I am a Physicist from Germany, and would like to use this site, to inform you of myself, my previous work, and my professional interests. I attended RWTH Aachen University and received my Bachelor of Science in Physics degree after three years. Known for its research excellence, I chose to attend the Technical University of Munich and graduated with a Master of Science in Physics and high distinction. During the last years I have focused on the field of Applied Physics with my interest now focusing on Medical Physics. It is exciting to me to see how concepts from particle and nuclear physics are applied in radiology and radiation oncology to diagnose and treat diseases effectively.
Under Work Samples I have compiled a list of my theses, presentations, and software. I welcome you to contact me with questions or inquiries anytime via email at christian.velten-AT-tum.de .
I have previously worked at the Forschungszentrum Jülich, a research center in Western Germany, preparing my Bachelor's thesis, and with several groups at the Technical University of Munich as a Research Assistant and Associate. I invite you to click on the headings below to toggle the visibility of each section.
Neutrons vs. Cancer
Most recently, I have worked at the Research Reactor Munich II (FRM-II) with the group operating the facility for medical applications (MEDAPP). There, fast neutrons taken directly from the fission of highly enriched uranium are used for computed tomography, fast neutron PGAA, and also to treat cancer patients.
Using particles instead of or together with radiotherapy is not uncommon, because the destructive power of an external beam can be much more localized to the cancer tissue than with common radiotherapy. The application of fast neutrons, however, remains a niche as proton therapy gets more common. Fast neutron radiation therapy proved very effective in salivary gland cancers and when tumors are inoperable or resistant against X-rays.
Like all other radiation treatment options a detailed plan needs to be developed by the treating physician and his/her team. Most radiation devices found in hospitals around the world get shipped with software which is able to calculate the ideal set of treatment sessions and configurations to maximize the damage to cancerous tissue while minimizing the damage to healthy tissue. Fast neutron facilities are found at either particle accellerators or research reactors and are in general the only one of their kind. The long-term goal at the FRM-II is to update and enhance the treatment planning system to make the fast neutron radiation therapy even more effective.
The first step is to simulate the interaction of neutrons and photons, which are also present at the irradiation point, in a water phantom. This phantom used to measure the energy deposition by neutrons and photons separately at various points of the beam field and different depths. For the simulation the open-source toolkit Geant4 was used, because it is modular and easily adaptable. Geant4 uses a Monte Carlo approach, which is using randomness to yield results to analytically hard-to-solve problems.
An additional factor, adding to the complexity of the problem, was the implementation of a adjustable multi-leaf collimator, with 38 leafs made from steel, borated polyethylene, and polyethylene.
Electric Fields and EDMs
Why is there something? Not just mankind, earth, our solar system, but why is there matter?! During the creation of our universe, shortly after the big bang, equal amounts of matter and antimatter have been created. Why? Because of symmetry! Physicists (and nature) love symmetries, so when the universe continued to expand to a point where no new matter was created, all the existing matter should(!) have annihilated with all the antimatter, because of symmetry! If that were true though, we wouldn't exist. So something must have gone wrong, something must've broken the symmetry and leave more matter than antimatter. Aside from using particle colliders, several groups try to take the other way towards low and lower energies, but higher precision. The existence of a permanent electric dipole moment for the neutron would provide an indicator and measure to a symmetry violating process.
In the search for electric dipole moments, one of course requires an electric field and since the quantities we set out to measure are that small, we need big fields and also the means to measure them and their stability. This task, as easy as it sounds at first, gets complicated by the experiments' constraints: one wants to measure in-situ, where the experiment is happening, and one must not introduce anything remotely magnetic, because any magnetic impurity would influence the measurement.
The primary idea is to utilize similar techniques which are employed in magnetometry, i.e. the measurement of magnetic fields. These use sophisticated laser systems and mercury or cesium gas to measure magnetic fields, so we just had to identify a concept that couples optical properties with the electric field. One of the most commonly known is the so-called Pockels effect, occurring in crystals with special symmetries (or better the lack thereof). When applying an electric field to birefringent (having refractive indices dependent on polarization and direction of light) crystals, like quartz, the electric field alters the crystal’s structure, altering the birefringence. Hence, I built an experiment from scratch in which polarized laser light is generated, routed through a quartz crystal, which sits between two high-voltage electrodes, and after this analyzed and processed.
