The next milestone on the road to the CHAOS launch is completed. On July 30th, two experts from ZARM visited us in Kiel to check on our integration process. We told them about the changes we have made since the CDR in Nordwijk and shown them our assembled instrument. The successful integration took place around two weeks ago, where we assembled the sensor head as well as the electronics to form a successfully measuring instrument. We are currently running a lot of test measurements to fully understand the instrument since it is the first time that an instrument-design as such has been integrated.
This current progress was presented to the experts from ZARM who were happy with the status. We hope to get the official pass soon. Then, the next step is the thermal vacuum test and the EAR (experiment acceptance review) at ZARM in Bremen, Germany in the first week of September. Until then, we will finalize the integration process and continue with the testing to minimize errors. Stay tuned for the journey!
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.
From 15th to 17th May 2024, we went to ESTEC (European Space Research and Technology Centre) in Noordwijk, Netherlands. There we had our CDR (Critical Design Review). Again, we presented progress we made on our experiment to the review board, which was made up of experts from DLR, ESA, ZARM and SSC. Even though the board had some useful comments and tips for us, we passed the review, and our experiment design was accepted. Now, we can focus all our efforts on integrating the CHAOS instrument. The next milestone will be the IPR (Integration Progress Review) at the end of July. Two experts from ZARM will visit us in Kiel and inspect the integration process.
Although it was an exhausting couple of days, we had a lot of fun in the Netherlands and used the time to explore the cities of Leiden and Amsterdam. Stay tuned for more information on CHAOS!
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.
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!
