What happens to a silica film at cryogenic temperatures?

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).

cryogenicPS

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.

Time-resolved Doppler spectroscopy

Positronium atoms created by implanting positrons into porous silica initially have ~ 1 eV kinetic energy, but subsequently cool by colliding with the inner surfaces of the porous network.  The longer spent inside the pores before being emitted to vacuum, the closer the Ps can get to thermalising with the bulk (i.e. room temperature, ~ 25 meV).

Once the positronium atoms make it out of the pores and into vacuum we can excite them using a 243 nm (UV) pulsed laser to n = 2, then ionize these with a 532~nm (green) laser. The amount of positrons resonantly ionised can be measured using SSPALS as the UV wavelength is slowly varied.  This gives us the 1s2p Doppler-width, from which we estimate the Ps energy.  The delay between implanting positrons and firing the 6 ns laser pulse was varied to try and see how the width changes when hitting the Ps cloud at different times.

2dplot

In the 3D plot above we see that at earlier times the Doppler width is broader than later on.  This is because Ps atoms that spend longer inside the silica have more collisions with the pores and therefore cool down further (narrowing the distribution at later times), mixed up with the simple fact that the fastest atoms reach the laser interaction region quickest, and pass through it more quickly too!

Rydberg Positronium and Stark broadening

We have recently produced Rydberg positronium atoms in a two step excitation process, using 243 nm light from our broad band pulsed dye laser to excite 2P states, as in our previous Ps spectroscopy measurements. Then, instead of photoionizing with 532 nm light, we used ~ 750 nm light to excite 2p-nd transitions. This process is shown in the energy level diagram below, you can also see a photograph of the green light produced by our Nd:YAG laser pumping the infra red laser.

greenlight

Rydberg

Once the Ps atoms have been excited to a Rydberg state, their lifetime is greatly increased, and they only annihilate once they collide with the vacuum chamber. This leads to a reduced delayed fraction in our positronium SSPALS signal, since there are less gamma ray events occurring on our delayed detection time (to read more about how we detect Ps, read here). This can be seen in our data below where we excited Ps atoms to n = 11.

Rydberg1kv2kv3kv

When atoms are subjected to a high electric field different states are separated and shifted leading to an overall broadening of the spectral line, this effect is known as Stark broadening,  the mixing and shifting of the states is proportional to the strength of the electric field being applied. We are able to observe this effect by varying the voltage  applied to our porous silica target from which Ps atoms are produced, and therefore changing the electric field that the Ps atoms are subjected to. As the voltage is increased, the broadening grows with the eclectic field, thus producing a signal over a wider range of infra red wavelengths, this is shown in the figure above where we plot the delayed fraction over a range of 5.6 nm, changing the voltage applied to our target from from 1 kV to 2 kV and 3 kV.

Positronium formation detected using annihilation radiation energy spectroscopy

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.

peakvalley3kV

We are able to measure gamma ray energies using a detector; in this case NaI(TI), or thallium doped sodium iodide. The data shown above were taken with a positron beam fired into a piece of untreated metal, from which we expect hardly any Ps to be made, and also in a silica film, which we know converts about 30% of the incident positrons into the Ps atoms. When the beam hits the metal many events at energies close to 511 keV are detected, since most positrons will annihilate directly into two 511 keV photons.
The production of Ps can be seen when we compare the two curves (red and black lines) shown in the figure. These spectra are normalised to have the same total area, so the excess of counts in the valley region (i.e., energies between about 300 and 500 keV), and the reduction of counts in the photopeak (at 511 keV) is exactly what you would expect if you made positronium: the decay into three photons means there are more photons with energies less than 511 keV.

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.

trace1

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.

Ps_243nm

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.

sspals

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.

Ps_drawing