The 64-channel Multianode Photo Multiplier (MaPMT) has been evaluated as a
candidate for the LHCb Ring Imaging Cherenkov (RICH) photo detectors. We
present result from data taken with a 3x3 array of closely packed MaPMTs
mounted onto the RICH 1 prototype vessel, exposed to charged particle beams at
CERN, and read out at LHC speed. Using a LED light source, we have performed
spatial light scans to study the light collection efficiency of the MaPMTs We
have also measured the performance of the MaPMTs as a function of the applied
high voltage. Different dynode resistor chains have been used to study the
tubes at low gains. In addition, we have studied the behaviour of the MaPMT in
magnetic fields.Comment: 5 pages, 6 figures, 8 .eps files, RICH2002 conference, Pylo

Albrecht

Chirikov-Zorn

F. Muheim

English

arXiv

The 64-channel Multianode Photo Multiplier (MaPMT) has been evaluated as a candidate for the LHCb Ring Imaging Cherenkov (RICH) photo detectors. We present result from data taken with a 3x3 array of closely packed MaPMTs mounted onto the RICH 1 prototype vessel, exposed to charged particle beams at CERN, and read out at LHC speed. Using a LED light source, we have performed spatial light scans to study the light collection efficiency of the MaPMTs We have also measured the performance of the MaPMTs as a function of the applied high voltage. Different dynode resistor chains have been used to study the tubes at low gains. In addition, we have studied the behaviour of the MaPMT in magnetic fields.The 64-channel Multianode Photo Multiplier (MaPMT) has been evaluated as a candidate for the LHCb Ring Imaging Cherenkov (RICH) photo detectors. We present result from data taken with a 3x3 array of closely packed MaPMTs mounted onto the RICH 1 prototype vessel, exposed to charged particle beams at CERN, and read out at LHC speed. Using a LED light source, we have performed spatial light scans to study the light collection efficiency of the MaPMTs We have also measured the performance of the MaPMTs as a function of the applied high voltage. Different dynode resistor chains have been used to study the tubes at low gains. In addition, we have studied the behaviour of the MaPMT in magnetic fields

Muheim, F.

