You may refer to Article 22 of ASME SEC V - SE-94, para 7.0 and table 1.
This provides guidelines related to energy of radiation source and corresponding thickness being radiographed. Hope this helps your needed detail.

Charlie's reference to ASME Section V Article 22 is the right place to look. That article covers radiographic examination, and the tables there show which radiation sources (X-ray or gamma) can be used for different material thicknesses.

The key principle is that radiographic sensitivity depends on radiation energy. Higher-energy sources can penetrate thicker material while still producing an image with acceptable contrast. ASME Section V Table SE-94 (in the 1977 and later editions through to current) maps material thickness ranges to the minimum radiation energy needed — typically measured in kilovolts (kV) for X-ray or in curie strength and isotope type for gamma sources.

For example, thinner sections need lower energy to achieve good sensitivity and definition, whereas thick sections require higher energy to generate enough exposure at the film or detector on the far side. If you try to radiograph a thick section with low-energy radiation, you get poor image quality or no image at all.

The exact table references and values depend on which edition of ASME Section V your code of construction requires (often ASME BPVC Section VIII Division 1 for pressure vessels will reference a specific edition of Section V). Check your project specification or the applicable ASME code case.

If you are setting up a radiographic procedure, your Level III or Qualified Inspector should confirm the thickness limits and source selection for your specific application and material.