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Janzos results on PeV and TeV gamma rays from SN1987A and the future plan for a TeV collaboration with Adelaide
Nuclear Physics B (Proc. Suppl.) 14A (1990) 183-187 North-Holland
JANZOS RESULTS ON PEV AND TEV G COLLABORATION WITH ADELAIDE
183
RAYS FROM SN1987A AND THE FUTURE PLAN FOR A TEV
TTarimod (JANZOS Collaboradon) National Laboratory for High Energy Physics (KEIL), Oho 1-1 Tsukuba, Ibaraki 305, Japan Very and Ultra high energy gamma rays from SN1987A are searched in New Zealand since 1987 . The new results measured in the both energy regions of very and ultra high energy gamma rays during 1988 are the end ofparticular, reported . In the Cherm*cv observation with large zenith angle provided the most restrict upper bound for -l00 TeV region gamma rays flux frorn SN1987A. Also, It is planned to construct an imaging TeV telescope for improving the TeV observation in the Southern hemisphere at era, Australia with the corporation of Adelaide University.
1. Introduction A Collaboration of Japan, Australia, and New Zealand
0
(JANZOS) started in late 1987 to monitor TeV and PeV region
13
gamma ray emission from SN1987A at the Black Birch Range in
0
New Zealand ( 1640 m above sea level, 41 0 45'S, 173°47"E). The TeV regions are measured by Cherenkov mirror teleszopos, and the PeV region gamma ray is done by air-shower counter
arrays . The first results are already publishedl 2, indicating that SN1987A is not an extraordit. rily strong source of very high energy gamma rays during the first one year since the explosion.
Our data during January 14 and 15, 1988 suggests that the source can be, however, occasionally active in the TeV region.
According to the recent opticalobservation, SN1987A may have
been identified as a sub-msecond pulsar 3 . From these observations, a continuation of the monitoring of SN1987A is very necessary, which motivates us to construct a rmaâ ng Cherenkov telescope.
2. Results from
JANZOS scintillation detectors
PeV region gamma ray is observed by a surface air-shower
array detector which consists of 76 scintillation counters . Fortyfive of the scintillation counters with 0.5 m2 area are viewed by
fast 2-inches photomultipliers (PMTs) in order to obtain the good
13
0
15
0 93
0 0 0
0 0
a
d--.
Fig. 1 The arrangement of detectors on the Black BirchRange. Circles with crosses and circles with p=usses, two groups of 0.5m2 deLec ors,1-m2detectors. open circles, minors. The trigger is caused by four fold coincidence of the 0.5 m2 counters, which threshold level is 1.8 times or the signal of one
particle passage in the detector. The trigger rate is about. 1 Hz . The effective exposure (defined as the product of effectivearea and observe-on time) wascalcuated by mante Cal° method for
time :esolution. On each of these counters, lead of 5 mm thickness is placed to convert gamma rays in air showers to
agamma ray from the directionof SN1987A, and estimated to b e 10 11 cm2s per day at 100TeV. The arrival direction of an air
particl,;-density measurement. The arrangement of counters is
artivil direction was estimated to be 1.73 0 (rms) for 100 TeV
electron positron pairs. Theother 31 scintillation couters with 1 m2 are viewed by 5-in photomultiplers, which are used for the shown in Fig. 1. 0020-5632/90%$03 .50 © Elsevier Science Publishers 13-V . (North-Holland)
shower is calculated by determining the shower front from the timing of tire 0.5 m2counters.The angular resolution of the
gamma rays . Details of the apparatû5 and the analysis was
rays from SN 1987A T. Tanimori, JANZOZ Colllaboration/JANZOZ results on PeV and TeV gamma
18 4
described in Ref.l. Also, the first result of the observation during Oct. 87 and Dec. 87 was already reported in Ref 1. In his report, we present the preliminary results of the
observation until Jan. 89. Figure 2 shows a right-Ascension scan of events near the declination of SN1987A. The angular window in this figure is defined as a circle of radius 1.730 centered at declination -69.3 0. No excess was observed around SN1987A, and new upper limit for 100 TeV gamma ray flux from SN1987A is estimated to be 4.0x10 -1 3cm-2 s-1 with 950
Boo
N
ô
U
400
confidence level.
