Nuclear-Chicago Model NC-205 BF3 Counter (ca. 1955-1965)
This neutron detector was produced by the Nuclear-Chicago Company of Des Plaines, Illinois. I have no information about this specific instrument, but the following general information comes from a Nuclear-Chicago bulletin that appeared in the December 1961 issue of Nucleonics: "BF3 gas proportional neutron detectors may be obtained in practically every size and shape. Their pressures range up to three atmospheres. All are sturdily constructed for long, rugged use... NC 218 is a large cosmic ray neutron detector having a guard ring insulator which assures low background. NC 251 is a general purpose thermal neutron detector with a ceramic insulator and stainless steel anode for high temperature operation." The presence of the exposed evacuation/fill tube leads me to suspect that it most likely dates from the late 1950s.
Size: ca. 5" long and 5/8" in diameter