What is the recommended method to reduce gamma ray exposure?

[Clara Chung]

A gamma ray source emits gamma ray to a photographic plate in a container. The book said the blacked region on the photographic plate wouldn't be affected if the gas pressure of the container was increased.
However, I think that the gamma ray will ionise more air molecules and lose more energy though ionising air molecules. Therefore, the photographic film will be hit by less photons of gamma ray and blackened less.
Am I right or wrong?

 

[Simon Bridge]

A gamma ray source emits gamma ray to a photographic plate in a container. The book said the blacked region on the photographic plate wouldn't be affected if the gas pressure of the container was increased.

Which book?
What is the significance of the blacked part of the photographic plate?
I cannot tell what the book is talking about without a more detailed description.

However, I think that the gamma ray will ... lose more energy though ionising air molecules. Therefore, the photographic film will be hit by less photons of gamma ray and blackened less.
Am I right or wrong?

...the bolded part is incorrect. Gamma rays do not lose energy from ionizing air molecules the way a massive charged particle does. Instead, gamma photons are either allowed to pass or absorbed completely by an atom it gets close to. The ray does lose more photons in denser air though... so what you want to consider is how dense the air would have to get for the weakened flux to show up on the plate?

What does it usually take to reduce gamma ray flux? What do people normally use for shielding?