Ps Spectroscopy

We have used our ultraviolet laser (a pulsed dye laser), in addition to a green laser, to ionize a significant fraction of the positronium (Ps) atoms produced by our beamline (read here for more details).

We first tune the UV laser to a wavelength of 243 nm, for which the photons have the same energy as the interval between the ground state and the first excited state of the positronium atom.

We carefully time the laser pulse to pass through the cloud of Ps atoms shortly after they’re created, and so many of the atoms absorb the light and become resonantly excited.  The photons in the green laser have sufficient energy to then ionise these excited atoms – separating the positron and electron.  This technique is known as resonant ionisation spectroscopy (RIS).

The positrons are very likely to fall back into the target and annihilate shortly after ionisation occurs, causing more gamma rays to be detected during the prompt peak of our SSPALS traces; with fewer o-Ps annihilations subsequently detected at later times.  An example SSPALS trace, with and without the laser, is shown below.


We quantify the ionisation by measuring the fraction of delayed annihilations in our SSPALS traces,

f = ∫(B→C)/∫(A→C)

and comparing it to a background measurement without the laser:

S = (flaser onflaser off)/flaser off

The figure below shows how this ionization signal, S, varies as we tune the UV laser across the resonant wavelength, 243nm, for the 1S-2P transition.


The width of the roughly Gaussian line-shape is caused by Doppler broadening, where the atoms moving towards, or away from the laser see a slightly shorter, or longer, wavelength.

The different coloured points represent different voltages applied to the Ps converter. This voltage creates an electric field that attracts and accelerates the positrons, implanting them into the material.  The highest target bias has the narrowest RIS line-shape as the Ps atoms form deep inside the sample and therefore experience more collisions as they make their way back to the surface, which slows them down and reduces the Doppler effect.

Positronium production

We have made positronium atoms by accelerating positrons from our trap into a porous silica target (which was supplied by Laszlo Liskay in Saclay). We observe the positronium atoms by single shot positronium annihilation lifetime spectroscopy (SSPALS – see here for more information). In this technique we record the gamma photons emitted as the positrons and positronium atoms annihilate. Approximately 50% of the positrons annihilate in the target, producing the large gamma peak observed within the first 20 ns of the trace below. Of the Ps atoms formed, around one quarter are in the very short-lived singlet state (para-positronium). With a lifetime against annihilation of only 120 ps, these atoms also contribute to the large peak. The remaining atoms occupy the triplet state (ortho-positronium) which, with a lifetime against annihilation of 142 ns, lives for over 1000 times longer than p-Ps. This increased lifetime leads to a long tail in the SSPALS trace which is characteristic of positronium generation.

The figure below shows two SSPALS data-sets: one taken with a porous silica target and one with a metal target. In both cases the implantation energy was 1 keV. In the case of the metal target (grey data) we observe no Ps formation, and there is only signal from positron annihilation. However, with  porous silica target (blue data) we record increased signal at times greater than 40 ns after implantation: the signature of Ps formation.


The porous silica target used in this experiment is an efficient source of cold Ps atoms. The incident positrons are accelerated into the bulk material, where they can either anniliate or form Ps. Once Ps has been formed it can diffuse out of the bulk and into the pores. Collisions with the walls of these pores cools the Ps atoms, ultimately to the lowest quantum state of the potential well. The cooled atoms can then diffuse out of the target through the interconnected pores and into the vacuum.