The Dual REAdout Method (DREAM) consists in comparing the

scintillation and Cherenkov light generated in the shower development process. By comparing the two, the electromagnetic fraction of the hadronic shower can be

measured event-by-event, to eliminate the effects of fluctuations in this fraction. In this paper the DREAM fiber calorimeter and its successor, the newDREAM

prototype that is currently under construction, will be described. We will also report on the efforts to study the Cherenkov component of the output of high-Z crystals and to realize a dual-readout electromagnetic section that can achieve outstanding electromagnetic resolution whitout compromising the hadronic resolution

Cascella, M.

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DOI 10.1393/ncc/i2011-11073-xColloquia: IFAE 2011IL NUOVO CIMENTO Vol. 34 C, N. 6 Novembre-Dicembre 2011Updates on the most recent results in dual readout calorimetryM. Cascella on behalf of the DREAM CollaborationUniversita` and INFN, Sezione di Pisa - Pisa, Italy(ricevuto il 29 Luglio 2011; pubblicato online il 6 Dicembre 2011)Summary. — The Dual REAdout Method (DREAM) consists in comparing thescintillation and Cherenkov light generated in the shower development process. Bycomparing the two, the electromagnetic fraction of the hadronic shower can bemeasured event-by-event, to eliminate the effects of fluctuations in this fraction.In this paper the DREAM fiber calorimeter and its successor, the newDREAMprototype that is currently under construction, will be described. We will also reporton the efforts to study the Cherenkov component of the output of high-Z crystalsand to realize a dual-readout electromagnetic section that can achieve outstandingelectromagnetic resolution whitout compromising the hadronic resolution.PACS 29.40.Vj – Calorimeters.PACS 06.20.fb – Standards and calibration.PACS 29.40.Ka – Cherenkov detectors.PACS 29.40.Mc – Scintillation detectors.1. – IntroductionThe DREAM Collaboration [1] has the goal of developing a calorimeter that ap-proaches the theoretical resolution limit in hadronic energy measurements. Thedual readout approach consists of measuring event-by-event both the scintillation andCherenkov light signal and use them to estimate the magnitude of the electromagneticcomponent of the shower.Scintillation light (S) is due to ionization while Cherenkov light (C) is produced byrelativistic particles, i.e. almost exclusively by the electromagnetic component of thehadronic shower. The simultaneous measurement of both components results in a betterresolution and linearity: the ratio of the two signals can be expressed as a function ofthe fraction of the shower energy transported by electromagnetic particles fem:CS=fem + λC(1− fem)fem + λS(1− fem) ,(1)λC and λS are the h/e ratios for the material that produces, respectively, the Cherenkovand the scintillation signal. This equation can be inverted to determine the value of fem.c© Societa` Italiana di Fisica 151152 M. CASCELLA on behalf of the DREAM COLLABORATIONFig. 1. – Cherenkov signal distribution for 100GeV π−(a) and distributions for three differentbins of fem(b). Signal distributions for high-multiplicity 200GeV “jets” in the DREAM before(c) and after (d) C/S-based corrections. To eliminate the effects of lateral leakage that dominatethe resolution in (d) energy constraints were imposed (e).2. – The dual-readout method with fiber calorimetersThe dual readout method has first been tested with the DREAM prototype, a sam-pling calorimeter with copper as the absorber medium, and scintillating and clear fibersas the active medium. The Cherenkov light produced in the high-density clear fibersis read out separately. Figure 1 shows an example of the dual-readout approach: theCherenkov signal for 100GeV π− is divided in 3 samples with different fem each with amuch smaller resolution. The many results obtained with the DREAM calorimeter havebeen presented in [1].2.1. The newDREAM prototype module. – The main factors that limit the DREAMcalorimeter performance were identified in the low Cherenkov photoelectrons yield (thatgreatly contributes to the C resolution), the lateral and longitudinal leakage and thesampling fluctuations. For this reason a new generation of fiber modules using eithercopper or lead as the absorber material are being tested. The newDREAM moduleshas a sampling fraction of 5% (almost two times that of DREAM) aluminized clearfibers with a larger numerical aperture and the final assembled calorimeter will have 99%lateral containment. Finally we plan to sample the time structure of the signals with aDRS-IV–based acquisition to correct for light attenuation effects.The Domino Ring Sampler chip. The DRS-IV chip [2] is the a multichannel samplerthat offers both a very fast sampling and a wide buffer. The chip has 8+1 channelseach with 1024 sampling capacitors on a single chip, a so-called “Domino wave” circuitgenerates a short pulse which opens analogue switches at the input of sampling cells ofeach channel. The Domino wave can be stopped by an external trigger, after which thesampling capacitors are read out and digitized by a commercial ADC.The excellent characteristics of the DRS-IV chip has enabled us to sample with highresolution the time structure of the PMT signals.UPDATES ON THE MOST RECENT RESULTS IN DUAL READOUT CALORIMETRY 153Fig. 2. – Experimental setup: placement of the crystals on the beam line (a). Developmentof the Cherenkov light cone in a crystal and placement of the UV and polarizing filters (b).Direction of the Cherenkov light polarization with respect to the light cone (c, d).3. – Cherenkov - scintillation separation in crystalsHigh-Z crystals such as PbWO4 (lead tungstate), BGO (bismuth germanate) and BSO(bismuth silicate) produce a significant amount of Cerenkov light (it contributes