Preventing time machine from thinning

I am pondering the idea to use Time Machine as an archiving tool for our lab data. I.e. I am not interested in disaster recovery, but in keeping a complete copy over every night’s status quo on an externa hard disk. I know how to set Time Machine to make a copy every night . What prevents me from using Time Machine right now, is that it reduced the density to one kept copy every week after a month by deleting old backups. Time Machine calls that “thinning.”

I am sure the rules for this are stored in some file. Does anyone know where and what setting, I have to change, to achieve the effect I want?