Materials Science. An Indian Journal. Full Paper. Color centers in lithium fluoride irradiated with high energy heavy ions ABSTRACT KEYWORDS

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1 Volume 13 Iue 8 ISSN : MSAIJ, 13(8), 2015 [ ] Color center in lithium fluoride irradiated with high energy heavy ion H.Benhacine*, A.Meftah LRPCSI, Univerity 20 août 55 Skikda, route El-Hadaeik, Skikda, (ALGERIA) hbenhacine@gmail.com ABSTRACT Single crytal ample of lithium fluoride were irradiated at room temperature with different energy ( and 840 MeV) Pb +53 ion in fluence range between and ion/cm 2. The optical aborption and photoluminecence (PL) technique were ued for characteriation of the irradiated ample. The damage i dominated by the creation of imple defect ( center) and aggregated defect ( n center). The concentration of -center increae a a function of fluence and energy. On the other hand two very intene PL band are oberved at around 545 and 665 nm which correpond to the emiion of + 3 and 2 center repectively and increae linearly in the ame fluence range Trade Science Inc. - INDIA KEYWORDS Lithium fluoride; Swift heavy-ion irradiation; Optical aborption; Photoluminecence INTRODUCTION The formation of defect in lithium fluoride (Li) and in other alkali halide created under variou type of irradiation, e.g. photon, electron, neutron [1] and ion [2-4] ha been tudied for long time by everal group. In Li primary radiation defect are -center (electron trapped by an anion vacancy) and H center (intertitial halogen atom) [5]. At higher irradiation temperature and higher fluence, thee primary defect are tranformed to more complex defect uch a n -aggregation and Li colloid. Uing the chemical etching technique, Young [6] ha oberved damage due to track of fiion fragment in irradiated Li crytal with heavy ion. Perez et al. [7] tudied Li crytal, irradiated with different ion with energy between 30 and 60 MeV/ u. Uing optical aborption pectrocopy it wa hown that the main defect in track are and 2 center. Moreover, Balanzat et al. [8] tudied variou crytal (Li, NaCl, and KBr) irradiated with heavy ion at low temperature, demontrated that the reulting exciton luminecence pectra and the dependence of the efficiency on temperature are the ame a under x-ray or optical excitation. The fluence dependent behavior of and 2 color center induced by SHI irradiation in Li crytal ha been tudied by Trautmann et al [9].The number of center per unit volume initially increae and aturate at higher fluence, the etimated track radiu from the aturation curve of center were found to be about 5 30 nm, depending upon ion energy. The ion-induced damage on the urface inpected by canning force microcopy i reported by Müller

2 268 Color center in lithium fluoride irradiated. with high energy heavy ion et al. [10, 11]. In thi report, we concentrate on ion-induced defect in Li by mean of optical aborption pectrocopy, complementing tudie performed with photoluminecence. EXPERIMENTAL PROCEDURE Li ample of variou thicknee, between 0.5 and 1mm, were cleaved along of the (100) plane from a ingle crytal block of high purity. The crytal of Li have been irradiated at room temperature under normal incidence with 208 Pb ion of 936 MeV energy delivered by the Medium Energy Line of the GANIL (Caen, rance). The fluence extended from to ion.cm -2 and the flux wa about ion cm -2-1 on a 1 cm 2 urface. Thin aluminum foil of different thicknee were placed in front of each ample in order to modify the initial energy of the ion and conequently the range R, and de dx e. The ion rang wa in all cae le than the ample thickne, the ion beam wa topped in the crytal. The main irradiation parameter, lited in TABLE 1, were deduced from TRIM 2005 [12] code calculation. The irradiated ample were analyzed by optical aborption pectrocopy (SHIMDZU 1700) in the pectral range nm and by photoluminecence (Perkin Elmer LS50B Luminecence Spectrometer). With the optical aborption pectrocopy technique it i poible to obtain information about the generated defect in the crytal by ion irradiation, and to calculate the concentration of defect. The concentration of defect can be determined by the Smakula formula, modified by Dexter [13] : Ion 208 Pb +53 e MSAIJ, 13(8) 2015 (1) Where f i the ocillator trength of the optical tranition, n the refractive index, á max the aborption coefficient meaured at the maximum of the band peak and W the band full-width at half maximum (WHM). A i a contant, which take ev -1 cm -2 for Gauian band. The aborption coefficient á max wa determined from the optical pectra of each ample uing the relation: where: (2) (3) Repreent the optical denity at band maximum and R i the ion range. RESULTS AND DISCUSSION Optical aborption pectrocopy TABLE 1 : Irradiation parameter of Pb in Li crytal E(MeV) de de Range E/R dx dx n (ìm) (kev/nm) (kev/nm) (kev/nm) Typical optical aborption pectra obtained with Li crytal irradiated with 2.6 MeV/u and 4.1 MeV/ u Pb ion at different fluence are reported in igure 1.a and igure 1.b. Aborption band how two predominant band around 245 and 445 nm correponding to and 2 center (two electron trapped in two neighboring anion vacancie) repectively. At higher fluence, of ion/cm 2 the pectra become more complex due to track overlapping, two band at about 315 and 375 nm, and two weak band at about 515 and 545 nm. All thee band are well known, and are commonly labeled a, R 1, R 2 ( 3 -center) and N 1, N 2 ( 4 -center) aborption repectively. The aborption band around 445 nm i a reult of the overlapped band correponding to the 2 and center (two electron trapped in a three neighboring anion vacancie). We oberved the area of the aborption pectrum for Li irradiated increae a a function of ion fluence and aturated at high fluence. The and 2 luence Ion/cm 2

