The county commissioners of each county joining in establishing detention facilities as provided for in this subchapter may make appropriations or incur or increase the indebtedness of the county, in the manner provided by law, to an amount sufficient to pay its proportionate part of the cost of acquiring a site and of erecting, constructing and equipping the joint detention facilities by issuing coupon bonds at a rate of interest not exceeding 7% and payable within 30 years from the date of issue. The county commissioners of the county shall levy an annual tax in an amount necessary to pay interest and sinking fund charges upon such bonds.