑉 = 𝐴𝐷𝐶 𝑉𝑎𝑙𝑢𝑒 𝑥
𝐴𝐷𝐶 𝑅𝑒𝑓𝑒𝑟𝑒𝑛𝑐𝑒 𝑉
𝐴𝐷𝐶 𝑟𝑒𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛
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
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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.
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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.
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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.
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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.
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Texas Tech University, Douglas Nichols, December 2016
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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.
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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
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Texas Tech University, Douglas Nichols, December 2016
Appendix B
Additional Printed Circuit Board
Figure 32: Rev 2 Design with Shield Cover Removed
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