The searches for ultra high energy gamma rays are done in other X-ray source in the southern hemisphere (LMCX-4, 3C273, SCO X-1, VELA PULSAR, VELA X-1) . For all source, no excess was observed. The preliminary results on -100 TeV gamma ray flux limits from these source obtained by this observation are given in Table 1.
Source
(preliminary)
LMC X-4
Flux Limit
(10 -1 3cm -2 s-1 ) 2.3
3C273
1.2
VELA X-1
Threshold Energy (TeV) 100
300
SCO X-1
2.3
100
9.5
50
VELA PULSAR
6.8
50
044
84
64
104
124
Right Ascension (deg.)
Fig. 2 The events observed from May to July 1988 at zenith angles are shown as a function of Right Ascension (RA). Only events in the declination strip -69.30 t0.5 0 are included. The bin width is 1 .50 in RA. The expected background level is drawn by the smooth curve. The live time of the observation is 39 hour:. The median energy of detectable shower is estimated to be 75 TeV. Cherenkov observation was recently suggested by Sommers and Elberrt4 . An air shower initiated by TeV gamma rays develops above -10 km altitude, and Cherenkov lights emitted from a shower pass this distance through thick atmosphere to the ground
The spread of Cherenkov lights is determined by the pressure of atmosphere, and typically 1 degree. Cherenkov light level.
3. The result from JANZOS Cherenkov telescope Our Cherenkov telescope consists of three mirrors with 2m
diameter and 2m focal length, which placed at the vertex of 80 m triangle . These mirrors are fixed, and are used by "drift scan
mode" At each mirror, ten fast 2-inches PMTS were arranged in
a strip viewing along the path of the objects. The field of view of each PMT was defined in 2.3 0x2 .3 0 by a light guide. Both
pulse height and timing of signals are measured in order to determine the energy and arrival direction of an air shower. In
particular, by using the timing difference between each mirrors, high angular resolution (-0.5 0 FWHM) is achieved. The detail of this experimental apparatus was described in Ref. 2.
SN1987A can be observed at two periods in one year, when it crosses the meridian with the highest and lowest elevation (small and large zenith angle) . The possibility of large zenith angle
from a vertical shower spreads within a circle of --200 m diameter, called "light pool", after it passes through -7 km depth of atmosphere. On the other hand, an inclined shower with large
zenith angle passes through large depth of atmosphere, and then
the light pool geometrically spreads to an extensively large area, whereas the threshold energy for detectable showers increases.
Therefore, we get a huge detection area for the observation of -100 TeV airshower compared with shower array counters.
The data was analyzed by the same way as that for the
previous result which was described in Ref. 2. Figure 2 shows the distribution of observed events as a function of Right Ascension (RA) in the declination strip -69.3 ± 0.50 where the
smooth curve indicates the expected number from the background cosmic rays . There is seen no significant excess at the position of SN 1987A (84.00 in RA).
T. Tanimori, JANZOZ Colllaboration/JANZOZ results on PeV and TeV gamma rays from SN 1987A In order to get the flux upperlimit and the luminosity upper
limit, the size of the light pool on our ground level was calculated
from the zenith angle and the depth of the maximum development of an air shower. In our case, the path length of the Cherenkov
light from the shower maximum to the observation point was
about 27 km, and the effective area was estimated to be 7.2x109 m2 . The median energy can be extrapolated from the measurement for vertical showers (2 TeV median energy), considering the light pool size and the absorption by the Rayleigh scattering (mean free pass in air 2974 g/cm2). The median
energy at 680 zenith angle was estimated to be 75 TeV for our observation. This median energy was consistent with the energy estimated from the background cosmic ray counting rate.
18 5
From these parameters, the upperlimit for the gamma ray flux from SN1987A was estimated to be 5.lxl0-14cm-2 s-1 for-75 TeV shower with 95% confidence level. This upper limit corresponds to the upper limit for the luminosity of 1037erg s-1, where the power law spectrum of 2.0 differential index with the
cutoffat 1017TeV was assumed. Figure 3 shows ourresults as well as the ones by other groups. As shown in this plot, this work provides the lowest limit for very high energy gamma rays
from SN1987A. The detail of this analysis will be published in ReE5 .
