The Experimental Setup

The CALICE W-AHCal Testbeam Campaign

The CALICE collaboration performed beam tests at the CERN Proton Synchrotron (PS) at the end of 2010 and the Super Proton Synchrotron (SPS) several times during

2011. The aim of this testbeam campaign was to test tungsten as an absorber for a hadronic sampling calorimeter, reusing the existing active layers of the steel analog hadron calorimeter (Fe-AHCal) prototype which is based on scintillators read out via SiPMs (cf. section 3.3, [38]). A mechanical stack was constructed holding layers of the tungsten absorber material in place while allowing to easily slide in the read out layers of the CALICE AHCal in between. The stack was designed to hold up to 40 layers of active material, while during the testbeam campaign not more than the first 38 layers were equipped. Hence the remaining slots were used to hold parasitic experiments like the T3B layer.

The first testbeam campaign with hadrons took place for two weeks in November 2010. The T9 beam line of the CERN Proton Synchrotron (PS) at the East Hall provided a mix of electrons, pions, muons and protons with a momentum range of 1 to 10 GeV. During this period the mechanical frame of the W-AHCal contained only 30 active and tungsten absorber layers. The remaining 8 layers were added for the beam tests performed at the SPS, which took place for a total of three weeks in June, July and September 2011 at the H8 beam line in the North Hall which was used to provide particles with a momentum between 10 and 180 GeV.

More details on the beam tests by the W-AHCal can be found in [45]. The T3B Layer with Scintillator Tile Readout

As it was described in section 3.5, the hadron calorimeter of the ILD as well as its prototype, the AHCal of the CALICE collaboration, both use scintillator as active medium which in turn is read out using Silicon Photomultipliers. As the focus of the T3B experiment lies on the measurement of the time structure of hadronic showers in the context of an ILD detector at CLIC, the same read-out technology was used. However, as already pointed out at the end of subsection 3.5.2, a few adaptations were made with respect to the CALICE prototype.

This includes the usage of blue sensitive Hamamatsu MPPCs [66] which are capable of directly recording the scintillation light. Thus it is now possible to use tiles which are directly coupled to the SiPM, in contrast to the coupling via a Wavelength Shifting Fiber (WLS) for the tiles of the CALICE AHCal. This modification was necessary, as the WLS introduces an additional time delay as well as an unwanted broadening of the signal over time, caused by the conversion of the blue scintillation light to green. To ensure a uniform response regardless on the position of a passing charged particle, a small hole (a “dimple”) was drilled into the scintillator tile [44]. The tile dimensions stayed the same with respect to the ones used in the CALICE AHCal prototype (30×30×5 mm3_).

Apart from a small hole for the SiPM, the scintillator tile was covered by a layer of reflective 3M foil [67] to maximize light collection. The SiPMs were mounted on a small PCB which contained a preamplifier and the necessary connectors. The size and form of the PCB was chosen to cover the attached scintillator tile, such that the tile, the SiPM and the PCB were one dedicated unit (Figure 4.1). Black plastic foil, normally used to cover photo multiplier tubes, was wrapped around the tiles including the SiPM to minimize noise from ambient light.

Figure 4.1: A T3B tile directly coupled to the SiPM on the preamplifier board. The tile is wrapped in black, light tight foil. The connections on the preamplifier board are (from left to right): bias voltage for the SiPM (+/-), signal, 5V for preamplifier (-/+).

beam spot

151413121110 9 8 7 6 5 4

scintillator tiles

(a) Sketch of the T3B layer (b) Foto of the opened T3B layer

Figure 4.2: The T3B layer. The left plot shows a sketch with the positions of the scintillator tiles shown in yellow. The expected position of the particle beam is marked as well. The right figure shows the opened T3B layer including the cables to the preamplifier boards.

1 mm thick aluminium layer (see Figure 4.2(b)). The size of 1.0×1.0 m2 _{was chosen to}

easily fit into one of the slots of the CALICE absorber structure stack with the central tile being at the foreseen beam center. A second aluminium plate was mounted on top to convert the layer into a cassette and protect the preamplifier boards both from damage and ambient light. A sketch of the layer can be seen in Figure 4.2(a).

The Readout

The T3B experiment is designed to record its data at a beam line at a testbeam facility which uses secondary particles created by shooting protons on a target. As there are many users of such a facility, the time is divided among them and each user gets assigned a “spill window”. Only inside this spill window particles are delivered. This window had a length of about 350 ms with a varying repetition rate of 5-15 s at the CERN PS, and about 10 s with a repetition rate of 45 s at the CERN SPS. Within the spill window between 102 _{and 10}4 _{and more particles were delivered.}

Most of these particles are hadrons which will almost always create a hadronic shower in the absorber of the CALICE hadronic calorimeter which is located upstream with respect to T3B. Each of these hadronic showers has to be recorded over a few microseconds with nanosecond precision. This requires fast digitizers with a recording speed of at least one giga sample per second per T3B channel. However, the particles are randomly distributed within the spill window and arrive with a rate of up to 1 kHz on average. It is difficult to transfer this amount of data directly onto a PC’s harddisk. Consequently the digitizers used for the T3B read out need to be able to trigger significantly faster and store each waveform in an internal buffer which is big enough to record all events within a spill window.

