Assessment of the Čerenkov light produced by a PbWO 4 crystal by means of the study of the time structure of the signal N. Akchurin 1, L. Berntzon 1, A. Cardini 2, L. Cavallini 3, R. Ferrari 4, S. Franchino 4, G. Gaudio 4, J. Hauptman 5, H. Kim 1, L. La Rotonda 6, M. Livan 4, C. Mancini 3, A. Mecca 3, E. Meoni 6, H. Paar 7, A.Penzo 8, D. Pinci 3, A. Policicchio 6, S. Popescu 9, G. Susinno 6, Y. Roh 1, W. Vandelli 9 and R. Wigmans 1 1 Texas Tech University, Lubbock, USA - 2 Università di Cagliari and INFN Sezione di Cagliari, Cagliari, Italy - 3 Università “La Sapienza” and INFN Sezione di Roma, Roma, Italy - 4 Università di Pavia and INFN Sezione di Pavia, Pavia, Italy - 5 Iowa State University, Ames, USA - 6 Università della Calabria and INFN Sezione di Cosenza, Cosenza, Italy - 7 University of California at San Diego, La Jolla, USA - 8 INFN Sezione di Trieste, Trieste, Italy - 9 CERN, Geneva, Switzerland Abstract On beam tests were carried out on PbWO 4 crystals. One of the aims of this work was to evaluate the contribution of the Čerenkov component to the total light yield. The difference in the timing characteristics of the fast Čerenkov signals with respect to the scintillation ones, which are emitted with a decay time of about 10 ns, can be exploited in order to separate the two proportions. In this paper we present the results of an analysis performed on the time structure of signals, showing how it is possible to detect and assess the presence and the amount of Čerenkov light. Since Čerenkov light is emitted only by the electromagnetic component of a hadronic shower, its precise measurement would allow to account for one of the dominant sources of fluctuation in hadronic showers and to achieve an improvement in the energy resolution of a hadronic calorimeter. Principle of Operation In order to assess the presence of Čerenkov light and to evaluate its ratio to the scintillation one, two of their main differences were exploited: 1. While the scintillating light, produced by molecular de-excitation, is emitted isotropically the Čerenkov light is produced in a cone with an opening angle θ = 1/βn as shown in figure below. 2. The emission process of the scintillating light has a characteristics decay time (about 10 ns for PbWO 4 crystal) while the Čerenkov light is produced prompt after the particle crossing. In order to measure the light produced, both sides of a PbWO 4 crystal were equipped with a Photo- Multiplier Tube (PMT). By varying the detector orientation with respect to the direction of the incoming particles (the angle  ), the number of Čerenkov photons reaching the L or R PMT varies, while the scintillation ones do not. In particular a maximum number of Čerenkov photons reaching the L (R) PMT is expected for  = +(-) 31.5 o. Signal Shapes In the beam measurements the signal shapes were acquired by means of a very fast Flash ADC with an e ff ective sampling frequency of 800 MHz or with an oscilloscope able to provide up to sample/s (one each 100 ps). In the figure below examples of m.i.p. signal shapes, averaged on 10 5 events, acquired on the two sides of the crystal are shown for two values of  : +30 o, when the Čerenkov light is collected on the L PMT and -30 o, when the Čerenkov light is collected on the R PMT. The green line, obtained as the difference of the two shapes described above, represents the Čerenkov signal as seen on both sides which are 27% of the total one in amplitude and about 10% in charge. The Čerenkov signals result to be very fast and short (8 ns in total). For this reason we expect that their contribute to the total light yield can be evaluated by studying the time structure and the time properties of the PMT signals. The methods for the analysis of the signal time structure The characteristics of the time structure of the signals are determined with three different methods. In the second method, we determine the precise time at which the pulse height exceeds a certain threshold level, e.g., -30 mV. An increase in the Čerenkov content of the signal will shift that point to an earlier moment. Results for the Single Crystal When the crystal is rotated towards values of θ > 0, Čerenkov light becomes a significant component of the signals measured by PMT R and the leading edge of the pulse shape steepens (τ L becomes smaller). For large angles, the acceptance of Čerenkov light decreases again and the leading edge becomes less steep, τ L increases. The precision on τ L, shown for the datum at 0 o, is completely dominated by photoelectron statistics and is such that the value of the leadtime