Monte Carlo yöntemi ile atmosferde ve yer yüzeyinde doz  hesaplamaları
Dose calculations on the atmosphere and ground surface by Monte Carlo method
Yazar: HATİCE DOĞRU
Danışman: PROF. DR. İSKENDER ATİLLA REYHANCAN
İstanbul Teknik Üniversitesi / Enerji Enstitüsü / Nükleer Araştırmalar Ana Bilim Dalı / Radyasyon Bilim ve Teknoloji Bilim Dalı
Derece: Yüksek Lisans, 2020
Konu: Nükleer Mühendislik = Nuclear Engineering

Özet:
Günlük hayatımıza devam ederken, çevrede çeşitli radyasyon formlarına maruz kalmaktayız. Bu radyasyonun yer seviyesindeki ana kaynaklarından biri kozmik ışınlardır. Yüksek enerjili bu kozmik ışınların kökeni, astrofizikte çözülememiş önemli bir sorun olmaya devam etmektedir. Birincil kozmik ışınların çoğunluğunu proton ve alfa oluştururken ikincil kozmik ışınlar; pion, kaon, müon, proton, gama, nötron, eletron, pozitron, nötrino ve antinötrinolardır. Zırh görevi gören Dünya atmosferi sayesinde, yüksek enerjili kozmik ışınların büyük çoğunluğu yer yüzüne ulaşamaz ve böylece deniz seviyesine inildikçe ikincil parçacıkların yoğunluğu ve doz miktarında azalma görülür. Bu sebeple uçak personelleri ve kutuplara yakın yaşayanlar, deniz seviyesinde olanlara göre daha fazla kozmik radyasyona maruz kalmaktadır.
Bu yüksek lisans tez çalışmasında GEANT4 simülasyon programından yararlanılmıştır. GEANT4 simülasyon programı, parçacıkların madde içerisinden geçişinin ve madde ile etkileşiminin simülasyonunun yapılmasını sağlayan yazılım paketidir ve Monte Carlo yöntemini kullanır. Monte Carlo yöntemi ise, rastgele sayılar kullanarak bir soruna makroskopik bir çözüm elde etmenin, istatistiksel bir yaklaşımıdır. Bu tez çalışmasında simülasyonlar iki aşamalı olarak gerçekleştirilmiştir. İlk olarak,  Dünya atmosferi modellenip deniz seviyesinde ve deniz seviyesinden 10 km yukarıda ikincil parçacıklardan gama, proton ve nötron akıları incelenmiştir. Simülasyon çalışmasının bu kısmında, yeryüzünden itibaren atmosferin, taban kenarları ve yüksekliği 80 km olan küp biçimindeki bölümü ele alınmış ve Dünya'nın yüzeyi düz kabul edilerek modellenmiştir. Ayrıca bu çalışmada Dünya'nın elektrik ve manyetik alanı hesaba katılmamıştır. Standartlara uygun bir şekilde, basınç, sıcaklık ve yoğunlukları ayrı ayrı belirtilerek atmosferin katmanları oluşturulmuştur. Atmosferin en üst tabakasından birincil parçacıklar gönderilmiş ve 2 farklı irtifada, ROOT analiz programıyla elde edilen verilere göre ikincil parçacıkların akısı incelenmiştir. Bu sonuçlara göre kozmik ışınların yoğunluğunun irtifa ile değiştiği, deniz seviyesinden yukarılara çıkıldıkça arttığı gözlemlenmiştir.
Simülasyon çalışmasının ikinci kısmında ise, 2 farklı irtifada elde edilen ikincil kozmik parçacıkların ortalama enerjileri kullanılarak, insan dokusu üzerindeki doz değerleri hesaplanmıştır. Bunun için GEANT4'te insan vücudu modellenmiş, bir önceki çalışmada elde edilen ortalama enerjideki nötron, gama ve protonlar, insanın belli bir mesafe yukarısından ayrı ayrı gönderilmiş, her biri için doz değerleri hesaplanmıştır. Elde edilen kozmik radyasyon dozu değerlerinin, deniz seviyesinden 10 km yukarıda çok daha fazla olduğu görülmüştür.

Abstract:
In our daily lives, we are exposed to various forms of radiation from the environment. One of the main sources of this radiation at ground level is cosmic rays. The origin of the cosmic rays is still not totally known. The majority of the primary cosmic rays are proton and alpha, while the secondary cosmic rays; pion, kaon, muon, proton, gamma, neutron, eletron, positron, neutrino and antineutrinos. By means of Earth's atmosphere, which acts as shield, the majority of high-energy cosmic rays cannot reach the surface, reducing the flux and dose of secondary particles. Therefore, aircraft personnel and those living close to the poles are exposed to more cosmic radiation than those living at sea level.
