Answer is: the original dose is 1180 μg.
m₀(Ba-142) = ?; initial mass of Ba-142.
m(Ba-142) = 9.25 μg; mass of remaining at time.
t = 10.6 min.; half-life of Ba-142.
t₁ = 1.25 h · 60 min/h.
t₁ = 75 min.
First calculate the radioactive decay rate constant λ:
λ = 0.693 ÷ t.
λ = 0.693 ÷ 10.6 min.
λ = 0.0654 1/min.
ln(m/m₀) = -λ·t₁.
ln(9.25μg/m₀) = -0.0654 1/min · 75 min.
ln(9.25μg/m₀) = -4.90.
m₀ = 1180 μg.
