operational performance of the hall a mirror aerogel cherenkov counter

Nuclear Instruments and Methods in Physics Research A 487 (2002) 346–352

Operational performance of the Hall A mirror aerogelCherenkov counter

E.J. Brasha,*, J. Hovdeboa, G.J. Lolosa, G.M. Hubera, R. van der Meera,b,Z. Papandreoua

aDepartment of Physics, University of Regina, Regina, Sask, S4S 0A2, CanadabThomas Jefferson National Accelerator Facility, Newport News, VA 23606, USA

Received 2 July 2001; accepted 16 November 2001

Abstract

We report the results of an operational test of the efficiency and position sensitivity of a silica-aerogel Cherenkov

detector installed in the HRS-E spectrometer in Hall A at Jefferson Lab. The calibration was performed with data

from elastic electron scattering from polarized 3He: The response of the photo-multiplier tubes was linearized

with a quadratic correction, allowing a unique number of photo-electrons (PEs) to be extracted. The result obtained

(B7:3 PEs) is consistent with the performance of the prototype detector tested earlier under ideal conditions.

r 2002 Elsevier Science B.V. All rights reserved.

PACS: 29.40.Gx; 29.40.Ka

Keywords: Nuclear and particle physics; Detectors; Cherenkov

1. Introduction

The aerogel Cherenkov detectors installed in theHRS spectrometers in Hall A at the ThomasJefferson National Accelerator Facility (JLab) areoptimized to separate pions from protons insingle-arm experiments. In addition, the detectorhas most recently been used to provide trigger-level pion rejection in coincidence experiments,where the accidental rates from pion contamina-tion are unacceptably large. As such, the aerogelCherenkov detectors constitute an additional

element in an array of particle identificationdetectors (PID). The combination of a mirrordesign, rather than a diffusion surface, and a highnumber of photo-multiplier tubes (PMTs) resultedin high efficiency, and agreed well with simula-tions, in the case of a six PMT prototype [1]. Basedon these proof of principle tests, the constructionof the actual detector was completed and testedusing an electron beam [2].

Due to electronics and aerogel supply limita-tions, these first tests of the actual detector wereconducted with a limited amount of aerogelobtained from Airglass [3] and only six, out ofthe 26, PMTs (BURLE 8854). The results wereconsistent with the six element prototype [1]and indicated an expected average yield of 7.5

*Corresponding author. Tel.: +1-306-585-4201; fax: +1-

306-585-5659.

E-mail address: [email protected] (E.J. Brash).

0168-9002/02/$ - see front matterr 2002 Elsevier Science B.V. All rights reserved.

PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 2 1 9 9 - 4

photo-electrons (PE) if all 26 PMTs had beeninstalled and used. At the same time, a high degreeof position sensitivity was demonstrated [2], animportant advantage over diffusion design ifmultiple tracks are recorded in the focal planeinstrumentation. However, the production detec-tor with all 26 PMTs instrumented and with thefinal SiO2 aerogel panels had never been tested.This paper describes the final performance char-acteristics of the detector under actual experimen-tal operating conditions and with all detectorelements in place.

2. Description of the detector

The physical and technical requirements for theaerogel Cherenkov detectors have been reported indetail earlier [1,2] and will only be described brieflyhere.

2.1. The enclosure

The detector consists of planar parabolicmirrors, symmetric around their center (ridge),which focus the reflected light onto 13 PMTs oneach side of the detector. A cross-sectionaldrawing is shown in Fig. 1. The removable tray,consisting of an aluminum frame covered with acarbon–fibre–epoxy skin, contains 9 cm of SiO2

silica aerogel. The enclosure was hermeticallysealed using a combination of Tedlar film [4] andmastic sealing compounds, together with rubbergaskets and black dye in the epoxy compoundsused in critical joint sections.

The tray is 195 cm long� 41 cm wide. The SiO2

silica aerogel panels are held in place by aremovable window frame supporting a net of fineKevlar string. This is necessary since the detectoris mounted at a 451 angle with respect to thehorizontal, the same angle as the focal plane of thespectrometer. The detector, when sealed, is con-tinuously flushed with dry CO2 at a pressureslightly above atmospheric, which is sufficient toprevent moist-air penetration and to overcomethe pressure differential along the length of thedetector due to the inclination. This keeps the

interior dry to the level of 10% (or less) relativehumidity at room temperature.