time does not provide statistically significant information about the contribution of Čerenkov light to the signal in question. The difference between the threshold crossing time on the two crystal sides (plot on the left) shows very well the effect of the Čerenkov photons on the signal timing with a maximum and a minimum for  =  30 o. For l  l > 50 o the Čerenkov light is not collected on the PMT's and the time difference becomes constant. We also tried to measure the difference between the crossing time by processing the PMT signals by means of a standard CAMAC discriminator and a TDC with cosmic rays. Although a maximum time difference of only about 1 ns was found, the effect is clearly visible. A third method to evaluate the Čerenkov contribution to the total light yield is to calculate the ratio between the light collected in the first few instants of the signal and the total one. The charge integrated in a gate including only the first nanoseconds of the signal peak (blue in the plot) is divided event by event by the total charge collected. This charge ratio (qRatio) is expected to increase when the Čerenkov light reaches the PMT because the signal becomes faster and higher. In the plot on the right the behaviour of qRatio as a function of the angle  is shown for the Left side signals. The value of qRatio is almost constant over the whole range of angles in which no Čerenkov light is collected by the PMT L. For  < 0 it starts to increase reaching its maximum for  = - 30 o. The maximum value is about 10% (once offset-subtracted) which is equal to the ratio between the Čerenkov and the total light calculated above. No effects due the effective particle path are found for large angles. The dependence of the time difference on the angle is smaller than the one of the single crystal, but still visible. The charge ratio is a variable sensitive to the Čerenkov light also for the crystal array. Conclusion Several variables sensitive to the effects of the Čerenkov light correlated with the signal time structure were studied both for the single crystal and for the array. The prompt Čerenkov photons give rise to a fast signal whose time characteristics (threshold crossing time and leading time) can give information about the presence and the amount of Čerenkov signal. The use of the qRatio method, which adds to the effects on the signal timing the increase of total charge when also the Čerenkov light is collected, can represent a novel and promising way to assess the Čerenkov light contribution to the total light yield also in homogeneous PbW0 4 calorimeters. Results for the Crystal Array A simple electromagnetic calorimeter made of 19 PbWO 4 crystals was also tested on particle beam. The crystals were arranged in a matrix and were readout on both sides by mean of two PMTs as in the figure below. The crystal array was exposed to electron and pion beams which have a certain probability to develop a shower in its 12.4 X 0 (for θ=0). Much of the shower energy is deposited by isotropically distributed electrons, produced in Compton scattering and photoelectric processes, the effects of that on the angular distribution of the emitted Čerenkov light are limited, since much of this takes place below the Čerenkov threshold. The main aim of the test was to verify if it is possible to evaluate the scintillation and the Čerenkov contribution to the total light signal also when the directionality of the Čerenkov light is “diluted”. We report the very preliminary results of the studies on the signal timing (left) and on the qRatio (right). Results very similar to the single crystal studies were obtained, even tough the absolute effects are smaller, which indicate that also in homogeneous PbW0 4 calorimeters the to light components can be separated Crossing time Threshold In a first method, the pulse leading edge is fitted to a Fermi- Dirac function: An increase in the Čerenkov content of the signal will manifest itself as a decrease in the value of τ L. The threshold crossing time was studied as a function of the crystal angle  and the results are shown in the plots on the right. On each side a minimum for the for  equal to the Čerenkov angle was found which confirms that the signal becomes faster when the Čerenkov photons are collected. The crossing time decreases also for large angles for slewing effects due to the increase of the effective path length of particles in the crystal. The figure below shows the value of the lead constant τ L measured for the 10 GeV electron signals on PMT R, as a function of the angle θ. preliminary results