We present the design, construction and performance of a compact subthermal neutron source to be operated at the Bariloche Electron LINAC. The design was based on the premise of keeping the moderator temperature stable at 77 K for at least seven hours without refilling liquid nitrogen. Two moderator materials in semi-cylindrical geometry were studied: polyethylene and mesitylene. The effective moderator volumes were optimized by Monte Carlo simulations. Experimental data and calculations corresponding to the neutron production as a function of the neutron energy for those materials at room temperature and at 77 K were compared. We observed that the rate of subthermal neutrons, compared to epithermal neutrons, increases up to five times for both moderators when they are cooled down.