The claim describes a fundamental definition of how cathode rays are produced in gas discharge tubes, representing a basic piece of historical physics common knowledge that does not require citation.
The claim is a standard, trivially true statement of historical physics regarding the production of cathode rays via gas discharge tubes. None of the retrieved papers discuss cathode rays in Crookes tubes; instead, they cover unrelated modern topics such as hydrology, superhydrophobic coatings, solar water heaters, and battery cathodes. Because the statement is common knowledge, no papers are attached.