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Influence of radioactivity on surface interaction forces
Journal of Colloid and Interface Science 350 (2010) 595–598
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Short Communication
Influence of radioactivity on surface interaction forces M.E. Walker a, J. McFarlane a, D.C. Glasgow b, E. Chung c, P. Taboada-Serrano a, S. Yiacoumi c, C. Tsouris a,c,* a
Separations and Materials Research Group, Nuclear Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6181, USA Radiochemical Analysis Group, Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6387, USA c School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0373, USA b
a r t i c l e
i n f o
Article history: Received 9 March 2010 Accepted 15 June 2010 Available online 20 June 2010 Keywords: AFM Adhesion force Surface force Particle transport Radioactivity
a b s t r a c t Although some differences have been observed, the transport behavior of radioactive aerosol particles has often been assumed to be analogous to the behavior of nonradioactive aerosols in dispersion models. However, radioactive particles can become electrostatically charged as a result of the decay process. Theories have been proposed to describe this self-charging phenomenon, which may have a significant effect on how these particles interact with one another and with charged surfaces in the environment. In this study, atomic force microscopy (AFM) was employed to quantify surface forces between a particle and a planar surface and to compare measurements with and without the involvement of radioactivity. The main objective of this work is to assess directly the effects of radioactivity on the surface interactions of radioactive aerosols via the measurement of the adhesion force. The adhesion force between a silicon nitride AFM tip and an activated gold substrate was measured so that any possible effects due to radioactivity could be observed. The adhesion force between the tip and the gold surface increased significantly when the gold substrate (25 mm2 surface area) was activated to a level of approximately 0.6 mCi. The results of this investigation will prompt further work into the effects of radioactivity in particle–surface interactions. Ó 2010 Elsevier Inc. All rights reserved.
1. Introduction Concern about the behavior of man-made radioactive particles may arise from a number of different scenarios, including nuclear reactor accidents (e.g., the Chernobyl accident), nuclear weapons surface tests, explosions of radioactive dispersal devices, leakage of radioactive solutions from temporary storage tanks, and treatment of nuclear waste. Current tests and analyses of radioactive-particle waste and contaminant transport do not take into consideration the increasing evidence that radioactive particles behave significantly differently than ‘‘cold” particles. Much of the evidence regarding these differences is derived from the long-term monitoring of the fate of aerosol particles released after the Chernobyl accident (via sampling stations scattered throughout European countries) [1,2] and from monitoring the fate of such particles after nuclear tests [3,4]. Extensive studies on the patterns of radionuclide distribution after the Chernobyl accident have demonstrated that radioactive aerosols do not follow traditional aerosol transport and deposition mechanisms [1]. However, the information obtained from monitoring and from conventional models of radioactive aerosol transport cannot be used to develop
* Corresponding author at: Separations and Materials Research Group, Nuclear Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6181, USA. Fax: 865 241 4829. E-mail address: [email protected] (C. Tsouris). 0021-9797/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2010.06.042
reliable tools for predicting possible scenarios of radioactive contaminant transport if nuclear reactor accidents or weapon-testing accidents were to occur [1–4]. Controlled fire experiments within the Chernobyl exclusion zone have shown that important secondary contamination was found in clean areas due to transport and deposition of aerosols in the fire plumes [5]. The use of traditional aerosol transport models to explain the results of the controlled fire experiments proved unsuccessful [6]. Aerosol dispersal models used in reactor accident scenarios tend to exclude the effects of radiation beyond radiolysis, because they assume that the event takes place in a high-radiation field, where particle charging is a minor effect. At the source, this is certainly the case, but effects of chemistry and charged particle interactions should play a major role during radioactive particle transport away from the source of contamination as well. A study of Chernobyl fallout over Tokyo reported that small radioactive particles, which had traveled halfway around the world, had agglomerated with atmospheric components in the urban environment [7]. The most likely explanation for the differences in behavior between radioactive and nonradioactive particles is that radioactive particles may develop charge through the following processes: (1) self-ionization from the decay of embedded radioactive atoms, as a result of either the emission of a charged subatomic particle or a secondary electron emission, and (2) collisions with ionized particles generated within the surrounding medium following
