Why debian Packages in Archive folder get deleted Automatically in Ubuntu

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http://askubuntu.com – I am using ubuntu 12.04 LTS. I want to save downloaded packages using aptoncd, so that I don't need to download all the packages again when I reinstall OS. Now, the problem is last week when I checked my /var/cache/apt/archives folder, there were more than thoousand packages about 1 GB in Total. Yesterday, when I again checked the archives folder, it had only 451 packages left about 500 MB in total. So, can anyone tell me how the packages in the Archives folder get deleted ?? 'cause I didn't delete any of them. Now, I can't take backup of those packages, 'cause I don't know which packages ar (HowTos)