A process for manufacturing a joint between two metal portions delineating an inner space, such junction allowing to obtain a high or ultra-high vacuum sealing and a radiofrequency contact both flush with the surface delineating the inner space, comprising the steps of: • providing a metal gasket the cross-section thereof has: a sealing head with conical shape, with a flat base constituting an inner face of the gasket and a conical body the upper and lower edges thereof (5, 6), adjacent to said base, are intended to be tightened between two metal portions therebetween a gasket compartment is formed having a pre-fixed depth, and a centering tail extending outwards from a vertex of said head opposite to said base, in perpendicular direction to said base for an extension so that the overall cross width of the head and of the tail correspond to said prefixed depth, the angle of opening (2α) of said conical shape being lower than 60°, and the head and the tail constituting a single body, without discontinuity, so that the gasket is constituted in a single piece; • arranging said gasket in said gasket compartment by resting the distal end of said tail onto the bottom of the gasket compartment so that the inner face of the gasket is flush with the surfaces of the metal portions to be joined by implementing therewith a continuous surface, in the profile of said tail and/or in the surface of said compartment suitable discontinuities being provided, so that the areas of the compartment above and below said tail are in communication therebetween; • tightening the two metal portions one against the other one by implementing the vacuum sealing and the radiofrequency contact and by interrupting the tightening before deforming the inner face of the gasket, so as to prevent a possible discontinuity between the joined surfaces. • providing a metal gasket the cross-section thereof has: a sealing head with conical shape, with a flat base constituting an inner face of the gasket and a conical body the upper and lower edges thereof (5, 6), adjacent to said base, are intended to be tightened between two metal portions therebetween a gasket compartment is formed having a pre-fixed depth, and a centering tail extending outwards from a vertex of said head opposite to said base, in perpendicular direction to said base for an extension so that the overall cross width of the head and of the tail correspond to said prefixed depth, the angle of opening (2α) of said conical shape being lower than 60°, and the head and the tail constituting a single body, without discontinuity, so that the gasket is constituted in a single piece; • arranging said gasket in said gasket compartment by resting the distal end of said tail onto the bottom of the gasket compartment so that the inner face of the gasket is flush with the surfaces of the metal portions to be joined by implementing therewith a continuous surface, in the profile of said tail and/or in the surface of said compartment suitable discontinuities being provided, so that the areas of the compartment above and below said tail are in communication therebetween; • tightening the two metal portions one against the other one by implementing the vacuum sealing and the radiofrequency contact and by interrupting the tightening before deforming the inner face of the gasket, so as to prevent a possible discontinuity between the joined surfaces.