An apparatus and method for downhole measurement during operation of a drilling system is disclosed. The apparatus (30) comprises: a first sensor (302) configured to detect a property of a subsurface formation (200); a radiation source (304) configured to, when triggered, emit radiation to excite a part (220) of the formation; a processing unit (308) configured to trigger the radiation source if a change of the property of the formation is detected; a second sensor (306) positioned at a known distance from the radiation source and operably connected to the processing unit, the second sensor is configured to detect said excited part of the formation and is more distant from a drill bit (104) of the drilling system than the radiation source is; the processing unit is further configured to, after the excited part of the formation is detected, calculate a rate of penetration of the drill bit based on the following: a time duration between the radiation source is triggered and the excited part of formation is detected, and the distance between the radiation source and the second sensor.