Dual Readout Calorimetry is a promising new technique for highresolution calorimetry. It is based on simultaneous measurements, in the shower development process, of the scintillation and Cherenkov light (only produced by the

electromagnetic shower component). In this way it is possible to measure, event by event, the contribution of the electromagnetic fraction, and to compensate for its fluctuations, that are the main source of degrading the hadronic energy resolution of calorimeters. In order to further improve both the electromagnetic and hadronic resolution, it is possible to reduce both the sampling fluctuations and the quantum fluctuations by using homogeneous, dense crystals. Promising results have been obtained by lead tungstate crystals (PbWO4), doped with small fractions of praseodymium or molybdenum. Results from the 2008 testbeam and perspective for the coming one this year will be shown

Franchino, S.

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DOI 10.1393/ncc/i2009-10478-4Colloquia: IFAE 2009IL NUOVO CIMENTO Vol. 32 C, N. 3-4 Maggio-Agosto 2009New crystals for Dual Readout calorimetryS. Franchino on behalf of the DREAM CollaborationINFN, Sezione di Pavia and Dipartimento di Fisica Nucleare e Teorica, Universita` di Paviavia Bassi 6, 27100 Pavia, Italy(ricevuto il 19 Settembre 2009; pubblicato online il 24 Novembre 2009)Summary. — Dual Readout Calorimetry is a promising new technique for high-resolution calorimetry. It is based on simultaneous measurements, in the showerdevelopment process, of the scintillation and Cherenkov light (only produced by theelectromagnetic shower component). In this way it is possible to measure, eventby event, the contribution of the electromagnetic fraction, and to compensate forits fluctuations, that are the main source of degrading the hadronic energy reso-lution of calorimeters. In order to further improve both the electromagnetic andhadronic resolution, it is possible to reduce both the sampling fluctuations and thequantum fluctuations by using homogeneous, dense crystals. Promising results havebeen obtained by lead tungstate crystals (PbWO4), doped with small fractions ofpraseodymium or molybdenum. Results from the 2008 testbeam and perspectivefor the coming one this year will be shown.PACS 29.00 – Experimental methods and instrumentation for elementary-particleand nuclear physics.PACS 29.40.Vj – Calorimeters.PACS 29.40.Mc – Scintillation detectors.PACS 29.40.Ka – Cherenkov detectors.1. – IntroductionThe Dual Readout Method (DREAM) allows to improve the performances in hadroniccalorimetry by measuring event by event the electromagnetic fraction of the hadroniccascades. The principle has been proved by the DREAM fiber calorimeter [1], but theperformances obtained were limited by different factors, such as shower containment,sampling fluctuations and quantum fluctuations. The latter two effects can be reducedby using homogeneous media, provided that an event-by-event separation of scintillation(S) and Cherenkov light (C) could be achieved.A description of the methods that were successfully used to achieve this separation inpast beam tests is given in sect. 2. A resume of the first results obtained with crystals usedin the DREAM project is given in sect. 3. In 2008, dedicated crystals for dual readoutwere developed and tested in particle beams. The results are presented in sects. 4 and 5,and projects planned for the 2009 testbeam are described in sect. 6.c© Societa` Italiana di Fisica 413414 S. FRANCHINO on behalf of the DREAM COLLABORATION2. – Separation of Cherenkov and scintillation lightsSeparation of C and S light in crystals can be achieved on the basis of the differentproperties of the two signals, in particular from the time structure, the emission spectrumand directionality. In fact, C light is prompt, exhibits a 1/λ2 spectrum, and is emittedat a characteristic angle (θC = arccos(1/βn)) by the relativistic (shower) particles thattraverse the detector. On the other hand, S light is isotropic, and is characterized by oneor several time constants and emission wavelenghts, that are characteristic of the crystal.3. – Results of DREAM with crystalsIn a recent paper [2], we have demonstrated that a significant fraction of the signalsfrom scintillating lead tungstate (PbWO4) crystals is due to Cherenkov radiation. Thiswas concluded from measurements of