Nuclear Instruments and Methods m Physics Research A302 (1991) 515-521

515North-Holland

Detection of ultraviolet Cherenkov light from high energycosmic ray atmospheric showers : a field test

B . BartoliINFN and University of Napoh, Italy

L. Peruzzo and G. SartoriINFN and University of Padoca, Italy

F. Bedeschi a, E. Bertolucci a, M . Mariotti a, A. Menzione a, L. Ristori a,

A. Scribano b , A . Stefanini a and F. Zetti a

° INFN and University of Pisa, Italyn INFN-Pisa and University of Siena, Italy

M. Budinich and F. LielloINFN and University of Trieste, Italy

Received 6 August 1990 and in revised form 27 December 1990

We present the results of a test with a prototype apparatus aimed to detect the ultraviolet Cherenkov light in the wavelength range2000-2300 A, emitted by high energy cosmic ray showers. The system consists of a gas proportional chamber, with TMAE vapour asthe photosensitive element, placed on the focal plane of a 1 .5 m diameter parabolic mirror . The test was done during the summer of1989 with cosmic ray showers seen m coincidence with the EAS-TOP experiment, an extended atmospheric shower charged particlearray now being exploited at Campo Imperatore, 1900 m above sea level, on top of the Gran Sasso underground Laboratory ofINFN . The results were positive and show that a full scale ultraviolet Cherenkov experiment with good sensitivity, angular resolutionand virtually no background from moonlight or even daylight can be envisaged

1 . Introduction

The detection of the Cherenkov light emitted by thesecondary electrons from a cosmic ray atmospheric

shower is a well known technique which has been usedfor many years to study high energy cosmic rays [1].This method is practically unique in the energy rangebetween 300 GeV, which is the maximum energy seenby satellite borne experiments, and 30 TeV, wherepunchthrough particles permit the use of charged par-ticle detector arrays on the ground surface. Its maindisadvantage is the necessity to work only during moon-less nights, hence with a very low duty cycle. To reducethe noise from night sky- or moon-light, it wasproposedto work in the utraviolet (UV) range since the Cheren-kov light spectrum increases in the UV [2]. It was alsonoted that in the far ultraviolet (wavelength range2000-3000 A) it could even be possible to detectCherenkov light from cosmic ray showers in full

Elsevier Science Publishers B.V . (North-Holland)

daylight, the ozone in the high atmosphere being com-pletely black at these wavelengths [3]. The first detec-tion of Cherenkov light in the ultraviolet range wasdone with solar blind photomultipliers [4] but, as far aswe know, no full fledged experiment was done withthem . Some time ago we proposed [5] the use of photo-sensitive gas proportional chambers of the type devel-oped for ring imaging Cherenkov (RICH) detectorsused in high energy accelerator physics. These chambersrespond to UV light around 2150 A, a shorter wave-length than solar blind photomultipliers. At these wave-lengths there is practically no background light from thesky even in full daylight, but there is no conclusiveinformation on the atmospheric transparency near

ground level, estimates for the absorption length vary-ing from 300 m at sea level with high ozone content to3000 m at mountain level with low ozone content.

In this article we describe tests done with a proto-type UV Cherenkov light detector using TMAE photo-

516

sensitive gas proportional chambers in comparison withan extended air shower array. Following the details ofthe apparatus, we describe the results which show thatthis technique can be a real improvement over visiblelight Cherenkov experiments .

2. Experimental apparatus

The detector, which is described in better detail inref. [6], is a square gas proportional wire chamber of25 x 25 cm2 sensitive area with a quartz (Ultrasil) frontwindow . In fig. 1 we show a section through the cham-ber. The gas used was an ethane/isobutane 4:1 mixturesaturated at 30 ° C with TMAE which is the photosensi-tive material [7] . The gas pipes and the chamber aremaintained at a temperature higher than 35 ° C to avoidTMAE condensation . The gas avalanches from the pho-toelectrons are detected via charge induction in theback cathode plane which consists of 16 x 16 squareconductive pads on a copper clad printed circuit board.The signals, after a low noise preamplifier, were analysedwith 2282 LeCroy ADCs in a CAMAC data acquisitionsystem . A sum signal (OR), sometimes used for trigger-ing purposes, was also taken from the anode wire plane.On the chamber side opposite to the quartz window, aplastic scintillator counter of the same area (25 x 25cmz) was used to detect charged particles traversing thechamber. In fig. 2 is shown the spectral sensitivity of thechamber and m fig. 3 the total charge distribution forlight pulses corresponding to an average number of 3.3photoelectrons . The peak quantum efficiency was about30% [6] .

