Design and Evaluation of a 3D Printed Ionization Chamber

(1)

IN

DEGREE PROJECT TECHNOLOGY, FIRST CYCLE, 15 CREDITS

STOCKHOLM SWEDEN 2019,

Design and Evaluation of a 3D Printed Ionization

Chamber

CAROLINE BOSTRÖM OLIVIA MESSLER

KTH ROYAL INSTITUTE OF TECHNOLOGY

SCHOOL OF ENGINEERING SCIENCES IN CHEMISTRY, BIOTECHNOLOGY AND HEALTH

(2)

(3)

i

This project was performed in collaboration with the division of nuclear physics at the department of physics, KTH Royal Institute of Technology

Supervisor at the division: Torbjörn Bäck

Design and Evaluation of a 3D Printed Ionization Chamber Design och utvärdering av en 3D-utskriven

jonisationskammare

C A R O L I N E B O S T R Ö M O L I V I A M E S S L E R

Degree project in medical engineering First level, 15 hp

Supervisor at KTH: Mattias Mårtensson, Tobias Nyberg Examiner: Mats Nilsson

School of Engineering Sciences in Chemistry, Biotechnology and Health KTH Royal Institute of Technology

SE-141 86 Flemingsberg, Sweden http://www.kth.se/cbh

2019

(4)

ii

(5)

Abstract

Ionizing radiation is often used within medicine for diagnosis and treatments. Because ionizing radiation can be harmful to the body, it is important to know how it affects the tissue. Dosimetry is the study of how ionizing radiation deposits energy in a material. To measure how much ionizing radiation is deposited in the body, gas-filled detectors are often used. An ionization chamber is a type of gas-filled detector and exists in different shapes and sizes, depending on what kind of measurements it is made for. Because ionization chambers are relatively expensive, it is often not possible to buy one for each type of measurement that is to be done. This results in ionization chambers being used for measurements they are not optimized for. This report evaluates the possibility of 3D printing ionization chambers to make it easier to optimize them for specific measurements. The process included creating models of ionization chambers using CAD-software, slicing them and then 3D printing them. The 3D printed models were then brought to the Swedish Radiation Safety Authority for measurements. The ionization chambers were connected to high voltage, and exposed to ionizing radiation in the form of high-intensity gamma-ray fields. The output current of the ionization chamber was measured, which is proportional to the field intensity.

The results were similar to those of a commercial ionization chamber. The conclusion is that it is possible to 3D print ionization chambers. However, to get more accurate results, the design has to be further optimized and more measurements need to be done.

Keywords: 3D printing, Ionization chamber, Gas-filled detectors

iii

(6)

Sammanfattning

Inom medicinska undersökningar och behandlingar används joniserande strålning. Joniserande strålning kan vara skadligt för kroppen och det är därför viktigt att veta hur den joniserande strålningen påverkar vävnad. Dosimetri är läran om hur joniserande strålning deponerar energi i material. För att kunna mäta hur mycket strålning som deponeras brukar gasfyllda detektorer användas. En jonisationskammare är en typ av gasfylld detektor och de kan ha olika form och storlek för olika typer av mätningar. Eftersom jonisationskammare är relativt dyra är det ofta inte möjligt att köpa en för varje typ av mätning som ska genomföras. Detta resulterar i att jonisationskammare ofta används till mätningar de inte är optimerade för. I denna rapport under- söks möjligheten att skriva ut tredimensionella jonisationskammare för att enklare kunna optimera dem för specifika mätningar. Olika modeller utformades i CAD-mjukvara och förbereddes för att skrivas ut. De utskrivna tredimensionella jonisationskamrarna togs till Strålsäkerhetsmyndigheten för att utföra mätningar med. Jonisationskamrarna kopplades till högspänning och exponerades för joniserande strålning i form av högintensiva gammastrålfält. Deras utsignaler mättes vilket är proportionerligt mot strålfältets intensitet. Resultatet blev likt det för en kommersiell jonisationskammare. Slutsatsen är att det går att skriva ut en tredimensionell jonisationskammare. För att ge mer korrekta och noggranna resultat bör designen utvecklas och fler mätningar bör utföras.

Nyckelord: 3D printing, Jonisationskammare, Gasfylld detektor

iv

(7)

Acknowledgement

We would like to thank our supervisor Torbjörn Bäck for expert advise and encouragement through- out the project, and Mie Wising and Jan Lillhök at the Swedish Radiation Safety Authority for using their expertise in the lab. We would also like to thank Pär Olsson, Chunxin Liu, and the staff at 3D-verkstan for valuable help with 3D printing.

v

(8)

vi

(9)

Abbreviations

ABS - Acrylonitrile Butadiene Styrene Ba - Barium

CAD - Computer-Aided Design CG- Collector-Guard

Cs - Cesium

G-code - General Code HV - High Voltage PLA - Polylactic Acid

RBE - Relative Biological Effectiveness

SSM - Strålsäkerhetsmyndigheten (Swedish Radiation Safety Authority) STL - Surface Tesselation Language

ix

(12)

x

(13)

1 Introduction

Ionizing radiation is an important tool in medicine for diagnosis and treatments of different ill- nesses [1]. It will most likely continue to be an important part of medical technology as research proceeds on finding future methods using ionizing radiation. However, ionizing radiation induces certain health risks and it is important to know its biological effect. Therefore, it is of large interest to measure the amount of ionizing radiation that is delivered to the tissue that is being radiated [2].

