Am Dienstag Mittag, nach dem Essen in der Uni-Mensa, sind wir in den Botanischen Garten gezogen, mit Spaten, Hämmern, Pürckhauer, Tüten und Dosen, um Bodenproben rund um den Neutronenmonitor auszubuddeln.
Das Wetter der letzten Monate war trocken, sehr trocken. Vorher hatte es über ein Jahr durchgeregnet, die Böden ware also sehr feucht. Nun beginnen die oberen Horizonte auszutrocknen.
Der Neutronenmonitor war lange kaputt. Dann wurde das Zählrohr ausgetauscht. Danach war die Zählrate deutlich niedriger als vorher. Das ist zu erwarten, wenn der Boden mehr Feuchtigkeit enthält. Oder liegt es doch am neuen Zählrohr?
In rot ist die gemittelte (2σ=4h) Zählrate der Monitors in Amerika zu sehen
Die blaue Kurve zeigt zum Vergleich die Zählrate des Kieler Neutronenmonitors, der die Intensität der kosmischen Strahlung anzeigt.
Beide Kurven sind um den Einfluss des Luftdrucks (grün) korrigiert.
Man sieht, daß die rote Kurve in diesem Frühjahr im Vergleich zur blauen Kurve wieder ansteigt. Die Bodenproben sollen nun hergeben, genau wie viel Wassen jetzt im Boden ist, bis in 30cm Tiefe und bis zu 150m Entfernung vom Monitor.
Der Neutronenmonitor zählt Neutronen, die von der kosmischen Strahlung im Boden erzeugt werden, als schnelle Neutronen durch die Gadolinium-Hülle des Rohres fliegen, in der Röhre von einem Plastikmoderator auf Umgebungstemperatur gebremst, und endlich von einem ¹⁰B Atom im Geiger-Müller-Zählrohr eingefangen werden. Dabei etsteht ein α-Teilchen, das im Zählrohr ein Signal auslöst. Wenn Wasser im Boden ist, dann werden die erzeugten Neutronen schon dort gebremst. Langsame Neutronen werden von der Gadoliniumhülle eingefangen und machen kein Signal im Zählrohr. Also, je höher die Zählrate, umso trockener ist der Boden.
The next extraterrestrial BEXUS project SETH shall include a new module called SETHAT, to measure the magnetic and gravity field vectors. Of course, this needs to be tested on a weatherballoon mission. The launch was scheduled on Tuesday, April 1st, 11 o’clock, after we watched the forecasts for weeks, while the landing was firmly predicted in the baltic sea between Gdańsk and Malmö. Now, it would land between Rendsburg and Hohenwestedt, safely right in middle of Schleswig-Holstein, with a clear blue sky.
Prediction for a launch on 11:00 CEST from Kiel: Red dot: launch position, green dot Landing, balloon pops at 40km: explosion.
Launch
At 10 o’clock the press arrived to follow the launch preparations. The reporters were underwhelmed by the crowd of three students and one graybeard. But then, unexpectedly, Matti and his Nawi class came along, from the Ricarda-Huch-Schule. The crowd was there, and the class was immediately put to work. Inflating the balloon, calculating and watching the Helium pressure, giding the payload gondola. But the graybeard (me) immediately recognized the bad omen, that Matti’s arrivals induced: all three of Matti’s own Balloons landed off shore.
Matti’s students did a good job, though, the liftoff on 10:42 was prefect.
The Helium bottle went back into storage, we had a quick lunch, and soon where on the road to Schachtholm airport, close to the Breiholz ferry. We got a coffee and some icecream, waiting for the balloon to land on the runway in front of us. In any case, we could quickly get to either side of the canal.
Tracking
The satellite tracker provides positions of our balloon every five minutes. It went to Wischhafen, a little further south than expected, but quickly came back north and turned west heading straight to the airport where we were sitting, almost exactly as expected. Even after the balloon popped, the satellite tracker was still providing positions, from heights up to 40km. Usually, there are no GPS solutions above a height of 20km from those surface trackers.
And those positions were rapidly going south, much faster than the forcasts predicted. Soon, we realized that it would land south of the river. Overhead of the Elbe, our box was still four kilometers high in the sky, … and then it turned west. It came down right in the water, close to Neufeld.
Recovery
We were on the way to the ferry in Glückstadt, when we turned around towards Brunsbüttel and Neufeld. There was only one boat in the harbour, belonging to Hafenmeister Hein Claußen. We got on bord, heading towards the last coordinates submitted by the satellite tracker.
In the beautifull weather it was no problem at all to spot the payload of our mission.
The parachute was full of sand, serving as an anchor. The box was very slowly drifting towards Hamburg with the tide. The anchor kept the box upside down, the custom payload electronics on the bottom of the box was held high above the water line. It turned out that is it still fully functional.
The avionics (SIM trackers, datalogger, video camera) are all corroded beyond repair. Salty water and with the batteries still connected quickly destroyed those units. Only the watertight satellite tracker survived unscratched.
All three storage cards in the gondola were recovered and are readable. We have video from launch and until the battery run out at 35 km height. The card from the datalogger provides pressure, temperatures, further environmental data, and precise GPS coordinates. The later are the source of the flight track shown above. The SD-card in the Raspberry Pi bord computer faithfully recored the output of the SETH Attitude sensor, the 3Diodes cosmic ray detector and furher environmental data.
The mission is a full success.
Video snapshots
Altitude and Weather logged by the Datalogger
Quicklook results straight from the Raspberry Pi
Outside temperature got as low as -50°C. The electronics stayed above 0°C. The sensor temperature is shown here with a wrong calibration. The battery voltage was good until the end. When the pressure reached 800mbar on descent, the unit turned itself off, to protect the electronics against fatal transients on impact. The storage cards are particularly sensitive to transient power disconnections .
