WELCOME-1

WELCOME-I was a balloon-borne hard X-ray and gamma-ray telescope utilizing a compound-eye configuration of well-type phoswich counters (hence its name). It was developed through collaboration between the Department of Physics at the University of Tokyo, the Institute of Space and Astronautical Science (ISAS), the National Laboratory for High Energy Physics (KEK), and Rikkyo University, all from Japan. The primary objective of the instrument was to conduct precise observations of hard X-ray and gamma-ray emissions from celestial sources, particularly within the energy range of 60 keV to approximately 80‰1000 keV.

In the image at left can be seen an scheme of the instrument (Click for more details). It consisted of 64 well-type phoswich counters in an 8×8 matrix, surrounded by 36 CsI(Tl) crystals for active shielding. Each counter featured a GSO(Ce) scintillator crystal (3.4 × 3.4 × 1.0 cm3) as the detection element, affixed to a CsI(Tl) well-shaped block for shielding and collimation. The well geometry was 14 cm long and 0.94 cm thick, with a bottom shield block measuring 5.5 × 5.5 × 8.0 cm3, providing nearly 4p steradian active shielding and efficient background suppression. Each unit was read out by a 2-inch Hamamatsu R1847-05 photomultiplier tube. The system used pulse shape discrimination based on different decay times (GSO: ~60 ns, CsI: ~1 µs) to distinguish valid detection events from background noise.

The data acquisition system employed VME-bus architecture with a 32-bit MC68000 processor. Data was recorded on a 2 GB EXABYTE 8mm tape and transmitted via 4 kB/s telemetry link. The telescope included a precise timing system for pulsar observations, recording photon arrival times with 32 µs resolution using an onboard quartz oscillator stabilized to less than 10?6 frequency variation over 10 seconds. UTC synchronization was achieved using ground-based GPS and Rubidium clock prior to launch, ensuring 2 µs timing accuracy across the 12-hour flight.

A specially built gondola housed the telescope and support systems, including power supplies, communications hardware, and a pointing mechanism. Elevation was controlled by a ball screw drive, and azimuth adjustments used a torque motor with a magnetometer-based stabilization system known as the Yorimodoshi method. For precise source targeting and tracking, the payload incorporated both a star camera (based on a microchannel plate coupled to a CCD) and a sun camera. These cameras digitized celestial images onboard and transmitted them for ground-based star reconstruction with an angular resolution suitable for accurate calibration of the pointing direction.

The telescope had an effective detection area of 740 cm² at 122 keV and 222 cm² at 511 keV. The energy resolution was approximately 28% at 122 keV and 12% at 511 keV, sufficient for resolving key gamma-ray lines. The field of view was about 7.4 degrees, defined by the geometry of the collimator well, and the time resolution of the detector system was 32 µs. The total weight, including the gondola, was 680 kg.

Balloon launched on: 12/3/1991 at 6:55 UTC
Launch site: Balloon Launch Sector, Cachoeira Paulista, Brazil  
Balloon launched by: CLB / INPE
Balloon manufacturer/size/composition: Zero Pressure Balloon 240.000 m3
End of flight (L for landing time, W for last contact, otherwise termination time): 12/3/1991 at ~ 17:00
Balloon flight duration (F: time at float only, otherwise total flight time in d:days / h:hours or m:minutes - ): 10 h
Landing site: In Itirapina, Sao Paulo, Brazil
Payload weight: 687 kgs

• A infraestrutura do INPE para lancar balao cientifico Elisete Rinke dos Santos, editorial Transtec, 1993
• Detector to study low-flux hard X-ray/gamma-ray sources Advances in Space Research, 13 (2), 165
• Hard X-Ray Emission from the Galactic Ridge THE ASTROPHYSICAL JOURNAL, 481:821–831, 1997 June 1
• New hard X-ray/gamma-ray telescope - Welcome-1 Proceedings Volume 1734, Gamma-Ray Detectors; (1992)
• Well-type phoswich counter for low-flux X-ray/ gamma -ray detection IEEE Transactions on Nuclear Science, vol. 40, no. 2, pp. 204