Are high energy proton beams ideal for AB-BNCT? A brief discussion from the viewpoint of fast neutron contamination control.

High energy proton beam (>8MeV) is favorable for producing neutrons with high yield. However, the produced neutrons are of high energies. These high energy neutrons can cause severe fast neutron contamination and degrade the BNCT treatment quality if they are not appropriately moderated. Hence, this study aims to briefly discuss the issue, from the viewpoint of fast neutron contamination control, whether high energy proton beam is ideal for AB-BNCT or not. In this study, D2O, PbF4, CaF2, and Fluental(™) were used standalone as moderator materials to slow down 1-, 6-, and 10-MeV parallelly incident neutrons. From the calculated results, we concluded that neutrons produced by high energy proton beam could not be easily moderated by a single moderator to an acceptable contamination level and still with reasonable epithermal neutron beam intensity. Hence, much more complicated and sophisticated designs of beam shaping assembly have to be developed when using high energy proton beams.

[On peculiarities of temperature dependences of water spectra in the terahertz frequency domain].

We analyzed spectra of light and heavy water at temperatures from 4 up to 50 degrees C in a frequency range of 0.15 to 6.5 THz. It was shown that the amplitude of high-frequency relaxation absorption band with its maximum at 0.5 THz extends with increasing, temperature and this temperature dependence for light water has a marked feature at 35-40 degrees C as a sharp growth. This fact is noteworthy because this range corresponds to physiological values of a body temperature of the warm-blooded organisms. At the same time, the analogous temperature dependence for heavy water in the considered temperature range lacks this particular feature. Thus, the water with its properties differs significantly not only from other fluids, but also from its own isotopologues.

Solvent response to solute photo-dissociation.

Ultraviolet-visible and infrared transient-absorption spectroscopy are used to investigate the transfer of energy from nitrite to water during the photo-dissociation of NO2-(aq). Nitrite is dissociated by photo-excitation at 200 nm. About 40% of the photo-fragments recombine and relax on a 3 ps timescale, while diffusive recombination accounts for another 10% of the fragments during the subsequent 50 ps. The infrared transient-absorption spectra of the photo-dissociation of nitrite solvated in H2O and D2O show no evidence of excited vibrations after 0.5 ps. Instead they reveal a sub-0.5 ps change in the infrared absorption similar to what is observed when the temperature of water is increased. Since this spectral change is associated with the weakening of the hydrogen-bond network, we infer that excess energy from the dissociation of nitrite is dissipated to the local hydrogen-bonded water network in less than 0.5 ps. The rapid change in the infrared absorption is followed by a slower (50 ps) component associated with the energy dissipation to the solvent as the photo-fragments diffusively recombine and relax.

The effect of dissolve gas concentration in the initial growth stage of multi cavitation bubbles. Differences between vacuum degassing and ultrasound degassing.

The sonochemical luminescence intensity from luminol was measured at a sampling rate of several kilohertz. This was noted at three different periods: first, the latent period in which no light emission occurs at all; second, the increased emission period from the start of light emission to the time when a steady state is reached; and third, the steady state period in which light emission occurs at the steady state value. When irradiated with ultrasound of different intensities, the times of the latent period and increased emission period are shorter for higher ultrasound intensities. To know how the dissolved oxygen content is involved in early-stage cavitation growth, an experiment was conducted using solutions with varying dissolved oxygen contents from 100% to 37%. For dissolved air content of 50% or less, it was found that the latent period was 30 times longer in a saturated condition. It was also found that the increased emission period was 10 times longer. However, the emission intensity in the steady state did not change at all even when the initial dissolved gas concentration of the sample was changed. From this, it was found that the reuse of collapsed bubbles takes place efficiently in the steady state. Dissolved oxygen was reduced by the use of a vacuum pump and by the degassing action of ultrasound, and it was discovered that the behavior of transient emission differed for the two ways of degassing.