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IL NUOVO CIMENTO VOL. 13 C, N. 2 Marzo-Aprile 1990

PLASTEX: the Palermo-Leeds Air Shower Tracking Experiment (*)(**).

G. AGNETTA, B. BIONDO, R. BUCCHERI, C. M. CAROLLO, O. CATALANO J. LINSLEY and A. MANGANO

Ist i tuto di Fisica Cosmica ed Applicazioni dell 'Informatica, C N R - Palermo

L. SCARSI

Ist i tuto di Fisica Cosmica ed Applicazioni dell 'Informatica, C N R - Palermo Dipartimento di Energetica e Applicazioni di Fisica dell 'Universit4 - Palermo

M. A. LAWRENCE, P. A. OGDEN, M. PATEL, R. J. O. REID and A. A. WATSON

Department o f Physics, Universi ty of Leeds - Leeds

(ricevuto il 31 Dicembre 1988)

Summary. - - An experiment is described in which air shower measurements are carried out by means of a ,,track detector,, placed in a test location near the center of a conventional array telescope currently in use for gamma-ray astronomy. This array, consisting of scintillators (GREX, operated at Haverah Park (U.K.) by the Department of Physics of the University of Leeds), provides, in the usual way, for each shower it selects, its own estimates of shower direction and shower particle density at the test location. Estimates of the muon density at the test location are provided by a large muon detector in operation nearby. The scintillator array triggers this muon detector and it is able to provide flexibly tailored trigger signals for the track detector. This detector named PLASTEX (acronym for Palermo Leeds Air Shower Tracking Experiment), consisting of tracking chambers above and below a thin sheet of lead, designed to provide data on charged particles incident from the air, on the stopping, scattering and multiplication of these particles in the lead sheet, and on charged particles created in the lead sheet by shower photons. From these data estimates are derived of the densities and directional properties of the electrons, photons and muons striking the

(*) Paper presented at the 4th Cosmic Physics National Conference, Capri, September 11-14, 1988. (**) To speed up publication, the proofs were not sent to the authors and were supervised by the Scientific Committee.

391

392 G. AGNETTA, B. BIONDO, R. BUCCHERI, C. M. CAROLLO, ETC.

track detector. These results are to be compared with those given by the conventional detectors and with theoretical predictions given by simulations that are being carried out. The objective is to develop and assess optimal procedures for employing clusters of PLASTEX-type detectors as telescopes for the observation of UHE cosmic rays, including cosmic gamma- rays, over a very wide energy range.

PACS 94.40 - Cosmic-ray interactions with the Earth.

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

The GREX array, operated by the University of Leeds at Haverah Park in England, is one of the new ,,telescopes), searching for point sources of UHE cosmic radiation. At energies in the UHE range ( E > 100TeV), particles reaching the Earth from such sources generate air showers capable of penetrating the atmosphere and reaching ground level. The observations are carried out by means of particle detectors deployed in large arrays. In order to resolve point sources from the nearly uniform background of showers due to ordinary cosmic rays, the arrays used for these searches need to afford the best possible angular resolution. In addition they should provide data on the muon content of the showers they detect, for the purpose of identifying the nature of the particles arriving from the point sources.

In the usual approach using scintillators, shower directions are determined from relative delays in the arrival time of the shower front. To determine the muon content, additional detctors, screened by many radiation lengths of solid absorber to remove the dominant electron-photon component. The accuracy of shower directions given by scintillator data is not much better than 1 degree, due to unavoidable ,(reception fluctuations~ in shower particle arrival times.

A better alternative, according to simulations by Poirier et al. [1], is to derive the directions from the average local directions of shower particles sampled by an array of tracking chambers. Tracking chambers can also separate air shower muons from electrons, when used in pairs separated by a few radiation lengths of dense material[2], with important advantages compared to absorption-type

o

muon detectors: one obtains not only the local density of muons, but also their directions. Furthermore, high-energy (E>100MeV) shower photons ma- terializing in the dense layer give information about the shower direction that might otherwise be lost.

