UV flame detector how it works see : "https://www.eetimes.com/document.asp?doc_id=1272315"
Theremino geiger adapter how it works see : "http://www.theremino.com/wp-content/uploads/2012/02/GeigerAdapter_Datasheet_ENG.pdf"
Don't look now, your candle is leaking!
Very interesting project! How well does it work? Can you detect any flame? How large of a flame? How far away from the flame can you detect the flame?