Category: Hardware

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First Vacuum Test

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.

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Preparations for the PDR

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.

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CHAOS-Junior in the Box

The sensor head for the CHAOS-Junior weatherballon mission was put in the styrofoam box. The area was made light tight with black ducttape. The CPU of the Raspberry Pi Zero and the FPGA of the RPiRENA DAQ board were glued to copper bands that extend through the styrofoam, where another copper foil was placed outside. The copper bands were fixed with copper tape and covered with a second surface mirror foil used for space missions. That foil emits heat radiation but reflects sunlight. Two GPS trackers and a battery box were placed in the box. One of the GPS trackers sends the NMEA messages to the serial port of the Raspberry Pi (via the FPGA), to be recorded in the data file. A STRATO3 datalogger will be added. Maybe we will find a camera to add as well. The Raspberry Pi is online.

CHAOS-Junior payload box with sensor head integrated.
Sun reflection foil.
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Successful Test of a Geiger-Müller Counter in Vacuum

For a Weatherballoon mission lead by the Ricarda-Huch-Schule we prepared and tested a Geiger-Müller Counter module. The challenge is to operate the high voltage supply of the tube in the partial vacuum of the stratosphere. We may encounter a Corona discharge, which may destroy the electronics or at least impact the measurements. We are all happy to leave Corona behind us. The second issue is the internal pressure of the tube. We passed all tests. Down to 1mbar pressure the unit worked flawlessly.

First, the tube was tested individually. It survived low pressure. The HV was calibrated to 400V as prescribed in the manual. Then we removed the calibration connecter and a power connector that were too close to the HV electrode of the tube. The board was coated twice with a space grade 2-component silicone encapsulant, in particular the HV supply and the area close to the HV end of the tube. We obtained an insulation distance of about 4cm.

The Paschen Curve tells us that we should be good down to a few mbar.

First, we tested in a glas chamber, to visually observe the test. But that pump could not go below 60mbar. Then we used our regular thermal vacuum test stand for balloon missions, shown above. The test was performed at room temperature. We used only the rough vacuum pump, which took the chamber pressure down to 1mbar. The output of the module was monitored with an oscilloscope. A ²⁰⁷Bi souce was place inside the chamber to increase the counting activity. It worked well. Unfortunately, we did not take any pictures.

Vio did the coating, see the log sheet above. Ronja soldered the feedthrough harness and performed the Vaccum tests. Sönke observed radiation safety. Matti provided the counter module.

Now we are waiting for the flight forecasts to predict a landing site on land.

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Gluing large BGOs

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.

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Solid State Detectors for Chaos-Jr

Today Myrdin and Stephan started to put together the sensor head of Chaos-Jr. Two spare Solo-EPD-HET-B detectors were put into a spare HET housing. The wires were soldered to the preamplifier board that was assembled by Sophie. The sensor is now connected to a RPiRENA on a Raspberry Pi1 and is taking data. Next we will put a ²⁰⁷Bi Source next to the sensor to perform a calibration with gamma and X-rays.

Observe how two preamplifiers are still unconnected. That is where the photodiodes from the BGO scintillator will be connected.

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Revival of an old Detector

The proposed CHAOS instrument needs a fairly large area Solid State Detector (SSD). All Detectors that we can use are spare parts from earlier space missions. For this large area SSD we put our eyes on a couple of recently rediscovered Soho Costep EPHIN F-Detectors, manufactured in the 1980ies.

A quick test revealed that all but one of them have excessive bias currents. The one good detector, unit F2, was subject of a C-V measurement, Capacitance versus bias Voltage. That kind of measurement tells about the impurity concentrations (dopands) and thickness of the detector.

The results confirm that the detector is about 700µm thick, and needs about 150V bias to be fully depleted, which is in range with the expectations. The diameter of the SSD is about 8cm.

This EPHIN F2 detector may become the anticoincidence counter of CHAOS. Currently it is tested in operation with cosmic muons and a gamma ray source.

C-V test instruction