Total Radioactivity exposure

I want to work out the total amount of radiation something will give off in it's whole-time, knowing it's halflife.  I can't remember simple calculus stuff, but it's ok coz i seem to remember there was a simple rule:

the rule went something like:
("starting radioactivity" * half-life * 2) * 2/3 = total amount of radiation given off over whole-life.

where "starting radioactivity" is the amount of radiation it's giving off at the start (Per whatever unit of time the Half-Life is also given in).

Anyone else remember what that rule was?

say if you have something that gives off Y Joules of heat every hour.   And it has a half-life of 60 hours.  How much heat will this thing give off in it's lifetime?  About 80 Y?

OP here.   maybe it was:
Total radioactivity = "starting radioactivity" * half-life * 1.5

?

Combining some formulas from Wikipedia, I get:
   num_particles = decays_per_second * halflife / ln(2)
Multiply with the energy per decay:
   num_particles * energy_per_decay = decays_per_second * energy_per_decay * halflife / ln(2)
Or:
   total_decay_energy = energy_per_second * halflife / ln(2)

Are you certain it isn't:

total_decay_energy = energy_per_second * halflife * 2 * ln(2)

?

Why would it be?

And 1/ln(2) = 1.44~, so you weren't too far off in >>2.

thx 4chan!

You are rewarded with this information:

Radioisotope thermoelectric generators (RTGs) are a kindof awesome battery that is sometimes used in space.  They are very simple and convert decay heat from a radioactive substance into electricity.  They supply power non-stop, limited only by the half-life of the material in them.  When you compare them with other compact sources of power, they are quite amazing.

In an RTG, a pellet of Plutonium-238 (halflife of 87 years) would give off approx 593,733.6 kWh/kg (2,137,441.1 MJ/kg) thermal over it's whole life.  As new, it can supply 124.2 W/kg (0.4471 MJ/kg) electric instant, and 136,558.7 kWh/kg (491,611.5 MJ/kg) electric total over it's entire life. 
That's a total energy density 682,794X better than the best Lithium ion batterys.

Strontium-90 (halflife 28.1 years) would give off approx 163,358.9 kWh/kg thermal (588,092.0 MJ/kg thermal) over it's whole life.  As new, it can supply 105.8 W/kg (0.3809 MJ/kg) electric instant, and 37,572.5 kWh/kg (135,261.2 MJ/kg) electric total over it's entire life.
That's a total energy density 188,295X better than the best Lithium ion batterys.

Think of the things you could do with that kindof energy density.  Mechas anyone?

I wants lots and lots of some delectable pot!

Marijuana MUST be legalized.