The upper (mirror) section is sealed from theenvironment by a thin Tedlar film, in addition tothe carbon–fiber–epoxy mirror structure. TheTedlar film is indicated in Fig. 1 by the dashedline. Under the conditions of slight overpressurerequired to satisfy the requirements above, andminimize CO2 usage at the same time, the Tedlarfilm provides an immediate visual confirmation ofoverpressure by its bulging shape. The small CO2

leakage rate around the PMTs and the bases tendsto prevent 4He penetration into the enclosure andthe PMTs, a problem which has led to degradationof PMTs in other devices within the HRS detectorhut due to cryogen escape from the nearestsuperconducting quadrupole of the spectrometer.

2.2. SiO2 panel preparation

The aerogel panels were obtained fromAirglass [3]. Since this type of aerogel material is

1 2 3 4 5 6 7 8 9 10 11 12 13

14 15 16 17 18 19 20 21 22 23 24 25 26

Incident Particles

FrameUpperSection

Mirrors

RidgeMiddle (PMT)

Section

SiO Aerogel2

Fig. 1. Cross-sectional drawing of the detector viewed along

(upper) and perpendicular to (lower) the direction of incident

particles (not to scale).

E.J. Brash et al. / Nuclear Instruments and Methods in Physics Research A 487 (2002) 346–352 347

hydroscopic, it was baked just before installation.The temperature was gradually raised, over aperiod of B10 h; to the maximum temperature ofthe oven, 3431C; and kept at that temperature for40 h: The aerogel was warmed up and cooleddown at the same rate of 351C/h to avoid thermalstresses. The entire operation was performed undernatural atmospheric conditions during a period ofdry weather (B35% relative humidity) within anair-conditioned building. Visual inspection ofidentical aerogel panels, one baked and the othernot, revealed a dramatic improvement in bothclarity and transparency. This was tested with bothnatural and blue light sources.

The panels, varying between 2.0 and 3:0 cmthickness, were arranged so as to cover the areaavailable without overlapping gaps. The nominalindex of refraction, quoted by the aerogel supplier,was 1.025. Immediately after baking, the aerogelpanels were inserted into the enclosure and thedetector was flushed for 24 h with dry CO2;pressurized, sealed, and transported to the spectro-meter. The detector was still pressurized at thetime it was reconnected to the CO2 system, 2 hlater. Thus, the aerogel panels were not exposed toany additional humidity, other than during theremoval from the baking oven and installationinto the enclosure.

3. Calibration

The Burle 8854 PMTs [5] utilized in the detectorgive good single PE resolution. However, the gainresponse of the PMT-base combination is non-linear. This non-linearity is caused by two factors.First, the impedance-matching resistor added tothe purely resistive dynode chain has induced anelement of non-linearity in the system [6]. Second,the near-maximum voltage applied to the PMTsð�2950 VÞ to maximize collection efficiency [1],gives rise to large pulse heights, which result inspace-charge effects in the last few dynodes of the14-stage dynode chain. Since the system isoptimized for efficiency rather than pulse-heightanalysis, such non-linearity is not a concern aslong as it is taken into consideration when thenumber of PEs is extracted from the data.

In the initial analysis, a linear correction derivedfrom the first few PE peaks was applied. While thisworked well for low number of PEs, the highdirectionality of the detector implies most of thePEs would be collected by a single PMT, thusresulting in large responses. The region of largeresponse is where the linear correction is thepoorest, so a higher order correction had to beused.

Data from experiment E94-010 at JLab, forelastic electron scattering from polarized 3He at aspectrometer setting of 1:7 GeV=c; were used tocalculate the quadratic correction to be applied tothe analog-to-digital converters (ADCs). Thespectrum for each ADC was fit with an exponen-tial or Gaussian background, with the PE peaksthen fit as Gaussians over the background. Thepositions of the PE peaks were then taken as thecentroids of the fit Gaussians. In Fig. 2, a samplefit of the background and PE peaks is shown. Inthis particular spectrum, the first to fourth PEpeaks are resolvable above background.

Using the positions of the first three, and insome cases four, PE peaks, as well as the ADCpedestal position, a correction to second order wascalculated to linearize the response at an arbitrary2000 channels per PE. In Fig. 3, sample calibrationfits are shown for four of the photo-tubes of thedetector. One can see that in some cases, theresponse is quite linear, whereas in others a

Fig. 2. Sample fit of background and PE peaks for determina-

tion of photo-plectron peak positions.

