Effect of construction steels on PMTs detection efficiency at JUNO

We study the impact of the carbon steel rebars and the steel TT bridge within the JUNO structure on the shielding effect of the coils. Our simulations demonstrate that despite the presence of carbon steel structures of the rebars of the water pool and the TT bridge within the central detector vicinity, the residual magnetic field experienced by the PMTs remains within the acceptable limit established by the JUNO experiment of 10% for CD-PMTs and 20% for Veto-PMTs, compared to the geomagnetic field. With the designed currents of the compensation coils, the maximum magnetic fields experienced by the CD-PMTs and Veto-PMTs are 9% and 18% of the geomagnetic field strength, respectively. These findings indicate that the residual magnetic field has some impacts on the PMTs detection efficiency. We investigate whether the residual magnetic field could be significantly reduced by adjusting the compensation coil currents. It is found that the maximum reduction of the residual magnetic field is achieved by reducing compensation coil currents to 94% the designed values, where the residual magnetic fields may be reduced to 6% and 13% the geomagnetic field in the CD-PMT and Veto-PMT regions, respectively. The implementation of the findings may lead to some increase in the PMTs photo detection efficiency and reduction in the power consumption of the compensation coils.