Designing an experiment to study absorption vs. dose for feedback enabled radiation therapy

Designing an Experiment to Study Absorption vs. Dose for Feedback Enabled Radiation Therapy

by

Hadrick Alexis Green

Submitted to the Department of Nuclear Science and Engineering in partial fulfillment of the requirements for the degree of

Bachelor of Science in Nuclear Science and Engineering at the

Massachusetts Institute of Technology June 2017

c

2017 Massachusetts Institute of Technology. All rights reserved.

Author . . . . Department of Nuclear Science and Engineering

May 18, 2017

Certified by . . . . Michael Short Assisstant Professor, Nuclear Science and Engineering Thesis Supervisor

Accepted by . . . . Michael Short Assisstant Professor, Nuclear Science and Engineering Chair, NSE Department Committee for Undergraduate Students

Designing an Experiment to Study Absorption vs. Dose for

Feedback Enabled Radiation Therapy

by

Hadrick Alexis Green

Submitted to the Department of Nuclear Science and Engineering on May 18, 2017, in partial fulfillment of the

requirements for the degree of

Bachelor of Science in Nuclear Science and Engineering

Abstract

In the field of radiation oncology, while there are simulations and devices that allow users to be relatively confident that radiation to the tumor and sparing of healthy tissue is being maximized, the inability to reliably measure and control the dose during radiation treatment is a major source of uncertainty. This uncertainty is due to issues such as organ movement, a lack of precise and constant knowledge of beam current at the target site, and the inability to correctly register dose during hardware or software failures; all of which result in radiation treatments being measured after the procedure or in a fault susceptible manner during the procedure. The integrating feedback f-center dosimeter (IF2D) is a dosimeter that would address these challenges and enable feedback during radiotherapy procedures, which would give doctors and patients confidence that the correct dose was delivered to the target sites without exceeding allowable doses to healthy tissue. An in-situ irradiator will be designed and later used to quantify the relationship between dose and f-center absorption. This design will help guide the future experiment and further the development of the IF2D.

Thesis Supervisor: Michael Short

Acknowledgments

I would like to thank Professor Short for his support, patience, and understanding throughout this project. Although it did not go as far as I had hoped, it was still a great opportunity to learn and experience something I would not have done in a typical classroom setting. I am extremely grateful for Cody Dennett and his guidance and vast knowledge of various experiment parts and practices as there were multiple times throughout this project that I was utterly lost. Lastly, I owe a lot of thanks to Peter Stahle for aiding me in the design of this project.

I would also like to take a moment to thank those who were not directly participating in the project but still provided in some form or way. I would like to thank Heather Barry for always letting my friends and I vent to her about our thesis struggles and for always giving great advice. I would also like to thank the establishment QMart for being QMart in its own special way.

