NeuRad detector prototype pulse shape study

The EXPERT setup located at the Super-FRS facility, the part of the FAIR complex in Darmstadt, Germany, is intended for investigation of properties of light exotic nuclei. One of its modules, the high granularity neutron detector NeuRad assembled from a large number of the scintillating ber is intended for registration of neutrons emitted by investigated nuclei in low-energy decays. Feasibility of the detector strongly depends on its timing properties de ned by the spatial distribution of ionization, light propagation inside the bers, light emission kinetics and transition time jitter in the multi-anode photomultiplier tube. The rst attempt of understanding the pulse formation in the prototype of the NeuRad detector by comparing experimental results and Monte Carlo (MC) simulations is reported in this paper.

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Each ber will have a square cross section of 3×3 mm 2 and a length of 1 m. Scintillating bers will be grouped into bundles tting the photosensors e.g. multi-anode photomultiplier tubes (MAPMT). The MAPMTs will be mounted to the bundles so that the area of the each pixel will completely overlap the front surface of one ber. The NeuRad will be placed 28 m downstream of the secondary target in such a way that the bers will be oriented along the beam direction [6]. It will provide sucient detection eciency and high angular resolution

Simulations and data processing
The simulation and data processing were performed within the EXPERTroot framework. A particle transport through the setup was performed using the GEANT4 [12]  Certain eects were not taken into account: any dependence of the pulse shape on the amplitude; the cross-talk as the charge sharing of a single photoelectron avalanche between two or more anodes; the partial collection of the light after a diuse scattering at the ber faces; the electronic noises; the pulse shape distortion in the readout line. Most of the parameters for the simulations were taken from the data-sheets of the bers and MAPMT. The single photoelectron pulse shape was tted to the experimental data. The single photoelectron amplitude spectrum was borrowed from [11], where a similar MAPMT H12700 has been studied. Several standard and rather simple data processing algorithms were implemented into the EXPERTroot framework, such as leading edge discriminator, dierent event selection lters, alignment and summing up of the pulses, etc. The implemented algorithms allowed us to study the summed pulse and to understand qualitatively its basic features. All algorithms could be applied in the same way to both simulated and experimental data. The parameters of the readout chain needs to be precisely measured using a fast laser. The actual single photoelectron pulse shapes and spectrum should be acquired. Bigger number of pixels should be read out at the same time in order to study all the eects more carefully. As the next step, multichannel electronics based on the TOFPET ASIC [13] will be used for the readout of the NeuRad prototype. A transfer function of these electronics will be implemented into the EXPERTroot software in other to compare the simulated and the experimental data.

Conclusion
The rst small-size prototype of the NeuRad detector was investigated using the γ-source.
The pulse shapes have been obtained using the DRS4 waveform digitizer board and have been studied individually and in average. Processing algorithms were implemented into the EXPERTroot software package. In order to understand the pulse formation mechanism, the measurements were simulated using the EXPERTroot.
The inuence of such parameters of the detector as light collection eciency, interber cross talk, detection threshold and luminescence kinetics on the signal shape are understood.
For a better understanding of the timing properties, quantitative validation of the MC simulations and complete feasibility study it is necessary to conduct new measurements with the NeuRad prototype recording bigger number of channels and using well calibrated electronics.
The results of this study will allow to choose the optimal readout electronics for the entire system.

Acknowledgement
The authors are grateful to I G Mukha for the stimulating interest and numerous useful advices. This work was partly supported by the Helmholtz Association under grant agreement