Smoke alarm ion neutralisation
From this paper, found the mechanism for neutralisation,


  Smoke particle charge interaction
  
  When smoke particles enter in the ionization chamber, they are
  subjected to the electric field generated by electrodes but also to the
  one generated by the charges. Close to the charges, the electric field is
  highly divergent resulting in dielectrophoretic force acting on smoke
  particles. The force tends to bind the smoke particles to the charges.
  When smoke particles are considered as isotropic neutral dielectric
  bodies linearly and homogeneously polarisable, dielectrophoretic
  force F can be expressed as
  
  $ F = \frac{1}{2} \alpha v \overrightarrow{grad} |E^{2} |$
  
  where $\alpha$ depends on smoke particle polarizability, $v$ is the smoke
  particle volume and $E$ is the electric field produced by the charges
  [11]. Coefficient $\alpha$ is typically of the order of $\epsilon_{0}/3$ [12] where $\epsilon_{0}$ is the
  dielectric constant of air. Volume $v$ is approximately $4 \mu m^{3} $ for a 1-$\mu m $
  diameter smoke particle.
  Amplitude of electric field $E$ produced by charges is given by
  coulomb’s law:
  
  $ E = \frac{1}{4\pi \epsilon_{0}} \frac{q}{r^{2}}$
  
  in which $r$ is the distance between the charge and the smoke particle.
  Due to the divergence of the field, the dielectrophoretic force
  decrease as $\frac{1}{r^{5}}$. If a smoke particle is close to a charge then they can bind. If the smoke particle is relatively far from the charge, the time
  of interaction may be relatively long and it is necessary that the
  charge drifts sufficiently slowly in the chamber to optimize the
  interaction. Indeed if the charge passes too rapidly there will be not
  enough time for the smoke particle to bind and thus nothing would be
  detected. In fact the capture radius reduces with the electric field and
  is of the order of 10 $\mu m $ for $E= 1  V/mm$.
  
  When the smoke particles attach to the free charges due to
  electrophoresis force, the charges become heavy bodies then the
  charge mobility $\mu$ decreases resulting in a drop of the electric current
  measured between the electrodes of the ionization chamber.
  Even if ionic detectors are very efficient, according to new
  standards they are no longer authorised in almost all of the developed
  countries because of the radioactive sources they include for
  generating the ions. Such radioactive sources are indeed difficult to
  collect and to recycle.