I think Jay knows this one:

In what page of "Prospects of Interestellar Travel" is that quote about the required meters of shielding?

Sorry for the delay on this one.

Section 8.5 (pp. 196-202) discusses the scientific and engineering aspects of radiation, focusing on the radiation induced by starship motion at relativistic speeds. Section 9.6 (pp. 225-227) discusses the biological effects of interstellar radiation and makes the important statement that biological protection from cosmic radiation particles is the determining factor for starship shielding. That is, if it weren't necessary to protect the crew from cosmic radiation, less shielding could be used.

The gist of the first section is that "forward radiation" (the effective radiation produced by a starship moving at relativistic speeds through stationary particle soup) is many, many times more intense than natural background radiation. However these energies are still in the MeV range -- dangerous, but not unmanageable. It also discusses other shielding needs, such as for dust erosion and thermal management.

The gist of the second section is that forward radiation is millions of times more intense than background radiation, but less penetrating. Even though background radiation occurs at far less intensity, its much higher energy -- 10 GeV and above -- dictates thicker shielding for biological purposes.

Although Mauldin does not say what kind of shielding he means, we can infer from his other statements that he means water. Nowhere does Mauldin suggest "six feet of lead," as is commonly attributed to him. In Chapter 8 he suggests several kinds of materials, but in Chapter 9 he specifies "solid shielding".

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A. C. Clarke has suggested in one of his novels to use ice as radiation shielding.
Large blocks of ice would be placed on the nose of the ship,
and during the voyage it would ablate away.
The advantage being that ice is easy to make and easy to handle,
so you could build a new "iceberg" at each stop.

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The Arthur C. Clarke book is called, "Songs of Distant Earth"