I have seen it said that because high-energy GCR particles cause a spray of secondary particles when they pass through material, there is more radiation behind even quite thick shielding that there is if there is no shielding at all. A paper by Vanessa Aulessa was used to produce the graph below in a lecture by Rob Mueller
But I have also been shown graphs that show cosmic radiation starting to drop as soon as it hits shielding. The graph below from a paper by Donald Rapp (7th page) shows that version:
This slide presentation by Martha Clowdsley comes to the same conclusion, and lists the transport models used.
I have seen both models referred to elsewhere. Why do they apparently contradict each other?