Contents

2.1.2 Electron Spin Resonance (ESR) Dosimetry . . . 17

2.1.3 MOSFET Dosimeters . . . 19

2.1.4 Optically Stimulated Luminescence (OSL) Dosimeters . . . 20

2.1.5 Summary of Dosimeters . . . 21

2.2 Integrating Feedback F-center Dosimeter . . . 22

2.2.1 Description of Device . . . 22

2.3 F-centers . . . 24

3 Design of Experiment 27 3.1 Experiment Overview and Housing Assembly . . . 27

3.2 Fiber Optics . . . 32

3.3 Salt Wheel . . . 35

4 Decay of F-centers 39 4.1 Estimated Decay Rate . . . 39

5 Conclusion 41 5.1 Implications . . . 41 5.2 Future Work . . . 41

List of Figures

2-1 Image of a TLD . . . 16

2-2 Glow Curve Using a Phosphor TLD . . . 17

2-3 Magnetic field vs. Energy for free electrons . . . 18

2-4 Diagram of a MOSFET . . . 19

2-5 Schematic of Dosimeter . . . 23

2-6 Process Diagram of Dosimeter . . . 24

2-7 F-center in an NaCl Crystal . . . 25

3-1 Launch Side Viewpoint of Design . . . 28

3-2 Collection Side Viewpoint of Design . . . 29

3-3 Beamline Viewpoint of Design . . . 30

3-4 Motor Feedtrhough and Cross Flange . . . 31

3-5 KT310 Spatial Filter . . . 32

3-6 LCP02 30mm to 60mm Cage Plate Adapter . . . 33

3-7 Infinity Corrected Lens Assembly . . . 33

3-8 Achromat Microscope Objective Assembly . . . 34

3-9 Collection Assembly . . . 34

3-10 Fiber Optic Cage Assembly . . . 35

3-11 Salt Wheel with Cover . . . 36

3-12 Salt Wheel Line Drawing . . . 37

List of Tables

Chapter 1

Introduction

1.1

Motivation

Five decades of dosimeter development has yet to enable real-time, fault-tolerant, feedback-controlled radiotherapy for cancer treatment. The current inability to re-liably measure and control the dose during radiation therapy treatments, whether by protons, x-rays, or implanted brachytherapy, is a major source of uncertainty in radiation oncology. Challenges such as organ movement on the timescale of seconds, precise and constant knowledge of radiation beam current at the target site, and the ability to correctly register dose during hardware or software failures mean that radiotherapy treatments are either measured after the procedure is finished, or in a fault-susceptible manner during the procedure. If a dosimeter existed that could overcome these challenges, it would enable feedback during radiotherapy procedures, which would give doctors and patients confidence that the correct dose was delivered to the target sites, without exceeding maximum allowable doses to healthy tissue [1]. The Integrating Feedback F-center Dosimeter (IF2_{D) has been designed to address}

these exact challenges but experiments must be conducted in order to aid with the de-velopment of a physical product. This paper will propose a design for an experiment to study the relationship between dose, measured in Gy, and the f-center absorption through the use of an in-situ irradiator. The experiment will take salt pellets and irradiate them so that there is a change in the color of the salts. Fiber optics and a

spectrometer will then be used to analyze the color shift in the salts.

1.2

Outline

Chapter 2 presents the background necessary to understand the purpose and signif-icance of the IF2D. The first sections will discuss the strengths and weaknesses of existing dosimeters such as TLDs, electron spin resonance (ESR) spectrometry, metal oxide semiconductor field effect transistor (MOSFET) dosimeters, and optically stim-ulated luminescent dosimeters (OSLD). The following sections will describe the IF2_D

and how it operates. Chapter 3 summarizes the experiment design and describes the parts used. Chapter 4 gives a spectrum that estimates the magnitude of color change for a few salts at various doses given the experiment set up. Chapter 5 presents the implications of the IF2_{D and future work.}

Chapter 2

Background

2.1

Existing Dosimeters

Current dosimeters can be grouped into two different classes: dosimeters that are not designed for in-vivo applications and dosimeters that are capable of in-vivo applica-tions and in some cases, real time feedback. This section will cover dosimeters from each of these two classes. Thermoluminescent dosimeters belong to the first class of dosimeters. Dosimeters that belong to the second class include electron spin reso-nance (ESR) dosimetry, MOSFET dosimeters, and optically stimulated luminescence (OSL) dosimetry. These methods of dosimetry are capable of accurately measuring dose given that the set up is correct and no part of the system malfunctions during operation. In general, the lack of in-vivo applications is a major drawback of the first class of dosimeters. This limits the use of these dosimeters to locations on the surface of the skin or near the patient. Therefore dose to tumors deep within the patient can not be accurately measured using the first class of dosimeters. For the second class of dosimeters, the inability of real time feedback or the vulnerability of not being fault tolerant is the main drawback. For dosimeters that are capable of real time feedback and are fault tolerant, other drawbacks such as the set up of such systems still exist.

2.1.1

Thermoluminescent Dosimeters (TLDs)

The standard method of dosimetry has been and continues to be thermoluminescent dosimeters (TLDs). These devices consist of a salt or other material which builds up point defects in response to incipient ionizing radiation [2]. TLD materials emit light as electrons return to their ground states, allowing for a very precise measurement of total (integrated) dose following a procedure [2]. TLDs must be heated to speed up the transition from excited to ground states [2]. Both skin and target site doses can be read by on-skin and implanted TLDs, respectively. However, due to the nature of TLDs, their accumulated dose can only be read post-procedure, using specialized equipment to capture the low levels of light emitted upon heating [3]. Another draw-back of TLDs is that placement is limited to the skin, in a bodily orifice, or behind the patient [1]. Thus, TLDs can only measure in-vivo dose of organs or tissue near the surface of the skin. The dose to a specific internal organ or tissue could be calcu-lated using TLD based doses taken at the skin or near the patient, but would have associated uncertainties that would make it sub-optimal. The post-processing and placement limitation aspects are the main reasons why TLDs are not viable for real time dosimetry. Fig.2-1 presents an illustration of a TLD and Fig. 2-2 displays the glow curve of a phosphor TLD.