M.E. Walker et al. / Journal of Colloid and Interface Science 350 (2010) 595–598
a radioactive particle and a nonradioactive surface, we measured the mirrored forces between a nonradioactive particle and a radioactive substrate. The gold foil substrate was activated to a level of 1 mCi prior to the measurements, giving an initial dose rate on unshielded contact of 90 mR/h. The main isotope produced in the reactor, 198 Au, decays through beta and gamma emission [16]. The gold sample was cleaned with ethanol and then affixed to the substrate mount on the atomic force microscope. A particle-like Veeco DNP silicon nitride cantilever/tip was attached to the cantilever mount on the AFM. The nominal spring constant of the cantilever was 0.06 N/m. Although the tip measured approximately 5.25 lm in length, the contact geometry was approximately a circle 0.12 lm in diameter. The tip was brought toward the gold surface at a rate of 0.25 lm/s. Once the tip and gold surface came into contact, the tip was automatically driven away from the gold surface by the AFM controls. As the tip was retracted, adhesion forces between the particle and the surface were obtained by multiplying the cantilever’s physical deflection due to particle–surface adhesion by its spring constant (Fig. 1). Elastic cantilever behavior was assumed and confirmed by 10 or 20 repetitive measurements for each set of conditions. Force measurements were taken at several random points on the gold surface, comprising a data set of adhesion force values corresponding to the radioactivity of the gold sample. Sets of adhesion force data were collected over a period of 2 weeks, as the activity of the gold decreased. Data sets were taken at unshielded contact dose rates of 53.8, 32.1, 11.5, and 1.9 mR/h. The retraction force curve for the 53.8 mR/h dose rate, showing an adhesion force of 13.9 nN, is presented in Fig. 2 as an example of the experimental measurements. A data set of surface forces between a DNP cantilever tip and a nonactivated gold foil sample was also collected as a control set. Each data set was measured with a new cantilever tip to mitigate any tip degradation during the measurements. All measurements were taken in air in a Plexiglas box that enclosed the atomic force microscope, with humidity levels controlled between 25% and 30%. The room temperature during the experiments was maintained at 295 K.
absorption of emitted radiation [8–12]. Macroscopic measurements of aerosol charge distributions have shown that the charge of radioactive particles can be either positive or negative, and can depend on the size of the particle [11]. No documented studies, however, have examined the charging effects of radioactivity on a microscopic scale, i.e., between two individual particles, or a particle and a surface. Also, forces that result from the charging of radioactive particles have never been measured. Such measurements are the topic of the present study. The hypothesis behind this work is that electrostatic charge generated by radioactivity contributes significantly to surface interactions between particles and environmental surfaces. This hypothesis was tested by employing atomic force microscopy (AFM) to directly measure interfacial forces. AFM has been used in previous work to obtain force–distance curves between particles and surfaces [13–15], making it possible to quantify the hypothesized electrostatic forces contributing to particle–surface interactions. Thus, the main objective of the present work is to assess directly the effects of radioactivity on surface interactions of radioactive aerosols via the measurement of surface forces. Direct measurement of surface forces allows unequivocal identification of the presence of an electrostatic component in interfacial interactions arising from radioactive decay. The magnitude of this component was quantified as a function of parameters, such as decay rate, included in a model of self-charging through radioactive decay. 2. Materials and methods 2.1. Materials All experimental data were collected with a Veeco Caliber atomic force microscope connected to Veeco SPMLab software. Cantilevers (DNP model) were purchased from Veeco, Inc. A sample of gold foil (5.0 5.0 0.1 mm) activated to a level of 1 mCi was used in the experiment as a substrate planar surface interacting with the AFM cantilever tip. Activation of the gold foil occurred in the Oak Ridge National Laboratory High Flux Isotope Reactor (HFIR) pneumatic transfer tube PT-2. A fluence monitor of dilute gold and manganese in aluminum foil was co-irradiated with the gold sample and counted to determine the neutron fluence. In this case, dilute manganese was the thermal monitor and dilute gold was the epithermal monitor. Care was taken to place the sample in the HFIR ‘‘rabbit” in such a way that the effects of fluence rate gradients were minimized for both the gold sample and the fluence monitors. The irradiation duration was less than 1 min in a fluence rate of 4.0 1014 thermal. The thermal/epithermal neutron ratio was calculated by simultaneous activation equations and was measured as 300. The irradiation position exhibits very few fast neutrons. After suitable decay to the specified activity, the gold foil was packed and delivered for surface force measurements.
