OSHPD had some rules about it that we had to follow. I don't have them now. But, we had to put a special type of lead lined drywall on the walls surrounding the room to contain it/ limit it.
I have wall and ceiling thickness information from the physicists. I am looking for information on mix design, joint placement, construction requirements, etc. Not sure if there is a spec out there like ASTM C637-20
Having worked for the design of hospital , i remember some details,
-The exterior walls RC wall with 200 mm thk.
- The interior walls dry walls with mm lead sheet
- Ceiling and floor covered with 2 mm lead.
I know some countries use heavy concrete ( the aggregate is iron ore instead of sand and gravel).
I will suggest you to check ( if there is ) local rules from health department and ask recommendation of eq. manufacturer . I remember there is an allowed radiation ( could be from sum light also..) and the wall thk. is determined to reduce the radiation to allowable level.