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New measurement of the electron flux from 10 GeV to 100 GeV with the bets instrument
Pergamon www elsewer nlllocateiasr
Adv Space Res Vol 26, No 11, pp 1823-1826,200O 0 2001 COSPAR Pubhshed by Elsevler Science Ltd All nghts reserved Pnnted In Great Bntain 0273-l 177/00 $20 00 + 0 00 PII SO273-1177(99)01229-6
NEW
TO
MEASUREMENT OF THE ELECTRON FLUX 100 GEV WITH THE BETS INSTRUMENT
S Toru’, T Tamural, N Tateyama’, K Yoshlda’, T Yamagaml 2, E Kamloka2, T Kobayash14,Y Komorl”, K Kasahara’, T Yuda7, and J Nlshlmura’ ’ 2 3 4 5 6 7 8
FROM
Y Saltol,
Faculty of Engzneerzng, Kanagawa Unzverszty, Yokohama, JAPAN Innstatute of Space and Astronautzcal Scaence, Sagamzhara, JAPAN Department of Physzcs, Rakkyo Unaversaty, Tokyo, JAPAN Department of Physacs, Aoyama Gakuan Unauersaty, Tokyo, JAPAN Kanagawa Prefectural College of Nursang and Medacad Technology, Yokohama, Shabaura Instatute of Technology, Ohmaya, JAPAN Instatute for Cosmac Ray Research, Unaversaty of Tokyo, Tanasha, JAPAN Yamagata Instatute of Techonology, Yamagata, JAPAN
10 GEV
H Murakam13,
JAPAN
ABSTRACT The BETS (balloon-borne electron telescope with scmtlllatmg fibers) instrument has been developed for high-altitude balloon flights to observe the cosmic ray electrons with energies of 10 GeV to several 100 GeV The detector 1s a Lead/SclFl sampling calorimeter conslstmg of 36 SclFl belts (each 280 mm wide) and 8 lead plates (each 5 mm thick) The electron ldentlficatlon 1s performed by trlggermg the electro-magnetic showers on board and by analyzing the three-dimensional shower images by an intensified CCD camera It IS demonstrated m the flight data m 1995 and 1997 that a reliable ldentlficatlon of the electron component against the proton background 1s achieved up to a few 100 GeV The performance of detector was tested by the CERN-SPS electron beams m 1996 and with the proton beams m 1997 The obtained energy spectrum 1s consistent with the recent observation by HEAT, although our result still has a little room for improvement The energy spectrum from 10 GeV to 1000 GeV which 1s obtained by combmmg these data and the emulsion chamber data ( Nlshlmura et al 1997)suggests that the diffusion constant 1s about 1 x 1028(E/GeV)o 3 cm2/sec m the energy range between 10 GeV and 1000 GeV A hump m the energy spectrum 1s observed around several hundred GeV, which 1s expected from a nearby source 0 2001 COSPAR Published by Elsevler Science Ltd All rights reserved INTRODUCTION It 1s commonly understood that high-energy electrons are accelerated m supernova remnants (SNRs), and they lose their energies by synchrotron radiation and by inverse Compton scattering during the propagation through the Galaxy The energy spectrum of electrons from 10 GeV to 100 GeV is, therefore, a key to Investigate the diffusion characterlstlcs m the Galaxy and the source spectrum Since the energy-loss rate by these mteractlons 1s proportional to the energy, hfe of electron becomes shorter with increasing of the energy It 1s , therefore, suggested that each of the nearby sources could be ldentlfied by the measurements of the spectral shape and the isotropy m the TeV region (See for example, Nlshlmura, 1994) However, observation of high-energy electrons 1s still a difficult task, although every effort has been made for the past 30 years The reason lies m the copious hadromc backgrounds which increase with increase m energy Instruments which mcorporate transition radiation detectors (Tang, 1984) or gas Cerenkov counters (Golden et al ,1984) have successfully observed electrons m the energy region lower than 100 GeV They are, however, inefficient m reJectIon of the background protons over several 100 GeV and m increasing the statistics due to the smaller solid angles Emulsion chambers (EC) have uniquely achieved observations of electrons m the TeV regton (Nlshlmura et al ,198O) They are, however, not suitable to use for very long duration observation because of accumulation of the backgrounds and to measure the amsotropy The Balloon Borne Electron Telescope with Scmtlllatmg Fiber (BETS) has been developed m order to study the sources and the diffusion characterlstlcs of high-energy electrons The telescope IS deslgned to have the capability of measuring the precise energy spectrum and the arrival directions m the energy region from 10 GeV to several 100 GeV
1823
S Tom et al
1824
BALLOON OBSERVATION Detector The BETS instrument consrsts of a shower detector mcorporating an imaging calorimeter and a trigger system, an electronics system, a data recording system and a telemetry system Details of the telescope and the method of data analysis are presented m Torn et al (1998a,1996) A recent study of the performance mcludmg an accelerator test at CERN-SPS 1s presented m a paper m this conference (Tamura et al ,1998) In Table 1, the basic performance of the BETS payload 1s summarized.. Table 1 Instrument performance summary 1 Energy Range md.l-1ra.l f%-tmr Gee..__-____ Proton/Electron Drscrrmmatron Enerev Resolutron Angular resolutron Weight Power Consumotion ___“__
-----Da
-------~----
1 10 GeV N several 100 GeV 1 I snn .d I3r 1 a few 10’ I-17% 1 degree typical N 320 kg 130 W (at maximum) _
_
_
_
___
The calorrmeter consrsts of 36 layers (18 m each of two orthogonal views) of the ScrFr belts with the lead plates of total depth of -7 r 1 Each belt IS composed of 280 round ScrFr’s (1 mm 4 each) m one millimeter pitch The total number of SCIFI’S1s 10,080 (5,040 for each dnectron) The guide part of fibers are made of clear fibers which have no efficiency to charged-particles m order to remove the noise The trigger system was optrmrzed by srmulatlon to enhance the ratio of electrons m the triggered events Three plastic scmtrllators were set at the top of detector (Sr), the depth of N 2 r 1 (Sz ) and the bottom (Ss ), respectively Each scmtrllator with a thickness of lcm IS viewed by a photomultrplier through a light guide For the observatron of electrons over 10 GeV and with a zenith angle smaller than 30 degrees, the highest proton-reJectron power of N 150 at 85% electron efficiency 1s realized rf we impose the numbers of particles observed m Sr, Sz and Sa be 0 7 N 5, 10 N 100, and 2 40 (m unit of single mmrmum romzmg particle of vertical incident), respectively The rejection power 1s enough to decrease the trigger rate during observatron as low as N 2 Hz Scmtrllatmg fibers outputs were observed with an image-mtensrfied CCD camera The CCD camera has an input window with a diameter of 10 cm and has a “shutter” function which 1s triggered by a gate signal from the trigger system An image on the input window IS reduced by one-twelfth m size on the sensing area of CCD The area contacted with the cross sections of all SclFr’s corresponds approximately to 512 x 512 pixels (5 816 x 5 96 mm m area) on CCD A video module especially developed, reads 512 x 256 prxels (for the 2 1 interlace CCD scanning method) and the signals whrch exceed the noise level are digitized wrth an 8-bit flash ADC By averaging the mtensltres m the adJacent two pixels, the amount