3. Results and discussion In the presence of a radioactive substrate, adhesion forces between the cantilever tip and the gold substrate increased significantly. At a dose rate of 53.8 mR/h on the gold foil, surface adhesion between the surface and the tip averaged 13.2 nN. The adhesion forces between the nonactivated gold foil and the tip averaged 6.8 nN. Data sets taken at intermediate levels of radioactivity imply a positive nonlinear trend between dose rate and adhesion force (Fig. 3). Overall, data measurements taken at all activity levels remained very consistent, despite force measurements being taken at random points on the gold surface. The lower 95% confidence bound for the 53.1 mR/h data set was still significantly higher than the upper 95% confidence bound for the nonradioactive data set. Thus, radioactivity does have a strong (90.25% confidence) statistical impact on particle–surface forces. The increased adhesion force between the cantilever tip and the activated substrate is believed to be the result of an electrostatic component that arises primarily from beta decay.
2.2. Methods Because of concerns about contamination control and friable radioactive particles, rather than measuring surface forces between
Cantilever
Substrate
Δz
Deflection (μm)
596
Approach
0
Δz
Retraction
Z Displacement Fig. 1. Schematic of experimental setup. The product of Dz and the spring constant of the cantilever is equal to the tip–surface adhesion force.
M.E. Walker et al. / Journal of Colloid and Interface Science 350 (2010) 595–598
597
Fig. 2. Force–distance curve taken during the retraction of the tip from the gold foil surface, contact dose rate 53.8 mR/h. The tip was initially at 0.35 lm, and was retracted in the negative direction. The measured adhesion force in this data sample is 13.9 nN, and is represented by the abrupt leap in the graph at 0.6 lm.
the gold surface occurred due to radioactive decay within the planar gold foil. 4. Conclusions
Fig. 3. Effect of radioactivity dose rate of a planar gold surface on the adhesion force between a silicon nitride cantilever tip and the gold surface. The red lines (wide lines and box) represent a conventional box plot—the middle red (wide) line for each data set is at the median, and each red box contains the interquartile range (25th to 75th percentile). The outer red (wide) lines represent the outer limits of each data set, excluding outliers. The middle (inside the box) blue (vertical) line in each data set marks the mean of the set ± one standard deviation. The outer blue ticks (narrow, horizontal) mark the 95% confidence limits of the data. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
The components of the adhesion force can be considered as the capillary, van der Waals, electrostatic, and image forces. The capillary force is ignored in this study because (1) the relative humidity was maintained at a level lower than that needed to form a liquid meniscus between the substrate and the cantilever and (2) the adhesion force measurement with AFM was performed soon after the two surfaces were brought into contact, before meniscus could form. The Coulombic electrostatic force from direct charging is also considered to be negligible because the surface charge on the conductive surface dissipates through a grounded path. The van der Waals force is independent of radioactivity dose rate. Therefore, it is likely that the increased adhesion force (13.2 6.8 = 6.4 nN) consists of only an attractive electrostatic image force. The image force between a spherical particle and a planar surface can be calculated using the relationship [17,18]: 2 F image ¼ a 16pqe R2 ; including parameters as the particle charge q, 0 the particle radius R, and the vacuum permittivity e0. The unitless constant a is determined by the properties and geometry of the particle; in this study it is assumed to be 3.6 for a discretely charged spherical particle. Assuming that the electrostatic image force is 6.4 nN as calculated above, we calculate the surface charge density of the AFM tip to be 0.52 lC/cm2 at a dose rate of 53.8 mR/h. Both the charge on the AFM tip and the image charge on
