Developing an ultra-high sensitivity electrostatic
collection radon monitor benefits the scientific experiments of
China Jinping Underground Laboratory. Here, a one cubic meter
electrostatic collection vessel with a multi-layer hemispherical
metal grid was designed to increase the collection efficiency of
positively charged Po-218 ions. The 3D model of the giant
electrostatic collection vessel was constructed using the COMSOL
Multiphysics simulation software, and the potential and electric
field distributions in the vessel were simulated. Numerical
simulation results were obtained according to the different radii
and voltages applied to the grid. The electric field between the
vessel wall and grid, between two grids, and between the grid and
surface of the PIPS detector must be set uniformly to reduce the
collection time of the positively charged Po-218 ions. Simulation
results showed that setting a charged metal grid in the vessel can
optimize the electric field distribution, and setting a two-layer
charged metal grid in the giant vessel can further increase the cost
performance. The average collection times of the electrostatic
collection vessel with the two-layer grid along the vertical and
oblique lines approximately 15% and 13% of that without the
grid. The rates of positively charged Po-218 ions that could pass
through the one and two-layer metal grids were 86.78% and
50%. Optimizing the electric field can greatly increase the
sensitivity of radon monitors and reduce the humidity restrictions.
Two models can be used to measure the radon exhalation rate
with a ventilation chamber. The first is the custom model with a
flow meter, and the second is the improved model without flow
meter. As the established radon exhalation standard facilities can't
simulate any medium to be measured by adjusting the radon exhalation
rate and effective decay constants as required, numerical simulation
is useful to compare the two models for measuring radon exhalation
rate under different effective decay constants. Three different
effective decay constants and two different radon exhalation rates
were simulated for measuring radon exhalation rates, and the
simulation results confirmed that the improved model is quick and
accurate even when the radon exhalation rate is very low. While the
custom model can only be used to calculate the radon exhalation
rates when the ventilation rate is far larger than the effective
decay constant, and it takes a long time for the radon concentration
inside the ventilation chamber to be stable. Therefore, the improved
model without flow meter is more accurate to measure the radon
exhalation rate under different effective decay constants.
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