LARGE AREA X-RAY SCINTILLATION DETECTORS

The objective of the flight was to study the performance of several phoswich designs for X-ray and gamma-ray detectors optimized for observations in the spectrum between 15 and 200 keV. Two of the detectors were of essentially identical design, except the central crystal-shield crystal materials were interchanged in order to study the optimum crystal configuration. The third detector was a large area unit of similar design. All three detectors were flown on an engineering balloon flight to study their performance in the cosmic-ray environment. The design and test of the detectors was in charge of the E. 0. Hulburt Center for Space Research of the Naval Research Laboratory from the United States Navy.

In the image at left we can see the distribution of the detectors in the flight gondola and below the designs of the three instruments (click to enlarge). The two small phoswich detectors are seen at the left of the gondola, positioned so that their apertures 'pointed' toward the zenith. A shutter assembly was utilized so that the detector apertures could be unobstructed, or could be blocked by either an active NaI(Tl) shutter, a plastic scintillator shutter or a passive shutter. The intrinsic detector background was measured with the NaI(Tl) shutter blocking the aperture, and in anticoincidence with the phoswich detector.

The primary X-ray detector (central crystal) consisted of a 3-mm thick by 5-inch. diameter CsI(Na) crystal. This was encapsulated between two 6-inch dia. NaI(Tl) crystals which provided the active shielding. The front NaI(Tl) crystal has 55 holes of 0,5 inch dia. drilled through it which acted as a collimator defining a 200 FWHM field-of-view. The rear NaI(Tl) crystal also served as a light pipe. The three crystals were optically coupled and viewed by a single 5-inch dia. photomultiplier tube.

The second small detector was of the same design except that the central crystal was NaI(Tl) and the shield crystal was CsI(Na).

The large area detector had a 15,5 inch dia. x 5-mm thick NaI(Tl) central crystal. The CsI(Na) collimator has 151 1-inch dia. holes drilled on 1 5/32-inch centers. This resulted in an effective area which was 63% of the total NaI(Tl) surface area. Seven 5-inch dia. phototubes viewed the crystal assembly. The effective area of this detector was 765 cm2.

The flight configuration for the large phoswich detector is shown at left (click to enlarge). The photomultiplier tubes were separated from the detector window by a 2-inch air gap to provide uniform response. External collimators, were co-aligned with each hole in the CsI collimator to reduce the field-of-view to 90 FWHM. The individual collimators consisted of graded lead, tin, and brass cylinders. A plastic anti-coincidence detector was used on top of the passive collimator to study background production in the graded shield by cosmic-ray interactions. Also, a plastic well detector surrounded the phototube assembly to investigate any background contribution due to direct cosmic-ray effects in the photomultiplier tubes.

Balloon launched on: 10/8/1973
Launch site: Columbia Scientific Balloon Facility, Palestine, Texas, US  
Balloon launched by: National Scientific Balloon Facility (NSBF)
Balloon manufacturer/size/composition: Zero Pressure Balloon Raven 314.319 m3 (15.24 Microns X-124)
Flight identification number: 787P
End of flight (L for landing time, W for last contact, otherwise termination time): 10/8/1973
Landing site: Balloon Failure
Payload weight: 512 kgs.

No scientific data was obtained due to the failure of the balloon. There is no information available on the nature of the failure. The mission was succesfuly acomplished on October 17, 1973

• National Scientific Balloon Facility Annual Report FY 1974 National Center for Atmospheric Research, December 1974