We have employed an experimental setup to quantify surface forces between a particle-like AFM cantilever tip and a radioactive gold substrate. The presence of radioactivity was found to increase the adhesion force between the cantilever tip and the gold surface in the ambient environment. This result constitutes a proof of the hypothesis of this work that electrostatic charge generated by radioactivity contributes significantly to interactions between particles and environmental surfaces. This work has implications on the prediction of transport properties for radioactive aerosols and hydrosols following release during a radiological incident. Thus, further studies are planned to examine the effects of radioactivity on various systems, including nonconductive surfaces in air as well as aqueous solutions, and to develop transport models that incorporate surface interactions in particle transport and deposition mechanisms. Acknowledgments This work was sponsored by the Seed Money Fund of the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory (ORNL). Activation of the gold film was performed at the High Flux Isotope Reactor at ORNL. Partial support was provided by the National Science Foundation, under Grant CBET-0651683. The support that Dunbar Lockwood of DOE NA-243 provided to Mark Walker is also gratefully acknowledged. This manuscript has been authored by UT-Battelle, LLC, under contract DE-AC05-00OR22725 with the US Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a nonexclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. References [1] R.K. Chesser, M. Bondarkov, R.J. Baker, J.K. Wickliffe, B.E. Rodgers, J. Environ. Radioact. 71 (2004) 147. [2] G. Lujaniene˙, V. Aninkevicˇius, V. Lujanas, J. Environ. Radioact. 100 (2007) 108. [3] R. Arimoto, J.L. Webb, M. Conley, Atmos. Environ. 39 (2005) 4745. [4] M.O. Adeyini, E.O. Oladiran, Radiat. Meas. 41 (2006) 330. [5] V.I. Yoshenko, V.A. Kashparov, V.P. Protsak, S.M. Lundin, S.E. Levchuk, A.M. Kadygrib, S.I. Zvarich, X.V. Khomutinin, I.M. Maloshtan, V.P. Lanshin, M.V. Kovtun, J. Tschiervsch, J. Environ. Radioact. 86 (2006) 143. [6] V.I. Yoshenko, V.A. Kashparov, S.E. Levchuk, A.S. Glukhovskiy, Y.V. Khomutni, V.P. Protsak, S.M. Lundin, J. Tschiersch, J. Environ. Radioact. 87 (2000) 260.
598 [7] [8] [9] [10] [11] [12] [13]
M.E. Walker et al. / Journal of Colloid and Interface Science 350 (2010) 595–598 H. Ooe, R. Seki, N. Ikeda, J. Environ. Radioact. 6 (1988) 219. C.F. Clement, R.G. Harrison, J. Aerosol Sci. 23 (1992) 481. C.F. Clement, R.A. Clement, R.G. Harrison, J. Aerosol Sci. 26 (1995) 1207. C.F. Clement, R.G. Harrison, J. Aerosol Sci. 31 (2000) 363. F. Gensdarmes, D. Boulaud, A. Renoux, J. Aerosol Sci. 32 (2001) 1437. Y.S. Mayya, S.N. Tripathi, A. Khan, J. Aerosol Sci. 33 (2002) 781. C.J. Chin, S. Yiacoumi, C. Tsouris, Environ. Sci. Technol. 36 (2002) 343.
[14] P. Taboada-Serrano, V. Vithayaveroj, S. Yiacoumi, C. Tsouris, Environ. Sci. Technol. 39 (2005) 6352. [15] P. Taboada-Serrano, V. Vithayaveroj, C.H. Hou, S. Yiacoumi, C. Tsouris, Ind. Eng. Chem. Res. 48 (2009) 6448. [16] P.W. Levy, E. Greuling, Phys. Rev. 75 (1949) 819. [17] D.A. Hays, J. Adhes. Sci. Technol. 9 (1995) 1063. [18] G.C. Hartmann, L.M. Marks, C.C. Yang, J. Appl. Phys. 47 (1976) 5409.
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Short Communication
Influence of radioactivity on surface interaction forces M.E. Walker a, J. McFarlane a, D.C. Glasgow b, E. Chung c, P. Taboada-Serrano a, S. Yiacoumi c, C. Tsouris a,c,* a
Separations and Materials Research Group, Nuclear Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6181, USA Radiochemical Analysis Group, Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6387, USA c School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0373, USA b
a r t i c l e
i n f o
Article history: Received 9 March 2010 Accepted 15 June 2010 Available online 20 June 2010 Keywords: AFM Adhesion force Surface force Particle transport Radioactivity
a b s t r a c t Although some differences have been observed, the transport behavior of radioactive aerosol particles has often been assumed to be analogous to the behavior of nonradioactive aerosols in dispersion models. However, radioactive particles can become electrostatically charged as a result of the decay process. Theories have been proposed to describe this self-charging phenomenon, which may have a significant effect on how these particles interact with one another and with charged surfaces in the environment. In this study, atomic force microscopy (AFM) was employed to quantify surface forces between a particle and a planar surface and to compare measurements with and without the involvement of radioactivity. The main objective of this work is to assess directly the effects of radioactivity on the surface interactions of radioactive aerosols via the measurement of the adhesion force. The adhesion force between a silicon nitride AFM tip and an activated gold substrate was measured so that any possible effects due to radioactivity could be observed. The adhesion force between the tip and the gold surface increased significantly when the gold substrate (25 mm2 surface area) was activated to a level of approximately 0.6 mCi. The results of this investigation will prompt further work into the effects of radioactivity in particle–surface interactions. Ó 2010 Elsevier Inc. All rights reserved.