The chamber was placed on the focal plane of aparabolic mirror . The mirror used was a searchlightmirror of 1.5 m diameter and 0.64 m focal length, which

B. Bartok et al / Detection of UV Cherenkov light

Fig. 1 . Cross section of the photosensitive chamber: (a) quartzwindow frame, (b) anode wires and cathode pads subassembly ;

(c) backplane and heating system .

v a

a._Z00

100

0

0

0

0

0

0 0

0

0

0

0

0

160 180 200 220 240 260

Wavelength

( nm )Fig. 2. Measured spectral sensitivity of the photosensitivechamber. The left hand cut is due to the quartz transparency,the right hand cut is due to the vanishing TMAE quantum

efficiency .

is shorter than the optimum. The mirror was recentlyfront-surface aluminized, the measured reflectance at2000 Â wavelength being 0.75 . A remeasurement of thereflectance after some months of data taking gave avalue of 0.65.

The mirror and chamber system was mounted on an

120

80

40

[Fq dLI

i

I

v ü-l

0

400 800Total charge ACC dise - ;r'

arb - rar .

Fig. 3. Total charge collected by the cathode pads (sum overthe nine pads involved) for light pulses corresponding to 3.3photoelectrons (210f 5 nm). The continuous curve is a fit to

the pedestal distribution .

10

B

C

EAS -TOP sheds

containers

B Bartoli et al / Detection

mirrork

16 m

I

i _ __10

20

30xim]

Fig 4 Plane view of the EAS-TOP sheds and our detectorlayout .

equatorial movement. When not in use, it was protectedto a standard container; it moved on rails to expose thedetector to the open sky during data taking . A secondcontainer was used for electronics, data taking com-puter, etc.

The measurements which we describe in the follow-ing were taken with the apparatus placed near an ex-tended atmospheric shower array (EAS-TOP) [8] atCampo Imperatore, 1900 mabove sea level (a .s .l .) placedon the vertical of the INFN underground Gran Sassolaboratory. For our testing purposes we used only fourof the counter sheds of EAS-TOP. Each shed contains16 liquid scintillator counters each seen by a photomul-tiplier, for a total sensitive area of 10 m2 per shed . Thefour sheds (named A, B, C and D) formed a 16 X 16 m2square, while our detector was placed on a side of the

square (see fig. 4) . Fourfold coincidences between thefour sheds were used as a trigger defining a cosmic rayextended shower . For any trigger event we registered,via CAMAC, the charge amplitudes of the 16 X 16 padsof the chamber and the relative time of the four EAS-TOP sheds. These times of flight, read by LeCroyTDCs, give the direction of the shower as defined byEAS-TOP.

3. Data analysis

3 1. Cherenkov light events

The data we show here were taken during somenights in the summer of 1989 . The trigger mostly usedwas the fourfold coincidence of the four EAS-TOP A,B, C and D sheds in coincidence also with the global(OR) signal from the photosensitive chamber. The scin-tillation counter behind the chamber was used as a vetoto reduce events due to charged particles of the showerin the chamber. A typical shower as seen on the on linecomputer is shown in fig. 5. The counting rate of thistrigger was 0.40 s -i , whereas the counting rate with the

of UV Cherenkov light

51 7

chamber quartz window obscured, was 0.07 s-1 . Eventswith the window covered do not come from casualcoincidences (the single counting rate of the chamberwas only 1 kHz) but from charged particles in theshower, because the scintillation veto counter has animperfect overlap for some incidence angles and be-cause of the conversion of photons in the aluminumback plane of the photosensitive chamber. On the otherhand, this small contamination can be sharply reducedbecause charged particles traversing the chamber areeasily distinguished from Cherenkov light events .Charged particles give a high signal to a small numberof pads whereas the Cherenkov image is more diffuse.This can be seen in fig. 6 where the maximum charge

Qmax in an event is plotted vs N.,� the number of padswith a charge larger than 20% of Qm, There are clearlytwo populations, the low Nc,,-high Qmex part corre-sponding to charged particles. This is confirmed by fig.7 which shows the same distribution for a run of chargedparticles, using the veto scintillation counter as a triggerwith the chamber obscured . A cut Qm,� < < 300 + 20N,leaves only a 4% charged particle contamination in thedata .

