PROCEEDINGS OF THE 31st ICRC, ŁODZ 2009 1

Scintillation hodoscope for muon diagnosticsI.I. Yashin, N.V. Ampilogov, I.I. Astapov, N.S. Barbashina, V.V. Borog, D.V. Chernov,

A.N. Dmitrieva, K.G. Kompaniets, A.A. Petrukhin, V.V. Shutenko and D.A. Timashkov

Moscow Engineering Physics Institute (State University), 115409 Moscow, Russia

Abstract. For development of experimental meth-ods of a new branch of solar-terrestrial physics –muon diagnostics, a new multichannel scintillationmuon hodoscope is proposed. Design of the newsetup is based on narrow long scintillation strips withoptical WLS fiber readout. Features of the hodoscopesetup are described and results of tests of a full-scaleprototype of basic detection module are discussed.

Keywords: muon diagnostic, wide aperture ho-doscope, scintillation strip

I. INTRODUCTION

One of the new and promising fields of the solar-terrestrial physics is muon diagnostics [1], intended toprovide continuous remote monitoring and forecastingof the conditions of the atmosphere and near-terrestrialspace. The experimental methods of muon diagnosticsare based on the simultaneous detection of muons fromall directions of celestial hemisphere. The muon fluxis generated in the upper atmosphere as a result ofinteractions of primary cosmic rays with the air atomicnuclei, and, on the one hand, brings information aboutactive processes in the heliosphere, which modulate theprimary cosmic ray flux, and, on the other hand, aboutprocesses of geophysical origin, which occur in theatmosphere. The analysis of spatial-angular variationsof muon flux detected in a real time mode gives apossibility to study such processes and to trace thedynamics of their changes, in particular, to reveal thedisturbed regions, to determine directions and speedsof their movement and to evaluate the time of theirappearance at a certain point.For the solution of these problems, elaboration of thedetectors with large sensitive area (> 10 m2) sufficientto provide required statistics in all directions of muonarrival, and also with high angular (< 1◦) and spatial(1 cm) accuracy is necessary.The first in the world such detector was scintillationmuon hodoscope TEMP (MEPhI, Russia) constructedin 1995 [2]. Detector consists of two two-coordinateplanes, mounted at a distance 1 m from each other on thetrunnion-type frame. The basic detection element whichdetermines the hodoscope spatial characteristics is thenarrow 3 m long scintillation strip with signal readout byseparate PMT. However the detection system is relativelyexpensive since it contains a large number of photomul-tipliers and costly strips manufactured from speciallypurified scintillator to provide a high transparency.The possibilities of muon diagnostics were demonstrated

by means of multilayer muon hodoscope URAGANwith total area 34 m2 assembled on the basis of thestreamer tube chambers [3]-[4]. However, the use ofgas-discharge chambers in muon hodoscopes for thecontinuous monitoring of processes in heliosphere, mag-netosphere and atmosphere of the Earth is unsuitable,since the efficiency of streamer tube operation dependson the conditions of the atmosphere, first of all, onpressure, temperature and humidity, i.e., on the basicparameters being investigated. Therewith, a high voltagesupply, automated system for gas mixture preparation,qualified personnel for the maintenance are required. Theoptimal detecting system for the muon hodoscope designis the assembly of scintillation strips with the lightcollection on the basis of the wavelength shifting (WLS)optical fibers. This experimental technique is widelyused for construction of the new generation of large areacoordinate-tracking detectors for particle physics [5]-[6].The main purpose of the proposed project is the de-velopment of the detecting system of new type wide-aperture muon hodoscope, intended for the solution ofthe problems of muon diagnostics.

II. HODOSCOPE DESIGN

Scintillation muon hodoscope is the multichannel sys-tem for detection and reconstruction in the real timemode of the track of each particle crossing the setupto provide the continuous zenith-azimuthal sensitivity tolow-level variations (∼ 0.1%) of muon flux.Hodoscope has a modular structure and is formed from

basic units of the detecting system – modules. The mod-ule is constituted by 64 strips read out by WLS fiberscoupled to one 64-pixel photodetector. All elements of

Fig. 1: Wide-aperture rotatable scintillation hodoscope.

