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.
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.
Last week the big BGOs have arrived. One alone weighs over 1kg. Currently, two BGOs are covered with photodiodes. One with two on opposite sides and one with three each at an angle of 120 degrees. Subsequently, the dependence of the light output (sum of the diode signals) on the number and arrangement of the diodes is to be determined experimentally.
After performing a calibration measurement for the two HET-B detectors, the BGO was now also integrated into the sensor head and the two glued-on diodes were connected to the two free slots on the preamplifier board. The gain was changed to 3 in advance. Additionally two temperature sensors were connected. One measures the temperature directly at the BGO mount, the other is intended to look out of the box during flight and measure the environmental temperature.
Trigger thresholds for each channel were set to 5mV. In coincidence, the two BGO diodes trigger from even 3mV. First measurements are running and results will follow shortly right here.
Today we have submitted our letter of intent to the BEXUS program marking our first steps to a possible participation in the next BEXUS-cycle
Our instrument CHAOS (Cherenkov Atmospheric Observation System) represents a scaled-down version of the ATHENA High Energy Particle Monitor (AHEPaM), which is designed to measure the background radiation of the ATHENA mission.
The preamplifier boards for CHAOSJunior were soldered according to slightly modified scematics of the High Energy Telescop (HET) experiment. For the detector setup of the balloon flight in September 2023, only four preamplifers are necessary and only those four were assembled for now. Since the HET preamp board has room for twelve preamps in total, more can be added at a later time for the detector setup of CHAOS. The assembled preamplifier board was tested for any short circuits and function of the operating points. The boards are working as desired and can now be tested further with attached detectors.
From September 25 to 30, 2023, the science festival “Highlights der Physik” will come to Kiel. As part of this event, we plan to fly a weather balloon with a CHAOSJunior experiment. This consists of a small hexagonal bismuth germanium oxide scintillator (BGO) with two photodiodes glued on opposite sides and two segmented solid state detectors (SSDs), which were adopted from the High Energy Telescop (HET) experiment. These HET-B detectors are placed in front of and behind the BGO and are used for coincidence measurements. In the past weeks, two photodiodes were glued to the BGO on opposite sides. The so prepared BGO was wrapped today in such a way that the light generated in the BGO is reflected on all inner sides and reaches the diodes. This BGO with diodes will be first tested in an already existing test can the following days. In the meantime, a pre-amplifier board for the flight is soldered and connected to the BGO diodes and the HET-B detectors. Then this setup will be tested.
BGO wrapped in teflon tape and put into old casing from HET