During the design phase of the T3B experiment different digitizers were tested. One option which fulfilled all the requirements is the PicoScope 6403 (Figure 4.3(b)) of the PS 6000 series created by Pico Technology [68], a digital oscilloscope operated via USB from a standard PC. In addition to the official manufacturer’s software it is possible to directly access the oscilloscopes via a software development kit (SDK) giving full control to a custom made C/C++ program working as a data acquisition (DAQ). The latter option was chosen and the created DAQ will be described in more detail in subsection 4.1.2. The PS 6403 has four channels with a sampling rate of 1.25 GS/s each and with a vertical resolution of 8 bit. It has a shared internal buffer of 1 GS which is large enough to store several ten thousand events with each waveform being recorded over 2₋3µs with the maximum resolution of 800 ps. The connection to the PC is done via a USB 2.0 interface which proved fast enough to transfer up to 10000 events recorded by four different oscilloscopes attached to the same PC in the time between two spill windows at the SPS (_≈35 s).

The data acquisition has to be started at the beginning of the spill window, and stopped at its end. Hence it is necessary to monitor the current status of the spill window. At CERN each beam line provides the necessary signal for the start and the stop of the particle extraction. These signals come at a maximum rate of around 1 Hz and can be recorded by the much cheaper PicoScope 2203 (Figure 4.3(a)) with a maximum sampling rate of 40 MS/s and a buffer which is only large enough to save a single waveform. Like the PS 6000 series it is a USB oscilloscope and is controlled and read out via the official manufacturer’s software or the SDK.

(a) Picoscope PS 2203 (b) Picoscope PS 6403

Figure 4.3: Picture of the PicoScope PS2203 (left) and the PS6403 (right) oscilloscopes used in the T3B experiment [68].

Temperature Monitoring

SiPMs are very sensitive towards temperature changes (cf. subsection 3.5.1). Thus each sensor was monitored individually by a PT-1000 platinum resistance, read out via a 4-wire connection using a custom made temperature monitoring board that is attached to the data acquisition PC via USB. Each sensor was read out about once per minute with a relative accuracy which is expected to be in the mK region.

Testbeam Setup

T3B was placed in one of the last layers of the W-AHCal absorber structure, acting as a parasitic experiment. The exact position changed during the different campaigns. At the PS beamlines T7 and T9, T3B was installed in slot 33, with slot 32 being empty, slot 31 containing a prototype of the MicroMegas layer [69], and slots 30 to including 1 were equipped with the AHCal sensitive scintillator layers. Only slots 1 to 30 were equipped with tungsten absorbers, while for slots 31 and up no absorber was used.

At the SPS beamline H8 T3B was installed in layer 40, with layer 39 being empty and layer 38 to 1 were equipped with the AHCal sensitive layers and tungsten absorber. Again layer 39 and 40 had no absorber plates installed.

As it was mentioned earlier it is essential to be able to synchronize the two data streams of the W-AHCal and the T3B experiment if one wants to use the information of the shower starting position from the AHCal. Therefore the T3B experiment was triggered by the W-AHCal, such that they both recorded the same events. A complete sketch of the experimental setup which was used both at the T9 and the H8 beamlines is shown in Figure 4.4.

The data acquisition (DAQ) of the W-AHCal monitors two 10×10 cm2 _{scintillators}

Particle Beam CALICE AHCal DAQ & Scintillator Coincidence 1x Data PS6000 #4 PS6000 #3 PS6000 #2 PS6000 #1 Fan In / Fan Out Gate Generator T3B DAQ PC CALICE W-AHCal T 3 B 4x USB PS2000 Spill Signal from Accelerator

USB Trigger

Signal

Ext. Trigger

3x Data 4x Data 4x Data 4x Data

10x10 cm Scintillators

15x SiPM Data

Figure 4.4: A sketch of the T3B experimental setup as part of the CALICE testbeam campaign at CERN.

particle in coincidence, the CALICE DAQ records the event and gives a signal to the T3B DAQ via the green trigger line. This signal is distributed to the external trigger input (EXT) of the four PS6000 PicoScopes by using a linear Fan-In/Fan-Out NIM module of type LeCroy model 428F. On arrival of the trigger signal at the EXT input, each PS6000 records the waveform with a total length of 2.4µs of each of its four channels and stores it in its internal buffer. These channels are connected to the 15 SiPMs (orange lines in Figure 4.4) of the T3B layer.

The PS2000 oscilloscope is attached to the spill signal which is provided by the beam line, in order to start and stop the data acquisition.

All PicoScopes are connected to the central DAQ PC via USB (cyan lines in Figure 4.4). The PC runs a self written data acquisition software which controls the oscilloscopes and will be described in detail in subsection 4.1.2.

Independent of the coincidence generated internally by the CALICE DAQ from the signal of the two scintillators, a NIM coincidence unit (Type LeCroy model 365AL) creates a similar signal which is fed into the last remaining channel of the four PS6000. This is used in the offline analysis to check that exactly one particle hit the detector setup and is needed for two reasons:

• Rejecting events with multiple particles within the recording time window.

• Rejecting calibration events generated internally by the CALICE W-AHCal DAQ without passing particles.

Another way of data taking is the usage of the NIM generated coincidence as trigger input instead of the one from the CALICE W-AHCal. This circumvents the long dead time of the CALICE DAQ after the recording of one event and allows a significant higher data acquisition rate. To ensure that the T3B experiment can cope with this high rate, the trigger is fed through a gate generator which is a NIM module of type LeCroy Model 222. This module ensures that once a trigger arrives, there is a minimum time span of 3µs before the next trigger signal is delivered to the four PicoScopes. This prevents the four oscilloscopes to run out of sync, as the time to recover from a recording (dead time) is not the same for all, which can lead to one oscilloscope recording an event, while another is still busy. This was useful to increase the statistics on the one hand and was used for beam tests together with the CALICE SDHCal, which uses steel instead of tungsten as absorber on the other. Details of this setup and the analysis which includes a comparison of steel to tungsten absorber is available in [70]. However, when running in this standalone mode no information on the shower starting point is available from the CALICE W-AHCal. Hence, in the analysis presented here only events which could successfully be synchronized to the W-AHCal were used.