2 I.I. YASHIN et al. SCINTILLATION HODOSCOPE FOR MUON DIAGNOSTICS

the module must be completed in one housing, havethe simple construction, and ensure reliable light insula-tion. The module represents the independent detectionsystem of the first level of the common hierarchicalstructure of muon hodoscope. Two such modules areassembled to construct the detection layer in order tocover 3.5 × 3.5 m2 sensitive area. Two layers of twomodules with the orthogonally oriented strips perform acoordinate plane providing X-Y track information. Fourplanes mounted on a common frame providing rotationaround both vertical and horizontal axes in wide rangeof zenith angles (±45◦) form multichannel precise muonhodoscope (see Fig. 1). In total, hodoscope contains 16basic 64-strip modules (1024 detection channels). Thedistance of 1 m between outer planes provides angularaccuracy of track reconstruction about 1.5 degree (fororthogonal incidence). Each detection channel representsscintillator strip, 3.5 m long, 10.6 mm thick, 26.3 mmwide, read by using WLS fiber and photodetector placedat one end of the fiber. The plastic scintillator is madeof polystyrene, 2% p-Terphenyl and 0.02% POPOP.As a basic type, the plastic scintillator strips producedby AMCRYS-H Company [7] are used. The particledetection principle and construction of scintillation stripsare shown in Fig. 2. The scintillator strips are obtainedby means of extrusion with a TiO2 co-extruded reflectivecoating for better light collection. A 3.5 m long groove(2.0 mm deep, 1.6 mm wide) in the centre of thescintillator strip houses the WLS fiber (Kuraray Y11-175multi-clad 1.0 mm [8]) which is glued in the groove withhigh transparency glue (BC-600, Saint-Gobain [9]). Theconstruction of basic module (BM) which consists of 64scintillation strips is presented in Fig. 3. As a photode-tector, a 64-anode Hamamatsu H7546 photomultiplier isused [10].

Fig. 2: Particle detection principle.

These PMTs have a matrix of 8×8 channels with a verycompact geometry (3× 3× 7 cm including the resistordivider and connectors). The mean gain at the voltage of850 V is about 106, providing efficient single photoelec-tron detection. The channel-to-channel gain difference(1÷3) is compensated by a readout electronics (speciallydesigned 64-channel ASIC chip MAROC2 [11]), whichallows the gain to be adjusted for each channel. The endsof the fibers are directly coupled to the PM Hamamatsu

H7546 through the end-cap which ensures the precisefiber positioning opposite the corresponding cells onthe photomultiplier cathode. The optical coupling ofWLS fibers and the PMT window is done by an opticalconnector (OC).

Fig. 3: Assembly of strips in basic module.

The rigidity of basic module construction is ensured ina way similar to the used for target tracker of OPERAexperiment [5]. The double face adhesive (thickness of1.1 mm) [12] is glued on two aluminium sheets with athickness 0.8 mm and with the sizes of 3500×1689 mm.On the surface of each strip above the grooves the Mylarscotch [13] 10 mm wide is glued for decreasing theloss of photons. After placing all 64 strips on adhesivesurface, the second aluminum sheet is glued to a striplayer. As a result it forms the dense “sandwich”.

III. CALIBRATION SYSTEM

To provide tests and monitoring of all electronicchannels, the LED flasher system is used. Two LEDs arederived in the area of end-cap strip coupling. Light fromLED flashes is injected in the WLS fibers. The LEDs arepulsed from outside by means of driver activated by anexternal flasher controller [14] (see Fig. 4). The flashercontroller provides independent smooth regulation oftwo LED luminosities in a wide dynamic range (upto 108 photons per LED flash). The LED flasher willprovide monitoring of each PMT channel up to about100 p.e. The LED flasher system will regularly beoperated during the experimental series.

IV. HODOSCOPE READOUT ELECTRONICS

The readout electronics of the scintillation hodoscopeis based on a 64-channel ASIC MAROC2 (Multi AnodeReadOut Chip) [11]. MAROC2 is the readout chipdesigned for the ATLAS luminometer [15]. It is usedto read out 64 channels of 64-anode Hamamatsu H7546photomultipliers. In total, 16 chips are necessary for the

PROCEEDINGS OF THE 31st ICRC, ŁODZ 2009 3

Fig. 4: General block-diagram of hodoscope electronics.

full hodoscope setup. Each MAROC2 channel provides a100% trigger rate for signal greater than 1/3 photoelec-tron. This corresponds to a charge of 50 fC for PMToperation at a gain of 106. The noise is less than 2 fC(for Hamamatsu H7546 PMT). The crosstalk betweenneighboring channels is less than 1%. Additionally, thecharge measurements should be feasible up to a signalof 30 photoelectrons with a linearity of 2%. ASICMAROC2 has been designed using the AMS SiGe0.35 µm technology.

Fig. 5: End-cap of the basic module prototype.

The chip has 64 trigger outputs, an analog and a digitalmultiplexed charge output. Each channel is made ofa variable gain preamplifier with low input tunableimpedance (50-100 Ohm), a low offset and a low biascurrent (20 µA) in order to minimize the crosstalk.This variable gain allows to compensate the PMT gaindispersion (up to a factor 4) to an accuracy of 6% with6 bits. An analog multiplexed charge measurement isdelivered with 5 MHz readout speed. A digital version ofthis measurement is also furnished by a 12-bit ADC. Adynamic range of the charge measurement correspond toabout 16 pC (∼ 100 p.e.) at PMT gain of 106. The front-end board of one basic module carries one MAROC2,which is directly plugged to the Hamamatsu H7546PMT. The general scheme of hodoscope electronics isshown in the Fig. 4.

