Temperature dependence of luminescence from a silica glass under in-reactor irradiation

We have investigated in-reactor luminescence (IRL) from a silica glass at temperatures ranging from 100 K to 250 K. The IRL consists mainly of a broad emission band peaked at 2.7 eV assigned to oxygen deficient centers produced in the silica glass under the in-reactor irradiation. The 2.7 eV emission intensity linearly increased with the irradiation time and its increasing rate was larger for higher irradiation temperatures. However, this temperature dependence is inconsistent with that for the defect production rate and the cause is not clear at present. The initial intensity of the 2.7 eV IRL band increases with temperature, showing an activation energy of ca 21 meV. This value is much lower than those observed in the temperature dependence of the 2.7 eV photoluminescence (PL) and the cathodeluminescence (CL) induced by 8 keV electron irradiation. These results suggest that in IRL, some electrons excited to higher energy levels than the luminescence level are likely transferred to the luminescence state without thermal activation, resulting in a lower activation energy in their temperature dependence.     

Low-temperature irradiation effects in lithium orthosilicates

The emission spectra of lithium orthosilicates (Li₄SiO₄₎ ceramics have been measured in the range of 1.8–5.8 eV under irradiation by 6–30 eV photons or 1–30 keV electrons at 6–300 K. The tunnel recombination phosphorescence, as well as luminescence, stimulated by 1.5–2.5 eV photons has been detected in the sample preliminarily irradiated at 6 or 80 K. The main peaks of thermally stimulated luminescence (TSL) in the irradiated ceramics have been observed at 72, 118 and 265 K. The creation spectra of the 118 K TSL peak, as well as the excitation spectrum of photostimulated luminescence (PSL) span the region of the intrinsic absorption of a lithium orthosilicate (9–30 eV). The intensity of PSL and the TSL peaks in Li₄SiO₄ ceramics prepared in hydrogen/argon atmosphere is several times lower than that in the mainly investigated Li₄SiO₄ ceramics prepared in the atmosphere of dry argon. The optical characteristics of Li₄SiO₄ are compared with the ones known for Li₂O and SiO₂. Low-temperature luminescent methods are promising for the investigation of electron–hole processes and radiation defects serving as the traps for tritium released in D–T fusion reactor blanket systems.     

Effect of γ-ray irradiation on Nd–Fe–B alloys

We report on how γ-ray irradiation affects the magnetic properties of a powder sample of Nd–Fe–B, which was irradiated at room temperature with doses up to 700 kGy. Both the magnetic properties and surface morphology were changed by the effects of the γ-ray irradiation. The unirradiated and irradiated samples were then characterized using the VSM, XRD and SEM techniques.     

Analysis of structural modifications in γ-irradiated PbO–B2O3–SiO2 glasses by FTIR spectroscopy

The effect of γ-irradiation on the structure of lead borosilicate glasses of varying composition has been probed by FTIR spectroscopy, before and immediately after γ-irradiation. The glasses were irradiated at Calliope ⁶⁰Co plant (RC ENEA Casaccia, Rome), and the spectra were recorded after absorbed doses of 50 Gy, 500 Gy, and 4 kGy. The structural analysis have been made considering both the effect of composition and of irradiation. The experimental results clearly indicate that after irradiation a significant change in structure of borosilicate glass network is observed.     

Observation of defect formation process in silica glasses under ion irradiation

We have performed in situ measurements of luminescence from silica glass induced by H⁺ beam at various energies to investigate the dynamic defect formation process in a silica glass. The luminescence spectra showed a broad emission band centered at around 2.7 eV assigned to B₂ oxygen deficient centers. The intensity of the 2.7 eV band rapidly increased to H⁺ fluence at first, and then gradually increased to a steady value. We found that the change in the intensity of the 2.7 eV band relates to two processes, fast and slow processes with different reaction rates for producing luminescence centers. The dependences of calculated reaction rates for faster and slower transformation versus H⁺ energy correspond well to those of electronic and nuclear stopping power, respectively. Consequently, the production of the luminescence centers under sub-MeV H⁺ irradiation is very likely dominated by electron excitation process at first, and subsequently, by nuclear collision process.     