Figure 2-1: Image of a TLD. A multitude of shapes for TLDs exist but most are on the scale of a few millimeters for each dimension [4].

Figure 2-2: Glow Curve Using a Phosphor TLD. Multiple peaks result as the material is heated and electrons trapped in shallow traps are released [5].

In Fig. 2-2 multiple peaks result as the material is heated and electrons trapped in shallow traps are released. This results in a peak as these traps are emptied. The light output drops off as these traps are depleted and as heating continues, the electrons in deeper traps are released which results in additional peaks. Usually the highest peak is used to calculate the dose equivalent and the area under the curve represents the radiation energy deposited in the TLD.

2.1.2

Electron Spin Resonance (ESR) Dosimetry

Irradiation results in long-lived electron spin resonances in tissue which can be de-tected and correlated to find the dose received; the ESR signal height exhibits a linear correlation with dose received [1]. ESR measurements are primarily made on teeth, nails, hair, and bones but not soft tissues since it is a method used for studying rad-icals formed in chemical reactions and the reactions themselves. Every electron has a magnetic moment and spin quantum number s = 1₂, with magnetic components ms = +1₂ and ms = −1₂. In the presence of an external magnetic field with strength

anti-parallel (ms = +1₂) to the field, with each alignment having a specific energy due to

the Zeeman effect [6]:

E = msgeµBB0 (2.1)

where µB is the Bohr magneton which is a physical constant and natural unit for

expressing the magnet moment of an electron caused by either its orbital or spin angular momentum. ge is the electron’s g-factor and is equal to 2.0023 for a free

electron [6]. The separation between the lower and upper state for unpaired free electrons is:

∆E = geµBB0 (2.2)

This equation implies that the splitting of the energy levels is directly proportional to the magnetic field’s strength as shown in Fig. 2-3 below:

Figure 2-3: Magnetic field vs. Energy for free electrons [7]

An unpaired electron can move between the two energy levels by either absorbing or emitting a photon of energy hν such that the resonance condition hν = ∆E is obeyed. This leads to the fundamental equation of ESR spectrometry [6]:

hν = geµBB0 (2.3)

ESR is mainly used for and it’s development as a post-irradiation technique, ESR is not fit for use in real time measurement of dose to tumors.

2.1.3

MOSFET Dosimeters

MOSFET is a special type of a field effect transistor that works by electronically varying the width of a channel along which charge carriers (electrons/holes) flow. The wider the channel, the better the device conducts. The charge carriers enter the channel at the source and exit via the drain. The width of the channel is controlled by the voltage on an electrode called the gate, which is located between the source and the drain and is insulated from the channel by a thin layer of metal oxide or glass [8]. The MOSFET can operate in depletion mode or enhancement mode. In drain mode, when there is no voltage on the gate, the channel exhibits its maximum conductance. As the voltage on the gate increases the channel conductivity decreases. In enhancement mode, when there is no voltage on the gate, there is effectively no channel and the device does not conduct. A channel is produced through the application of a voltage to the gate where the greater the voltage, the better the conduction. The main advantage of the MOSFET is that the gate is electrically insulated from the channel such that no current flows between the gate and the channel regardless of the gate voltage. This gives the MOSFET infinite impedance. Fig. 2-4 shows a diagram of a MOSFET.

Figure 2-4: Diagram of a MOSFET [9]

Ionizing radiation creates electron hole pairs in the gate of the MOSFET [1]. Com-pared to the electrons, the holes have low mobility and some travel to the interface

between the gate and thin layer of insulating material and become trapped. The buildup of positive charge in this area will modify the formation of a depletion layer when the gate is biased, which will change the voltage required to open the channel. The change in threshold voltage can then be correlated to the dose received at the gate [1]. Due to the holes that are trapped having longevity, a bias does not have to be constantly applied which means that MOSFET dosimeters can passively collect information about radiation dose. It can be used as a cumulative charge detector as measurements are made at regular intervals.

