POSITRON-NEGATRON (e±) EXPERIMENT

The objective of the POSITRON-NEGATRON (E±) EXPERIMENT was to measure the cosmic-ray positron and negatron spectra between 15 and 1500 MeV using a Magnetic Spectrometer which measured the charge sign and magnetic rigidity (momentum divided by charge) of charged particles by determining their deflection in a magnetic field. The instrument was developed at the California Institute of Technology.

In the figure at left we can see a basic scheme of the detector used between 1970 and 1973 (click to enlarge). It consisted of a magnetic spectrometer utilizing a 2300-gauss permanent magnet, scintillation counters, a lucite Cerenkov counter and an additional gas Cerenkov counter. An "event" (observation of a charged particle) was defined by a triple coincidence between Telescope Counter (Tl), Telescope Counter (T2), and the Lucite Cerenkov Counter (LC), and the absence of a pulse from any of the guard counters. This coincidence produced the fast-gate pulse (FG) which triggered the high voltage to the spark chambers and initiates the data read-out cycle. The two 4-gap spark chambers were used to define the particle's trajectory before and after passing through the gap of the permanent magnet.

The Lucite Cerenkov counter was designed to discriminate against upward-moving splash albedo particles and to eliminate the nucleonic component of the cosmic rays in the rigidity interval of interest. The gas Cerenkov counter (GC) was added to this improved version of the detector in order to eliminate contamination due to cosmic-ray nuclei above the LC threshold (because of the larger magnet used these particles would be indistinguishable from high-energy electrons) and to further discriminate against upward-moving particles. It contained a gas mixture of sulfur hexafluoride at 2.2 atmospheres absolute pressure. The two flat mirrors served to reflect the Cerenkov light into the phototube faces. The mirrors were constructed of 1/8-inch lucite and were aluminized on their upper surfaces. The conical mirrors were made of spun aluminum with their interior surfaces aluminized. Each of the two phototubes of the counter acted independently; a coincidence between the fast-gate pulse and the output of one phototube generated a data bit which was recorded as part of the event's data word.

The specially designed Magnet Guard Counter covered the top of the magnet and completely lined the gap volume leaving a 3-cm x 12-cm open passage. That guard counter eliminated particles which might interact or scatter in the magnet pole pieces and, together with Tl and T2, defined the acceptance cone of the detector. Additional guard counters surrounded the sides of the instrument and covered the top with the exception of the telescope aperture. These counters eliminated charged particles which entered the detector from outside the acceptance cone and might subsequently interact, producing particles which triggered the telescope counters. All guard counters were in active anti-coincidence.

The atmospheric pressure during flight was monitored by a Wallace-Tiernan aneroid barometer which was photographed, together with a clock and a thermometer, at 5-minute intervals. An additional low-pressure gauge was read out electronically.

The entire instrument, with the exception of the barometer and silver-zinc batteries, was enclosed in a pressure-tight aluminum gondola during flight.

Balloon launched on: 7/15/1973
Launch site: Fort Churchill Airport, Manitoba, Canada  
Balloon launched by: Raven Industries Inc.
Balloon manufacturer/size/composition: Zero Pressure Balloon  
End of flight (L for landing time, W for last contact, otherwise termination time): 7/20/1973
Balloon flight duration (F: time at float only, otherwise total flight time in d:days / h:hours or m:minutes - ): 5 d
Landing site: Recovered near the city of Yakutsk in Siberia, USSR

The balloon was launched succesfully from Fort Churchill, Manitoba as part of 1973 Skyhook program on 15 July. Both, the instrument and balloon perfomed well. However, the balloon control instrumentation failed and the flight could not be terminated with the parachute release of the gondola. The balloon, with the payload still attached, went out of control and continued its westerly drift across Canada at an altitude of about 150.000 ft. It continued westward over Alaska and crossing the Bering strait it entered Soviet airspace into Siberia, on July 18, 1973. At that time, the Soviet Foreign Minister was advised of the balloon through diplomatic channels. According to representatives of the soviet space program INTERKOSMOS, the flight ended -in an undetermined fashion- near the city of Yakutsk, Siberia about July 20th, 1973. Pieces of the heavily damaged flight train were brought to Moscow where they were inspected by a Caltech representative in September. The Caltech e± spectrometer gondola plus several flight barographs were totally destroyed. However, magnetic tape from onboard recorder that survived damage and contained scientific data collected during the flight was saved.

Besides the recovery efforts, the Caltech positron-negatron research program was terminally crippled as at the time no funds weare available to rebuild the spectrometer.

According to Caltech's Semi-Annual Status Report to NASA for the period 1 October 1973 - 31 March 1974, post flight analysis showed that "...the loss occurred due to technical and operational deficiencies in the NASA/ONR operated Skyhook program. It is particularly regrettable that a simple safety device for emergency termination of an out of control flight, allegedly for reasons of "economy", carried ordinary dry cells, whose operational temperature range was totally inadequate for typical environmental conditions, instead of the recommended Nicad devices. Thus, "savings" of a few dollars led to the irreplaceable loss of an instrument worth several hundred thousand dollars and the termination of a productive and vigorous research project..."

• Aeronautics and Space Report of the President, fiscal year 1973 edition Pag. 28 NASA History Program Office
• RECOVERY OF ERRANT SCIENTIFIC BALLOON Diplomatical Note published in Wikileaks
• REQ FOR HELP IN RECOVERING NASA BALLOON PAYLOAD Diplomatical Note published in Wikileaks
• SOVIET RECOVERY OF NASA/ONR SCIENTIFIC BALLOON Diplomatical Note published in Wikileaks
• Voyager Scientist Celebrates 50 Years at Caltech - Via Pasadena Now Interview with Alan Cummings on which he mentions the incident of the balloon that fell in Siberia in 1973