of data 1s reduced by a factor of two, 1 e 256 x 256 pixels Balloon Flight The instrument with a total weight of 320 kg and the power consumptron of 130 W was launched m June, 1997 from Sannku Balloon Center m Japan, and was flown for about 4 5 hours at an altitude over 35 km The event trigger was carried out either by three-hold comcrdence of Sr, Sz and Sa (SH mode) or by two-fold of &and Sz (SI mode) The SH mode for shower events was used mostly m electron observatron, and the SI mode was for heavy particles The observatron of heavy tracks was planned for the cahbratron of posrtron The trigger rate for showers over the threshold energy was N 1 7 Hz at the level flight The number of triggered events 1s about 28,000
Electron selection by the image data analysis was achieved for the triggered events by the followmg steps 1 2 3 4
Shower axis passes through all ScrFr layers The zenith angle 1s less than 30 degrees Charge of mcrdent particle 1s single Ratio of energy-deposrtlon wrthm 5mm from the shower axis to total (RE) 1s higher than certain value
1825
NewMeasurementof the ElectronFlux From 10 GeV to 100 GeV
For By the reductions up to the step of 3, mcludmg on-board tngger, about 99% protons are reJected showmg how much reJectIon power can be improved by the analysis of energy concentration m the step 4, the simulated results are presented m Fig 1 The slmulatlon events were “triggered” by reahstlc condltlons If we select the events of which RE 1s higher than 0 7, the which are almost same as the observations The total rejection power agamst the backprotons are rejected by 95% with 85 % electrons remaining The observed dlstrlbutlon presented m Fig 2 ground protons W, therefore, estimated to be a few thousands 1s consistent with the slmulatlon, and the number of electron candidates m RE > 0 7 IS nearly 600
Simulated Events
-------
Flight Events at Level ( > 35 km )
Electrons(E>lOGeV)
60. 40. 20.
02
04
06
06
Rabo of Energy Cqw~lbcm wkhln 5mm from Shower AXIS
Ratio of Energy Deposition withm Gun from Shower Axis
Fig 1 Dlstrlbutlon of REs for slmulatlon events which are “triggered” by same condition with the observation
ELECTRON
Fig 2 Observed RE dlstrlbutlon for flight events at an altitude over 35 km
FLUX
The energy spectrum of observed electrons was obtained by consldermg the energy dependence of energy resolution and geometrical factor by the accelerator tests and slmulatlons Also, m the estlmatlon, correction factors were calculated for the secondary electrons and gamma-rays, and the maldentlfied protons However, the flux 1s still prehmmary, and it may be changed by nearly 10% mainly by correction of dead time of the detector, which 1s not considered m this paper It 1s seen m Fig 3, our flux IS considerably smaller than previous observations m the energy region of 10 GeV to 100 GeV The recent measurement by the HEAT detector (Barwlck et al, 1998) IS, however, consistent with ours
et al
loo
IO’
lo2 ENERGY
(Radio)
‘80
IO4 ( GeV
)
Fig 3 Companson of our results (BETS ‘99) with HEAT and previous observations Solid line presents calculated spectrum by dlffuslon model for all sources, dotted the nearby sources
S Tom era1
1826
The best fit curve of the model calculations presented m Fig 4 gives a diffusion constant of 1 x 1028(E/GeV)o 3 cm2/sec for the SN rate of once per 30 years (Nlshlmura et al, 1997) The estimated magnetic field by companng the electron flux around 10 GeV to the r&o flux 1s obtamed to be N 7 PG (Komon, 1998)
-,10'
7
:
_
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--
-
5.
c
7
.
