peak energy broadening in a photon simulation

[ckong]

Hello, I'm a graduate student using Serpent 2.

I made an input consisted of an NaI detector and a Co gamma source.
I set the pulse-height detector.

Now I'd like to broaden peak energies using a function that
FWHM(E) = a + b * sqrt(E+c*E^2),
and I found Serpent2 has such function in the attached figure.

How can I use that function?
I saw a post that Serpent 2.1.31 supports that kind of function.
Thank you.

Best regards,
Chidong Kong