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Radiation Measurements 36 (2003) 225–228www.elsevier.com/locate/radmeas

AFM observations of latent !ssion tracks on surfaces:amorphous SiO2 and quartz

G. Espinosaa ;∗, J.I. Golzarria, C. V1azquezb, R. Fragosob, L.T. Chaddertonc, S.A. Cruzd

aInstituto de F��sica, UNAM. Apdo. Postal 20-364, M�exico, DF 01000, M�exicobDepartamento de F�isica, CINVESTAV, M�exico, DF, Mexico

cAtomic and Molecular Physics Laboratories, Institute of Advanced Studies, Australian National University,Canberra, ACT 2601, Australia

dDepartamento de F��sica, Universidad Aut�onoma Metropolitana-Iztapalapa, Apdo. Postal 55-534, M�exico, DF, Mexico

Received 21 October 2002; accepted 22 April 2003

Abstract

Preliminary results of a systematic AFM experimental investigation of the surface ‘track’ e<ects produced by the passageof !ssion fragments from a californium (252Cf ) source into amorphous SiO2 and quartz are described. Fission fragments fromthe source were collimated using a 10 �m thick aluminum foil and comprised fragments with the usual binary distributionof energies—light and heavy—79.4 and 103:8 MeV. Irradiations and AFM measurements were carried out in air at normalroom temperature and pressure. Remarkably high sputtering yields/fragment were discovered, and in the case of crystallinequartz the ejecta was found to be arranged in an ordered manner. A brief discussion is given of a part likely to be played byelectronic energy loss induced Coulomb explosion of target atoms for each point of fragment entry.c© 2003 Elsevier Ltd. All rights reserved.

Keywords: Latent track; SiO2; Atomic force microscope (AFM)

1. Introduction

Radiation e<ects associated with !ssion (Chadderton andTorrens, 1969) and speci!cally the induced damage at thesurface of materials due to irradiation with !ssion fragments,and subsequent latent track formation, is a topic of renewedand growing interest (Tombrello, 1994; Havancsak et al.,1997; Yamamoto et al., 1997; Vacik et al., 1999). Neverthe-less the further development of new nanometer-scale instru-mental measurement methods for material surfaces, whichyield more accurate and speci!c direct information aboutsurface e<ects due to the impact of energetic charged parti-cles is only slowly being realized. As part of a program in-volving applications of atomic force microscopy (AFM) tosurface radiation damage generally, we have !rst carried outpreliminary studies of amorphous SiO2(commercial optical

∗ Corresponding author. Tel.: +52-55-56225051;fax: +52-55-56161535.

E-mail address: espinosa@!sica.unam.mx (G. Espinosa).

!ber material) and natural crystalline quartz. Our primaryinterest has been to determine the nature of the damageproduced on these surfaces by individual !ssion fragmentsi.e. latent surface tracks.

2. Experiment

An atomic force microscope (from TM Microscopes-Veeco Metrology GroupJ, model Autoprobe CP Research)was used in the contact mode for the surface topographyanalysis. The AFM probe consisted in a silicon cantileverwith a silicon conical tip of height 5–7 �m and radius ofcurvature 10 nm. In the scanning process a constant forceof 10 nN was set up. The cantilever used is factory cal-ibrated with a force constant of 0:26 N=m. CommercialSiO2 optical !ber material (Nokia CableJ) and naturalquartz samples were carefully cut into pieces appropriatefor AFM analysis. The pristine materials were then scannedand characterized in the AFM, and the corresponding topo-graphic surface images recorded. The same materials were

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226 G. Espinosa et al. / Radiation Measurements 36 (2003) 225–228

afterwards exposed to a 252Cf source with a homogenousactivity of ’= 98 < min−1 cm−2. The exposure was madein air at normal temperature, an aluminum foil cylinderof 10 �m thickness being placed between the californiumsource and the target in order to achieve as near normalparticle incidence as possible.

