OBJECTIVES radiation-sensitive thermoluminescent (TL) color- forming solid-state dielectric and transparent compounds Lithium Fluoride (LiF)

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1 Point defects in Lithium Fluoride films induced by gamma radiation M. Montecchi, S. Baccaro ENEA, Irradiation Technologies Laboratory, C.R. Casaccia, Via Anguillarese 301, S. Maria di Galeria,Rome (Italy) E. Nichelatti ENEA, Applied Physics Div., C.R. Casaccia, Via Anguillarese 301, S. Maria di Galeria, Rome (Italy) F. Bonfigli, T. Marolo, R.M. Montereali ENEA, Applied Physics Div., C.R. Frascati, Via E. Fermi 45, Frascati, Rome (Italy) 7th International Conference Advanced technology and particle Physics Villa Olmo, Como October 2001

2 OBJECTIVES Many radiation-sensitive thermoluminescent (TL) and colorforming solid-state dielectric and transparent compounds are utilized for radiation dosimetry. Among them, Lithium Fluoride (LiF) is one of the most used and investigated in different forms: pure and doped LiF crystals; LiF powders; microcrystalline LiF dispersions in plastic matrices; LiF films The great interest for new radiation detectors based on this material prompted us to a careful investigation of the optical properties of gamma irradiated LiF films grown by thermal evaporation on silica substrates. The preliminary results are encouraging to consider LiF films as sensors in compact, cheap and versatile dosimeters. OUTLINE Introduction Color centers in LiF crystals Color centers in LiF films Experimental LiF film: growth and charaterization Formation of color centers by gamma irradiation Optical and spectroscopic characterization Results Refractive index of uncolored and colored LiF films Optically Stimulated Luminescence (OSL) on colored LiF films Defect concetrations in gamma colored LiF films Conclusions and future perspectives

3 COLOR CENTERS IN LITHIUM FLUORIDE Color centers (CCs): point defects in insulating crystals. Alkali halides (AH): ionic crystals with face centered cubic structure, optically transparent from near UV to IR. Nearest neighbour distance (Å) Melting point ( C) Density (g/cm 3 ) Molecular weight Refractive 640 nm, RT Solubility (g/100g H 2 25 C) Transmission range (µm) Their irradiation causes coloration, i.e. the formation of discrete optical absorption bands, generally located in the visible spectral range, due to the presence of anionic lattice vacancies occupied by electrons. Among AH, LiF is of particular interest, from the applicative point of view, because it is almost non-hygroscopic; it can host point defects stable at RT; it can host optically active CCs emitting in a broad wavelength range in the visible and near infrared. The high efficiencies of green and red photoemissions from the F 3 + and F 2 defects (two electrons bound to three and two adiacent anion vacancies, respectively), when excited in their almost overlapping absorption bands located around 450 nm allows to utilize Optically Stimulated Luminescence (OSL) as useful method of dosimetry for this inorganic crystalline material.

4 Color Centers in LiF at RT Center E a (ev) λ (nm) E e (ev) λ (nm) HWa(eV) HW e (ev) τ(ns) Absorbed dose range (Gy) F F 2 F 3 + F 2 + F 2 - F3(R1) F 3 (R 2 ) F 3 - (R1 - ) F3 - (R2 - ) F 4 (N 1 ) F4(N2) colloidi x unstable x x Spectrophotometry of the darkening is a useful mean of determining large absorbed doses beyond the normal range of LiF thermoluminescence dosimetry. By selecting the optical wavelength corresponding to the maximum absorption of a suitable radiation-induced CC, a broad range of absorbed doses from 10 2 to 10 7 Gy may be monitored in pure LiF single crystals.