CERN Document Server

1Multianode Photo Multiplier Tubes as Photo Detectors for Ring ImagingCherenkov DetectorsF. Muheimaedin]Department of Physics and Astronomy, University of EdinburghMayeld Road, Edinburgh EH9 3JZ, Scotland/UKa[The 64-channel Multianode Photo Multiplier (MaPMT) has been evaluated as a candidate for the LHCb RingImaging Cherenkov (RICH) photo detectors. We present result from data taken with a 3x3 array of closely packedMaPMTs mounted onto the RICH 1 prototype vessel, exposed to charged particle beams at CERN, and readout at LHC speed. Using a LED light source, we have performed spatial light scans to study the light collectionefficiency of the MaPMTs We have also measured the performance of the MaPMTs as a function of the appliedhigh voltage. Different dynode resistor chains have been used to study the tubes at low gains. In addition, wehave studied the behaviour of the MaPMT in magnetic fields.1. IntroductionWe have evaluated the 64-channel MultianodePhoto Multiplier (MaPMT) as a candidate photodetector for the LHCb Ring Imaging Cherenkov(RICH) counters. The MaPMT contains a 8x8array of 64 dynode chains enclosed in a single vac-uum envelope. The window is made of UV glassinstead of borosilicate which extends the trans-parency down to 200 nm. According to the man-ufacturer, the bi-alkali photo cathode has a quan-tum eciency of 22% at a wavelength of 380 nm.The pitch between anode pixels is 2.3 mm. Whenaccounting for the inactive 0.2 mm gaps betweenpixels, the MaPMT has an active area coverageof only 38%. The full angular coverage with 85%active area can be restored by mounting a fusedsilica lens with one flat and one spherical surfacewith a radius of curvature of 25 mm in front ofthe tube. The lens has a demagnication factorof about two-third and focuses the incoming pho-tons onto the sensitive area of the MaPMT.2. Test Beam ResultsThe sensitivity of the MaPMT to Cherenkovphotons has been tested with particle beams of120 GeV/c pions at the SPS accelerator at CERN.A detailed account of these tests is described inreference [1]. Here we present the main results. A3x3 cluster of closely packed MaPMTs has beenmounted onto a prototype of the RICH1 detectorfor the LHCb experiment. The radiator gas wasCF4 at a pressure of 700 mbar and the photocathode was set at -1000 V. To demonstrate thatthe MaPMTs will work at the LHC the data wererecorded with the APVm chip running at 40 MHz.Figure 1. Cherenkov ring measured with theMaPMT array with (right plot) and withoutlenses (left plot) mounted in front of the tubes.In Figure 1 we show the results for two runs of6000 events each, one with and one without the2lenses mounted in front of the MaPMTs. In bothcases, a ring of Cherenkov photons is clearly vis-ible. The ratio of yields with and without lensesis 1.55 as is expected from a simulation. Withthe lenses the full Cherenkov ring is capturedwhich demonstrates that MaPMTs can be closelypacked to restore full geometrical acceptance. Weobserve 6.96 0.33 photo electrons per event, invery good agreement with the simulation. Thecorresponding gure of merit for a RICH detec-tor is N0 = 110  7 cm−1. In Figure 2 we showFigure 2. MaPMT pulse height spectrum witha superimposed t (solid line). Also shownis the single photo electron component (dashedline). The vertical line indicates the 5σ cut abovepedestal.the pulse height spectrum for a single pixel. Thedata were corrected as follows. A common-modebaseline has been subtracted and signals due tocross-talk stemming from the read-out electron-ics have been removed. We clearly distinguishbetween the pedestal peak and the signal contain-ing mainly one photo electron. Overlaid is a twhere λ is the mean number of photo electrons.3. LED Scans, Signal Shape, Low GainsWe have also studied the performance ofMaPMTs in the laboratory. As a light sourcewe used a blue LED coupled to a mono-mode -bre which could be moved with respect to theMaPMT by semi-automated optical stages. Tworead-out systems were employed. One is the 40MHz read-out with the APVm chip also used forthe beam tests, and another one is based on CA-MAC ampliers and ADCs.In addition to the nine tubes studied with testbeams we obtained two more MaPMTs which, ac-cording to the manufacturer, have an increasedquantum eciency of maximum 25 - 27% at360 nm. The focusing of these new tubes hasbeen improved. Firstly, the acceptance at theoutside edges of the active area of the MaPMThas been increased, and another focusing wirehas been added. Secondly, the distance betweenthe focusing grid and the entry slits to the dyn-ode chain has been reduced. We have studiedthese new MaPMTs with a LED light scan. Thedata have been taken by moving the light sourcein steps of 0.1 mm along the front face of theMaPMT, with the photocathode at -900 V. The02040608010012014016020 40 60 80 100scan across MaPMT surface : [1/10 mm]signal-pedestal  [ADC]new focussingold focussing00.050.10.150.20.250.30.350.420 