GAMMA RAY AND NEUTRON MEASUREMENTS

Objective of the flight was to survey the southern hemisphere sky in the photon energy range of 30 keV to 1.5 MeV to observe emissions from discrete or extended sources that have or have not been detected at balloon altitudes. The balloon carried a large area SCINTILLATION TELESCOPE sensitive to photons developed by the Physics Department of th University of New Hampshire.

The spectrometer-telescope used in this experiment can be seen schematicaly in the picture at left (click for more details). It was composed by multiple components designed to optimize the detection and measurement of high-energy photons, such as X-rays and gamma rays, in a balloon-borne observational platform. The instrument incorporated a primary detector, or spectrometer, which used a thallium-activated sodium iodide (NaI(Tl)) scintillation crystal measuring 10 inches in diameter and 1.425 inches in thickness. This crystal was hermetically sealed in a special aluminum housing, with a 0.020-inch thick aluminum front window that allowed photon detection down to 15 keV. Although the crystal had a total surface area of 500 cm², an aluminum flange at the front edge reduced the sensitive area to 450 cm² for lower-energy photons.

Four RCA 7151N photomultiplier tubes, positioned to view the crystal through four quartz windows on the side of the housing, detected the light produced by the scintillation events. This configuration, optimized for a back-to-back detector design, limited the total photocathode area available for light collection, thereby reducing the energy resolution of the spectrometer.

The spectrometer detected photons based on their interaction with the NaI(Tl) crystal, utilizing the photoelectric effect and Compton scattering as the dominant interaction processes in the energy range of 17 keV to 1.5 MeV. The 1.425-inch thickness of the spectrometer represented at least one interaction length up to 600 keV and approximately 0.6 interaction length at 1.5 MeV.

A secondary component, the guard detector, served as an additional NaI(Tl) scintillation crystal measuring 10 inches in diameter and 1.5 inches in thickness. Housed in a regular aluminum casing with a 10-inch optical window at the bottom, the guard detector was viewed through a 5-inch EMI 9530 photomultiplier tube via a cone-shaped diffuse optical chamber. This detector shielded the spectrometer from background atmospheric X-rays, gamma rays, and charged particles approaching from behind. It absorbed X-rays, stopped most gamma rays, and triggered an anticoincidence rejection signal if a scattered photon was detected in both the guard detector and the spectrometer.

A graded photon shield surrounded the spectrometer and guard detector. This cylindrical shield, measuring 10 inches in height and 12 inches in internal diameter, was composed of three coaxial layers of high atomic number (Z) metals: 1/4-inch lead (Pb), 1/32-inch tin (Sn), and 1/64-inch copper (Cu). These layers worked together to block background radiation, particularly fluorescence X-rays generated by high-energy photon interactions with lead.

The charged particle shield consisted of a cylindrical shell made of 1-inch thick NE 102 plastic scintillator, enclosing the spectrometer and graded shield. This shield, with a height of 10 inches and an internal diameter of 12.5 inches, was housed in an aluminum can with polished internal surfaces to enhance light transmission. Four 2-inch EMI 6097 photomultiplier tubes at the bottom end detected scintillation events caused by charged particles, allowing efficient discrimination of background events. The plastic scintillator effectively detected charged particles with nearly 100% efficiency, while X-rays and gamma rays passed through with minimal interaction.

A brass collimator positioned above the spectrometer defined the telescope's field of view. Constructed from 1/32-inch thick brass plates in an egg-crate structure, the collimator measured six inches in height and covered the full surface area of the spectrometer. Each rectangular cell within the collimator defined a geometrical aperture of 6° x 10° in half angles. By absorbing off-axis photons, the collimator limited the angular response of the telescope and improved directional sensitivity.

The spectrometer-telescope's electronic system performed several critical functions, including signal processing, energy range selection, event discrimination, pulse-height analysis, and telemetry formatting. Detector signals, generated as pulses by photon or particle interactions, were first summed (for detectors with multiple photomultiplier tubes), shaped, and amplified. A set of discriminators defined the lower and upper energy limits of each detector. The spectrometer accepted pulses corresponding to energy deposits between 17 keV and 1.5 MeV, while the guard detector and charged particle shield had their own discriminator thresholds to reject unwanted events.

Auxiliary systems included fluxgate magnetometers for measuring the azimuthal aspect of the telescope. Mounted orthogonally, these magnetometers provided sinusoidal voltage outputs that varied with the telescope's orientation relative to Earth's geomagnetic field, enabling precise determination of pointing direction. A temperature sensor, positioned between the telescope and electronics, monitored system temperature.

The entire instrument was housed in a cylindrical aluminum gondola, pressurized to one atmosphere and thermally insulated with plastic foam. The gondola's hemispherical top featured a 0.020-inch thin aluminum window over the telescope aperture to minimize X-ray absorption. A rotator, consisting of a DC motor and clutch assembly, controlled the gondola's orientation. Power was supplied by onboard silver-zinc batteries, ensuring continuous operation throughout the balloon flight.

Balloon launched on: 11/11/1971 at 7:00 local
Launch site: Second Air Brigade, Paraná, Entre Rios, Argentine  
Balloon launched by: National Center for Atmospheric Research (NCAR) / Comision Nacional de Investigaciones Espaciales (CNIE)
Balloon manufacturer/size/composition: Zero Pressure Balloon Winzen 10.600.000 ft3 (0.7 / 0.7 mil. Stratofilm)
Flight identification number: 64N
End of flight (L for landing time, W for last contact, otherwise termination time): 11/11/1971 at 17:55 local
Balloon flight duration (F: time at float only, otherwise total flight time in d:days / h:hours or m:minutes - ): 10 h 55 m
Landing site: Near Piquillin, Cordoba, Argentine
Campaign: GALAXIA 71  
Payload weight: 905 lbs

• NCAR Scientific Balloon Facility Annual Report, 1971 National Center for Atmospheric Research, February 1972