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Electronic stopping and etched particle tracks in polymers
Nuclear Instruments and Methods in Physics Research B 91 (1994) 168-171 North-Holland
Electronic
NOMB
Beam Interactions with Materials&Atoms
stopping and etched particle tracks in polymers
L.T. Chadderton a,*, J.L. Zhu a,b, S.A. Cruz a,c, D. Fink a,d and S. Ghosh w a Division of Applied Physics, CSIRO, and Research School of Physical Sciences, Institute for Advanced Studies, Australian National University, GPO Box 4, Canberra, ACT 2601, Australia ‘Applied Radiation Institute, Shanghai University of Science and Technology, Shanghai, China ’ Departamento de Fisica, Universidad Autonoma Metropolitana-Iztapalapa, Al’ 55-534 09340, Mexico, DF, Mexico d Hahn- Meitner-Institut GmbH, Dep. P-3, Glienickerstr. 100, D-14109, Berlin, Germany e Department of Chemistry, North Eastern Hill University, Shillong, India
The existence of a maximum in the electronic stopping power when an energetic ion reaches some depth in a solid target is well known. Experiments are described in which lithium ions - in the energy range 0.6 to 3 MeV - were used to bombard the common polymer and particle dosimeter CR-39, so that the position of the stopping power peak relative to the polymer surface could be systematically varied. Differences in etched surface track diameters measured by optical microscopy and corresponding to differences in energy _ 0.2 MeV could be readily distinguished. Maximum etched track diameters clearly coincided with the intersection of maximum electronic energy losses with the CR-39 surface. Implications of this to more general stopping are discussed, including a linear relationship between the maximum electronic stopping power at a surface and the atomic number of the projectile ion.
1. Introduction When a very energetic highly charged ion strikes a solid it loses by far the greater part of its energy in exciting or ionizing electrons. In the case of polymer targets this dominance of electronic energy loss processes is the cause of characteristic radiation damage such as chain scission and cross-linking. Other experimental studies of ion-bombarded polymers [I,21 also indicate a damage distribution parallel to latent tracks - as a function of depth - for which electronic energy losses can be held responsible. But there is also a peak in the electronic energy loss processes - in the stopping power - a peak whose profile and depth below the target surface will, to a first approximation bring about a maximum in the radiation damage at that point. It follows that, if E,, is that ion energy at which the maximum electronic stopping cross section is attained for a given projectile-target combination, then for an ion energy E = E,, at the entry surface, we must also expect maximum damage. This rather obvious prediction is simply due, of course, to the quite direct correlation of the maximum in the electronic stopping cross section (the solid) with the maximum in energy delivery (the particle). In general, however, the
* Corresponding author.
possibility that this useful criterion might also predict those threshold ion energies which maximise the etched track diameters at polymer surfaces, used as detectors and/or dosimeters, does not appear to have been followed up. Different polymer detectors, of course, have different sensitivities for different particles at different energies, and also to different chemical reagents. But in every case the detector surface, and the specific concentration of damage there, is the primary point of etchant chemical attack. The rate of track development must therefore certainly be optimized from the very beginning at the point of strike and entry, when E = E max, and also be considerably enhanced in subsequent stages of the etch. Experiments are described for the combination of lithium ions striking the familiar polymer and detector CR-39. The effect predicted is observed in practice.
2. Experiment CR-39 foils at room temperature were irradiated with Li+ ions at energies between 0.6 and 3 MeV up to doses between 2 x lo3 and 2 X lo5 ions/cm’. These low doses were achieved by backscattering the primary ion beam (up to 5 MeV energy and - 1 nA current) from a thin Au target on a polyimide backing at well-defined angles (90”, 160”, 170”, and 179.5”). Pri-
0168-583X/94/$07.00 0 1994 - Elsevier Science B.V. All rights reserved SSDZ 016%583X(93)E0991-0
L.T. Chadderton et al. /Nucl.
