Patent Number: 
Section: description

In FIG. 1, a DC positron beam 14 is generated in a vacuum chamber 26. The positrons can be produced by moderation of positrons emitted from radioactive isotopes, or they can be produced as a positron beam from an electron linear accelerator, for example. More particularly, the beam 14 is made up of positrons having a monochromatic energy of, for example, 10 eV. The positrons move in approximately the same direction due to the presence of a magnetic field B. The positrons 14 strike the sample 17, which may be a thin solid film, a film substrate interface, or a substrate, for example. The sample 17 is mounted in a metallic sample holder 18 that can be maintained at a selected electrical potential. The sample is thus maintained at the sample holder potential. When the mono-energetic positrons 14 bombard the sample 17, secondary electrons 15 are ejected from the sample. The detection of the secondary electrons 15 by a shielded multi-channel plate (MCP) electron detector 20 is used to generate the start signal for the positron lifetime spectrometer. The MCP detector 20 is typically mounted in an electrically shielded housing 21 that is maintained at a potential V2. The. MCP electron detector 20 and housing 21 comprise a MCP electron detector assembly 27. The stop signal of the spectrometer is derived from the detection of the gamma radiation 22 from the positron annihilations. This is accomplished with a scintillation detector 24, such as a BaF2 detector, for example, and an associated PM tube 25. The scintillation detector 24 is shielded by a collimator 23 that is used to align the detector 24 with the sample 17. Measurements of the number of annihilation events as a function of the difference between the start time and the stop time are then used to construct the positron lifetime spectrum. The above described elements and functions are considered to be known in the art. The present invention has to do with providing a means for simultaneously accelerating both the incoming positrons and the secondary electrons by a single potential difference between the sample and an entrance grid in front of the sample. The invention also separates the path of the secondary electrons from that of the positrons by tilting the acceleration direction away from the incident positron direction. More particularly, as shown in FIG. 1, the incident positron beam 14 is initially passed through an entrance grid 16 that has been carefully positioned in front of the sample 17. The incident positrons 14 are accelerated to the desired implantation energy by the potential V1 applied to the sample holder 18. The potential V1 is lower than the potential V2 applied to the entrance grid 16. The potential difference V1xe2x88x92V2 accelerates the secondary electrons 15 away from the sample 17 toward the multi-channel-plate (MCP) electron detector 20. The entrance face 19 of the electron detector assembly 27 is positioned parallel to the sample 17. The energy of the secondary electrons 15 is distributed in a range between approximately 0 and 40 eV, depending on the sample material. For example, a 10 eV spread of energy would cause hundreds of psec worse time resolution for 600 V acceleration. With the acceleration of electrons provided by this invention, for example 10 kV, the time spread induced by the 10 eV electron energy uncertainty is reduced to less than 100 psec. Another benefit of the simultaneous acceleration method is that no additional bias is needed for accelerating the electrons, which would decelerate the incoming positrons. Another advantage of the simultaneous acceleration (dual acceleration) method is less contribution of backscattering positrons since the amount of apparatus in front of sample is minimized. Because of this advantage, no abnormal or xe2x80x9cghostxe2x80x9d structures are produced. FIG. 1 also illustrates a second major aspect of the invention, the separation of the secondary electron path from the path of the incoming positrons. The sample 17, sample holder 18, entrance grid 16, and entrance face 19 of the multichannel plate electron detector assembly 27 are made to be parallel to each other, and are arranged at a tilt angle xcex8 to the axis of the positron beam 14. Because of the potential difference V1xe2x88x92V2, the incoming positrons 14 strike the sample 17 approximately normal to its surface. This is shown in more detail in the computer simulation graph of FIG. 2. The tilt angle xcex8 of the sample 17, sample holder 18, entrance grid 16, and entrance face 19 of the multichannel plate electron detector assembly 27 effectively separates the path of the secondary electrons 15 from the path of the incident positrons 14. The tilt angle is selected such that the electron detector assembly 27 does not block the incoming positron beam 14. An angle of about 45 degrees was found to be practical in some test embodiments of the invention. A large number of positron beam characterization facilities have found it useful to maintain the sample at ground or positive electrical potential. In our invention, V1 can be maintained at 0 V or at a positive potential. The potential V2 would still be maintained at a higher potential than V1. FIG. 2 is a computer simulation graph illustrating the trajectories of both the incident positrons 14 and the secondary electrons 15 in the spectrometer. In this calculation, the incident positrons have an energy of 10 eV. The energy spread of secondary electrons initially emitted from the surface is considered to be 0-10 eV. The sample potential is xe2x88x9210 kV and the potential applied to the entrance grid is 0 V. A 50 gauss magnetic field is assigned to the system, with its direction parallel to the direction of the incoming positrons. The tilt angle xcex8 is 45xc2x0. The trajectories show that the flight paths of the electrons and the positrons are clearly separated. Considering the distance between the entrance grid and the sample to be 10 mm, and the distance between the entrance grid and the electron detector to be 60 mm, the time spread induced by the 10-eV energy spread of the secondary electrons is only 51 psec in this calculation. FIG. 3 is a block diagram illustrating a conventional delayed coincidence timing system that may be used with the DC beam positron lifetime spectrometer of this invention. The secondary electron signal is obtained through a capacitor 31 coupled to the MCP electron detector 20 and sent to the start input of time-amplitude-converter (TAC) 32. A time delay module 33 may be inserted between the MCP detector 20 and the TAC 32 if it is desired to use the electron signal as the stop signal. The gamma radiation signal from the PMT 25 is shaped by a constant fraction differential discriminator 34 and then sent to the stop input of the TAC 32. A computer 35 records and stores the number of events as a function of the difference between the start time and the stop time positron lifetime spectrum. FIG. 4 is a representative positron lifetime spectrum of a Cu-capped porous SiO2 film measured using the spectrometer of the present invention; The positron energy on the sample is 7 keV. The time resolution for the measured spectrum is 570 psec. Two lifetime components are observed: 465 psec and 1.9 nsec, shown as straight lines. A time range of 25 nsec is shown in FIG. 4. The remainder of the data, out to 300 nsec, is not shown in FIG. 4, but is completely without abnormal structures. Positron backscattering, which can cause abnormal structures in some spectrometers, is not a problem in this invention because no ExB separator is used in front of the sample. The 570 psec or even higher time resolution is achieved by the elimination of most of the time spread induced by energy distribution of the secondary electrons. In the valid time range of this test of the invention (0-300 nsec), the signal-to-noise ratio was 3300. While there has been shown and described what are at present considered the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications can be prepared therein without departing from the scope of the invention defined by the appended claims.