Bryan M. van der Ende

Safeguards measures are employed at nuclear reactor facilities worldwide, to ensure that nuclear material is not diverted from peaceful uses. Typical safeguards measures involve periodic inspections, off-line verification and video surveillance of fuel cycle activities. Real-time verification of the fissile contents via stand-off monitoring can enhance continuity of knowledge for non-traditional reactor types, including research reactors and small modular reactors. Here we demonstrate the feasibility of using large-area neutron detectors for monitoring nuclear reactors at stand-off distances up to 100 m outside reactor shielding, as a potential reactor safeguards tool. Since the neutron yield per unit reactor power depends upon the isotopic composition of the reactor core, declared changes in fissile composition can be verified without accessing the core. The supporting results of experiments conducted at the National Research Universal reactor in Canada, are presented.

Energy losses in solar cells caused by the spectral mismatch can be reduced by adapting the solar spectrum using a downconversion material where one higher energy visible photon is ‘cut' into two lower energy near-infrared photons that both can be absorbed by the solar cell. Downconversion with the (Pr³⁺, Yb³⁺) couple in YF₃ is investigated. Based on analysis of luminescence and diffuse reflectance spectra it is evident that two-step energy transfer takes place from the ³P₀ level of Pr³⁺ (around 490 nm) exciting two Yb³⁺ to the ²F_5/2 level giving emission around 980 nm. The transfer efficiency increases with Yb³⁺ concentration and is 86% for YF₃ doped with 0.5% Pr³⁺ and 30% Yb³⁺. Due to concentration quenching the intensity of emission from Yb³⁺ is strongly reduced and the ²F_5/2 emission intensity reaches a maximum for the sample with 0.5% Pr³⁺ and 2–5% Yb³⁺ at 300 K. Temperature dependent measurements reveal the role of the Pr^{3+ 1}G₄ level in the energy transfer between Pr³⁺ and Yb³⁺. Back-transfer of excitation energy from the Yb^{3+ 2}F_5/2 level to the ¹G₄ level of Pr³⁺ occurs and quenches the Yb³⁺ emission. The quenching is shown to become more efficient between 4 and 50 K due to faster phonon-assisted energy transfer between the Yb³⁺ donors. Upon raising the temperature from 50 to 300 K, the luminescence life time of the Yb³⁺ emission increases again because the small energy difference between the Pr³⁺ (¹G₄) level and the Yb³⁺ (²F_5/2) level (∼300 cm⁻¹) which makes the ¹G₄ less efficient as a trap for the excitation energy. The present results give insight into factors involved in the concentration quenching in downconversion materials based on the (Pr³⁺, Yb³⁺) couple.

Energy losses inherent to the conversion of sunlight to electricity in solar cells are mainly due to the so-called spectral mismatch: low energy photons are not absorbed while the energy of high energy photons is only partly used by the solar cell. The losses can be significantly reduced by adapting the solar spectrum. A promising avenue is the use of a downconversion material where one higher energy visible (blue-green) photon is ``cut'' into two lower-energy near-infrared photons that both can be used by the solar cell. Here the efficiency of downconversion for the (Nd3+,Yb3+) couple in YF3 is studied to investigate if efficient two-step energy transfer occurs from the G49/2 level of Nd3+ (situated around 21\,000$\hskip0.3em${}cm$-${}1 or 470 nm) exciting two neighboring Yb3+ ions to the F25/2 level (around 10\,000$\hskip0.3em${}cm$-${}1 or 1000 nm). Optical measurements of YF3 doped with Nd3+ and Yb3+ show that there is efficient energy transfer from Nd3+ to Yb3+, but downconversion from the G49/2 level does not occur due to fast multiphonon relaxation. Relaxation from this level to lower-energy levels populates the F43/2 level of Nd3+ from which efficient one-step energy transfer to Yb3+ occurs. Analysis of the luminescence decay curves for different Yb3+-concentrations using Monte Carlo simulations reveals a high nearest neighbor transfer rate (3.3\ifmmode\times\else\texttimes\fi{}105$\hskip0.3em${}s$-${}1) through a dipole-dipole interaction mechanism. Downconversion is observed from the D43/2 level (situated in the UV, around 28\,000$\hskip0.3em${}cm$-${}1 or 360 nm) with an estimated quantum efficiency up to 140%. For application in solar cells this UV to 2 NIR downconversion will only result in a marginal reduction of spectral mismatch losses.

The use of lanthanide ions to convert photons to different, more useful, wavelengths is well-known from a wide range of applications (e.g. fluorescent tubes, lasers, white light LEDs). Recently, a new potential application has emerged: the use of lanthanide ions for spectral conversion in solar cells. The main energy loss in the conversion of solar energy to electricity is related to the so-called spectral mismatch: low energy photons are not absorbed by a solar cell while high energy photons are not used efficiently. To reduce the spectral mismatch losses both upconversion and downconversion are viable options. In the case of upconversion two low energy infrared photons that cannot be absorbed by the solar cell, are added up to give one high energy photon that can be absorbed. In the case of downconversion one high energy photon is split into two lower energy photons that can both be absorbed by the solar cell. The rich and unique energy level structure arising from the 4fⁿ inner shell configuration of the trivalent lanthanide ions gives a variety of options for efficient up- and downconversion. In this perspective an overview will be given of recent work on photon management for solar cells. Three topics can be distinguished: (1) modelling of the potential impact of spectral conversion on the efficiency of solar cells; (2) research on up- and downconversion materials based on lanthanides; and (3) proof-of-principle experiments. Finally, an outlook will be given, including issues that need to be resolved before wide scale application of up- and downconversion materials can be anticipated.