An observation was done forVela X-1(a 128.40A -45.00)
by the Cherenkov mirror during Jan.88May88. In this period,
Vela X-1 passes at midnight nearly through the zenith of the sky. to Fig. 4, the ratio of the observed events to the expected ore is
shown. The median energy for Vela X-1 was estimated to be 2
TeV. There canbe seen no significant excess around the Vela X-
-9 -10
1. From this data, the upper limit for-2TeV gamma ray flux was
P
1
ai0
-11
T
LL
J3
for SN1987A. This upper limit, as a do flux,
4
T
E
w
obtained to be 6.05x 10-12cm-2s-1 , and the upper limit for the luminosity was 9.02x1034 erg s-1 using the same assumption as
D
S -il
-12
contradicts with the results of Roubenheim et al. which reported the observation of gamma ray flux from Vela X-1 with 1.5_2.0xl0-llcm-2 s-1 (2 TeV) . Although any excess was not observed in the R.-A distribution, the period analysis is now
a
T
-13
strongly
being examined for the same data.
J5 /`
-14 -15 10
12
.
14
16
LOG E (eV)
Fig.3 The present result with other previous results are shown. The present result designated by ' J5'. 'J2' and 'J3' are our previous TeV results2 , corresponding to the upperlimit for DC signals obtained by the observation during December 1987 and January 1988 and the burst of January 14-15th 1988, respectively. The 100 TeV result obtained by the JANZOS scintillation detector array is indicated by 'J1', and 'J4' is the updated data . 'A' , 'P' ;D' and 'S' represent the results of Adelaide groroup6 , Potchestroom group , Durham group8 and SPASE9 .
Right Ascension (deg .)
Fig. 4 The ratio of observed to expected count is plotted as a function of RA for the observation from January 1988 to May 1988 . The bin width is 1 .50 in RA. Only events in the declination strip -45.00 ±0.50 are included .
18 6
T. Tanimori, JANZOZ Colllaboration/JANZOZ results on PeV and TeV gamma rays from SN 1987A
4 Future Plan : Collaboration of Australia and Nip n for Gamma Ray Observation at Woomera (CANG OO Collaboration) Some of the JANZOS Collaboration proceed thenext plan to
construct a fine imaging Cherenkov telescope for continuing and improving the TeV gamma ray observation in the Southern
hemisphere in corporation with Adelide University in Australia (CANGAROO Collaboration) . The telescope will be located at Woomera, where is about 1000 km north from Adelaide in Australia.
A parabolic mirror of 3.8-m diameter with f-number equal to
one, which was used by optical astronomers for lunarranging, is
to be used as a light collector. this mirror has a good surface
quality (Blur" 1 ") to resolve the fine structure of the Cherenkov
light image . A image camera placed at focal plane will consistof 450 fast PMTs of 3/8 inches square, which has a large field of
The event trigger is a multiplicity of the number of fired PMTs, which is a good measure of the amount of Cherenkov light in the present case of small amount of photoelectrons compared to the large number of PMTS . In order to avoid a trigger due to the night sky background which follow a
Poissonian distribution with the mean value of about 30 photoelectrons, a spatial clustering of the fired PMTs will be
further required. The cluster consists of about 10 PMTs, and has
a about 1 .50 x1 .5° field of view which is almost equal to the size of a shower induced by a gamma ray shower. It will be required
at the nigger level that almost of photon concentrate into a few clusters . This trigger scheme will enable us to reject the almost
of the fake niggers caused by the night sky background. Then, we expect that the threshold of the trigger can be set lower than 0.5 TeV for an air shower induced by a gamma ray, which
view with 40 (each PMT views 0.150x0.150). In Fig.5, the
corresponds about 10 Hz trigger rate for an air shower induced
Monte Carlo simulationl 0 was superimposed onto the filed of
the telescope of Whipple observatory (10-m diameter mirror),
the field of view of 40. By square shaped PMTS, the dead space of a camera can be decreased to 15% of the total field. The
achieved . Furthermore, we will be able to reject about 2/3 of the
Cherenkov light image of 250 GeV gamma ray obtained by view, where squares represent views of PMTs and the circle is
number of photoelectrons collected from 1 TeV gamma ray is
estimated to be about 85 ( reflection coefficient of mirror -0.7,
quantum efficiency of a PMT -0.26 for400 nm photon), and the background from night sky is also estimated to be about 30 for 10 ns ( NSB - 780 m-2ns-1 sr~l, quantum efficiency for 500 nrn photon-0.2).