The most common way to measure ionizing radiation is by using gas-filled detectors [3].

There are many different types of gas-filled detectors and many fields of application [3]. According to Torbjörn Bäck, Associate professor in Experimental Nuclear Physics at KTH, the performance of the detectors vary depending on their usage, and they are often not optimized for their application.

Therefore there is a demand for a more flexible and cheaper way to produce and customize the structure of gas-filled detectors.

There is currently an ongoing research project within microdosimetry and nanodosimetry at KTH in collaboration with the Swedish Radiation Safety Authority (SSM). The response from gas-filled detectors of different size and types of gas are being studied [2]. In the KTH/SSM project, there is an interest to study novel methods of building ionization chambers. One possible method to investigate is 3D printing the detectors.

1.1 Aim

The main objective of this project was to 3D print an ionization chamber that could detect gamma radiation and measure its response in a high-intensity field of gamma radiation. The focus was on investigating the method of 3D printing rather than optimizing design for detector performance.

1.2 Limitations

• The study was limited to producing spherical ionization chambers

• The ionization chamber only operated in current mode

• The gas used was air at atmospheric pressure

1

(14)

2 Background

In this report radiation is playing an important role. Radiation can be divided into two different types: ionizing radiation and non-ionizing radiation. The difference between the two types is how the radiation interacts with matter [4]. Ionizing radiation is the type of radiation used in this report. If biological tissue is being exposed to ionizing radiation the tissue can be harmed and it is therefore important to measure the dose to the tissue [5].

2.1 Radioactivity and Ionizing Radiation

Ionizing radiation can be divided into two groups: directly ionizing radiation and indirectly ionizing radiation. Directly ionizing radiation consists of fast-moving charged particles, e.g. protons or alpha particles [6]. Their energy is delivered directly through Coulomb interactions to the medium along their path. Indirectly ionizing radiation, on the other hand, consists of uncharged particles, e.g. high energy photons or neutrons [1]. When the uncharged particles pass through matter they can transfer their energy to charged particles. These charged particles become fast-moving due to their energy gain and thereafter behave as directly ionizing radiation [6].

Gamma Radiation

A common source of gamma radiation is radioactive materials prone to beta decay. Beta sources can directly decay to their stable ground state through beta emission, but in many cases, they first decay to an excited state of the daughter nucleus. The nuclei in the excited state then emits gamma-rays to transition to their stable ground state or another excited state. The energy of the gamma photons usually ranges between a few hundred keV and a few MeV [7]. Gamma radiation is a form of indirectly ionizing radiation [6].

Decay of Cesium-137

The radioisotope Cesium-137 (¹³⁷Cs) decays through beta decay. As shown in Figure 1, about 5.4% of ¹³⁷Cs directly reaches the stable ground state of ^137mBa, but the majority, 94.6 %, of

137Cs decays to the metastable state of Barium-137m (^137mBa) [6]. The m indicates that it is an isomer [1]. From that state, 89.9 % of^137mBa then emits a gamma-ray to reach the stable state of

137Ba. This means that around 85 gamma-rays are emitted for 100 decays of ¹³⁷Cs. The energy of the gamma-rays is 662 keV [6].

Figure 1: Decay scheme of Cesium-137. From ref. [6]. Reprinted with permission.

2

(15)

2.2 Photon Interaction

Photons, in the energy ranges of interest here, mainly interact with matter through Compton scattering, photoelectric effect and pair production [8]. The most common process will depend on the energy of the photons [5].

Photoelectric effect is the dominant process at energies lower than 30 keV. The incoming photon is absorbed entirely and releases an electron from its orbit around the nucleus. The electron released can originate from any orbit. The atom enters a highly excited state because it misses an electron in the shell, which will be filled by another electron of higher energy. This causes a characteristic X-ray to be emitted by the atom. Some of the energy of the incoming photon is used to overcome the binding energy of the electron, the rest of the energy becomes kinetic energy of the electron [5].

In Compton scattering, the incoming photon collides with an electron in the material. The energy and momentum is conserved in the process. Some of the energy and momentum is transferred from the incoming photon to the electron, which results in both the electron and photon being scattered [5]. The change in wavelength of the photon depends on the scattering angle [9]. Compton scattering is the dominant process at energies between 30 keV and a few MeV [5].

Pair production is the dominant mode for photons to interact with matter at very high energies over a few MeV. If a photon passes a nucleus closely enough it might convert to an electron and a positron. For this to happen the energy needs to be higher than the total mass of the electron and positron which is 1.022 MeV [5].

