In pursuit of cold Ps

Experiments involving positronium (Ps), such as laser excitation for Rydberg state production, precision spectroscopy and positronium chemistry can benefit from a slow Ps source. The technical word for slow is cold. As the mass of positronium is twice the mass of an electron, even room temperature Ps have speeds on the order of 100 km/s. In comparison, atomic beams produced from supersonic jets have speeds of roughly 2 km/s. Atom optics techniques such a decelerating the Ps with electric fields to slow down the atoms may be possible, but at the cost of loss in intensity. An ideal alternative would be to fabricate a target that intrinsically produces slow positronium, and efficiently too.

In most of our experiments, Ps is produced from a mesoporous silica target grown on a silicon substrate. Ps, initially with energy of 1 eV, is emitted from the same side as the positrons enter the converter (“reflection geometry” production) and one can generally obtain Ps with final energy of approximately 50 meV.  Once produced inside granulated powders, such as silica or magnesium oxide (MgO), Ps will lose some of its energy by making collisions with the surrounding internal surfaces before being emitted into vacuum. Generally, more collisions means greater amounts of energy loss, therefore colder Ps. A silica target was fabricated so that the Ps traverses the whole thickness of the target to be emitted out of the opposite side after making the maximum amount of collisions with the internal surfaces (transmission geometry).  However, the internal spacing of this target was too large to efficiently cool Ps below 200 meV (~200 km/s), which is still hotter than Ps emitted from mesoporous silica. Ps can also be produced from MgO, albeit with a slightly higher initial energy of around 4 eV. Previous studies have indicated that this 4 eV energy is reduced down to energies of around 300 meV in a 6 μm thick MgO layer. So, that gave us an idea. If we make a thicker MgO layer to increase collisional cooling, Ps atoms could be produced with energies lower than 50 meV (Positronium emission from MgO smoke nanocrystals).

MgOblog
A scanning electron microscope (SEM) image of smoked MgO.

To make the MgO target, we set fire to a strip of magnesium ribbon and collected the smoke on a suitable substrate (tinted goggles and a fume cupboard are necessary as you might imagine). This procedure produces perfect cubic crystals, in a 30 μm MgO layer, that have a wide size distribution, as evident from the SEM image above. The substrate of our choice was a 50 nm silicon nitride film, as it allows us to implant the positrons into the SiN side, to make positronium in the SiN-MgO boundary, which is then emitted into vacuum from the opposite side in transmission geometry. In this configuration, Ps atoms will travel through the 30 μm thick MgO, making the maximum amount of collisions. For comparison, we can rotate the target by 180° so the positron hits the MgO side (“reflection geometry”) and Ps makes the least amount of collisions, for positrons which are implanted to a relatively shallow depth of only 100 nm. We expect colder Ps to be produced in the former case compared to the latter, due to the distance travelled by the Ps (30 μm vs 100 nm). These two orientations are shown below, including the positron pulse and the excitation lasers. VT and VG refer to voltages applied to the target and grid electrode to control the positron energy and electric field.

MgOblog2
Experimental setup showing the target utilised in reflection and transmission geometry.

Once positronium atoms are emitted in either of the two set-ups, they are excited with UV and IR lasers to measure the Doppler profile, which gives an indication of their kinetic energy, KE. In the “reflection geometry” configuration, VT controls how deep into the MgO layer the positrons are implanted. Higher voltages results in deeper implantation, hence larger amount of collisions made by the Ps on their way out and colder Ps. However in the “transmission” set-up, around 2 keV voltage on VT is enough to make the positrons go through the SiN and form Ps in MgO. We found that for the 2 – 5 keV range we measured, positron penetration depths into the MgO layer in transmission set-up were essentially the same, meaning that Ps always travelled through 30 μm of MgO in this configuration. The resultant KE obtained from the Doppler profiles are shown below.

KEfit2
Left: An example of a Doppler profile. Right: Kinetic energies obtained from the Doppler profiles.

Surprisingly, Ps appears to be emitted with the same energy regardless of the amount of collisions it makes with the smoked MgO surfaces. This is inconsistent with the idea that Ps is formed in MgO with 4 eV of energy and reduction in energy takes place due to the collisions. If this is true then we expect KE in reflection set-up, when VT is 1 kV, to be very close to 4 eV. However, Ps kinetic energy is always around 400 meV. This is because Ps, in smoked MgO, is intrinsically produced with around 300 meV of energy with a wide distribution due to the grain sizes, and because of the large open volumes between the MgO crystals, cooling is rather inefficient.

This rules out smoked MgO as a candidate for cold Ps production but there are many experiments where the Ps production and interaction region needs to be separated from the positron beam line. In such experiments, a simple and easily producible MgO target as discussed here, in “transmission geometry” configuration, can be employed where, for example, a scattering gas cell can be installed without interfering with the incoming positrons.

We have also noticed some interesting observations when the lasers are fired into the MgO rather than travelling just in front of the MgO surface. More on that in the next blog post.

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