1. PbWO4 Cherenkov light contribution to Hamamatsu S8148 and Zinc Sulfide–Silicon avalanche photodiodes signals
F. KOCAK, I. TAPAN
Department of Physics, Uludag University, 16059, Bursa-TURKEY
Introduction
Simulation and Results
The scintillation light detection using crystal–avalanche photodiode (APD) system is important techniques in research, medicine and industry. Also, for some selected applications, mainly in high-energy physics. As an example, the CMS collaboration has chosen the Hamamatsu S8148 silicon APDs as photodetectors for the lead tungstate (PbWO4) crystals light in the barrel of the CMS electromagnetic calorimeter (ECAL) at the Large Hadron Collider at CERN [1].
The charge signal generation process in well-defined the Hamamatsu S8148 APD and the ZnS–Si APD geometries were simulated using a Single Particle Monte Carlo technique [4]. In the simulation, incident photons create electron-hole pairs in the depletion layer. After photo conversion the charges produced are tracked further through the device to the avalanche region where avalanche gain is produced by successive impact ionization processes in the high field region. Each charge carrier released by impact ionization is also tracked to check whether it generates further ionization. The charge carriers eventually depart from the avalanche region and are likely to be collected by the contact electrodes.
The mean value of the charge signal, which is the average number of the charge carriers collected, depends on the wavelength dependent quantum efficiency, QE(λ), and wavelength dependent avalanche gain, M(λ), of the detector and can be given by
S(λ)=QE(λ).M(λ). Nph
where Nph is the number of incident photons on to the detector surface which is related the number of photons at the end of the crystal given in Figure 2. The mean charge signals of the APDs were simulated for Cherenkov, scintillation and total number of photons produced in the PbWO4 crystal by single electron traversing at different energies (Figure 5).
The light generated in the PbWO4 crystal could be split into scintillation and Cherenkov components As the scintillation light is emitted in the wavelength region of 320 nm to 600 nm peaking at around 420 nm, the Cherenkov light is emitted with a characteristic λ-2 spectrum. The wavelength dependence of the Cherenkov and scintillation photons collected at the end of the crystal are shown in Figure 1.
Fig. 1. Cherenkov, scintillation and total photons spectrum
GEANT4 simulation showed that the Cherenkov photons was considerably contributed to the total number of the photons at the end of the crystal. The simulated size of the crystal is same size used in CMS ECAL. Figure 2 shows the number collected photons at the end of the crystal for up to 1 GeV single electron traversing.
Fig. 2. The number of collected photons as a function of incident electron energy
Fig. 5. APDs signal variations for electrons at different energies traversing the crystal
Structures of the APDs
The simulation showed that the Cherenkov photons was contributed more than 20% to the total signal of the APDs placed at the end of the crystal (Figure 6).
The Hamamatsu APD type S8148 is a silicon avalanche photodiode that consists of successive layers of p+, p, n, n- and n+-type silicon layers. The properties and working principles of the structure were given in Ref [2]. The zinc sulfide–silicon (ZnS–Si) APD structure has been developed by Tapan et al. [3]. The ZnS has been chosen as surface layer for the transmission of incident light into the Si depletion layer.
The structures and quantum efficiencies for both the Hamamatsu S8148 and the ZnS–Si APDs are seen in Figures 3 and 4, respectively.
Fig. 6. Signal distributions of the Hamamatsu S8148 (left) and the ZnS-Si APD structures (right) for 200 MeV electron traversing the crystal
Fig. 3. Sketches of Hamamatsu S8148 Silicon APD Structure (left) and ZnS-Si APD Structure (right)
The ZnS-Si APD structure has higher quantum efficiency, so the bigger number of primary charge carriers gives bigger signals even same number of incident photons.
Acknowledgment
This work is supported by Turkish State Planning Organization (DPT) under the grant DPT2006K
Fig. 4. Quantum efficiency as a function of wavelength for both the S8148 and the ZnS-Si APD structures
References
[1] CMS Collaboration, The electromagnetic calorimeter technical design report, CERN/LHCC 97-33, (1997).
[2] K.Deiters, et al., Nucl. Instr. and Meth. A 453, 223 (2000).
[3] I. Tapan, M.A. Ahmetoglu, F. Kocak, Nucl. Instr. and Meth. A 567, 268 (2006).
[4] I. Tapan, R. S. Gilmore, Nucl. Instr. and Meth. A 454, 247 (2000).