2.3 Medical Applications of Ionizing Radiation

Ionizing radiation is used within several areas in medicine such as imaging, radiotherapy and sterilization of medical devices [1]. Examples of techniques using ionizing radiation in diagnostic radiology are X-ray imaging, computed tomography, and gamma camera [5]. Radiation therapy is often used to treat cancer, during which the cancerous cells are exposed to ionizing radiation.

Examples of forms of radiation therapy are proton beam therapy and gamma knife. Ionizing radiation can also be used to sterilize single-use medical devices [10].

Health Risks

Exposure of ionization radiation can affect biological tissue. Ionizing radiation can make a cell carcinogenic as the radiation can harm the healthy cell and make it malignant [5]. How much the radiation affects the tissue depends on several variables. To understand how different types of radiation affect the biological tissue the term ”relative biological effectiveness” (RBE) is used [2].

Absorbed dose is also a measurement that explains the biological risk of radiation. It is defined as the amount of energy deposited in the matter from the ionization [5].

Dosimetry

Dosimetry is the determination and calculation of the absorbed dose in the tissue of the radiation.

The type, energy and intensity of radiation and for how long the tissue is exposed are important variables in dosimetry. In dosimetry, the radiological effects of absorbed dose on biological tissue can only be predicted and not known for certain [2].

3

(16)

2.4 Gas-filled Detectors

One way to detect radiation is by using gas-filled detectors. A voltage is applied across two electrodes of the detector, producing an electric field [1]. The incoming ionizing radiation causes ionization of the gas molecules in the detector which results in the formation of ion pairs. Because of the existing electric field, the positive ions move to the electrode of lower potential, and the electrons move to the electrode of higher potential, also called the ”collector”. This produces a current that can be measured by an electrometer. The current is proportional to the radiation and voltage applied to the electrodes. It will also depend on other factors such as pressure and type of gas [11].

Voltage Regions

The amount of voltage applied to the electrodes can be divided into six different regions: the recombination region, the ionization chamber region, the proportional region, the limited proportional region, the Geiger-Müller region, and the continuous discharge region, shown in Figure 2.

The ions and electrons will behave in different ways in the different regions, affecting the measured current [7, 11].

Figure 2: Curve representing the ionization current for different voltage regions of gas-filled detectors. ”A” represents the recombination region, ”B” represents the ionization chamber region, ”C” represents the proportional region, ”D” represents the limited proportional region, ”E”

represent the Geiger-Müller region and ”F” represents the continuous discharge region. From ref. [11]. Reprinted with permission.

In the recombination region, represented by ”A” in Figure 2, the voltage applied is very low [11]. If the voltage is increased in this region the pulse is also increased. The low voltage makes it possible for some of the ion pairs to recombine. Therefore, not all of the electrons produced will be detected in this region [7].

In the ionization chamber region, represented by ”B” in Figure 2 [11], all the ions that have been produced will be collected at the electrodes which gives a saturation current. The saturation current is proportional to the number of ions produced [7]. The current remains almost the same with changing voltage within this region [11].

In the proportional region, represented by ”C” in Figure 2, more ions are collected than initially 4

(17)

produced [11]. The ions migrating along the electric field towards the electrodes will be able to further ionize the gas. This causes a multiplication of the number of ions and is called gas amplification [7].

The limited proportional region is represented by ”D” in Figure 2 [11]. This region is similar to the proportional region, but here the gas amplification does not increase linearly with increase of voltage. This causes difficulties in measuring radiation and therefore this region is not used [7].

In the Geiger-Müller region, represented by ”E” in Figure 2 [11], the size of the pulse increases as voltage increases [7]. When the voltage is increased beyond the limited proportional region, the Geiger-Müller region is reached. The electrons are accelerated towards the collector and hit it with such force that ultraviolet (UV) light is emitted. The UV light then further ionizes the gas or hits the chamber wall, causing emission of more electrons. These electrons are also accelerated towards the collector, and causes more ultraviolet light to be emitted when they hit the collector.

Through this process, electron avalanches are generated in the Geiger-Müller region. The positive ions, however, move much slower and will build up around the collector, which causes the voltage to drop and the avalanche to be terminated. Recovery occurs when the ions move toward the electrode of lower potential [11].

In the continuous discharge region, represented by ”F” in Figure 2, spontaneous discharge occurs.

This happens because the voltage is increased past the Geiger Müller region and every ionization leads to repetitive discharge of electricity in the detector’s chamber. A detector operating in this region will get damaged and not function correctly [11].

Ionization Chambers

Ionization chambers are gas-filled detectors operating in the ionization chamber region and are widely used for measurements of ionizing radiation. The device has a long history and new designs are developed for special aims. There are three ways for an ionization chamber to operate: current mode, pulse mode and electrostatic or charge integration mode. Current mode is the mode used in this project. In current mode the electrode collects electrons and ions which causes an electrical current flow. The flow is registered during the time of observation [3]. Ionization chambers usually operate in the voltage region of 50-300 V [11].

