Vol. 48 1 
spacecraft operating since August 1995. More details of the INTERBALL Project 
can be found at www.iki.rssi.ru/interball.html. The RF­15I instrument has been 
developed as common venture between the Astronomical Institute, Czech Academy 
of Sciences, Space Research Center, Polish Academy of Sciences and IKI, Russia. 
2. The Instrument 
The RF­15I solar X­ray photometer is equipped with two detector systems: 
ffl Proportional argon gas­filled (350 Torr) detector of aperture 4.5 mm 2 
equipped with Be filter of 150 ¯m thickness. The largest cross sectional 
area of the detector for gamma radiation is ¸ 10:5 cm 2 . 
ffl The Na(I) scintillation detector of 15.2 cm 2 aperture and crystal thickness 
8 mm. 
Detailed description of the instrument is in preparation (Sylwester et al. in prepa­ 
ration). The proportional detector nominally registers the soft X­ray solar flux in 
the three energy channels 2--3--5--8 keV every 2 s. By the time of the reported 
observations the appropriate boundaries of energy channels moved to ¸ 0:7--1.0-- 
1.7--2.7 keV due to aging of the calibration Fe 55 radioactive source. 
The scintillation detector registers every 0.125 s the hard X­ray flux in five en­ 
ergy bands 10--15--30--60--120--240 keV provided that appropriate rate thresholds 
are exceeded in each of the upper four energy ranges. At the time of reported 
observations all these channels have been active due to substantial particle back­ 
ground. In the lowest energy range 10--15 keV, the data are collected every 2 s 
simultaneously with the softer proportional detector channels. 
The normal to the detectors' windows is pointed to the Sun within \Sigma10 ffi thanks 
to spin­stabilisation of the spacecraft. One satellite revolution takes ¸ 118 s. 
3. The Observations 
At the time of the reported measurements the INTERBALL­tail satellite was on 
its elongated elliptic orbit (183 000 \Theta 22 000 km) at the distance of ¸ 93 400 km 
from the Earth center as is schematically sketched in Fig. 1. Relevant directions 
are also indicated. The altitude data indicate that the satellite was within the Earth 
magnetosphere at the time of burst measurements. 
Based on the directional scheme and satellite attitude data we conclude that 
before entering the detectors the investigated radiation pulse had to cross substantial 
amount of satellite shielding and structure, estimated as few cm aluminum, and in 
addition, ¸1 mm of steel cover for the proportional detector. 
Approximately 60 hours before the reported pulse a strong X1.0 long duration 
solar flare has been observed which caused substantial increase of the SEP particle 
flux propagating through the Earth vicinity. GOES satellite measured high levels of 
energetic particles most probably related with this flare. Also the RF­15I detector