PROBLEMA GENERAL

The resolution of the reconstructed position is strongly dependent on the incident photon energies. The obtained resolutions at the beamtime test in Bonn vary from 3.9 mm up to 4.8 mm at 1 GeV depending on the position of the beam. This is in a good agreement with results from an earlier beamtime test in Mainz which are shown in Fig. 4.4. However, both resolutions are not corrected for the spot size of the photon

Figure 4.4: The resolution of the reconstructed position as a function of the incident photon energy measured at MAMI [40].

beam. It has to be considered that

σx= σBeam⊗ σDetector. (4.4)

Therefore, at the highest energies the obtained resolutions seem to reflect the dimen- sions of the photon beam. Hence, these resolutions are not the limit which will be achievable for the Barrel EMC. Better resolutions of the reconstructed position in the order of σx,y ≈ 1.1 mm have been obtained at a beamtime test of the PROTO60

together with prototype tracking detectors of the PANDA MVD [27].

4.3 PROTO120

Despite the fact, that the first generation of prototype, the PROTO60, had achieved a sufficient performance which almost satisfied the requirements of the PANDA EMC TDR, a new generation prototype, the PROTO120, had been constructed. The in- herent difference is the read-out electronics. In particular, the specially developed APFEL ASIC preamplifier whose performance test was the principal task of the beamtime test with the PROTO120 presented within this work. The main require- ment of the preamplifier deduces from the PANDA physics program which relies on the detection of low energetic photons. Therefore, the read-out electronics is required to feature a low electronic noise level. A sufficiently low noise level had been stated by the developers at GSI in Darmstadt which had to be verified under real experi- mental conditions in a beamtime test with a prototype detector. Such a test has been performed with the PROTO120 at the MAMI-facility in Mainz. The obtained relative energy resolution of a 3 × 3 crystal array can be parametrized by

σ E  PROTO120 = 0.16% E/GeV⊕ 2.46% pE/GeV ⊕ 2.32% , (4.5)

where the first component estimates a rather low electronic noise contribution, the second a sufficient stochastic term and the third a rather large constant term.

noise equivalent of the ASIC are calculated explicitly and compared in the following in order to judge the performance of the ASIC. Here, the electronic noise is defined as the standard deviation of a trace without the presence of a signal. In order to guarantee that, the baseline of each event was used to calculate the standard devia- tion. The obtained maxima of the distribution leads to an electronic noise level of

σnoise = 31.5 ch = 0.72 MeV.

The intrinsic photon equivalent noise equivalent of the ASIC can be calculated by the following formula:

σnoise,ASIC =

Dynamic Range · Noise · Amplification

Maximal Input Charge . (4.6)

At the beamtime test, the ASIC saturated at around 24000 channels in the high gain branch which corresponds to a dynamic range of approximately 745 MeV. The ASIC amplification during the beamtime test deviated from the design value of 16 and amounted to 11. The large discrepancy is not understood until now. However, it has no influence on the analysis because it is constant. The noise of the ASIC had been measured at the laboratory at GSI for a maximal input charge of 8 pC and amounted to 0.74 fC. Finally, the photon equivalent noise level of the ASIC amounts

to σnoise,ASIC = 0.76 MeV.

Hence, the noise level of the high gain branch is comparable to the intrinsic noise level of the ASIC. This indicates that the shielding and grounding concept of the PROTO120 is more than sufficient to reduce the influence of pick-up noise. There- fore, the ASIC can be regarded as the limiting factor of the noise level.

The electronic noise can be translated directly to the summation threshold of a single crystal. The summation threshold corresponds to the level where most of the noise contribution is rejected. The noise distribution is expected to be Gaussian. Therefore, more than 99.7% of the events are within a 3 · σ-range. This range is usually set as the summation threshold. For the beamtime test a summation threshold of 2.7 MeV has been set to reject the noise contribution.

The summation threshold is below the single crystal threshold of 3 MeV which al- lows a cluster reconstruction threshold of 10 MeV required by the TDR. However, the summation threshold does not imply a sufficient low signal to noise ratio for photons below 3 MeV. In addition, the noise distribution showed a tail to higher energies.

The discrepancy to an expected Gaussian shape indicates a correlated noise. In or- der to get rid of the high noise level, a different feature extraction method had been tested. Adding the original traces of the two LAAPDs before the feature extraction steps, showed a significantly lower noise level. Nevertheless, the high energy tail did not vanish and the relative energy resolution obtained by this method did not show an improvement.

However, the trigger-less readout of the final Barrel EMC has to be considered. On the one hand, a single crystal threshold of 2.7 MeV would avoid a trigger of sin- gle channels which is caused by misidentified electronic noise events. On the other hand, the threshold makes it impossible to recover small pulses which are rejected below the threshold. Alternatives to the independent triggering are currently being discussed. A cluster trigger readout would feature the recovery of channels within the cluster which are below the threshold. Then again, the readout of a cluster would challenge the data processing. Therefore, the reduction of the threshold is considered as well. This method would enable the recovery of small pulses but would go along with an increased count rate of the detector which is limited by the rate capability of the electronics.

Furthermore, it should be accounted for the observed increasing noise level at CMS (see Sec. 4.1). A similar behavior at PANDA would foil the goals of the physics program.