the time structure of the signals and from the non-isotropic nature of the light generated by high-energy electrons traversing the crystal.Even better results were obtained with a BGO crystal [3]. Unfortunately BGO is amuch brighter scintillator than PbWO4 and, therefore, C only represents a small fraction(less than 1%) of the light produced in this material by relativistic ionizing particles.However, since the spectra of the two types of light are very different, we managed toobtain an excellent separation by filtering the light. In addition, the relatively longdecay time of the S component (300 ns) was helpful in separating it from the prompt Ccomponent(1).Based on this experience, we decided to explore the possibility to combine the advan-tages of BGO with the intrinsically much higher C fraction of PbWO4. To reach thisgoal, we developed a number of dedicated PbWO4 crystals doped with small fractionsof impurities chosen such as to achieve a shift of the scintillation spectrum to longerwavelengths, and a longer decay time [5].The measurements described were performed in the H4 beam line of the SPS at CERNwith a beam of 50GeV electrons.Two different types of crystals were studied in these tests: PbWO4 crystals dopedwith Mo, and Pr(2). The aim of the test was to understand if doping the PbWO4 crystalwould improve its properties for dual readout calorimeters. In evaluating the results,we focused our attention on the possibility to separate the S and the C componentsthanks to their different characteristics in time and spectral emission. We studied alsothe attenuation of the light, and the number of C p.e. (Cherenkov light yield).Figure 1 shows the radioluminescence spectra [6, 7] measured on small samples ofdoped and undoped PbWO4. Both dopings create a shift of the scintillation wavelength,making it possible to use simple filters to separate the C from the S components. Forundoped PbWO4 crystals, instead, it is very hard to obtain a reasonable separationbecause the S peaks on the blue light, where there is also the maximum of the C spectrum.In both cases, we adopted a readout scheme which consists of one PMT at each side ofthe crystal, associated with a short-pass filter that selects Cherenkov light, and long-passfilter for scintillation light(3).(1) This separation was harder with the PbWO4 crystal with 10 ns [4] of scintillating component.(2) We tested six crystals doped with Mo at 1% and 5%, with Pr at 0.5%, 1%, 1.5%, and anundoped crystal as reference.(3) For C we used UG11 filter, called in the following “UV”, selects λ < 400 nm and BG3, Bluefilter “B”, with a cut-off of 500 nm; for S we tested different filters. Measurements reported hereused a GG495 filter, yellow “Y”, which selects λ > 495 nm.NEW CRYSTALS FOR DUAL READOUT CALORIMETRY 415Fig. 1. – Radioluminescence emission spectra of PbWO4 samples, doped with Mo (a) and Pr (b).4. – PbWO4 doped with molybdenumAs suggested by the radioluminescence measurements, essentially all light selectedwith the UV filter should be the result of pure C radiation, while the region above 500 nm(Y filter) is strongly dominated by S light. This is confirmed by our measurements ofthe time structure of the signal.In fig. 2a the time structure of the signal passing through the UV filter (blue line),and on the Y filter (red line) are shown. These two time structures, which were measuredfor the same sample of events, are spectacularly different; in particular, the UV filteredsignals show a very narrow peak and return to the baseline within few ns. On the otherhand, signals selected with the Y filter show the typical exponential tail of scintillation.A C/S ratio as good as 5 can thus be obtained. We also measured the decay timecomponents to be 26 and 59 ns. This time scale is ideal for our calorimetric applications,and makes it much easier to separate the C prompt peak from the S exponential decaysignal than in the case of undoped crystals. Unfortunately, the Mo doping also shifts theabsorption edge of the crystal to longer wavelengths, thus reducing the available windowFig. 