What is a positron moderator?

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2 What is a positron moderator? The advent of slow positron beams has resulted in nondestructive depth profiling of defects in surfaces and interfaces, low energy positron diffraction and positron remission microscopy studies on surfaces. The production of slow positron beam is essentially determined by a moderator foil, from which mono-energetic slow positrons are emitted. Moderator foils can be used either in transmission or reflection geometry. W (100) single-crystal foil has been a workhorse for use in positron beams as moderators.

3 Different Moderator geometries (courtesy Paul Coleman)

4 How does a moderator work? The moderator works based on the negative workfunction for positrons in certain solids. Positrons from the radioactive source fall onto the moderator foil. Since, the thickness of the moderator foil is much smaller than the mean penetration depth of the positrons entering the foil, only a few thermalizes, diffuses and reaches the surface. The thermalized slow positrons are emitted from the surface of the foil spontaneously owing to the negative positron workfunction (φ+). The workfunction of W (110) foil is 3.0 ev and that of W (100) is ev.

5 Work function of electron and positron

6 The commonly used foils in the slow positron beam production are the single crystal tungsten foil in (100) or (110) orientation with thickness of a few µm. The important criterion for a moderator foil to be used in the slow positron beam production is the moderator efficiency, ε. It is defined as the ratio of the slow positron flux from the moderator to the fast positrons from the radioactive source. For the W(100) foil, the moderator efficiency is ~ Polycrystalline tungsten foils have also been used successfully as moderators. Solid rare gas moderators of Ne, Ar, Kr and Xe had also been reported with efficiencies in the range These are obtained by depositing neon or krypton or argon layer on a source foil at low temperatures. They have extremely good efficiencies of the order of 10-2 but preparation and maintenance of the foil at low temperatures (~ 40 K) is rather involved process. (courtesy Paul Coleman)

7 (courtesy Paul Coleman)

8 (courtesy Paul Coleman)

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10 Energy distribution of positrons in a β-decay process and the energy distribution of the positrons after moderation.

11 W (100) foil in a variable low energy positron beam Sample at negative high voltage 0-20 kv End point energy 545 kev Fast + slow positrons Fast + slow positrons slow positrons -HV Tunable positron energy 0-20 kev Radioactive Na-22 positron source Moderator foil W(100) Electrostatic extraction & focusing slow/fast filter and Electrostatic or magnetic transport Target chamber HPGe Detector Detects the 511 kev annihilation signal MCA Records the Doppler broadened annihilation spectrum at each positron beam energy (E p )

12 Issues in the use of W(100) moderator As-received W(100) foil needs to be heated at higher temperatures for optimal yield. What is the ideal temperature of heating? How long it needs to be heated? Since slow positron emission is surface process, what is the role of surface chemical impurities? How do they influence the moderation yield?

13 Determination of Φ + Analysis chamber using electrostatic beam at Brandeis Moderator for analysis Source Linear Transporter Moderator W(100) Electron Gun Auger CMA Soa Gun Zoom Lens Sample Mirror Chamber NaI detector 3 x3

14 Experimental determination of φ+

15 Positron yield of single crystalline W foil Electron beam W, 2 Min ; 2400 K Base Pressure 4x10-10 torr a) Annihilation γ-ray counts, b) surface carbon concentration and c) surface oxygen concentration as a function of annealing temperature. Filled circles correspond to values soon after flashing and open circles corresponds to that taken 24 h after flashing

16 a) Annihilation γ-ray counts, b) surface carbon concentration and b) c) surface oxygen concentration as a function of time after eight flashes.

17 Degradation in the beam is associated with long term accumulation of carbon on the surface Differential longitudinal energy spectra of reemitted positron beam at different times after moderator foil surface preparation

18 Reduced surface contamination after hydrogen treatment. Reduction in FWHM of the beam Comparison of differential energy spectra of reemitted positron beam after annealing foil in oxygen and hydrogen atmosphere G. Amarendra et al., J. Appl. Phys. 80, 4660 (1996).

19 Intense Positron beamline at AIST, Tsukuba, Japan

20 AIST, Tsukuba, Japan

21 AIST, Tsukuba, Japan

22 Time spectra of polycrystalline W foil after 2000 C, 10 s annealing with and without oxygen partial pressure of 1 x 10-5 Torr.

23 Time spectra of W (110) after 2000 C, 10 s annealing with and without oxygen partial pressure of 1 x 10-5 Torr. R. Suzuki et al., Appl. Surf. Sci. 149, 66 (1999).

24 Positron reemission from tungsten surfaces R-parameter (bar) and Ps fraction (dot and line) on polycrystalline W for various heat and gas treatments

25 R-parameter (bar) and Ps fraction (dot and line) on polycrystalline W for various heat and gas treatments after seven days of exposure to air. R. Suzuki et al., Appl. Surf. Sci. 149, 66 (1999).

26 Influence of defect-impurity complexes on slow positron yield of a Tungsten moderator Variation of reemitted slow positron fraction (R-parameter) and Ps fraction with annealing temperature

27 Variable Energy Positron Beam S-parameter study Variation of normalized S-parameter for virgin and annealed W foil with positron beam energy

28 Box profiles of the normalized S-parameter corresponding to the WC and W layers and their thicknesses for virgin and annealed W foil

29 Auger Electron Spectroscopy (AES) study of W foil Auger spectra of W foil in virgin and annealed conditions

30 Secondary Ion Mass Spectroscopy (SIMS) study of W foil Concentration depth profile of carbon for virgin and annealed W foil G. Amarendra et al., Phys. Rev. B 69, (2004).

31 Kr moderator in the high brightness beam at Brandeis Schematic of the modified primary gun for incorporating a Kr moderator

32 Retarding field measurements with NaI (Tl) detector D. Vasumathi et al., Appl. Surf. Sci. 85, 154 (1995).

33 Summary W(100) is stable and robust moderator yielding ε ~ 10-5, with quite small E T. Inert gas moderators provide high ε ~10-3 but E T is quite large - moderator preparation is rather involved. Need for more focused research for higher efficiency moderators.