1. Introduction Concern about the behavior of man-made radioactive particles may arise from a number of different scenarios, including nuclear reactor accidents (e.g., the Chernobyl accident), nuclear weapons surface tests, explosions of radioactive dispersal devices, leakage of radioactive solutions from temporary storage tanks, and treatment of nuclear waste. Current tests and analyses of radioactive-particle waste and contaminant transport do not take into consideration the increasing evidence that radioactive particles behave significantly differently than ‘‘cold” particles. Much of the evidence regarding these differences is derived from the long-term monitoring of the fate of aerosol particles released after the Chernobyl accident (via sampling stations scattered throughout European countries) [1,2] and from monitoring the fate of such particles after nuclear tests [3,4]. Extensive studies on the patterns of radionuclide distribution after the Chernobyl accident have demonstrated that radioactive aerosols do not follow traditional aerosol transport and deposition mechanisms [1]. However, the information obtained from monitoring and from conventional models of radioactive aerosol transport cannot be used to develop
* Corresponding author at: Separations and Materials Research Group, Nuclear Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6181, USA. Fax: 865 241 4829. E-mail address: [email protected] (C. Tsouris). 0021-9797/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2010.06.042
reliable tools for predicting possible scenarios of radioactive contaminant transport if nuclear reactor accidents or weapon-testing accidents were to occur [1–4]. Controlled fire experiments within the Chernobyl exclusion zone have shown that important secondary contamination was found in clean areas due to transport and deposition of aerosols in the fire plumes [5]. The use of traditional aerosol transport models to explain the results of the controlled fire experiments proved unsuccessful [6]. Aerosol dispersal models used in reactor accident scenarios tend to exclude the effects of radiation beyond radiolysis, because they assume that the event takes place in a high-radiation field, where particle charging is a minor effect. At the source, this is certainly the case, but effects of chemistry and charged particle interactions should play a major role during radioactive particle transport away from the source of contamination as well. A study of Chernobyl fallout over Tokyo reported that small radioactive particles, which had traveled halfway around the world, had agglomerated with atmospheric components in the urban environment [7]. The most likely explanation for the differences in behavior between radioactive and nonradioactive particles is that radioactive particles may develop charge through the following processes: (1) self-ionization from the decay of embedded radioactive atoms, as a result of either the emission of a charged subatomic particle or a secondary electron emission, and (2) collisions with ionized particles generated within the surrounding medium following
M.E. Walker et al. / Journal of Colloid and Interface Science 350 (2010) 595–598
a radioactive particle and a nonradioactive surface, we measured the mirrored forces between a nonradioactive particle and a radioactive substrate. The gold foil substrate was activated to a level of 1 mCi prior to the measurements, giving an initial dose rate on unshielded contact of 90 mR/h. The main isotope produced in the reactor, 198 Au, decays through beta and gamma emission [16]. The gold sample was cleaned with ethanol and then affixed to the substrate mount on the atomic force microscope. A particle-like Veeco DNP silicon nitride cantilever/tip was attached to the cantilever mount on the AFM. The nominal spring constant of the cantilever was 0.06 N/m. Although the tip measured approximately 5.25 lm in length, the contact geometry was approximately a circle 0.12 lm in diameter. The tip was brought toward the gold surface at a rate of 0.25 lm/s. Once the tip and gold surface came into contact, the tip was automatically driven away from the gold surface by the AFM controls. As the tip was retracted, adhesion forces between the particle and the surface were obtained by multiplying the cantilever’s physical deflection due to particle–surface adhesion by its spring constant (Fig. 1). Elastic cantilever behavior was assumed and confirmed by 10 or 20 repetitive measurements for each set of conditions. Force measurements were taken at several random points on the gold surface, comprising a data set of adhesion force values corresponding to the radioactivity of the gold sample. Sets of adhesion force data were collected over a period of 2 weeks, as the activity of the gold decreased. Data sets were taken at unshielded contact dose rates of 53.8, 32.1, 11.5, and 1.9 mR/h. The retraction force curve for the 53.8 mR/h dose rate, showing an adhesion force of 13.9 nN, is presented in Fig. 2 as an example of the experimental measurements. A data set of surface forces between a DNP cantilever tip and a nonactivated gold foil sample was also collected as a control set. Each data set was measured with a new cantilever tip to mitigate any tip degradation during the measurements. All measurements were taken in air in a Plexiglas box that enclosed the atomic force microscope, with humidity levels controlled between 25% and 30%. The room temperature during the experiments was maintained at 295 K.
