NOvA detector used liquid scintillator to create the signals of incoming particles, and in each cell, a looped fiber may absorb the scintillated light and transport them to the readout Avalanche Photo Diode (APD). APD has a high quantum efficiency and intensity versus a much wider range of different wavelengths of the incoming photons than traditional PMTs. Also the size of APD is small enough to be installed on high dense scintillator detector such as the Far Detector that Nova has. At -15C, noise was reduced to 10:1 as the signal to noise ratio at the furthest end of each detector cell. Under this temperature, the bulk dark current of APDs was lower than 0.3pA, which is the best limit on the output we can reach. The chilled water with a temperature at 15C do not connect APD directly, it carry away the heat from thermoelectric cooler, which turn the APD temperature from 15C to -15C. APD has to work under a high voltage to maintain a sufficient gain, we supply a 425V on each APD, thus if the circumstance is not dry enough around APD, it may get more easier to melt the APD. Since water molecules are easy to permeate through PVC materials, which most part of the detector is made of, we have to use an active dry gas flow to keep the APD works properly.
One High voltage wire connects to 8 di-blocks of the detector, each HV channel are supplying 64 APDs. So if some of the APDs start to melt, and no one responds promptly, we have to shut off the whole detector and will definitely delay the process of Nova experiment. Kirk and Biao perform as experts of Detector Control System on the APD protection. There are currently two parallel systems for monitoring and controlling the water-cooling and dry gas systems, ifix and acnet. We use synoptic system to monitor the status of the power supply, gas flow, cooling water and APD conditions. Once there are some alarms shown in the remote control room, we use ifix to check the problem details first and then depend on the issue we may disable some part of equipment or alter to a spare one. Acnet is better to be used as a command line tool to record the status of the each module by using camonitor. It records the value of drive current or APD temperature every 5 seconds. Acnet also has function to change the status of each channel in water-cooling or dry gas system by using caput. We make a list of command history of useful commissioning under the home directory of novadcs@novadcs-far-master-02. With progress of the whole experiment, experts are also able to commit new scripts to meet the challenging requirement.