3 MSAIJ, 13(8) 2015 H.Benhacine and A.Meftah 269 igure 1.a : Optical aborption pectra of Li irradiated with 540 MeV Pb ion of variou fluence igure 1.b : Optical aborption pectra of Li crytal irradiated With Pb ion (4.1 MeV/u) of variou low fluencie center band of Li were analyzed by mean of Eq. (1). The evolution of N a a function of the ion fluence i preented in igure 2. In the low fluence regime, N follow a linear increae. or fluence higher than ion.cm -2, N aturate reaching a maximum value of cm -3. Uing the imple model propoed by Thévenard et al. [1], the ituation can be implified auming that -center are homogeneouly ditributed in a cylindrical volume around the ion trajectory. The -center creation obtained from thi model i in the form: (4) where N (cm-3 ) i the total -center concentration obtained for an irradiation with an ion fluence (ion.cm -2 ), N (cm -3 ) i the aturated concentration of -center in each individual track, and r i the radiu of the track. Another intereting parameter deduced from the fit of the -center curve in igure 2 i the aturation level cm -2, correponding to a volume concentration of about cm -3. The concentration of 2 centre tart aturating at a fluence of ion.cm -2. The reulting aturation wa due to the overlapping of neighboring track [9]. We found that radii of the -center halo increae

4 270 Color center in lithium fluoride irradiated. with high energy heavy ion MSAIJ, 13(8) 2015 igure 2 : Concentration of -center (triangle) and 2 -center (circle) a a function of the fluence of Pb ion with an energy of 1.2 MeV/u, and concentration of -center (quare) at an energy 0.7 MeV/u igure 3 : Track radii in Li deduced from optical aborption band of -center plotted veru the mean energy lo ( ), track radii of thi work ( ) with the mean energy lo of the Pb ion from 9 nm up to 22 nm. In igure 3, the radii are preented a a function of the mean energy lo, i.e., the total ion energy i divided by the range. The track radii of - center in Li irradiated with Pb at different energie are compared with thoe of Li irradiated variou ion pecie [9]. In order to compare the evolution of and 2 aggregate center per track a a function of the fluence at the different energie, the number per ingle track of center can be determined by: [14],where n i given in cm -2. Since the different band in the nm region trongly overlap were it i difficult to analyze individual contribution eparately due to the unknown ocillator trength of the different center. The creation of all

5 MSAIJ, 13(8) center ha been etimated from the parameter a which i deduced from the integrated aborption: normalied by the ion fluence H.Benhacine and A.Meftah 271 (5) (6) Where: a the integral aborption. The dependence of n and a on the irradiation fluence and with different energie are diplayed in igure 4 and igure 5. In all cae the number n and a per ion a a function of the fluence take the ame evolution. At energie 144 MeV and 255 MeV the number n of color center produced per incident ion decreae with fluence. However, the n aggregate center per ion increae in the firt tage of the fluence, reach a maximum and finally decreae at higher fluence. Thi maximum appear at fluence ion.cm -2 at an energy correponding to 0.7 MeV/u, and ion.cm -2 with 1.2 MeV/u. The number of color center produced per ion track decreae at higher fluencie. Thi decreae give an igure 4 : Concentration of -center ( Pb ion at different energie n ) per ion track a a function of fluence for Li crytal irradiated with igure 5 : Concentration of aggregate (a ) per ion track a a function of fluence for Li crytal irradiated with Pb ion at different energie

6 272 Color center in lithium fluoride irradiated. with high energy heavy ion MSAIJ, 13(8) 2015 igure 6 : Number of center i expreed in ion/cm 2 ) n per ion a a function of the mean energy lo at high-fluence radiation (fluence igure 7 : Number of n aggregate center a per ion a a function of the mean energy lo at high-fluence indication that the recombination procee of electron and hole center become dominant in thi regime. Comparing the reult of Li to thoe obtained earlier for Li irradiated of Ni ion at 170 MeV and with Xe ion 300 MeV [15], we oberve very imilar effect. Alo the evolution of n and a a a function of fluence i imilar with Mg 2 [15]. igure 6 and 7, diplay -center and n aggregate center per track a a function of the mean energy lo at high ion fluence. The concentration of -center ( n ) and aggregate (a ) per ion track are proportional to the mean energy lo, and inverely proportional to the fluence. It hould be mentioned that the number of n and a per ion continue to increae with incident energy. The value of the energy 4.1M ev /u (S e ~27