40 60 80 100scan across MaPMT surface : [1/10 mm]λ-PoissonFigure 3. Scan across an MaPMT. Shown are theaverage single photon pulse height s (top) andthe mean number of photo electrons λ (bottom)versus the relative position.3pulse height spectra have been t with a Gaus-sian curve as single photo electron signal shape.Shown in Figure 3 are the average single photonpulse height, s, and λ, the mean number of mea-sured photo electrons as a function of the relativeposition along the tube. The gain is proportionalto s and the collection eciency is closely relatedto λ. We observe that the edge pixels have alarger acceptance for the new MaPMT (solid his-tograms) with respect to the standard focusingtube (dashed histograms). Due to the improvedfocusing, both the gain and the collection e-ciency are more homogeneous.Most front-end chips developed for the LHChave been optimised for signals from Silicon sen-sors or micro-strip gas chambers with a charge of26000 - 40000 e. At a voltage of 800 V betweenanode and photocathode the gain of the MaPMTis around 300000 which is about 10 times higher.To make use of one of these front-end chips forthe the MaPMT a gain adaptation is needed. Thepreferred solution is to modify a preamplier toa lower gain. A second possibility is to make useof an attenuator network. This has been usedfor the APVm read-out by means of an AC cou-pler. However this approach suers from crosstalk problems. A third way that we have investi-gated is the feasibility to run the MaPMTs at alower gain.To reduce the gain of the MaPMT we havechanged the values of the resistors between thephoto cathode, the rst, second and the thirddynode, respectively. The basic idea is to keepthe gain at the rst dynode while lowering theoverall gain. The values for the default, a mediumand a low gain option are given in Table 1. Thesignal width is mainly due to the Poisson distri-bution for multiplication at the rst dynode andcan be estimated as g1 = s2/σ2 where σ is theGaussian width of the single photon signal. Us-ing the CAMAC read-out we have measured theoverall gain of the MaPMT for a center and aborder pixel as a function of the high voltage forthe three dierent resistor chains. In Figure 4top) we plot s as a function of the applied highvoltage for the three dierent resistor chains. Ata given voltage the gain of the MaPMT reducesby a factor of four for the low gain option. In theMaPMT HV scan : for different resistor chains1010 2750 800 850 900 950 1000HV [volts]signal-pedestal  [ADC] default gain   : 3-2-2-1-...-1-2-5 medium gain : 4-2-2-1-...-1-2-5 low gain         : 4-3-3-1-...-1-2-5 center pixel border pixelhigh gain ≈ 1.65 * medium gainmedium gain ≈ 2.35 * low gain0123456710 2(gain) signal [ADC]g 1 = signal2  / width2Figure 4. High voltage scan for three dierentresistor chains. Top) Average single photon pulseheight s versus high voltage. Bottom) Gain atthe rst dynode g1 = s2/σ2 versus s.bottom plot we plot g1 = s2/σ2 as a function ofs. As expected we observe that g1 increases withs. However there are no dierences between thedierent resistor chains. We have repeated thesemeasurements with the APVm read-out and theresults agree.We have studied the loss of signal below thethreshold cut of 5 σped where σped is the Gaussianwidth of the pedestal peak. We have employeddierent t methods. Besides a Gaussian signalshape we have implemented a t which uses aPoisson distribution for the rst two photo elec-trons [2]. This method also allows for photonswhich pass through the photo cathode to be con-verted into photo electrons at the rst dynode.A Gaussian or a Poisson t to the pulse heightspectra are inadequate to describe the region ofsmall signals close to the pedestal. In Figure 5we show a pulse height spectra recorded with theCAMAC read-out and the photo cathode at -900V. The superimposed t (dotted line) allows for4Table 1Resistances in[102 kΩ]in front of each dynode.Dynode 1 2 3 4 5 6 7 8 9 10 11 12 AnodeStandard gain 3 2 2 1 1 1 1 1 1 1 1 2 5Medium gain 4 2 2 1 1 1 1 1 1 1 1 2 5Low gain 4 3 3 1 1 1 1 1 1 1 1 2 5Pulse Height [ADC Counts]Entries / [ADC Count]11010 210 30 50 100 150 200 250 300 350Figure 5. MaPMT pulse height spectrum. Theoverlaid t is described in the text.photo electrons to be produced at the rst dyn-ode. This t still does not follow the data pointscorrectly for small signals. There is a broad addi-tional contribution of small pulse heights, but noevidence for a second peak in the data. Includinga contribution due to production of photo elec-trons at the rst dynode improves the descriptionof the data. However, the t improves the over-all description of the data and, for the spectrumshown above, yields a gain at the rst dynode ofK1 = 6.1 which is in agreement with expecta-tions whereas the Gaussian and Poisson t giveK1  g1 = 3.2 and K1 = 4.1, respectively. Theparameter λ is independent of the t method.Due to gain variations for pixels within a tubethe loss of signal below the