All posts by tmertzi

Isoscalar and isovector spin response in sd-shell nuclei

The spin magnetic-dipole transitions and the neutron-proton spin-spin correlations in sd-shell even-even nuclei with N=Z are investigated by using shell-model wave functions taking into account enhanced isoscalar (IS) spin-triplet pairing as well as the effective spin operators. It was shown that the IS pairing and the effective spin operators gives a large quenching effect on the isovector (IV) spin transitions to be consistent with data observed by (p,p’) experiments. On the other hand, the observed IS spin strengths show much smaller quenching effect than expected by the calculated results. The IS pairing gives a substantial quenching effect on the spin magnetic-dipole transitions, especially on the IV transitions. Consequently, an enhanced IS spin-triplet pairing interaction enlarges the proton-neutron spin-spin correlation deduced from the difference between the IS and the IV sum-rule strengths. The β-decay rates and the IS magnetic moments of the sd shell are also studied in terms of the IS pairing as well as the effective spin operators.

Figure 7

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Generic features of the neutron-proton interaction

We show that fully aligned neutron-proton pairs play a crucial role in the low-energy spectroscopy of nuclei with valence nucleons in a high-j orbital. Their dominance is valid in nuclei with valence neutrons and protons in different high-j orbitals as well as in N=Z nuclei, where all nucleons occupy the same orbital. We demonstrate analytically this generic feature of the neutron-proton interaction for a variety of systems with four valence nucleons interacting through realistic, effective forces. The dominance of fully aligned neutron-proton pairs results from the combined effect of (i) angular momentum coupling and (ii) basic properties of the neutron-proton interaction.

Figure 6

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Probing the fusion of neutron-rich nuclei with re-accelerated radioactive beams

We report the first measurement of the fusion excitation functions for 39,47K+28Si at near-barrier energies. Evaporation residues resulting from the fusion process were identified by direct measurement of their energy and time of flight with high geometric efficiency. At the lowest incident energy, the cross section measured for the neutron-rich 47K-induced reaction is ≈6 times larger than that of the β-stable system. This experimental approach, both in measurement and in analysis, demonstrates how to efficiently measure fusion with low-intensity re-accelerated radioactive beams, establishing the framework for future studies.

Figure 2

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The Journey of Actinium-225: How Scientists Discovered a New Way to Produce a Rare Medical Radioisotope

Inside a narrow glass tube sits a substance that can harm or cure, depending on how you use it. It gives off a faint blue glow, a sign of its radioactivity. While the energy and subatomic particles it emits can damage human cells, they can also kill some of our most stubborn cancers. This substance is actinium-225.

Fortunately, scientists have figured out how to harness actinium-225’s power for good. They can attach it to molecules that can home in on only cancer cells. In clinical trials treating late-stage prostate cancer patients, actinium-225 wiped out the cancer in three treatments.

This picture shows three different images of a single patient with end-stage prostate cancer.

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Αχ, πού’σαι, νιότη, που’δειχνες, πως θα γινόμουν άλλος.

Πάλι μεθυσμένος είσαι, δυόμισι η ώρα της νυχτός.
Κι αν τα γόνατα σου τρέμαν, “εκρατιόσουνα” στητός
μπρός στο κάθε τραπεζάκι. – Γεια σου Κωνσταντή βαρβάτε!
-Καλησπερούδια αφεντικά, πώς τα καλοπερνάτε;

Ένας σου’δινε ποτήρι κι άλλος σου’δινεν ελιά.
Έτσι πέρασες γραμμή της γειτονιάς τα καπηλιά.
Κι αν σε πείραζε κανένας –αχ, εκείνος ο Τριβέλας!
έκανες, πως δεν ένιωθες και πάντα εγλυκογέλας.

Χτες και σήμερα ίδια κι όμοια, χρόνια μπρός, χρόνια μετά…
Η ύπαρξή σου σε σκοτάδια όλο πηχτότερα βουτά.
Τάχα η θέλησή σου λίγη, τάχα ο πόνος σου μεγάλος;
Αχ, πού’σαι, νιότη, που’δειχνες, πως θα γινόμουν άλλος.

(του Βάρναλη)

Keeping it radioactive: This is how a molten salt nuclear reactor works

Radioactive elements produce heat as they decay. Nuclear plants draw power from this process, and typically stabilize the temperature with water. But during a power outage, H2O—which needs pumps to flow—can’t always prevent meltdowns. Molten salt reactors, which instead control heat with melted lithium and potassium fluorides, have a fail-safe: If the electricity dies, a plug will melt, causing the salts to seep down a safety drain and solidify around the uranium, preventing overheating. After a decades-long lull in development, countries from China to Denmark are building new molten salt reactors.

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Quantum electrodynamics and the proton size

Finding discrepancies between the predictions of fundamental theories and experimental observations is the main driver to develop physics further — the route to more advanced theories (‘new physics’) that fix the discrepancies. In that sense, quantum electrodynamics (QED) is currently seen as the most advanced fundamental theory, serving as the blueprint for any other quantum field theory. Progress is expected to come from ever more precise testing through comparison of theoretical predictions and experimental data. A good test compares values that can be both computed and measured with high accuracy. Some QED predictions excel in that respect, such as for the transition frequencies of atomic hydrogen and the gyromagnetic ratio of the electron.

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The Heidelberg compact electron beam ion traps

Electron beam ion traps (EBITs) are ideal tools for both production and study of highly charged ions (HCIs). In order to reduce their construction, maintenance, and operation costs, we have developed a novel, compact, room-temperature design, the Heidelberg Compact EBIT (HC-EBIT). Four already commissioned devices operate at the strongest fields (up to 0.86 T) reported for such EBITs using permanent magnets, run electron beam currents up to 80 mA, and energies up to 10 keV. They demonstrate HCI production, trapping, and extraction of pulsed Ar16+ bunches and continuous 100 pA ion beams of highly charged Xe up to charge state 29+, already with a 4 mA, 2 keV electron beam. Moreover, HC-EBITs offer large solid-angle ports and thus high photon count rates, e.g., in x-ray spectroscopy of dielectronic recombination in HCIs up to Fe24+, achieving an electron-energy resolving power of EE > 1500 at 5 keV. Besides traditional on-axis electron guns, we have also implemented a novel off-axis gun for laser, synchrotron, and free-electron laser applications, offering clear optical access along the trap axis. We report on its first operation at a synchrotron radiation facility demonstrating the resonant photoexcitation of highly charged oxygen.



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