CONCEPTO DE LA OPERACIÓN

The scintillator cells used for the T3B experiment are similar to the ones used in the CALICE AHCAL. Nevertheless, there are differences due to the optimization of T3B for timing measurements and due to technological advances since the AHCAL was assembled in 2005/2006. Figure 4.1 shows how the different components were assembled into the final T3B cell module.

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3

30

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Figure 4.1: Assembly process and components of a T3B scintillator cell. Left: Hama- matsu SiPM of the type MPPC-50 P-Type. Center: Scintillator tile with attached SiPM and inserted dimple. Right: Completely assembled T3B cell attached to a preamplifier board. All dimensions are given in millimeter.

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Figure 4.2: Picture of the dimple drilled into the T3B tiles and of the SiPM coupled to it. All dimen- sions are in millimeter.

In the production procedure, a silicon photomultiplier of the type MPPC-50 P-type [54] with translucent casing is coupled directly via air gap to one side of a plain scintillator tile of the type Bicron-420 [57] (see Section 3.3.3). The SiPM is inserted into a gap that matches the dimensions of its casing. That way the SiPM is aligned to the center of the side face of the tile. The SiPM-tile entity is then enclosed on all tile faces by a highly reflective mirror foil [68] and a low reflective and low transmissive black absorber foil [69]. In the final step, the packaged tile is soldered to a custom preamplifier board designed and produced at the Max-Planck-Institute for Physics in Munich (MPI) [70]. The board needs a supply voltage of 5V to power the amplifier and a bias voltage of ∼ 70V that is supplied and device specific to the powered SiPM. The SiPM signal is amplified by a factor of 8.9 and read out by a SMB connector (for details on the board design see [52]). Table 4.1 shows a summary of the individual components and their specifications.

In contrast to the scintillator tiles used for the CALICE AHCAL, the T3B tiles are designed without embedded wavelength shifting fiber (WLS). This is made possible by the advent of blue-sensitive silicon photomultiplier in the last years (e.g. the MPPCs from Hamamatsu). It simplifies the production procedure significantly, but comes for the price of a non-uniform signal response meaning that number of photons collected by the SiPM varies largely depending on where a particle traverses the tile. To regain the uniformity a dimple of optimized shape is drilled into the tile at the SiPM coupling position (see Figure 4.2). More details on tile uniformity optimization studies can be found in [71] or [14]. Avoiding the WLS also improves the timing performance of the T3B cells. The absorption and re-emission of light by the wavelength shifter introduces a decay time constant of the order of a few nanoseconds that delays the time of arrival

Detector Component Manufacturer Dimensions Specifications MPPC-50 P-Type Hamamatsu Case: 4 × 3mm2,

Sens area: 1 ×

1mm2

50 micrometer pix- el pitch

BC-420 Scintillator Tile Saint Gobain 30×30×5mm3 _{SiPM insertion gap,}

uniformity dimple Radiant Mirror Foil 3M encloses all tile

faces

> 90 % reflectivity for >400nm Black Al Foil BKF24 Thorlabs encloses all tile

faces

< 20 % reflectivi- ty for380−850nm, small transmission Preamplifier Board custom prod. ≈ 6×6cm2 _minus

cut outs

Amplification fac- tor of 8.9 in rele- vant bandwidth Table 4.1: Main specifications of the components used within the T3B cell modules.

of photons on the SiPM. That way the rise time of the detected signal is stretched which reduces the accuracy to time stamp the energy deposition of tile traversing particles (see Figure 4.3, left). Furthermore, the total signal width and therefore the time necessary to integrate over a large fraction of the signal is increased significantly. A large scale calorimeter assembled with tiles with embedded WLS would need to integrate longer to achieve the same energy resolution compared to a calorimeter that uses directly coupled tiles (see Figure 4.3, right).

For T3B, MPPCs with a pixel pitch of50µm were used. This guarantees a relatively high photon detection efficiency (PDE) of above20 % for an overbias of≈1.5V. At the same time the afterpulsing probability is with less than 25 % moderately small [72] and the darkrate with≈500kHz at an acceptable level [54] as we will see in Chapter 5 (compared to the other available MPPC devices with25µm and100µm pitch, respectively). These three parameters are most relevant for the timing performance of the T3B cells. A high PDE allows for the detection of even small energy depositions within the T3B cell. A high afterpulsing probability and dark rate, on the other hand, can fake late energy depositions and therefore increase the systematic errors on the timing measurements. Bicron-420 is with a light emission rise time of0.5ns and pulse width of1.3ns (FWHM) one of the faster scintillator materials on the market. These time constants make the scintillator one of the factors limiting the signal time stamping accuracy and the width of the detected light signal. Another factor is the unwanted but existing self scintillation of the mirror foil which might have a long time constant and cause the late emission of photons. Furthermore, timing effects of the SiPM contribute some of which will be eliminated during the calibration procedure discussed in Chapter 5. A minor contribution originates from the photon travelling time within the scintillator which is prolonged by the high reflectivity of>90 % of the attached mirror foil.

Time [sek] -20 -10 0 10 20 30 -9 10 × Amplitude [V] -0.01 0 0.01 0.02 0.03 0.04 0.05 MPPC25 with WLS MPPC25 Direct Coupling MPPC25 Darkrate at 1p.e. Time [sek] -20 -10 0 10 20 30 -9 10 ×

Fraction of Integrated Energy Deposition [%] 0 0.2 0.4 0.6 0.8 1 MPPC25 with WLS MPPC25 Direct Coupling MPPC25 Darkrate at 1p.e.

Figure 4.3: Photon signal delay introduced by a wavelength shifting fiber. Left: The averaged signal of traversing electrons originating from a Sr-90 source for a tile of the CALICE AHCAL with WLS (red) and a directly coupled tile with BC-420 scintillator and applied dimple (blue). The signal of a single photon equivalent is shown for comparison (green). Right: Fraction of the total signal detected at a certain point in time. In the case of direct coupling, it takes 15.1ns for the signal fraction to increase from 10 % to90 %. With embedded WLS it takes 19.5ns.

transmission that minimizes the probability for ambient light to reach the SiPM. Otherwise, ambient light would represent an additional source of error on the timing measurements.