If results from PLASTEX confirm the simulations the door will be open to constructing air shower telescopes with angular resolution a few tenths of a degree. Not only will these be superior to existing telescopes for the study of UHE point sources, they will also be capable of observing and comparing shadows cast by the Sun and Moon, the latter through simple absorption but the

PLASTEX: THE PALERMO-LEEDS AIR SHOWER TRACKING EXPERIMENT 393

former through a combination of absorption and magnetic deflection. Thus the differential effect, Sun minus Moon, can provide information both on the large- scale heliomagnetic field and on the charge spectrum of cosmic-ray nuclei [3, 4].

The first goal of PLASTEX being improved angular resolution, the second goal is to demonstrate a new method of separating shower muons from electrons in EAS analysis. If point-source showers are produced by gamma rays,, their muon content should be less than that of normal, nucleus-initiated showers by a factor of 10 or more, unless there is an unexpected, strong deviation in the standard theory of high-energy interactions. But if the point-source particles are not gamma-rays, then by default they are exotic, a result that would be almost equally unexpected and shocking[5, 6].

PLASTEX is designed to give the directions of shower muons as well as their density. The directions of muons arriving at moderate-to-large core distance (R > a few tens of m) reflect the geometric height at which a shower starts to develop. Event-by-event estimates of this height can be used to correct for fluctuations in shower size, for a given energy, due to variations in the starting height, thus improving greatly the energy resolution of an air shower telescope made up of PLASTEX-type tracking chambers, compared to that of scintillator array telescopes, which is limited by the presence of these fluctuations. By making use of this feature, together with the favourable statistical accuracy of muon-content estimates when all detectors are muon counters, a PLASTEX- based array telescope will have superior primary mass resolution for showers initiated by nuclei [7]. Since for these nuclei the charge is proportional to the mass, estimating the mass is crucially important for interpreting results on the energy spectrum and anisotropy, in light of influences by the galactic magnetic field.

Finally, such an array will be will suited for measuring, as well as detecting, ,,giant,, air showers, with primary energy in the range (101s + 102~ eV, striking outside the array at considerable distance from the center ([4b] and references therein). Thus such an array, although small in actual dimensions (diameter a few hundred m), can have a counting rate, for giant showers, comparable to that of the Fly's Eye detector[8], core distances (impact parameters) and shower directions being found by closely analogous principles, using muons instead of photons from atmospheric fluorescence.

2. - The GREX array .

The GREX array at Haverah Park, near Leeds, shown in fig. 1 comprises 32 scintillators plus 4 in process of being added. The central portion, which will be somewhat enlarged as a result of the additional detectors, has a 30 m spacing, and responds to cosmic-ray showers initiated by primaries of energy about 400 TeV. The outer portion covered by scintillators with a 50 m spacing has a

394 G. AGNETTA, B. BIONDO, R. BUCCHERI, C. M. CAROLLO~ ETC.

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Fig. 1. - Layout of the original 32 detectors of the GREX array. The detectors are laid out on a 30 m triangular grid near the center, and on a 50 m grid in the outer region. Before the end of 1988, 4 additional scintillators will be added so as to enlarge the central portion (shown 0). Detectors 15, 16 and 25, 32 are side-by-side so that measurement uncertain- ties can be assessed directly. Detector size not to scale.

threshold of about 600 TeV. Both ar rays have been shown to have an angular resolution be t t e r than 1 degree. The P L A S T E X tracking chamber assemblies have been placed within a few meters of scintillators 15 and 16, in the central portion of the array. The ra te of events within 30 ~ of vert ical and having cores within 30 m of the tes t location is about 70 per day. The overall event ra te is 13 per minute. G R E X has been in operation since March 1986 with an <,on-time>> of over 80%; this is expected to increase to at least 90% within next year. In addition to the scintillator array, a 40 square me te r de tec tor opera ted by the Univers i ty of Not t ingham (shielded liquid scintillator, muon ene rgy threshold 400MeV), is located adjacent to t ex t location. The total facility thus makes possible in well-measured air showers a realistic tes t of the P L A S T E X tracking chamber assembly.

3 . - The P L A S T E X d e t e c t o r .

