  A custom-made 228Th source of several MBq activity was produced for the
Borexino experiment for studying the external background of the detector. The
aim was to reduce the unwanted neutron emission produced via (alpha,n)
reactions in ceramics used typically for commercial 228Th sources. For this
purpose a ThCl4 solution was converted chemically into ThO2 and embedded into a
gold foil. The paper describes the production and the characterization of the
custom-made source by means of gamma-activity, dose rate and neutron source
strength measurements. From gamma-spectroscopic measurements it was deduced
that the activity transfer from the initial solution to the final source was
>91% (at 68% C.L.) and the final activity was (5.41+-0.30) MBq. The dose rate
was measured by two dosimeters yielding 12.1 mSv/h and 14.3 mSv/h in 1 cm
distance. The neutron source strength of the 5.41 MBq 228Th source was
determined as (6.59+-0.85)/sec.
