6. A LiF TLD chip gives a reading r after receiving an absorbed dose of 4.5 Gy to the LiF from 60Co rays. Later the same reading is obtained after the chip is struck perpendicularly by a beam of 500-keV electrons on its 3.2 3.2-mm2 face. The chip thickness is 0.9 mm. What is the electron fluence, neglecting backscattering