Vacuum ultraviolet scintillators: sodium salicylate and p-terphenyl

Imaging Extreme Ultraviolet Radiation Using Nanodiamonds with Nitrogen-Vacancy Centers

Extreme ultraviolet (EUV) radiation with wavelengths of 10-121 nm has drawn considerable attention recently for its use in photolithography to fabricate nanoelectronic chips. This study demonstrates, for the first time, fluorescent nanodiamonds (FNDs) with nitrogen-vacancy (NV) centers as scintillators to image and characterize EUV radiations. The FNDs employed are ∼100 nm in size; they form a uniform and stable thin film on an indium-tin-oxide-coated slide by electrospray deposition. The film is nonhygroscopic and photostable and can emit bright red fluorescence from NV0 centers when excited by EUV light. An FND-based imaging device has been developed and applied for beam diagnostics of 50 nm and 13.5 nm synchrotron radiations, achieving a spatial resolution of 30 μm using a film of ∼1 μm thickness. The noise equivalent power density is 29 μW/(cm2 Hz1/2) for the 13.5 nm radiation. The method is generally applicable to imaging EUV radiation from different sources.

Evaluation of the Photophysical Properties of Two Scintillators: Crystalline Para-terphenyl and Plastic-Embedded 2,5-Diphenyloxazole Dye (EJ-276) at Room and Cryogenic Temperatures

The photophysical characterization of two dyes used as scintillators, crystalline para-terphenyl and EJ-276, a plastic heavily doped with 2,5-diphenyloxazole (DPO), was investigated with steady-state absorption, time-resolved emission, and transient absorption at room and cryogenic temperatures. Application of time-gated emission spectroscopy allowed for the measurement of phosphorescence spectra and their temporal dynamics. The photophysical properties of plastic-embedded DPO are not substantially altered compared to those previously determined for this dye in solvents. Notably, the amount of delayed fluorescence is always greater than that of phosphorescence. However, our study of crystalline para-terphenyl suggests that a second phase called β (perhaps comprising more planar molecules) functions as a triplet trap and decreases the amount of delayed fluorescence relative to phosphorescence. While the "main form" of para-terphenyl dominates absorption, the emissive properties (fluorescence, phosphorescence, and delayed fluorescence) are dominated by the β-phase. Studies of the para-terphenyl crystal performed with femtosecond time-resolved transient absorption demonstrate that excitation from the main form of the para-terphenyl crystal is promptly transferred to the β-phase with a time constant of roughly 300 ps. This work provides insight into the photophysical properties of two scintillators utilized to differentiate γ-ray- and neutron-induced signals.

Nitrogen-Vacancy Centers in Diamond for High-Performance Detection of Vacuum Ultraviolet, Extreme Ultraviolet, and X-rays

Fluorescent nanodiamonds (FNDs) containing nitrogen-vacancy (NV) centers as built-in fluorophores exhibit a nearly constant emission profile over 550-750 nm upon excitation by vacuum-ultraviolet (VUV), extreme ultraviolet (EUV), and X-radiations from a synchrotron source over the energy (wavelength) range of 6.2-1450 eV (0.86-200 nm). The photoluminescence (PL) quantum yield of FNDs increases steadily with the increasing excitation energy, attaining a value as great as 1700% at 700 eV (1.77 nm). Notably, the yield curve is continuous, having no gap in the VUV to X-ray region. In addition, no significant PL intensity decreases were observed for hours. Applying the FND sensor to measure the absorption cross-sections of gaseous O₂ over 110-200 nm and comparing the measurements with the sodium-salicylate scintillator, we obtained results in agreement with each other within 5%. The superb photostability and broad applicability of FNDs offer a promising solution for the long-standing problem of lacking a robust and reliable detector for VUV, EUV, and X-radiations.

Vacuum UV Polarization Spectroscopy of p-Terphenyl

p-Terphenyl is used as a component in a variety of optical devices. In this investigation, the electronic transitions of p-terphenyl are investigated by synchrotron radiation linear dichroism (SRLD) spectroscopy in the range 30000-58000 cm^-1 (330-170 nm) on molecular samples aligned in stretched polyethylene, thereby extending the region investigated by polarization spectroscopy into the vacuum UV. The resulting partial absorbance curves reveal that the vacuum UV band system with a maximum at 55000 cm^-1 (180 nm) is predominantly short axis-polarized. This result is of interest in the optical applications of p-terphenyl, for example as a wavelength shifter. The observed polarization spectra are compared with the results of quantum chemical model calculations. Convoluted versions of the transitions predicted with the semiempirical ZINDO method and with the long-range-corrected time-dependent density functional theory (TD-DFT) procedures TD-CAM-B3LYP, TD-LC-ωPBE, and TD-ωB97XD are in similar qualitative agreement with the observed partial absorbance curves throughout the investigated spectral regions, while TD-B3LYP fails to predict qualitatively the spectrum of p-terphenyl in the region above 40000 cm^-1 (250 nm).

Thin-film photodetectors for the vacuum ultraviolet spectral region

We report on thin-film photodetectors optimized for detecting the vacuum UV and rejection of the visible spectrum of electromagnetic radiation. The devices are made of hydrogenated amorphous silicon and silicon carbide on a glass substrate. At room temperature the photodetectors exhibit quantum efficiencies of 52% at lambda = 58.4 nm, 1% at lambda = 400 nm, and 0.1% at lambda = 650 nm. The response time for UV pulses from an N(2) laser gives signals of 6-mus full width at half-maximum and 500-ns rise time.

Soft-x-ray damage to p-terphenyl coatings for detectors

The organic phosphor p-terphenyl is used as a wavelength-converter coating in some soft-x-ray detectors. We have measured the absolute photoluminescent efficiency of p-terphenyl as a function of incident photon energy from 36 to 191 eV. We have also measured changes in the efficiency caused by soft-x-ray fluence (total photons absorbed per unit area) at several photon energies in this range. We find that efficiency drops rapidly as a function of fluence, with the rate of decrease increasing with higher soft x-ray energies.

Phosphor film characterization measurements in the vacuum UV using a multichannel detector

A linear microchannel plate-reticon array detector is used to characterize phosphor films. These films upconvert vacuum UV radiation into the detector's sensitive spectral range (wavelengths >350 nm). The methodology for measuring the sensitivity as well as the angular dependence of fluorescence of films is described, and absolute as well as relative efficiencies for several phosphors is given. The use of this device circumvents some of the problems inherent in such characterization measurements.