Still a good starting point after all these years

https://public.wsu.edu/~collins/Phys415/writeups/massabscoeff.gif

For brick you may refer to:

Article Simulations of mass attenuation coefficients for shielding m...

Please have a look at Fig. 2a for the energy dependence of the x-ray mass attenuation µ/rho of brick. Taking the density rho of brick from table 1 you are able to calculate the appropriate µ= (µ/rho)*rho and from that the half- or tenth-value thickness.

XCOM is also mentioned there.

You may calculate the mass attenuation via XCOM by yourself taking the elemental composition of brick according to table 1.

Link for NIST/XCOM:

https://physics.nist.gov/PhysRefData/Xcom/html/xcom1.html

Please use the 'Mixture' box *) and at the next page put in the elemental fractions of the elements of brick according to table 1 in the upper box. Examples are displayed above the box.

Please add your photon energy 0.208 in MeV at the 'additional energy box' and exclude the standard grid *).

You will find the result table at the bottom of the next page (please scroll that page).

*) press 'the 'Submit Information' box.

I got for 208 keV (=0.208MeV): µ/rho= 0,1241cm²/g

For gypsum you may proceed in a similar way via chemical composition and density of your shielding material.

Good luck

Use the XCOM database which freely online in webpage.