It may be important for your purposes to know only approximately the neutron fluence rate from an AmBe source or you may need data about a specific source. See NBS Handbook No. 72 (1960) (= NCRP Report No. 23) for some discussion of sources and their fluence rates. NBS Handbook No. 85 (1964) (= ICRU Report 10b) may also be of help.