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Incorporation of chlorine in polymer films due to MeV proton bombardment
Nuclear Instruments and Methods in Physics Research B 166±167 (2000) 632±636
www.elsevier.nl/locate/nimb
Incorporation of chlorine in polymer ®lms due to MeV proton bombardment N. Menzel, K. Wittmaack
*
GSF-Forschungszentrum, Institut f ur Strahlenschutz, Postfach 1129, 85758 Neuherberg, Germany
Abstract Adsorption of Cl on polymer ®lms and enhanced uptake due to bombardment with 1.1 MeV protons has been studied after an occasional contamination of an analysis chamber for external proton-induced X-ray analysis (PIXE). The contamination occurred as a result of cleaning the Be window of the Si(Li) detector with CCl4 . Changes in areal density of Cl on and in polymer ®lms were measured by PIXE, at Cl partial pressures between about 10ÿ4 and 10ÿ5 mbar in the He back®lling gas. The partial pressure of gases with atomic numbers Z P 10 (Ne) was measured by gas phase PIXE. After a few hours of exposure to CCl4 at 5 10ÿ4 mbar, areal densities of 1 lg/cm2 of Cl were observed on cellulose acetate, cellulose nitrate and polycarbonate ®lms. Detailed studies at about 2 10ÿ5 mbar showed that uptake of Cl in proton bombarded cellulose acetate is enhanced by several orders of magnitude compared to the non-bombarded ®lm. After exposure to low bombardment ¯uences, 1014 p/cm2 , approximately two Cl atoms per incident proton were incorporated in 2 min. This rate increased by a factor of 3.3 after bombardment to 2 1015 p/cm2 . Longterm exposure (45 min) in He after low-¯uence bombardment showed that the rate of uptake decreases with increasing time. The results suggest that proton-induced damage produces active sites in the polymer ®lm that can bond Cl atoms or Cl containing molecules. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 61.80Jh; 82.35.+t; 82.80.Yc Keywords: PIXE; Radiation damage; Chlorine uptake
1. Introduction Polymer ®lms are common backings for collecting aerosol particulate matter. Multi-element analysis of the sampled material may conveniently be performed by proton-induced X-ray emission
*
Corresponding author. Tel.: +49-89-3187-2439; fax: +4989-3187-2949. E-mail address: [email protected] (K. Wittmaack).
spectrometry (PIXE). We have recently described a new external-PIXE chamber which was designed for routine, high-throughput analysis of aerosol deposits in a He atmosphere [1]. The novel feature of the chamber is a rotating target holder which allows total-sample analysis of material collected along a circular ring (50 mm diameter) on the backing, such as in Berner-type impactors. The chamber can also be used for analysis of largearea targets, without rotation [2]. After about one year of routine work using the chamber it ap-
0168-583X/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 8 7 0 - 8
N. Menzel, K. Wittmaack / Nucl. Instr. and Meth. in Phys. Res. B 166±167 (2000) 632±636
peared that some degradation in sensitivity had occurred. Inspection of the Be entrance window of the Si(Li) detector of a Link ISISTM Series 200 Microanalysis System (Oxford) revealed partial coverage of the window with some kind of black debris. Upon suggestion of the service group of the manufacturer, the window could be cleaned successfully by rinsing with CCl4 . In course of this ®rst-time cleaning procedure, a sizeable amount of CCl4 got spilled inside the analysis chamber. During the ®rst series of experiments on bare polymer ®lms we noted high levels of Cl in the respective PIXE spectra. These signals were clearly related to the Cl contamination of the chamber. In previous work involving backscattering analysis we found that O is incorporated in proton bombarded polypropylene ®lms exposed to air [3]. This observation prompted the present study which aimed at exploring the idea that a similar incorporation process might occur with Cl present in the gas surrounding the analysed sample. 2. Experimental Details of the analysis chamber have been described before [1]. Brie¯y, the proton beam produced by a Van de Graa accelerator enters the analysis chamber through a 4 lm Al0:96 Mg0:04 window (4.5 mm diameter). The proton beam passes through the back®lling gas of He at ambient pressure and hits the target at normal incidence, with an energy of 1.1 MeV on impact. The Si(Li) X-ray detector views the target at 135° to the direction of beam propagation. The X-ray ®lter preceding the detector is made of 100 lm Mylar, with a central 1 mm bore. This study diered from the work on O incorporation [3] in two ways. First, PIXE was used throughout for determining the areal density of Cl. Hence there is no direct information on the depth distribution of Cl in the polymer ®lms. Second, the ®lms were bombarded and stored under standard PIXE conditions, i.e., in the He ambient. Further analysis after exposure to air was not carried out.
