The instrument (also known as MASS-2 or MASS-91 in the specialized literature) was designed to measure antiprotons in the range of energies between 4 and 20 GeV and positrons from about 4 to 10 GeV. It used the same configuration of the MASS-1 experiment flown in 1989, except that the tracking system was improved by installing drift chambers.
The instrument design -as many others of those years- was an adaptation of the basic configuration of the Balloon-Borne Magnet Facility (BBMF), a payload developed by New Mexico State University and NASA's Goddard Space Flight Center to perform research on high energy particles using a magnet spectrometer.
A schematic diagram of the instrument used in this experiment is shown at left (click to enlarge). It was composed by a superconducting magnet spectrometer, a time-of-flight device, a gas cherenkov counter and a streamer tube imaging calorimeter.
The magnet spectrometer consisted of a single coil superconducting magnet that produced a maximum field strength of 2.2 Tesla, along with a hybrid tracking system that consisted of 2 modules of drift chambers and 8 planes of multiwire proportional chambers (MWPC) adding up to a total height of 110 cm. Each drift chamber (DC) module consisted of 6 layers of hexagonal drift cells in the bending direction and 4 layers in the non-bending direction, operating with a gas fill of CO2. The MWPCs were filled with a mixture of Argon-Isobutane-Freon and read out by a cathode coupled delay line system. Three MWPC layers were placed at the bottom of the tracking system, three were located between the two DC modules, and two layers were kept at the top of the tracking device. Four MWPCs were instrumented to read out both coordinates, and the rest measured positions in the bending direction.
The time-of-flight device (TOF) was made of two planes of scintillators separated by 2.36 m. The upper one was at the top of the gondola and it consisted of two layers of scintillators, each segmented into 5 paddles of 20 cm width and variable lengths to match the round section of the cylindrical payload structure. Each paddle was viewed by a phototube at the opposite end in each layer. The bottom plane of the TOF system was located just below the tracking device and had one layer of scintillator segmented into 2 paddles, viewed by phototubes at both ends. The coincidence between signals from the two planes provided the trigger for data acquisition.
Signals from each paddle were independently digitized for the TOF measurement to determine the direction and velocity, and for the pulse-height analysis to obtain the ionization loss in the scintillators.
The Cherenkov radiator was located at the top of the gondola, just bellow the upper TOF plane in a 1 m tall cylinder isolated from the rest of the structure. It was filled with Freon-12 at a pressure of 760 Torr at the ground. The Cherenkov light was reflected and focused by 4 segmented spherical mirrors onto four phototubes.
The imaging calorimeter was located below the bottom scintillator and was filled with 50 cm long brass streamer tubes of cross-dimension 9 x 7 mm2. There were 40 horizontal layers, each consisting of 64 streamer tubes. The tubes in alternate layers were arranged perpendicularly to each other in order to obtain a three dimensional view of an event passing through the calorimeter.
Balloon launched on: 9/23/1991 at 14:00 utc
Launch site: Scientific Flight Balloon Facility, Fort Sumner, (NM), US
Balloon launched by: National Scientific Balloon Facility (NSBF)
Balloon manufacturer/size/composition: Zero Pressure Balloon N29I-8/8/8/8T-28.40
Balloon serial number: R28.40-3-117
Flight identification number: 316N
End of flight (L for landing time, W for last contact, otherwise termination time): 9/24/1991 at 12:49 utc
Balloon flight duration (F: time at float only, otherwise total flight time in d:days / h:hours or m:minutes - ): 23 h
Landing site: 23 miles W of Corona, New Mexico, US
This balloon was launched by dynamic method using a crane as launch vehicle at 8:00 local time on September 23, 1991.
After a nominal ascent phase, the balloon reached the float altitude of 31.7 km at 10:12 am. Ten hours into the flight the instrument magnet failed.
After a total flight time of near 23 hours the balloon was terminated.
No data is available about the landing place.
During the flight the instrument performed well except by the fact that after 10 hours the magnet ceased to work. Also one MWPC chamber was not working during the flight.
During the 10 hours of flight during which the magnet worked a total of 552 electrons between 5 and 40 GeV, and 37 positrons between 5 and 14 GeV were detected.
The observed positron flux in the energy range 7-16 GeV was approximately an order of magnitude lower than that of electrons, as measured in other experiments at various energies.