Antihydrogen atoms with K or sub-K temperature are a powerful tool to precisely probe the validity of fundamental physics laws and the design of highly sensitive experiments needs antihydrogen with controllable and well defined conditions. We present here experimental results on the production of antihydrogen in a pulsed mode in which the time when 90% of the atoms are produced is known with an uncertainty of ~250 ns. The pulsed source is generated by the charge-exchange reaction between Rydberg positronium atoms—produced via the injection of a pulsed positron beam into a nanochanneled Si target, and excited by laser pulses—and antiprotons, trapped, cooled and manipulated in electromagnetic traps. The pulsed production enables the control of the antihydrogen temperature, the tunability of the Rydberg states, their de-excitation by pulsed lasers and the manipulation through electric field gradients. The production of pulsed antihydrogen is a major landmark in the AE $$\bar{g}$$ḡ IS experiment to perform direct measurements of the validity of the Weak Equivalence Principle for antimatter.

The depth profile of open volume defects has been measured in Si implanted with He at an energy of 20 keV, by means of a slow-positron beam and the Doppler broadening technique. The evolution of defect distributions has been studied as a function of isochronal annealing in two series of samples implanted at the fluence of $5\times {10}^{15}$ and $2\times {10}^{16}{\mathrm{He}\mathrm{}\mathrm{cm}}^{-2}.$ A fitting procedure has been applied to the experimental data to extract a positron parameter characterizing each open volume defect. The defects have been identified by comparing this parameter with recent theoretical calculations. In as-implanted samples the major part of vacancies and divacancies produced by implantation is passivated by the presence of He. The mean depth of defects as seen by the positron annihilation technique is about five times less than the helium projected range. During the successive isochronal annealing the number of positron traps decreases, then increases and finally, at the highest annealing temperatures, disappears only in the samples implanted at the lowest fluence. A minimum of open volume defects is reached at the annealing temperature of $250°\mathrm{C}$ in both series. The increase of open volume defects at temperatures higher than $250°\mathrm{C}$ is due to the appearance of vacancy clusters of increasing size, with a mean depth distribution that moves towards the He projected range. The appearance of vacancy clusters is strictly related to the out diffusion of He. In the samples implanted at $5\times {10}^{15}{\mathrm{cm}}^{-2}$ the vacancy clusters are mainly four vacancy agglomerates stabilized by He related defects. They disappear starting from an annealing temperature of $700°\mathrm{C}.$ In the samples implanted at $2\times {10}^{16}{\mathrm{cm}}^{-2}$ and annealed at $850–900°\mathrm{C}$ the vacancy clusters disappear and only a distribution of cavities centered around the He projected range remains. The role of vacancies in the formation of He clusters, which evolve in bubble and then in cavities, is discussed.

We report on laser cooling of a large fraction of positronium (Ps) in free flight by strongly saturating the $1^{3}S\text{−}{2}^{3}P$ transition with a broadband, long-pulsed 243 nm alexandrite laser. The ground state Ps cloud is produced in a magnetic and electric field-free environment. We observe two different laser-induced effects. The first effect is an increase in the number of atoms in the ground state after the time Ps has spent in the long-lived $2^{3}P$ states. The second effect is one-dimensional Doppler cooling of Ps, reducing the cloud’s temperature from 380(20) to 170(20) K. We demonstrate a 58(9)% increase in the fraction of Ps atoms with $v_{1\text{D}}<3.7\times {10}^4\mathrm{m}{\mathrm{s}}^{-1}$.

Si $p$-type (100) samples were coimplanted at room temperature with ${\mathrm{He}}^{+}$ ions at $30\mathrm{keV}$ with a dose of $1\times {10}^{16}\text{ions}∕{\mathrm{cm}}^2$ and successively with ${\mathrm{H}}^{+}$ ions at $24\mathrm{keV}$ with a dose of $1\times {10}^{16}\text{ions}∕{\mathrm{cm}}^2$. A series of samples was thermally treated for $2\mathrm{h}$ from $100\text{to}900°\mathrm{C}$ at $100°\mathrm{C}$ steps to study the evolution of pointlike and extended defects by two complementary techniques: positron Doppler broadening spectroscopy and transmission electron microscopy. Depth profiling the samples with a positron beam led to the identification of five different traps and the evolution of their profile distributions with thermal treatments. The positron traps were identified as decorated vacancy clusters of different sizes. Their decoration by implanted ions and in some case by oxygen was probed by coincidence Doppler broadening spectroscopy. Up to $300°\mathrm{C}$ annealing temperature positrons probe three distributions of different decorated defects covering regions of the sample down to $400–450\mathrm{nm}$. Starting from $300°\mathrm{C}$ annealing temperature no defects were revealed by positrons in the region next to the peak of the implanted ions distributions positioned around $280\mathrm{nm}$, where extended defects are expected; this indicates complete filling of the defects by H and He. From $300\text{to}600°\mathrm{C}$ decorated vacancy clusters of different sizes appear progressively in the region below $280\mathrm{nm}$, with a distribution moving deeper into the sample. Comparison with previous measurements on He-implanted samples points out the chemical action of H. Hydrogen atoms interact with the previous damage by He, producing more stabilized vacancylike defects distributed through the damage region of the sample. Electron microscopy shows the transformation of the extended defects from platelets to blisters and cavities.