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Positronium Annihilation Lifetime Spectroscopy (PALS) Basics

PALSMeasures positronium (Ps) annihilation lifetimes and intensities, which can be related to the size and amount of defect structures, such as voids or pores in the range of several angstroms to tens of nanometers.   Suitable for insulating materials while not good in the bulk of metals since Ps cannot be formed in the latter.   

Beam-PALS:  A beam of monoenergetic low-energy positrons (several keVs) are used for thin film study.  Thickness of the film can vary from several nanometer to a few microns.  The energy of the implanted positrons are tunable, which enables PALS to depth-profile thin films.  For comparison, positrons typically used for bulk PALS have an implantation depth of 0.2 ~ 0.3 millimeters and as such are unsuitable for probing thin films. Beam-PALS technique is  crucial to the success of this research.

Positronium (Ps):  Neutral atom, bound state of a positron (similar to electron in all aspects except positive charge) and an electron.  

Ps interactions with condensed matter (Figure 1)

  • Reemitted positrons

  • Positron free annihilation:  Lifetime less than 500 picosecond (10-12 sec)

  • Backscattered Ps:  formed by positrons diffuse back to the vacuum and capture an electron on the way.   Lifetime is ~ 142 nanosecond (10-9 sec, ns)

  • Secondary electrons:  positrons can knock off secondary electrons while entering the solid material.  (the signal is used as lifetime start signal in PALS)

  • Singlet state (para-Ps or p-Ps):   lifetime ~ 125 psec, 

  • Triplet state (ortho-Ps or o-Ps) in vacuum: lifetime ~ 142 nsec, 

  • Triplet state (ortho-Ps or o-Ps) in condensed matter: prefers to stay in defects or pores. Lifetime is reduced by interaction with molecular electrons during collisions with the pore surface.   The collisionally reduced o-Ps lifetime is correlated with void size and forms the physical basis for probing pore structure with PALS.

 

Figure 1 .  Positron and Ps interactions with condensed matter

Lifetime measurement

  • Start Signal:  While being implanted into the film, positrons knock off secondary electrons which starts the timing clock. (The time for positrons to form Ps is on the order of picosecond, i.e., 10-12 sec, which can be neglected compared to Ps lifetime on the order of nanosecond.) 

  • Stop signal:  Ps annihilates into gamma-rays, which stops the timing clock,  

Typical PALS spectrum (histogram, Figure 2):  

  • The prompt peak: includes events such as para-Ps, free positron annihilation (each with lifetime less than 0.5 ns).  These signals, which can account for 50 90 % of the total events, unfortunately, do not provide any information about the properties of the materials to be studied 

  • Ortho-Ps components:  exponential decay, each corresponding to a type of ortho-Ps behavior in open-volume in the solid.  The lifetime relates to void or pore size while the intensity relates to the amount of such defects.  

  • Backscattered Ps: Ortho-Ps in vacuum, with lifetime around 140 ns.  Everpresenting signal in beam-PALS in thin-film study.  The intensity is reversely proportional to the beam implantation energy.  

 


 Figure 2.  A typical PALS spectrum with three film Ps lifetimes fitted using POSFIT.  The points for each channel have been connected with a line for visual clarity.  Each channel corresponds to 1.25 ns.  

 

Depth-profiling capability

The major advantage of beam-PALS is the ability to control positron implantation by varying the beam energy.  The implantation distribution for several beam energies is shown in Figure 3 for a density of 1.0 g/cm3.  Samples can be depth-profiled by varying the incident beam energy and hence the mean implantation depth.  This is the critical feature that allows analysis of surfaces, thin films and inhomogeneous films.  

Figure 3.  Positron implantation profile at different beam energies assuming a film density of 1.0 g/cm3.