Radioactive Emission: Determining Half-Life

[henryc09]

Homework Statement

A radioactive source emits electrons. The number of electrons emitted per second
N(t) decreases with time according to the equation:

dN/dt = −1.8 × 10⁻⁵N(t) ,
where t is expressed in seconds. If N = 12 electrons/sec at t = 0, after what time t
has N fallen to the value 4.0 electrons/sec? [7]
What is the half-life of the radioactive source in seconds? [3]

Homework Equations

The Attempt at a Solution

I have the first part done, but am confused about the second part. How can you tell the half life of the material from the information given? N is just the amount of electrons given off per second, and the half life is the amount of time taken for the amount of substance to halve right? I don't see how they can be directly related. Any ideas?

 

[Stonebridge]

One way is to just work out how long it takes for the number of electrons given off per second to drop from 12 to 6. That represents the half-life time.

 

[henryc09]

OK I was thinking that must be the only way. Thanks :)