Design of a positron microprobe using magnetic lenses

Abstract

We have designed a positron microbeam using magnetic lenses based on the commercial scanning electron microscope (SEM). Employing a source with a diameter of 1 mm and an immersion lenses, the diameter of primary beam can be reduced to 200 µm. This could be further reduced to less than 10 µm using the SEM optics. By introducing a beam pulsing section, positron lifetime measurement using a pulse beam will be also enabled. The extraction of positron beam from the magnetic field was simulated numerically.1 Introduction Lateral scanning of a positron microbeam can provide the spatial distribution of positron annihilation near the subsurface region. This technique is useful for the study of small domains, such as cracks, whiskers and voids. Several positron microbeams have been already developed [1][2][3][4][5][6][7]. Munich group successfully formed a positron microbeam of diameter of 2 µm using the remoderation technique and an in-lens type objective lens [1][2][3]. Bon group also successes to form a positron microbeam of diameter of 20 µm using small source and an out-lens type objective lens of the scanning electron microscope (SEM) [4,5]. To obtain such positron microbeam, a well-focused and monoenergetic positron beam and an objective lens, which is commonly used in the electron microscopy, are required. We have attempted to form the positron microbeam using a high brightness positron beam for the positron diffraction experiment [8,9]. The beam diameter is ~80 µm and counting rate is ~50 cps, which is not enough for the detail two-dimensional scan of samples. In addition, positron lifetime measurement is hardly carried out because of the DC beam. In this work, we therefore designed a new positron microbeam and carried out the feasibility study.

Defect imaging and detection of precipitates using a new scanning positron microbeam

“…At Munich a scanning positron microscope for lifetime measurements has been developed providing a diameter of 20 µm [47] which could be improved to 5 µm with a count rate of 100 cps and, at the expense of count rate, further reduced to about 2 µm [37]. In Japan, a positron microbeam with a diameter of 80 µm and count rate of 50 cps [48] was improved to a beam diameter of about 4 µm with 30-40 cps for positron lifetime or DB measurements [49].…”