3.2 . Shower direction

The relative times between the four EAS-TOP shedscan be used to find the shower direction. This is thestandard method used in extended air shower arrayexperiments, and the typical precision reached is about1.5 ° ; we are far from this value because we use onlyfour counters instead of the whole EAS-TOP apparatus.Moreover, since we used a low threshold for the coun-ters, we have many low energy showers marginallydetected by the four sheds and these have a larger timespread for the incoming electrons. A reasonable esti-

ventRun = 44

Event =

117PH--

7 6910 -5454 -9999 - 143 t >

143 - 188 â~1 88 - 233233 - 277277 - 322322 - 367367 - 411411 - 456456 - 501501 - 545545 - 590590 - 635635 - 679679 - 724724 - 700

Fig. 5. Typical shower event as seen on the on-line dataacquisition computer . The grid reproduces the 16 X 16 pads of

the chamber cathode plane.

518

TE}CS n

3000

2000

1000

0

Fig. 6 . Maximum charge Qmax vs N,� the number of pads withcharge larger than 20% of Q.,�, . Cosmic ray showers events.

mate for the angular definition given by the time offlight analysis can be obtained from fig . 8a where thetime difference between sheds A and B (TA - TB ) isshown vs the time difference (TD - Tc ), for a run withABCD as a trigger . The four sheds being on a square,

TOF(TA-TB)

[ns]

50

0

-50

Nch

- 50

0

50 [ris]

B. Bartoh et al / Detection of UV Cherenkou light

TOF(TO-TE)

v

3000

2000

1000

0

0

40

80 NchFig. 7 . Maximum charge Q,_ vs N.,,, the number of pads witha charge larger than 20% of Qmax . Charged particle events,taken with the chamber obscured and using the veto counter as

a trigger .

these time differences should be identical. In fig . 9a weplot the quantity (TA - TB ) - (TD - Tc ) : from its rmswidth of 8.0 ns we can infer that the four EAS-TOPsheds can be used to define the cosmic ray shower

TO F(TA-TB )[ns]

50

0

ns

-50

-50

0

50 (ns]TOF

(TO-TE )

Fig. 8 . Time difference (TA-TB) vs (TD-Tc) : (a) for a run with the fourfold ABCD coincidence as a trigger; (b) for a run with thechamber in coincidence, i.e, with trigger ABCD-OR_,,,.

120

60

40

-40 -20 0 20 40[ns]

Fig . 9 The distribution of (TA - TB)-(TD - T(-), for the same events of figs . 8a and 8b

direction with a rms error of 4.3 ° in the projectedangle.A completely independent measure of the shower

direction is given by our Cherenkov detector. When werestrict ourselves to events with the photosensitivechamber m the trigger (trigger ABCD OR), we selectshowers with a direction near the axis of the mirror.This can be seen in fig . 8b, which is the same plot as fig.8a but for events with the chamber in the trigger. Theevents fall in a much smaller region, centered becausethe mirror has an angular acceptance of f 10 ° and waspointed to the zenith . For these events the showerdirection can be inferred by the position of the lightspot in the chamber, a point m the chamber correspond-

[ rad ].2

0

- 2 L_ ~

I

80

60

40

20

ing to light coming from a definite direction. As a firstapproximation, we define the direction of the shower asthat corresponding to the charge center on the chamber.The chamber, with its square grid of pads, has the sameorientation as the square defined by the four A, B, C,and D EAS-TOP sheds (see fig . 4) . If we choose coordi-nates on the chamber as shown to fig . 4, then thecoordinate of the charge centroid (X,~ = EX Q,1Qioi,where X, is the coordinate of the i th pad and Q, itscharge) corresponds to the same shower direction pro-jected angle which is measured by the time difference(TA - TB) - (TD - Tc). Fig. 10 shows the correlationbetween the projected shower direction angles measuredby the EAS-TOP sheds and those measured by the

[rad ]

0

- 2

- 3

-t, �� I r, nr-IJ

b)

519

-40 -20 0 20 40[ns]

b)

- 1

0

1

.2[rad ]

Fig . 10 . Shower projected direction angles measured with time of flight vs the same angles measured with the Cherenkov detector .(a) Projection on the x-z plane (z is the vertical axis) ; (b) projection on the y-z plane.