7 MSAIJ, 13(8) 2015 KeV/nm) and 2.6 MeV/u (S e ~ 27.4 KeV/nm) are ituated on the right and the left of the Bragg peak. Photoluminecence meaurement The photoluminecence technique allow the H.Benhacine and A.Meftah 273 poibility to tudy with more preciion the 3 and 2 center. The emiion band of and 2 center are well reolved contrary to their aborption band. The excitation with a 445 nm wavelength-photon induce emiion band centred at about 553 nm and 680 nm aigned to and 2 center repectively [16, 17]. Moreover, The linear dependence between luminecence intenity and the number of related radiation defect i only valid at ignificantly lower value of optical denity (<0.4). Thu only 2 and with optical denity lower than 0.5 i conidered. igure 8 how an example of photoluminecence pectra of Li ingle crytal irradiated with 144 MeV Pb ion at the value of fluence 6, 8, 1, 8 and ion. cm -2. A mentioned above the pectra how two emiion band with maxima at about 2.17 ev (570 nm) and 1.95 ev (640 nm) due to the luminecence of and 2 center [18,19] repectively. The line in the figure i the fit of the experimental data uing two Gauian peak. The fit re- igure 8 : Photoluminecence emiion pectra of Li crytal irradiated with 144 MeV Pb ion at different fluence (fluence i expreed in ion/cm 2 ) igure 9 : PL intenity 2 and color center a a function of fluence

8 274 Color center in lithium fluoride irradiated. with high energy heavy ion MSAIJ, 13(8) 2015 veal approximately the ame emiion band width with increaing fluence and electronic topping power. The 2 and intenitie linearly increae with increaing fluence a hown in igure 9. igure 10 : PL intenity 2 and color center a a function of mean energy lo CONCLUSIONS Thi paper report on the damage produced in ingle crytal of Li by irradiation with lead ion at different energie in the range of everal hundred MeV. Single defect uch a -center are produced in a large halo of 9 22 nm around the ion trajectory and track radii increae with the mean energy lo. Photoluminecence pectrocopy reveal that center aggregate (, 2 ) increae linearly with fluence, but not linearly with mean energy lo. Increae in PL intenity of the irradiated ample indicate an increae in high energy induced radiation defect in the ample. ACKNOWLEDGEMENT The author are particularly indebted to Pr. A. Meftah to have placed at our dipoal the ingle crytal of Li ued in thi tudy within the framework of project CMEP, Tail 03, MDU 573, and Dr. L. Gerbou from Nuclear Reearch Center of Algier for photoluminecence meaurement. REERENCES [1] P.Thevenard, G.Guiraud, C.H.S.Dupuy, B.Delaunay; Radiat.Eff., 32, 83 (1977). [2] K.Schwartz, G.Wirth, C.Trautmann, T.Steckenreiter; Phy.Rev.B, 56, (1997). [3] C.Trautmann, K.Schwartz, J.M.Cotantini, T.Steckenreiter, M.Toulmonde; Nucl.Intr.and Meth.B, 146, 367 (1998). [4] C.Trautmann, K.Schwartz, O.Geiß; J.Appl.Phy, 83, 3560 (1998). [5] N.Itoh, K.Tanimura; J.Phy.Chem.Solid, 51, 717 (1990). [6] D.A.Young; Nature (London), 183, 375 (1958). [7] A.Perez, E.Balanzat, J.Dural; Phy.Rev.B, 41, 3943 (1990). [8] E.Balanzat, S.Bouffard, A.Caimi, E.Dooryhee, L.Protin, J.P.Grandin, J.L.Doualan, J.Margerie; Nucl.Intr.and Meth.B, 91, 134 (1994). [9] C.Trautmann, M.Toulemonde, K.Schwartz, J.M.Cotantini, A.Müller; Nucl.Intr.And Meth.B, 164, 365 (2000). [10] A.Müller, R.Neumann, K.Schwartz, C.Trautmann; Nucl.Intr.and Meth.B, 146, 393 (1998). [11] A.Müller, C.Müller, R.Neumann,.Ohneorge; Nucl.Intr.and Meth.B, 166, 581 (2000). [12] J..Ziegler, P.Bierack, U.Littmark; in: The Stopping and Range of Ion in Matter, J..Ziegler Pergamon, New York, (1985). [13] W.B.owler; Editor, Phyic of Color Center, Aca-

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