Instr. and Meth. in Phys. Res. B 91 (1994) 168-171
mary ion beam doses - lOlo to 10” ions/cm’ were required in order to obtain the necessary backscattering yields - some lo4 ions/cm’ arriving at the CR-39 target foils within a few seconds. Maximum angular spread of the backscattered ions was approximately 3.5”. This corresponds to an energy spread of the incident ions of less than 1%. A further energy spread due to backscattering at different depths in the Au foil (thickness - 1000 A) was estimated at less than 6%. The Au foils which we used had a thickness - 300 A. Irradiated foils of CR-39 were etched in 6N NaOH at 55.2 f O.l”C for times varying between 0.5 and 15 h, and the ion track diameters subsequently measured in through-focal transmission by optical microscopy. In each irradiation, the diameters of more than 600 tracks were measured. The ion track number density increased in proportion with the particle fluence; and it did not vary with etching time. The maximum diameter for etched tracks (- 2.3 urn) was obtained for energies of approximately 1.1 MeV, and after etching for three hours. This energy coincides closely with that corresponding to the broad maximum in the electronic energy transfer, predictable using the computer code TRIM [3]. The track diameters increased with etching time at a roughly constant rate of about (0.52 f 0.05) Fm/h, up to etching times of around 7 h. For longer times the etch track diameter growth was so swift and unpredictable as to invalidate the experiment. Finally, etching times in the interval between 3-5 h were chosen as standard for the work, since the maximum in the track diameter could be readily obtained as a function of incident ion energy. Differences in surface track diameters corresponding to differences in energy - 0.2 MeV could be distinguished.
3. Results and discussion The time evolution for the surface diameter of etched tracks as a function of ion energy is shown in Fig. 1. Clearly, a maximum diameter is observed for a projectile energy E N 1.1 MeV, which corresponds closely to the energy (E,, - 1.3 MeV) at which the maximum in electronic stopping power (see dotted line) is predicted for Li+ ions incident on CR-39 [3]. Supporting evidence for this identification comes from recent experiments by Bernardi et al. [4] who describe etched tracks in CR-39 following proton bombardment at normal and oblique incidence in the energy range 0.2-2.3 MeV. When their measured track diameters for normal incidence are plotted against proton energy, a trend similar to that shown in Fig. 1 is observed, indicating good correlation with the position of the peak of the electronic stopping maximum. This is in spite of the considerably longer etching times (2 5 h) required for observable tracks to appear for protons.
169
3 3
2
1.5
35
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Energy (MeV) Fig. 1. Time evolution of the diameter of chemically etched tracks as a function of energy of Li’ ions incident on CR-39. The dashed curve represents the energy behaviour of the corresponding electronic stopping cross section on the right hand scale (see text).
Plots of our experimental mean track diameters against etching time showed a linear dependence. This trend is of significance in defining the sensitivity of any track detector, characterized by the etch ratio S = r_+/z+,,where z+ and ut, are the particle track and bulk etch-rates, respectively. Therefore, since electronic stopping is the dominant mechanism of energy loss here, in the bulk, it is reasonable to conclude that the damage density in the surface is also substantially due to electronic processes. Suppose we assume that the main mechanism of damage production in a polymer is indeed that due to electronic processes. Then the primary deposited energy (EJ at a certain depth (x) may be written to a first approximation [5] as: E* =
/ &,(E)P(&
-E,
X> dE,
(1)
where P(E, -E, x) is the probability for an energy transfer AE to take place at depth x. At the surface, P(E, -E, 0) = S(E, -E), hence: ~(0)
= S,(&).
