Prompt x-rays emitted in neutron-induced fission help unveil the evolution of fission fragment charge yields as a function of incident neutron energy.

Nuclear fission is accompanied by the prompt emission of neutrons, gamma rays and x-rays. It has been known since the sixties that fission prompt x-rays originate essentially as a consequence of the internal conversions occurring in the prompt gamma deexcitation cascades of fission fragments.

This work presents for the first time a measurement of the prompt fission x-ray yields in ²³⁸U(n,f) for average incident neutron energies ranging from 3 to 200 MeV. Fission fragment charge distributions are derived from the measured x-ray yields using x-ray emission probabilities per fragment obtained in an earlier work on low energy fission. The results are found to be in a remarkable agreement with the Wahl phenomenological systematics for fission product yields, as well as with the more sophisticated GEF fission model. More detailed comparisons demonstrate that x-ray emission evolution with increasing incident neutron energy tends to be dominated by the transition towards lighter fragments which on average are closer to closed-shell nuclei and are thus less subject to internal conversion.

Non-linear optical processes of atoms and molecules such as multiphoton absorption and tunnelling ionization are very attractive issues in current atomic, molecular and optical sciences. The recent development of free electron laser (FEL) sources enabled us to investigate such non-linear optical processes in the extreme ultraviolet (XUV) wavelength regions Our group demonstrated that we can determine absolute values of a two-photon ionization cross section of atomic species and its wavelength dependence by using an XUV FEL light source. This was achieved by introducing an internal reference for the cross section measurements and by the frequency tunability of the FEL light source.

The FEL light source we used is the SPring-8 Compact SASE Source test accelerator in RIKEN, Harima Institute, equipped with a couple of compact vacuum undulators, having a unique advantage of its high peak intensity and frequency tunability in the 50 ~ 62 nm region.

We measured the wavelength dependence and the light field intensity dependence of the absolute values of two-photon ionization cross section of He at 53.4, 58.4, 56.0 and 61.4 nm, covering the 1s2p and 1s3p resonances in the light field intensities range of 5×1012 ~ 5×1013 W/cm2 by measuring simultaneously one-photon ionization signal of H2 mixed in the sample as reference.

We showed through the critical comparison with the theoretically obtained cross sections that, in the resonance wavelength regions, dressed state formation through the strong coupling between the intermediate 1snp resonance state and the 1s2 ground state needs to be taken into account if the XUV light field intensity becomes larger than ~1012 W/cm2. We are now entering into the stage of quantitative non-linear spectroscopy in the XUV wavelength region.

Longer wavelength InGaN emitters (~600nm) are important for some potential applications such as optoelectronic tweezers and visible light communication. The primary obstacle for developing InGaN light-emitting diodes (LEDs) at longer wavelengths, however, is because it is difficult to incorporate a high indium composition (for extending the emission wavelength) while maintaining good epitaxial quality. Indium in high proportion tends to aggregate. High indium InGaN structures also show strong piezoelectric fields, which in turn induce a reduced wavefunction overlap between electrons and holes, and a consequently weakened emission. To overcome these effects, new epitaxial InGaN structures of high-indium content are grown, in which an electron reservoir layer is introduced to enhance the indium incorporation and the light emission, but retain conventional (1000) orientation.

Photoluminescence measurements reveal that the emission wavelengths of these high-In quantum well structures can be tuned from 560nm to 600nm, depending on actual indium composition. Yellow-green and amber devices in an array-format are developed based on these wafer structures, where each LED pixel can be individually addressable. Power measurements indicate that the power density of the yellow-green (amber) device per pixel is up to 8W/cm² (4.4 W/cm²), much higher than that of conventional broad-sized LEDs made under the same condition, and nearly an order higher than that required by optoelectronic tweezers, validating the feasibility of using these micro-LEDs for tweezing. Nevertheless, it is found that the emission wavelength is strongly blueshifted upon injection current increase, up to ~50nm. Numerical simulations reveal that this is caused by screening of the quantum Stark effect and a band filling effect, thus further optimisation of the growth conditions and epitaxial structures is needed.