The minority‐electron diffusion length in commercialn+psilicon solar cells was determined from measurements of transient photocurrent generated by low‐energy x rays. The results show an order of magnitude increase in diffusion length over a time interval inversely proportional to the square of the injection rate. This accounts for the occurrence of dose rate sensitivity of silicon detectors of x rays at very low injection rates. The strong rate dependence observed suggests a space‐charge trapping mechanism.