The P L A S T E X detector , shown in fig. 2, consists of 2 equal par ts (,<stacks>>) placed side by side. Each stack is made up of two equal t racking chambers (2.35

P L A S T E X : THE PALERMO-LEEDS AIR SHOWER TRACKING EXPERIMENT 395

.9 cm

Fig. 2. - The P L A S T E X t rack ing c h a m b e r assembl ies .

m • 2.5m) placed above and below a thin-layer dense material (0.9cm thick lead). Each tracking chamber (fig. 3) consists of 3 planes of 1 cm width streamer tubes. Each plane has orthogonal sets of readout strips giving both X and Y coordinates of discharges caused by the passage of charged particles. The streamer tubes, the arrangement of readout strips, and the electronic readout

40 cm f

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40 cm

40 cm

A B

A B

A B C A B

A B

Fig. 3. - Track ing c h a m b e r plane dis t r ibut ion.

A = LST B = supporting structure C = lead

40 cm

. . . A ~ A I I / l l l l l / l l l l l l l l i l l l l l l l J I I I j j j ~ e l l L B

396 G. AGNETTA, B. BIONDO, R. BUCCHERI, C. M. CAROLLO, ETC.

system are similar to those used in the NUSEX experiment [9] and in R422-ISR accelerator experiment [10].

The mechanical structure is made from steel tubing and plate, and supports the 12 planes in two adjacent stacks of six planes. It is composed of

1) base frame, 2) vertical columns, 3) brackets, 4) support planes for the limited tubes (LST).

The support planes, which are 2.5m x 2 . 3 5 m x 5cm, rest on the brackets which are mounted on the eight vertical columns. The spacing of the planes can be varied in 4 cm steps, and is presently 40 cm. The two central planes in each stack are in contact with a layer of LST on top and a 0.9cm layer of lead and another layer of LST in between. The rigidity of the support planes, which are made of a paper honeycomb faced with glass fibre, is such that the deformation of the leaded plane is very small (< 3mm).

The LST are flushed continuously at low pressure (1 mb over atmospheric) with a mixture of 75% isobutane 25% argon. The gas is supplied to each of the 24 semi-planes individually. Each semi-plane supply is equipped with a pressure monitor which closes an electrovalve should the pressure exceed 10mb over atmospheric. The hazardous nature of the gas used requires a number of safety measures. In particular:

1) Each semi-plane has an oil-filled bubbler to prevent the entry of air if the gas flux is stopped.

2) The isobutane bottles are located external to the main experimental area.

3) Three explosive-gas detectors are located in the vicinity of the experiment, and are linked to a central controller with the following functions:

a) shut off the gas flux,

b) operate acoustic and visual warnings,

c) switch on extractor system to remove leaked gas from building.

A CAEN SYS27 high-voltage supply provides the EHT (Extra High Tension) for the 24 semi-planes individually. Twelve low-tension power supplies are used to power the LeCroy STOS 4200 read-out boards at + 5 V, and a single 4 output adjustable power supply maintains the threshold voltage for the X and Y boards of the two stacks.

The control system comprises a Compaq Deskpro 286 computer interfaced to the readout boards via an IBM type board which contains the logic for the control of the STOS 4200 boards (clock, load, test, shift). In addition the computer is interfaced to a Phase IV streamer tape unit, with a maximum capacity of 2


streamer tube tracking detector & read-out boards

control and

internal and GREX programmable trigger

data acquisition _ tape - recorder

Fig. 4. - PLASTEX data acquisition and control flow diagram.

Gbytes, on which the data from the stacks plus the event time (provided by GREX) will be stored (fig. 4).

The gas detector electronics are ba t te ry-backed to ensure operation in the case of main power failure.

The t r igger input is a very flexible system, with 16 possible t r igger sources.

read-out control trigger data input

Y Y I terminatiO~panel [termination I p a n e l I terminatiOnpanel

t I I

EPLD programmable trigger circuit I/O parallel port

I

clock and load generator

Fig. 5. - Data acquisition procedure.

bus translation and control logic

? P C bus

398 G. AGNET'rA, B. BIONDO, R. BUCCIIERI, C. M. CAROLLO, ETC.

Any combination on triggers may be selected by software, for example a single plane or group of planes, and there is also provision for the GREX external trigger.

