I don't really care for an external probe. I've purchased my 320+ just for a rough indication of radiation. For precise measurements I'll use a Victoreen with pancake or a Berthold contamination monitor.
more hardware improvement ideas on new hand held model: 1. wider scale range. The CPM should use 4 bytes data type, it can store CPM up to 4,294,967,295. 2. Dual tube on board capability. So that to increase the sensitivity. 3. Multi tube type capability. User may install other tube easily. Such as pancake tube. 4. WIFI capability. So that user can submit the data to web server without connection to a computer. 5. GPS option Provide more accurate location information. 6. GPRS option Provide wireless internet connection from remote area via phone sim card data link. 7. Bluetooth option 8. Longer the battery life. The unit should run at least one month without charge the unit. 9. Larger screen size. That can provide more information at once. 10. External probe option. 11. More internal storage. more...
Dual tube, WiFi and GPS sound like nice improvements. Perhaps also a better quality plastic housing with IPX-3 water resistance? The current GMC 320+ really has an "indoor" look & feel. I prefer a more rugged design. And instead of the fixed wristband, simply provide a 'hole' so users can attach a lanyard of their own liking.
But you can build in two tubes already, as I did. What will be the difference?
Oh yeah, the housing: It would be REALLY GOOD if you use thinner plastic! The sensitivity will increase GREATLY! Maybe go for really thin around the tube. With glasswool in the plastic. The manufacturers of multimeters use this kind of plastic to make them really rugged.
GMC-300E+ V4.20 with sbt-11a alpha tube
My statements are "stuff-a-hobbyist-says" and not in any way professional.