Purpose of the flight and payload description

SMILE (Sub-MeV gamma-ray Imaging Loaded-on-balloon Experiment) was a series of experiments to observe sub-MeV gamma rays from celestial objects by using a balloon-borne camera. It was developed in middle 2000s decade at the Kyoto University in Japan.

Working principle

Unlike visible light or X-rays, gamma rays at MeV wavelengths are too short to focus by a mirror or lens. Previous Compton telescopes (space-based and balloon-borne) localized the incident direction of gamma rays on an event circle superposed on the sky, by measuring the direction of a scattered gamma ray and the energy of both a Compton recoil electron and the scattered gamma rays. However, a significant background reduction could be achieved by restricting the incident direction to a reduced arc on the Compton circle, by measuring the track of the Compton recoil electron. Based on that principle, the Kyoto group developed under the SMILE program a new MeV gamma-ray imaging detector capable of measuring three dimensional tracks of Compton-recoil electrons in order to explore the all sky in the band between 0.1 and 30 MeV.

The insrument

In the figure above left can be seen a scheme of the SMILE-2+ prototype of electron-tracking Compton camera (ETCC). It had a sensitive volume of 30 x 30 x 30 cm3 filled with an argon-based gas at a pressure of 2 atm. To make a precise three-dimensional tracking of electrons for the gamma-ray reconstruction, a time-projection chamber (TPC) with a micro-pixel chamber (µ-PIC) and a gas electron multiplier insulated by 100 µm liquid crystal polymer was adopted.

As the gamma-ray absorber, were selected Ce-doped gadolinium oxyorthosilicate GSO(Ce) pixel scintillator arrays (PSAs), each containing 8 x 8 pixels. The pixel size was 6 x 6 mm2. The GSO scintillator was 26 and 13 mm thick at the bottom and sides of the electron tracker, respectively. To efficiently absorb the scattered gamma-rays, were placed 36 PSAs at the bottom and 18 PSAs at each side of the tracker. The total number of scintillation pixels was 6912. For the photo readout, a four-channel (ch) charge division method with a resistor network and multi-anode photomultiplier tubes was adopted. The PSAs were placed inside the TPC vessel while at the top of it was installed a 5 mm thick plastic scintillator to reduce the number of triggers by charged particles.

The SMILE-2+ ETCC was set above the control system, as shown in the figure. The control system included a central processing unit (CPU) for communication with the balloon control system, two CPUs with 1 TB solid-state drives for data acquisition, the trigger control unit, four high-voltage units for the TPC, a power management system with DC/DC converters, and lithium batteries. SMILE-2+ also had a receiver with a global positioning system, an atmospheric pressure gauge, two clinometers, and three geomagnetic aspectmeters (GAs) to measure the gondola attitude.

The entire instrument was sealed in a pressured vessel maintained at 1 atm. The outer vessel was covered on its side by multilayered insulators for temperature maintenance and placed on a small aluminum gondola. It carried an independent piggyback sensor that measured the attitude with three GAs, three accelerometers, and a gyroscope that was later checked for consistency against the attitude data determined by the SMILE-2+ sensors. The entire gondola weighed 511 kg, plus ballast.

Details of the balloon flight

Balloon launched on: 4/7/2018 at 6:24 acst
Launch site: Australian Balloon Launching Station, Alice Springs, Australia  
Balloon launched by: Japan Aerospace Exploration Agency (JAXA)
Balloon manufacturer/size/composition: Zero Pressure Balloon model B500 500.000 m3
Flight identification number: B18-02
End of flight (L for landing time, W for last contact, otherwise termination time): 4/9/2018 at 11:07 acst
Balloon flight duration (F: time at float only, otherwise total flight time in d:days / h:hours or m:minutes - ): 28 h 43 m
Landing site: 190 km SSE of Alice Springs, Australia

The balloon was successfully launched from the Australian balloon launch station in Alice Springs, on April 7, 2018, at 06:24 Australian Central Standard Time (ACST). After a nominal ascent of near 2 hours it reached a float altitude of 39.6 km and remained in flight fro 29 hours. Mission was terminated at 10:53 ACST on April 8, 2018.

External references

Images of the mission

      Image of the balloon taken by David McDonald who captured it from Alice Springs using a Canon T3i digital camera coupled to a Skywatcher 2700 mm Cassegrain-Maksutov telescope.  

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