Delayed emission of cold positronium from mesoporous materials

Abstract

It is well known that ortho-positronium (ortho-Ps) atoms are emitted with high efficiency from various porous materials following the implantation of positrons. Since the ortho-Ps lifetime in a mesoporous material may be a substantial fraction of the ortho-Ps vacuum lifetime (142 ns), the time dependence of Ps emission may have to be considered when conducting certain types of experiments, such as time of flight measurements or pulsed ortho-Ps-laser interactions, when using this kind of target as a positronium source. By taking into account the positron implantation profile and subsequent Ps diffusion and decay in a mesoporous film we calculate the time dependent ortho-Ps emission rate (t), which in turn allows us to establish the total annihilation rate, arising from the decay of ortho-Ps both inside and outside the sample. Using time-delayed laser spectroscopy and single-shot lifetime measurements we have directly probed the rate at which Ps is emitted into vacuum from a target with ∼3-nm diameter pores and have observed delayed ortho-Ps emission that is consistent with our model. From the ortho-Ps decay spectrum we find that, whereas a simple two-component lifetime fit gives a short lifetime of 25.3 ± 0.3 ns, an analysis that properly takes into account the emission rate yields an ortho-Ps lifetime inside the porous material of 32.3 ± 1.2 ns, demonstrating that the ortho-Ps escape rate into vacuum can significantly modify the apparent lifetime of ortho-Ps inside a mesoporous material. Our measurements yield a Ps diffusion coefficient D = 0.07 ± 0.01 cm 2 s −1 , which is consistent with a tunneling limited diffusion process.

Experimental progress in positronium laser physics

“…The latter scenario takes place with a much lower diffusion coefficient than the former [110]. This means that the time it takes for thermalized Ps atoms to be emitted from a mesoporous film can be significantly longer than that of non-thermal atoms.…”

“…This work has also led to the use of thin mesoporous films that can be used as Ps converters [109,110,[246][247][248][279][280][281][282][283][284]. These materials produce Ps in a similar way to the powders discussed previously [35,36].…”

“…This can be done in different geometries to obtain the Ps energies perpendicular or parallel to the surface of Ps converters [109,113]. Such measurements can be used, with appropriate additional parameters, such as laser delays, to obtain Ps thermalization and emission rates [110]. An example of Ps Doppler profiles measured using a mesoporous silica film converter is shown in Figure 13 [109].…”

Positronium emission from mesoporous silica studied by laser-enhanced time-of-flight spectroscopy

“…Both of these scenarios are difficult to model and require detailed knowledge of the pore structure that is usually not available. We note that our discussion here refers only to silica films, but many other materials can be used to create mesoporous structures (e.g., [25]) and in general our arguments would apply to those as well.Because quantum confinement can set a lower limit on the minimum energy with which Ps atoms can be emitted into vacuum, it is not always possible to correlate the free Ps kinetic energy with other parameters (e.g., implantation depth) to obtain information about the Ps dynamics in a porous network [26]. We show here that time-of-flight (TOF) measurements can reveal Ps cooling and emission times directly, without any modeldependent assumptions concerning Ps dynamics prior to emission.…”

Positronium emission spectra from self-assembled metal-organic frameworks

“…As a cross check we took data at different slit positions (10, 15, 20 mm) achieving consistent results. From these measurements, one can also determine the delay time for Ps emission into vacuum due to the diffusion to the surface [33,34]. This can be estimated by extrapolation to a slit position of 0 mm.…”