Test stand for measurements of the NOvA electronics
The NOvA test stand was made at JINR for studying native electronic responses through Avalanche PhotoDiode and Front-End board chain. The bench was used to perform investigations for physical signals in the detectors also. Main activities of the NOvA-JINR stand are connected with signal shaping studies, simulation of the electronic response on hypothetical magnetic monopole signals, sag measurements and other required to do simulations and precise energy reconstruction.
One can see the interesting hit structures in the picture below. They were called Flashes. The main feature of the flashes is that most of the FEB channels in the same FEB produce signals over thresholds simultaneously but these events take place only in high energy region. It makes them really important for high energy deposition and may take effect on background estimation (cosmic muons) or some Exotics (monopoles etc).
That's why one of the main reasons to have test bench was the investigation of hit structures but of course it is not only one. It is possible to measure the shaping parameters of the electronics and electronics' response to different initial light signals. It is very important for the MC simulation and right understanding of the electronics behaviour.
General technical information
NOvA test bench at JINR consists of a few parts. First is the native NOvA electronics -- Avalanche PhotoDiode and Front-End Board. Second is the special hardware -- download cable, DCM-emulator, LED, Low and High Voltage Sources, Pulse generator and cooling system. Third is a PC with necessary software.
All sensitive devices were put into a black metal box. The black box allows to perform all the measurements with photodetectors like APD and PMT and screens all external electromagnetic noises. Since APD in NOvA operate at -15 degrees we employed the cooling system based on Nitrogen evaporation flow.
Pulse generator waits for the FEB trigger and sends the electric pulse to the LED. After that the light pulse from the LED comes through the fibers to the APD and FEB is reading at this moment.
Cross-talk measurements and dynamic range extension
When test bench had been built we started with APD response on different light intensities measurements. It was very important for understanding the detectors performance for high energy dissipations. By sending very high intensity light into a single APD pixel which saturates ADC we found out that the same small inverted signals occur in most of channels. First we checked FEB by injecting a huge charge into a single FEB channel and we observed normal cross-talk in neighbouring channels that drops exponentially. The next step was to check APD feeding chain. We performed measurement of APD gain with different temperatures to study APD operation voltage at different temperatures. In NOvA detectors all the APDs operate with the gain about 100. We employed different capacitors into FEB PCB and obtained different value of negative cross-talk pulse. The suggestion was that capacitance blocks a voltage drop which was named signal sag.
Sag is a value of the amplitude of the inverted signal in the neighbouring channels in case of huge amplitude in the primary one. We put different amplitudes to APD and measured the value of the sag. The first result was that relative value of the sag doesn't depend on the amplitude. It is equal to 1.89 %. Sag doesn't depend on the temperature and only thing that can define it is bypass capacitor. If we replace original capacitor by 9.4 nF we could reduce the effect sevenfold. This effect could be very helpful for extending of the ADC dynamic range. Sag can be measured in many channels (up to 31) which gains precision. As the result we can increase the statistics and expand the dynamic range by 10 times.
Monopole simulation and Response to long signals
The approximate scheme of the special analog simulation of the electronic's response on hypothetical magnetic monopole signals. "Ordinary" response is the standard behaviour of our electronics for the external signal. Also we have a theoretically predicted monopole signal. Since monopole leaves a huge energy and travels slowly through the detector the light pulse duration may reach level of a few microseconds. After the convolution with electronic shaping it was anticipated to obtain signal like it shown in the picture below. During the simulation we noticed that the tail of the signal was longer than it's expected and it depends on the amplitude. The behaviour of this effect is linear.
Special kind of experimental setup was made for measurements with long signals. In addition to the ordinary setup PMT and Digitizer were added. PMT was used to monitor the shape of the initial light pulse and ADC with fast sampling to digitize PMT's signals.
The main idea was to send light with the same integral intensity which corresponds to the constant charge. We tried to generate rectangular-like light pulses. The ASIC shaper integrates the APD pulse width and converts into a Rise time and shaping parameters vary on pulse amplitude. We thank Dr. Martin Frank for useful discussions and help during this measurement.
Improved Monopole simulation and Response to long signals
Main parameters of the previous modeling were:
1) Rectangular shape electric pulse (this shape went from the suggestion of high ionization power of the monopole) – it converts into rectangular light pulse and this light pulse goes to PMT and APD – this assumption is correct.
2) For the first time point (20 ns) we chose the signal with amplitude that didn’t saturate the APD and had 20 ns width. We calculated charge and kept it constant for all other time points – increased the width and decreased the amplitude – for improved modeling we have to select primary light (and then electric) pulse amplitude according to NOvA soft simulation output.
NOvA soft simulation has several features:
1) Several possible dE/dx for monopoles (high, nominal and low). They are connected with monopole energy loss uncertainties.
2) Two possible APD gains – low (100) and high (140). They are connected with two sets of NOvA data taking.
We have to extract primary light pulse after the scintillator and fiber, put it inside the bench and investigate what is the lowest possible signal that NOvA readout chain can detect.
Results for the high dE/dx and high gain (highest energy deposition) and low dE/dx and low gain (lowest energy deposition) are as follows:
High dE/dx and high gain EVD and planes signals (planes 7 and 21, cell 192 – it is the beginning of the detector along the monopole track):
Low dE/dx and low gain EVD and planes signals (planes 7 and 21, cell 192 – it is the beginning of the detector along the monopole track):
Light signal extraction
Light signal extraction has been made for several betas of slow magnetic monopoles. Output plots look like Npe (photoelectrons number) vs Time for all cells along the monopole track (sum) and average number of Npe for all cells. Second one is used like input parameter for the light signal in test stand modeling. Preliminary resulting plots of APDs outputs for two FEB operation modes (Oscope (left) and DCS MP (right)) are:
For simulation of the detectors performance NOvA collaborators asked us to find all the shaping parameters for both detectors. It was necessary for computer modelling and simulations. The concept of the measurement was the same that aforementioned. The fall time isn't fit to the table value. It linearly depends on the amplitude. The Rise time doesn't fit to the table value either but it doesn't vary or the variation is negligible.
We develop the powerful tool for direct manipulation and measurements with NOvA electronics. A lot of issues have been solved and JINR-NOvA stand can solve many problems or clarify some misunderstood features in the future.