The sensor head of our CHAOS experiment contains various different detector types. One of them is the Bismuth-Germanium-Oxide (BGO) scintillator. We want to observe galactic cosmic rays. When the particles travel through the scintillator, photons are created. These mass-less, charge-less and super fast photons are the particle manifestation of light. They propagate through the BGO and are measured at two photodiodes glued to the outside surfaces of the BGO. But how exactly do they propagate through the scintillator? And what factors influence the signal yield at the photodiodes? Investigating this is the subject of my bachelor’s thesis, in which I wrote a Python-based program that simulates the photons’ path through the scintillator. Stay tuned for the results.
The CHAOS particle telescope will be able to differentiate between heavy particles and lighter particles in the Galactic Cosmic Rays (GCRs). The heavier particles are protons and different nuclei while electrons for example are among the lighter particles in the GCRs. To differentiate between those different kinds of high energetic particles the CHAOS telescope is going to use a Cherenkov detector in which particles who are moving faster than the speed of light in the medium are creating so called Cherenkov-radiation. At similar energies the heavier and the lighter particles in the GCRs have vastly different velocities. Therefore only the lighter particles who are moving fast enough will create Cherenkov-radiation in the medium.
In our Cherenkov detector, the material in which the Cherenkov-radiation is created will be aerogel. Aerogels are porous solid bodies whose volume can be made of up to 99.98% pores. Therefore aerogels are among the lightest materials available.
On 22 April, the eagerly awaited blocks of aerogel finally arrived in Kiel. The two custom made blocks from the aerogel factory in japan had to undergo strict inspections to check if they fulfill several quality criteria especially wether or not the deviations in dimension are of a tolerable amount. Because the aerogel is so dust sensitive and brittle the inspection had to be done in our dust free clean room. When we inspected the aerogel we had to undergo several steps to ensure that we don’t carry any dust or electrostatic charge into the clean room. Only in the clean room the aerogel’s packaging was opened and we first saw the sky blue color of the unpacked blocks. The 62mm * 62mm * 40 mm big blocks of aerogel weigh only 26.6 g. While inspecting, the aerogel blocks had to be handled with great caution not to damage or even scratch the material. Afterwards we decided that the aerogel blocks meet our requirements, the deviation in dimension are sufficiently small and. Now we can finally start experimenting with them and getting them ready for flight.
On Thursday 25 April, two schoolgirls visited us as part of Girls’ Day. In the morning we inspected the room in our building and glued one of the photodiodes to the bismuth germanium oxide scintillator. Then we showed and explained two of our current experimental setups. The set-up with the BGO crystal was going to be placed in the vacuum chamber. Together with the students, we took all the necessary precautions and transferred the whole setup to the laboratory with the vacuum chamber. In preparation, several cables were soldered from the test cell to the vacuum feedthrough and further to the readout electronics. These were extensively tested by the girls before commissioning, as were all the other feedthroughs and the power supply. After the lunch break, the measurement setup was put into operation and after successful tests, the vacuum chamber was closed and the air pumped out. As the light output in the BGO is temperature dependent, the next step is temperature calibration. Unfortunately, there is currently a problem with the cooling system, so this test will be delayed. However, with the help of the two students, the test setup in the pump-down vacuum chamber was successfully put into operation.
A key component of our experiment is the bismuth germanate oxide (BGO) scintillator. Scintillators are materials that emit light when ionising particles pass through them, and deposit energy. This light is then complexly reflected and scattered until it is detected by the attached photodiodes. It is interesting to investigate the overall yield and the individual yield of the glued on photodiodes depending on where a particle flies through the BGO. This was done by a student in our team as part of his bachelor thesis.
The focus was on a BGO with 2 photodiodes glued to two opposite sides. In order to be able to make a statement about the trajectories, 8 square photodiodes with a sensitive area of 10 mm x 10 mm were attached above and below, acting as trigger diodes. To ensure that the positions of the photodiodes were well defined, a frame was designed and then printed using a 3D printer.
A 21-day measurement was carried out using cosmic muons at sea level as the particle source. The investigations revealed a rather strong trajectory dependent yield. Particles that fly very close to a readout photodiode through the BGO deposit significantly more energy in this diode via the scintillation light, while the readout photodiode further away sees a smaller signal. The measured signals are subjected to statistical processes. For comparison, the most probable value mpv is determined for each curve.
Another way of comparing the signals is to plot the signals from diodes B1 and B2 against each other. If both signals are the same, the points in a 2D histogram will lie approximately on a straight line with a slope of 1. If one diode has a stronger signal, the dots are shifted in one direction. If a particle hits the BGO in the centre, both diodes will see the same signal. If the hit is closer to one diode, the closer diode will see a significantly higher signal than the other diode. If the sum of the two signals were the same, this would not be a problem. But the sum of the signals is lower in the centre of the BGO (position 9) compared to the sides where the diodes are glued (positions 2 and 16).
An even lower signal was detected on the sides where no diode is attached (positions 7 and 11). The strangest behaviour can be seen at position 12. Although readout diode B2 is closer to the particles with the corresponding coincidence, B1 has seen a larger signal compared to B2. A possible explanation would be that the photons generated were reflected at the side edge towards photodiode B1, resulting in a larger signal being measured.
