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There is a stream of neutrons with a kinetic energy of 0.0327 eV. If the half life of neutrons is 700 seconds, what fraction of neutrons will decay before they travel a distance of 10 m ?
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The kinetic energy of neutron is $$\displaystyle \frac{1}{2}mu^2=0.0327\times 1.6\times 10^{-19}J$$        --{the unit eV is converted to J by multiplying with 1.6×10−19

The mass of neutron is $$1.675 \times 10^{-27 }$$

Therefore $$\displaystyle u^2=\frac{2 \times0.0327 \times 1.6 \times 10^{-19}}{1.675 \times 10^{-27} }$$    =625×10

∴, the speed of neutrons is u=2500m/sec

Time taken to travel 10 m is $$\displaystyle \frac {\text {Distance}}{\text {speed}}= \frac{10}{2500}=0.004 \: sec$$

$$\displaystyle \frac{dN}{N}=\lambda .dt$$

The fraction of neutron will decay before they travel a distance of 10 meters is  $$\displaystyle \frac{dN}{N}=\frac{0.693}{700}\times 0.004=3.96\times 10^{-6}$$

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