I have not yet closed the Android radiation monitor case. Besides building a stand-alone circuit with parts that I have at home - sooner or later - I need to locate a source that does not require walking through security checks. One of the Chinese big online sellers has something that might come handy in the form of a self-luminescent keychain. Today I ordered one for about 12€.
Results in a couple of weeks, when the sample will arrive. Testing will be straightforward.