SUPERCONDUCTING MAGNET SPECTROMETER

The objective of the balloon flight was to search for antiprotons in the cosmic rays using a superconducting magnet spectrometer developed by the Physical Science Laboratory in Las Cruces based on a previous apparatus developed by the same scientific group at the NASA's Johnson Space Center.

A basic scheme of the magnet spectrometer and the container gondola is shown in the figure at left (click to enlarge). The spectrometer has two main components: the SUPERCONDUCTING MAGNET and the MULTIWIRE PROPORTIONAL COUNTERS (MWPC) stack. The magnet was a scaled-up version of the first superconducting magnet to fly successfully on a balloon by the NASA JSC group back in 1969-1970. The new magnet was 61 cm in diameter and produced a magnetic field of ~40 kilogauss at the center of the coil when operating at a current of 120 Amperes. It weighed 150 kg and was maintained at a temperature a few degrees above the absolute zero inside a cryostat cooled by liquid helium. The purpose of the magnet was to deflect the course of heavy particles reaching the detector from outer space, so that the charge and momentum of each particle -its characteristic signature- can be measured.

The MWPC stack, consisted of eight identical 50 x 50 cm2 MWPCs modules staggered horizontally so that the top chambers are closer to the magnet face. This increased the effective maximum detectable rigidity of the spectrometer. The MWPCs position measurements were digitized using distributed inductance delay lines coupled to the chamber wires. The chambers were oriented with the anode wires perpendicular to the magnet axis so that most of the bending in the magnetic field occurs along the cathode coordinate axis. Position measurement, in each coordinate axis, was obtained by measuring the time of arrival of the signal at each end of the delay lines.

A gas Cherenkov counter, was located above the MWPC stack. The detector was filled with air or sulfur hexafluoride depending on the flight objectives. Its aperture was divided equally by four spherical mirrors, each of which was viewed by a 12.5 cm diameter RCA 4525 photomultiplier (PM) tube through a conical mirror that acted as a high transmission lens. The overall length of this detector was 1 meter, with an effective path length of 80 cm.

All scintillators, S1, S2, P1-P7, were 0.6 cm thick Pilot Y. Scintillators S1 and S2 were each viewed by two 5 cm diameter RCA 4523 PM tubes through two adiabatic light pipes, and all other scintillators were viewed by single RCA 4523 PM tubes through conventional light pipes. Except for P1, which was 45 x 45 cm2, all of the scintillators were 50 x 50 cm2. The scintillators P1 - P7 were each separated by 1.2 radiation length of lead thereby forming a shallow shower counter of 1.2 radiation lengths depth. This counter provided a rough energy determination (30%) of the electron events and was crucial in reducing the proton contamination of positron measurements.

The signals from the scintillators and the Gas Cerenkov system were pulse height analyzed and digitized with 12-bit resolution. These values, along with the outputs from the MWPC time digitizers, were telemetered and recorded on ground along with a flag to indicate the type of coincidence event. In addition, the time of flight between S1 and P1 was measured to determine the direction of motion of the incoming particle. The digitized data, together with 100 channels of sub-commutated engineering data (voltages, temperatures, pressures and logic state) were transmitted to the ground via a 48 kbit/s telemetry link. A 100-channel command link was used to control the payload on the ground and during flight.

The availability of the data in real time, and the possibility of actively controlling discriminators, detector voltages and logic circuitry pushed the development of a mini-computer based real-time data processing and experiment control system. A data van equipped with a Digital Equipment Corporation PDP-8E and PDP-11/10 computers was used for interactive payload control. Scientific data was presented to the experimenters in graphic and tabular form. Information was also available for examination in real-time on pulse height spectra, MWPC resolution, etc. Commands were issued to the payload to control the logic and detector operating conditions in order to optimize key parameters (e.g., chamber resolution and experiment dead-time). All commands were issued by the computers in response to keyboard mnemonics typed by the observing teams. In a typical flight about one hundred keyboard command sequence mnemonics were typed. These result in roughly one thousand actual two-digit commands being sent to the payload. Experience from first flights demonstrated that this interactive control technique was an invaluable aid in achieving high quality data from the payload.

The instrument and all its components, including electronics and batteries were encased in an insulated gondola 5 meters tall and 2.4 meters in diameter with a total mass of 2100 kg.

Balloon launched on: 5/20/1976
Launch site: Columbia Scientific Balloon Facility, Palestine, Texas, US  
Balloon launched by: National Scientific Balloon Facility (NSBF)
Balloon manufacturer/size/composition: Zero Pressure Balloon Winzen 612.093 m3 (22.86 microns - Stratofilm)
Flight identification number: 969P
End of flight (L for landing time, W for last contact, otherwise termination time): 5/22/1976
Balloon flight duration (F: time at float only, otherwise total flight time in d:days / h:hours or m:minutes - ): F 40 h
Landing site: In the Navarro Mills Lake, Navarro County, Texas, US
Payload weight: 3011.5 kgs.

The balloon was launched from the National Scientific Balloon Facility in Palestine, Texas as mission 969-P in the morning of May 20, 1976. There is a confusion in the dates published in the papers regarding the date of the balloon flight. After checking the National Balloon Facility Records for the epoch, I concluded that the correct date of the flight was May 20, 1976. Flight 969-P also established a new record for the balloon facility using the dynamic launch technique with a gross lift of 5243.7 Kg (counting payload and balloon).

After a nominal ascent at a speed of 244 meters per minute, the balloon reached the predetermined altitude of 34.4 kilometers. Initially the balloon moved slowly east of Palestine, then changed direction in the light winds of the turnaround period and moved west. Night passed, then another day. By the second night the balloon has been aloft more than 25 hours when finally the batteries in the payload ran down and the experiment was turned off.

When the Sun rose the next day, the balloon was hovering over Waco, Texas. After calling FAA for autorizathion to terminate the flight, the chase plane proceeded to send the termination command but nothing happened. A second plane was sent after the balloon with another equipment to terminate the flight but also that failed. At 14:30 the team that was following the balloon prepared for the activation of the onboard backup timer that would separate the payload from the balloon but this system also failed. A that moment there was only one remaining system: if the balloon could be brought down to 16.5 kilometers, a cutdown system triggered by barometric pressure would be activated. From Palestine, the balloon was commanded to vent helium. That control worked and the balloon, which was by then moving west toward Hillsboro, Texas, slowly started to descent. When it hit 16.5 kilometers nothing happened. The barometric cutdown had also failed to separate the balloon and parachute. More helium was vented to lower it to 14.6 kilometers.

After a quick consultation, it was decided that simultaneous failure of several redundant systems was highly improbable; the difficulty must be a hang-up in the mechanical separation unit itself. It was also decided that a rapid descent of the system would cause the extended parachute partially to open, relieving pressures on the mechanical fitting which should allow separation. Commands were transmitted to open the helium exhaust valves and, after a short reaction time, the system began a rapid descent. At about 4.5 kilometers the parachute billowed and separation occurred.

The payload descended under parachute right into Navarro Mills Lake. The recovery team, guided by radio from the chase plane, arrived at the scene quickly and borrowed a motorboat from a bass fisherman and towed the payload to shore. Then the instrument was loaded onto a truck and driven back to Palestine where technicians checked it. It was in excellent condition and working properly.

The primary objective of the flight was to track positrons and develop a more useful data base for adding the data to the one obtained in previous flights. A secondary goal was to look for the first evidence of anti-protons (protons with a negative charge) in the cosmic radiation.

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