Preliminary spectrum shifter design for intermediate energy nuclear data benchmark experiments with DT neutrons

Integral benchmark experiments with DT neutrons are not always sufficient for nuclear data benchmarking in the MeV region, below 10 MeV. A neutron spectrum shifter, which will be placed between a sample and a DT neutron source, is effective to moderate DT neutrons incident to the sample. In order to estimate effects of the spectrum shifter, the ratio of the contribution of 14 MeV neutrons in the leakage neutron and gamma-ray spectra was calculated with MCNP-4C for an experimental configuration at FNS of JAEA, Japan. The calculations were carried out for a Li₂TiO₃ sample with a Be, D₂O, or ⁷LiD spectrum shifter. It was found out that the Be shifter was superior to others and the Be shifter was effective to decrease the contribution of 14 MeV neutrons especially for secondary gamma-ray spectrum measurements.     

Polarization response of two-dimensional metallic photonic crystals studied by terahertz time-domain spectroscopy

The polarization characteristics of a terahertz (THz) wave transmitted through two-dimensional (2-D) metallic photonic crystals (MPCs) are investigated. The 2-D MPCs studied in this paper are metal slabs perforated periodically with circular holes. We measured the polarization characteristics of the THz wave using a THz time-domain spectroscopic system with wire grid polarizers in the time and frequency domains. The linearly polarized incident THz wave changes its polarization direction and becomes elliptic after it transmits through the sample. This phenomenon is highly sensitive to the incident angle. It is shown that the frequency range at which the polarization rotation occurs is related to the lattice constant of a photonic crystal, indicating the importance of photonic band modes of the 2-D MPC in the mechanism of the phenomenon.     

Increase of heat-shock protein and induction of gamma/delta T cells in peritoneal exudate of mice after injection of live Fusobacterium nucleatum

The herpes simplex virus-1 (HSV-1) capsid shell has 162 capsomers arranged on a T = 16 icosahedral lattice. The major capsid protein, VP5 MW = 149,075) is the structural component of the capsomers. VP5 is an unusually large viral capsid protein and has been shown to consist of multiple domains. To study the conformation of VP5 as it is folded into capsid promoters, we identified the sequence recognized by a VP5-specific monoclonal antibody and localized the epitope on the capsid surface by cryoelectron microscopy and image reconstruction. The epitope of mAb 6F10 was mapped to residues 862-880 by immunoblotting experiments performed with (1) proteolytic fragments of VP5, (2) GST-fusion proteins containing VP5 domains, and (3) synthetic VP5 peptides. As visualized in a three-dimensional density map of 6F10-precipitated capsids, the antibody was found to bind at sites on the outer surface of the capsid just inside the openings of the trans-capsomeric channels. We conclude that these sites are occupied by peptide 862-880 in the mature HSV-1 capsid.     

Fabrication of CaCO3-Coated Vesicles by Biomineralization and Their Application as Carriers of Drug Delivery Systems

We fabricated CaCO₃-coated vesicles as drug carriers that release their cargo under a weakly acidic condition. We designed and synthesized a peptide lipid containing the Val-His-Val-Glu-Val-Ser sequence as the hydrophilic part, and with two palmitoyl groups at the N-terminal as the anchor groups of the lipid bilayer membrane. Vesicles embedded with the peptide lipids were prepared. The CaCO₃ coating of the vesicle surface was performed by the mineralization induced by the embedded peptide lipid. The peptide lipid produced the mineral source, CO₃²⁻, for CaCO₃ mineralization through the hydrolysis of urea. We investigated the structure of the obtained CaCO₃-coated vesicles using transmission electron microscopy (TEM). The vesicles retained the spherical shapes, even in vacuo. Furthermore, the vesicles had inner spaces that acted as the drug cargo, as observed by the TEM tomographic analysis. The thickness of the CaCO₃ shell was estimated as ca. 20 nm. CaCO₃-coated vesicles containing hydrophobic or hydrophilic drugs were prepared, and the drug release properties were examined under various pH conditions. The mineralized CaCO₃ shell of the vesicle surface was dissolved under a weakly acidic condition, pH 6.0, such as in the neighborhood of cancer tissues. The degradation of the CaCO₃ shell induced an effective release of the drugs. Such behavior suggests potential of the CaCO₃-coated vesicles as carriers for cancer therapies.     

Photoconductive Emission and Detection of Terahertz Pulsed Radiation Using Semiconductors and Semiconductor Devices

Recent studies on the techniques and development of photoconductive (PC) semiconductor devices for efficient generation and detection of terahertz (THz) pulsed radiation are reported. Firstly, the optimization of PC antenna design is discussed. The PC detection of THz pulsed radiation using low-temperature grown GaAs with 1.55-μm wavelength probe is then described. Finally, the enhancement of THz radiation from InSb by using a coupling lens and magnetic field is investigated. These results reveal valuable insights on the design of an efficient, compact, and cost-effective THz time-domain spectroscopy system based on 1.55-μm fs laser sources.     