threshold varies frompixel to pixel. Using the Poisson t includingthe \1st dynode" eect gives the most reliableresults. The average signal loss of the 64 pixelsof an MaPMT is 13%. We have also measuredthat the signal loss below threshold scales withs. Consequently, running at lower gains increasesthis loss.4. Beetle Chip StudiesThe APVm read-out is not compatible with theLHCb architecture. A possible candidate to readout the MaPMT is the Beetle chip. It can be runin analogue and binary mode. The binary modehas several advantages, as it ties in better withthe Pixel HPD development for LHCb, describedin reference [3], which also has a binary read-out, and for which a design of the O-DetectorElectronics (ODE) is underway. In addition therewould be substantial cost savings.A program has been undertaken to develop aBeetle adaptation to the MaPMT. By adding acharge and a voltage attenuator at the input ofthe preamplier we have lowered the gain of theBeetle chip. Test structures have been producedand studied for this design. For the charge at-tenuator, measurements with a test charge agreevery well with the simulation. The dynamic rangeof the preamplier is 10 photo electrons. Thispreamplier has been mounted onto a MaPMTand a pulse height spectrum from an LED lightsource has been measured. The signal frommostly single photo electrons is nicely separatedfrom the pedestal. In this approach the load ca-5pacitance aects the gain which would require tokeep the input capacitances of all channels withina few pF.5. Magnetic Field StudiesA consequence of the optimisation of the LHCbspectrometer [3] is that now the RICH 1 photo de-tectors will be placed in a region where the mag-netic eld strength is about 40 mT. We have stud-ied the sensitivity of the MaPMT to longitudinaland transverse magnetic elds up to B = 35 mT.A set-up with 4 LEDs has been built to allow fora diuse illumination of the MaPMT. The APVmread-out system was used.We observe that the gain and the collection ef-ciency of a MaPMT decreases with increasing Beld. At B = 3 mT, this eect is already sizablefor the two edge rows of the tube. Averaged forall pixels of a tube the number of measured photoelectrons for B  3 mT is below 90% of that mea-sured at 0 mT. The MaPMT can be shielded byenclosing it laterally with a µ-metal case. Thisshielding should extend beyond the photo cath-ode of the tube. In Figure 6 we show the mea-sured number of photo electrons for all pixels of aMaPMT relative to its value at 0 mT for the fol-lowing set-ups: No shielding, 0.9 mm thick shield-ing extending by 13 mm and 20 mm, and 1.8 mmthick shielding extending by 13 mm and 20 mm.We observe that the shielding of the tube is eec-tive. Already for the single 0.9 mm thick shieldingthe MaPMT functions in longitudinal magneticelds up to 10 mT. We have also studied the be-haviour of the MaPMT in transverse elds. Thesemeasurements show that the MaPMT is insensi-tive to transverse magnetic elds up to 25 mT.6. ConclusionsUsing particle beams at the SPS at CERN wehave successfully tested a 3x3 array of multianodephotomultiplier tubes. We have demonstratedthat by means of lenses mounted in front of theclosely packed MaPMTs the Cherenkov photonsare focused onto the sensitive area of the de-vices. Using a laboratory with LED light sourceswe have studied the performance and the signalField (mT)-40 -30 -20 -10 0 10 20 30 40# of entries00.20.40.60.811.2 no shield13mm shield13mm double shield20mm shield20mm double shieldTotal Number of Entries against fieldFigure 6. The number of observed photo electronsrelative to B = 0 mT versus magnetic eld.shape of the MaPMTs in detail. A change in thefocusing of the photo electrons onto the rst dyn-ode improved the homogeneity of the gain and ofthe light collection. The gain and width of thesignal as well the signal loss have been measuredfor voltages between −700 and −1000 V. We havedemonstrated that running the MaPMT at lowergains signicantly increases the signal loss belowthreshold. Finally, we have evaluated the sensi-tivity of the MaPMT to longitudinal and trans-verse magnetic elds up to 35 mT.7. AcknowledgementsI thank Reinhardt Chamonal, Stephan Eisen-hardt, and Dave Websdale for the help in prepar-ing this talk. This workshop was dedicated tothe memory of Tom Ypsilantis whom I had theprivilege to work with.REFERENCES1. E. Albrecht et al., Nucl. Instrum. Meth. A488, (2002) 110-130.2. I. Chirikov-Zorn et al., Nucl. Instrum. Meth.A 456, (2001) 310-324.3. S. Easo, contribution to these proceedings.

Multianode Photo Multiplier Tubes as Photo Detectors for Ring Imaging Cherenkov Detectors

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Multianode photo multiplier tubes as photo detectors for Ring Imaging Cherenkov detectors

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Multianode Photo Multiplier Tubes as Photo Detectors for Ring Imaging Cherenkov Detectors

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