In this experiment, !elds of 100×100 �m2 were selectedin each case, and systematically searched in the AFM in or-der to !nd single damaged zones over an area of 5×5 �m2,and down to 2 × 2 �m2. Thus, assuming a search areaA= 2:5 × 10−7 cm2 we may estimate the exposure time to!ssion fragments in terms of the probability to achieve atleast one impact within this area. If we consider the irradia-tion process and impacts as a series of uncorrelated, randomevents, the probability for at least one impact in the area Aiout of the total number of !ssion fragments impinging inthe total observation area in a time t is given (Lengar et al.,2002; Ilic and Najzer, 1990) as

P = 1 − e−Ai’t : (1)

In 1 week the total number of fragments impinging upon thetotal area A0 = 100 × 100 �m2, is given by

N0 = ’tA0 = 98 (< min−1 cm−2) (10080 min)

×(10−4 cm2) = 98:78 < (2)

and for the area Ai the argument in the exponential term inEq . (1) becomes

Ai’t = (2:5 × 10−7 cm2) (10080 min)

×(98 < min−1 cm−2) = 0:24696 < : (3)

Hence, P = 0:2188; and the number of zones of areaA= 2:5× 10−7 cm2 having at least one impact is estimatedas

Nimp = (A0=Ai)P = 400P = 87:5: (4)

(a) (b)

Fig. 1. Microphotography of the optical !ber obtained by the AFM, showing the surface topography of the SiO2 amorphous: (a) beforeexposure to radiation, and (b) after being exposed to !ssion fragment source. Area of 5 × 5 �m2. Total vertical scale of 136:5 JA (a) and1190 JA (b).

Thus the number of estimated impacts per week in each ofthese zones is obtained by simply dividing the number offragments over the total area A0 (=98:78 <) by this numberof zones:

Number of impacts = 98:78 < =87:5 = 1:1 < : (5)

The number of zones with one impact/zone increases pro-portionately with irradiation time up to about 2.5 weeks,after which the probability for 2,3,4,5 impacts increasesyielding the total number of zones damaged in 20 weeks(with an average of !ve impacts/zone). After this time, morethan !ve impacts per zone are predicted. From these esti-mates we !nd that the optimal exposure time necessary forone impact per zone is between 7 and 20 days.

3. Results

3.1. Amorphous SiO2 (commercial optical :ber SiO2

surfaces)

Fig. 1(a) displays the AFM 3D image of the original sur-face of pristine SiO2 commercial optical !ber. This surfaceis clearly relatively smooth and featureless, with a roughnessof 32:2 JA. In Fig. 1(b) we show the same material imme-diately after being exposed to the !ssion fragment source.In this irradiated sample, we !nd a random and irregulardisplay of nonuniform height peaked features (heights upto ∼ 700 JA), and a considerable amount of ejected materialdeposited on the surface of the sample, product of the !ssionfragment collisions. The calculated rms roughness is 115 JA.

3.2. Natural quartz (crystalline SiO2)

Fig. 2(a) is the 3D image display with the AFM ofpristine natural quartz. Once again, as with the amorphousSiO2 material, we have a smooth topology in the naturalcrystalline quartz, with features of 15 JA height and rms

G. Espinosa et al. / Radiation Measurements 36 (2003) 225–228 227

(a) (b)

Fig. 2. Microphotography obtained by the AFM, showing the surface topography of the crystalline quartz: (a) before exposure to radiation,and (b) after being exposed to !ssion fragments source.

roughness of 4:4 JA. In Fig. 2(b) (sample irradiated) we now!nd a remarkable rather well ordered array of surface fea-tures, creating a x; y pattern. In this case, the height of thepeaks is variable along the formation, with a peak height of125 JA, almost 8 times higher than the original surface level,and a rms roughness of 22:5 JA.

4. Discussion and conclusion

The sputtering of surfaces from energetic electronic en-ergy loss processes (dE=dx)e in the stopping of heavy ener-getic ions is not well understood. The suggestion has beenmade (Chadderton, 2003) that whilst one may have somegeneral understanding of stopping in the bulk from simplemacroscopic thermal spike concepts, for electronic sputter-ing at surfaces it is more likely that we should have resourceto the Coulomb explosion concept for surface tracks. This issimply because there is space available outside the surfaceinto which a plume of back-sputtered material may go—arelaxation can take place before core ions are either neu-tralized or shielded, as they are for latent tracks in the bulk.The energized electrons appear !rst, but they drag—!rst thelighter ions and then the heavier ions—behind them. And ofcourse we must also expect to !nd a simultaneous atomiccluster emission , possibly reLecting order in the crystallinetarget.

