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Re: [Phys-l] Ionization type smoke detectors.

At 01:21 PM 9/28/2006, BC, you wrote:

too cryptic; slowed or stopped would involve transfer of energy to the
particle likely resulting in further ionization increasing the current
instead of a decrease. The e attachment is the dominant effect
evidently. Not incidentally, this corroborates the previous
descriptions, as the attached es are still ions, just slower (less mobile).

Let's see if I can make this naive thought process more transparent then:
though I cannot claim to have quite the insight of people who have
been involved with Plutonium by-products, one of which is
Americium 241 in the Neptunium breakdown series.

Americium 241 emits helium nuclei at about 5.48 Mega electron volts.
Charged particles of this energy can travel about 0.04 m in air.
We may ask the question, how far would a Helium nucleus travel
through a carbon particle? It turns out that the relative stopping
power for carbon is not very much greater than for air.
Very well then: how many charged species might one expect
from a single alpha? Taking the simplistic ratio
of alpha energy - 5.48 MeV divided by the first ionisation
potential of O or N - let us say 14 eV : we estimate
5.5E6/14 or around 400 thousand charges.
We would expect these ions to drift
rather slowly to the electrodes, impeded by numerous collisions,
It appears then, that the sensing of a smoke event depends on replacing
a certain number of air molecules (so to speak) with a similar number
of carbon particles. On general principle one supposes a smoke particle
might be as fine as 0.1 micron or as large as ten microns (I plucked these
sizes out of the air, knowing only that cigarette smoke has been said
to penetrate ultra fine filters formerly used on disk drives.)

In this situation, the carbon particle might be thought of as still in space,
compared to the speed of the positively ionized air molecule - one
supposes that the electrons saltate even more rapidly towards the anode,
as compared with the slow drift of the molecules and the glacial
movement of the soot.

This simple model suggests that fixing the positive ions due to smoke ingress
might essentially halve the current.


Fixed (permanently installed) unvented heaters are illegal in CA.
Ha! At first this looks like
Thesis: kerosene heaters are available at most hardware stores.
Antithesis: Fixed unvented heaters are illegal in California.

Perhaps I can offer another understanding:

1) Kerosene heaters are available at most hardware stores
2) Fixed unvented heaters are illegal in California
3)The kerosene heaters available in most hardware stores are
unvented and (nominally) portable.

You will recognize this as the usual format of a syllogism.

And this is as far as I can sensibly go in this discussion, I suspect.

Brian W

>>... stopping the Alphas would have the
>>opposite of the intended effect, a "proof" of his explanation.
> [Brian]
>Alphas are stopped in large carbon particles - Oh, all right,
>greatly slowed?


>>... I discovered a very curious effect when I lived in an
>>old flat in a depressed part of England. I would use a liquid paraffin
>>heater when I ran out of coal. /// I discovered the heater would
>>flame out after a time. /// I suspect if the heater was not self
>>extinguishing w/
>>low O2 supply, I wouldn't be writing this.

>Kerosene unvented heaters are on sale at most hardwares in the US.
>The makers apparently take care to minimize the lethal CO production
>(boasting of "blue flame") and the CO2 production is physiologically
>observable - unlike the O2 depletion. This might be a factor in
>BC's survival.

Ah yes: I have observed people pass out insensate and without resistance
in hypobaric chambers when the oxygen supply is lost. I have observed
people pant energetically when the CO2 level increases.
I think this must be a debatable point - because I have heard people
claim similar observables for low O2 as for high CO2 just as Mike did
earlier in this thread.

Brian Whatcott Altus OK Eureka!