I've built a number of these and it would be a challenge to make a robust, portable one. Mass flow controllers are expensive and sensitive, and carrying around

I've built a number of these and it would be a challenge to make a robust, portable one. Mass flow controllers are expensive and sensitive, and carrying around a supply of ethane or other gas would be inconvenient; even if cool. The detectors themselves also tend to be quite sensitive (to motion, RFI, and everything else), though much of that could probably be overcome.