633
3. Results and discussion Fig. 1 shows a comparison of PIXE spectra of two neat cellulose acetate ®lms from the same batch (Sartorius SM 11107, pore size 0.2 lm). The thick line relates to a measurement prior to cleaning with CCl4 . The spectrum is dominated by a continuous background which is attributed to bremsstrahlung generated by knock-on electrons. A number of characteristic X-ray lines due to impurities in the ®lm are easily identi®ed, such as S, Cl, K, Ca, Ti and Fe. The actual amount of these impurities varies from sample to sample and across a sample by up to several 10%. High levels of Fe occasionally observed are presumably due to external contamination introduced during handling of the ®lms. After the CCl4 cleaning procedure described in Section 1, another set of polymer samples was introduced into the sample chamber. The spectrum of cellulose acetate observed after storage in the He-back®lled chamber for about 2 h is plotted in Fig. 1 as a thin line with solid circles. Compared to the ÔcleanÕ spectrum, the (background-corrected) Cl signal has increased by more than two orders of magnitude. Undoubtedly, the enhanced Cl signal must be attributed to adsorption of CCl4 , or residues thereof, still present in the analysis chamber at that stage of outgassing.
Fig. 1. PIXE spectra of cellulose acetate, either clean or exposed for several hours to CCl4 at a partial pressure of about 5 10ÿ4 mbar.
634
N. Menzel, K. Wittmaack / Nucl. Instr. and Meth. in Phys. Res. B 166±167 (2000) 632±636
Samples of other polymers mounted on the same target holder also revealed contamination with Cl. The amount of Cl present on/in the different ®lms diered by up to about a factor of 5, being smallest for cellulose nitrate (Sartorius SM 11304-47, pore size 0.8 lm), intermediate for polycarbonate (Millipore HTTP 03700, pore size 0.4 lm) and largest for cellulose acetate. Apparently, the sticking coecient of the contaminant gas on the dierent polymer materials is not the same. In view of these results it was clearly desirable to know the partial pressure of Cl containing molecules in the He gas. For this purpose we made use of the results of a prior study devoted to neon gas phase analysis by PIXE and backscattering. PIXE spectra observed after introducing neon into the evacuated analysis chamber through a leak valve are shown in Fig. 2. The proton beam was passing through a large hole in the sample holder and only the X-rays emitted due to the interaction of protons with the low-pressure gas were viewed by the detector. The spectrum is dominated by the characteristic Ne-Ka line at 0.85 keV. The observed Ar-Ka; b doublet at 2.96 and 3.19 keV is attributed to argon present in the residual air (with the standard pump con®guration, the minimum pressure in the analysis chamber is only about 10ÿ4 mbar). The Al line was found to be due to spurious
Fig. 2. PIXE spectra of Ne introduced into the evacuated analysis chamber at two dierent pressures.
X-ray emission originating from the Al±Mg window. Meanwhile, the Al contamination in the spectrum has been removed by shielding with a graphite tube. In this work, however, the Al turned out to be useful in that it served to normalise the Ne spectra (with He back®lling, the primary ion ¯uence is controlled by backscattering of protons from He atoms [4]; this procedure is not applicable in vacuum). PIXE spectra of the Cl-contaminated He gas are shown in Fig. 3, for dierent times after use of CCl4 . It should be noted that due to the presence of the 12 lm Be window of the detector and the additional X-ray ®lter the detection sensitivity for Ne is lower by an extrapolated factor of about 20 compared to Cl (see [2]). Hence the partial pressure of Cl relating to the upper spectrum in Fig. 3 (thin line and solid circles) is estimated to be 5 10ÿ4 mbar. For the lower spectrum (thick line) the pressure was a factor of about 20 lower. The eect of proton bombardment on the uptake of Cl was explored 3 days after CCl4 exposure, i.e. at the lower Cl partial pressure of 2 10ÿ5 mbar. Fig. 4 shows four spectra out of a total of 13 measured at the same position (A) on cellulose acetate, with about 3.5 min between two consecutive spectra (2 min per measurement, 1.5 min to save data and to initiate the next spectrum).
Fig. 3. PIXE spectra of the chlorine gas component in the He back®lling gas of the analysis chamber, at two dierent levels of the chlorine partial pressure.
N. Menzel, K. Wittmaack / Nucl. Instr. and Meth. in Phys. Res. B 166±167 (2000) 632±636
Fig. 4. Expanded PIXE spectra of cellulose acetate after repeated exposures to the probing proton beam.