2.1.4

Optically Stimulated Luminescence (OSL) Dosimeters

OSL dosimetry makes use of electrons trapped between the valence and conduction bands in the crystalline structure of certain minerals, mostly quartz and feldspar [10]. Ionizing radiation produces electron hole pairs where electrons are in the conduction band and holes are in the valence band [10]. Electrons that have been excited to the conduction band may become entrapped in the electron or hole traps [10]. Under stimulation of light the electrons can free themselves from the trap and enter the conduction band. From the conduction band they recombine with holes in the hole traps and if the hole is a radiative recombination center, light is emitted [10]. Photons are then detected using a photomultiplier tube and the signal from the tube is used to calculate the dose that the material had absorbed. OSL material is similar to TLD material and what separates the two materials is that OSL material does not have to be heated up to accelerate the light emitting process. Landauer Inc. has developed a dosimeter that utilizes OSL materials to provide real time feedback [1]. Although it is proven to be accurate, this specific dosimeter has a complex system and requires lasers, optics, and a photomultiplier tube [1]. OSL dosimeters are better than TLDs in the sense that they are more adaptable to in-vivo use but are still susceptible to dose information being lost if there are errors in the signal collection component of

2.1.5

Summary of Dosimeters

Table 1 gives a quick summary of the different types of dosimeters covered and the strengths and weaknesses of each [1].

Technology In-vivo Real-time

Fault-tolerant Drawbacks Signal to Noise Ratio Expense TLD Limited No Yes Requires post pro-cessing, integral dose only Good Low ESR

Spec-troscopy Yes No Yes

Best for hair, nails, teeth, bones

Poor High

MOSFET Yes Yes Yes

Need elec-trical con-nections or wireless power/data Good Low

OSL Yes Yes No

Information lost if any part of the system breaks down Good Medium

Table 2.1: Summary of Dosimeters

Each dosimeter has its strengths and weaknesses in terms of in-vivo capability, real time feedback, and fault tolerance. Those that excel in all of those categories have other drawbacks that currently limit their application. A category that has not been studied but carries much significance is toxicity and patient comfort. Even if a dosimeter is well designed for in-vivo applications and able to provide real time feedback while being fault tolerant, it will not succeed if it is toxic to the patient or requires invasive procedures each time it is used. Such procedures would take a lot of time and cause much discomfort and distress to the patient. Toxicity would

depend mainly on the materials the dosimeter is made up of and the goal would be to use biocompatible and biodegradable materials whenever possible. Another goal would be to design a dosimeter that has the capability to be reused for instances that require multiple irradiation sessions. That way, any invasive procedure would only have to be done once. If both of those goals are incorporated alongside the other goals of in-vivo capability, real time feedback, and fault tolerance, an ideal dosimeter can be designed.

2.2

Integrating Feedback F-center Dosimeter

The IF2D, through the combination of size and implantability of scintillating fibers with TLD materials, provides simultaneous proportional and integral dose informa-tion directly at the site of irradiainforma-tion [1]. The device relies on the creainforma-tion of f-centers, which are color changes due to crystalline defects trapping electrons, caused by irradi-ation. The salt changes color passively with irradiation so the dosimeter’s continued operation does not depend on any data acquisition or signal processing hardware to continue acquiring dose information [1]. Groups of tiny dosimeters can be arranged in a 3D configuration by coupling salts within a fiber optic bundle [1]. Whenever the dosimeter is interrogated with a known white light source, the device’s ability to differentiate the inherently integral dose information allows for feedback controlled radiation therapy [1].

2.2.1

Description of Device

A biocompatible casing contains the halide salt core. Incoming radiation causes dam-age to the structure of the salt, resulting in a temporary color shift of the salt. Fiber optics are used to analyze the color shift experienced by the salt. Broad spectrum white light with a known spectrum is sent into the device through one fiber optic cable [1]. Light then exits through a second fiber optic cable which sends it to a color spectrometer for analysis, where the spectrum of the exiting light changes based on

ing light to an unirradiated baseline light, the color shift of the salt is characterized [1]. The color shift can then be correlated to the extent of radiation damage in the salt which can finally be correlated to the dose that the salt has sustained. This information can be collected continuously during a radiation procedure, allowing for continuous feedback about dose received at the device’s locations [1]. Fig. 2-5 and Fig. 2-6 illustrate a basic schematic of the IF2_{D and a process diagram for its usage}

respectively.

