Does boron trifluoride have to be replaced in a proportional chamber detector with ${\rm BF}_3$? Does boron trifluoride have to be replaced in a proportional chamber detector with ${\rm BF}_3$?
Boron-10 reacts with a neutron through alpha decay
$$^{10}{\rm B}+{^1}n \rightarrow {^7}{\rm Li}+{^4}\alpha.$$
So you're losing boron over time and accumulate lithium. Or is this change negligible?
 A: Transmuting a chemically significant amount of material is really hard.
I have been involved in a couple of statistics-limited neutron capture experiments which have stopped an entire cold neutron beam in a target.  In both cases we were looking for part-per-billion effects and so needed $10^{18}$ or $10^{19}$ neutrons — a microgram of radiation. Integrated beam time was a couple of years for each experiment. In my dissertation experiment the target was about 20L of liquid hydrogen, for which the produced deuterium was completely negligible. In the follow-on experiment the target was a few STP-liters of helium-3; the accumulating tritium contamination was something we discussed, but we didn’t wind up doing any flushing or purifying until the experiment was over and our collaborators wanted their helium-3 back.
Proportional chambers are usually transmission devices: most of the neutrons pass through without capturing or scattering at all. You should figure out how much boron there is in the detector which interests you, and how rapidly it is consumed by neutrons in the application where you are using it.  Typically such devices are designed to have lifetimes of decades; however they may also be actively in use for decades, or may be relocated to newer facilities with more neutrons where their useful life is shorter.
