Smoke alarm ion neutralisation

Smoke alarms are a necessary part of many modern day buildings, especially in laboratories and of course, the family home.


According to the World Nuclear Association page [Smoke Detectors and Americium](http://www.world-nuclear.org/info/non-power-nuclear-applications/radioisotopes/smoke-detectors-and-americium/) (2014), a major part of the operation of a smoke detector is when the Americium-241 emits alpha particles collide with atmospheric oxygen and nitrogen, ionising them - which form part of an electric current between two electrodes.


When there is smoke (from the article) - bolding mine: 



> 
> When smoke enters the space between the electrodes, **the smoke particles attach to the charged ions, neutralizing them**. This causes the number of ions present – and therefore the electric current – to fall, which sets off an alarm.
> 
> 
> 


**What is the chemical process that causes smoke to neutralise the charged oxygen and nitrogen ions in smoke alarms?**




From [this](https://www.google.co.in/url?sa=t&rct=j&q=&esrc=s&source=web&cd=9&ved=0CFAQFjAI&url=http%3A%2F%2Fwww.ismtii2011.org%2Farticle%2Fxml%2Fsub%2Ffile_download.kin%3Fmain_no%3D202%26mode%3Dpdf&ei=4mBDVZGhNY63uQS8toHYCQ&usg=AFQjCNH4KEifjFcA8P6pE9KxmsMyLXjbyA&sig2=hgnbBozls-uaDFqTWlOHew&bvm=bv.92189499,d.c2E&cad=rja) 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](http://en.wikipedia.org/wiki/Dielectrophoresis) 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. 
> 
> 
> 


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