E.J. Brash et al. / Nuclear Instruments and Methods in Physics Research A 487 (2002) 346–352348

significant quadratic correction is required. Priorto the data taking period for this experiment, all 26photo-tubes were approximately gain-matched inhardware by adjusting the applied high voltage toplace the first PE peak in the region of channel2000. This resulted in some tubes having highervoltages than others, and thus, for the reasonsmentioned earlier, we expect the non-linearity tobe more pronounced for some photo-tubes.

Following this fitting procedure, the Hall A dataanalysis package (ESPACE) was modified toperform a quadratic correction in place of theusual linear one.

4. Efficiency

Defining the detection efficiency, Z; as theprobability of collecting an average number ofPEs, Navg; greater than zero, we have

Z ¼ 1� e�Navg :

The average number of PEs is obtained from asum of the calibrated ADC responses for eachevent. With the calibration performed, the datafiles could be reanalyzed. The average of the ADCsum spectrum thus produced then corresponds toa unique average number of PEs.

A sample ADC sum is shown in Fig. 4, wherethe summed ADC value has been converted tonumber of PEs by dividing the ADC channel by2000 (the gain/PE). For the experimental runshown in the figure, all the PMTs were at thenominal (maximum) operating voltage of�2950 V: Of course in such an arrangement thePMTs were not gain-matched, as mentioned ear-lier. However, it is expected this configuration willresult in the largest number of PEs, for the reasonsexplained in Section 3. A bin-wise average wascalculated from the ADC sum to give Navg: Theresulting average number of PEs was Navg ¼ 7:3with a resultant efficiency for a b ¼ 1 particle ofZ ¼ 99:93%: This is in excellent agreement with theextrapolated number expected from the six PMTmeasurements of the same detector in Ref. [2].

Under normal operating conditions, it mayprove useful to impose a threshold requirementto suppress accidental Cherenkov events triggeredby the noise in a single PMT exceeding thehardware discriminator threshold. This is morelikely when the logic is in the OR mode, whereonly one PMT may act as a Cherenkov event.Nevertheless, the PMT ‘‘dark count-rate’’, withdiscriminator thresholds set just below the singlePE output, rarely exceeds 1 kHz; thus eliminatingaccidental coincidences as a factor of concern.With the summed ADC threshold set just aboveone PE, the efficiency was measured to be Z ¼99:36%: This value was obtained by integrating thesummed ADC spectrum both above and below thethreshold.

Fig. 3. Examples of ADC calibration.

Fig. 4. Sample ADC sum spectrum.


In some situations, the use of a higher threshold,for example three PEs, can be advantageous indistinguishing between two different particleswhich both have b values above the critical valuefor the detector (e.g. pions and kaons). This higherPE threshold can be used to preferentially rejectthe lower b particle. With the threshold set justabove three PEs, the efficiency was measured to beZ ¼ 92:9%:

5. Determination of effective index of refraction

During experiment E94-010, a large amount ofdata was accumulated over a wide range ofmomentum settings of the electron spectrometer.In general, for a given setting, both electrons andp� particles will be focused onto the focal planedetector package. For the purposes of determiningthe effective index of refraction of the SiO2

aerogel, several runs were analyzed in the regionwhere we expect the aerogel detector to becomeefficient for detecting pions. Using the nominalindex of refraction quoted by the manufacturer,n ¼ 1:025; this corresponds to a threshold b ¼0:976; which corresponds to a pion of momentumof 0:620 GeV=c:

In Fig. 5, we show the determined efficiencyusing data taken in the region of the expectedthreshold. In order to distinguish pions fromelectrons in each data set, a threshold Gas

Cherenkov detector, which is also part of theelectron arm detector stack, was used. The indexof refraction of the gas used in this detector isB1:003; and thus its momentum threshold forpions is much greater than for the aerogel detector.Particles were identified as electrons if they weredetected in the Gas Cherenkov, and as pionsotherwise. In this region of momentum, thecontamination from other particles, such as kaons,is expected to be very small. The pion efficiencydata in Fig. 5 were fit with a function of the form

Z ¼ 1� e�ðp�p0Þ=G:

The resulting fit gave p0 ¼ 0:536 GeV=c and G ¼0:121 GeV=c: We define the effective momentumthreshold to be the momentum where the efficiencyreaches 0.50. Using the above fit values, thiscorresponds to pthreshold ¼ 0:619 GeV=c: Thus, theeffective index of refraction is 1:025070:0004; inexcellent agreement with the nominal value.