The data acquisition procedure is as follows: when there is trigger (previously selected) the shift registers on the read-out boards are commanded to load the data from the streamer tube discriminators. Then the data are shifted out of the registers in parallel by the clock instruction, and transferred to the tape (fig. 5).

Presently the experiment has undergone the process of commissioning:

1) All the streamer tubes are operational and conditioned.

2) Repeated measurements of the operating EHT plateau give consistent results. A plateau region of 200V with knee at about 4.60KV is found, with a cosmic-ray background rate of 200m-2s -~ above the leaded plane.

3) The variation of count rate with threshold voltage has been investigated and a wide acceptable range found.

4) The data taking has started and a session of measurement aiming to qualify the performance of the detector is now in progress. This session is planned to continue for twelve months.

The LST detectors and their read-out electronics are part of the BCF (Bologna-CERN-Frascati) experiment R-422 (Prof. Zichichi's group), with which a fruitful collaboration has started and is going on.

R E F E R E N C E S

[1] J. POIRIER, J. LINSLEY and S. MlCOCKI: University of Notre Dame Preprint GRAND 87-90 (1987).

[2] J. LINSLEY: Proceedings of the XX International Cosmic Ray Conference (Moscow, 1987), p. 138.

[3] J. LLOYD EVANS: Proceedings of the XIX International Cosmic Ray Conference (La Jolla, 1985), paper 0G5.1-9.

[4] a) J. LINSLEY: Proceedings of the XIX International Cosmic Ray Conference (La Jolla, 1985), paper OG 9,4-9; b) J. LINSLEY: in Techniques in Ultra High Energy Gamma Ray Astronomy, edited by R. G. PROTHEROE and S. A. STEPHENS (Department of Physics, University of Adelaide, Australia).

[5] W. OCHS and L. STODOLSKY: Phys. Rev. D, 33, 1247 (1985). [6] B. L. DINGUS et al.: Phys. Rev. Lett., 61, 1906 (1988). [7] J. LINSLEY: in Origin of Cosmic Rays, edited by G. SETT et al. (Reidel, Dordrecht,

1981), p. 53. [8] R. M. BALTRUSAITIS et al.: Nucl. Instrum. Methods A, 240, 410. [9] G. BATTISTONI et al.: Nucl. Instrum. Methods A, 245, 277 (1986).

[10] M. BASILE et al.: Nucl. Instrum. Methods A, 235, 74 (1985).


�9 R I A S S U N T O

In questo articolo viene descritto un esperimento (PLASTEX) in cui vengono effettuate misure di sciami cosmici per mezzo di un rivelatore posto al centro di un telescopio convenzionale per gamma-astronomia alle alte energie (GREX, Haverah Park, Leeds, Gran Bretagna). U telescopio, costituito da un array di scintillatori, fornisce, per ogni sciame rivelato, le stime della direzione di arrivo e della densit~ di particelle in corrispondenza alla posizione del rivelatore di PLASTEX. La stima del contenuto muonico dello sciame in tale posizione ~ invece data da un rivelatore di muoni posto nelle vicinanze. L'array di scintillatori pilota tale rivelatore di muoni e fornisce gli opportuni segnali di trigger al rivelatore di PLASTEX. Quest'ultimo consiste in due camere a tracciamento di muoni separate da un sottile strato di piombo, che possono fornire dati sulle particelle cariche incidenti, sull'arresto, la dispersione e la moltiplicazione di tali particelle nello strato di piombo, e sulle particelle cariche create nel piombo dai fotoni appartenenti allo sciame incidente. Da questi dati vengono derivate le stime delle densit~ e delle propriet~ direzionali degli elettroni, dei fotoni e dei muoni che colpiscono il rivelatore che sono poi confrontate con quelle ottenute dagli scintillatori di GREX e con le predizioni teoriche ricavate da simulazioni at tualmente in corso. L'obiettivo ~ di studiare la possibilit~ dell'impiego di un array di rivelatori tipo PLASTEX come telescopi per l'osservazione di radiazione cosmica di energia molto alta.

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