- Email: [email protected]
Development of a filter for operation of atmospheric Cherenkov telescopes close to the Moon
Nuclear Instruments
and Methods
in Physics
Research
A 385 (1997) 258-264
NUCLEAR INSTRUMENTS a METHODS IN PHYSICS REi%iF
ELSEWIER
Development of a filter for operation of atmospheric Cherenkov telescopes close to the Moon H.M. Badrana’c’*, M. Urbanb, T.C. Weekes” “WhippIe Observatory, Harvard-Smithsonian CfA, P.O. Box 97, Amado, AZ 856450097, “LPNHE Ecole Polytechnique IN2P3ICNRS, F-91128, Palaiseau, Cedex, France ‘Department of Physics, Faculty of Science, Tanta University, Tanta, Egypt Received
2 July 1996; revised form received
16 September
USA
1996
Abstract A filter has been developed and tested for the ARTEMIS experiment with solar-blind photomultiplier tubes. The filter consists of CoSO,, NiSO,, and 2,7-dimethyl-3,6-diazacyclohepta-2,6-diene perchlorate solutions in water. The filter transmission is about 65% in the wavelength range 230-270 nm. The Ni-Co solution allows a factor of 10 reduction in the background light from the Moon and the dye introduces another factor of 20. This reduction reduces significantly the systematics due to the position of the Moon. The new arrangement allows the center of the camera to be as close as 2” from the Moon center without any bias to a specific direction from the moon at any phase.
1. Introduction
protons and anti-protons TeV.
A major disadvantage of the atmospheric Cherenkov technique [ 1,2] is that the measurements can only be carried out on moonless clear nights. The improvement of the atmospheric Cherenkov technique to allow observations to be carried out even in the presence of the fullmoon or when the source of interest lies in a field of bright stars is an important goal. To achieve this aim some experiments are specifically designed to record only the UV component (or together with a very compressed visible component) of the light flashes generated by TeV air shower; the CLUE experiment [3] using gas proportional chamber with TMAE vapour, a hybrid version of the Whipple visible camera [4], and the ARTEMIS experiment (Anti-matter Research Through the Earth Moon Ion Spectrometer) [5] using solar-blind photomultipliers in the Whipple imaging camera. While this aim can be considered as an improvement of the atmospheric Cherenkov telescope (ACT) performance with the major concern of detecting TeV y-ray from celestial objects, it is a fundamental issue for the ARTEMIS experiment since the telescope has to operate within 2” (center to center) of the Moon. The experiment makes use of the Moon’s shadowing effect of cosmic ray
* Corresponding author. Tel.: + 1 20 40 344352; fax: + 1 20 40 350804. 0168~9002/97/$17.00 Copyright PII SO 168-9002(96)00896-O
01997
to measure the ratio p/p around 1
2. Background light The night-sky provides a background (more literally a foreground) against which faint images must be detected. The many components of the light in the night sky are: integrated starlight, zodiacal light, airglow, aurora, diffuse galactic light, and moonlight [6]. Clearly, the most serious component is the moonlight. Moonlight at quarter-phase adds roughly about fourth magnitude per square degree, but at full phase it makes the sky more than ten times as bright. Atmospheric extinction is a function of wavelength and air mass. Near the zenith, the extinction at sea level is about 15% at 550 nm if the air is very clear. It rises steeply in the blue and UV, becoming more than 90% below 300 nm. Light from any source outside the atmosphere is heavily attenuated in the range 200-300 nm and below 200 nm mainly by the ozone layer and oxygen [7-91, respectively. The atmospheric shielding below 300 nm has less effect on the Cherenkov light since it is radiated in the lower layers of the atmosphere (< 10 km). The light spectrum from the Moon as well as the atmospheric transmittance in the spectrum range of interest (200-700 nm) has been calculated using LOWTRAN7 [lo]. The program includes the most recently absorption
Elsevier Science B.V. All rights reserved
H.M. Budran et al. I Nucl. Instr. and Meth. in P&s. Res. A 385 (1997) 258-264
data for molecular
oxygen and ozone. It has band models
for H,O, 0,. N,O, CH,, CO. O,, CO,, NO, NO,, NH,, and SO, in addition to 13 minor gases. While there is negligible light from the Moon below 300 nm at altitude 2.3 km (altitude of Mt. Hopkins, Arizona), a sizeable fraction of photons generated in air showers at altitude as high as IO km is able to survive the atmospheric extinction. The Cherenkov light spectrum generated below the ozone layer extends from UV to visible wavelengths. Most of the emitted light is concentrated in the UV region. The
200
300
400
259
ratios of the light emitted at 200-300, 300-400. 400-500, and 500-600 nm are 5:2.5:1.5:1. Unfortunately, these ratios change at the detection level.