~I
by acosmic ray proton. In spite of a small mirror compared with very low threshold energy as same as that of Whipple will be
night sky background against the signal arriving within 3-5 ns, by using the timing of fired PMTs . The pulse-height information
are necessary to reconstruct the image pattern like the one shown in Fig.5 . The major axis of the image is determined to infer the arrival directionof gamma ray within an accuracy of a few arc of minutesl 1.
The proposed Cherenkov telescope is expected to discriminate
gamma rays from photon showers by exploiting the structure of the observed image as demonstrated by Whipple group. 12 We
""imm"""""""r-m" ir NEMESES u" u"a mons """""numa! mai
also expect that the present telescope can reduce and monitor the systematic errors during the observation, because separate groups of PMTs will be used for on- and off-source
measurement, simultaneously, and a large amount of night sky Insu"""""üjT
background data can be utilized to monitorthe sky condition as """""
!
i\0"""""""m"""" !" mri c""""". """""""""I . "~"""" 1 "o 2 "1 "
Fig. 5 the arrangement of photomltipliers of the imaging camera, where a image of a shower induced by 250 GeV gamma ray and a field of view with 40 diameter represent.
well as the gain change of PMTs.
T. Tanimori, JANZOZ Colllaboration/JANZOZ results on PeV and TeV gamma rays from SN 1987A efferences 1. M. J. Conway et al., Phys . Rev. Len.
(1988) 1110.
2. I. A. Bond et al., Phys. Rev. Lett. 61 (1988) 2292. 3. J. Krstian et al ., Nature 338 (1989) 234. 4. P. Sommers and J. W. E1bert, J. Phys. G: Nucl. Phys. 13 (l987) 553. 5. I. A. Bond et al., io be published in Ap. J. Letters. 6. D. Ciampa, et al ., Astrophys. J. 326 (1988) L9.
187
7. B. C. Raubenheimer et al., Astron. Asttophys.,193 (1988) Lll. 8. P. M. Cadwick et al., Astrophys. J., 333 (1988) L19. 9. T. K. Gaisser et al., Phys. Rev. Lett., 62 (1989) 1425. 10. A. M. Hillas and J. R. Patterson, in Very High Energy Gamma Ray Astronomy, edited by K. E. Zurver (D. Reidel publishing Company, 1987). 11 . A. M. Hillas, a Talk presented in the Crimea Workshop (1989). 12 . T.C. Weeks et al ., Astrophys. J. July 1, (1989) .
JANZOS RESULTS ON PEV AND TEV G COLLABORATION WITH ADELAIDE
183
RAYS FROM SN1987A AND THE FUTURE PLAN FOR A TEV
TTarimod (JANZOS Collaboradon) National Laboratory for High Energy Physics (KEIL), Oho 1-1 Tsukuba, Ibaraki 305, Japan Very and Ultra high energy gamma rays from SN1987A are searched in New Zealand since 1987 . The new results measured in the both energy regions of very and ultra high energy gamma rays during 1988 are the end ofparticular, reported . In the Cherm*cv observation with large zenith angle provided the most restrict upper bound for -l00 TeV region gamma rays flux frorn SN1987A. Also, It is planned to construct an imaging TeV telescope for improving the TeV observation in the Southern hemisphere at era, Australia with the corporation of Adelaide University.
1. Introduction A Collaboration of Japan, Australia, and New Zealand
0
(JANZOS) started in late 1987 to monitor TeV and PeV region
13
gamma ray emission from SN1987A at the Black Birch Range in
0
New Zealand ( 1640 m above sea level, 41 0 45'S, 173°47"E). The TeV regions are measured by Cherenkov mirror teleszopos, and the PeV region gamma ray is done by air-shower counter
arrays . The first results are already publishedl 2, indicating that SN1987A is not an extraordit. rily strong source of very high energy gamma rays during the first one year since the explosion.