Irradiation effects of swift heavy ions in actinide oxides and actinide nitrides: Structure and optical properties

Actinide oxides have been used as nuclear fuels in the majority of power reactors working in the world and actinide nitrides are under investigation for the fuels of the future fast neutron fission reactors developed in Forum Generation IV. Radiation damage in actinide oxides UO₂, (U_0.92Ce_0.08)O₂, and actinide nitride UN has been characterized after irradiation with swift heavy ions. Fluences up to 3 × 10¹³ ions/cm² of heavy ions (Kr 740 Mev, Cd 1 GeV) available at the CIRIL/GANIL facility were used to simulate irradiation in reactors by fission products and by neutrons. The macroscopic effects of irradiation remains very weak compared with those seen in other ceramic oxides irradiated in the same conditions: practically no swelling can be measured and no change in colour can be observed on the irradiated part of a polished face of sintered disks. The point defects in irradiated actinide compounds have been characterized by optical absorption spectroscopy in the UV–Vis–NIR wavelength range. The absorption spectra before and after irradiation are compared, and unexpected stability of optical properties during irradiation is shown. This result confirms the low rate of formation of point defects in actinide oxides and actinide nitrides under irradiation. Actinide oxides and nitrides studied are >40% ionic, and oxidation state of the actinides seems to be stable during irradiation. The small amount of point defects produced by radiation (<10¹⁶ cm⁻²) has been identified from differences between the absorption spectrum before irradiation and the one after irradiation: point defects in oxygen or nitrogen lattices can be observed respectively in oxides and nitrides (F centres), and small amounts of U⁵⁺ would be present in all compounds.     

Defect creation caused by the decay of cation excitons and hot electron–hole recombination in wide-gap dielectrics

Insufficient radiation resistance of construction materials is the Achilles heel for thermonuclear energetics. In wide-gap dielectrics, Frenkel defects are created not only because of the knock-out (impact) mechanism but also because of the decay of the electronic excitations formed during the irradiation (i.e. due to nonimpact mechanisms). The processes of the defect creation at the irradiation of highly pure LiF single crystals at 6–8 K by 1–30-keV electrons, X-rays, or synchrotron radiation of 12–70 eV have been investigated. The annealing processes of these defects in a temperature range up to 200 K have been studied as well. In LiF, creation has been revealed for the following: (1) F–H pairs caused by the decay of anion excitons or by the recombination of electrons and holes, (2) F′–H–V_K and F–I–V_K defect groups at the decay of cation excitons (62 eV), or (3) 20-keV electron irradiation. The mechanism of defect creation at the recombination of hot holes and hot electrons has been discussed for -SiO₂ crystals with an energy gap between the subbands of a valence band. One of the possible ways to suppress this mechanism (“luminescent defence”) is doping a material by luminescent impurities able to capture a part of the energy of hot carriers before their relaxation and recombination (e.g. in MgO:Cr).     

Heavy-ion-induced luminescence of amorphous SiO2 during nanoparticle formation

Silica glass was implanted with negative 60 keV Cu ions at an ion flux from 5 to 75 μA/cm² up to a fluence of 1 × 10¹⁷ ions/cm² at initial sample temperatures of 300, 573 and 773 K. Spectra of ion-induced photon emission (IIPE) were collected in situ in the range from 250 to 850 nm. Optical absorption spectra of implanted specimens were ex situ measured in the range from 190 to 2500 nm.

IIPE spectra showed a broad band centered around 560 nm (2.2 eV) that was assigned to Cu⁺ solutes. The band appeared at the onset of irradiation, increased in intensity up to a fluence of about 5 × 10¹⁵ ions/cm² and then gradually decreased indicating three stage of the ion beam synthesis of nanoclusters: accumulation of implants, nucleation and growth nanoclusters. The IIPE intensity normalized on the ion flux is independent on the ion flux below 20 μA/cm²at higher fluences. The intensity of the band increased with increasing samples temperature, when optical absorption spectra reveal the increase of Cu nanoparticles size.     

Analysis of molecular oxygen formation in irradiated glasses: a Raman depth profile study

Depth profiles experiments have been performed by Raman spectroscopy on three alkali (Na, Li, K) borosilicate glasses irradiated with 1.8 MeV electrons at 1 and 3 GGy. These experiments show that molecular oxygen produced under β irradiation is concentrated near the glass surface according to a depth depending on the irradiation dose. Moreover, we observed that the polymerisation increase is the same in the entire volume sample. The average Si–O–Si angle decrease under irradiation is also homogeneous in the whole irradiated glass volume. From all results, we demonstrate that oxygen migrates up to the glass surface during irradiation without strong interaction with the glass network. Migration of oxygen and probably alkalis takes place through percolation channels with a possible departure of oxygen in some cases.     