3. UV filter The chemical compounds that reduce the transmission of visible light and allow the UV component are well known [I 11.
xl0
600
700
Wavdsnglh @ml Fig. I. Measured transmission CoSO;7H,O
(I
cm pathlength) of (a) NiS0;6H,O
solutions with concentrations 100, 200, 300,400.
solutions with concentrations 25, 100. 125, 150, 200. 300, 400, and 500 g/l, diene iodide solutions with concentrations 0.1 and 0.2 g/I.
500, and 700 g/l, (b)
and Cc) 2.7-dimethyl-3.6-diazacyclohepta-
1,6-
260
H.M. Badran
et al. I Nucl. Instr. and Meth. in Phys. Res. A 385 (1997) 258-264
which has the desired characteristics was unsuccessful. The main reason is that the known UV filters are developed in such a way as to provide very low transmission in visible and IR frequencies. To achieve this low transmission, high concentrations of NiSO, and/or CoSO, are normally used. This allows an almost clear spectrum above a certain limit but also has the major disadvantage of the reduction of the transmission in the lower wavelengths. The maximum transmission in the range 250-300 nm is 30% to 40% in the best case [ 121. This is not necessary in our case since the maximum sensitivity of the photomultiplier (PM) tube occurs below 250 nm and drops rapidly with increasing wavelength to a considerably lower value above 400 nm. Hence, the best strategy is to use as low a concentration of the chemicals as possible in such a way that the combined effect of the filter (2 cm thickness) and the quantum efficiency of PM
NiSO;6H,O: It has good UV transmission characteristics from 230 to 330 nm. However, it has a high transmission peak at 500 nm. CoS0;7H,O: This compound also transmits in the UV and absorbs better in the visible and IR. It has a strong absorption at 500 nm, where NiSO, has high transmission. iodide/ 2,7-dimethyl-3,6_diazacyclohepta1,6-diene -2,6-diene perchlorate (dye): Both have a very strong transmission in the range 245 to 290 nm and above 340 nm. The use of this filter depends on the fact that it absorbs strongly in the range 300 to 340 nm. We preferred to use the perchlorate rather than the iodide compound because its transparency band extends to lower wavelengths [ 121. The search
in the literature
for an appropriate
filter
(a) Ni-Co
loo-,,,,,,,,,,,,I
,,,,,,,,I,,
(b) Ni-Co-dye
8 I I 200
300
400
500
600
700
Wavelength (nm) Fig. 2. Measured
(100
transmission
and dye
and 0.2
cm pathlength)
mixtures of
Ni (400
and Co
100, and
g/l) and
Ni (400
Co
H M. Badran et al. I Nucl. Instr. and Meth. in Phys. Res. A 385 (19971 258-264
would provide both high transmission below 300 nm and acceptable reduction of the light with longer wavelengths.
3.1. Luboratoty
measurements
Solutions of different concentrations of CoSO;7H,O, NiS0;6H,O. and dye (2.7.dimethyl-3,6-diaza cyclohepta1.6-diene perchlorate) have been prepared. The solutions are stored by covering with Al-foil to ensure that they are not subject to any source of light and kept at room temperature. Measurements of the solutions have been carried out using Cary 5E LJV-Vis-NIR spectrophotometer coupled to a PC. The light source was a tungsten halogen lamp with quartz window for the visible and a deuterium arc for the UV Fig. la shows the measured transmission of NiSO;6H,O with 100, 200, 300, 400, 500, and 700 g/l. The results for CoSO;7Hz0 are shown in Fig. I b for
261
concentrations 25, 100, 125, 150, 200, 300. 400, and 500 g/l and that for dye (0.1 and 0.2 g/l) are shown in Fig. lc. Clearly, one has to chose a concentration of at least 200 g/l in case of NiSO, and 100 g/l in case of CoSO, to get zero transmission around 400 and 500 nm, respectively. There is no major difference between the tested concentrations of the dye. A combination of the three chemicals can be used to achieve our goal: 1. CoSO,-NiSO, Filter: The ratio of cobalt to nickel should be kept as low as possible to achieve good absorption in the visible and IR. The recommended values [ 1I] for a 5 cm filter are 240 and 45 g/l for NiSO, and CoSO,, respectively. The transmission in this case is -50% between 240 and 320 nm with a cutoff around 350 nm. As the contribution from cobalt increases the ability of the filter to remove visible light increases. This is also accompanied with loss in 240-320 nm (see Fig. 2a).
