Radiation units are fiendishly tricky to convert between. Here, the only indication is that after decontamination their hair was still reading 300 counts per minute. CPM are instrument-specific and doesn't mean that's the correct number of disintegrations per second, nor easily converted to absorbed dose units, and this is after decontamination, and disregarding the amount of water they ingested.
All that disclaimer aside: a banana produces about 15 Bq (which is s^-1), i.e. 900 cpm.
All that disclaimer aside: a banana produces about 15 Bq (which is s^-1), i.e. 900 cpm.