As a result of repeated bombardment all impurity signals increase. The very large enhancement seen for Cl is attributed to continuous incorporation from the gas phase, with an incorporation rate that increases with increasing damage in the ®lm (see below). For impurities other than Cl one observes a rapid initial increase in signal. Thereafter, the signals tend to become constant, as shown in more detail in Fig. 5. The initial enhancement for these impurities is dicult to explain. One might speculate that there is some impurity transport to the surface. It is also likely that the eect is related to the porous structure of the ®lm. The eect of bombardment time and exposure time can be distinguished on the basis of the results of Fig. 6 which shows two second spectra (#2), one recorded at position A and the other one at position B on the same sample. The dierence between the two is that spectrum #2 at A was measured right after the ®rst spectrum (as discussed with reference to Fig. 4) whereas #2 at B was taken 47 min after #1, with no additional bombardment in between. Clearly the Cl content is higher after a longer period of exposure to the contaminated gas, but the dierence in Cl content is much smaller than the dierence in time. The importance of proton bombardment is also evident from the fact that, for about the same time of exposure since spectrum #1, the Cl content is 4.5 times higher with 10 consecutive periods of bombardment than
635
Fig. 5. Time (¯uence) dependence of impurity peak heights measured after repeated exposure to the proton beam at position A. The superscript B denotes peak heights measured at another position B on the same sample, with beam interruption for about 45 min.
without, as illustrated more clearly in Fig. 5 (full and open circles, respectively). In an attempt to evaluate aspects of the incorporation mechanism, the Cl signal have been corrected for the background and the initial Cl level. Corrected signals are presented in Fig. 7 as a function of bombardment time, which is directly proportional to the bombardment ¯uence. The straight line through the data points re¯ects a
Fig. 6. Expanded PIXE spectra for the second run at positions A and B, after dierent beam-o periods.
636
N. Menzel, K. Wittmaack / Nucl. Instr. and Meth. in Phys. Res. B 166±167 (2000) 632±636
ration rate per spectrum. After a proton ¯uence of 1:3 1015 cmÿ2 the areal density of incorporated Cl corresponds to 1:5 1016 Cl atoms/cm2 . This high number implies that Cl atoms must be distributed over a signi®cant depth. At a proton ¯uence of 1:3 1015 cmÿ2 , the dierential incorporation rate is 3.3 times higher than at 1 1014 cmÿ2 . Apparently, the damage responsible for Cl uptake has not saturated at a ¯uence of 1:3 1015 cmÿ2 . 4. Conclusion Fig. 7. Background-corrected peak heights versus time (or bombardment ¯uence).
power dependence, i.e. the Cl content N increase with time t as N / tp , with p 1:37 0:03. This non-integer p-value indicates a complex incorporation mechanism, i.e., the Cl content measured in the evaluated experiment apparently depended on more than one variable. In fact, the results presented above provide clear evidence that Cl uptake increases with increasing bombardment ¯uence as well as with exposure time. In the experiment of Fig. 7, these two parameters were not independent of each other. Hence the details of the incorporation mechanism cannot be determined. Based on the results of a previous calibration exercise [2], we can use the present data to derive incorporation rates as a function of bombardment ¯uence and exposure time, for the particular experiment under study. The proton ¯uence per spectrum was calibrated to be 1 1014 cmÿ2 . The areal density of Cl incorporated during the ®rst spectrum at this ¯uence was 2 1014 cmÿ2 , i.e. about two Cl atoms were incorporated within 2 min as a result of the damage produced by one incident proton. After an additional 45 min of exposure to the contaminated gas, the areal density increased to 6 1014 cmÿ2 , corresponding to three Cl atoms incorporated by the impact of one proton. It can be expected that further exposure of the slightly damaged sample to the gas would result in even higher Cl uptake. Extended bombardment, on the other hand, enhances the dierential incorpo-
The accidental contamination of our PIXE analysis chamber with Cl has been utilised to explore adsorption and incorporation of this species in polymer ®lms, notably in cellulose acetate. It turned out that the proton bombardment generates active sites in the ®lm which can eciently bond or trap Cl atoms or Cl containing molecules. The amount of Cl actually incorporated was found to depend on the bombardment ¯uence as well as on the availability of Cl, i.e. the respective partial pressure. The transport mechanism into the bulk of the ®lm may be another factor controlling the ÔspeedÕ of Cl uptake. Last but not least, bombardment-induced charging of the sample surface(s) may have an eect on the sticking coecient of the Cl carrying molecules. Combining the present results with those previously obtained for O we conclude that incorporation of ÔreactiveÕ gases into damaged polymer ®lms is a common phenomenon that may well be worth further studies.