Figure 2-5: Schematic of Dosimeter. A biocompatible casing contains salt that changes color with irradiation. White light of a known spectrum is sent in through a

fiber optic cable and comes out of another fiber optic cable with a spectrum that reflects the change in color of the salt. The new spectrum is sent to a spectrometer

Figure 2-6: Process Diagram of Dosimeter. It shares the benefits of a targeted measure dose of a scintillating dosimeter combined with the passively integrating nature of a TLD. Another benefit is that it does not have to be removed between

multiple treatment sessions [1].

2.3

F-centers

An f-center, also known as a Farbe center or color center, is a type crystallographic defect in which a vacancy in a crystal is filled by one or more unpaired electrons [11]. Electrons in such a vacancy tend to absorb light in the visible spectrum such that a material that is usually transparent, becomes colored [11]. This is used to identify many compounds, especially zinc oxide, which turns yellow when this type of defect takes place [11]. F-centers can occur naturally in compounds such as metallic oxides because when heated to high temperature, the ions become excited and are displaced

vacated spaces [11]. F-centers are often paramagnetic and can be studied by electron paramagnetic resonance techniques. The greater the number of f-centers, the greater the intensity the color of the compound is. A way of producing f-centers is to heat a crystal in the presence of an atmosphere of the metal that constitutes the material, such as heating NaCl in a metallic Na atmosphere shown in Fig. 2-7 below:

Figure 2-7: F-center in an NaCl Crystal [11]

In Fig. 2-7 NaCl is heated in a metallic Na atmosphere. Then Cl- _{vacancies are}

generated because of the excess N a+_{. These vacancies capture available electrons to}

maintain charge neutrality and form f-centers. Ionizing radiation can also produce f-centers which is why this phenomenon can be utilized in the form of a dosimeter [11]. The IF2_{D is by default passively integrating, as the device changes color with}

irradiation regardless of its accompanying data acquisition system. The total dose at the implantation site can therefore be read online at any time, or the information can be transformed to a proportional dose counter by taking the derivative of integral dose information [1]. The IF2_{D is therefore inherently fault-tolerant, as the failure}

of any component in the data acquisition chain does not change the response of the IF2_{D device.}

Chapter 3

Design of Experiment

The purpose of the experiment is to study the relationship between absorption and dose of various salts when they are irradiated. A factor that heavily influenced the design of the experiment is the short excitement time of the salts. As a result, the experiment had to be designed in a way such that the salts can be irradiated and immediately analyzed within the chamber. The following sections will illustrate the experiment which can be broken up into three sections: The housing assembly, the fiber optics, and the salt wheel. The first section gives an overview of the whole experiment and lists the parts that house the fiber optics and salt wheel. Then, the fiber optics will be presented in a piece by piece assembly. Afterwards, the salt wheel will be presented.

3.1

Experiment Overview and Housing Assembly

The fiber optics and salt wheel will be housed in a 10” elongated nipple. A 10” to 8” straight reducer nipple will be used to connect the experiment to the beamline. On the other side of the experiment, the rotary motor feedthrough will be placed as well as a 2.75 inch 4 way CF cross which will be used for instrumentation feedthroughs. The whole assembly will be supported by a beam that will be welded to the collection side of the chamber. The next set of figures present the complete design of the experiment from different vantage points.

Figure 3-1: Launch Side Viewpoint of Design. The orange arrow represents the beam. From this perspective, the various components of the fiber optics from the

Figure 3-2: Collection Side Viewpoint of Design. The collection side of the fiber optics can be seen [12].

Figure 3-3: Beamline Viewpoint of Design. Figure gives a visual of how the salt wheel is aligned such that one hole is aligned with the beam line while the following

Figure 3-4: Motor Feedthrough and Cross Flange. Presents a close up view of the components connected to the flange on the collection side of the experiment. The components consist of the motor feedthrough for the motion of the salt wheel and

3.2

Fiber Optics

The fiber optics obtain the color shifts in the excited salts caused by f-centers. The assembly begins with a fiber launch system which has the ability to couple a free space laser into single mode, multi-mode, or polarization-maintaining fiber [13]. Since the optics assembly takes on the form of a cage assembly, the spatial filter system (KT310) from the cage system kits was used. The spatial filter is ideal for producing a clean Gaussian beam as the input side consists of a z-axis translator that will hold a diffraction-limited lens to focus the laser through a pinhole, such as the hole containing excited salts [13]. Fig. 3-5 illustrates the spatial filter.

