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High energy density physics 32, 63 - 69 (2019). doi:10.1016/j.hedp.2019.05.002, We present results from the SPring-8 Angstrom Compact free electron LAser (SACLA) X-ray free electron laser (XFEL) facility, using an X-ray pump, X-ray probe scheme to observe ultrafast changes in the structure of heated graphite. The 9.8 keV XFEL beam was focused to give an intensity on the order of ∼ 10$^{19}$ W/cm$^2$, and the evolution of the diffraction pattern observed up to delays of 300 fs. The interplanar diffraction peaks weaken significantly within 10s of femtoseconds, but in-plane diffraction orders i.e. those with Miller Index $(hk0)$, persist up to 300 fs, with the observed signal increasing. We interpret this as nonthermal damage through the breaking of interplanar bonds, which at longer timescales leads to ablation by removal of intact graphite sheets. Post-experiment examination of the graphite samples shows damage which is comparable in size to the range of the excited photoelectrons. These results highlight the challenges of accurately modelling X-ray driven heating, as it becomes a routine approach to generating high energy density states., Published by Elsevier, Amsterdam [u.a.]

High intensity X-ray Free Electron Lasers (XFEL) are an ideal tool to heat materials directly and isochorically, which can cause them to be modified and damaged irreversibly. During XFEL-matter interactions, the energy of an XFEL beam will be mainly absorbed by photoionization, creating numerous high-energy photo- and Auger electrons. Modelling this process is quite complex since both atomic physics and plasma physics are involved. Atomic collisional-radiative (CR) codes such as FLYCHK are widely used to simulate such processes. However, the CR codes typically assume local thermodynamic equilibrium (LTE) and are limited to zero dimension. In order to understand the sample damaging mechanisms, we performed two-dimensional kinetic particle-in-cell simulations with radiation transport (PIC-RT) to retrieve the temporal processes of XFEL-matter interactions. The dynamics of XFEL-Matter interactions can roughly divide into three different time scales: 1) electron heating and photoionization by XFEL in ~ 10 fs ; 2) collisional heating and ionization by high-energy photo- and Auger electrons with several keV energy in tens of fs to sub-ps; and 3) collective hydrodynamic explosion driven by ~ TPa thermal pressure from ~100 fs to nanoseconds. The simulation results are compared to our recent experiment that a variety of samples were irradiated by Japanese XFEL SACLA with intensities on the order of 10^20 W/cm^2. The post-analysis of the irradiated samples showed that large holes with radius sizes more than one order of magnitude higher than the XFEL spot were created for metallic samples. The hole size is also much larger than the stopping range of high energy electrons. According to our PIC-RT simulations, we attribute the generation of such large holes to the micro-explosion process. Kinect simulation of the hole generation with multiple time scales is also useful and complementary to understand the change of X-Ray diffraction pattern in the experiment that infers significant material structural change on femtosecond timescales.

The COBRA demonstrator, a prototype for a large-scale experiment searching for neutrinoless double beta-decay, was built at the underground laboratory Laboratori Nazionali del Gran Sasso (LNGS) in Italy. It consists of an array of 64 monolithic, calorimetric CdZnTe semiconductor detectors with a coplanar-grid design and a total mass of 380g. It is used to investigate the experimental challenges faced when operating CdZnTe detectors in low-background mode, to identify potential background sources and to show the long-term stability of the detectors. The first data-taking period started in 2011 with a subset of the detectors, while the demonstrator was completed in November 2013. To date, more than 250kg d of data have been collected. This paper describes technical details of the experimental setup and the hardware components., 8 pages, 10 figures, submitted to NIM A

Scientific reports 9(1), 4196 (2019). doi:10.1038/s41598-019-40782-5, We investigated the high-pressure behavior of polyethylene (CH2) by probing dynamically-compressed samples with X-ray diffraction. At pressures up to 200 GPa, comparable to those present inside icy giant planets (Uranus, Neptune), shock-compressed polyethylene retains a polymer crystal structure, from which we infer the presence of significant covalent bonding. The A2/m structure which we observe has previously been seen at significantly lower pressures, and the equation of state measured agrees with our findings. This result appears to contrast with recent data from shock-compressed polystyrene (CH) at higher temperatures, which demonstrated demixing and recrystallization into a diamond lattice, implying the breaking of the original chemical bonds. As such chemical processes have significant implications for the structure and energy transfer within ice giants, our results highlight the need for a deeper understanding of the chemistry of high pressure hydrocarbons, and the importance of better constraining planetary temperature profiles., Published by Macmillan Publishers Limited, part of Springer Nature, [London]