Figure 3: Illustration of a cavity ionization chamber. Both spherical and cylindrical forms are shown. From ref. [6]. Reprinted with permission.

There are different types of ionization chambers for different purposes. One type of ionization chamber is cavity ionization chambers. An example of the design is shown in Figure 3. This type of chamber can have different shapes but usually has a solid shell and a gas-filled volume inside [6].

In the gas-filled volume an electric field exists to collect the ion pairs [1]. The most common shape of a cavity ionization chamber is thimble-type chamber. Thimble-type chambers include spherical

5

(18)

or cylindrical shapes. The advantage of a spherical ionization chamber is that it is isotropic, the direction of the radiation does not matter because of the round shape [6].

Spherical Ionization Chamber

A spherical ionization chamber consists of a spherical chamber wall or shell, collector or center electrode, guard electrode, high voltage (HV) insulator and a collector-guard (CG) insulator which can be seen in Figure 3. There are also often vent holes in the spherical chamber wall in order to make it possible to change the pressure and type of gas inside the ionization chamber. The diameter of the collector and its length inside the shell can be chosen by the relationship shown in Figure 4 [12]. The size of ionization chambers commonly varies between a couple of millimeters and a few decimeters in diameter. A negative high voltage is usually applied to the shell while the guard electrode and collector are grounded. This results in the collector having a higher potential than the shell. A signal relative to ground is detected by the collector. When radiation interacts with the detector, the chamber gas is ionized. The positive ions are collected by the shell and the negative electrons are collected by the collector due to their electric potentials. The collector is connected to an electrometer to measure the collected electric current. In the stem, insulating material is used between the collector and the chamber wall to separate these from each other. This material is usually divided by a guard electrode made of conductive material to collect leakage current across the insulating material. The insulating material between the collector and the guard electrode is called the collector-guard insulator, and the insulating material between the guard electrode and the chamber wall is called the HV-insulator [6].

Figure 4: Design of a spherical ionization chamber. ”R” represents the inner radius of the shell,

”r” represents the radius of the collector, ”w” represents the shell thickness and ”D” represents the diameter of the stem of the ionization chamber. From ref. [12]. Reprinted with permission.

2.5 3D printing

3D printing is a way to produce 3D objects by building them up layer by layer. The process can be divided into three general steps: creating a 3D computer model, slicing the model, and printing the object [13].

3D Computer Model

3D computer models can be retrieved by downloading existing models or created by either scan- ning an existing object or designing one using computer-aided design (CAD) software. There are many different types of CAD-software and some are relatively simple to use while others are more

6

(19)

advanced [13]. The most common type of 3D computer model file format is Surface Tesselation Language (STL) [14]. STL files contain information about the surface of an object in the form of a list of triangles that together make up the whole surface [13].

Slicing

The STL file can not be directly translated to commands by the 3D printer. The model first needs to be ”sliced”, which means that a slicing program divides the model into layers that the printer later will produce. Before the STL file is sliced, several settings need to be set such as temperature, speed and how much infill is to be used when printing [13]. At the end of slicing, General code (G-code) is produced which is a file format that consists of commands for the 3D printer [14].

Printing

The G-code can be transferred to the printer in different ways such as via USB or SD-card, depending on the printer. After the G-code has been transferred to the printer, the print can be started [13]. The printer drives filament into an extruder with a hot end. The hot end melts the filament, which is then extruded through a nozzle onto a build plate, according to the commands in the G-code [15]. Printing usually takes a number of hours [13]. Some 3D printers have two extruders, making it possible to print with two different filaments in the same print. This is called dual extrusion printing [15] and was used in this project.

Settings

There is a setting called prime tower which is used when the 3D print has more than one material.

The setting makes the printer extrude a circle of the filament outside the object before it prints another layer of the object. When the nozzle has created the circle it makes a fast movement before printing a layer of the object to make sure the nozzle is clean from strings. The layers of circles together form a prime tower [16]. Another setting widely used when using more than one filament is Z hop retraction. When it is time for the printer to switch filament, the build plate moves down so the nozzle does not touch the object. This prevents the object from getting extra filament on its surface [17].

Filaments

There are different filaments that can be used by a 3D printer [16]. The most common ones are plastic filaments such as acrylonitrile butadiene styrene (ABS) and polylactic acid (PLA). There also exists conductive versions of ABS and PLA. The plastic filaments are made conductive by adding carbon or other conductive materials [18].

7

(20)

3 Method and Materials

The method and list of materials describe the most important parts of the project. Preparatory work such as learning how to use the CAD-software and 3D printers is not described as it is not of importance for the results.

3.1 List of Materials

Materials were used both for producing the ionization chambers and for doing measurements. The materials are listed below.

