The important properties of thermal ovens for producing intense beams of F atoms by thermal dissociation of are analysed. After reviewing previous constructions a new source made of a single crystal of is described and characterized. This source has been tested for more than 3000 h at temperatures up to with a mixture of 10% in Ar at pressures up to 12 bar. A degree of dissociation of 50% and a F atom beam velocity spread of about 7% were achieved.