One shot pulse generator (5ns)
There are several "retriggerable monostable multivibrator" ICs in production, which could be used for the purpose stated. Searching for the term in italics above will yield several options. 

For instance, the 74LV123 would meet your requirements: 


Minimum pulse width 3.0 nS for 3 volt operation, 2.5 nS at 5 volts.
Output pulse width configured by external R/C, typically 470 microseconds
Retrigger time 45 nS (3 volts) to 40 nS (5 volts).


There are two monostables in the package in case you need another monoshot output.

The part is available in DIP as well as TSSOP, thus usable for both breadboard and production.



Alternatively, the venerable 555 family of multivibrator ICs provide a simple means of obtaining a one-shot pulse of duration configured by an external R/C:

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If you must use a 555, consider the ultra low power CSS555C timer. Specifically, note the "C" at the end of the part number: This indicates an internal capacitor integrated on the chip, hence only the external resistor is needed for setting the time constant.