B Bartoh et al / Detection of UV Cherenkov light

NV) CC OOU

Ua)

520

3.3 . Energy threshold

cOU

40

20

a)

25

5

- 5

- 25

0

25

5[rad]

[rad]

Fig 11 . Difference between the projected shower direction angles of fig. 10 .

-5

- 25

0

Cherenkov detector . In fig. 11 we plot the differences

between the angles measured with the two methods. Therms width of this distribution turns out to be 4.5 ° ,almost identical to the width we found from the time of

flight method alone: it is evident that the error intro-

duced by the Cherenkov detector is much smaller. The

intrinsic optical precision of the mirror and detector

system is much better, as shown by a separate measure-

ment we have performed with a point source (hydrogen

UV lamp) placed 100 m away from the mirror andproducing a pointlike light distribution on the chamber.

The primary cosmic ray energy threshold of our

apparatus is obviously quite difficult to estimate . The

threshold of a fourfold coincidence (high level of dis-

mCOU

100

10

10 photoelectrons 100

B Bartoli et al / Detection of UV Cherenkoo light

Fig. 12 . Photoelectron number to the photosensitive chamberfor cosmic ray showers.

cOU

40

20

criminators) in EAS-TOP is - 30 TeV. In the case of

our data taking conditions, their simulation indicates a

value around 10 TeV for the energy threshold of protoninitiated showers. This rather low value is justified bythe fact that a low discrimination level (about 20 MeVper shed) for the EAS-TOP counter signal was used . A

similar value for the threshold can be obtained compar-

ing the counting rate with the known primary cosmic

rays flux . Of all the ABCD coincidences, those with the

shower direction (defined by time of flight method)

falling inside the mirror acceptance were detected bythe Cherenkov light detector in 30% of the cases. Thisloss is partly due to the cut on charged particles ap-plied. In fig. 12 we show the amount of light seen for

these shower events . The number of photoelectrons seen

is higher than 12, which is the threshold set on the OR

signal from the chamber wire anode plane used in thetrigger. Considering the measured possibility [6] to set athreshold at about 3 photoelectrons, we expect that ourdetector can see, at these altitudes, showers of energy as

low as 3-5 TeV.

4. Conclusions

b)

We have demonstrated the feasibility of UVCherenkov light detection of cosmic ray atmospheric

showers with TMAE photosensitive gas proportional

chambers . These detectors, even in this prototype form,have given no particular problems, being able to work

in open air in a rather severe environment with strong

winds, large thermal variations and humidity changes.They seem to be sufficiently reliable for a full scaleexperiment . These chambers seem to be preferable to

solar blind photomultipliers in having a better quantum

efficiency and being much cheaper than a phototube

matrix with the same area and granularity . We are at

present designing a gamma ray telescope with a largenumber of detectors based on this prototype; the possi-bility to work with the moon and perhaps even in fulldaylight, combined with the good angular resolutionachievable with such a high granularity and wide open-ing detector should finally give a significant improve-ment to the well established but perhaps a little staticfield of Cherenkov light gamma ray astronomy.

Acknowledgements

We would like to thank all the people of the EAS-TOP Collaboration for permitting us to use their ap-paratus and P. Fleury, J.M. Gaillard, F. Plouin, M.Urban, and the technical group of Ecole Polytechnique,for their kind contribution to a phase of the data takingof the present work, on the Gran Sasso, and for themany helpful discussions.

B Bartok et al / Detection of UV Cherenkov light

References

52 1

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[3] K. Ya. Kondratyev, Radiation in the Atmosphere (Acad-emic Press, New York, 1969).

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[51 G. Apollmari et al, Nucl . Instr. and Meth . A263 (1988)255

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p-