A super-simple ionization detector - just the thing for finding Polonium (a very small chamber suitable for alphas). Probably good enough for most targets of certain governments, although improving the design is easy.
The cute thing about the Darlington pair is that it is just two bipolar transistors combined to cascade amplification. In the early days these sometimes were packaged in a single case. Sid Darlinton of Bell Labs patented it for two or three transistors on a single chip of silicon ... if he would have done it for an arbitrary number, Bell Labs may have held the patent to the integrated circuit.