HEGCS (High Energy Gas Cerenkov Spectrometer)

HEGCS was the acronym for HIGH ENERGY GAS CERENKOV SPECTROMETER a large-area, high-resolution, balloon-borne cosmic ray telescope optimized for charge and energy measurements of high-Z nuclei. Its design integrated a multi-layered detection system combining position-sensitive scintillation tracking with a high-efficiency gas Cherenkov calorimeter. The objective of the instrument was to directly determine the differential energy spectra of cosmic ray nuclei with atomic numbers between 15 and 28, particularly focusing on iron nuclei, in the energy range from 50 to 200 GeV per nucleon. It was developed at the Laboratory for High Energy Astrophysics of the NASA Goddard Space Flight Center.

In the image at left we can see an schematic view of the experiment (click to enlarge). The instrument had a large geometric factor of 4.0 m² sr and was constructed as a drum-shaped detector measuring 3 meters in diameter and 4.5 meters in height. The total instrument weight was approximately 4400 pounds.

The internal structure of the detector consisted of three optically isolated light diffusion chambers. The top and bottom chambers each housed a hodoscope array composed of 24 equilateral triangular NE-114 plastic scintillators, each measuring 75 by 75 by 1.05 cm. These were arranged in a hexagonal configuration. RCA 4525 photomultiplier tubes were optically coupled at the vertices of the triangles, with each PMT linked to 2, 3, or 6 scintillators depending on its location. A cosmic ray passing through a triangle produced signals in the three associated PMTs, and the amount of light collected by each PMT was a function of both geometric distance and internal reflections. The signal distribution from the PMTs allowed the reconstruction of the incident particle's position with a resolution of approximately ±4 cm. This positional information was then used to correct the scintillation path length for accurate charge determination. The top and bottom hodoscope chambers were painted internally with barium sulfate-based white reflective paint, turning them into light diffusion boxes. Approximately 23 percent of the scintillation light escaped through the top and bottom surfaces of the scintillators, and about 10 percent of that escaped light was collected by 12 additional RCA 4525 PMTs arranged around the circumference of each chamber. This configuration enabled charge resolution better than 0.3 charge units for iron nuclei.

The central chamber of the detector was a pressure vessel containing Freon-12 gas at 3 psi. This chamber served as a gas Cherenkov detector with a threshold of 50 GeV per nucleon, defined by the refractive index of the gas. The interior surface of the central drum was also coated with barium sulfate white reflective paint, which had an average visible reflectance of 95 percent. However, since the ultraviolet region of the Cherenkov spectrum had lower reflectivity, the vertical walls of the chamber were additionally coated with a thin fluorescent waveshifter film that converted ultraviolet photons down to 220 nm into visible light near 425 nm. This conversion process preserved about 90 percent of all UV photons, significantly enhancing photon collection efficiency.

Cherenkov light within the central chamber was detected by 24 RCA 4522 photomultiplier tubes mounted near the upper circumference of the drum. Each 5-inch diameter PMT was fitted with an 8-inch hemispherical fisheye lens made of material with a refractive index of 1.43. These lenses improved light collection in the diffusion-box environment by a measured factor of 2.2. Overall, approximately 24 percent of all Cherenkov photons generated between 220 and 500 nm were collected by the phototube array, with about 70 percent of those photons having been wavelength-shifted.

The design and optical performance of the detector ensured high precision in both charge and energy measurement of individual heavy cosmic ray nuclei. The instrument was intended for long-duration balloon flights, during which it was expected to collect roughly 200 iron events above 200 GeV per nucleon in a 24-hour period, allowing the spectral index in that energy region to be determined with a precision of approximately ±0.1.

For thermal control in the intense sunlight at flight altitude, the HEGCS instrument was painted orange in the exterior. The HEGCS team couldn't help but notice that it greatly resembled a huge pumpkin. The resemblance was further improved by the addition of a Jack-O-Lantern face.

Balloon launched on: 9/30/1983 at 12:57 utc
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 30.970.000 cuft (0.8 Mils - Caps 1.5 Mils x2 - Stratofilm) SF434.92-080-NSCR-01
Balloon serial number: W30.97-2-01
Flight identification number: 1344P
End of flight (L for landing time, W for last contact, otherwise termination time): 10/1/1983 at 00:26 utc
Balloon flight duration (F: time at float only, otherwise total flight time in d:days / h:hours or m:minutes - ): 12 h
Landing site: 24 miles N of Shreveport, Louisiana, US
Payload weight: 6331 lbs

• Cosmic ray nuclei of energy 50 GeV/NUC 19th Intern. Cosmic Ray Conf., Vol. 2 p 44
• Cosmic-Ray Detector for High Energy Iron Nuclei 17th International Cosmic Ray Conference, Paris, France. Volume 8., p.54
• Measurement of the iron spectrum from 60 to 200 GeV per nucleon 19th Intern. Cosmic Ray Conf., Vol. 2 p 40-43 (1985)