## Photoemission of Ps from single-crystal p-Ge semiconductors

The production of positronium in a low-temperature (cryogenic) environment is in general only possible using materials that operate via non-thermal processes. In previous experiments we showed that porous silica films can be used in this way at temperatures as low as 10 K, but that Ps formation at these temperatures can be inhibited by condensation of residual gas, or by laser irradiation.

It has been known for several years now that some semiconductors can produce Ps via an exciton-like surface state [12]. Si and Ge are the only semiconductors that have been studied so far, but it is likely that others will work in a similar way. The electronic surface state(s) underlying the Ps production can be populated thermally, resulting in temperature dependent Ps formation that is very similar to what is observed in metals (for which the Ps is actually generated via thermal desorption of positrons in surface states). Since laser irradiation can also populate electronic surface states, and is known to result in Ps emission from Si at room temperature, the possibility exists that this process can be used at cryogenic temperatures.

We have studied this possibility using p-type Ge(100) crystals. Initial sample preparation involves immersion in acid (HCl) and this process leaves the sample with Chlorine-terminated dangling bonds which can be thermally desorbed. We attached the samples to a cold head with a high temperature interface  that can be heated to 700 K and cooled to 12 K. The heating is necessary to remove Cl from the crystal surface, which otherwise inhibits Ps formation. Fig 1 shows the initial heating cycle that prepares the sample for use. The figure shows the delayed annihilation fraction (which is proportional to the amount of positronium) as a function of temperature.

FIG. 1:  Delayed fraction as a function of sample temperature after initial installation into the vacuum system. After the surface Cl has been thermally desorbed the amount of Ps emitted at room temperature is substantially increased.

As has been previously observed [2] using visible laser light at 532 nm can increase the Ps yield. This occurs because the electrons necessary for Ps formation can be excited to surface states by the laser. However, these states have a finite lifetime, and as both the laser and positron pulses are typically around 5 ns wide these have to be synchronized in order to optimise the photoemission effect. This is shown in FIG 2.  These data indicate that the electronic surface states are fairly short lived, with lifetimes of less than 10 ns or so. Longer surface states were observed in similar measurements using Si.

FIG 2: Delayed fraction as a function of the arrival time of the laser relative to the incident positron pulse. These data are recorded at room temperature.  The laser fluence was ~ 15 mJ/cm$^2$

When Ge is cooled the Ps fraction drops significantly. This is not related to surface contamination, but is due to the lack of thermally generated surface electrons. However, surface contamination does further reduce the Ps fraction (much more quickly than is the case for silica. This effect is shown in FIG 3. If a photoemission laser is applied to a cold contaminated Ge sample two things happen (1) the laser desorbs some of the surface material and (2) photoemission occurs .This means that Ge can be used to produce Ps with a high efficiency at any temperature, and we don’t even have to worry about the vacuum conditions (within some limits).

FIG 3: Delayed fraction as a function of time that the target was exposed to showing the effect that different laser fluences has on the photoemission process. During irradiation, the positronium fraction is noticeably increased.

There are many possible applications for cryogenic Ps production within the field of antimatter physics, including the formation of antihydrogen formation via Ps collision with antiprotons [3], Ps laser cooling and Bose Einstein Condensation [4], as well as precision spectroscopy.

## 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).

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.

## 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.

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.

## 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.