An attempt was made to influence the photon paths in the BGO by roughening one of the side surfaces. No improvement could be seen, on the contrary, the deviations have increased. In order to cover the position dependency as well as possible, a BGO with 6 small diodes is currently being prepared and the measurements will be repeated.
Last week, a few of us went to Greifswald, Germany, to take part in the Spring Conference of the German Physics Society. Hannes presented our instrument and the BEXUS campaign, while Tom talked about his Bachelor’s thesis, regarding the pathways of photons in the BGO scintillator crystal. Jasper had prepared a poster for the mechanical setup of the experiment.
This was a great experience where we could get in contact with physicists from all over Germany and got to hear some very interesting talks from different topics. But now, we will be heading back to work on the instrument and prepare for our journey to the CDR in the Netherlands!
We are happy to report that we passed the Preliminary Design Review (PDR)! The next step will be the Critical Design Review (CDR) taking place in May. We will be travelling to the European Space Research and Technology Centre (ESTEC) in Noordwijk, the Netherlands, to present our final design in front of a board of experts.
This weekend, a segment of our group traveled from Hamburg to Kiruna, Sweden, to participate in the Student Training Week hosted at the Esrange Space Center. Upon arrival, we met with other BEXUS/REXUS teams at the airport and proceeded together to the center via bus.
The week commenced with a series of informative presentations about Esrange and the associated Space Agencies on Monday. A significant part of our schedule was dedicated to the Preliminary Design Review (PDR), during which we presented the current status of our experiment to a panel of experts. The session was interactive, with a focus on our project’s high voltage systems and pressure housing, prompting a detailed discussion and questions from the panel.
Throughout the week, we plan to provide additional information based on the feedback received and hope to progress to the next phase of the project.
In addition to project-related activities, the week included a tour of the Esrange site and further presentations, offering us the opportunity to learn more about space exploration and to network with teams from across Europe.
Stay tuned for more insights from our trip to Sweden!
I am Tom Ruge and I am writing my bachelor’s thesis about a part of our instrument CHAOS and in the following I will tell you what we have been doing lately.
For our particle detector, which we are currently building, we are using a transparent BGO scintillator crystal. When a charged particle flies through it, light is emitted, which is then measured. Based on how light is emitted and how it is ultimately measured, conclusions can then be drawn about the particle. Depending on where exactly the particle flies through the BGO scintillator crystal, I investigate how exactly the light is emitted and then measured by the photodiodes. And that’s exactly what my bachelor’s thesis is about, which I started a few weeks ago. So we are trying to better understand our new BGO crystals.
I spent the last few weeks putting together the test setup in our electronics lab with the help of our team and was finally able to start my first measurement on Friday and everything seems to be working well so far. Woohoo! Now you might be wondering which particles we are actually measuring in our electronics lab when we are not using a radioactive source? They are mainly muons, which originate from high-energy particles from outer space and then fly down to us. I will now continue to work on my bachelor’s thesis and find out new things. See you soon!
We successfully launched a weatherballon with our instrument CHAOSjunior as part of “Highlights der Physik”, a science festival taking place in Kiel. The clear blue sky allowed a launch at around 11:45 am. Many spectators followed the launch and interesting questions were asked, especially by kids. We even had a send-off by a friend and colleague from the Department of Extraterrestrial Physics who accompanied our ballon with his plane. He took the pictures of the balloon from the air. Currently, we are on our way to Rostock to recover CHAOSjunior. Stay tuned for further information on CHAOSjunior and visit our social media channels and the channels of “Highlights der Physik”.
CHAOSjunior is a reduced version of the CHOAS experiment. The sensor head consists of a scintillator of Bismuth Germate Oxide (BGO) and two silicon detectors taken from High Energy Telecope (HET-B) in front of and behind the BGO. Two photodiodes were glued on the hexagonal BGO at opposite sides. The energy deposited in the detectors is proportional to the signals from the detectors. During configuration, the calibration parameter is determined. For this purpose a radioactive source of bismuth isotope 207 Bi is used. The characteristic lines of this source are known relatively precisely and can therefore be used for the calibration. First, the calibration of the HET-B detectors was carried out without BGO. The calculation of the channel names was chosen in following way. CHAOSjunior basically consists of three detectors, one HET-B, the BGO and another HET-B. These are named A,B and C in the direction of particle pass through. The information from the BGO (B) is shared between two channels. These are designated B1 and B2. The two HET-B detectors have only one channel each, these are designated AA and CC respectively. For calibration of preampfilters the sensor units were stimulated by a radioactive source of bismuth isotope 207 Bi. This isotope was placed in front of the sensor head. The prominent emission lines were taken from NUDAT3 (https://www.nndc.bnl.gov/nudat3/ ). The Compton effect of the γ lines at 569.698 keV and 1060 keV could be well observed for both HET-B signals where the first edge is much more precise. The data recorded was adapted by three models, one for the x-ray peaks and one for each Compton-edge. The models deliver the noise s and the calibration parameters u, which allow the conversion of the signals into keV.
