Description of the payload

An instrument optimized for the measurement of the cosmic ray antiproton flux up to at least a range of energy of 50 GeV, that identifies particles through the combination of a superconducting magnet, a drift tube hodoscope (DTH), time-of-flight scintillators (ToF), and multiple energy loss wire chambers (dE/dx). At left can be seen a scheme of it (click to enlarge)

Its an evolved version of a similar experiment that was focused on the detection and identification of cosmic-ray electrons and positrons, flown succesfully in 1994 and 1995.

The superconducting magnet, with two circular Niobium-Titanium coils held at 4.2 K in a liquid-helium bath, provides an approximately uniform field of about 1 Tesla within a rectangular room-temperature bore. The cryostat employs a vapor-cooled radiation shield and superinsulation to isolate the inner helium vessel from the ~300 K environment. The total liquid helium capacity for the system is 260 liters, with a maximum hold time of ~7 days.

The drift-tube hodoscope (DTH) consists of 479 thin-walled drift tubes, each 2.5 cm in diameter, arrayed in 26 layers. Of these, 18 layers are aligned with the tube axes parallel to the magnetic field, and provide trajectory measurements used to determine rigidity. The remaining 8 layers, aligned with axes normal to the magnetic field, provide the additional information required to reconstruct trajectories in three dimensions. A drift gas based on CO2 with hexane as an organic quencher flows through the tubes.

The time-of-f1ight detector discriminates against upward particles, which can masquerade as antiprotons. In addition, ToF signals are combined with coincident hits in S (a separate layer of scintillator in the magnet bore just above the DT hodoscope) to provide a fast trigger. The ToF detector consists of two 1 m x 1 m layers of plastic scintillator separated by 2.76 m. Each scintillator layer is divided into four paddles 1 cm x 25 cm x 1 m in size and each end of a paddle is viewed by a photomultiplier tube (PMT). Each PMT detects approximately 40 photoelectrons for a vertical atmospheric muon traversing the center of the paddle. For a triggered event, timing and charge-integrated signal size are digitized for each participating PMT. Thus, upward and downward going particles are easily distinguished by the TOF detector.

The dE/dx detector system contains a total of 140 multiwire proportional chambers, each with a thickness of 1 cm, and is divided into two stacks, one above and one below the magnet. A combination of Xenon and Methane (95% and 5% of the mixture respectivelly) provides the highest possible velocity resolution and stability. Anode wires are connected in groups of 4-6 and read out by an AMPLEX amplifier. Custom 8-bit ADCs digitize the amplified signals.

Details of the balloon flight and scientific outcome

Launch site: Scientific Flight Balloon Facility, New Mexico, US  
Balloon launched by: National Scientific Balloon Facility (NSBF)
Balloon manufacturer/size/composition: Zero Pressure Balloon 1.100.000 m3 - SF3-39.57-.8/.8/.8/.8-NHR
Balloon serial number: W39.57-3-23
Flight identification number: 483N

The balloon was launched by dynamic method with assistance of launch vehicle (Big Bill) in the morning of 21 May. 2000. Weather conditions for the launch were good and launch operations were nominal. The balloon was released from the spool and rose overhead in a normal fashion.

The balloon developed a hole immediately after the launch collar was released. The payload was launched and the balloon system began ingesting air. It ascended to approximately 33.000 ft. where it began to show an anomalous ascent profile. The flight was eventually terminated at 44.000 ft.

An investigation was started. The comitee found no specific anomalies with the balloon or launch process and concluded that the most likely cause of balloon failure was the inability of the balloon to adjust to, absorb and/or tolerate the stresses present during the dynamic launch process.

The balloon was carrying the maximum allowable payload of 3629 kg. and had been inflated to a gross inflation of 6591 kg. The committee found a correlation between payload weight and failure rate which indicated that the success rate of balloon flights diminishes with the increase in Launch Stress Indices and gross inflations.

After this flight they recommended that heavy payloads continue to be flown with the realization that flights with heavy payloads, high Launch Stress Indices and high gross inflations carry a higher probability for failure.

No data was obtained from the flight due to the balloon failure.

After the mission was aborted the flight was terminated and the payload landed close to the launch base essentially undamaged. The field crew then removed the instrument from the shell and performed an improvised hydrostatic test to ensure that the gondola, which had taken some dents landing, was still sound. Simultaneously, another team exhaustively checked, refurbished, and tested the instrument, which was determined to be in perfect working order.

Within 72 hours the instrument was back in the gondola, working perfectly, and again ready to be launched.

It was succesfully reflown two weeks later from the same place.

External references and bibliographical sources

Images of the mission

HEAT properly mounted in his gondola ready to go to the launch pad. The balloon is released. Notoriously, the balloon fabric is sailing A detailed view of the balloon bubble during ascent The ascending balloon has an strange envelope shape. Minutes later was confirmed that he has a leak. A perfect landing: he gondola hit ground almost vertically.