• 3D printers: CraftBot 3, Craftunique and Dreamer, Flashforge

• Conductive filament: Conductive PLA filament 1.75mm, ECO

• PLA filament: PLA white 1.75mm, Colorfabb

• Computer with CAD software: Fusion 360 by Autodesk and slicer software: Craftware by Craftunique and Simplify3D by Simplify3D

• Cs-137 source with dose rate 4.332 µGr/s where ionization chamber is placed, Comecer

• Electrometer: ADA4530-1R-EBZ-TIA-ND, Analog Devices Inc.

• Multimeter: 34470A, Keysight

• Power supply: 2701C, Valhalla Scientific

• Coaxial cables

• Resistors

• Retort stand

• Rubber covered 3-jaw clamp

• Small drill

• Scalpel

• Soldering iron

• Solder

• Jig saw

3.2 Method

The method consisted of several parts: designing the models, 3D printing them and lastly doing measurements with them at SSM. This process was followed for two complete models.

Printing Method and Choice of Filaments

The method chosen was to print the non-conductive and conductive parts in one run. Here, the choice of filaments is crucial. The filament chosen for the conductive parts was ECO conductive PLA filament and normal PLA filament was chosen for the isolating parts. Alternative methods, for example printing the collector separately, could be easier but would be a limited factor in the future if more unique geometries were to be investigated. Before printing the dual material models, a number of basic prototypes to investigate the collector and the detector geometries were done.

8

(21)

Producing the First Model

3D-models of the different parts of an ionization chamber (shell, collector, HV-insulator, guard electrode, and CG-insulator) were made using CAD-software (Fusion 360) and saved as STL files.

Rectangles were joined with guard electrode, collector and shell in the stem to form places for contact, which can be seen in Figure 5. The dimensions of the different parts of the model are shown in Table 1. The parts were then imported to the slicer software and put together. The filament material for the two extruders were set and the parts were matched to the correct filament/extruder.

Support was added to the model to keep it stable when printing. The temperature settings were changed to match the recommended printing temperatures for the filaments. The layer height was set to 0.15 mm and the printing of a prime tower was enabled. All settings are shown in Table 2.

The model with its settings was then sliced and converted to G-code, and saved to a USB-stick.

The filaments were then loaded to the 3D printer. After that, the G-code was imported by the 3D printer Craftbot 3 and the model was printed.

(a) (b)

Figure 5: CAD of Model 1. Subfigure (a) shows all the CAD parts put together. The collector is marked ”A”, CG-insulator ”B”, guard electrode ”C”, HV-insulator ”D” and shell ”E”. Sub- figure (b) shows the CAD parts of the collector marked ”A”, CG-insulator marked ”B”, guard electrode marked ”C” and HV-insulator marked ”D”..

9

(22)

Table 1: Dimensions of parts in Model 1. Their diameter and length in the stem and their length inside the spherical part of the shell are listed. The different parts is marked with the same letters that is used in Figure 5..

Parts Diameter in stem

[mm]

Length inside sphere [mm]

Length outside sphere [mm]

Collector (A) 5 32.5 125

CG-insulator (B) 9 8 120

Guard electrode (C) 15 8 115

HV-insulator (D) 19 4 110

Shell (E) 25 0 105

Table 2: 3D print settings used for Model 1..

Setting Value

Temperature 230^◦C

Speed 60 mm/s

Layer height 0.15 mm

Z hop Off

Measurements with the First Model

The model was brought to SSM for measurements. A vent hole was made in the outer sphere by using a small drill. Then the contact between different parts was checked by using a multimeter to measure resistance. The stem was cut off using a jig saw because of suspected print errors in that part which will be discussed later in the report. The resistance between different parts was measured again. One of the sides of two coaxial cables were cut off and the isolation was trimmed down using a scalpel to expose the conductive parts. The outer conductive layers of the two coaxial cables were connected to each other and soldered to the guard layer of the ionization chamber.

The inner wire of one of the coaxial cables was soldered to the collector, and the inner wire of the other coaxial cable was soldered to the shell. The connection is shown in Figure 6. A retort stand with a rubber covered 3-jaw clamp was used to hold the ionization chamber shown in Figure 8.

Figure 6: Soldered connections in Model 1. The coaxial cables are soldered to the collector, guard and shell..

10

(23)

The coaxial cable connected to the shell was connected to two resistors with resistances of 100 kΩ each which were connected to a power supply. The collector was connected to an electrometer which was connected to a multimeter. This setup is shown in Figure 7. The electrometer that was used displayed voltage.

Figure 7: The circuit used for Model 1 in the measurements at SSM..

The resistance of the electrometer was 500 GΩ. Using Ohm’s law, the relationship between the detector current and the multimeter voltage can be written as:

I[A] = U [V]

500[GΩ]. (1)

A voltage of -300 V was applied to the shell while the collector and guard electrode were connected to ground. The displayed voltage of the electrometer was noted. The guard electrode was disconnected from ground and the displayed voltage was noted again. The shell was cut apart and strings between the shell and collector were removed. The shell was then melted back together by using a soldering iron. The ionization chamber was put back and connected again. A voltage of -150 V was applied to the shell and the displayed voltage was noted. The voltage was then changed to -50 V and the leakage current was calculated. The radiation source was brought out and the ionization current was calculated.