Our data during January 14 and 15, 1988 suggests that the source can be, however, occasionally active in the TeV region.
According to the recent opticalobservation, SN1987A may have
been identified as a sub-msecond pulsar 3 . From these observations, a continuation of the monitoring of SN1987A is very necessary, which motivates us to construct a rmaâ ng Cherenkov telescope.
2. Results from
JANZOS scintillation detectors
PeV region gamma ray is observed by a surface air-shower
array detector which consists of 76 scintillation counters . Fortyfive of the scintillation counters with 0.5 m2 area are viewed by
fast 2-inches photomultipliers (PMTs) in order to obtain the good
13
0
15
0 93
0 0 0
0 0
a
d--.
Fig. 1 The arrangement of detectors on the Black BirchRange. Circles with crosses and circles with p=usses, two groups of 0.5m2 deLec ors,1-m2detectors. open circles, minors. The trigger is caused by four fold coincidence of the 0.5 m2 counters, which threshold level is 1.8 times or the signal of one
particle passage in the detector. The trigger rate is about. 1 Hz . The effective exposure (defined as the product of effectivearea and observe-on time) wascalcuated by mante Cal° method for
time :esolution. On each of these counters, lead of 5 mm thickness is placed to convert gamma rays in air showers to
agamma ray from the directionof SN1987A, and estimated to b e 10 11 cm2s per day at 100TeV. The arrival direction of an air
particl,;-density measurement. The arrangement of counters is
artivil direction was estimated to be 1.73 0 (rms) for 100 TeV
electron positron pairs. Theother 31 scintillation couters with 1 m2 are viewed by 5-in photomultiplers, which are used for the shown in Fig. 1. 0020-5632/90%$03 .50 © Elsevier Science Publishers 13-V . (North-Holland)
shower is calculated by determining the shower front from the timing of tire 0.5 m2counters.The angular resolution of the
gamma rays . Details of the apparatû5 and the analysis was
rays from SN 1987A T. Tanimori, JANZOZ Colllaboration/JANZOZ results on PeV and TeV gamma
18 4
described in Ref.l. Also, the first result of the observation during Oct. 87 and Dec. 87 was already reported in Ref 1. In his report, we present the preliminary results of the
observation until Jan. 89. Figure 2 shows a right-Ascension scan of events near the declination of SN1987A. The angular window in this figure is defined as a circle of radius 1.730 centered at declination -69.3 0. No excess was observed around SN1987A, and new upper limit for 100 TeV gamma ray flux from SN1987A is estimated to be 4.0x10 -1 3cm-2 s-1 with 950
Boo
N
ô
U
400
confidence level.
The searches for ultra high energy gamma rays are done in other X-ray source in the southern hemisphere (LMCX-4, 3C273, SCO X-1, VELA PULSAR, VELA X-1) . For all source, no excess was observed. The preliminary results on -100 TeV gamma ray flux limits from these source obtained by this observation are given in Table 1.
Source
(preliminary)
LMC X-4
Flux Limit
(10 -1 3cm -2 s-1 ) 2.3
3C273
1.2
VELA X-1
Threshold Energy (TeV) 100
300
SCO X-1
2.3
100
9.5
50
VELA PULSAR
6.8
50
044
84
64
104
124
Right Ascension (deg.)
Fig. 2 The events observed from May to July 1988 at zenith angles are shown as a function of Right Ascension (RA). Only events in the declination strip -69.30 t0.5 0 are included. The bin width is 1 .50 in RA. The expected background level is drawn by the smooth curve. The live time of the observation is 39 hour:. The median energy of detectable shower is estimated to be 75 TeV. Cherenkov observation was recently suggested by Sommers and Elberrt4 . An air shower initiated by TeV gamma rays develops above -10 km altitude, and Cherenkov lights emitted from a shower pass this distance through thick atmosphere to the ground
The spread of Cherenkov lights is determined by the pressure of atmosphere, and typically 1 degree. Cherenkov light level.
