-- Note that lighter elements will be easier to separate from the decay
product and thus incur less maintenance/recycling overhead.


Let's put the upper weight to 250 g. The greater the energy and longer
half-life, the better (infrastructure related).

Sr-90 (manmade):
Specific power in W/g at t=0: 0.447073
Specific power in W/g at t=28.79 y: 0.223537

	May pose some infrastructure problems due to its relatively
	short half-life. Still, 100g of this emits 447 W, decreasing to 224 W
	after 28.79 years, which should be both long and sufficient enough
	for great amounts of lighting.

Ra-226 (natural):
Specific power in W/g at t=0: 0.0285473
Specific power in W/g at t=1600 y: 0.0142737
	
	This is on the other end of the scale. 1 kg gives us 28 W, but it'll
	last practically forever. It's also likely quite easy to get one's
	paws on.

	(TODO: Find how often it dumps 14C with a decay energy of 28.199 MeV.
		3E-9.
		4.871*(1-(3E-9)) + 28.199*(3E-9) = 4.87100007 - negligible)

Am-241 (manmade):
Specific power in W/g at t=0: 0.114695
Specific power in W/g at t=432.2 y: 0.0573476

	(1 kg gives us 114 W, 57 W after 432 years)

	Americium-241 seems to be a good compromise, at least for lighting
	purposes. Assuming a 20 W light output that is to last 100 years,
	and assuming the lamp holds, with 10 years for infrastructure safe
	margins, 

	W/g at t=110 y : 0.114695 * 2^(-110/432.2) = 0.09615

	20/0.09615 = 208

	The battery should be of mass 208 grams. At the very start, it'll
	dump 23.8W . Our max specified weight 250 grams gives
	27 W at the start, 24 W after 110 years, and 14.3 W after half-life.)

Ar-39 (manmade):
Specific power in W/g at t=0: 0.114249
Specific power in W/g at t=269 y: 0.0571247
Specific power in W/g at t=110 y: 0.086051

	This is very light compared to the other elements (but for all I know,
	quite expensive as well). To power the lamp example above, 233 grams
	is required.

Th-229:
Specific power in W/g at t=0: 0.00651562
Specific power in W/g at t=7340 y: 0.00325781

Specific power in W/g at t=110 y: 0.00644828
	
	10 kg gives us 64 W, and it lasts a long time.

({H-3, Si-32, Ra-226} - {ten-year, hundred-year, millennial} ?)

And just for fun:

K-40 (natural):
(Decay energy avg = 1.3318 MeV)
Specific power in W/g at t=0: 5.531e-08
Specific power in W/g at t=1.277e+09 y: 2.7655e-08

	0.006 W/metric ton, in other words. Forget it.

C-14 (natural):
Specific power in W/g at t=0: 0.00412065
Specific power in W/g at t=5730 y: 0.00206033
	
	4W/kg. Nah.