The Google NGRAM graph above shows the relative frequency for use of the term "neutron depth profiling" each year in print articles from 1975 to 2019.
A few words of caution in interpreting the graph. The graph timeline can be extended back to the year 1800. And, there would appear to be uses of the phrase "neutron depth profiling" many years before 1981, however that does not seem to be true. Instead appearance of the phrase before 1983 (believed to be the first use of the term) indicates, for instance, when a journal began publication or the journal copyright date. An example is there appears the use of the term in a Chinese Journal for the year 1933, and that is the journal copyright date and not an article date that includes the term for NDP.
Another caution is that the term NDP, meaning "neutron depth profiling" can not be reliably search using the Google NGRAM Viewer. The term "NDP" has many other meanings, simply look at the WIKIPEDIA listing for NDP. Therefore, a search for the term "NDP" will include all uses of this acronym and not only for our "neutron depth profiling" analytical analysis technique.
Also note, It is quite likely, the reason for the reduced use of "neutron depth profiling" in publications beginning about 1999 is that authors use the term "NDP" exclusively in the publication and do not spell out "neutron depth profiling" in the article. The number of articles published using the technique has remained constant or increased in the years after 1999.
To open a web link to the Google site NGRAM Viewer and apply additional variables click here.