10'
7
LLbZp1-"
-7
r
cimmgn /\
i’
I
IO’
“.,,_I loo
Cg
1iq I 10'
“...,.I
f I
.,..,I lo2
Energy
J y;
;
““”
10'
10'
10S
(GcV)
Fig 4 The BETS results (crosses) and the emulsion data (filled circles) are compared urlth Each of the the best fit curve of dlffuslon model for D = 1 x 102s(E/1GeV)o 3crn2/sec contnbutlons
from near-by
sources IS denoted
by dotted
lmes with the source name
The azimuthal-angle lstnbutlon of the electron candidates was measured for energies lower than 20 GeV As expected for negatively-charged particles, the &stnbutlon has promment excess from the east (Toni et al, 1998) The result rmght be an mdlrect proof that we could have successfully reJected the proton backgrounds SUMMARY
AND DISCUSSION
A new telescope which adopts a scmtlllatmg-fiber lmagmg-calonmeter has been successfully developed to observe electrons from 10 GeV to a few 100 GeV Although the full rejection power has not been demonA crude analysts of the strated yet, BETS has a potential power m excess of lo4 up to the TeV region balloon expenment m 1997 has shown that the shower profiles are efficient to chscrnnmate electrons from the proton background up to several 100 GeV The observed flux above 10 GeV under 5 g cm-’ of average residual atmosphere 1s consistent with the HEAT result Final conclusions of the electron energy spectrum will be pubhshed after some corrections A senes of balloon experiments will be carned out to observe the It 1s also planned to use a BETS-type detector for electrons up to several 100 GeV with sufficient statlstlcs the TeV observation m space (Toni et al, 1998b) ACKNOWLEDGMENTS We We are indebted to the crew of the Sanrlku Balloon Center, ISAS for the successful balloon fight express smcere thanks to the techmcal staffs m Kanagawa Umverslty for their helps m manufacturing of the detector This work 1s partially supported by Grants-m-Aid for Scientific Research (A) REFERENCES Barwlck S W et al, Astrophyszcal Journal, 498, 779 (1998) Golden, R L et al, Astrophyszcal Journal, 287, 622 (1984) Komorl Y , pnvate commumcatlon (1998) Nlshlmura J et al, Astrophyszcal Journal, 238, 394 (1980) Nlshlmura J , Proc of Towards a Major Atmospherzc Cerenkov Detector III, echted by T Klfune, pp l-10, Universal Academy Press Inc , Tokyo (1994) Nlshlmura J et al, PTOCof 25th Internataonal Cosmzc Ray Conference (Durban), 4, 233 (1997) Tamura T et al, this conference (1998) Tang K K, Astrophyszcal Journal, 278, 881 (1984) Toni, S et al, Proc of the Internatzonal Soczety for Optzcal Engzneenng, 2806, 145 (1996) Toru, S et al, Proc of 20th Internataonal Symposzum on Space Technology and Sczence (1998a) (in press) Toni, S , et al, thrs volume (1998b) Toru, S et al, Proc of the Internatzonal Soczety for Optzcal Engzneenng, 2806, 145 (1996)
Adv Space Res Vol 26, No 11, pp 1823-1826,200O 0 2001 COSPAR Pubhshed by Elsevler Science Ltd All nghts reserved Pnnted In Great Bntain 0273-l 177/00 $20 00 + 0 00 PII SO273-1177(99)01229-6
NEW
TO
MEASUREMENT OF THE ELECTRON FLUX 100 GEV WITH THE BETS INSTRUMENT
S Toru’, T Tamural, N Tateyama’, K Yoshlda’, T Yamagaml 2, E Kamloka2, T Kobayash14,Y Komorl”, K Kasahara’, T Yuda7, and J Nlshlmura’ ’ 2 3 4 5 6 7 8
FROM
Y Saltol,
Faculty of Engzneerzng, Kanagawa Unzverszty, Yokohama, JAPAN Innstatute of Space and Astronautzcal Scaence, Sagamzhara, JAPAN Department of Physzcs, Rakkyo Unaversaty, Tokyo, JAPAN Department of Physacs, Aoyama Gakuan Unauersaty, Tokyo, JAPAN Kanagawa Prefectural College of Nursang and Medacad Technology, Yokohama, Shabaura Instatute of Technology, Ohmaya, JAPAN Instatute for Cosmac Ray Research, Unaversaty of Tokyo, Tanasha, JAPAN Yamagata Instatute of Techonology, Yamagata, JAPAN
10 GEV
H Murakam13,
JAPAN