Time (days) Fig. 3. Measured average transmission in Ni-Co-dye
(above) and Ni-Co
(down) filters. The wavelength range are given in each case.
H.M. Badran et al. I Nucl. Instr. and Meth. in Phys. Res. A 385 (1997) 258-264
262
Calculation of the expected effect suggested that the best combination is 400 and 100 g/l for NiSO, and CoSO,, respectively. This combination with somewhat higher concentrations has been used for ARTEMIS experiment [13] and with the Whipple visible camera [4] and it was found to reduce the background light by a factor -10. The main problem associated with this choice is due to the light transmitted in the range 300-350 nm. filter: The addition of dye to the 2. CoSO,-NiSO,-dye previous filter would absorb completely light above 300
2
,,,,,,,,,,,,,,,,,,,L,,,,,,,,,,,,,,
a) WithoutFttter
-----z-“--’
nm but also slightly decreases the transmission below 300 nm (Fig. 2b). The good feature of this filter over the previous one is that it does not transmit photons with wavelengths 300-350 nm. This allows the UVcamera to be almost noise free as will be shown later. 3.2. Time stability The stability of the filter is a point of major concern. For this purpose solutions of both single chemical and mixtures have been examined for long period (up to 50 days) after
_-___-__ ___ ___=---‘____ .---__?---_-_ ----__-_-_.___
Lunar ---2-s ‘-5=___
-_=.-_ w
0
I-
skvfloise
PmtOl
-2
-\_-_-__
Fig. 4. The lunar spectrum (full moon, 8 = 0”) and sky noise integrated in 30 n for 10 and 15 PMs in each case (a) without filter, (b) with 2 cm Ni-Co filter, and (c) with 2 cm Ni-Co-dye filter. For comparison, the average light from 1 TeV proton simulated showers is also shown.
H.M. Badran et al. I Nucl. Instr. and Meth. in Phw. Res. A 385 (1997) 258-264
the preparation. The average transmission in different wavelength ranges are shown in Fig. 3 for both filters. No real change in the filters transmission has been found. In addition, the filters were subject to three-quarters to fullmoon for more than 10 h during the test of the camera. Samples were taken after different time intervals. Again, no changes have been found.
3.3. Simulated performance
of the W-jilter
The performance of Co-Ni and Co-Ni-dye filters has been examined for the Cherenkov light spectrum generated from simulated 1 TeV proton showers as well as the sky background light. The shower and telescope simulation code used in this work is described in Ref. [l4]. Fig. 4 shows the expected average spectrum in the solar-blind PM from proton showers and the integrated sky noise and full-moon light (0 = 0”) in 30 ns for 10 and I5 PMs to make the background light comparable to a typical TeV image. The telescope response (PM’s quantum efficiency and mirror reflectivity) is taken into account in all cases. When the Co-Ni-dye filter is used the UV signal will be reduced by 20%, while the background will be down by a factor -20 with respect to the previous filter, a total
Uban.
3-e
M. et al. (1998)
. .
. .
2-
0
lr 0
i
o-
reduction factor of -200 Moon.
of the background
263
light from the
3.4. Moon profile measurements Measurements of the camera response to the Moon light after adding the new Ni-Co-dye filter has been carried out under different moon phases and elevations during the season 1995-96 using the Whipple 10 m reflector with the solar-blind PMs. Fig. 5 shows the average measured flux from the Moon for phases 92-94% and elevations 35-40” as a function of the angle between the Moon center and the center of the PM. Previous measurements [ 131 of the same system without the filter are also shown. In the region of interest (> I”) we observed a reduction factor of 100 instead of the expected 200. The measured relation drops with decreasing moon phase and/or moon elevation.