(2) Thus, if we let E, = Em,, the deposited energy at the surface will be proportional to S,,,. There are direct implications from this simple observation for the development of radiation damage at the surface of a given polymer. The maximum electronic stopping cross secII. ENERGETIC HEAVY IONS
170
L.T. Chadderton et al. /Nucl. In&r. and Meth. in Phys. Res. B 91 (1994) 168-171
tion (S e,max) is plotted in Fig. 2 as a function of projectile atomic m.nnber (Z,), for a given target material, suggesting the simple linear relationship: Se,maX==L
(3)
with K a constant. A~r~ngly,
from Eq. rz>:
Q(O) = S,(&l)
= m1, (4) and so the radiation damage at the surface should have the same linear dependence on projectile atomic number. Moreover, if we are to assume that the etching velocity for a track is proportional to the amount of damage, then we should also observe a linear relationship for the etching velocity of tracks formed by different ions with incident energy E = E,, in the same target material. It is interesting that electronic stopping power maxima for different polymers may be compared in terms of their co~esponding bulk densities, closely following the simple relation:
S,,,,(Tawt 1) &,&Target
2) a
[
Ptarget 1 -Ptargetz
l/2
1
(5)
*
1 Thus Fig. 3 is a plot of S_,,p-1/2 as a function of 2, for a random selection of familiar polymer targets (lexan, polypropylene, PVC, Kapton and CR-39). In each case values for S,,, were obtained by applica-
14
27
41
54
Zl Fig. 3. Scaling properties of S,,,,P-~/’ as a function of projectile atomic number, where p is the polymer density. Symbols correspond to the same materials listed in Fig. 2.
tion of the SR routine of the TRIM-90 computer code [3]. The scaling law obtained, after linear regression analysis of the data, reads as follows:
se,max =~l’~(18.2862~
- 7.613).
(6) This relation, it must be stressed, is satisfied not only for the specific polymers selected, but applies equally well over a wide class of polymer targets. From Eqs. (2) and (6), for a given projectile (Z,), the rate of surface damage created in two quite different polymers should scale as:
610
e,(target
1)
E,(target2)
Ptarget1 X i PtargetZ.i
l/2
’
(7)
It follows that plastic targets with similar densities but neglecting differences in their chemical structure would respond similarly to the maximum energy deposition at the surface for the same charged projectile.
210:
100 1
14
27
41
54
Zl Fig. 2. TRIM calculation [3] of the maximum electronic stopping cross section as a function of projectile atomic number for various polymer targets: Lexan (+ 1, Polypropylene (X1, PVC (El), Kapton (A >,and CR-39 (0).
4. Conclusion The diameters of etched ion tracks in CR-39 scale directly with the amount of electronic energy transferred to the surface at the point of incidence of the ions into the polymer. For 0.6-3 MeV lithium ion bombar~ent of CR-39 there is a pronounced maxi-
L. T. Chadderton et al. /Nucl. In&r. and Meth. in Phys. Res. B 91 (1994) 168-171
mum etch track diameter at N 1.1 MeV. This corresponds to that energy for which the maximum electronic stopping cross section is attained. Computer simulations show that the maximum electronic energy transfer for different ions in the same target material is proportional to the corresponding projectile atomic number. This behaviour is evidently related to the number of electrons - projectile and target - participating in the stopping process, at the maximum of the electronic energy loss curve. It is precisely here that the electronic clouds of both projectile and target are strongly involved in competing mechanisms of energy loss - excitation, ionization, and charge exchange. A preliminary analysis on other classes of inorganic targets indicates a similar linear on projectile atomic number. dependence of S,,,
Acknowledgements Two of the authors (L.T.C. and S.A.C.) are grateful to Australia and Mexico for assistance under the aegis
171
of the bilateral science and technology agreement, and also from the Autonomous Metropolitan University of Mexico (Mexico DF). J.L.Z is appreciative for the support of the International Atomic Energy Agency (Vienna). The office of CONACYT (Mexico) supports the work of S.A.C. under Research Contract No. 1405E9207. This direct assistance is both warmly appreciated, and clearly acknowledged.
References ill J. Davenas, G. Boiteux and M. Fallavier, Nucl. Instr. and
Meth. B 39 (1989) 796. La D. Fir& M. Muller, A. Schmoldt, J.K. Zhou, L.T. Chadderton and X.L. Xu, Nucl. In&. and Meth. B 65 (1992) 432. 131J.P. Biersack, Z. Phys. A 315 (1982) 95. 141 L. Bernardi, A. Cecchi, C. Gori, F. Lucarelli and R. Renzi, Nucl.Instr. and Meth. B 53 (1991) 61. [51 J.F. Gibbons, Proc IEEE 60 (1972) 1062.
