IL NUOVO CIMENTO

NOTE BREVI

VOL. 15 C, N. 3 Maggio-Giugno 1992

Imaging of Atmospheric EAS Cherenkov Light at EAS-TOP.

M. AGLIETTA (1,2), B. ALESSANDRO (2), F. ARNEODO (2), L. BERGAMASCO (2,3), A. CAMPOS FAUTH (3,4), C. CASTAGNOLI (i,2), A. CASTELLINA (i,2), C. CATTADORI (5), A. CHIAVASSA (2,3) G. CINI (z.~), B. D'ETrORRE PIAZZOLI (~), W. P~GIONE C'2), P. GALEOTH (2,3), P.L GHIA (,,2) G.MANNOCCHI (12), C. MORELLO (1,2), G. NAVARRA (2,3), L. RICCATI (2), O. SAAVEDRA (2,~) G.C.TRINCHERO (1,2), p. VALLANIA (~,2) and S. VERNETTO (1,2)

(L) Isti tuto di Cosmo-Geofisica (tel CNR, Torino, Italia (2) I N.F.N., Sezione di Torino, Italia ('~) Isti tuto eli Fisica deU'Universitd di Torino, Italia (4) Insti tuto di Fisica, Universidazle Estadual de Campinas, Brazi l (~) I.N.F.N., Sezione di Milano, Italia (6) Dipart imento di Scienze Fisiche deU'Universith di Napoli and 1NFN Sez. di Napoli, Italia

(ricevuto il 2 Aprile 1992; approvato il 21 Aprile 1992)

Summary - Images of atmospheric EAS Cherenkov light have been recorded in connection with the EAS-TOP experiment. We describe the technique (based on a multianode photomultiplier Philips XP4702) and present some preliminary data analysis.

PACS. 94.40 - Cosmic-ray interactions with the Earth.

1 . - I n t r o d u c t i o n .

The importance of measuring the angular distribution of the atmospheric Cherenkov light f rom Extensive Air Showers (EAS) for studies of VHE 7- ray as t ronomy and shower development was recognized long time ago [1]. Theoret ical [2,3] and experimental [4,5] work begun at that time, and first images of the Cherenkov light spots were obtained by means of the image intensifier technique [4].

A significant result based on EAS atmospheric Cherenkov light imaging has been recently obtained using as de tec tor an array of photomultipliers in the focus of a large area and long focal distance mirror [6]. By selecting ((narrow), spots from showers originated high in the a tmosphere and developing parallel to the optical axis of the mirror [7], a clear TeV 7 - ray signal f rom the Crab Nebula has been detected [8].

357

358 M. AGLIETTA, B. ALESSANDRO, F. ARNEODO, L. BERGAMASCO~ ETC.

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Fig. 1. - Image of a 4 mm lamp beam placed at = 50 m from the detector and focussed on the XP4702 photocatode.

�9 Imaging o f a t m o s p h e r i c C h e r e n k o v light is a m o n g the main a ims [9] o f the mu l t i component EAS-TOP detector [10,11] at Campo Imperatore (Gran Sasso) , both for m e a s u r e m e n t s o f y - ray as tronomy in the 10 TeV energy region, and studies o f EAS deve lopment in connec t ion with the e lectromagnet ic , m u o n (GeV and TeV) and hadron detectors .

Results o f EAS Cherenkov light tests at EAS-TOP (both in co inc idence with the e lec tromagnet ic [12] detector and in correlat ion with the deep underground m u o n detector MACRO [ 13]) have already been published. In this note w e present the first results obtained with an imaging technique, based on a mult ianode photomult ipl ier (Philips XP4702). The m e a s u r e m e n t s have been performed at the Astronomical Observatory o f Campo Imperatore (C.I., 2200 m a.s.l.) and at the Astronomica l Observatory o f Pino Torinese (P.T., 600 m a.s.l.).

2. - T h e t e c h n i q u e .

The XP4702 is a 64 pixels mult ianode photomultipl ier. The full p h o t o c a t o d e dimen-

LMAGING OF ATMOSPHERIC EAS CHERENKOV LIGHT AT EAS-TOP 359

sions are (2 x 2) cm2; its quantum efficiency is 12% at ~t = 400 nm. Gain is 10 ~ at the opera t ion voltage of 1400 V [14,15].

The light col lector is a parabol ic mir ror of 90 cm diameter , 67 cm focal length (field of v iew of individual pixels = 0.2 • 0.2 degrees, full field of view = 1.7 degrees). Dynode charges are read by two ADCs CAEN C205 (sensitivity 0.03 pC per channel) [16].

