Educación en la ciudad de Riobamba en el siglo XIX

In the last step of the full detector simulation, the response of the detectors and the electronics is simulated. This “digitization” step is based on the information provided by the Geant4 simulation of particles traversing the detector and the resulting ionization

energies in the various materials. This includes:

• Detector thresholds and noise, such as darkpulses in SiPMs.

• Dependence of the detector response on changes in temperature, applied voltage and other environmental conditions.

• Detector and read-out specific features or shortcomings such as Afterpulsing or saturation in the case of SiPMs.

The goal is to convert the knowledge of the deposited energy into a signal as it would have been recorded at a real experiment. As this is highly specific to the detector and the used read-out, no generic software is available. Instead for each detector the corresponding digitization software has to be written anew, using very detailed knowledge of the detector behaviour.

Measurement of the Time

Development of Hadronic Showers

in Tungsten

One of the challenges to perform precision measurements at CLIC is the high bunch crossing frequency of 2 GHz (0.5 ns) which makes it more difficult to correctly correlate a measurement of one of the subdetectors to a certain event. This is especially true for energy depositions of hadronic showers in the calorimetric system, as hadronic showers, unlike electromagnetic ones, have a non-instantaneous component (cf. section 3.3). These “late energy depositions”, albeit small, can occur tens of ns up to several microseconds after the actual interaction and thus can be wrongly assigned to one of the succeeding bunch crossings during reconstruction. This is especially true for the CLIC ILD detector concept where the calorimetric system uses tungsten as absorber, in which the amount of evaporating neutrons causing these late energy depositions is relatively high.

There are several techniques to ensure a correct assignment between bunch crossing and detected energy deposition. One of them is searching for clusters in the calorimeters over a larger time frame and then using a single timestamp for the entire cluster of hit cells, assuming they all originate from the same event. This technique was tested amongst others against a full detector simulation based on Geant4 (cf. subsection 3.6.2) plus a

sophisticated digitization simulating detector effects based on experiences obtained from testbeam data like the one from the CALICE collaboration (cf. section 3.5). Obviously such a comparison relies on the accuracy of the simulation. Therefore a small experiment was designed to actually measure the timing structure of hadronic showers and compare them to the predictions of the simulation. As the absorber used in the calorimetric system of the ILD detector for CLIC is tungsten, this testbeam experiment was called “Tungsten Timing Testbeam” (T3B).

This chapter will show the details of this experiment, starting with the experimental setup in section 4.1. In section 4.2 the calibration and reconstruction of the data will be discussed, while section 4.3 gives details on the simulation. Finally, the results including a comparison between data and Monte Carlo are shown in section 4.4.

4.1

T3B: The Tungsten Timing Testbeam Experi-

ment

The measurement of the time structure of hadronic showers is quite challenging. De- pending on the spatial resolution, a prototype of a hadronic calorimeter already needs several hundred or thousand read out channels. For example, the CALICE Fe-AHCal with a depth of about 5.3λI has already about 7800 channels, using varying cell sizes

with the smallest being 3×3 cm2 _{[38]. To measure the timing of hadronic showers each}

read out channel has to be monitored with a very good accuracy in the order of one nanosecond over a period of several microseconds. This does not only lead to a huge amount of data, but is on the other hand very expensive to construct. Consequently the number of channels that can actually be monitored with this high timing precision is limited.

One way to get a comparable lateral resolution by using only a limited number of channels is to place all tiles in a straight line, starting perpendicular from the shower axis. Given enough recorded events such a detector can measure the average development of hadronic showers by using the radial symmetry around the showers axis. As the dimensions of the used tungsten absorber plates were 80×80 cm2_{, a strip of 15 tiles,}

each with dimensions of 3_×3 cm2 _{and thus covering a radius of 45 cm, is sufficient for}

the measurement of hadronic showers.

As it was discussed in subsection 3.3.4, hadronic showers are statistical processes. Especially the shower starting point, which we shall define as the first hard interaction with the creation of secondary particles, differs on an event-by-event basis and its distribution is defined by a falling exponential (cf. Equation 3.5). Thus a single detector layer placed after a sufficient depth of absorber material sees every single development stage of hadronic showers, starting from no hadronic interaction of punch-through and thus MIP-like particles, over the central part of the cascade down to the tails of hadronic showers starting at the front of the calorimeter. If the distance between the detector layer and the shower starting position is known by exterior means on an event-by-event basis, the measurements of these 15 channels are sufficient to describe the average response of hadronic showers.

During the testbeam campaigns T3B was always parasitic to a fully functional prototype of the Tungsten Analog Hadronic Calorimeter (W-AHCal) of the CALICE collaboration, which is able to determine the shower starting position and provide it to the T3B experiment for offline analysis. Hence, some effort was made in order to synchronize the data streams and thus the event recordings of the two independent experiments.