2. – (Colour on-line) a) Average time structure of the signals from a PbWO4 crystal 1%Mo-doped with filters UV (blue line) and Y (red line) (a), and 0.5% Pr-doped with filter B (b)and Y (c).416 S. FRANCHINO on behalf of the DREAM COLLABORATIONin which C light can be detected. This results in strong self-absorption of the C signalin the crystal, which is not acceptable for real calorimeters. Due to this absorpion, alsothe C light yield is affected; in fact we estimated it to be 8 p.e./GeV, that is similar tothe value obtained in the case of the fiber DREAM calorimeter.We have also tested the 5% Mo-doping, but the signal on the UV side is much moreattenuated by the self-absorption of the crystal.5. – PbWO4 doped with praseodymiumThe time spectrum of PbWO4 doped with 0.5% Pr, read through the B and Y filterrespectively, is shown in fig. 2b. With respect to the Mo-doping, one observes two peculiarbehaviors. The first one is that, interestingly, also the yellow signal has a dominantprompt component, just like the blue signal, where we expect such a component as a resultof the Cherenkov radiation. In order to investigate the nature of this signal component,we studied its angular dependence, and we observed that this prompt component isreally caused by the tail of the C radiation that is transmitted by the Y filter. Thesecond effect is that, even after few μs, the signal passing through the Y filter does notreach the baseline. This is due to the fact that the Pr-doping increases the scintillationdecay time to a level not compatible for most of high-energy physics devices.The contamination of C in the S signals could be kept at the level of a few percentby integrating the signal in a time window that is far from the prompt peak. In fact, thetail of the time signal is only due to the scintillation process. In this way it was possibleto obtain a C/S ratio of 2. Self-absorption of C light, in the case of Pr-doping, does notseem to be an important problem.6. – New ideas for the 2009 test beamThe results shown here and, in more detail, in [8] are extremely encouraging. Nonethe-less, an increase of the wavelength gap between crystal absorption edge and filters cutoffis needed in order to avoid light attenuation and increase the C light yield, still main-taining a good separation in both time structure and spectral properties. To do that, wehave chosen two possible candidates for the 2009 testbeam, that we are now preparing:– Mo-doped PbWO4 (0.1%, 0.2%, 0.3%): it has been shown that a lower Mo concen-tration allows to maintain the shift of the scintillation emission spectrum to higherλ with respect to undoped PbWO4, but reduces the shift of the absorption cut-offto higher wavelength with respect to higher Mo concentrations. By combining thesefeatures with a higher QE PMT and C filters with transmission region extendedthrough higher wavelenths, we expect to reduce the problems of self-absorption andlow C light yield.– BSO: we proved BGO to be a fairly good candidate for DREAM technique. Wefound BSO(4) shows even more suitable characteristics for dual readout, in partic-ular a lower-wavelength absorption cut-off.(4) BSO has almost the same chemical composition as BGO, but with silicon atoms instead ofgermanium ones: Bi4Ge3O12.NEW CRYSTALS FOR DUAL READOUT CALORIMETRY 4177. – ConclusionPbWO4 crystals doped with Mo or Pr have been tested and shown to represent aconsiderable improvement with respect to undoped ones, on the basis of the temporaland spectral differences of the light emitted in the Cherenkov and scintillation processes.However, in order to make them realistic candidates for application in practical calorime-ters, further improvements would be needed. In particular, the scintillation process inthe Pr-doped crystals is unacceptably slow and the self-absorption of Cherenkov light inthe Mo-doped crystals is unacceptably large. Further studies to improve on these aspectsare ongoing, and new results are expected from the 2009 test beam.REFERENCES[1] Akchurin N. et al., Nucl. Instrum. Methods A, 537 (2005) 537.[2] Akchurin N. et al., Nucl. Instrum. Methods A, 582 (2007) 474.[3] Akchurin N. et al., Nucl. Instrum. Methods A, 595 (2008) 359.[4] Akchurin N. et al., Nucl. Instrum. Methods A, 584 (2008) 273.[5] Nikl M. et al., J. Appl. Phys., 91 (2002) 2791.[6] Korzhik M. V. et al., Proc. IEEE Nucl. Sci. Symp., Dresden (2008).[7] Nikl M. et al., J. Appl. Phys., 104 (2008) 093514.[8] Akchurin N. et al., Nucl. Instrum. Methods A, 604 (2009) 512.

New crystals for Dual Readout calorimetry

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New crystals for Dual Readout calorimetry

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