5 ALKALI HALIDE THIN FILMS Little attention has been paid to alkali halide films with the only exception of fluorides for their potential applications in optics ( coatings in the ultraviolet) microelectronics ( insulators on semiconductors ) sensors (dosimetry, nuclear sensors for neutrons,.. ) The opportunity to color only a thin layer located at the surface of a crystalline materials, bulk and/or film, by using low penetrating ionizing radiations has been investigated for basic and applied research, mainly making use of charged particles like low energy electron beams and ion beams, but also neutrons, x-rays, UV light,... Less investigation has been devoted to film irradiation with penetrating radiation due to the unavoidable effects of substrate coloration. The present trends on miniaturization calls for an exhaustive investigation of the optical properties of CCs produced in thin films of insulating materials. The same kinds of defects host in an AH polycrystalline matrix directly grown as thin films by several deposition methods on different types of substrates implies a good knowledge and control of the used growth methods an accurate investigation of the structural and morphological properties of the as grown materials. The formation efficiency of F-aggregate centers is higher in polycrystalline films than in single crystals and depends on the film growth conditions, which in turn influence the surface to volume ratio, the void presence and the preferred orientation of crystallites.

6 LiF FILM DEPOSITION BY THERMAL EVAPORATION Polycrystalline films are grown by thermal evaporation on amorphous (glass, silica, silica on silicon, ) and crystalline (LiF single crystals, NaF, MgF 2, silicon, ) substrates. oscillating quartz vacuum pump substrate holder crucible Deposition parameters: Pressure < 10-6 mbar Evaporation rate R = nm/s Total film thickness t = µm Substrate temperature Ts= C The structural, morphological and optical properties of the films are strongly dependent on the nature of the substrate the depositon parameters: T s, t, R s AFM scansion of a 2.6 µm thick LiF film thermally evaporated at T S =250 C on a glass substrate. The film is polycrystalline and the image shows a smooth surface with typical root mean square roughness of 17 nm over an area of 4x4 µm 2.

7 IRRADIATION AT CALLIOPE PLANT AT ENEA Casaccia The LiF films were exposed to gamma radiation from a 60 Co source at doses in the range Gy in air. Cell dimensions 7x6x3.9 m Maximum permitted activity 3.7x10 15 Bq (100kCi) Actual activity (30/6/2001) 1.03x10 15 Bq (28 kci) Maximum dose intensity 2.4 kgy/s (1.3Mrad/h) Rack with 60 Co sources Co 1.17 MeV 1.33 MeV β γ γ Ni* Ni Ni* Red-Perspex (5-40 kgy) and radiochromic (1 kgy 3 MGy) dosimeter

8 SPECTROPHOTOMETRIC CHARACTERIZATION OF UNCOLORED LiF FILMS A comprehensive model of an inhomogeneous thin film bounded by rough, unparallel interfaces has been used for characterization of uncolored LiF films from reflectance and trasmittance at normal incidence, measured by Perkin Elmer λ19 spectrophotometer. Wedged interfaces roughness roughness air substrate -Inhomogeneity: the refractive index of the film is not costant along the growth axis z, n film = n film (z). -Roughness: random deviations of the film interfaces from smooth planes. -Wedged interfaces: the film interfaces are not mutually parallel. 1.0 Reflectance and transmittance experimental model Wavelength (nm) T R Fit parameter Value Film thickness (3.06 ± 0.02) µm <n> at nm 1.385±0.6 n/n (1.8 ± 0.7) % Film roughness (19.0 ± 0.2) µm Wedged interfaces (1.0 ± 0.3) %

9 SPECTROPHOTOMETRIC CHARACTERIZATION OF GAMMA COLORED LiF FILMS Trasmittance (%) bare SiO 2 substrate F F 2 LiF on SiO 2, F + 3 initial after 1.16 x 10 5 Gy Wavelength (nm) 10 8 Reflectance (%) initial after 1.16 x 10 5 Gy Wavelength (nm)

10 LASER INDUCED LUMINESCENCE The OSL differs from TSL because the light emitting defect is not removed by the analytical measurement with the exciting light as it is with the heat induced glow curve used for TL analysis. It does not matter which portion of the uniformly irradiated film is read. Wavelength (nm) Luminescence (arb. units) F 2 LiF film 60 Co 1.16 x 10 5 Gy λ P = 458 nm RT Experiment Best fit F Energy (ev) Only after the instrumental calibration a best fit of the emission spectra as a sum of two gaussian bands with proper peak positions and halfwidths is obtained. So, apart the formation efficiency, the main spectroscopic features of defects do not show any substantial differences between films and bulk.

