Who do you believe? I'm doing some shielding calcs for Cs-137 (662 KeV gamma) in stainless steel (Fe seems close enough) with ux (mfp) in the range of 5 to 15. Shultis and Faw in Radiation Shielding (2000) treat Photon buildup pretty well. Their coefficients for the Berger approximation agree pretty well with the tables in Martin's Physics for Radiation Protection. (2006) A lot better than the single and double term Taylor forms. But Shultis and Faw say the Berger may be off by up to 45%. Glasstone and Sesonske (4th ed) say the Berger is more accurate than the Taylor. Shultis and Faw say the Geometric Progression approximation is the cat's meow, but they don't treat it well enough for me to get a handle on it. Is there a generally accepted table of buildup factors, or formula and coefficients, for photon buildup in shielding that most people use or accept as standard? Is there a good description of the Geometric Progression Approximation available on the web somewhere? For now I'm using the Berger approximation. Thanks!