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.
The heart of our experiment CHAOS is the Cherenkov detector. High energy particles travelling through this detector will create photons and because only few photons are created, a Photomultiplier Tube (PMT) is needed to detect them. This makes the use of High Voltages (HVs) necessary to operate the PMT and poses a security risk for our experiment. During the BEXUS balloon flight CHAOS will be exposed to low pressure environments. The combination of high voltages and low pressures can lead so-called corona discharges which could possibly harm the experiment. The easiest way to mitigate this risk is to place the experiment inside a pressure housing which ensures the same ambient pressure as on ground.
Here you can see the CHAOS pressure housing. It consists of an aluminum base plate on which a die-cast aluminum box is screwed. We use two additional U-profiles which press the box onto the base plate.
But we cannot simply put our pressure housing onto the BEXUS balloon. First, we have to prove that our design really works. This is why we performed several tests:
In a first step, we inflated our pressure housing with a tire inflator. Maybe not the most scientific way, but it worked. Sometimes scientists have to be creative. We were able to show that our pressure housing has no problems to withstand an additional pressure of 1.2 bar. During the BEXUS balloon flight the maximum pressure difference between the inside and outside of the pressure housing can be 1 bar.
But that was not enough. We decided to put our experiment in the vacuum chamber at our university to perform a professional vacuum test. For the test we placed a pressure sensor inside the pressure hosuing as seen in the following pictures:
The pressure housing was put in the vacuum chamber which was then evacuated to a pressure of 0.1 mbar. We left the pressure housing inside the vacuum chamber for a total of four days. The results can be seen in the following plot:
The red curve is the temperature inside the pressure housing and the blue curve the pressure. The big spikes in temperature and pressure are caused by the heating plate inside the vacuum chamber which we turned on for several hours. The measured pressure was corrected for the temperature using the ideal gas law. This is the green curve. We can see a linearly decreasing pressure. This leakage was expected because a perfectly airtight pressure housing is hard to build. But we only lost around 100 mbar in four days. This is totally acceptable because the BEXUS balloon flights only last several hours. To quantify our results, we performed a linear regression on our pressure measurements. This regression was then used to calculate a leakage rate of around Q = -0.003 mbar*l/s.
We are very happy with the results of our tests. Hopefully, they will convince the board of experts at our Critical Design Review (CDR) as well. The CDR will take place at ESTEC in the Netherlands in May. So, stay tuned and follow us on our journey!
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!
This afternoon, the styrofoam boxes, which we ordered, arrived at our institute. We plan to place our experiment CHAOS inside one of these boxes for thermal control during the flight on the stratospheric BEXUS balloon. This approach was successully used by other BEXUS experiments from our Department before. These boxes were designed to transport and cool food and drinks, not to be flown on a stratospheric balloon. To test wether the styrofoam withstands the expected low pressure environments, we spontaneously decided to put it into our vacuum chamber and start a test. The styrofoam encapsulates tiny bubbles of air. In a low pressure environment this air wants to expand and can become critical if the syrofoam loses its structural integrity because of this expansions. Unfortunately, we could only place the lid of a box inside the chamber because of the size of the box.
The results of our test can be seen below. The plot shows the pressure of the backing pump (p2) and within the chamber (p1). We can see that we reached pressures of less than 0.01 mbar. This is three orders of magnitude below the expected pressure environment during the BEXUS flight. In the last picture we see the comparison of the tested styrofoam (top) with an untested lid (bottom). The tested lid shows some open pores, but the styrofoam kept its structural integrity. Therefore, we conclude that the styrofoam boxes are ready for flight.
CHAOS has successfully started into the year 2024. The next big milestone on our journey is the Preliminary Design Review (PDR) in Kiruna, Sweden at the beginning of February. At the PDR we will present the current status of our experiment design. But first, we have to hand in our first version of the SED (Students Experiment Documentation) at the end of January. This document includes all relevant information regarding our experiment. Currently, we are finalizing the design of CHAOS to include it in the SED and present it at the PDR. It is a lot of work but also a lot of fun. We have been told that it might be cold in Kiruna, but the polar lights make up for it. Therefore, the work will be definitely worth it. Stay tuned for more information on CHAOS and our journey to Sweden.
CHAOS has been selected for the 15th cycle of the BEXUS program! The next step of our journey will be the Preliminary Design Review (PDR) in February next year in Kiruna, Sweden, where we will be presenting the current status of our experiment. The final flight will take place next fall, also in the Esrange Space Center in Kiruna.
Thank you @germanaerospacecenter for selecting our experiment, we are very excited about this opportunity!
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!