11

(24)

Figure 8: Measurements with Model 1. The picture shows the setup of the model in front of the radiation source. .

Producing the Second Model

A second ionization chamber was designed using the same process as described for the first model, but with different dimensions which are shown in Table 3. The slicer used this time was Simplify3D.

The settings were similar as for the first model, shown in Table 4, but this time the Z hop setting for when the extruder is retracting was enabled. The CAD of Model 2 are shown in Figure 9. The model was printed by the 3D printer Dreamer FlashForge.

Table 3: Dimensions of parts in Model 2. Their diameter and length in the stem and their length inside the spherical part of the shell are listed. The different parts is marked with the same letters that is used in Figure 9..

Parts Diameter in stem

[mm]

Length inside sphere [mm]

Length outside sphere [mm]

Collector (A) 2.5 28.75 80

CG-insulator (B) 6.5 8 75

Guard electrode (C) 8.5 4 70

HV-insulator (D) 12.5 4 65

Shell (E) 14.5 0 60

12

(25)

(a) (b)

Figure 9: CAD of Model 2. Subfigure (a) shows all the CAD parts put together. The collector is marked ”A”, CG-insulator ”B”, guard electrode ”C”, HV-insulator ”D” and shell ”E”. Sub- figure (b) shows the CAD parts of the collector marked ”A”, CG-insulator marked ”B”, guard electrode marked ”C” and HV-insulator marked ”D”..

Table 4: 3D print settings used for Model 2.

Setting Value

Temperature 230^◦C

Speed 60 mm/s

Layer height 0.15 mm

Z hop On

Measurements with the Second Model

The printed model was then brought to SSM for measurements. A loose piece of plastic was noticed inside the shell so the shell was cut open using a scalpel. The piece of plastic was removed and the shell was put back together using the soldering iron to melt the PLA. The contact between the different parts was checked, and the stem was cut off similar to the previous ionization chamber.

The contact was checked again, and then the wires for the cables were soldered to the ionization chamber, shown in Figure 10. As shown in Figure 11, the ionization chamber was hanging from the retort stand this time. This was only because it was easier to set up this way and should not have impacted the measurements. The setup of the equipment was almost the same as for the first model. The only difference was that the guard electrode was connected to ground for Model 2 which is shown in Figure 12. The radiation source was brought out. A voltage of -50 V was applied and the ionization current was calculated. The measurement was repeated for decreasing voltages up to -300 V.

13

(26)

Figure 10: Soldered connections in Model 2. The coaxial cables are soldered to the collector, guard and shell..

Figure 11: Measurements with Model 2.

The picture shows the setup of the model in front of the radiation source..

Figure 12: The circuit used for Model 2 in the measurements at SSM.

14

(27)

4 Results

To estimate the expected current for the measurements before irradiating the detectors, standard values given by the SSM staff from ionization chambers of similar shape were taken, scaled with detector volume. This estimation gave us an expected ionization current of 9.2 pA in the radiation field of interest.

4.1 Results for Model 1

Based on the exterior, the first printed model seemed to have been printed successfully. The printed model is shown in Figure 13.

Figure 13: The 3D print of Model 1.

Before the stem was cut off, resistance in the order of kΩ to MΩ, was found between the collector, shell and guard electrode contacts. This was a clear indication of print errors, creating electrical contact. These print errors were confirmed after cutting off parts of the stem. When the shell was opened, connecting strings between collector and shell were discovered and removed. After the whole stem was cut off, contact between the shell and the collector was well separated but the guard electrode could not be used and was disconnected from ground. The detector could only operate at voltages up to 50 V. Above this value the detector current was too high for the electrometer to measure. This was an indication of current leakage between the shell and the collector.

Table 5: Measurement of ionization current for Model 1. The measured leakage current and ionization current are listed for the applied voltage difference between collector and shell. .

Voltage difference[V] Leakage current [pA] Ionization current [pA]

50 0.36 9.6

As shown in Table 5, the ionization current (with gamma radiation) was measured to 9.6 pA when there was a voltage difference between collector and shell of 50 V. The leakage current with no

15

(28)

gamma radiation was measured to 0.36 pA. It was not possible to measure the ionization current for voltage differences above 50 V.

4.2 Results for Model 2

For the second model, the prime tower collapsed into the ionization chamber during printing, causing some strings of both materials to be misplaced. Figure 14 shows the printed model and the misplaced strings. A loose piece of plastic was found inside the shell when it was cut open.

Figure 14: The 3D print of Model 2. The collapsed prime tower is seen on the outside of the model.

