Arab J Sci EngDOI 10.1007/s13369-013-0847-z

RESEARCH ARTICLE - PHYSICS

Aerogel Efficiencies of Threshold Cherenkov Counters

Baris Tamer Tonguc · Sule Citci

Received: 12 November 2012 / Accepted: 31 May 2013© King Fahd University of Petroleum and Minerals 2013

Abstract We present the aerogel efficiencies of the Cher-enkov counters used as a sub-component of Bates largeacceptance spectrometer toroid. The events of the elasticchannel of electron–proton scattering from hydrogen is uti-lized. The counters’ aerogel efficiencies change between(91.5 ± 0.3) and (98.6 ± 0.6) % based on a time-of-flightscintillation paddle. In addition, up to ∼7 % variation in thelocal efficiencies per paddle is attributable to statistical fluc-tuations and no local inefficient aerogel pockets are encoun-tered.

Keywords Cherenkov detectors · Aerogel · Scintillationdetectors · Data analysis · Elastic electron scattering

1 Introduction

Cherenkov light, produced by a charged particle passingthrough an insulator with a speed greater than the speed oflight in the same medium, consists of photons emitted by theelectrons in the medium [1]. The charged particle polarizes

B. T. Tonguc (B) · S. CitciDepartment of Physics, Sakarya University, Sakarya 54187, Turkeye-mail: btonguc@sakarya.edu.tr

the electrons with its own electromagnetic field when mov-ing across. While returning to their earlier states after theparticle has left, the polarized electrons emit photons whichconstructively interfere with each other. This intensifies theCherenkov light coming out conically.

A Cherenkov counter (CC) collects the Cherenkov lightproduced in the medium using photomultiplier tubes (PMTs)in particle detection. The number of photoelectrons Np.e.

knocked out at PMTs gives a measure on the calibre of aCC.

Threshold CCs distinguish a light particle of mass m froma heavier particle of mass M up to a momentum threshold pt

by choosing a transparent medium with the index of refrac-tion n such that it equals 1/β where β(=v/c) value is deter-mined using M and pt . Up to this pt value, Cherenkov lightwould be produced by only the lighter particle species whichallows for separation. Beyond this value the light is also pro-duced by the other particle species. The number of photo-electrons produced per path length Np.e./L for the lighterparticle is approximately found [2,3] using

Np.e./L ≈ 0.90 m−1 M2 − m2

p2t + M2

(1)

CCs are mainly used in particle identification based onCherenkov effect and come in two types: threshold or imag-ing [2]. Between the two types, threshold CCs vary in termsof the refraction index of translucent material used as theradiator of which can be in the form of solid, liquid and gas.Low index of refraction and solid-state feature make aerogelssuitable for radiators among the threshold counters. Aerogelscome in large variety depending on the constituent materialsuch as silica, metal oxide, organic and carbon aerogels. Inparticular, silica aerogel is the most commonly known andextensively studied type and can be either hydrophobic or

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Fig. 1 A drawing of seven aerogel tiles add up to make aerogel layerfor larger counters

hydrophilic [4]. It stands out with the features of high homo-geneity and long-lasting optical stability. On the other hand,fragile and brittle silica aerogel can be in the form of blocksthat can be chipped and cracked at the edges [5].

2 Motivation

The number of photons, produced in aerogel layers, is oneof the essential elements for the particle separation featureof CCs. Hydrophobic Matsushita SP30 aerogel units wereused in the formation of the layers of the CCs (Fig. 1). Anydamage on these units can significantly reduce the photonyield and, in turn, can negatively affect general efficiencyof the CCs. Accordingly, systematic errors may be intro-duced into experimentally measured quantities. Therefore, athorough reexamination becomes necessary on the aerogellayers.

3 The Cherenkov Counters and TOF Scintillators

The Bates large acceptance spectrometer toroid (BLAST)was built to study the spin-dependent electromagnetic inter-actions in few nucleon systems and located in the South HallStorage Ring of Bates Linear Accelerator Center, a Labora-tory of Massachusetts Institute of Technology, USA. Longi-tudinally polarized electrons were accelerated in the facilityup to ∼1 GeV with currents of ∼175 mA and lifetimes of∼25 min. Polarization was maintained in the storage ring by aSiberian snake and was monitored by a Compton polarimeter.An atomic beam source polarized target nuclei, proton and

Fig. 2 A sketch of relevant components of the BLAST detector system(not scaled). A full and scaled version can be found at [7]

deuteron and hence allowed for double polarization measure-ments. Target being in gas form was blown into a tube whichis co-axial with the beam and at the centre of the spectrometer.Left/right symmetric, having large acceptance and multipur-pose BLAST detector consisted of a number of sub-detectorsystems (Fig. 2): wire chambers (WCs) for determining thekinematical quantities of the charged particles in a regionwith a toroidal magnetic field of 3800 G, Cherenkov counters(CCs) for the electron-pion separation, time-of-flight scintil-lators (TOFs) for the timing measurements of particles, andneutron counters (NCs) for the neutron identification [6].

Three diffusely reflective aerogel CCs, labeled from for-ward to backward as CC0, CC1 and CC2, are used to distin-guish electrons from pions for each sector up to the momen-tum of 700 MeV/c. A fourth CC used to be present previouslywas removed in favour of detection of elastic deuterons. Thecounters were built and placed in such a way that symmetryaccording to beam axis was preserved. The dimensions of theCCs change to some extent due to the volume constraints inthe system. The larger counters are 1.00 m wide, 1.50 m longand 0.30 m deep (Fig. 3). While the most forward ones have0.07 m thick aerogel layer with refraction index n = 1.02,the others have 0.05 m thick layers with n = 1.03 [3]. Thecounters are equipped with 6, 8 and 12 PMTs with 0.127 mdiameter and cover 20◦–65◦ polar angle with 15◦ portions,respectively. The PMTs are shielded against the high mag-netic field. Cherenkov light produced in the aerogel layerrefracted into a closed space coated with diffusely reflectivematerial inside. All the components were bundled in an alu-minium case [8].