Cosmic rays are basically divided into three main groups: Galactic Cosmic Rays, Solar Cosmic Rays and Abnormal Cosmic Rays. The Galactic Cosmic Rays are charged particles coming out of our solar system. While these rays are mainly composed of proton and helium nuclei, about 1% are made up of heavier nuclei. They move in a wide speed range close to the speed of light in the galaxy. Since the energy of particles in the solar wind caused by the expansion of the solar corona is relatively small, the Solar Cosmic Rays are easily deflected by the Earth's magnetic field and do not affect the Earth's atmosphere. In some cases, however, the surface of the Sun emits sudden bursts of energy in the form of gamma rays, X-rays and radio waves. Abnormal cosmic rays, on the other hand, are most likely produced by neutral atoms in interstellar space. Each of the cosmic rays classified above is divided into two main groups, primary cosmic rays and secondary cosmic rays. Primary cosmic rays are stable charged particles accelerated with great energies by astrophysical sources somewhere in our universe. Primary high energy cosmic ray particles, mostly protons, interact with the atmospheric substance primarily through strong interaction and form a large number of particles called secondary cosmic rays. Secondary cosmic ray components can be divided into three groups: muonic, hadronic, electromagnetic. 
The atmosphere consists of approximately 78% nitrogen, 21% oxygen, 0.93% argon, 0.034% (average) carbon dioxide and trace amounts of other gases. Atmospheric layers, from bottom to top; it is in the form of troposphere, stratosphere, mesosphere, and exosphere and is defined according to temperature-density changes. Troposphere; It contains about 80% of the mass of the atmosphere and is where most of the daily air observed from the ground takes place. It is the layer where the gases are most intense. Stratosphere; It extends up to about 50 km from the troposphere. The ozone layer is in this layer. Ozone and oxygen in the stratosphere absorb most of the UV rays from the Sun. Mesosphere is the layer of the atmosphere between 50 km and 80 km elevation above sea level. The mesosphere layer protects the earth from meteorites from space, and when meteors enter this layer, they burn. The exosphere is the highest atmospheric layer. This is where the earth's atmosphere meets the interplanetary space. 
Upon entering the atmosphere, primary cosmic radiation is exposed to the electrons and nuclei of the atoms and molecules that make up the air, and as a result, the radiation composition changes as it diffuses through the atmosphere. All particles lose energy through hadronic and / or electromagnetic processes. Electromagnetic showers can be started by gamma rays, electrons or positrons.  When a high-energy hadron, mostly proton, interacts with atmospheric nuclei with strong force, a hadronic shower occurs. Energetic protons and neutrons often lose energy with strong interactions, heavy nuclei are broken down in collisions with the nuclei of the air molecules, and electrons and photons are exposed to electromagnetic energy processes. In addition, all charged particles are subject to ionization losses.
GEANT4 simulation program was used in this master thesis study. GEANT4 simulation toolkit  is a software package that allows the simulation of the passage of particles through the substance and its interaction with the substance using Monte Carlo method. The Monte Carlo method is a statistical approach to obtaining a macroscopic solution to a problem using random numbers. 
In the Monte Carlo simulation, radiation transport occurs by moving the particles in different steps and interacting with various particles along the way. The fact that the radiation stone is a random process by definition is an excellent example of the Monte Carlo technique.
GEANT4 provides a comprehensive array of particles, detector geometry, material, tracking method, detector response, visualization and user interface. It also provides physics processes such as electromagnetic, hadronic, and optics to describe the interaction of particles with matter over a wide range of energy. GEANT4 has the ability to create a geometric model with components in a number of different shapes and materials and identify sensitive parts that record the information needed to simulate detector responses. Particle interaction in matter is related to particle types and energies. After the primary particles start with appropriate physics processes, they are monitored in the system until they hit another particle, transfer all their energy and stop, decay or go out of the world volume, this is called track. The production of primary particles can be done using the event generator or the particle shooter class, which can form a beam of particles combined with the type, location, direction of movement and kinetic energy of the particle. In order for the particles to move between the two interaction points, their energies must be updated as well as the space and time coordinates. For this reason, it would be useful to distinguish a certain initial and final state in a particle interaction or decay. 
Event management is the main unit in the simulation. Event provides event producers with an abstract interface. The event generator provides the primary particles that define a physics event. This event class avoids hiding temporary information that is not required after an event has completed the process. Includes primary vertices and primary particles before event processing. After processing, it has hits and digitizations created by the simulation and can optionally store the trajectories of simulated particles.
