Given an O'Neill "Island Three" style cylindrical habitat located at Mercury–Sun L5 orbit, how much radiation shielding would be required to protect the occupants, and how would it need to be structured?
The Island Three O’Neill Cylinder design has two counter-rotating cylinders aligned with the axes pointed towards the sun, with a sun shade/solar power facility at the sunward end, and mirrors to reflect light into the the habitats. A sun-shade and radiator seemed to be enough for the recent MESSENGER mission.
I haven't found a source for how much radiation to expect in Mercury orbit, which comes within 46 million kilometers of the Sun.
In Orbital Space Settlement Radiation Shielding (Globus 2016), Globus recommends a limit of 20 mSv/year exposure for the general population, and 6.6 mGy/year exposure for pregnant women.
Globus recommends water as the most effective shielding material, but his calculations are for the LEO environment, where Earth's radiation belts are more of a concern than GCR.
Ignoring the cost, assuming all the materials needed, and the technology to construct them, what radiation shielding would be required to limit the risk of life-long occupants, especially pregnant women, of such a habitat, and how would the shielding need to be configured?