3. The result from JANZOS Cherenkov telescope Our Cherenkov telescope consists of three mirrors with 2m
diameter and 2m focal length, which placed at the vertex of 80 m triangle . These mirrors are fixed, and are used by "drift scan
mode" At each mirror, ten fast 2-inches PMTS were arranged in
a strip viewing along the path of the objects. The field of view of each PMT was defined in 2.3 0x2 .3 0 by a light guide. Both
pulse height and timing of signals are measured in order to determine the energy and arrival direction of an air shower. In
particular, by using the timing difference between each mirrors, high angular resolution (-0.5 0 FWHM) is achieved. The detail of this experimental apparatus was described in Ref. 2.
SN1987A can be observed at two periods in one year, when it crosses the meridian with the highest and lowest elevation (small and large zenith angle) . The possibility of large zenith angle
from a vertical shower spreads within a circle of --200 m diameter, called "light pool", after it passes through -7 km depth of atmosphere. On the other hand, an inclined shower with large
zenith angle passes through large depth of atmosphere, and then
the light pool geometrically spreads to an extensively large area, whereas the threshold energy for detectable showers increases.
Therefore, we get a huge detection area for the observation of -100 TeV airshower compared with shower array counters.
The data was analyzed by the same way as that for the
previous result which was described in Ref. 2. Figure 2 shows the distribution of observed events as a function of Right Ascension (RA) in the declination strip -69.3 ± 0.50 where the
smooth curve indicates the expected number from the background cosmic rays . There is seen no significant excess at the position of SN 1987A (84.00 in RA).
T. Tanimori, JANZOZ Colllaboration/JANZOZ results on PeV and TeV gamma rays from SN 1987A In order to get the flux upperlimit and the luminosity upper
limit, the size of the light pool on our ground level was calculated
from the zenith angle and the depth of the maximum development of an air shower. In our case, the path length of the Cherenkov
light from the shower maximum to the observation point was
about 27 km, and the effective area was estimated to be 7.2x109 m2 . The median energy can be extrapolated from the measurement for vertical showers (2 TeV median energy), considering the light pool size and the absorption by the Rayleigh scattering (mean free pass in air 2974 g/cm2). The median
energy at 680 zenith angle was estimated to be 75 TeV for our observation. This median energy was consistent with the energy estimated from the background cosmic ray counting rate.
18 5
From these parameters, the upperlimit for the gamma ray flux from SN1987A was estimated to be 5.lxl0-14cm-2 s-1 for-75 TeV shower with 95% confidence level. This upper limit corresponds to the upper limit for the luminosity of 1037erg s-1, where the power law spectrum of 2.0 differential index with the
cutoffat 1017TeV was assumed. Figure 3 shows ourresults as well as the ones by other groups. As shown in this plot, this work provides the lowest limit for very high energy gamma rays
from SN1987A. The detail of this analysis will be published in ReE5 .
An observation was done forVela X-1(a 128.40A -45.00)
by the Cherenkov mirror during Jan.88May88. In this period,
Vela X-1 passes at midnight nearly through the zenith of the sky. to Fig. 4, the ratio of the observed events to the expected ore is
shown. The median energy for Vela X-1 was estimated to be 2
TeV. There canbe seen no significant excess around the Vela X-
-9 -10
1. From this data, the upper limit for-2TeV gamma ray flux was
P
1
ai0
-11
T
LL
J3
for SN1987A. This upper limit, as a do flux,
4
T
E
w
obtained to be 6.05x 10-12cm-2s-1 , and the upper limit for the luminosity was 9.02x1034 erg s-1 using the same assumption as
D
S -il
-12
contradicts with the results of Roubenheim et al. which reported the observation of gamma ray flux from Vela X-1 with 1.5_2.0xl0-llcm-2 s-1 (2 TeV) . Although any excess was not observed in the R.-A distribution, the period analysis is now
a
T
-13
strongly
being examined for the same data.
J5 /`
-14 -15 10
12
.
14
16
LOG E (eV)
Fig.3 The present result with other previous results are shown. The present result designated by ' J5'. 'J2' and 'J3' are our previous TeV results2 , corresponding to the upperlimit for DC signals obtained by the observation during December 1987 and January 1988 and the burst of January 14-15th 1988, respectively. The 100 TeV result obtained by the JANZOS scintillation detector array is indicated by 'J1', and 'J4' is the updated data . 'A' , 'P' ;D' and 'S' represent the results of Adelaide groroup6 , Potchestroom group , Durham group8 and SPASE9 .