11 SPECTROFLUORIMETRIC CHARACTERIZATION OF GAMMA COLORED LiF FILMS The PL spectra are obtained using a Jobin-Yvon Fluorolog-3 spectrofluorometer with a front-face detecting geometry. Luminescence (arb. units) λ P = 458 nm LiF 60 Co 10 3 Gy LiF 60 Co 10 4 Gy LiF 60 Co 1.16x10 5 Gy LiF 60 Co 10 6 Gy λ P 20 PM T Wavelength (nm) F + 3 F 2 Net fluorimeter reading Dose in air (Gy)

12 SIMPLIFIED OSL SET-UP f = 50mm SAMPLE LiF on α-sio 2 LED King Bright 53 MBD f = 110mm GLASS FILTER SHOTT RG630 PMT MONOCHROMATOR Jobin Yvon H10 double UV λ = 650 nm ν = 50 nm 1.5 LiF 60 Co 10 6 Gy absorption LiF 60 Co 1.16 x 10 5 Gy absorption King Bright 53 MBD LED emission Normalized intensity Wavelenght (nm)

13 INTEGRATED OSL RESULTS Integrated red emitted light intensity vs absorbed doses shows a linear response in the investigated range up to values of 2x10 5 Gy. 100 LiF film 60 Co Signal (mv) Signal/noise Dose in air (Gy)

14 REFRACTIVE INDEX MODIFICATION The formation of high concentrations of F and F-aggregate defects in LiF induces a consistent modification of the material refractive index Extinction coefficient LiF film 60 Co 1.16 x 10 5 Gy Wavelength (nm) A simple dipole-field interaction model (M.Montecchi, E.Nichelatti, A.Mancini and R.M.Montereali, J.Appl.Phys. 86, 6 (1999) 3745) has been developed to quantitatively evaluate the single contributions of different kinds of electronic defects formation to the overall refractive index change of the colored volume of a low-energy electron irradiated LiF crystals. It allows to determine the wavelength dependence of the refractive index for the colored layer from: the measurements of a single transmission spectra; the knowledge of LiF color centers optical properties (absorption band peak position and half width).

15 DEFECT CONCENTRATIONS IN GAMMA COLORED LiF FILMS Absorption (cm -1 ) F F 2, F 3 + LiF 60 Co 10 6 Gy LiF 60 Co 1.16 x 10 5 Gy Wavelength (nm) From the Smakula formula 17 Nf = n N = center concentration in cm -3 f = oscillator strength n = refractive index α max = absorption coefficient in cm -1 W = absorption band halfwidth in ev n + 2 α 2 max W -3 Centre N(cm -3 ) N 1E6 Gy /N 1E5 Gy 10 5 Gy 10 6 Gy F 1.4x x ±0.7 F 2 3.4x x ±0.7 F x x ±0.7

16 CONCLUSIONS Polycrystalline LiF thin films, 3 µm thick, thermally evaporated on silica substrates have been optically characterized before and after gamma irradiation in the dose range from 10 3 to 10 6 Gy. It gives rise to the stable formation of: - primary F centers; - aggregate F 2 and F 3 + defects. Their efficient visible emissions have been measured at RT by different apparata. Emitted light intensity vs absorbed doses shows a linear response in the investigated range up to values of 2* 10 5 Gy both for green and red emissions separately, as measured by a spectrofluorimeter. The same behaviour has been obtained by detecting only the red part of the emitted spectrum by using a simplified luminescence set-up with a diode pumping, which sensitivity can be further improved in order - to extend the readout to lower doses; - and/or to simplify its scheme. A careful and accurate spectrophotometric characterization of the unirradiated and irradiated LiF samples allows to estimate the defect concentrations in the more colored layers, showing that the lack of a linear response at the highest doses seems due to a reduction in the active defects formation efficiency rather than to concentration quenching phenomena and can be calibrated. The thin films geometry supplies either single integrated dose readings or two-dimensional radiographic images. The obtained results are promising for the realization of new radiation detectors based on LiF thin films directly grown on different substrates.