Figure 3-5: KT310 Spatial Filter [13]

The 1 inch post and mounting plate on the bottom will be removed since it is unnec-essary and space is limited within the chamber. The iris assembly on the front side of the spatial filter will also be removed since the launcher will connect in-line to the cage assembly.

Following the fiber launch system is the cage assembly which houses the lenses and objectives used to analyze the salts. From the launch side, there is a cage plate adapter (LCP02) which provides a convenient means for coupling 30mm and 60mm cage plates via cage rods (ER) [13]. This particular cage plate adapter couples the spatial filter to the 60mm threaded cage plate (LCP01T). The threaded cage plate

(ITL200). The infinity corrected tube lens has an effective focal length of 200mm and is optimized for widefield imaging of visible wavelengths; it has a working distance of 148mm over the design wavelength range of 400nm-750nm [13]. Fig. 3-6 gives an illustration of the cage plate adapter and Fig. 3-7 illustrates the infinity corrected tube lens assembly.

Figure 3-6: LCP02 30mm to 60mm Cage Plate Adapter [13]

Figure 3-7: Infinity Corrected Lens Assembly. Consists of a 60mm threaded cage plate which houses an infinity corrected lens adapter and infinity corrected tube lens.

The infinity corrected lens assembly is followed by another cage plate adapter which is used to connect the SM1 threaded cage plate (CP02T), which houses the achromat microscope objective (RMS4x), to the cage assembly. The achromat microscope ob-jective is for visible wavelengths and provides 4x magnification, making it suitable for focusing or collimating laser light; it has a working distance of 18.5mm [13]. Fig. 3-8 shows the achromat microscope objective encased in the SM1 threaded cage plate.

Figure 3-8: Achromat Microscope Objective Assembly. Consists of a threaded cage plate which houses an achromat microscope objective.

Located 18.5mm from the achromat microscope objective is the salt wheel, which will be elaborated on in the next section. The cage assembly is mostly symmetric about the salt wheel so 18.5mm beyond the salt wheel, another achromat microscope objective assembly is located. From there, another cage plate adapter is located but instead of an infinity corrected lens assembly there is a collection assembly. The collection assembly consists of a cage plate adapter that houses a SM1 threaded adapter (AD12F) and triplet fiber optic collimator (TC18FC-405). Fig. 3-9 illustrates the collection assembly.

Figure 3-9: Collection Assembly. Consists of a cage plate adapter which houses a threaded adapter and triplet fiber optic collimator.

will have the 1 inch post and mounting plate, iris assembly, optics, and pinhole removed. Fig. 3-10 presents the fiber optic cage assembly.

Figure 3-10: Fiber Optic Cage Assembly. Consists of all elements of the fiber optics excluding the launchers on both sides.

3.3

Salt Wheel

The salt wheel contains the salts being tested and is designed with eight holes. One hole is dedicated to housing the Faraday cup which will be used to determine the beam current. With 7 holes remaining, the salt wheel is designed such that a hole in the salt wheel will be aligned with the beam and then rotated 45◦ so that it is aligned with the fiber optic assembly. Due to space limitations, the salt wheel can not be rotated 360◦ due to the fact that the Faraday cup will not clear the fiber optic assembly. The holes of the salt wheels will contain thermocouples along with the salts in order to measure temperature. The holes themselves will be counter-bore holes with a 13.1mm diameter on the side of the larger hole. This dimension was based off of the size of the salt pellets themselves, which were designed to be 13mm diameter pellets. On the side of the 13.1mm diameter holes, there will be a cover with 13mm diameter holes that will be attached to the wheel through screws. With this design, the 13mm diameter pellets can be placed into the salt wheel and then secured in place with the cover, thus eliminating any risk of the salt pellets falling out

of the salt wheel during the experiment. As for motion of the salt wheel, it will be rotated remotely using a rotary motor feedthrough and motor. By remotely rotating the wheel, there will be no need to go through the timely process of manually rotating the wheel and thus decrease the probability that the salts will de-excite before they can be analyzed by the fiber optics. Fig. 3-11 shows the salt wheel with its cover. Fig. 3-12 and Fig. 3-13 are line drawings of the salt wheel and salt wheel cover respectively.