II. ENERGETIC HEAVY IONS
Electronic
NOMB
Beam Interactions with Materials&Atoms
stopping and etched particle tracks in polymers
L.T. Chadderton a,*, J.L. Zhu a,b, S.A. Cruz a,c, D. Fink a,d and S. Ghosh w a Division of Applied Physics, CSIRO, and Research School of Physical Sciences, Institute for Advanced Studies, Australian National University, GPO Box 4, Canberra, ACT 2601, Australia ‘Applied Radiation Institute, Shanghai University of Science and Technology, Shanghai, China ’ Departamento de Fisica, Universidad Autonoma Metropolitana-Iztapalapa, Al’ 55-534 09340, Mexico, DF, Mexico d Hahn- Meitner-Institut GmbH, Dep. P-3, Glienickerstr. 100, D-14109, Berlin, Germany e Department of Chemistry, North Eastern Hill University, Shillong, India
The existence of a maximum in the electronic stopping power when an energetic ion reaches some depth in a solid target is well known. Experiments are described in which lithium ions - in the energy range 0.6 to 3 MeV - were used to bombard the common polymer and particle dosimeter CR-39, so that the position of the stopping power peak relative to the polymer surface could be systematically varied. Differences in etched surface track diameters measured by optical microscopy and corresponding to differences in energy _ 0.2 MeV could be readily distinguished. Maximum etched track diameters clearly coincided with the intersection of maximum electronic energy losses with the CR-39 surface. Implications of this to more general stopping are discussed, including a linear relationship between the maximum electronic stopping power at a surface and the atomic number of the projectile ion.
1. Introduction When a very energetic highly charged ion strikes a solid it loses by far the greater part of its energy in exciting or ionizing electrons. In the case of polymer targets this dominance of electronic energy loss processes is the cause of characteristic radiation damage such as chain scission and cross-linking. Other experimental studies of ion-bombarded polymers [I,21 also indicate a damage distribution parallel to latent tracks - as a function of depth - for which electronic energy losses can be held responsible. But there is also a peak in the electronic energy loss processes - in the stopping power - a peak whose profile and depth below the target surface will, to a first approximation bring about a maximum in the radiation damage at that point. It follows that, if E,, is that ion energy at which the maximum electronic stopping cross section is attained for a given projectile-target combination, then for an ion energy E = E,, at the entry surface, we must also expect maximum damage. This rather obvious prediction is simply due, of course, to the quite direct correlation of the maximum in the electronic stopping cross section (the solid) with the maximum in energy delivery (the particle). In general, however, the
* Corresponding author.
possibility that this useful criterion might also predict those threshold ion energies which maximise the etched track diameters at polymer surfaces, used as detectors and/or dosimeters, does not appear to have been followed up. Different polymer detectors, of course, have different sensitivities for different particles at different energies, and also to different chemical reagents. But in every case the detector surface, and the specific concentration of damage there, is the primary point of etchant chemical attack. The rate of track development must therefore certainly be optimized from the very beginning at the point of strike and entry, when E = E max, and also be considerably enhanced in subsequent stages of the etch. Experiments are described for the combination of lithium ions striking the familiar polymer and detector CR-39. The effect predicted is observed in practice.
2. Experiment CR-39 foils at room temperature were irradiated with Li+ ions at energies between 0.6 and 3 MeV up to doses between 2 x lo3 and 2 X lo5 ions/cm’. These low doses were achieved by backscattering the primary ion beam (up to 5 MeV energy and - 1 nA current) from a thin Au target on a polyimide backing at well-defined angles (90”, 160”, 170”, and 179.5”). Pri-
0168-583X/94/$07.00 0 1994 - Elsevier Science B.V. All rights reserved SSDZ 016%583X(93)E0991-0
L.T. Chadderton et al. /Nucl.