4. Conclusions Using the developed filter the UV-camera can be pointed directly to the full-moon. The elimination of the background light has the advantage of providing a uniform field of view around the Moon. Without achieving this, a systematic effect may be present in the data since the ARTEMIS measurements are not always carried out in full-moon. For phases lower than full-moon it is expected that the background light in one direction from the Moon can differ from that on the opposite direction when the camera points close to the Moon’s center. The ARTEMIS experiment requires measurements -2” east (i region) and west (p region) from the Moon as well as north and south (control regions). As it has been demonstrated, for angle - 1” there is hardly any light detected by the solar-blind PM. The promising noise reduction (factor of - 100) is due to the addition of the dye in the filter solution. It remains to be seen what the actual sensitivity of the camera will be to the Cherenkov light generated by cosmic particles; it is planned to carry out ARTEMIS measurements using this filter in the winter 1996-97.
-1 -
Acknowledgments -2 0
oo
Moon Phase=92-94%
Fig. 5. Moon profile (phase 92-94’S and elevation 35-40”) measured with the Ni-Co-dye filter (open circles) compared with the previous measurements (ciosed circles) [ 131.
for We are indebted to the Whipple Collaboration permission to use the IO m telescope. We are thankful to D. Huffman for making the spectrophotometer available to us and L. Broadfoot for his help with LOWTRAN program. We appreciate the technical help provided by K. Harris and T. Lappin. HMB acknowledges a fellowship from the Smithsonian Astrophysical Observatory. This work is supported by the U.S. Department of Energy.
264
H.M. Badran et al. / Nucl. Instr. and Meth. in Phys. Res. A 385 (1997) 258-264
References 111 M.F. Cawley and T.C. Weekes, Exp. Astron. 6 (1995) 7. PI T.C. Weekes, Space Sci. Rev. 75 (1996) 1. 131 E. Anassontzis et al.. Nucl. Instr. and Meth. A 315 (1992) 267. r41 M.C. Chantell et al., submitted for publication. PI M. Urban et al., Nucl. Phys. (Proc. Suppl.) B 14 (1990) 223. 161 F.E. Roach and J.L. Gordon, The Light of the Night Sky (Reidel, Dordrecht, Holland, 1973). and Radioactivity (Academic [71 C.E. Junge, Air Chemistry Press, New York, 1963). PI K.Ya. Kondratyev, Radiation in the Atmosphere (Academic Press. New York, 1969).
[91 A.D. Jursa (ed.), Handbook of Geophysics and Space Environments (Air Force Geophysics Lab., U.S. Air Force, 1985). 1101 F.X. Kneizys et al., Users Guide to LOWTRAN7, Air Force Geophysics Lab., AFGL-TR-88-0177 (1988). [Ill M. Kasha, J. Opt. Sot. America 38 (1948) 929. 1121 J.C. Scaiano (ed.), CRC Handbook of Organic Photochemistry, V I (CRC Press. 1989) [I31 M. Urban et al., Nucl. Instr. and Meth. A 368 (1996) 503. [I41 M.P. Kertzman and G.H. Sembroski, Nucl. Instr. and Meth. A 343 (1994) 629.
and Methods
in Physics
Research
A 385 (1997) 258-264
NUCLEAR INSTRUMENTS a METHODS IN PHYSICS REi%iF
ELSEWIER
Development of a filter for operation of atmospheric Cherenkov telescopes close to the Moon H.M. Badrana’c’*, M. Urbanb, T.C. Weekes” “WhippIe Observatory, Harvard-Smithsonian CfA, P.O. Box 97, Amado, AZ 856450097, “LPNHE Ecole Polytechnique IN2P3ICNRS, F-91128, Palaiseau, Cedex, France ‘Department of Physics, Faculty of Science, Tanta University, Tanta, Egypt Received
2 July 1996; revised form received
16 September
USA
1996
Abstract A filter has been developed and tested for the ARTEMIS experiment with solar-blind photomultiplier tubes. The filter consists of CoSO,, NiSO,, and 2,7-dimethyl-3,6-diazacyclohepta-2,6-diene perchlorate solutions in water. The filter transmission is about 65% in the wavelength range 230-270 nm. The Ni-Co solution allows a factor of 10 reduction in the background light from the Moon and the dye introduces another factor of 20. This reduction reduces significantly the systematics due to the position of the Moon. The new arrangement allows the center of the camera to be as close as 2” from the Moon center without any bias to a specific direction from the moon at any phase.
