A radioactive substance of half-life 6 min is placed near a Geiger counter which is found to resister ${\rm{1024}}$ particles per minute. How many particles per minute will it register after ${\rm{42}}\,{\rm{ min}}$ ?

${\rm{8 }}\,{\rm{per }}\,{\rm{min}}$

${\rm{4 }}\,{\rm{per }}\,{\rm{min}}$

${\rm{5 }}\,{\rm{per }}\,{\rm{min}}$

${\rm{7 }}\,{\rm{per }}\,{\rm{min}}$

JEE PHYSICS MCQ: NUCLEI

1 Medium

1 of 1

Application +4 | -1

A radioactive substance of half-life 6 min is placed near a Geiger counter which is found to resister

\mathrm{1024}

particles per minute. How many particles per minute will it register after

\mathrm{42}\,\mathrm{ min}

View answer and explanation

Correct option: B

\mathrm{8 }\,\mathrm{per }\,\mathrm{min}

Option B (

\mathrm{8 }\,\mathrm{per }\,\mathrm{min}

) is correct because after 42 minutes (7 half-lives), the activity reduces to 1/128 of the initial activity.