absorption of emitted radiation [8–12]. Macroscopic measurements of aerosol charge distributions have shown that the charge of radioactive particles can be either positive or negative, and can depend on the size of the particle [11]. No documented studies, however, have examined the charging effects of radioactivity on a microscopic scale, i.e., between two individual particles, or a particle and a surface. Also, forces that result from the charging of radioactive particles have never been measured. Such measurements are the topic of the present study. The hypothesis behind this work is that electrostatic charge generated by radioactivity contributes significantly to surface interactions between particles and environmental surfaces. This hypothesis was tested by employing atomic force microscopy (AFM) to directly measure interfacial forces. AFM has been used in previous work to obtain force–distance curves between particles and surfaces [13–15], making it possible to quantify the hypothesized electrostatic forces contributing to particle–surface interactions. Thus, the main objective of the present work is to assess directly the effects of radioactivity on surface interactions of radioactive aerosols via the measurement of surface forces. Direct measurement of surface forces allows unequivocal identification of the presence of an electrostatic component in interfacial interactions arising from radioactive decay. The magnitude of this component was quantified as a function of parameters, such as decay rate, included in a model of self-charging through radioactive decay. 2. Materials and methods 2.1. Materials All experimental data were collected with a Veeco Caliber atomic force microscope connected to Veeco SPMLab software. Cantilevers (DNP model) were purchased from Veeco, Inc. A sample of gold foil (5.0 5.0 0.1 mm) activated to a level of 1 mCi was used in the experiment as a substrate planar surface interacting with the AFM cantilever tip. Activation of the gold foil occurred in the Oak Ridge National Laboratory High Flux Isotope Reactor (HFIR) pneumatic transfer tube PT-2. A fluence monitor of dilute gold and manganese in aluminum foil was co-irradiated with the gold sample and counted to determine the neutron fluence. In this case, dilute manganese was the thermal monitor and dilute gold was the epithermal monitor. Care was taken to place the sample in the HFIR ‘‘rabbit” in such a way that the effects of fluence rate gradients were minimized for both the gold sample and the fluence monitors. The irradiation duration was less than 1 min in a fluence rate of 4.0 1014 thermal. The thermal/epithermal neutron ratio was calculated by simultaneous activation equations and was measured as 300. The irradiation position exhibits very few fast neutrons. After suitable decay to the specified activity, the gold foil was packed and delivered for surface force measurements.
3. Results and discussion In the presence of a radioactive substrate, adhesion forces between the cantilever tip and the gold substrate increased significantly. At a dose rate of 53.8 mR/h on the gold foil, surface adhesion between the surface and the tip averaged 13.2 nN. The adhesion forces between the nonactivated gold foil and the tip averaged 6.8 nN. Data sets taken at intermediate levels of radioactivity imply a positive nonlinear trend between dose rate and adhesion force (Fig. 3). Overall, data measurements taken at all activity levels remained very consistent, despite force measurements being taken at random points on the gold surface. The lower 95% confidence bound for the 53.1 mR/h data set was still significantly higher than the upper 95% confidence bound for the nonradioactive data set. Thus, radioactivity does have a strong (90.25% confidence) statistical impact on particle–surface forces. The increased adhesion force between the cantilever tip and the activated substrate is believed to be the result of an electrostatic component that arises primarily from beta decay.
2.2. Methods Because of concerns about contamination control and friable radioactive particles, rather than measuring surface forces between
Cantilever
Substrate
Δz
Deflection (μm)
596
Approach
0
Δz
Retraction
Z Displacement Fig. 1. Schematic of experimental setup. The product of Dz and the spring constant of the cantilever is equal to the tip–surface adhesion force.
