what is a positron moderator? · ¾the important criterion for a moderator foil to be used in the...

The advent of slow positron beams has resulted in nondestructivedepth 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 reflectiongeometry.

W (100) single-crystal foil has been a workhorse for use in positron beams as moderators.

What is a positron moderator?

Different Moderator geometries

(courtesy Paul Coleman)

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 -2.48 eV.

Work function of electron and positron

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 ~ 10-4.

Polycrystalline tungsten foils have also been used successfully as moderators.

Solid rare gas moderators of Ne, Ar, Kr and Xehad also been reported with efficiencies in the range 10-3 – 10-2. 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)

(courtesy Paul Coleman)

(courtesy Paul Coleman)

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

Radioactive Na-22

positron source

Moderator foil

W(100)

Electrostatic extraction &focusing

slow/fast filter and

Electrostatic or magnetic

transport

Target chamber

HPGeDetector

- HV

0-20 kV

End point energy 545 keV

Fast + slow positrons slow positrons

Tunable positron energy 0-20 keV

Fast + slow positrons

Sample at negative high voltage

Detects the 511 keV

annihilation signal

MCA

Records the Doppler

broadened annihilation spectrum at each positron

beam energy (Ep)

W (100) foil in a variable low energy positron beam

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 ?

Analysis chamber

Moderator for analysis

Electron Gun

Source

Moderator W(100)

Soa Gun

SampleZoom Lens

Mirror Chamber

Linear Transporter

3”x3”NaIdetector

Auger CMA

Determination of Φ+ using electrostatic beam at Brandeis

Experimental determination of φ+

Positron yield of single crystalline W foil

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

Base Pressure 4x10-10 torr

Electron beam heating @50 W, 2 Min ; 2400 K

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

Differential longitudinal energy spectra of reemitted positron beam atdifferent times after moderator foil surface preparation

Degradation in the beamis associated with long term accumulation of carbon on the surface

G. Amarendra et al., J. Appl. Phys. 80, 4660 (1996).

Comparison of differential energy spectra of reemitted positron beam after annealing foil in oxygen and hydrogen atmosphere

Reduced surface contamination after hydrogen treatment.

Reduction in FWHM of the beam

Intense Positron beamline at AIST, Tsukuba, Japan

AIST, Tsukuba, Japan

AIST, Tsukuba, Japan

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

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).

Positron reemission from tungsten surfaces

R-parameter (bar) and Ps fraction (dot and line) on polycrystalline W for various heat and gas treatments

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).

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

Variation of normalized S-parameter for virgin and annealed W foil with positron beam energy

Variable Energy Positron Beam – S-parameter study

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

Auger spectra of W foil in virgin and annealed conditions

Auger Electron Spectroscopy (AES) study of W foil

Concentration depth profile of carbon for virgin and annealed W foil

G. Amarendra et al., Phys. Rev. B 69, 094105 (2004).

Secondary Ion Mass Spectroscopy (SIMS) study of W foil

Kr moderator in the high brightness beam at Brandeis

Schematic of the modified primary gun for incorporating a Kr moderator

Retarding field measurements with NaI (Tl) detector

D. Vasumathi et al., Appl. Surf. Sci. 85, 154 (1995).

Summary W(100) is stable and robust moderator yielding ε ~ 10-5, with quite small ET.

Inert gas moderators provide high ε ~10-3 but ET is quite large -moderator preparation is rather involved.

Need for more focused research for higher efficiency moderators.