1. Introduction
protons and anti-protons TeV.
A major disadvantage of the atmospheric Cherenkov technique [ 1,2] is that the measurements can only be carried out on moonless clear nights. The improvement of the atmospheric Cherenkov technique to allow observations to be carried out even in the presence of the fullmoon or when the source of interest lies in a field of bright stars is an important goal. To achieve this aim some experiments are specifically designed to record only the UV component (or together with a very compressed visible component) of the light flashes generated by TeV air shower; the CLUE experiment [3] using gas proportional chamber with TMAE vapour, a hybrid version of the Whipple visible camera [4], and the ARTEMIS experiment (Anti-matter Research Through the Earth Moon Ion Spectrometer) [5] using solar-blind photomultipliers in the Whipple imaging camera. While this aim can be considered as an improvement of the atmospheric Cherenkov telescope (ACT) performance with the major concern of detecting TeV y-ray from celestial objects, it is a fundamental issue for the ARTEMIS experiment since the telescope has to operate within 2” (center to center) of the Moon. The experiment makes use of the Moon’s shadowing effect of cosmic ray
* Corresponding author. Tel.: + 1 20 40 344352; fax: + 1 20 40 350804. 0168~9002/97/$17.00 Copyright PII SO 168-9002(96)00896-O
01997
to measure the ratio p/p around 1
2. Background light The night-sky provides a background (more literally a foreground) against which faint images must be detected. The many components of the light in the night sky are: integrated starlight, zodiacal light, airglow, aurora, diffuse galactic light, and moonlight [6]. Clearly, the most serious component is the moonlight. Moonlight at quarter-phase adds roughly about fourth magnitude per square degree, but at full phase it makes the sky more than ten times as bright. Atmospheric extinction is a function of wavelength and air mass. Near the zenith, the extinction at sea level is about 15% at 550 nm if the air is very clear. It rises steeply in the blue and UV, becoming more than 90% below 300 nm. Light from any source outside the atmosphere is heavily attenuated in the range 200-300 nm and below 200 nm mainly by the ozone layer and oxygen [7-91, respectively. The atmospheric shielding below 300 nm has less effect on the Cherenkov light since it is radiated in the lower layers of the atmosphere (< 10 km). The light spectrum from the Moon as well as the atmospheric transmittance in the spectrum range of interest (200-700 nm) has been calculated using LOWTRAN7 [lo]. The program includes the most recently absorption
Elsevier Science B.V. All rights reserved
H.M. Budran et al. I Nucl. Instr. and Meth. in P&s. Res. A 385 (1997) 258-264
data for molecular
oxygen and ozone. It has band models
for H,O, 0,. N,O, CH,, CO. O,, CO,, NO, NO,, NH,, and SO, in addition to 13 minor gases. While there is negligible light from the Moon below 300 nm at altitude 2.3 km (altitude of Mt. Hopkins, Arizona), a sizeable fraction of photons generated in air showers at altitude as high as IO km is able to survive the atmospheric extinction. The Cherenkov light spectrum generated below the ozone layer extends from UV to visible wavelengths. Most of the emitted light is concentrated in the UV region. The
200
300
400
259
ratios of the light emitted at 200-300, 300-400. 400-500, and 500-600 nm are 5:2.5:1.5:1. Unfortunately, these ratios change at the detection level.
3. UV filter The chemical compounds that reduce the transmission of visible light and allow the UV component are well known [I 11.
xl0
600
700
Wavdsnglh @ml Fig. I. Measured transmission CoSO;7H,O
(I
cm pathlength) of (a) NiS0;6H,O
solutions with concentrations 100, 200, 300,400.
solutions with concentrations 25, 100. 125, 150, 200. 300, 400, and 500 g/l, diene iodide solutions with concentrations 0.1 and 0.2 g/I.
