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Since we are interested in making positronium atoms we are always looking to shoot positron beams at various materials, and under different conditions. In some cases we might need our Ps atoms to be made in a cold environment, so they can be excited to Rydberg states without being harassed by black body radiation. One of the best positronium formation targets we have used are porous silica films, which we get from collaborators in Paris (Laszlo Liskay and co-workers from CEA Saclay) [1]. Because of the way these materials make Ps they are not very sensitive to the temperature, so it should be possible to cool them down without changing the amount or character of Ps produced after a positron beam is implanted. This has already been seen at around 50 K [2] but we decided to have a look for ourselves at a slightly lower temperature (12K) to see if the impact of the positron beam might cause some damage at these temperatures (it can happen [3]).

With a cold head installed in our new positronium interaction chamber, we have cooled one of Lazslo’s silica films [1] to 12 Kelvin (~261˚C) which is about 100˚C colder than the dark side of the moon. It turns out that our positron beam didn’t do any damage at all and the sample was basically fine, so just for fun we decided to blast it with a laser beam (UV light, at 243 nm).

When you cool something down any gas in the region will tend to freeze on it. In ultra-high vacuum there isn’t that much gas around, but there is always a bit (known as residual gas, for obvious reasons) and after a while we do observe some fairly minor effects from all this freezing gas. Fortunately this takes a long time, and the sample is still useable for a week or so, and if you warm it up it will be restored to its original condition (since the frozen gas just evaporates away from the target). Once you start shooting the silica with a laser, however, things are not so stable, as shown in the figure. We observe a drastic reduction in the positronium formation efficiency after the silica is irradiated at low temperature (nothing happens at room temperature).

The delayed fraction f (black data points) measured for different sample temperatures (solid red lines), with the UV laser fired during the times indicated. Since f measures the amount of long-lived Ps present (it is more or less proportional to the fraction of incident positrons that form positronium) the sharp drop indicates that either less Ps is being created, or that it is being destroyed shortly after creation. The latter process is consistent with the experiment of Saito et al. Note that there is no effect from the laser at room temperature, and that the paramagnetic centers created at low temperature can be annealed out when the temperature is raised.

This is not very surprising, researchers in Japan already saw this many years ago [4]. Although they did not use lasers, and their experiments were done with slightly different samples (not thin films as we have been using) the physical mechanism is expected to be essentially the same. At low temperatures disturbed molecules are not able to repair themselves and so if they are distorted in some way by radiation they tend to remain in that configuration. This can create something called a paramagnetic center which is bad news for positronium atoms. Why? Well paramagnetic centers are essentially unpaired spins, and interactions with these makes it very easy for a long-lived (triplet) Ps state to be converted into a short-lived (singlet) state. In other words, paramagnetic centers kill positronium atoms. These killer centers are not stable at room temperature, and molecular thermal fluctuations can restore the system to its normal state (which generally does not contain any paramagnetic centers). This means that after we create these troublesome centers with a laser all we have to do to get rid of them is to warm the target up. When we do this (see figure above) we get an annealing/recovery process quite similar to the results of Saito et al [4].

Refs.

[1] L. Liszkay, C. Corbel, P. Perez, P. Desgardin, M.-F. Barthe, T. Ohdaira, R. Suzuki, P. Crivelli, U. Gendotti, A. Rubbia, M. Etienne, and A. Walcarius (2008). Positronium reemission yield from mesostructured silica films. Applied Physics Letters 92, 063114. http://dx.doi.org/10.1063/1.2844888.

[2] Paolo Crivelli, Ulisse Gendotti, André Rubbia, Laszlo Liszkay, Patrice Perez, and Catherine Corbel (2010). Measurement of the orthopositronium confinement energy in mesoporous thin films. Physical Review A 81, 052703. http://dx.doi.org/10.1103/PhysRevA.81.052703.

[3] D. B. Cassidy and A. P. Mills Jr (2007). Radiation damage in a-SiO2 exposed to intense positron pulses. Nuclear instruments and Methods B, 262 59. http://doi.org/10.1016/j.nimb.2007.04.220.

[4] Saito, Haruo and Nagashima, Yasuyuki and Hyodo, Toshio and Chang, Tianbao (1995). Detection of paramagnetic centers on amorphous- SiO₂ grain surfaces using positronium. Phys. Rev. B 52 R689(R). http://dx.doi.org/10.1103/PhysRevB.52.R689.

When a positron and an electron annihilate directly, instead of forming a Ps atom, all of their energy is converted into two gamma ray photons, each with 511 keV (the rest mass energy of the electron/positron). However, if an electron and a positron form a Ps atom the annihilation can occur either into two or three photons, depending on the spin state of the Ps atom. The longer-lived Ps state is called ortho-positronium (o-Ps), and in this system the electron and positron spins point in the same direction, so the total spin of the atom is 1. This means that o-Ps has to decay into an odd number of gamma rays in order to conserve angular momentum. Usually this means three photons, as single photon decay can only happen if there is a third body present (this has been observed). The three photon energies are spread out over a large range (but they always add up to 2 x 511 keV). The short-lived Ps state is called para-positronium (p-Ps) and this usually decays into two photons. It is possible for a three photon state to have zero angular momentum, so singlet decay into three photons is not ruled out by momentum considerations, but this mode is suppressed and to a good approximation p-Ps decays into two gamma rays with well-defined energies (i.e., 511 keV). This means that p-Ps decays look very similar to direct electron-positron decays. It also means that we can detect the presence of o-Ps by looking at the energy spectrum of annihilation radiation, as is shown in the graph below.