ABSTRACT The BETS (balloon-borne electron telescope with scmtlllatmg fibers) instrument has been developed for high-altitude balloon flights to observe the cosmic ray electrons with energies of 10 GeV to several 100 GeV The detector 1s a Lead/SclFl sampling calorimeter conslstmg of 36 SclFl belts (each 280 mm wide) and 8 lead plates (each 5 mm thick) The electron ldentlficatlon 1s performed by trlggermg the electro-magnetic showers on board and by analyzing the three-dimensional shower images by an intensified CCD camera It IS demonstrated m the flight data m 1995 and 1997 that a reliable ldentlficatlon of the electron component against the proton background 1s achieved up to a few 100 GeV The performance of detector was tested by the CERN-SPS electron beams m 1996 and with the proton beams m 1997 The obtained energy spectrum 1s consistent with the recent observation by HEAT, although our result still has a little room for improvement The energy spectrum from 10 GeV to 1000 GeV which 1s obtained by combmmg these data and the emulsion chamber data ( Nlshlmura et al 1997)suggests that the diffusion constant 1s about 1 x 1028(E/GeV)o 3 cm2/sec m the energy range between 10 GeV and 1000 GeV A hump m the energy spectrum 1s observed around several hundred GeV, which 1s expected from a nearby source 0 2001 COSPAR Published by Elsevler Science Ltd All rights reserved INTRODUCTION It 1s commonly understood that high-energy electrons are accelerated m supernova remnants (SNRs), and they lose their energies by synchrotron radiation and by inverse Compton scattering during the propagation through the Galaxy The energy spectrum of electrons from 10 GeV to 100 GeV is, therefore, a key to Investigate the diffusion characterlstlcs m the Galaxy and the source spectrum Since the energy-loss rate by these mteractlons 1s proportional to the energy, hfe of electron becomes shorter with increasing of the energy It 1s , therefore, suggested that each of the nearby sources could be ldentlfied by the measurements of the spectral shape and the isotropy m the TeV region (See for example, Nlshlmura, 1994) However, observation of high-energy electrons 1s still a difficult task, although every effort has been made for the past 30 years The reason lies m the copious hadromc backgrounds which increase with increase m energy Instruments which mcorporate transition radiation detectors (Tang, 1984) or gas Cerenkov counters (Golden et al ,1984) have successfully observed electrons m the energy region lower than 100 GeV They are, however, inefficient m reJectIon of the background protons over several 100 GeV and m increasing the statistics due to the smaller solid angles Emulsion chambers (EC) have uniquely achieved observations of electrons m the TeV regton (Nlshlmura et al ,198O) They are, however, not suitable to use for very long duration observation because of accumulation of the backgrounds and to measure the amsotropy The Balloon Borne Electron Telescope with Scmtlllatmg Fiber (BETS) has been developed m order to study the sources and the diffusion characterlstlcs of high-energy electrons The telescope IS deslgned to have the capability of measuring the precise energy spectrum and the arrival directions m the energy region from 10 GeV to several 100 GeV
1823
S Tom et al
1824
BALLOON OBSERVATION Detector The BETS instrument consrsts of a shower detector mcorporating an imaging calorimeter and a trigger system, an electronics system, a data recording system and a telemetry system Details of the telescope and the method of data analysis are presented m Torn et al (1998a,1996) A recent study of the performance mcludmg an accelerator test at CERN-SPS 1s presented m a paper m this conference (Tamura et al ,1998) In Table 1, the basic performance of the BETS payload 1s summarized.. Table 1 Instrument performance summary 1 Energy Range md.l-1ra.l f%-tmr Gee..__-____ Proton/Electron Drscrrmmatron Enerev Resolutron Angular resolutron Weight Power Consumotion ___“__
-----Da
-------~----
