CC2650 very accurate Bluetooth interrupt on RX event?


I only can find examples where a callback is called when a Bluetooth packet is received. 

Of course this caues more delay and is less accurate than a direct interrupt when a packet arrives. For a project I need a very accurate RX timestamp. 

Can you please give me a hint how to access the RX interrupt directly? Is there maybe a even better idea to have a very accurate timestamp when the packet arrives?