The comic ray count rates are highest at around 50 to 80mbar. This is called the Regener-Pfotzer maximum. (This pressure sensor is not rated for pressures below 10 mbar.) The colors denote various trigger conditions. The yellow line is the overall trigger rate.
The pulse height spectra for the whole mission. Each detector diode (A, B, and C) individually count about the same number of hits, with a broad spectrum due to variable path lengths in the sensitive volume. Coincidences for vertically aligned diodes A and B show a narrow sprectrum (blue line, purple dots), due to the contrained path lengths. Coincidences for diodes A and C (red line, green dots) detect particles from a zenith angle around 45°. The path lengths are longer, the spectrum is shifted to larger energy deposits. The projected area is smaller and the flux from that angle may be different. The dependence of the flux on the zenith angle depends on the height, which is not resolved in this picture.
Es sieht hoffnungsvoll aus, daß wir am Samstag Mittag starten können. Die Wettermodelle sagen etwas blauen Himmel voraus und eine Landung zwischen Rostock und Stettin.
Gezeigt sind die Landeorte je nach Startzeitpunkt. Angefangen heute um 7 Uhr, wenn die Landung noch mitten in der Ostsee passieren würde, wie während der ganzen letzten Woche. Die eingezeichneten Flugrouten sind für Starts um 12 Uhr am Samstag. Einmal links, für einen kurzen Flug, wenn der Ballon sehr früh platzen sollte, schnell aufsteigt, und der Fallschirm verheddert kaum Wirkung zeigt. Zum anderen rechts, wie die meisten Flüge bisher verlaufen sind, der Ballon platzt unerwartet spät in 40km Höhe.
Morgen, am Samstag werden um 10 Uhr die Startvorbereitungen auf dem Rathausplatz beginnen. Der Start sollte dann zwischen 12 und 13 Uhr passieren, sobald genug blauer Himmel zu sehen ist und die Flugsicherung die Starterlaubnis erteilt.
Diese Sonde mit Fallschirm sollen fliegen. Darin ist ein Instrument zur Messung primärer und sekundärer kosmischer Strahlung.
Last Wednesday, September 6, the Ricarda-Huch-Schule in Kiel launched their third Weatherballon mission.
And just as with the last two missions, this mission ended in the water as well. We filled an extra large volume of Helium into the balloon, 93bar×50l, to make sure it would rise fast and burst early, to safely land close to Oldenburg in Holstein.
That was the prediction. We reached 5m/s rise speed, but the balloon burst at 40.7km height, five kilometers higher than predicted. The real flight dropped our sensor box about six nautical miles east of Dahme.
Of course, we did not know about that at first. We spend the time waiting for the GPS trackers to come in range in an ice cream cafe in Lensahn. When the satellite radio tracker finally send the first messages descending into GPS range below 10km height, we got the first idea that the track may be way to far east. The message after touchdown confirmed that. The satellite radio tracker did not send further messages, because the box was floating at very low speed that did not trigger the motion alert. Anyhow, we went to Grömitz and hired a boat to look for it. The boat with one teacher and three pupils returned empty handed after more than five hours.
The next morning, we got a scheduled daily message from the tracker, the box drifted north over night. Shall we make a hike along the beaches from Dahme to Großenbrode? No, the next morning the box reached the Fehmarn Belt and set of to continue through the Store Belt. But here the currents were so fast, that we got new positions every five minutes. Another boat was put behind a car, went to Fehmarn, launched into the water and picked the box out of the water.
What did we find inside the box? The electronics was mostly destroyed by some salty water and the voltages still applied. The SDcards of the video camera and the primary payload, an Arduino reading the pulses from a Geiger-Müller counter, were readable. But the Arduino stopped working properly before the balloon launched. There was no data except for an hour before launch. The raw video awaits processing and compression. The STRATO3 datalogger is in very bad shape. Its SDcard is very exposed and the contacts were eroded. Two contacts were gone. But that has never been a problem in the past. We scratched the plastic a little, exposed all contacts, and soldered an adapter to the card, that could be inserted into an SDcard reader. The data was fully recovered, covering the whole flight. That is how we know where our balloon was flying. The datalogger has a specially hacked GPS chip, that works up to a height of 50km.
The pressure data shows a very remarkable feature. At a height between 20km and 30km the pressure did not fall monotonically. There were strange fluctuations. Seen here in purple, with the battery voltage in yellow, and inside (green) and outside (blue) temperatures.
Looking at the GPS data, the strange fluctuations happened while the balloon was tumbling around the Bungsberg, the highest mountain of Schleswig-Holstein, at 168m.
So, what happened to the first two missions? The first mission landed sixty nautical miles west of Rømø. The balloon did not burst at all when it arrived at 40km height. It was floating at that height for another hour before it finally burst. It was found after a month by two girls in Sweden. All SDcards were readable in the end.
The second mission dropped down one nautical mile east of Schleimünde. We went to Maasholm, found a boat and recovered the box after a few hours. The video coverage was complete and showed how the crew of a sailing boat found the box, looked at it and put it back into the water. The big letters on the box read: “Safe Scientific Experiment”. The small print talked about recovery and phone numbers. The crew was afraid to disturb important measurements. The datalogger SDcard failed. The datalogger was later fixed. It turned out that a regulator was damaged by corrosion and put the full battery voltage to the SDcard, instead of a much lower supply voltage.
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.
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.
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.
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.
Welcome to the internet presence of the Chaos project. For the next two years you will find updates of the project in this place. The server is a virtual maschine provided by netcup.de.
