The intensity of gamma radiation emitted by a radioactive source decreases with distance. A student uses a Geiger-Müller tube and a counter to measure the radiation at different distances.
The relationship between the recorded count rate and the distance from the source is given by the formula:
count rate=kd2 \text{count rate} = \frac{k}{d^2} count rate=d2kwhere d d\,d is the distance from the source in metres (m) and k k\,k is a constant.
When the Geiger-Müller tube is placed 0.60 m from the source, the recorded count rate is 75 000 counts per minute75\,000\text{ counts per minute}75000 counts per minute.
Calculate the count rate registered by the detector when it is moved to a distance of 1.5 m from the source.