Alpha/Beta radiation and photoionisation of air Which type of radiation is more likely to lower breakdown threshold when using focal lense focused pulse laser,for example 1064nm Nd:YAG.
I reed three studies that said free electrons are most important to initiate the avalanche breakdown,since beta radiation are free electrons,I would think that beta radiation would have biggest impact on the air breakdown threshold.I also heard conflicting opinion that alpha radiation will have biggest effect becose it penetrates less far,it will interact more with the small area of focal point where photoionisation happens.
I would like to represent alpha radiation by Americium 241,alpha emiter used in smoke alarms,and beta radiation with Caesium 137,a beta emiter used for example in Raytheon CK1097-7 spark gap tube to aid ionisation.Which of these two radioactive isotopes will have greater effect on laser induced breakdown when placed in close proximity to the focal point area ( few milimeters ) in quantity around 1 to 100 miligrams?
Right now I am of opinion that for conducting current as sort of bridge between anode and cathode that are connected to power source,a alpha radiation with its stronger charge and more concentrated area of effect would be stronger than beta,but thats different that photoionisation in air,there is no anode,cathode or power source,becose like I written at the begining,the studies said that electrons specificaly have greatest impact,a alpha radiation is helium 4 nucleus without electrons,while its strongly charged,it doesnt "bring" new electrons into the focal point area like beta emiter would.
To clarify,I ask what type of radiation will make the air more opaque to the laser beam,the problem in photoionisation threshold is to first get the gas to start absorbing the beam by turning opaque.I understand alpha radiation have more ionisation energy,but does that translate to making the air more opaque than beta?
Just want to mention that dust particles, lower photoionisation threshold by multiple orders of magnitude! Thats massive difference,and it doesnt matter if the particles are conductive or electrical insulator,thats becose photoionisation is much different than your spark gap ionisation with electrodes.Its about absorbtion of photons,not electrical conductivity.
 A: The ionization density of alphas is much higher as they range out than that of electrons. So, if your need it to make a region of gas significantly ionized then alphas are a good bet.
On the other hand, alphas range out over very short distances even in air while betas will carry for modest distance, so if your need to to have a non-trivial length of track with some ionization then betas are a better choice.
A: If you actually have both of these sources, or if your design is very sensitive, then you should just do the experiment.  One possibility is that each source has a slightly different geometry where it is most effective at producing ionization.  My intuition is that an alpha source would be better, but I wouldn't be surprised either way.
The conduction electrons in the air don't come from the radiation source directly, but from its interactions with the air.  Ionizing one air molecule takes some finite amount energy that you can look up, probably from the NIST data websites, one the scale of about 10 eV.  So you can roughly predict the number of electron-ion pairs produced in air per decay by looking up the energy that the charged particles carry away.
Americium-241 emits 5.4 MeV alpha particles.  Cesium-137 emits beta particles with a mean energy of a few hundred kilo-eV. So if you treat the scattering probabilities the same way (not correct, but not terrible) then you'd expect perhaps twenty times as many electron-ion pairs in the air from each alpha particle than from each beta particle.
However there are lots of sneaky things that can happen in systems like this --- which is why if you really care, you should obtain both sorts of source and make a comparison measurement.