The resistance between the shell, collector and guard electrode was in the order of a few kΩ to MΩ before the stem was cut off. After the whole stem was cut off, the resistance was too large to measure indicating no electrical contact. For this model, the guard electrode was connected and fully operating. The leakage current had a value of 14 fA and did not increase with increasing high voltage. Ionization current with gamma radiation was measured with high voltage differences between 50 and 300 V, see Table 6. The current saturates around 8.5 pA as the high voltage is increased which can be seen in Figure 15.

16

(29)

Table 6: Measurement of ionization current with Model 2. The table shows the values of the measured ionization current for a range of voltage differences between collector and shell.

Voltage difference [V] Ionization current [pA]

50 7.80

70 8.00

100 8.30

150 8.40

180 8.42

200 8.44

250 8.46

300 8.48

50 100 150 200 250 300

7.5 8 8.5 9

Voltage difference [V]

Current[pA]

Figure 15: Graph of the ionization current for Model 1 with varying voltage difference between collector and shell. The plotted values are the measured ionization currents for different voltages listed in Table 6..

17

(30)

5 Discussion

The leakage current was measured to 0.36 pA for Model 1 when 50 V was applied to the chamber wall and no gamma-ray field was present, as shown in Table 5. There was no working guard electrode in this model as there was a connection between the guard electrode and collector inside the model, and the guard electrode was disconnected from ground. The leakage current probably originated from current that traveled across surfaces due to different electrical potential in the model such as between the shell and the collector. This leakage current was voltage dependent and increased rapidly when increasing the voltage difference above 50 V, which is why measurements with a higher voltage difference could not be done.

When Model 1 was placed in the gamma-ray field, the current increased due to the ionization in the chamber, but also because of unwanted ionization between the collector and shell electrodes. The ionization current that originates from within the sphere of the chamber is the only current that is supposed to be measured. However, the collector electrode and shell electrode that come out of the stem form an electric field between them, creating a “second ionization chamber” which is illustrated in Figure 16. When the ionization chamber was placed in the gamma-ray field, gamma photons ionized air between the collector electrode and shell electrode outside the chamber. The electrons were collected by the collector electrode and therefore added to the measured ionization current. This gave an error in the ionization current from Model 1. This effect gets bigger when increasing the voltage, as a higher voltage difference causes the electric field between the shell and collector electrode to expand, which increases the volume of the “second ionization chamber”.

The leakage current for Model 2 had the impressively low value of 14 fA at 50 V, a value comparable to commercial ionization chambers, and did not change when the voltage difference was increased.

The difference between these measured leakage currents of Model 1 and Model 2 must have been due to the use of a guard electrode in Model 2 as the guard electrode will collect most of the leakage current. This result illustrates the important role of including a guard electrode in the design.

When Model 2 was placed in the gamma-ray field, the “second ionization chamber” instead formed between the guard electrode and the shell electrode, illustrated in Figure 17. The electrons resulting from ionization of the air between the guard electrode and shell electrode were in this case collected by the guard electrode. The current on the guard electrode was not measured and did therefore not affect the ionization current, leading Model 2 to give a more accurate measurement. This explains why there was a difference between the measured ionization currents for Model 1 and Model 2 even after the measured leakage currents were subtracted, when used at the same voltage difference in the same intensity field.

As shown in Table 6 and Figure 15, for voltage differences between 50 and 150 V the current increased with the voltage differences which indicates that recombination occurred for the lower voltage differences. The graph in Figure 15 can be compared to the graph in the recombination region and ionization chamber region in Figure 2. The saturation current seems to be achieved between 150 V and up to at least 300 V as the current did not change much between those measurements. This also means that the ionization current measured with Model 1 was not a saturation current, and some recombination occurred in that measurement.

The method to print the whole structure in one print, using dual extrusion, turned out to be a challenge in itself. Several problems arose such as collapse of prime tower and stringing when the extruder went between printing the collector and the shell. The collapse of the prime tower is probably what caused the loose piece of plastic in the second model which had to be removed. It could also be the source of the contact between the conductive parts of the model before the stem was cut off. Enabling the Z hop setting when retracting seemed to help with the stringing problem, as the second model had no stringing. Examples of parameters that can further be optimized are

18

(31)

printing temperatures, speed, and layer thickness.

Figure 16: An illustration of the right half of a cross section of the stem of Model 1. The left side of the image represents the center of the stem, i.e. the collector. The potential difference between collector electrode and shell electrode creates a ”second ionization chamber”.

Electrons from the ionized air are collected by the collector electrode, affecting the measured ionization current. Increasing the voltage applied to the ionization chamber causes the electric field between collector electrode and shell electrode to expand, resulting in a larger

”second ionization chamber”.

Figure 17: An illustration of the right half of a cross section of the stem of Model 2. The left side of the image represents the center of the stem, i.e. the collector. The potential difference between the guard electrode and the shell electrode creates a ”second ionization chamber” between those electrodes. Electrons from the ionized air are collected by the guard electrode, and the measured ionization current is unaffected by the ”second ionization chamber”

.

