A radiation detector is used to monitor the radiation from a radioactive source. A ratemeter records the number of counts per minute.
The source is brought close to the detector. The table below shows how the ratemeter reading varies with time.
The bottom row gives the count rate due only to the source.
Fill in the missing values.
Even when the source is a long way from the detector, the ratemeter registers a reading of 
. Suggest a cause of this .