In the simulation, each particle is moved from the environment step by step. G4Step class coordinates store changes in track properties between start and end points, including volume, energy, and momentum. In the physics process, process properties are shown by following three monitoring actions: particle decay when stopped, energy loss or secondary particle production during the step, and post-step decay, or secondary particle production by interaction. 
In the GEANT4 simulation program, a hit is an image of a physical interaction, or accumulation of traces (or traces) interactions, in a sensitive detector component. On the other hand, the term "digit" represents a detector output, for example an ADC or TDC count or a trigger signal. A digit is created from one or more hits and / or other digits. GEANT4 provides only abstract classes for both detector sensitivity and hit or digit. A sensitive detector creates  hit using the information provided in the current step.There are three main visualization drivers integrated into the GEANT4 toolkit, such as OpenGL 37, DAWN 38 and HepRep (High Energy Physics Can Be Represented). In this thesis, OpenGL interface was used.
In this master's thesis, simulations were carried out in two stages. In the first, the Earth's atmosphere is modeled and gamma, proton and neutron fluxes from secondary particles are investigated at sea level and 10 km above sea level. In this part of the simulation study, the surface of the atmosphere from the earth and the cube-shaped part of the ground, whose height is 80 km, is handled and modeled by considering the surface of the Earth as flat.  Also in this study, the electric and magnetic fields of the Earth are not taken into account. Firstly, the physical processes, which are the most important part of the simulation, were determined and included in the program. While the standard electromagnetic package included in GEANT4 is used for electromagnetic interactions, different models of GEANT4 are used for hadronic interactions depending on the energy range. For high energies (> 10 GeV), the quark-gluon string model was chosen, and the binary intranuclear cascade model for nucleons with energies less than 10 GeV. For neutrons with energy less than 20 MeV, the G4NeutronHP model based on the ENDF database was used. In simulation G4HadronPhysicsQGSP_BIC_HP contains all packages for hadronic interactions. In addition, G4RadioactiveDecayPhysics has been used for decay interactions of radioactive ions. 
Atmosphere geometry was created by overlapping 19 cubic geometry layers, separated by 1 km between sea level and 10 km, 5 km between 10-30 km, and 10 km between 30-80 km. Temperature, pressure and density values of each atmospheric layer were calculated based on "U.S Standard Atmosphere 1976". 99% of the primary cosmic rays reaching the earth's atmosphere constitute the nucleus of the proton and helium atom. With the help of balloons, proton and helium fluxes with different energy values were obtained in the measurements obtained in the upper atmosphere. Using these fluxes, proton and alpha particles were sent separately from the top layer of the atmosphere, 80 km above the ground, perpendicular to the earth's atmosphere. In this study, the flux of only secondary gamma, neutrons and protons formed at two different levels as sea level and 10 km from sea level were analyzed with ROOT analysis program, and Origin drawing program graphics were obtained.
In the second part of the simulation, the average energy and count values obtained in the first stage of the simulation study were used for dose calculations. For this, a new simulation study was made and a new geometry was designed.  These dose calculations have been studied on human tissue, and an average human body is designed for this (G4_A-150_TISSUE material used for tissue sample was used in GEANT4), and the average energy and count values of the secondary particles obtained were sent randomly from an area of 1 m2, about 1 m above the human body for two different altitudes.
The absorbed dose obtained in GEANT4 was converted into equivalent doses according to the recommendation of the International Radiological Protection Council, because for different types of radiation, although the absorbed energy is the same, the biological effects it generates are different. According to the results, it has been observed that the intensity of the cosmic rays varies with altitude, and increases as it rises above sea level and also it has been observed that the obtained cosmic radiation dose values are higher than the standards because of sending directly from one direction directly above the human body from 1 m above and the electric and magnetic fields are not taken into account.
The maximum flux of particles occurs at a height of 15 km or just above aircraft altitudes. Flying at high altitudes exposes aircraft personnel and those who travel frequently to higher doses of cosmic radiation than ground-level ones, since the energy of particles decreases as they approach the ground surface. Both primary high-energy particles and secondary particles can have negative health effects on humans. Cosmic radiation can break down DNA and produce free radicals that can alter cell functions. 























