Elsevier

Applied Surface Science

Volume 252, Issue 9, 28 February 2006, Pages 3121-3125
Applied Surface Science

Design of a new type positron beam system

https://doi.org/10.1016/j.apsusc.2005.08.047Get rights and content

Abstract

A new type positron beam system is being constructed in Wuhan university. The goal of this project is to build a positron beam which can measure positron lifetimes and has high moderation efficiency. The system utilizes a magnetically guided incident positron beam and the sample is biased to a high negative potential to achieve the desired implantation energies. A conventional tungsten moderator is replaced by a solid Ne moderator with high moderation efficiency (about 1%). A multi-functional target chamber for slow beam studies is designed, which can be used for positron annihilation lifetime spectroscopy (PALS), Doppler broadening (DB) and coincidence Doppler broadening (CDB) measurements.

Introduction

Slow positron beams are of interest as a probe for nuclear physics and material science investigations [1]. In existing beam systems, they mainly are isotope-based DC positron beams, which only measure Doppler broadening (DB) spectra. Furthermore, the intensity of slow positron beams produced by radioactive sources is very low, due to low moderation efficiency.

High moderation efficiency and long-term stability of positron moderators have been important factors in the development of variable energy positron beams. The moderation efficiency is defined as the ratio of the number of extracted slow positrons to the number of positrons emitted in the source per unit time. The low efficiency of current positron moderators is responsible for the somewhat limited use of the depth selective positron method for the study of surfaces and thin layers [1]. In most cases, annealed very thin tungsten single crystals or polycrystalline foils serve as moderators [2]. Some experiments showed that solid noble gases [3] surpass tungsten in slow positron yield, e.g. solid neon is an attractive moderator that has been used to produce cold positron beam in the study of atomic physics and antihydrogen production [4], [5]. Improvements have also been pursued by developing field-assisted moderators, but without practical breakthroughs so far.

Beam lifetime measurements have previously been made using the timing start signal provided either by the pulsing system of the position beam, either by chopping and bunching a DC positron beam (Schodlbauer et al. [6]) or a pulsed linac beam (Suzuki et al. [7]). These methods can provide time resolutions of the order of 300 ps, but are rather complicated and expensive. The quite high cost and complexity makes it unaffordable for small laboratories (e.g. universities). Lynn et al. [8] developed a relatively simple and low cost spectrometer that utilized secondary electron timing that could be used with a DC positron beam and similar devices have been utilized by other workers (Gidley et al. [9], Szpala et al. [10] and Moxom and Xu [11]). It is based on the use of an appropriate detector (e.g. a microchannel plate, MCP) to detect secondary electrons produced when the positron beam strikes the sample surface and to generate the start signal. The stop signal is provided by one of the annihilation γ-rays.

In this paper, a new type of positron beam system with high moderation efficiency is described. A multi-functional target chamber for slow beam studies is designed, which can be used for positron annihilation lifetime spectroscopy (PALS) using secondary electron techniques and is capable of alternatively performing DB/coincidence Doppler broadening (CDB) measurements.

Section snippets

Positron beam line

We construct a new type of positron beam, which can measure the positron lifetime and has high moderation efficiency. The lifetime spectrometer is similar to that employed by Lynn et al. [8] and Moxom and Xu [11] and also uses secondary electrons emitted from the surface of the sample by the primary positrons to generate timing signals from a DC positron beam. The system utilizes a magnetically guided incident positron beam and the sample is biased to a high negative potential to achieve the

Design of target chamber

Fig. 1 shows that the chamber is a horizontal cylinder with a diameter of 250 mm. The length of the cylinder on the front side of the target position is 200 mm. The cylinder is joined to the 70 mm at exit of first part with a 100 mm one tube of length 500 mm. The manipulator for the sample holder is positioned on top of the cylinder. Two sample holders are used for PALS and CDB measurements.

Fig. 2 shows the geometrical design for PALS and CDB measurement in the target chamber. For CDB measurements

Acknowledgements

This research is supported by the Natural National Science Foundation of China (A03524501, 10475062) and partly by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry.

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