Positron beam

Reasons for using a beam

The high sensitivity of positrons to open surface defects makes desirable using Positron Annihilation Spectroscopy (PAS) for studying the defect strucure of thin foils and sub-surface layers. This is impossible with e+ directly coming from a radioactive source of  22Na, since they are very fast  (kinetic energy up to 0.5 MeV) and are implanted very deeply (tens of microns) into the sample. The chance of annihilation near to the free surface or even emission as positronoum (Ps) from the surface is practically null. The remedy is to use a monoenergetic beam, tunable from 0.1 keV up to 20 keV. This allows one to explore selectively subsurface layers from a few tens of nanometres up to about 1 micrometer (in Si). The depth resolution is limited by the width of the implantation profile and by the diffusion of the positrons after thermalisation. Most positrons implanted at less than about 100 nm may return to the surface by diffusion and be emitted as bare positrons or as Ps. This is interesting for obtaining Ps in vacuum.

The L-NESS beam

The beam is a recent acquisition by L-NESS, operative since 2010. It gives a positron current at the sample 103 e+/s on a spot of about 5 mm, with energy tunable from 0.1 to 20 keV. It is a fully electrostatic system, comprised of the following parts: a) Primary radioactive source (15 mCi  of 22Na); b) Moderator (W foil, 1 micrometer thick), where a small fraction (about 10-3) of the positron flux thermalises and is emitted from the surface at the energy corresponding to the negative workfunction  of the positrons in W (1.5 eV); c) electron optics for transport of fixed energy (1.5 keV); d) energy filtering by beam bending (suppression of the high energy background); d) electron optics for energy tuning and final focusing. Automatic energy scan is implemented. Planned addition: side chamber for periodical moderator annealing without breaking the vacuum.

Slow positron beam. 1. Radioactive source; 2. Electrostatic optics; 3. Sample chamber; 4. HpGe detectors; 5. Cryostat; 6. High voltage protection cage; 7. Power suppliers; 8. Detector electronics.

The positron beam is equipped with HpGe detectors for momentum distribution measurements, which can be operated in both single and coincidence mode. Samples are kept in high vacuum (~10-9 mbar) and their temperature can be varied from 10 to 1100 K. The slow positron beam has been calibrated for positronium fraction measurements.

Depth Profile

Makhov profile showing the implantation profile as a function of positron energy in Si. The dashed lines correspond to the mean penetration depth z.

more information about positronbeams