Thermal stability of radiation-induced optical centers in non-stoichiometric spinel crystals

The successive anneals of the neutron-irradiated non-stoichiometric spinel crystals MgO · 2.2Al₂O₃ leads to incremental change of optical spectra that demonstrates two main bands whose intensity and spectral position depends on annealing temperature. While temperature increases from 450 to 750 K, one of the bands shifts from photon energy of 4.2–5.1 eV. Another one shifts in the opposite direction from 6.4–5.6 eV. This effect can be attributed to coagulation of radiation induced defects near cation vacancies and change of the energy levels and transitions in F⁺- and F-centers in neutron-irradiated crystals. The final position of these two bands 5.1 and 5.6 eV corresponds to transitions in F⁺- and F-centers in non-stoichiometric spinel, respectively. The investigation of optical centers induced at subsequent UV-illumination of neutron-irradiated crystals annealed to temperature of 750 K, and comparison with as-grown crystals, shows the existence of residual concentration mainly of the antisite defects and partially of the anionic vacancies in the neutron irradiated and annealed crystals. The existence of two temperature stages where optical centers at antisite defects are effectively destroyed may indicate the presence of spatially correlated and isolated antisite defects in irradiated spinel.     

Effects of 160 MeV Ni ion irradiation on polypyrrole conducting polymer electrode materials for all polymer redox supercapacitor

Electronically conducting polymers are suitable electrode materials for high performance supercapacitors, for their high specific capacitance and high dc conductivity in the charged state. Supercapacitors and batteries are energy storage and conversion systems which satisfies the requirements of high specific power and energy in a complementary way. Ion beam {energy > 1 MeV} irradiation on the polymer is a novel technique to enhance or alter the properties like conductivity, density, chain length and solubility.

Conducting polymer polypyrrole thin films doped with LiClO₄ are synthesized electrochemically on ITO coated glass substrate and are irradiated with 160 MeV Ni¹²⁺ ions at different fluence 5 × 10¹⁰, 5 × 10¹¹ and 3 × 10¹² ions cm⁻². Dc conductivity measurement of the irradiated films showed 50–60% increase in conductivity which is may be due to increase of carrier concentration in the polymer film as observed in UV–Vis spectroscopy and other effects like cross-linking of polymer chain, bond breaking and creation of defects sites. X-ray diffractogram study shows that the degree of crystallinity of polypyrrole increases in SHI irradiation and is proportionate to ion fluence. The capacitance of the irradiated films is lowered but the capacitance of the supercapacitors with irradiated films showed enhanced stability compared to the devices with unirradiated films while characterized for cycle life up to 10,000 cycles.     

Study of 160 MeV Ni ion irradiation effects on electrodeposited polypyrrole films

Conducting polymer polypyrrole thin films doped with LiCF₃SO₃, [CH₃(CH₂)₃]₄NBF₄ and [CH₃(CH₂)₃]₄NPF₆ have been electrodeposited potentiodynamically on ITO coated glass substrate. The polymer films are irradiated with 160 MeV Ni¹²⁺ ions at three different fluences of 5 × 10¹⁰, 5 × 10¹¹ and 3 × 10¹² ions cm⁻². An increase in dc conductivity of polypyrrole films from 100 S/cm to 170 S/cm after irradiation with highest fluence is observed in four-probe measurement. X-ray diffractogram shows increase in the crystallinity of the polypyrrole films upon SHI irradiation, which goes on increasing with the increase in fluence. Absorption intensity increase in the higher wavelength region is observed in the UV–Vis spectra. The SEM studies show that the cauliflower like flaky microstructure of the surface of polypyrrole films turns globular upon SHI irradiation at fluence 5 × 10¹¹ ions cm⁻² and becomes smooth and dense at the highest fluence used. The cyclic voltammetry studies exhibit that the redox properties of the polypyrrole films do not change much on SHI irradiation.     

Measurement of β-ray maximum energy with an HPGe detector by means of β-annihilation photons coincidence

A β⁺-ray detection system free from summation of annihilation photons has been constructed for the determination of Q_EC-values. It consists of an HPGe β-ray detector and two pairs of BaF₂ scintillation detectors for annihilation photons. A Q_EC-value of 4.83(4) MeV is obtained for ¹²⁶Cs separated with the JAERI on-line isotope separator.     

Stopping power and Cherenkov radiation in photonic crystals

Electron energy loss spectroscopy (EELS) induced by fast electrons in electron microscopes are used to probe photonic structures. Some of the loss features are shown to be associated to the excitation of radiative modes in the samples (Cherenkov radiation), from where information on photonic bands is extracted. The case of a 1D crystal is qualitatively discussed to explain the physics behind Cherenkov radiation in photonic crystals. For an electron beam collimated on one of the pores of 2D crystal consisting of a porous alumina film with 100-nm lattice constant, theory predicts a Cherenkov feature at around 6–9 eV, which is in excellent agreement with experiment. Finally, the features of the loss spectra are shown to be strongly correlated with the density of photonic states, suggesting the potential application of this technique to probe the quality and actual performance of photonic crystals.     