500, and 700 g/l, (b)
and Cc) 2.7-dimethyl-3.6-diazacyclohepta-
1,6-
260
H.M. Badran
et al. I Nucl. Instr. and Meth. in Phys. Res. A 385 (1997) 258-264
which has the desired characteristics was unsuccessful. The main reason is that the known UV filters are developed in such a way as to provide very low transmission in visible and IR frequencies. To achieve this low transmission, high concentrations of NiSO, and/or CoSO, are normally used. This allows an almost clear spectrum above a certain limit but also has the major disadvantage of the reduction of the transmission in the lower wavelengths. The maximum transmission in the range 250-300 nm is 30% to 40% in the best case [ 121. This is not necessary in our case since the maximum sensitivity of the photomultiplier (PM) tube occurs below 250 nm and drops rapidly with increasing wavelength to a considerably lower value above 400 nm. Hence, the best strategy is to use as low a concentration of the chemicals as possible in such a way that the combined effect of the filter (2 cm thickness) and the quantum efficiency of PM
NiSO;6H,O: It has good UV transmission characteristics from 230 to 330 nm. However, it has a high transmission peak at 500 nm. CoS0;7H,O: This compound also transmits in the UV and absorbs better in the visible and IR. It has a strong absorption at 500 nm, where NiSO, has high transmission. iodide/ 2,7-dimethyl-3,6_diazacyclohepta1,6-diene -2,6-diene perchlorate (dye): Both have a very strong transmission in the range 245 to 290 nm and above 340 nm. The use of this filter depends on the fact that it absorbs strongly in the range 300 to 340 nm. We preferred to use the perchlorate rather than the iodide compound because its transparency band extends to lower wavelengths [ 121. The search
in the literature
for an appropriate
filter
(a) Ni-Co
loo-,,,,,,,,,,,,I
,,,,,,,,I,,
(b) Ni-Co-dye
8 I I 200
300
400
500
600
700
Wavelength (nm) Fig. 2. Measured
(100
transmission
and dye
and 0.2
cm pathlength)
mixtures of
Ni (400
and Co
100, and
g/l) and
Ni (400
Co
H M. Badran et al. I Nucl. Instr. and Meth. in Phys. Res. A 385 (19971 258-264
would provide both high transmission below 300 nm and acceptable reduction of the light with longer wavelengths.
3.1. Luboratoty
measurements
Solutions of different concentrations of CoSO;7H,O, NiS0;6H,O. and dye (2.7.dimethyl-3,6-diaza cyclohepta1.6-diene perchlorate) have been prepared. The solutions are stored by covering with Al-foil to ensure that they are not subject to any source of light and kept at room temperature. Measurements of the solutions have been carried out using Cary 5E LJV-Vis-NIR spectrophotometer coupled to a PC. The light source was a tungsten halogen lamp with quartz window for the visible and a deuterium arc for the UV Fig. la shows the measured transmission of NiSO;6H,O with 100, 200, 300, 400, 500, and 700 g/l. The results for CoSO;7Hz0 are shown in Fig. I b for
261
concentrations 25, 100, 125, 150, 200, 300. 400, and 500 g/l and that for dye (0.1 and 0.2 g/l) are shown in Fig. lc. Clearly, one has to chose a concentration of at least 200 g/l in case of NiSO, and 100 g/l in case of CoSO, to get zero transmission around 400 and 500 nm, respectively. There is no major difference between the tested concentrations of the dye. A combination of the three chemicals can be used to achieve our goal: 1. CoSO,-NiSO, Filter: The ratio of cobalt to nickel should be kept as low as possible to achieve good absorption in the visible and IR. The recommended values [ 1I] for a 5 cm filter are 240 and 45 g/l for NiSO, and CoSO,, respectively. The transmission in this case is -50% between 240 and 320 nm with a cutoff around 350 nm. As the contribution from cobalt increases the ability of the filter to remove visible light increases. This is also accompanied with loss in 240-320 nm (see Fig. 2a).
Time (days) Fig. 3. Measured average transmission in Ni-Co-dye
(above) and Ni-Co
(down) filters. The wavelength range are given in each case.