The triggering sys tem was provided by a threefold coincidence (At = 40 ns) with two similar optical sys tems equipped with photomult ip l iers XP2041 (field of view 10 ~ at C.I., or XP3462 (field of view reduced to 2 ~ at P.T.. The mirrors were posi t ioned within 10 m f rom each other. The count ing rate of each photomult ip l ier was = 20 Hz at C.L and = 2000 Hz at P.T. where the discrimination threshold was reduced by a fac tor 2 and the background luminosi ty was larger. In both cases the chance coincidence rate was kept at a low level (< 10 ~ Hz). A gate of 40 ns width has been used, which, combined with an individual pixel current of = 1 ~A, gives --- 0.04 pC, i .e. = 1 ADC count f rom nightsky background. Relative ad jus tment of pixels gains was pe r fo rmed through the m e a s u r e m e n t of the individual cm~rents with the pho toca thode looking at the sky.

The angular resolut ion of the de tec to r has been checked by recording images of a

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Fig. 2. - EAS Cherenkov light images recorded at Pino Torinese. The z-axis is in units of ADC counts: five ADC counts correspond, roughly, to one photoelectron.The content of each bin (corresponding to one pixel of the photocatode) has been corrected for channels gains differences and for ADC pedestals.

360 M. AGLIETrA, B. AI,ESSANI)RO, F. ARNEODO, L. BERGAMASCO, ETC.

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Fig. 3. - The same as fig. 2 for the Campo Imperatore.

b r igh t star, and of a l amp loca t ed a t 50 m f r o m the mirror . One o f such images is s h o w n in fig. 1, s h o w i n g tha t = 60% of the cha rge can be c o n t a i n e d inside one p ixe l o v e r the who le field o f view.

3 . - R e s u l t s .

The co inc iden t even t ra te r anges f r o m 0.02 Hz a t C.I., to 0.2 Hz at P . T . . T h e r a t e s w e r e s tab le dur ing o p e r a t i o n s and the n u m b e r o f co l l ec ted even t s is = 400 in the C.I. run and = 1000 in the P.T. run. A few even t s r e c o r d e d in the t w o runs a re s h o w n in fig. 2 and 3. Ind iv idua l images a re in all c a se s wel l def ined; b a c k g r o u n d and noise a re negligible.

As we can see f r o m the C.I. events , the co l l ec ted n u m b e r o f p h o t o n s r anges f r o m 103 to 104/m 2 c o r r e s p o n d i n g to p r i m a r y ene rg ie s E 0 = (1013 + 1014) eV. By a s s u m i n g an e f fec t ive a r e a A -- 104 m '~, the e x p e c t e d ra te o f even t s a b o v e 5 �9 1013 eV is -- 0.03 Hz, in g o o d a g r e e m e n t wi th the e x p e r i m e n t a l one.

Such ene rgy has b e e n r e d u c e d to = 101'~ eV in the P.T. run, and can be fu r the r r e d u c e d wi th an i m p r o v e d t r igger ing s y s t e m and in the d a r k e r cond i t ions o f o p e r a t i o n o f C.I . .

IMAGING OF ATMOSPHERIC EAS CHERENKOV LIGHT AT EAS-TOP 361

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, 150

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a)

i I . . . . . i

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Fig. 4. - Distributions of the ((azwidth)) (a)), (,width)) (b)) and (dength)) (c)) parameters: continu- ous line: data collected during the Pino Torinese run; dashed line: experimental data from Weekes et al. [8]; dotted line: simulated data from Hillas [7].

For a first compar i son of these p ic tures with expectat ions: i) in fig. 4 a), b), c) the ((azwidth)), ((width)) and (dength)) distr ibutions are shown

toge ther with calculat ions [7] and exper imenta l da ta [8] (al though concerning lower p r imary energies: E 0 -- (0.5 + 1) TeV). The ((width>> and ((length)) are defined, respectively, as the dispers ion along the major and minor axis of the elliptical image. The (<azwidth~ is the dispers ion measured perpendicular to the radius passing through the image centroid. The smaller field of view with respec t to the Mt. Hopkins de tec tor (3.5 ~ can account for the lack of large values in some of our exper imenta l distributions.

ii) In fig. 5 isophote curves (at 33% and 10~ of m a x i m u m intensity) of four events are shown.

iii) In table I the m a x i m u m and min imum elongation of the measured 10% isophotes of such events are com pa red with calculat ions [2,3], showing good agreement with the expecta t ions .

3 6 2 M. A G L I E ~ I ? A , B. A L E S S A N D R O , F . A R N E O D O , L. B E R G A M A S C O , E T C .