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Fig. 3 A schematic view of CC1 on the left (CCL1) and a scatteredelectron passing through

TOFs with detection efficiency better than 99 % providedtiming information for triggering system. The TOF detectorin each sector consisted of 16 vertical scintillator paddlesand was placed behind the CCs. All the paddles are madeof 0.0254 m thick Bicron plastic scintillation material. Themost forward four paddles covering CC0s (TOF0–TOF3) are1.19 m high and 0.15 m wide, while the rest of the paddlescovering CC1s (TOF4–TOF7) and CC2s (TOF8–TOF11) are1.80 m high and 0.26 m wide. Two PMTs were mountedon top and bottom of each paddle. The PMTs enable oneto determine the approximate vertical position of a particlehitting it [6].

4 Analysis and Results

The analysis was performed using 1H→

(e→

, e′p) reaction chan-nel. The main reason is that much higher statistics can beachieved. The elastic channel events are selected by utiliz-ing various cuts to electron–proton scattering data. These cutscan be categorized on the basis of electron and proton identifi-cation, detector acceptance, origination of particle tracks andvertices from the target and coplanarity of electron–protontracks.

Cherenkov counter efficiencies are mainly based on TOFscintillator paddles. In this work, the efficiencies are obtainedby normalizing the events from every counter to the ones fromeach of the four corresponding paddles. In the analysis eachpaddle is divided into 0.05 m long horizontal slices. Thisenables an aerogel surface to be checked at 80 points.

-0.6 -0.4 -0.2 0 0.2 0.4 0.60.5

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Fig. 5 The aerogel efficiency results for CCR0

Fig. 6 Typical examples for the edge effects: a an electron hits CCR1instead of CCR0 before hitting TOFR3, b an electron hits CCR2 insteadof CCR1 before hitting TOFR7, c an electron misses CCR2 beforehitting TOFR11

The key parameter for obtaining the efficiencies per aero-gel sub area is the vertical position knowledge of an electronpassing through a TOF paddle. Time difference between the

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Fig. 7 The aerogel efficiency results for all CCs

arrivals of scintillation light to top and bottom PMTs of thepaddle provides the information. The numbers of CC andTOF hits are filled in one-dimensional histograms depend-ing on position. These histograms are identical in terms ofbin size and bin number. The efficiency per bin correspond-ing to a sub area is determined by dividing the CC histograminto the TOF one. Aerogel efficiencies of CCR0, CC0 on theright sector, in front of TOFR0, the most forward TOF on theright, are given in Fig. 4.

In the figure, y = 0 points to right in the middle of theTOF0. Most of the elastic events are concentrated on the pad-dle centre and, in general, on forward TOFs. The errors are<∼1 % for −0.35 m < y < 0.35 m region, while these areup to ∼7(∼4) % for −0.50(0.35) m < y < −0.35(0.50) m.Efficiencies per bin fluctuate between 93 and 96 %. In addi-tion, the weighted average is found to be (94.9 ± 0.1) %.The events are filled in two-dimensional histograms to obtainthe efficiency per bin for whole aerogel surface of a CC. The

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Fig. 8 The aerogel efficiencies per sector

second dimension is reserved for the TOF paddle number. Inthe case of CCR0, it changes from 0 to 3 (Fig. 5).

Efficiency per bin using first three TOFs fluctuate around95 % with the minimum of 87 % for −0.45 m < y <

−0.40 m. On the other hand, the average efficiency usingthe last paddle, TOFR3, is (57.6 ± 0.2) %. The loss in theefficiency results from the fact that the trajectories of elec-trons are bent inward. This is known as “edge effect”. Inthis case, a substantial number of TOFR3 events come fromCCR1 (Fig. 6a). The amount of loss due to this effect isdetermined to be (31.2 ± 0.1) %. Accordingly, the com-bined value, (89.2 ± 0.2) %, is comparable to the resultsobtained for other forward TOFs on the right.

Aerogel efficiencies are close to 95 % with (1−4) % errorsper bin and fluctuate within statistical limits for all counters(Fig. 7). Similar edge effects also exist for aerogel regionscorresponding to TOF7s and TOF11s (Fig. 6b, c).

Aerogel efficiency per sector is obtained by combining ofthree CCs (Fig. 8). Efficiencies seem to remain nearly con-stant, and the edge effects between the counters are substan-tially eliminated. The case persists for TOF11s since thereare no other CCs to check the amount of missing events (Fig.6c).

5 Summary and Conclusions

In this study, aerogel efficiency was investigated for allBLAST CCs. To begin with, the efficiencies per sub areawere obtained along a TOF paddle. In general, 93–96 %with a few per cent errors per sub area was found. After-ward, the process is repeated for a CC aerogel layer with fourconsecutive TOF paddles. The efficiency figures fluctuated

87–95 % for 3 paddles and decreased to ∼58 % in averagefor the fourth one due to the edge effect. Similar behaviourwas observed for the other counters. At last, when all CCefficiencies as a whole were looked per sector, edge effectsdisappeared and efficiencies for those regions increased tothe level comparable to the others. In summary, zero- or low-efficiency pockets corresponding insensitive or less sensitiveaerogel regions were not encountered. Thus, that no chippingand/or cracking occurred was understood clearly. In conclu-sion, these high-efficiency figures entitled CCs to be utilizedin analyses and data acquisition system.

Acknowledgments We would like to thank Professor Ricardo Alar-con for his comments and support.

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