During a materials test, a researcher is at a monitoring position behind a shield. Without the shield, the dose rate at this position would be 4.0 μSv4.0\ \mu\mathrm{Sv}4.0 μSv per minute. The shield transmits 20% of the radiation. The researcher receives a dose of 6.4 μSv6.4\ \mu\mathrm{Sv}6.4 μSv.
What is the exposure time?
0.32 minutes0.32\ \mathrm{minutes}0.32 minutes
2 minutes2\ \mathrm{minutes}2 minutes
1.6 minutes1.6\ \mathrm{minutes}1.6 minutes
8 minutes8\ \mathrm{minutes}8 minutes