1 10 GeV N several 100 GeV 1 I snn .d I3r 1 a few 10’ I-17% 1 degree typical N 320 kg 130 W (at maximum) _
_
_
_
___
The calorrmeter consrsts of 36 layers (18 m each of two orthogonal views) of the ScrFr belts with the lead plates of total depth of -7 r 1 Each belt IS composed of 280 round ScrFr’s (1 mm 4 each) m one millimeter pitch The total number of SCIFI’S1s 10,080 (5,040 for each dnectron) The guide part of fibers are made of clear fibers which have no efficiency to charged-particles m order to remove the noise The trigger system was optrmrzed by srmulatlon to enhance the ratio of electrons m the triggered events Three plastic scmtrllators were set at the top of detector (Sr), the depth of N 2 r 1 (Sz ) and the bottom (Ss ), respectively Each scmtrllator with a thickness of lcm IS viewed by a photomultrplier through a light guide For the observatron of electrons over 10 GeV and with a zenith angle smaller than 30 degrees, the highest proton-reJectron power of N 150 at 85% electron efficiency 1s realized rf we impose the numbers of particles observed m Sr, Sz and Sa be 0 7 N 5, 10 N 100, and 2 40 (m unit of single mmrmum romzmg particle of vertical incident), respectively The rejection power 1s enough to decrease the trigger rate during observatron as low as N 2 Hz Scmtrllatmg fibers outputs were observed with an image-mtensrfied CCD camera The CCD camera has an input window with a diameter of 10 cm and has a “shutter” function which 1s triggered by a gate signal from the trigger system An image on the input window IS reduced by one-twelfth m size on the sensing area of CCD The area contacted with the cross sections of all SclFr’s corresponds approximately to 512 x 512 pixels (5 816 x 5 96 mm m area) on CCD A video module especially developed, reads 512 x 256 prxels (for the 2 1 interlace CCD scanning method) and the signals whrch exceed the noise level are digitized wrth an 8-bit flash ADC By averaging the mtensltres m the adJacent two pixels, the amount of data 1s reduced by a factor of two, 1 e 256 x 256 pixels Balloon Flight The instrument with a total weight of 320 kg and the power consumptron of 130 W was launched m June, 1997 from Sannku Balloon Center m Japan, and was flown for about 4 5 hours at an altitude over 35 km The event trigger was carried out either by three-hold comcrdence of Sr, Sz and Sa (SH mode) or by two-fold of &and Sz (SI mode) The SH mode for shower events was used mostly m electron observatron, and the SI mode was for heavy particles The observatron of heavy tracks was planned for the cahbratron of posrtron The trigger rate for showers over the threshold energy was N 1 7 Hz at the level flight The number of triggered events 1s about 28,000
Electron selection by the image data analysis was achieved for the triggered events by the followmg steps 1 2 3 4
Shower axis passes through all ScrFr layers The zenith angle 1s less than 30 degrees Charge of mcrdent particle 1s single Ratio of energy-deposrtlon wrthm 5mm from the shower axis to total (RE) 1s higher than certain value
1825
NewMeasurementof the ElectronFlux From 10 GeV to 100 GeV
For By the reductions up to the step of 3, mcludmg on-board tngger, about 99% protons are reJected showmg how much reJectIon power can be improved by the analysis of energy concentration m the step 4, the simulated results are presented m Fig 1 The slmulatlon events were “triggered” by reahstlc condltlons If we select the events of which RE 1s higher than 0 7, the which are almost same as the observations The total rejection power agamst the backprotons are rejected by 95% with 85 % electrons remaining The observed dlstrlbutlon presented m Fig 2 ground protons W, therefore, estimated to be a few thousands 1s consistent with the slmulatlon, and the number of electron candidates m RE > 0 7 IS nearly 600
Simulated Events
-------
Flight Events at Level ( > 35 km )
Electrons(E>lOGeV)
60. 40. 20.