Instr. and Meth. in Phys. Res. B 91 (1994) 168-171
mary ion beam doses - lOlo to 10” ions/cm’ were required in order to obtain the necessary backscattering yields - some lo4 ions/cm’ arriving at the CR-39 target foils within a few seconds. Maximum angular spread of the backscattered ions was approximately 3.5”. This corresponds to an energy spread of the incident ions of less than 1%. A further energy spread due to backscattering at different depths in the Au foil (thickness - 1000 A) was estimated at less than 6%. The Au foils which we used had a thickness - 300 A. Irradiated foils of CR-39 were etched in 6N NaOH at 55.2 f O.l”C for times varying between 0.5 and 15 h, and the ion track diameters subsequently measured in through-focal transmission by optical microscopy. In each irradiation, the diameters of more than 600 tracks were measured. The ion track number density increased in proportion with the particle fluence; and it did not vary with etching time. The maximum diameter for etched tracks (- 2.3 urn) was obtained for energies of approximately 1.1 MeV, and after etching for three hours. This energy coincides closely with that corresponding to the broad maximum in the electronic energy transfer, predictable using the computer code TRIM [3]. The track diameters increased with etching time at a roughly constant rate of about (0.52 f 0.05) Fm/h, up to etching times of around 7 h. For longer times the etch track diameter growth was so swift and unpredictable as to invalidate the experiment. Finally, etching times in the interval between 3-5 h were chosen as standard for the work, since the maximum in the track diameter could be readily obtained as a function of incident ion energy. Differences in surface track diameters corresponding to differences in energy - 0.2 MeV could be distinguished.
3. Results and discussion The time evolution for the surface diameter of etched tracks as a function of ion energy is shown in Fig. 1. Clearly, a maximum diameter is observed for a projectile energy E N 1.1 MeV, which corresponds closely to the energy (E,, - 1.3 MeV) at which the maximum in electronic stopping power (see dotted line) is predicted for Li+ ions incident on CR-39 [3]. Supporting evidence for this identification comes from recent experiments by Bernardi et al. [4] who describe etched tracks in CR-39 following proton bombardment at normal and oblique incidence in the energy range 0.2-2.3 MeV. When their measured track diameters for normal incidence are plotted against proton energy, a trend similar to that shown in Fig. 1 is observed, indicating good correlation with the position of the peak of the electronic stopping maximum. This is in spite of the considerably longer etching times (2 5 h) required for observable tracks to appear for protons.
169
3 3
2
1.5
35
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Energy (MeV) Fig. 1. Time evolution of the diameter of chemically etched tracks as a function of energy of Li’ ions incident on CR-39. The dashed curve represents the energy behaviour of the corresponding electronic stopping cross section on the right hand scale (see text).
Plots of our experimental mean track diameters against etching time showed a linear dependence. This trend is of significance in defining the sensitivity of any track detector, characterized by the etch ratio S = r_+/z+,,where z+ and ut, are the particle track and bulk etch-rates, respectively. Therefore, since electronic stopping is the dominant mechanism of energy loss here, in the bulk, it is reasonable to conclude that the damage density in the surface is also substantially due to electronic processes. Suppose we assume that the main mechanism of damage production in a polymer is indeed that due to electronic processes. Then the primary deposited energy (EJ at a certain depth (x) may be written to a first approximation [5] as: E* =
/ &,(E)P(&
-E,
X> dE,
(1)
where P(E, -E, x) is the probability for an energy transfer AE to take place at depth x. At the surface, P(E, -E, 0) = S(E, -E), hence: ~(0)
= S,(&).
