Half life

[cloak25]

A material undergoes first-order decay by emitting photons. Initially the intensity is 131 nams (1 nam = 1000 photon/sec). If after 18 minutes the intensity is exactly 100 nams, the half-life is:

A. less than 10 minutes
B. between 10 and 18 minutes
C. between 18 and 36 minutes
D. greater than 36 minutes

answer is: D

I'm pretty bad at half life problems. can someone explain this to me? thanks

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[loveoforganic]

1st order decay means a fixed percentage decays per unit time. 31/131 ~ .25 decays in 18 minutes.

In the next 18 minutes, half of what is now present will degrade. 100*.25 = 25

25+31 = 56, which is less than half of 131 (i.e. you have not yet reached 1 half life after 36 minutes)