An alternative method would be to print a few separate parts, for example a separate collector and to mount the parts together after the printing process. However, the dual-material printing method has a number of advantages. For example it could be possible to print several ionization chambers on the same build plate and to produce a large number of ionization chambers in a short time. Also, this method has the future potential to experiment with unique geometries for the collector and chamber volume.

The successful results of this project mean that different detector designs can be explored in the KTH/SSM project. The method of 3D printing detectors could be used for finding new and optimized detector geometries for micro- and nanodosimetry. Micro- and nanodosimetry would make it possible to see how the radiation energy is distributed in the body on a micro/nanometer scale. This can give information on how tissue is damaged on a cellular as well as sub-cellular level which could potentially be used to minimize health risks connected to radiation. Micro- and nanodosimeters could be used for better monitoring low-level doses of radiation to hospital staff, and for improved dose measurements in diagnostic radiology and radiation therapy.

19

(32)

6 Conclusion

The focus of this report was on investigating if it was possible to measure ionization with a 3D printed ionization chamber. The results show that it is possible to 3D print a working ionization chamber. The use of a guard electrode is of high importance to minimize the leakage current. For a reliable method of producing the ionization chamber, more work needs to be done on optimizing 3D print settings. The design of the model should also be optimized to give more accurate measurements.

20

(33)

7 References

[1] E. B. Podgoršak, Introduction to Modern Physics. Cham: Springer International Publishing, 2016, pp. 1–78. [Online]. Available: https : //doi.org/10.1007/978− 3 − 319 − 25382 − 41 [May 21, 2019]

[2] T. Bäck, “Prestudy: Detectors for variance measurements in the nanometer range,” SSM, 13, 2018.

[3] G. Steinhauser and K. Buchtela, “Chapter 3 - gas ionization detectors,”

in Handbook of Radioactivity Analysis, third edition ed., M. F. L’Annunziata, Ed. Amsterdam: Academic Press, 2012, pp. 191–231. [Online]. Available:

http : //www.sciencedirect.com/science/article/pii/B9780123848734000037[May 21, 2019]

[4] J. W. Cherrie, R. Howie, S. Semple, and I. Ashton, Monitoring for health hazards at work, Chichester, West Sussex ; Malden, MA: Blackwell., 2010, vol. 4.

[5] P. P. Dendy and O. W. E. Morrish, Physics for diagnostic radiology, 3rd ed., ser. A Taylor Francis Book, 2012.

[6] F. H. Attix, Introduction to Radiological Physics and Radiation Dosimetry. Weinheim, Germany: Wiley‐VCH Verlag GmbH, 2007.

[7] M. K. Zaidi and S. Naeem, “Gas-filled and plastic scintillation detectors: Advantages and disadvantages,” in New Techniques for the Detection of Nuclear and Radioactive Agents, G. A. Aycik, Ed. Dordrecht: Springer Netherlands, 2009, pp. 181–192.

[8] C. Grupen and B. Shwartz, Particle Detectors, 2nd ed., ser. Cambridge Monographs on Par- ticle Physics, Nuclear Physics and Cosmology. Cambridge University Press, 2008.

[9] H. D. Young, R. A. Freedman, and A. L. Ford, University Physics with Modern Physics, Global Edition. Pearson Education M.U.A., 2015.

[10] T. K. Gupta, Instrumentation and Its Applications in Nuclear Medicine. Berlin, Hei- delberg: Springer Berlin Heidelberg, 2013, pp. 451–494. [Online]. Available:

https : //doi.org/10.1007/978− 3 − 642 − 34076 − 59 [May 21, 2019]

[11] G. B. Saha, Gas-Filled Detectors. New York, NY: Springer New York, 2006, pp. 71–80.

[Online]. Available: https : //doi.org/10.1007/978− 0 − 387 − 36281 − 67

[12] A. F. Bielajew, F. Tessier, and I. El Gamal, “The inverse-square gamma-irradiation anomaly of the Nuclear Enterprises 2575 large-volume ionisation chamber,” Radiation Protection Dosimetry, vol. 167, no. 4, pp. 385–391, 10 2014. [Online]. Available:

https://doi.org/10.1093/rpd/ncu306

[13] J. Horvath, Mastering 3D Printing, 1st ed. Berkeley, CA: Apress, 2014.

[14] J. Micallef, Exploring Design Techniques for 3D Printing. Berkeley, CA: Apress, 2015, pp.

31–66. [Online]. Available: https://doi.org/10.1007/978-1-4842-0946-22

[15] B. Evans, Practical 3D Printers, ser. Technology in action Practical 3D printers, 2012.

[16] S. Klomp, “Printing conductive and non-conductive materials simultaneously on low-end 3d printers,” Department of Industrial System and Product Design, 2015.

[17] “Ultimaker cura travel settings | ultimaker.” [Online]. Available: Internet:

https://ultimaker.com/en/resources/52507-travel [May 18, 2019]

[18] J. Horvath, 3D Printing with MatterControl, ser. Technology in action, 2015.

21

(34)

TRITA CBH-GRU-2019:073

www.kth.se