H.M. Badran et al. I Nucl. Instr. and Meth. in Phys. Res. A 385 (1997) 258-264
262
Calculation of the expected effect suggested that the best combination is 400 and 100 g/l for NiSO, and CoSO,, respectively. This combination with somewhat higher concentrations has been used for ARTEMIS experiment [13] and with the Whipple visible camera [4] and it was found to reduce the background light by a factor -10. The main problem associated with this choice is due to the light transmitted in the range 300-350 nm. filter: The addition of dye to the 2. CoSO,-NiSO,-dye previous filter would absorb completely light above 300
2
,,,,,,,,,,,,,,,,,,,L,,,,,,,,,,,,,,
a) WithoutFttter
-----z-“--’
nm but also slightly decreases the transmission below 300 nm (Fig. 2b). The good feature of this filter over the previous one is that it does not transmit photons with wavelengths 300-350 nm. This allows the UVcamera to be almost noise free as will be shown later. 3.2. Time stability The stability of the filter is a point of major concern. For this purpose solutions of both single chemical and mixtures have been examined for long period (up to 50 days) after
_-___-__ ___ ___=---‘____ .---__?---_-_ ----__-_-_.___
Lunar ---2-s ‘-5=___
-_=.-_ w
0
I-
skvfloise
PmtOl
-2
-\_-_-__
Fig. 4. The lunar spectrum (full moon, 8 = 0”) and sky noise integrated in 30 n for 10 and 15 PMs in each case (a) without filter, (b) with 2 cm Ni-Co filter, and (c) with 2 cm Ni-Co-dye filter. For comparison, the average light from 1 TeV proton simulated showers is also shown.
H.M. Badran et al. I Nucl. Instr. and Meth. in Phw. Res. A 385 (1997) 258-264
the preparation. The average transmission in different wavelength ranges are shown in Fig. 3 for both filters. No real change in the filters transmission has been found. In addition, the filters were subject to three-quarters to fullmoon for more than 10 h during the test of the camera. Samples were taken after different time intervals. Again, no changes have been found.
3.3. Simulated performance
of the W-jilter
The performance of Co-Ni and Co-Ni-dye filters has been examined for the Cherenkov light spectrum generated from simulated 1 TeV proton showers as well as the sky background light. The shower and telescope simulation code used in this work is described in Ref. [l4]. Fig. 4 shows the expected average spectrum in the solar-blind PM from proton showers and the integrated sky noise and full-moon light (0 = 0”) in 30 ns for 10 and I5 PMs to make the background light comparable to a typical TeV image. The telescope response (PM’s quantum efficiency and mirror reflectivity) is taken into account in all cases. When the Co-Ni-dye filter is used the UV signal will be reduced by 20%, while the background will be down by a factor -20 with respect to the previous filter, a total
Uban.
3-e
M. et al. (1998)
. .
. .
2-
0
lr 0
i
o-
reduction factor of -200 Moon.
of the background
263
light from the
3.4. Moon profile measurements Measurements of the camera response to the Moon light after adding the new Ni-Co-dye filter has been carried out under different moon phases and elevations during the season 1995-96 using the Whipple 10 m reflector with the solar-blind PMs. Fig. 5 shows the average measured flux from the Moon for phases 92-94% and elevations 35-40” as a function of the angle between the Moon center and the center of the PM. Previous measurements [ 131 of the same system without the filter are also shown. In the region of interest (> I”) we observed a reduction factor of 100 instead of the expected 200. The measured relation drops with decreasing moon phase and/or moon elevation.
4. Conclusions Using the developed filter the UV-camera can be pointed directly to the full-moon. The elimination of the background light has the advantage of providing a uniform field of view around the Moon. Without achieving this, a systematic effect may be present in the data since the ARTEMIS measurements are not always carried out in full-moon. For phases lower than full-moon it is expected that the background light in one direction from the Moon can differ from that on the opposite direction when the camera points close to the Moon’s center. The ARTEMIS experiment requires measurements -2” east (i region) and west (p region) from the Moon as well as north and south (control regions). As it has been demonstrated, for angle - 1” there is hardly any light detected by the solar-blind PM. The promising noise reduction (factor of - 100) is due to the addition of the dye in the filter solution. It remains to be seen what the actual sensitivity of the camera will be to the Cherenkov light generated by cosmic particles; it is planned to carry out ARTEMIS measurements using this filter in the winter 1996-97.
-1 -
Acknowledgments -2 0
oo
Moon Phase=92-94%
Fig. 5. Moon profile (phase 92-94’S and elevation 35-40”) measured with the Ni-Co-dye filter (open circles) compared with the previous measurements (ciosed circles) [ 131.
for We are indebted to the Whipple Collaboration permission to use the IO m telescope. We are thankful to D. Huffman for making the spectrophotometer available to us and L. Broadfoot for his help with LOWTRAN program. We appreciate the technical help provided by K. Harris and T. Lappin. HMB acknowledges a fellowship from the Smithsonian Astrophysical Observatory. This work is supported by the U.S. Department of Energy.
264
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