Mean excitation energy for the stopping power of light elements

We have evaluated the mean excitation energy or I value for Coulomb excitations by swift charged particles passing through carbon, aluminum and silicon. A self-consistent Kramers–Kronig analysis was used to treat X-ray optical spectra now available from synchrotron light sources allowing us to carry out Bethe’s original program of evaluating I from the observed dielectric response. We find that the K and L shell are the dominant contributors to I in these light elements and that the contribution of valence electrons is relatively small, primarily because of their low binding energy. The optical data indicate that Si and Al have nearly equal I values, in contrast to Bloch’s Thomas–Fermi result, I ∝ Z. The optically based I values for C and Al are in excellent agreement with experiment. However, the dielectric-response I value for Si is 164 ± 2 eV, at variance with the commonly quoted value of 173 ± 3 eV derived from stopping-power measurements.     

Co γ-ray irradiation effects on dielectric characteristics of tin oxide films of different thicknesses on n-type Si(1 1 1) substrates

We have tried to determine the effects of ⁶⁰Co gamma irradiation on properties of Au/SnO₂/n-Si (MOS) structures such as dielectric constant (ε′), dielectric loss (ε″), tangent loss (tan δ) and ac conductivity (σ_ac). Three samples were fabricated with different deposition time. The samples were irradiated using a ⁶⁰Co γ-ray source irradiation with the total dose range of 0–500 kGy at room temperature. Capacitance and conductance (C–G–V) measurements were performed at a frequency of 500 kHz in the dark and at room temperature before and after irradiation. The experimental data were analyzed using complex permittivity and electric modulus. The values of ε′, ε″, tan δ and σ_ac showed a strong dependence on the applied voltage and irradiation dose. The dielectric properties of MOS structures have been found to be strongly influenced by the presence of dominant radiation-induced defects. Experimental results show that the interfacial polarization contributes to the improvement of dielectric properties of Au/SnO₂/n-Si (MOS) Schottky diodes.     

Helium retention of vanadium alloy after energetic helium ion irradiation

Helium irradiation experiments of V–4Ti alloy were conducted in an ECR ion irradiation apparatus by using helium ions with energy of 5 keV. The ion fluence was in the range from 1 × 10¹⁷ He/cm² to 8 × 10¹⁷ He/cm². After the helium ion irradiation, the helium retention was examined by using a technique of thermal desorption spectroscopy (TDS). After the irradiation, the blisters with a size of about 0.1 μm were observed at the surface, and the blister density increased with the ion fluence. Two desorption peaks were observed at approximately 500 and 1200 K in the thermal desorption spectrum. When the ion fluence was low, the retained helium desorbed mainly at the higher temperature regime. As increase of the ion fluence, the desorption at the lower temperature peak increased and the retained amount of helium saturated. The saturated amount was approximately 2.5 × 10¹⁷ He/cm². This value was comparable with those of the other plasma facing materials such as graphite.     

Kinetic electron emission in the interactions of slow ions with MgO surfaces

We report experimental energy distributions and yields of electrons emitted from MgO surfaces under the impact of slow noble gas and sodium singly charged ions at varying incident energies.

At impact energies below 1 keV, electron spectra are nearly independent of ion type and energy. A tail of high-energy electrons is observed to grow at higher impact energies.

The results are explained in terms of promotion of oxygen-2p electrons during binary projectile-oxygen collisions populating continuum and excitonic states. Excitons can significantly contribute to electron emission due to the negative electron affinity of the surface.     

Peak identification of L X-ray spectra of elemental Np and U

The L X-ray photons emitted by transuranic (TRU) elements are expected to be useful for developing nondestructive TRU monitors. Energy spectra of L X-rays emitted by ²⁴¹Am, ²³⁸Pu and ²³⁹Pu sources were measured by a transition edge sensor (TES) microcalorimeter, which allowed precise peak identification with high energy resolution. In the measurements using the TES microcalorimeter, the full width at half-maximum energy resolution was 62.6 eV at 17.222 keV for ²³⁹Pu source, 62.5 eV at 17.222 keV for ²³⁸Pu source and 60.9 eV at 17.751 keV for ²⁴¹Am source. This study demonstrates the separation of ²⁴¹Am and plutonium isotopes by L X-ray spectroscopy using a TES microcalorimeter.     

Gamma-Ray Spectra of Fission Products Observed with Lithium Drifted Germanium Detectors

The γ-ray spectra of fission products from thermal neutron irradiation of natural U were observed with an encapsulated Ii-drift-Ge γ-ray spectrometer. The spectra were recorded at various periods after irradiation—1, 2, 5, 12, 30, 250 days and 3 years. The γ-ray spectra of eight individual fission products including ²³⁹Np were also studied. Due to the high resolution obtained with the detector, many nuclides were identified which would have been undetectable without resorting to chemical separation.