This setup was used to detect both very slow and very fast changes in the electric field using different pieces of software. Though I developed them originally for this experiment, they are now also being used by the group working on magnetic field measurements.
While the measurement sensitivity remains to be improved, I have shown that a fully optical measurement of the electric field and its stability is feasible.
PGAA as a precision instrument
I had the opportunity to work as part of the Nuclear Data group at the FZ Jülich, to support them in their endeavor to measure partial and total neutron absorption cross-sections for actinides like Uranium, Plutonium, and Americium. Those cross-sections, another word would be interaction probabilities, describe how likely it is for the nuclei of those elements to absorb a neutron, which is "passing" them within a certain distance and time. This kind of data has been both theoretically calculated and measured for decades for light and stable elements. However, the aforementioned elements occur in high quantities primarily in nuclear facilities like nuclear power plants, and are radioactive. Additionally, the primary driver for Neutron Activation Analysis (NAA) has been archaeology, which focuses on other elements and isotopes over the actinides.
In recent years, with increased interest in spent fuel reprocessing, and novel reactor designs, the need for more precise data on the properties of the actinides has emerged. The more precise the data, the better the results extracted from simulations, the better the design of new reactors, leading to more efficient producers of clean energy.
The goal of my work has been to validate the measurement technique which was to be employed for the actinides using something well-characterized, in this case: sodium chloride, salt. Following the same protocol, I prepared several samples (small pressed pellets in between thin glass sheets), and irradiated them at the research reactors in Munich, Germany and Budapest, Hungary. This means that we exposed the small salt pellets to a vast number of neutrons for several hours and measured their response during that time and afterwards. When a nucleus absorbs a neutron it is in an excited state and in order to reach its preferable state, it emits characteristic radiation on two different time scales, essentially instantaneous and over a course of hours to days. From the analysis of this data and comparison with established references, I was able to prove that the proposed measurement technique was feasible for actinides and more importantly that its results could be trusted!
During my graduate studies at the Technical University of Munich I worked with the PGAA group in Munich, where a new PGAA detector was being set up, which utilizes fast neutrons over the commonly used cold (slowish) neutrons. This enables us to observe inelastic scatter reactions, fission reactions, and some more exotic reactions in which the sample element's nucleus absorbs the neutron, but emits a helium nucleus! However, at the same time the number of radiative absorption processes decreases immensely. As such it is not replacing cold neutron PGAA, but complementing it!
In addition to supporting the initial set-up, I primarily focused on characterizing the instrument and evaluating measurements from various metal foils, including depleted uranium. To know which dot and spike corresponds to what specific energy, one has to make an energy calibration. This is usually achieved by using radioactive test sources, which are well defined e.g. by the NIST; most commonly used are radioactive isotopes of Barium, Europium and Cesium. To extend the calibration range, one also usually irradiates a sample containing high amounts of either chlorine or nitrogen, whose radiative neutron absorption processes are well-known.
However, it was also necessary, to use gamma lines from the spectra that were to be analyzed themselves. Since there is a high abundance of iron near the detector, gammas from neutron absorption by iron nuclei was used.
After the characterization, several metals like niobium, zirconium, titanium, cobalt, cadmium, and many more were irradiated. Subsequently their spectra were analyzed and the results compared to one of the only existing databases for fast neutron irradiation. Many data points matched closely with previous (i.e. 1960s) measurements, but we also found many unique gamma lines, which have not been accounted for.
During this first round of measurements I identified several areas that needed improvement, to be eventually be able to succeed at building a fast neutron PGAA library, which then would truly complement PGAA in its day-to-day application.
2016: M.Sc., Physics, Technical University of Munich
2013: B.Sc., Physics, RWTH Aachen University