02
04
06
06
Rabo of Energy Cqw~lbcm wkhln 5mm from Shower AXIS
Ratio of Energy Deposition withm Gun from Shower Axis
Fig 1 Dlstrlbutlon of REs for slmulatlon events which are “triggered” by same condition with the observation
ELECTRON
Fig 2 Observed RE dlstrlbutlon for flight events at an altitude over 35 km
FLUX
The energy spectrum of observed electrons was obtained by consldermg the energy dependence of energy resolution and geometrical factor by the accelerator tests and slmulatlons Also, m the estlmatlon, correction factors were calculated for the secondary electrons and gamma-rays, and the maldentlfied protons However, the flux 1s still prehmmary, and it may be changed by nearly 10% mainly by correction of dead time of the detector, which 1s not considered m this paper It 1s seen m Fig 3, our flux IS considerably smaller than previous observations m the energy region of 10 GeV to 100 GeV The recent measurement by the HEAT detector (Barwlck et al, 1998) IS, however, consistent with ours
et al
loo
IO’
lo2 ENERGY
(Radio)
‘80
IO4 ( GeV
)
Fig 3 Companson of our results (BETS ‘99) with HEAT and previous observations Solid line presents calculated spectrum by dlffuslon model for all sources, dotted the nearby sources
S Tom era1
1826
The best fit curve of the model calculations presented m Fig 4 gives a diffusion constant of 1 x 1028(E/GeV)o 3 cm2/sec for the SN rate of once per 30 years (Nlshlmura et al, 1997) The estimated magnetic field by companng the electron flux around 10 GeV to the r&o flux 1s obtamed to be N 7 PG (Komon, 1998)
-,10'
7
:
_
/*
--
-
5.
c
7
.
10'
7
LLbZp1-"
-7
r
cimmgn /\
i’
I
IO’
“.,,_I loo
Cg
1iq I 10'
“...,.I
f I
.,..,I lo2
Energy
J y;
;
““”
10'
10'
10S
(GcV)
Fig 4 The BETS results (crosses) and the emulsion data (filled circles) are compared urlth Each of the the best fit curve of dlffuslon model for D = 1 x 102s(E/1GeV)o 3crn2/sec contnbutlons
from near-by
sources IS denoted
by dotted
lmes with the source name
The azimuthal-angle lstnbutlon of the electron candidates was measured for energies lower than 20 GeV As expected for negatively-charged particles, the &stnbutlon has promment excess from the east (Toni et al, 1998) The result rmght be an mdlrect proof that we could have successfully reJected the proton backgrounds SUMMARY
AND DISCUSSION
A new telescope which adopts a scmtlllatmg-fiber lmagmg-calonmeter has been successfully developed to observe electrons from 10 GeV to a few 100 GeV Although the full rejection power has not been demonA crude analysts of the strated yet, BETS has a potential power m excess of lo4 up to the TeV region balloon expenment m 1997 has shown that the shower profiles are efficient to chscrnnmate electrons from the proton background up to several 100 GeV The observed flux above 10 GeV under 5 g cm-’ of average residual atmosphere 1s consistent with the HEAT result Final conclusions of the electron energy spectrum will be pubhshed after some corrections A senes of balloon experiments will be carned out to observe the It 1s also planned to use a BETS-type detector for electrons up to several 100 GeV with sufficient statlstlcs the TeV observation m space (Toni et al, 1998b) ACKNOWLEDGMENTS We We are indebted to the crew of the Sanrlku Balloon Center, ISAS for the successful balloon fight express smcere thanks to the techmcal staffs m Kanagawa Umverslty for their helps m manufacturing of the detector This work 1s partially supported by Grants-m-Aid for Scientific Research (A) REFERENCES Barwlck S W et al, Astrophyszcal Journal, 498, 779 (1998) Golden, R L et al, Astrophyszcal Journal, 287, 622 (1984) Komorl Y , pnvate commumcatlon (1998) Nlshlmura J et al, Astrophyszcal Journal, 238, 394 (1980) Nlshlmura J , Proc of Towards a Major Atmospherzc Cerenkov Detector III, echted by T Klfune, pp l-10, Universal Academy Press Inc , Tokyo (1994) Nlshlmura J et al, PTOCof 25th Internataonal Cosmzc Ray Conference (Durban), 4, 233 (1997) Tamura T et al, this conference (1998) Tang K K, Astrophyszcal Journal, 278, 881 (1984) Toni, S et al, Proc of the Internatzonal Soczety for Optzcal Engzneenng, 2806, 145 (1996) Toru, S et al, Proc of 20th Internataonal Symposzum on Space Technology and Sczence (1998a) (in press) Toni, S , et al, thrs volume (1998b) Toru, S et al, Proc of the Internatzonal Soczety for Optzcal Engzneenng, 2806, 145 (1996)