Dave T is right. Before inexpensive transistors and integrated circuits, neon lamps were used for ring counters, timers, sweep generators, circuit testers, and countless other clever adaptations. I don't remember radioactivity being used to lower the starting voltage. However, ambient light has that effect. The frequency of a neon oscillator varies slightly with ambient light. vpwphoto is also right; I've seen many erratic neon lamps over the past 60+ years.