Low-energy neutron spectrometer using position sensitive proportional counter—Feasibility study based on numerical analysis

There is no direct technique to measure a neutron energy spectrum, particularly in the lower energy region, because the reaction Q value for detection is much larger than the neutron energy to be measured. However, such techniques are becoming a necessity, for example, in medical applications such as boron neutron capture therapy. In this study, a new spectrometer to measure low-energy neutrons (from thermal to 100 eV) is investigated numerically. We propose a unique approach of estimating the neutron energy spectrum by analyzing the distribution of neutron detection depths in the detector using an exact relation between the neutron energy and nuclear reaction cross-section. The proposed spectrometer has been established to be feasible to manufacture. The conversion performance of the neutron detection depth distribution to the neutron energy spectrum has also been proven to be acceptable, with the unfolding process based on Bayes’ theorem, even though the detector response function is non-distinctive (without peaks or edges). The present spectrometer is now under development, and its practical performance will be reported as soon as the prototype detector is completed.     

Saturation and Polarization Characteristics of 1.56 μm Optical Probe Pulses in a LTG-GaAs Photoconductive Antenna Terahertz Detector

The characteristics of low temperature-grown GaAs photoconductive antenna (PCA) terahertz detectors probed by 1.56 μm laser pulses are investigated. The influence of TM and TE polarized probe, as well as the saturation characteristics are studied for 2 μm- and 5 μm-gap PCA’s. Different polarization characteristics at low probe powers and at the saturation regimes were observed. Results are explained in terms of the polarization-dependent photocarrier distribution at the PCA gap arising from tight focusing. This work also demonstrates using a 1.56 μm probe for a GaAs PCA to achieve ~60 dB SNR; matching its performance characteristics for above-bandgap probes.     

Highly Sensitive Terahertz Sensing of Glycerol-Water Mixtures with Metamaterials

We demonstrate the highly sensitive terahertz (THz) sensing of a water-glycerol mixture poured on a metamaterial surface using reflection-based THz time domain spectroscopy. The reflectance at the resonant dip frequency changes significantly with glycerol concentration. The detection sensitivity is about 8 times higher than that compared to the case without a metamaterial structure. We also investigate how sensitivity depends on the Q factor of the metamaterial resonance for highly absorptive media, such as a water solution in the THz region.     

Speed-up of cross-section collapsing with weight-window for subcritical burnup calculation

In the author’s group, a fusion–fission (FF) hybrid energy system has been analyzed using our own burnup calculation system consisting of Monte Carlo transport code MCNP-4C and point burnup code ORIGEN2.1. Since the neutron energy spectrum changes along with progress of burnup in a subcritical system, it is necessary to update one-group cross-section library in each burnup step. The one-group cross-sections are normally updated by collapsing the evaluated nuclear data such as JENDL and ENDF using a neutron flux calculated by an appropriate transport code such as MCNP. The collapsed cross-sections are handed over to ORIGEN, and the reaction rates for burnup of elements are thereafter estimated accurately.As well known, MCNP generates track-length (TL) data in the neutron transport calculation, which are base data to estimate the neutron flux. We thus use the track-length data directly instead of the calculated neutron flux, in order to evaluate the reaction rate as accurately as possible. However, the number of TLs becomes extremely large and thus it takes a longer computation time. We therefore reduce the number of TLs used in the cross-section collapsing process as far as the accuracy is conserved. However, in some energy region the number of TLs is inversely too small to conserve the original cross-section accuracy of the evaluated nuclear data files, because the number of TL data per unit energy is smaller than that of the nuclear data.In the present study, the weight-window (WW) technique of MCNP was applied to our burnup calculation system in order to control the number of TLs in such an energy region artificially and to complete the collapsing process accurately in the whole energy region. As a result, the variance of the calculated neutron flux thus deteriorates slightly, but the number of TLs could be successfully adjusted to conserve the accuracy of the nuclear data file in the whole energy region. And the accurate reaction rate estimation for burnup with MCNP was finally realized and simultaneously the computation time could be saved reasonably.     

Direct neutron spectrum measurement to validate Zr(n,2n) reaction cross-section at 14 MeV

Lithium zirconate, Li₂ZrO₃, is known as a candidate blanket material in a fusion reactor. Various neutronics benchmark experiments for zirconium have thus been carried out so far. According to the independent benchmark studies by two parties, the neutron spectrum calculations show fairly large overestimation for most evaluated nuclear data libraries. However, the reason has not yet been made clear up to now. The author's group expects it would be due to a problem of evaluation for the ^natZr(n,2n) reaction cross-section, because the cross-section measurement is basically not possible with the foil activation method for zirconium isotopes except for ⁹⁰Zr.In the present study, two neutrons emitted from ^natZr(n,2n) reaction have been measured directly to investigate the reason for the above overestimation. The measurement was done with our own special technique of detecting angle-correlated neutrons by the coincidence detection technique and the pencil-beam DT neutron source of FNS, JAEA. Angle-correlated energy differential cross-sections for ^natZr(n,2n) reaction were successfully measured. The obtained total cross-section above the emitted neutron energy of 800 keV was fairly larger than the one evaluated in JENDL-3.3. The total cross-section of ^natZr(n,2n) reaction was estimated by extrapolating the spectrum down to zero energy taking into account the nuclear temperature. The estimated cross-section value with the nuclear temperature of 1 MeV, which is larger than the one adopted in JENDL-3.3, was in acceptable agreement with JENDL-3.3. It is suggested from the result that the disagreement pointed out in the previous benchmark studies may be due to inappropriate nuclear temperature used in the evaluation. Further investigation of the nuclear temperature employed in the nuclear data evaluation should thus be carried out once again.