Patent Number: 
Section: claims

1. A spectrometer for positron lifetime characterization studies, wherein a DC positron beam is directed onto a sample surface to produce annihilation gamma radiation and secondary electrons from the sample, the annihilation gamma radiation is detected by a gamma detector, and the secondary electrons are detected by an electron detector, comprising: wherein: an entrance grid situated in the incident positron beam, said entrance grid positioned parallel to the sample surface, and said entrance grid arranged to have a higher electrical potential than the sample potential;  the entrance face of the electron detector assembly is situated parallel to the entrance grid,  the entrance face of the electron detector assembly is arranged to have the same potential as the entrance grid,  and the sample surface, said entrance grid, and said entrance face of said electron detector assembly are disposed at a tilt angle to the incident DC positron beam. 2. The spectrometer of  claim 1  wherein said tilt angle is such that the electron detector does not physically block the incoming positron beam. claim 1 3. The spectrometer of  claim 1  wherein the secondary electrons generated by the bombardment of the primary positrons on the sample start the positron lifetime clock, and the detection signal of the annihilation gamma radiation of the positrons stops the positron lifetime clock. claim 1 4. The spectrometer of  claim 1  further including a time delay that delays the secondary electron signal so that the detection signal of the annihilation gamma radiation of the positrons starts the positron lifetime clock, and the secondary electrons generated by bombardment of the primary positrons on the sample stops the positron lifetime clock. claim 1