V. PROTOTYPE OF HODOSCOPE

The working prototype of the basic module with 16strips was made for the development of procedures ofassembling, optimization of construction and study ofcharacteristics of measuring scintillation strip channelswith the light collection on the basis of the WLS fibers.

Hamamatsu H8711 PMT was used as a photodetector.The features of the prototype end-cap design are shownin Fig. 5. Strips are glued to the aluminum sheets bymeans of the double face adhesive. The prototype end-cap represents a thin boxing, whose internal volume isdivided into two parts (see Fig. 5). The part neighboringto the strips is intended for coupling of strips to opticalconnector. In another part of the end-cap, the housing forPMT, OC, PMT itself and the cable layout are located.Measuring electronics of the prototype is located outsidethe BM. In Fig. 6 the photo of the prototype end-capwith coupled fibers is presented. Assembled prototype(without upper aluminium sheet) is shown in Fig. 7.

Fig. 6: End-cap with coupled fibers.

The PMT Hamamatsu H8711 (4×4 of pixels) was testedon the specialized facility equipped with LED flasherwith two blue LED drivers. Signals from PMT wereanalyzed by means of 4-channel CAEN VME digitaloscilloscope V1729. For each channel of PMT, the out-put charge distribution obtained at single electron modeof the light flash level (detection efficiency ∼ 10% withthe threshold of 1/3 p.e.) was measured at UHV = 850 V.The value of gain differences between channels wasobtained as about 2 (maximum gain was found forch. #16 as 2.9×106 at 850 V). Scheme of prototypedetection system is presented in Fig. 8. For PMT testsa LED flasher with two LED drivers and two-channelcontroller was used. The LEDs were mounted directlyinside the area of fiber coupling (Fig. 5).For study of PMT response at detection of scintil-lation flashes generated in the strips by penetratingmuons, the calibration telescope (CT) was used (see

4 I.I. YASHIN et al. SCINTILLATION HODOSCOPE FOR MUON DIAGNOSTICS

Fig. 8). Telescope consists of two scintillation counters(200×100×20 mm) with FEU-85 PMT (Russia). To

Fig. 7: Assembled prototype (without upper sheet).

Fig. 8: General block-diagram of prototype electronics.

0 1 2 3 4 5 6 7 8 9 100

5

10

15

20

25

Channel #13 Q = (3.78 ± 0.04), pC

Q, pC

Num

ber o

f eve

nts L = 300 mm

Fig. 9: Charge distribution of the signals from 13th stripof prototype (distance to telescope axis L = 300 mm).

suppress the soft component, the block of lead (5 cm)was placed between the counters. The transverse sizesof CT counters make it possible to simultaneously testfour strips. The distribution of charges of output PMTsignals for channel #13 at distance L = 300 mm toCT location is shown in Fig. 9. The graphs illustratingthe dependences of the light yields (in pC) of the strips##13, 14, 15, 16 on the distance from the beginning offibers to the CT axis are presented in Fig. 10. Curvesrepresent results of two-exponential fit:

y(x) = eA1−x/l1 + eA2−x/l2 . (1)

This function is a sum of two exponents: the firstexponential curve (short component l1) is responsible forabsorption and re-emission of the scintillation photons inthe fiber, and the second one determines the attenuationof re-emitted photons. Average attenuation length oflight in the fiber (l2) was estimated to be about 5.5 m.

0 500 1000 1500 2000 2500 3000 35001.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5y(x) ~ ea

1-x/l

1+ea2-x/l

2

Q, p

CDistance, mm

Q13: l2 = 5625 ± 652 mm Q14: l2 = 5614 ± 3489 mm Q15: l2 = 5493 ± 1689 mm Q16: l2 = 5406 ± 590 mm

Fig. 10: Dependences of the light yields on the distanceto CT axis for four strips ##13, 14, 15, 16.

VI. CONCLUSION

The scintillation hodoscope for development of ex-perimental methods of muon diagnostics of processes inthe heliosphere, magnetosphere and atmosphere of theEarth is proposed. The prototype of the hodoscope basicmodule detecting system with the light read by means ofWLS optical fibers is developed and created. Results ofprototype characteristic study demonstrated its ability toprovide reliable detection of muon tracks. The prototypecan serve as the basis for full-scale hodoscope.

VII. ACKNOWLEDGMENTS

The research was conducted in Scientific and Edu-cational Centre NEVOD with the support of the De-partment of science and industrial policy of Moscowgovernment (project 8/3-308n-08).

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