In our daily lives, we are exposed to various forms of radiation from the environment. One of the main sources of this radiation at ground level is cosmic rays. The origin of the cosmic rays is still not totally known. The majority of the primary cosmic rays are proton and alpha, while the secondary cosmic rays; pion, kaon, muon, proton, gamma, neutron, eletron, positron, neutrino and antineutrinos. By means of Earth's atmosphere, which acts as shield, the majority of high-energy cosmic rays cannot reach the surface, reducing the flux and dose of secondary particles. Therefore, aircraft personnel and those living close to the poles are exposed to more cosmic radiation than those living at sea level.
Cosmic rays are basically divided into three main groups: Galactic Cosmic Rays, Solar Cosmic Rays and Abnormal Cosmic Rays. The Galactic Cosmic Rays are charged particles coming out of our solar system. While these rays are mainly composed of proton and helium nuclei, about 1% are made up of heavier nuclei. They move in a wide speed range close to the speed of light in the galaxy. Since the energy of particles in the solar wind caused by the expansion of the solar corona is relatively small, the Solar Cosmic Rays are easily deflected by the Earth's magnetic field and do not affect the Earth's atmosphere. In some cases, however, the surface of the Sun emits sudden bursts of energy in the form of gamma rays, X-rays and radio waves. Abnormal cosmic rays, on the other hand, are most likely produced by neutral atoms in interstellar space. Each of the cosmic rays classified above is divided into two main groups, primary cosmic rays and secondary cosmic rays. Primary cosmic rays are stable charged particles accelerated with great energies by astrophysical sources somewhere in our universe. Primary high energy cosmic ray particles, mostly protons, interact with the atmospheric substance primarily through strong interaction and form a large number of particles called secondary cosmic rays. Secondary cosmic ray components can be divided into three groups: muonic, hadronic, electromagnetic. 
The atmosphere consists of approximately 78% nitrogen, 21% oxygen, 0.93% argon, 0.034% (average) carbon dioxide and trace amounts of other gases. Atmospheric layers, from bottom to top; it is in the form of troposphere, stratosphere, mesosphere, and exosphere and is defined according to temperature-density changes. Troposphere; It contains about 80% of the mass of the atmosphere and is where most of the daily air observed from the ground takes place. It is the layer where the gases are most intense. Stratosphere; It extends up to about 50 km from the troposphere. The ozone layer is in this layer. Ozone and oxygen in the stratosphere absorb most of the UV rays from the Sun. Mesosphere is the layer of the atmosphere between 50 km and 80 km elevation above sea level. The mesosphere layer protects the earth from meteorites from space, and when meteors enter this layer, they burn. The exosphere is the highest atmospheric layer. This is where the earth's atmosphere meets the interplanetary space. 
Upon entering the atmosphere, primary cosmic radiation is exposed to the electrons and nuclei of the atoms and molecules that make up the air, and as a result, the radiation composition changes as it diffuses through the atmosphere. All particles lose energy through hadronic and / or electromagnetic processes. Electromagnetic showers can be started by gamma rays, electrons or positrons.  When a high-energy hadron, mostly proton, interacts with atmospheric nuclei with strong force, a hadronic shower occurs. Energetic protons and neutrons often lose energy with strong interactions, heavy nuclei are broken down in collisions with the nuclei of the air molecules, and electrons and photons are exposed to electromagnetic energy processes. In addition, all charged particles are subject to ionization losses.
GEANT4 simulation program was used in this master thesis study. GEANT4 simulation toolkit  is a software package that allows the simulation of the passage of particles through the substance and its interaction with the substance using Monte Carlo method. The Monte Carlo method is a statistical approach to obtaining a macroscopic solution to a problem using random numbers. 
In the Monte Carlo simulation, radiation transport occurs by moving the particles in different steps and interacting with various particles along the way. The fact that the radiation stone is a random process by definition is an excellent example of the Monte Carlo technique.
GEANT4 provides a comprehensive array of particles, detector geometry, material, tracking method, detector response, visualization and user interface. It also provides physics processes such as electromagnetic, hadronic, and optics to describe the interaction of particles with matter over a wide range of energy. GEANT4 has the ability to create a geometric model with components in a number of different shapes and materials and identify sensitive parts that record the information needed to simulate detector responses. Particle interaction in matter is related to particle types and energies. After the primary particles start with appropriate physics processes, they are monitored in the system until they hit another particle, transfer all their energy and stop, decay or go out of the world volume, this is called track. The production of primary particles can be done using the event generator or the particle shooter class, which can form a beam of particles combined with the type, location, direction of movement and kinetic energy of the particle. In order for the particles to move between the two interaction points, their energies must be updated as well as the space and time coordinates. For this reason, it would be useful to distinguish a certain initial and final state in a particle interaction or decay. 