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Fig. 5. - Isophote curves of four typical events. The lines mark the boundaries at which light intensity goes down to 33% and 10% of the maximum.

TABLE. I - M e a s u r e m e n t s o f the m a x i m u m a n d m i n i m u m e l o n g a t i o n o f the 10% i sopho te are compared w i t h ca lcu la t ions p e r f o r m e d f o r vert ical showers , E o = 1012 eV, core d i s tance R = 100 m, sea

level observation.

minimum elongation (degrees) maximum elongation (degrees)

Castagnoli et al. [3] 0.5 1.4 Zatsepin et al. [2] 0.14 1.0 this experiment 0.6 + 0.8 1.2 + 1.4

4 . - C o n c l u s i o n s .

Images of a tmospher ic Cherenkov light f rom EAS have been obta ined by means of a mul t ianode photomult ipl ier . They are of quite good quality and can be easily analysed.

A larger field of view can be obta ined by using photomult ip l iers with a grea ter n u m b e r of pixels such as H a m a m a t s u H4140 (256 pixels, 16 cm 2, squared) [17] or Philips XP1704 (96 pixels, 6 cm 2, nearly circular) [18].

Such de tec tor will be the basic unit of each of the eight t e lescopes of the EAS-TOP Cherenkov ar ray at Campo Impera to re [9].

We wish to thank the Directors of the INFN Gran Sasso National Laborator ies and of the Astronomical Observator ies of Campo Impera to re and Pino Torinese.

The prec ious technical col laborat ion of C. Barattia, R. Bertoni, M. Canonico, G. Giuliani, A. Giuliano, G. Pirali is gratefully acknowledged.

We also wish to thank E. Capulli for his suppor t during the Campo Impera to re run.

IMAGING OF ATMOSPHERIC EAS CHERENKOV LIGHT AT EAS-TOP 363

REFERENCES

[ 1] J.V. JELLEY: Progress in Elementary Particle Physics and Cosmic Ray Physics (North Holland, 1967), p.105.

[2] V.I. ZATSEPIN: Zh. Eksp. Teor. Fiz., 47, 689 (1964). [3] C. CASTAGNOLI, M.A. LoccI and P. PICCRI: Nuovo Cimento B, 9, 213 (1972). [4] D.A. HILL and N.A. PORTER: Nature, 191,690 (1969). [5] C. CASTAGNOLI, R. HABEL, G. MARANGONI and V. MONTEL: CNR, Internal Report no. 34, FISCOT

(1967). [6] M.F. CAWLEY, D.J. FEGAN, K. HARRIS, A.M. HILLAS, P.W. KWOK, R.C. LAMB, M.J. LANG, D.A. LEWIS,

D.MACOMB, P.T. REYNOLDS, D.J. SCHMID, G. VACANTI and T.C. WEEKES: EXp. Astron.,1, 173 (1990). [7] A. M. HILLAS: Proc. IXX ICRC (La JoUa), 3, 445 (1985). [8] T.C. WEEKES, M.F. CAWLEY, D.J. FEGAN, K.G. GIBBS, A.M. HILLAS, P.W. KWOK, R.C. LAMB, D.A.

LEWIS, D. MACOMB, N.A. PORTER, P.T. REYNOLDS and G. VACANTI: Astrophys. J., 342, 379 (1989). [9] M. AGLIETTA et al. (EAS-TOP Collaboration): Internal Report no. 213/89, Istituto di Cosmo-

geofisica del CNR, Torino. [10] M. AGLIETTA et al. (EAS-TOP Collaboration): Nuovo Cimento C, 9, 262 (1986). [11] M. AGLIETTA et al. (EAS-TOP Collaboration): Nucl. Instrum. Methods A, 277, 23 (1989). [12] M. AGLIETTA et al. (EAS-TOP Collaboration): Proceedings of the Hinter"national Workshop on

Neutrino Telescopes, Venezia, 1990, edited by M. Baldo Ceolin, p. 195. [ 13] M. AGLIETTA et al. (EAS-TOP Collaboration and MACRO Collaboration): Proceedings of the H

International Workshop on Neutrino Telescopes, Venezia, 1990, edited by M. Baldo Ceolin, p. 209.

[14] Philips, Data Handbook, PC04 1990. [15] G. COMBY and R. MEUNIER: Nucl. Instrum. Methods A, 269, 246 (1988). [16] CAEN, General Catalog (1991). [ 17] Hamamatsu Technical Data (1990). [18] Philips, preliminary specification (June 1991).