WELCOME-1 (MK.2)

WELCOME-I (Mk-2) was an improved version of 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 800–1000 keV.

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.

As the improved version counted with reduced background interference and a narrower field of view, it demanded higher pointing precision with a system's control accuracy within ±0.5° and capable of following targets moving 8° in 10 minutes and rotating 20° within 1 minute.

A specially built gondola housed the telescope and support systems, including power supplies, communications hardware, and a pointing mechanism. The pointing system in the MK-2 version included a reaction wheel to provide torque and a torsion relief motor to manage accumulated angular momentum and decouple from balloon-induced rotation. The azimuth control was achieved using both analog and digital control circuits. A magnetic field sensor (MAG-03MS) and solar sensors were used to determine the absolute direction. A novel feature was a fixed magnetic sensor system that processed signals electronically, thus avoiding the issues of previous rotating sensor setups.

The telescope had an effective detection area of 360 cm². The energy resolution was approximately 8% at 662 keV. The field of view was about 3.8 degrees, narrower than in the previous iteration of the instrument.

Balloon launched on: 6/5/1993 at 7:50 JST
Launch site: Sanriku Balloon Center, Iwate, Japan  
Balloon launched by: Institute of Space and Astronautical Science (ISAS)
Balloon manufacturer/size/composition: Zero Pressure Balloon Winzen - 180.000 m3
End of flight (L for landing time, W for last contact, otherwise termination time): 9/5/1993 at 23:25 JST
Balloon flight duration (F: time at float only, otherwise total flight time in d:days / h:hours or m:minutes - ): 15 h
Payload weight: 580 kgs

• Azimuth Control System with A Reaction Wheel and A Torsion Relief Motor for Hard X-ray/?-ray Detector Welcome-1 (mk2) ISAS departmental bulletin paper
• Balloon observations in Space Research in Japan, 1994
• Scientific observations with balloons Institute of Space and Astronautical Science Annual Handbook 1993 (In Japanese)