At GSI/FAIR, significant efforts and resources are dedicated to developing internal targets for use in the ultra-high vacuum environments of particle storage rings and traps. A major breakthrough in this field has been the ability to generate liquid jets just a few micrometers wide, enabling highly precise experiments. This expertise has also allowed us to tackle fundamental challenges in crossdisciplinary research, with a particular focus on climate science.
Internal targets, created by expanding gas or liquid through a small aperture (a "nozzle") into a vacuum, offer unmatched efficiency in terms of luminosity. These advantages stem from the large areal density at a well-defined interaction point they provide. As a result, internal-target beams play a crucial role in numerous experiments planned within the SPARC/APPA research pillar at the Experimental Storage Ring (ESR), the CRYRING@ESR, the HESR, and HITRAP.
Our team has also pioneered the development of microscopic liquid jets. Originally introduced by Robert Grisenti at the Max Planck Institute for Fluid Dynamics in Gö ttingen, these jets were further developed at the ATP division of GSI and the Institute for Nuclear Physics (IKF) at Goethe University in Frankfurt. They have already been successfully employed as high-density internal targets at the ESR, where, for the first time, interactions between highly charged ions and droplets were explored. Beyond their applications in nuclear and atomic physics, these microscopic liquid jets are also instrumental in climate science research, particularly in studying crystal nucleation and the behavior of supercooled water - key factors in climate models.

One of the most powerful ways to explore the fundamental structure of matter is through collision experiments. These experiments typically involve a charged particle (the projectile) colliding with a target made of solid, gaseous, or liquid material. By analyzing the resulting reaction products - such as emitted photons or electrons - we can gain valuable insights into the underlying physical processes.
At GSI, we currently operate two internal gas-jet targets, one at the ESR and the other at CRYRING@ESR. These targets stand out for their ability to deliver a wide range of target area
densities, precisely tailored to experimental requirements, while maintaining the ultra-high vacuum conditions essential for storage ring operations. Their flexibility and efficiency make them indispensable tools for cutting-edge research in atomic and nuclear physics.

Liquid jets are formed by expanding a chosen target liquid - under carefully controlled temperature and pressure conditions - through a nozzle into vacuum. Our ability to produce liquid jets just a few micrometers wide has opened up exciting opportunities in interdisciplinary research. One prominent example is the study of supercooled water - water that remains in a liquid state below its freezing point. Supercooled water droplets are naturally present in the upper layers of Earth’s atmosphere, where they play a critical role in cloud formation and climate dynamics. Understanding how these droplets transition into ice is essential for improving climate models.
When a liquid jet travels through a vacuum, its surface rapidly evaporates, inducing a cooling effect known as evaporative cooling. Using this technique in combination with Raman spectroscopy, our team, along with colleagues at DESY, recently succeeded in measuring the refractive index of supercooled water down to a temperature of 230 K. This parameter is crucial for understanding how light interacts with clouds, influencing atmospheric reflection and refraction processes.
Studying ice formation in supercooled water presents an even greater challenge. Ice formation begins with the spontaneous creation of a tiny crystalline seed, but testing the well-established classical theory of crystal nucleation has proven difficult. Recently, our team used liquid jets to conduct the most precise experimental test of this theory, focusing on two simple liquids - krypton and argon. Since noble gases interact via well-understood Lennard-Jones potentials, theoretical predictions should be highly accurate.
By conducting experiments at the European XFEL, we probed the liquid jets with high-intensity Xray pulses, allowing us to measure the rate of crystal formation with unprecedented precision. Surprisingly, we found that classical nucleation theory overestimated the actual nucleation rates by a factor of 100 to 1000. These findings suggest the need for non-classical extensions of nucleation theory, potentially leading to a deeper understanding of this fundamental phase transition.