M.E. Walker et al. / Journal of Colloid and Interface Science 350 (2010) 595–598
597
Fig. 2. Force–distance curve taken during the retraction of the tip from the gold foil surface, contact dose rate 53.8 mR/h. The tip was initially at 0.35 lm, and was retracted in the negative direction. The measured adhesion force in this data sample is 13.9 nN, and is represented by the abrupt leap in the graph at 0.6 lm.
the gold surface occurred due to radioactive decay within the planar gold foil. 4. Conclusions
Fig. 3. Effect of radioactivity dose rate of a planar gold surface on the adhesion force between a silicon nitride cantilever tip and the gold surface. The red lines (wide lines and box) represent a conventional box plot—the middle red (wide) line for each data set is at the median, and each red box contains the interquartile range (25th to 75th percentile). The outer red (wide) lines represent the outer limits of each data set, excluding outliers. The middle (inside the box) blue (vertical) line in each data set marks the mean of the set ± one standard deviation. The outer blue ticks (narrow, horizontal) mark the 95% confidence limits of the data. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
The components of the adhesion force can be considered as the capillary, van der Waals, electrostatic, and image forces. The capillary force is ignored in this study because (1) the relative humidity was maintained at a level lower than that needed to form a liquid meniscus between the substrate and the cantilever and (2) the adhesion force measurement with AFM was performed soon after the two surfaces were brought into contact, before meniscus could form. The Coulombic electrostatic force from direct charging is also considered to be negligible because the surface charge on the conductive surface dissipates through a grounded path. The van der Waals force is independent of radioactivity dose rate. Therefore, it is likely that the increased adhesion force (13.2 6.8 = 6.4 nN) consists of only an attractive electrostatic image force. The image force between a spherical particle and a planar surface can be calculated using the relationship [17,18]: 2 F image ¼ a 16pqe R2 ; including parameters as the particle charge q, 0 the particle radius R, and the vacuum permittivity e0. The unitless constant a is determined by the properties and geometry of the particle; in this study it is assumed to be 3.6 for a discretely charged spherical particle. Assuming that the electrostatic image force is 6.4 nN as calculated above, we calculate the surface charge density of the AFM tip to be 0.52 lC/cm2 at a dose rate of 53.8 mR/h. Both the charge on the AFM tip and the image charge on
We have employed an experimental setup to quantify surface forces between a particle-like AFM cantilever tip and a radioactive gold substrate. The presence of radioactivity was found to increase the adhesion force between the cantilever tip and the gold surface in the ambient environment. This result constitutes a proof of the hypothesis of this work that electrostatic charge generated by radioactivity contributes significantly to interactions between particles and environmental surfaces. This work has implications on the prediction of transport properties for radioactive aerosols and hydrosols following release during a radiological incident. Thus, further studies are planned to examine the effects of radioactivity on various systems, including nonconductive surfaces in air as well as aqueous solutions, and to develop transport models that incorporate surface interactions in particle transport and deposition mechanisms. Acknowledgments This work was sponsored by the Seed Money Fund of the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory (ORNL). Activation of the gold film was performed at the High Flux Isotope Reactor at ORNL. Partial support was provided by the National Science Foundation, under Grant CBET-0651683. The support that Dunbar Lockwood of DOE NA-243 provided to Mark Walker is also gratefully acknowledged. This manuscript has been authored by UT-Battelle, LLC, under contract DE-AC05-00OR22725 with the US Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a nonexclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. References [1] R.K. Chesser, M. Bondarkov, R.J. Baker, J.K. Wickliffe, B.E. Rodgers, J. Environ. Radioact. 71 (2004) 147. [2] G. Lujaniene˙, V. Aninkevicˇius, V. Lujanas, J. Environ. Radioact. 100 (2007) 108. [3] R. Arimoto, J.L. Webb, M. Conley, Atmos. Environ. 39 (2005) 4745. [4] M.O. Adeyini, E.O. Oladiran, Radiat. Meas. 41 (2006) 330. [5] V.I. Yoshenko, V.A. Kashparov, V.P. Protsak, S.M. Lundin, S.E. Levchuk, A.M. Kadygrib, S.I. Zvarich, X.V. Khomutinin, I.M. Maloshtan, V.P. Lanshin, M.V. Kovtun, J. Tschiervsch, J. Environ. Radioact. 86 (2006) 143. [6] V.I. Yoshenko, V.A. Kashparov, S.E. Levchuk, A.S. Glukhovskiy, Y.V. Khomutni, V.P. Protsak, S.M. Lundin, J. Tschiersch, J. Environ. Radioact. 87 (2000) 260.
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