Clearly the amount of sputtered material depends not onlyon the properties of the projectile (especially the e<ectivecharge Ze< ) but also on the electronic properties of the tar-get, especially the nature of the electronic bond. For all tar-gets, however, the fundamental sputtering process is one inwhich the forward momentum of the projectile is convertedinto reverse momentum of target components—comprisingsurface ejecta—which are inevitably neutralized and rede-posited on the target surface. In the sputtering of condensedgases by swift ions (v¿ v0 (the Bohr velocity)), it has been

noted (Ritchie and Claussen, 1982) that the sputtering yieldY is not a decreasing function of the energy as might beexpected, but that

Y ∼[(

dEdx

)e

]n(6)

where n ∼ 2, or greater, depending on experimental condi-tions. In further experiments of the kind described here wewill quantify the yield for all our projectile/target combina-tions. It is already clear, however, that the yield per fragmentis extraordinarily high, and that deposition of the materialejected, whilst being random for the amorphous SiO2 tar-gets, is ordered for the case of quartz. This may well be dueto atomic surface migration and di<usion and nucleation ofgrowing deposits on regular crystal surface features (steps,intersecting stacking faults, etc.). And the possibility of epi-taxy is diNcult to ignore. These experimental observationsopen new windows to the nuclear track formation theoreti-cal modeling.

Acknowledgements

One of (LTC) us wishes to thank the Departamento deF1Osica, Universidad Aut1onoma Metropolitana-Iztapalapa, fortheir support, and also CSIRO (Australia). The authors wishto thanks to A. Lara, for providing us the quartz crystal, toMetronet, S.A. de C.V., for providing us the optical !bermaterial; to A. S1anchez and A. Garc1Oa for their technicalhelp, and to Dr. Eduardo Ayala Riestra, as medical doctorhelp in my health (GE) to continue working in science.

References

Chadderton, L.T., 2003. Nuclear tracks in solids: registrationphysics and the compound spike. Radiat. Meas., this issue,doi:10.1016/S1350-4487(03)00094-5.

228 G. Espinosa et al. / Radiation Measurements 36 (2003) 225–228

Chadderton, L.T., Torrens, I.McC., 1969. Fission Damage inCrystals. Methuen, London.

Havancsak, K., Biro, L.P., Gyulai, J., Didyk, A.J., 1997. SMA andAFM observations of damage produced by Swift Ne and Krions in graphite. Radiat. Meas. 28, 65–70.

Ili1c, R., NajRzer, M., 1990. Image formation in track-etch detectors—I. The large area signal transfer function. Nucl. Tracks Radiat.Meas. 17, 453–460.

Lengar, I., Skvarc, J., Ilic, R., 2002. Fast neutron detection withcoincidence counting of recoil tracks in CR-39. Nucl. Instrum.Methods Phys. Res. B 192, 440–444.

Ritchie, R.H., Claussen, C., 1982. Core plasma model of chargedparticle track formation in insulators. Nucl. Instrum. MethodsPhys. Res. A 198, 133–138.

Tombrello, T.A., 1994. Predicting latent track dimensions. Nucl.Instrum. Methods Phys. Res. B 94, 424–428.

Vacik, J., Cervena, J., Hnatowicz, V., Fink, D., Yobayashi, Y.,Hirata, K., Apel, P.Y., Stratuss, P., 1999. Study of latent andetched tracks by a charged particle transmission technique.Radiat. Meas. 31, 81–84.

Yamamoto, M., Yasuda, N., Kaizuka, Y., Yamagishi, M., Kanai,T., Ishigure, N., Furukawa, A., Kurano, M., Miyahara, N.,Nakazawa, M., Doke, T., Ogura, K., 1997. CR-39 sensitivityanalysis on heavy ion beam with atomic force microscope.Radiat. Meas. 28, 227–230.