(2) Thus, if we let E, = Em,, the deposited energy at the surface will be proportional to S,,,. There are direct implications from this simple observation for the development of radiation damage at the surface of a given polymer. The maximum electronic stopping cross secII. ENERGETIC HEAVY IONS
170
L.T. Chadderton et al. /Nucl. In&r. and Meth. in Phys. Res. B 91 (1994) 168-171
tion (S e,max) is plotted in Fig. 2 as a function of projectile atomic m.nnber (Z,), for a given target material, suggesting the simple linear relationship: Se,maX==L
(3)
with K a constant. A~r~ngly,
from Eq. rz>:
Q(O) = S,(&l)
= m1, (4) and so the radiation damage at the surface should have the same linear dependence on projectile atomic number. Moreover, if we are to assume that the etching velocity for a track is proportional to the amount of damage, then we should also observe a linear relationship for the etching velocity of tracks formed by different ions with incident energy E = E,, in the same target material. It is interesting that electronic stopping power maxima for different polymers may be compared in terms of their co~esponding bulk densities, closely following the simple relation:
S,,,,(Tawt 1) &,&Target
2) a
[
Ptarget 1 -Ptargetz
l/2
1
(5)
*
1 Thus Fig. 3 is a plot of S_,,p-1/2 as a function of 2, for a random selection of familiar polymer targets (lexan, polypropylene, PVC, Kapton and CR-39). In each case values for S,,, were obtained by applica-
14
27
41
54
Zl Fig. 3. Scaling properties of S,,,,P-~/’ as a function of projectile atomic number, where p is the polymer density. Symbols correspond to the same materials listed in Fig. 2.
tion of the SR routine of the TRIM-90 computer code [3]. The scaling law obtained, after linear regression analysis of the data, reads as follows:
se,max =~l’~(18.2862~
- 7.613).
(6) This relation, it must be stressed, is satisfied not only for the specific polymers selected, but applies equally well over a wide class of polymer targets. From Eqs. (2) and (6), for a given projectile (Z,), the rate of surface damage created in two quite different polymers should scale as:
610
e,(target
1)
E,(target2)
Ptarget1 X i PtargetZ.i
l/2
’
(7)
It follows that plastic targets with similar densities but neglecting differences in their chemical structure would respond similarly to the maximum energy deposition at the surface for the same charged projectile.
210:
100 1
14
27
41
54
Zl Fig. 2. TRIM calculation [3] of the maximum electronic stopping cross section as a function of projectile atomic number for various polymer targets: Lexan (+ 1, Polypropylene (X1, PVC (El), Kapton (A >,and CR-39 (0).
4. Conclusion The diameters of etched ion tracks in CR-39 scale directly with the amount of electronic energy transferred to the surface at the point of incidence of the ions into the polymer. For 0.6-3 MeV lithium ion bombar~ent of CR-39 there is a pronounced maxi-
L. T. Chadderton et al. /Nucl. In&r. and Meth. in Phys. Res. B 91 (1994) 168-171
mum etch track diameter at N 1.1 MeV. This corresponds to that energy for which the maximum electronic stopping cross section is attained. Computer simulations show that the maximum electronic energy transfer for different ions in the same target material is proportional to the corresponding projectile atomic number. This behaviour is evidently related to the number of electrons - projectile and target - participating in the stopping process, at the maximum of the electronic energy loss curve. It is precisely here that the electronic clouds of both projectile and target are strongly involved in competing mechanisms of energy loss - excitation, ionization, and charge exchange. A preliminary analysis on other classes of inorganic targets indicates a similar linear on projectile atomic number. dependence of S,,,
Acknowledgements Two of the authors (L.T.C. and S.A.C.) are grateful to Australia and Mexico for assistance under the aegis
171
of the bilateral science and technology agreement, and also from the Autonomous Metropolitan University of Mexico (Mexico DF). J.L.Z is appreciative for the support of the International Atomic Energy Agency (Vienna). The office of CONACYT (Mexico) supports the work of S.A.C. under Research Contract No. 1405E9207. This direct assistance is both warmly appreciated, and clearly acknowledged.
References ill J. Davenas, G. Boiteux and M. Fallavier, Nucl. Instr. and
Meth. B 39 (1989) 796. La D. Fir& M. Muller, A. Schmoldt, J.K. Zhou, L.T. Chadderton and X.L. Xu, Nucl. In&. and Meth. B 65 (1992) 432. 131J.P. Biersack, Z. Phys. A 315 (1982) 95. 141 L. Bernardi, A. Cecchi, C. Gori, F. Lucarelli and R. Renzi, Nucl.Instr. and Meth. B 53 (1991) 61. [51 J.F. Gibbons, Proc IEEE 60 (1972) 1062.
II. ENERGETIC HEAVY IONS