Event management is the main unit in the simulation. Event provides event producers with an abstract interface. The event generator provides the primary particles that define a physics event. This event class avoids hiding temporary information that is not required after an event has completed the process. Includes primary vertices and primary particles before event processing. After processing, it has hits and digitizations created by the simulation and can optionally store the trajectories of simulated particles.
In the simulation, each particle is moved from the environment step by step. G4Step class coordinates store changes in track properties between start and end points, including volume, energy, and momentum. In the physics process, process properties are shown by following three monitoring actions: particle decay when stopped, energy loss or secondary particle production during the step, and post-step decay, or secondary particle production by interaction. 
In the GEANT4 simulation program, a hit is an image of a physical interaction, or accumulation of traces (or traces) interactions, in a sensitive detector component. On the other hand, the term "digit" represents a detector output, for example an ADC or TDC count or a trigger signal. A digit is created from one or more hits and / or other digits. GEANT4 provides only abstract classes for both detector sensitivity and hit or digit. A sensitive detector creates  hit using the information provided in the current step.There are three main visualization drivers integrated into the GEANT4 toolkit, such as OpenGL 37, DAWN 38 and HepRep (High Energy Physics Can Be Represented). In this thesis, OpenGL interface was used.
In this master's thesis, simulations were carried out in two stages. In the first, the Earth's atmosphere is modeled and gamma, proton and neutron fluxes from secondary particles are investigated at sea level and 10 km above sea level. In this part of the simulation study, the surface of the atmosphere from the earth and the cube-shaped part of the ground, whose height is 80 km, is handled and modeled by considering the surface of the Earth as flat.  Also in this study, the electric and magnetic fields of the Earth are not taken into account. Firstly, the physical processes, which are the most important part of the simulation, were determined and included in the program. While the standard electromagnetic package included in GEANT4 is used for electromagnetic interactions, different models of GEANT4 are used for hadronic interactions depending on the energy range. For high energies (> 10 GeV), the quark-gluon string model was chosen, and the binary intranuclear cascade model for nucleons with energies less than 10 GeV. For neutrons with energy less than 20 MeV, the G4NeutronHP model based on the ENDF database was used. In simulation G4HadronPhysicsQGSP_BIC_HP contains all packages for hadronic interactions. In addition, G4RadioactiveDecayPhysics has been used for decay interactions of radioactive ions. 
Atmosphere geometry was created by overlapping 19 cubic geometry layers, separated by 1 km between sea level and 10 km, 5 km between 10-30 km, and 10 km between 30-80 km. Temperature, pressure and density values of each atmospheric layer were calculated based on "U.S Standard Atmosphere 1976". 99% of the primary cosmic rays reaching the earth's atmosphere constitute the nucleus of the proton and helium atom. With the help of balloons, proton and helium fluxes with different energy values were obtained in the measurements obtained in the upper atmosphere. Using these fluxes, proton and alpha particles were sent separately from the top layer of the atmosphere, 80 km above the ground, perpendicular to the earth's atmosphere. In this study, the flux of only secondary gamma, neutrons and protons formed at two different levels as sea level and 10 km from sea level were analyzed with ROOT analysis program, and Origin drawing program graphics were obtained.
In the second part of the simulation, the average energy and count values obtained in the first stage of the simulation study were used for dose calculations. For this, a new simulation study was made and a new geometry was designed.  These dose calculations have been studied on human tissue, and an average human body is designed for this (G4_A-150_TISSUE material used for tissue sample was used in GEANT4), and the average energy and count values of the secondary particles obtained were sent randomly from an area of 1 m2, about 1 m above the human body for two different altitudes.
The absorbed dose obtained in GEANT4 was converted into equivalent doses according to the recommendation of the International Radiological Protection Council, because for different types of radiation, although the absorbed energy is the same, the biological effects it generates are different. According to the results, it has been observed that the intensity of the cosmic rays varies with altitude, and increases as it rises above sea level and also it has been observed that the obtained cosmic radiation dose values are higher than the standards because of sending directly from one direction directly above the human body from 1 m above and the electric and magnetic fields are not taken into account.
The maximum flux of particles occurs at a height of 15 km or just above aircraft altitudes. Flying at high altitudes exposes aircraft personnel and those who travel frequently to higher doses of cosmic radiation than ground-level ones, since the energy of particles decreases as they approach the ground surface. Both primary high-energy particles and secondary particles can have negative health effects on humans. Cosmic radiation can break down DNA and produce free radicals that can alter cell functions.