Right Ascension (deg .)
Fig. 4 The ratio of observed to expected count is plotted as a function of RA for the observation from January 1988 to May 1988 . The bin width is 1 .50 in RA. Only events in the declination strip -45.00 ±0.50 are included .
18 6
T. Tanimori, JANZOZ Colllaboration/JANZOZ results on PeV and TeV gamma rays from SN 1987A
4 Future Plan : Collaboration of Australia and Nip n for Gamma Ray Observation at Woomera (CANG OO Collaboration) Some of the JANZOS Collaboration proceed thenext plan to
construct a fine imaging Cherenkov telescope for continuing and improving the TeV gamma ray observation in the Southern
hemisphere in corporation with Adelide University in Australia (CANGAROO Collaboration) . The telescope will be located at Woomera, where is about 1000 km north from Adelaide in Australia.
A parabolic mirror of 3.8-m diameter with f-number equal to
one, which was used by optical astronomers for lunarranging, is
to be used as a light collector. this mirror has a good surface
quality (Blur" 1 ") to resolve the fine structure of the Cherenkov
light image . A image camera placed at focal plane will consistof 450 fast PMTs of 3/8 inches square, which has a large field of
The event trigger is a multiplicity of the number of fired PMTs, which is a good measure of the amount of Cherenkov light in the present case of small amount of photoelectrons compared to the large number of PMTS . In order to avoid a trigger due to the night sky background which follow a
Poissonian distribution with the mean value of about 30 photoelectrons, a spatial clustering of the fired PMTs will be
further required. The cluster consists of about 10 PMTs, and has
a about 1 .50 x1 .5° field of view which is almost equal to the size of a shower induced by a gamma ray shower. It will be required
at the nigger level that almost of photon concentrate into a few clusters . This trigger scheme will enable us to reject the almost
of the fake niggers caused by the night sky background. Then, we expect that the threshold of the trigger can be set lower than 0.5 TeV for an air shower induced by a gamma ray, which
view with 40 (each PMT views 0.150x0.150). In Fig.5, the
corresponds about 10 Hz trigger rate for an air shower induced
Monte Carlo simulationl 0 was superimposed onto the filed of
the telescope of Whipple observatory (10-m diameter mirror),
the field of view of 40. By square shaped PMTS, the dead space of a camera can be decreased to 15% of the total field. The
achieved . Furthermore, we will be able to reject about 2/3 of the
Cherenkov light image of 250 GeV gamma ray obtained by view, where squares represent views of PMTs and the circle is
number of photoelectrons collected from 1 TeV gamma ray is
estimated to be about 85 ( reflection coefficient of mirror -0.7,
quantum efficiency of a PMT -0.26 for400 nm photon), and the background from night sky is also estimated to be about 30 for 10 ns ( NSB - 780 m-2ns-1 sr~l, quantum efficiency for 500 nrn photon-0.2).
~I
by acosmic ray proton. In spite of a small mirror compared with very low threshold energy as same as that of Whipple will be
night sky background against the signal arriving within 3-5 ns, by using the timing of fired PMTs . The pulse-height information
are necessary to reconstruct the image pattern like the one shown in Fig.5 . The major axis of the image is determined to infer the arrival directionof gamma ray within an accuracy of a few arc of minutesl 1.
The proposed Cherenkov telescope is expected to discriminate
gamma rays from photon showers by exploiting the structure of the observed image as demonstrated by Whipple group. 12 We
""imm"""""""r-m" ir NEMESES u" u"a mons """""numa! mai
also expect that the present telescope can reduce and monitor the systematic errors during the observation, because separate groups of PMTs will be used for on- and off-source
measurement, simultaneously, and a large amount of night sky Insu"""""üjT
background data can be utilized to monitorthe sky condition as """""
!
i\0"""""""m"""" !" mri c""""". """""""""I . "~"""" 1 "o 2 "1 "
Fig. 5 the arrangement of photomltipliers of the imaging camera, where a image of a shower induced by 250 GeV gamma ray and a field of view with 40 diameter represent.
well as the gain change of PMTs.
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