The first isotope of plutonium discovered was plutonium-238. It is used to power batteries for heart pacemakers. A sample of plutonium-238 weighing $2.8 \times 10^{-6} \mathrm{~g}$ decays at the rate of $1.8 \times 10^{6}$ disintegrations per second. What is the decay constant of plutonium- 238 in reciprocal seconds (/s)?