In Fig.3-11 the salt wheel is the thicker, gray part and the cover is the thinner, golden part. As can be seen, the holes in the salt wheel are counterbore holes of diameters 13mm and 13.1mm. The two smaller holes closer to the centers are for the screws that will hold the cover and salt wheel together after the salts have been placed in the salt wheel. The central hole is for the rod connected to the rotary motor system.

Chapter 4

Decay of F-centers

4.1

Estimated Decay Rate

An important factor that will affect various facets of the future experiment is the decay rate of f-centers in the salts. If the decay rate is known, factors like salt thickness, beam current, and wheel rotation speed can be decided. The following equation illustrates the estimated decay rate in the form of a first order differential equation.

dN

dt = −λN + φσn (4.1)

The decay rate of f-center is dependent on the decay and production of f-centers. The decay term is modeled in −λN where λ is the decay rate of f-centers and N is the total number of atoms of the salt. The production term is modeled in +φσn where φ is the flux of the accelerator, σ is the cross section for f-center creation in a salt, and n is the number density of the salt being used. A search of literature for decay rates of f-centers proved unsuccessful and is something that will be continued in future work. Number of atoms of a salt and number density can be easily calculated using known values. The DANTE accelerator has a beam current of 10 microamps and the f-center cross sections can be assumed to be the same as the elastic scattering cross sections of the salt. If numerical values for the decay rates of f-centers can be located, graphs

Chapter 5

Conclusion

5.1

Implications

The IF2_{D utilizes light spectroscopy to measure color shift in irradiated salts, which}

is then correlated to dose received. By encasing salts in a biocompatible encasing and coupling it with appropriate optics, it is possible to create a dosimeter that can be used to measure dose at specific locations, such as a tumor in a patient’s body, in an integral and fault-tolerant manner in real time. With the aspect of real time, medical staff can monitor and adjust dose throughout treatment sessions and confirm that the tumor and surrounding healthy tissue are each receiving their prescribed doses. The device can be made for single use, where it dissolves inside of the patient post treatment, or adapted to last multiple sessions [1]. Since the salt resets after each exci-tation, the same device can be used for multiple sessions thus minimizing invasiveness and discomfort to the patient.

5.2

Future Work

Future work will include the construction, conduction, and analysis of the experiment. Construction of the experiment is expected to take the most time due to the amount of time it takes to get parts shipped and the large quantity of those parts that need to be machined to fit the needs of the experiment. In the meantime, another attempt

Bibliography

[1] Michael Short, Rajiv Gupta, Sara Ferry, and Cody Dennett. Fault Tolerant, Pas-sively Integrating In-vivo Dosimeters for Feedback Enabled Radiation Therapy, August 2015.

[2] DeWerd L.A., Bartol L., and Davis S. Thermoluminescence Dosimetry. Presen-tation, AAPM Summer School, June 2009. Accessed 15, 2017.

[3] D.C. Weber et al. Assessment of Target Dose Delivery in Anal Cancer using In-vivo Thermoluminsecent Dosimetry. European Society of Radiotherapy and Oncology, 59:39–43, April 2001. Accessed 15, May 2017.

[4] Faiz M. Khan. The Physics of Radiation Therapy. LWW, fourth edition edition, 2009.

[5] NukeWorker. TLD, OSLD, Film Badge. https://www.nukeworker.com, August 2007. Accessed 15, May 2017.

[6] B. Odom and D. Hanneke. New Measurement of the Electron Magnetic Mo-ment Using a One-Electron Quantum Cyclotron. Physical Review Letters, 2006. Accessed 15, May 2017.

[7] E. Zavoisky. Spin-magnetic Resonance in Paramagnetics, 1945. Accessed 15, May 2017.

[8] Yuhua Cheng and Chenming Hu. MOSFET Classification and Operation, 1999. Accessed 15, May 2017.

[9] Stack Exchange. P-channel MOSFET series configuration, 2016. Accessed 15, 2017.

[10] Edward Rhodes. Optically stimulated luminescence dating of sedminets over the past 20,000 years. Annual Review of Earth and Planetary Sciences, 39:461–488, May 2011. Accessed 15, May 2017.

[11] Berzina B. Formation of self trapped excitons through stimulated recombination of radiation induced primary defects in alkali halides. Journal of Luminescence, 76-77:389–391, February 1998. Accessed 15, May 2017.

[12] Peter Stahle. Designs of experiment, April 2017.