Purpose of the flight and payload description

The large area solid angle double scatter neutron telescope was first conceived in 1968 by the Physics Department and Institute of Geophysics and Planetary Physics of the University of California, Riverside. The objective of the experiment was to measure the flux, energy and direction of albedo neutrons from the earth and to search for solar neutrons.

A schematic cross section of the neutron telescope is shown at left along with a photograph of the assembled detector, showing the location of the anticoincidence sheets and light pipes without the gondola cover (click to enlarge). The aluminum tanks, S1, and S1, were filled with liquid scintillator NE 223 (decaline). The tanks were 100 cm x 50 cm x 15 cm. They were mounted coplanar with a separation of 100 cm center to center. Each tank was optically separated into eight identical cells and each cell was viewed by a 5 inch Amperex XP 1040 photomultiplier tube. For optimum light collection the cell walls were painted white and each photomultiplier tube face was in contact with the liquid scintillator. A reservoir tank with both positive and negative pressure relief valves was attached to each of the main tanks to allow for thermal expansion and contraction of the liquid scintillator.

Extraneous material which might attenuate either the incident or scattered neutrons was avoided wherever possible. The walls of the tanks and the optical dividers were as thin as feasible structurally and the photomultipliers were mounted along two sides of the tanks, rather than on the top or bottom. The thickness of the top and bottom of the tank was 0.32 cm. The resultant dimension of the tank-photomultiplier tube assembly was 102 cm x 110 cm x 16 cm.

The overall dimensions of the detector were limited by the requirements placed on energy response, resolution and efficiency. The efficiency of the detector was determined by the areas and thicknesses of S1 and S2 and the solid angle subtended at one tank by the other. With the additional constraint on up/down symmetry, the optimum detector thickness was about 15 cm for neutron energies between 2 and 100 MeV. If it were thicker, there would be some increase in efficiency but multiple neutron scattering would degrade the angle measurement.

Charged particles were rejected by completely enclosing each liquid scintillator tank with a box of Pilot F plastic scintillator, 6 mm thick. Each anticoincidence scintillator assembly had sheets 104 cm x 114 cm top and bottom, and four sheets interleaved to form the sides. The four side sheets were curved at the ends to produce overlap at the corners of the box. Each was viewed with a 2 inches photomultiplier tube (RCA 8575) via an adiabatic light guide. The corners of the "box" were rounded to reduce the maximum dimension of the detector. The top sheet of the anticoincidence scintillator box was viewed by two 5 inches photomultipliers (Amperex XP 1040) via adiabatic light guides, and the bottom sheet was viewed by two 2 inches photomultipliers (RCA 8575) via tapered light guides.

The liquid scintillator tanks were supported at the corners by 2.5 cm square cross section, 6 mm wall thickness chrominum-molybdenum steel tubing. These strong support members were required for the 360 Kg detector to survive the parachute landing following a balloon flight. The tubing penetrated the lower sheet of the top anticoincidence shield (A12) and the top sheet of the bottom anticoincidence shield (A21). It was cut to a "U" cross section where it penetrated the scintillator to reduce the amount of scintillator removal. The frame was attached to the base of the aluminum gondola. The gondola maintained the instrument at atmospheric pressure and near room temperature during a balloon flight. It had a 3.2 mm cylindrical wall and 1.6 mm spherical end caps and was surrounded by 15 cm of expanded polystyrene.

Details of the balloon flight

Balloon launched on: 9/26/1971 at 12:30 utc
Launch site: Columbia Scientific Balloon Facility, Palestine, Texas, US  
Balloon launched by: National Center for Atmospheric Research (NCAR)
Balloon manufacturer/size/composition: Zero Pressure Balloon Winzen - 10.600.000 cuft (0.7 Mil - Cap. 0.8 Mil. Stratofilm)
Flight identification number: 633P
Balloon flight duration (F: time at float only, otherwise total flight time in d:days / h:hours or m:minutes - ): F 26 h 30 m
Payload weight: 2371 lbs

On 26 September 1971, the neutron detector was flown from NCAR balloon base in Palestine, Texas. The auxiliary radio equipment and ballast were hung onto the sides of the gondola to eliminate material both above and below the detector. The balloon was launched at 7:30 EDT and reached its float altitude of about 120,000 feet in about three hours. The flight was a success: data were accumulated during the balloon ascent and about 15 hours of levelled flight at maximum altitude, although the balloon went out of telemetry range before the desired 36 hours of flight.

External references

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