X-RAY DETECTOR

The objective of the flight was to determine the X-ray spectrum of sources in the Cygnus region, extending previous observations from 58 keV up to 130 keV. The project was carried out by researchers from the Cosmic Ray Working Group at the Royal Dutch Academy of Sciences' Kamerlingh Onnes Laboratorium in Leiden, Netherlands.

The detector -that we can see in the scheme at left- consisted of a sodium iodide (NaI) crystal, 75 mm in diameter and 5 mm thick, coupled to a photomultiplier assembly. This assembly was mounted inside a cylindrical tin cup with 2 mm thick walls that provided shielding against background X-rays. The collimator was made of two coaxial tin cones (2 mm thick) with an 18-degree half-angle at the top, corresponding to the zenith angle of Cyg XR-1 at its meridian passage at their latitude of 52° N. The inner surfaces of both the cup and collimator were covered with 0.3 mm thick copper foil to absorb K-X-rays potentially generated in the tin. The maximum effective area for source detection was 26 cm².

A key feature of the detector was a rotating tin disk (3 mm thick) that periodically masked part of the opening angle, revolving around the collimator axis with a period of about 100 seconds. This chopping mechanism allowed the detection of X-ray sources by creating a modulation in the counting rate while maintaining a constant background level due to the detector's axial symmetry. A sun sensor was mounted on top of the disk to track its position relative to the source. The entire payload was designed with damped torsional oscillation for stability.

The detector included a plastic scintillator guard counter surrounding the main detector and collimator to reject background events from charged particles. Energy calibration was performed using Cadmium-109, achieving resolutions of 50% and 30% full width at half-maximum at 22 keV and 88 keV respectively. The system incorporated a 32-channel pulse height analyzer that digitally telemetered the pulse height of each X-ray event.

Balloon launched on: 4/5/1966
Launch site: De Bilt, Netherlands  
Balloon launched by: Royal Netherlands Meteorological Institute (KNMI)
Balloon manufacturer/size/composition: Zero Pressure Balloon  
End of flight (L for landing time, W for last contact, otherwise termination time): 4/5/1966

The balloon was launched from De Bilt, the Netherlands, on April 5, 1966, and reached a ceiling altitude corresponding to 5 gm/cm² residual atmosphere, at 6:45 a.m. local time where it could simultaneously observe three known X-ray sources in the Cygnus region: Cygnus XR-1, Cyg XR-2, and Cyg-A.

• Balloon Observation of the X-Ray Sources in the Cygnus Region in the Energy Range 20-130 KEV Astrophysical Journal, vol. 147, p.391