Ben Crowell wrote:
> Bryan Derksen wrote:
>> Ben Crowell wrote:
>>> It would make sense for it to be nuclear, since nothing else would
>>> give the necessary energy density. But a nuclear reactor is still
>>> a heat engine.
>>
>> Could be generating electricity directly from beta decay.
>
> I'll admit the theoretical possibility that beta decay could be used
> to create electrical energy without a heat engine, but I don't think
> it could possibly do the job here. To get the kind of power output
> we're talking about from beta decay, rather than a chain reaction,
> you'd need a blazing-hot beta source with a short half-life, and
> I'd think the amount of material would have to be extremely large --
> large enough that all the betas would be stopped internally in the
> source, and their energy thermalized, so you'd end up with a heat
> engine again.
The kinetic energy of the electron in beta decay has a spectrum (since
the decay energy is shared with a neutrino) that has a maximum of about
1 MeV = 1.6 x 10^-13 J -- let's use that as a typical energy to get a
conservative estimate for the next figure.
To get a "few gigawatts," then you need to have 2 x 10^22 of these
decays happen every second. SI even has a unit for this -- the
becquerel -- so your radioactive power source will have to have an
activity of 2 x 10^22 Bq (20 ZBq if you want to use a neat prefix).
As a comparison, the total amount of radiation released in the Chernobyl
accident is about 14 EBq -- 1.4 x 10^19 Bq. You'd be walking around
with a thousand Chernobyls in your pocket. (And that's a deliberately
underestimate of the required activity -- typical electron energies in
beta decay are more like 0.2 MeV.)
--
Erik Max Francis && max@[EMAIL PROTECTED]
&& http://www.alcyone.com/max/
San Jose, CA, USA && 37 18 N 121 57 W && AIM, Y!M erikmaxfrancis
I needed sun****ne in my day / Something to wash away the pain
-- Zhane
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