It depends on the energy of the gamma rays.
6cm of lead would do a decent job with most things though.
(
my other username is a porsche , posting shit pictures so you don't have to,
Sat 2 May 2009, 23:16,
archived)
Well I've commissioned a helmet made exclusively from 6cm-thick lead.
I hope you're right and I hope my neck muscles grow significantly before it arrives.
(
zoldergoose,
Sat 2 May 2009, 23:18,
archived)