SNMO-5 is a neutron detector (proportional counter) filled with boron trifluoride enriched with boron-10 – Fig. 1. The tube is 10.5″ long and has a diameter of 1.375″. These dimensions are comparable to the size of helium-3 filled SNM-18 tube. BF3 gas pressure in SNMO-5, however, is rather low, 1/3 atm.
Due to the fairly low gas pressure the sensitivity of SNMO-5 tube to thermal neutrons as measured with 5 mCi Po-Be source is only ~1 CPS. This is not impressive and amounts to about 10% of the sensitivity of SNM-18 helium-3 filled tube, which has almost identical dimensions. SNMO-5 low sensitivity is almost entirely caused by the much lower gas pressure (1/3 atm) vs. 2-3 atm in SNM-18. Sensitivity reduction due to boron-10 is a relatively minor factor: boron-10 has ~30% lower cross section for capturing thermal neutrons compared to helium-3.
Thermal neutron spectrum of SNMO-5 is shown on Fig. 2. X-ray peak is evident in the first few channels of the spectrum. Thermal neutron peak is non-existent due to low gas pressure.
Conclusion
Given the availability of reasonably priced SNM-18 counter tubes it makes no sense to go with boron trifluoride, especially at low pressure.