6. Position sensitivity and multiplicity

One of the characteristic differences betweenmirror and diffusion box aerogel Cherenkovdetector designs is that the former collects mostof the PEs primarily on a single PMT (called herethe primary PMT), whereas the latter illuminate allthe PMTs more evenly. Thus, even if the totalnumber of PMTs is the same for both types, thePE distribution among the PMTs is quite different.In reality, diffusion by the SiO2 aerogel panels,which depends on the wavelength of the light, theindex of refraction, and the total thickness of theaerogel, will result in a higher multiplicity, even inmirror aerogel designs. Additional factors con-tributing to more than one PMT sharing the lightreflected by the mirrors are geometry, the size ofthe Cherenkov cone, as well as the location wherethe particle traversed the aerogel material withrespect to the mirror-PMT geometry. All of thesefactors result in a high probability that more thanone PMT will collect the emitted photons. Theeffective diffusion results have been investigated indetail and reported in Ref. [2].

In high rate experiments, as is common in HallA, multiple track events in the spectrometers may

Fig. 5. Determination of effective index of refraction of the

aerogel. The circles represent electron data, and the triangles

represent the pion data.


be of concern. Specifically, the ability to use theposition sensitivity of the Cherenkov detector toisolate which of the two possible tracks might berejected is a distinct advantage.

The position sensitivity under actual beamconditions is shown in Fig. 6. These data wereobtained in an inelastic scattering run whereelectrons were distributed rather uniformly overthe focal plane. This figure demonstrates thecorrelation between the Cherenkov PMT withthe largest ADC value and the x-position (dis-persive direction of the spectrometer) recon-structed from vertical drift chamber (VDC)information. The width of each ‘‘locus’’ in thefigure is B20 cm; or about twice the diameter ofthe PMT face itself. This is consistent withexpectations based on the diffusion that will occurin the 9 cm of aerogel in the detector. Thus, onewould be able to distinguish between multipletracks provided that the separation between them(in the dispersive plane) was \20 cm:

The actual data analysis based on single trackevents in the VDCs shows that the multiplicitydistribution is consistent with expectations, and isshown in Fig. 7. From this figure, the requirementof an ‘‘AND’’ coincidence for at least two PMTsto ‘‘fire’’ for a valid Cherenkov event to be

processed results in an efficiency of 97.6%. Thisresult is very encouraging, as it allows one to usethe ‘‘AND’’ requirement to reduce accidentalCherenkov events triggered by the single PMTnoise without greatly sacrificing efficiency.

7. Conclusions

The Hall A aerogel Cherenkov detectors havebeen installed and tested under operational condi-tions at Jefferson Lab. The detector has beencalibrated, and the extracted performance char-acteristics have met, and in some cases exceeded,the design goals. The combination of a mirrordesign, rather than a diffusion surface, and a highnumber of photo-multiplier tubes (PMTs) resultsin very high efficiencies and makes it possible touse the detector in a wide range of experimentalsituations where accurate particle identification isan important factor.

Acknowledgements

We gratefully acknowledge the assistance andsupport of the JLab Hall A physics and technical

Fig. 6. Correlation between PMT with largest ADC signal and

reconstructed x-position from VDC information. PMT num-

bers 1–13 correspond to the right side of the detector, and PMT

numbers 14–26 correspond to the left side of the detector (see

Fig. 1). Fig. 7. The multiplicity probability distribution with thresholds

set just below the single PE pulse-height for each PMT.


staff. This work was supported in part by theNatural Sciences and Engineering Research Coun-cil of Canada, and also by DOE contract DE-AC05-84ER40150 under which the SoutheasternUniversities Research Association (SURA) oper-ates the Thomas Jefferson National AcceleratorFacility.

References

[1] L.C. Alexa, et al., Nucl. Instr. and Meth. A 365 (1995) 299.

[2] G.J. Lolos, et al., Nucl. Instr. and Meth. A 385 (1997)

403.

[3] Airglass AB, Box 150, S-24500, Staffanstorp, Sweden.

[4] DuPont Canada Inc., Box 2200, Streetsville, Mississauga,

ON L5M 2H3, Canada

[5] Burle Industries GmbH, Postfach 1349, D-52492 Baeswei-

ler, Germany.

[6] G.J. Lolos, et al., Nucl. Instr. and Meth. A 309 (1991)

552.


operational performance of the hall a mirror aerogel cherenkov counter

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