to upto 15% of the signal generated by high-energy particles traversing a PbWO4 crystal [3]),which may be separated from the scintillation light by exploiting differences in timestructure, directionality, spectral properties and polarization.The experimental setup used in the crystal studies is detailed in fig. 2a which showsa schematic of the placement of the crystals on the beam line. Crystals are readout onboth sides, usually with a different filter setup on each side.3.1. Time structure. – The Cherenkov light is a very fast signal while the scintillationsignal is characterized by longer decay constants. This can be exploited using two gatesto measure the Cherenkov and scintillation signal in the same channel as shown in fig. 3a.If the timing structure of the signal is available a template fitting technique can be usedto obtain an even better result.Fig. 3. – The time structure of a typical shower signal measured in the BGO electromag-netic calorimeter equipped with a UV filter (a), in a PbWO4 crystal with no filters wherethe Cherenkov signal is estimated by taking the difference between the signal on the two sides(b) and in the same crystal readout with a UV filter on the −30◦ side and a yellow filter on theother (c).154 M. CASCELLA on behalf of the DREAM COLLABORATIONFig. 4. – Average amplitude (a) and integral (b) of the signals of 180GeV π+ traversing a BSOcrystal as a function of the angle θ. Results are given for the Cherenkov signals corrected forthe path length, with the transmission axis of the polarization filter oriented in the horizontalplane (C0), the vertical plane (C90) and for the scintillation signals with the transmission axisin the horizontal plane (S0).3.2. Direction. – By placing the crystal at the Cherenkov angle with respect to thedirection of the beam we can exploit the directionality of Cherenkov light collect most of iton one side (see fig. 2b). Figure 3b shows how the Cherenkov signal can be reconstructedas the difference of the light measured at the two ends of a PbWO4 crystal.3.3. Wavelength. – Cherenkov light spans the UV and blue regions of the spectrumwhile scintillation light is mainly produced in the visible range. We take advance ofthis by using a yellow filter to only select scintillation light and a UV filter to readoutCherenkov light [5]. In this way, an excellent separation of the two components can beobtained as shown in fig. 3c.An example of the spectral and directional separation can be seen in fig. 5c showingthe angular dependence of the ratio of the Cherenkov and scintillation signals (selectedusing UV and yellow filters) in BSO and BGO crystals.Fig. 5. – The energy resolution of a BGO DREAM calorimeter as a function of energy plotted sep-arately for the scintillation and Cherenkov components and for the total signal (a). Comparisonof a BSO and a BGO crystal: average profile of the light output (b) and Cherenkov/Scintillationratio as a function of the angle (c).UPDATES ON THE MOST RECENT RESULTS IN DUAL READOUT CALORIMETRY 1553.4. Polarization. – The characteristic polarization of Cherenkov light (see fig. 2cand 2d) has been exploited to separate the two light components [6].Figure 4a shows better how large the effect of the polarization filter is. The Cherenkovcomponent is reduced by a factor of about 3.5 as a result of the rotation of the transmis-sion axis of the polarization filter from the horizontal to the vertical plane. The sameeffect is visible, even if less pronounced, in the integrated signal (fig. 4b).4. – Toward a dual-readout homogeneous DREAM calorimeterA full-size dual readout electromagnetic calorimeter made of 100 BGO crystals recov-ered from the L3 experiment was tested using the time structure method to separate theCherenkov from the scintillation signal [7]. Since the characteristics of the setup limitedthe achievable resolution (fig. 5c shows the electromagnetic resolution) the matrix hasbeen equipped with a new array of PMT with and improved optical coupling and iscurrently under test.A number of BSO crystals have been also characterized and compared with BGOcrystals. BSO represent a viable alternative to BGO crystals and present a number ofadvantages namely faster scintillation (see fig. 5b) and a much higher C/S ratio (fig. 5c)around the Cherenkov angle [8].After a careful work of characterization of a series of doped PbWO4 crystals [9] asmall matrix of 7 crystals read on the two sides with UV and yellow filters has been builtand is undergoing testing as an alternative for a dual-readout electromagnetic stage.5. – Final remarksThe dual readout of Cherenkov and scintillation light make it possible to estimateevent-by-event the fem of the hadronic shower reducing the effect of its fluctuationson the resolution. This technique has been extensively tested on both sampling andhomogeneous calorimeters. The DREAM Collaboration keeps exploring and addressingall the factors that limit hadronic calorimetry resolution.REFERENCES[1] Akchurin N. et al., Nucl. Instrum Methods A, 533 (2004) 305; 536 (2005) 29; 537 (2005)537; 548 (2005) 336; 550 (2005) 185.[2] Ritt S. et al., Nucl. Instrum. Methods A, 623 (2010) 486.[3] Akchurin N. et al., Nucl. Instrum. Methods A, 582 (2007) 474.[4] Nikl M. et al., J. Appl. Phys., 91 (2002) 2792.[5] Akchurin N. et al., Nucl. Instrum. Methods A, 595 (2008) 359.[6] Akchurin N. et al., Nucl. Instrum. Methods A, 638 (2011) 47.[7] Akchurin N. et al., Nucl. Instrum. Methods A, 610 (2009) 488.[8] Akchurin N. et al., Nucl. Instrum. Methods A, 640 (2011) 91.[9] Akchurin N. et al., Nucl. Instrum. Methods A, 621 (2010) 212.

Updates on the most recent results in dual readout calorimetry

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Updates on the most recent results in dual readout calorimetry

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