References [1] H. Halder, N. Menzel, B. Hietel, K. Wittmaack, Nucl. Instr. and Meth. B 150 (1999) 90. [2] N. Menzel, B. Hietel, M. Leirer, W. Szymczak, K. Wittmaack, Nucl. Instr. and Meth. B 150 (1999) 96. [3] B. Hietel, K. Wittmaack, International Symposium on Materials Science Applications of Ion Beam Techniques, Seeheim, Germany, 1996. [4] B. Hietel, N. Menzel, K. Wittmaack, Nucl. Instr. and Meth. B 109/110 (1996) 139.
www.elsevier.nl/locate/nimb
Incorporation of chlorine in polymer ®lms due to MeV proton bombardment N. Menzel, K. Wittmaack
*
GSF-Forschungszentrum, Institut f ur Strahlenschutz, Postfach 1129, 85758 Neuherberg, Germany
Abstract Adsorption of Cl on polymer ®lms and enhanced uptake due to bombardment with 1.1 MeV protons has been studied after an occasional contamination of an analysis chamber for external proton-induced X-ray analysis (PIXE). The contamination occurred as a result of cleaning the Be window of the Si(Li) detector with CCl4 . Changes in areal density of Cl on and in polymer ®lms were measured by PIXE, at Cl partial pressures between about 10ÿ4 and 10ÿ5 mbar in the He back®lling gas. The partial pressure of gases with atomic numbers Z P 10 (Ne) was measured by gas phase PIXE. After a few hours of exposure to CCl4 at 5 10ÿ4 mbar, areal densities of 1 lg/cm2 of Cl were observed on cellulose acetate, cellulose nitrate and polycarbonate ®lms. Detailed studies at about 2 10ÿ5 mbar showed that uptake of Cl in proton bombarded cellulose acetate is enhanced by several orders of magnitude compared to the non-bombarded ®lm. After exposure to low bombardment ¯uences, 1014 p/cm2 , approximately two Cl atoms per incident proton were incorporated in 2 min. This rate increased by a factor of 3.3 after bombardment to 2 1015 p/cm2 . Longterm exposure (45 min) in He after low-¯uence bombardment showed that the rate of uptake decreases with increasing time. The results suggest that proton-induced damage produces active sites in the polymer ®lm that can bond Cl atoms or Cl containing molecules. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 61.80Jh; 82.35.+t; 82.80.Yc Keywords: PIXE; Radiation damage; Chlorine uptake
1. Introduction Polymer ®lms are common backings for collecting aerosol particulate matter. Multi-element analysis of the sampled material may conveniently be performed by proton-induced X-ray emission
*
Corresponding author. Tel.: +49-89-3187-2439; fax: +4989-3187-2949. E-mail address: [email protected] (K. Wittmaack).
spectrometry (PIXE). We have recently described a new external-PIXE chamber which was designed for routine, high-throughput analysis of aerosol deposits in a He atmosphere [1]. The novel feature of the chamber is a rotating target holder which allows total-sample analysis of material collected along a circular ring (50 mm diameter) on the backing, such as in Berner-type impactors. The chamber can also be used for analysis of largearea targets, without rotation [2]. After about one year of routine work using the chamber it ap-
0168-583X/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 8 7 0 - 8
N. Menzel, K. Wittmaack / Nucl. Instr. and Meth. in Phys. Res. B 166±167 (2000) 632±636
peared that some degradation in sensitivity had occurred. Inspection of the Be entrance window of the Si(Li) detector of a Link ISISTM Series 200 Microanalysis System (Oxford) revealed partial coverage of the window with some kind of black debris. Upon suggestion of the service group of the manufacturer, the window could be cleaned successfully by rinsing with CCl4 . In course of this ®rst-time cleaning procedure, a sizeable amount of CCl4 got spilled inside the analysis chamber. During the ®rst series of experiments on bare polymer ®lms we noted high levels of Cl in the respective PIXE spectra. These signals were clearly related to the Cl contamination of the chamber. In previous work involving backscattering analysis we found that O is incorporated in proton bombarded polypropylene ®lms exposed to air [3]. This observation prompted the present study which aimed at exploring the idea that a similar incorporation process might occur with Cl present in the gas surrounding the analysed sample. 2. Experimental Details of the analysis chamber have been described before [1]. Brie¯y, the proton beam produced by a Van de Graa accelerator enters the analysis chamber through a 4 lm Al0:96 Mg0:04 window (4.5 mm diameter). The proton beam passes through the back®lling gas of He at ambient pressure and hits the target at normal incidence, with an energy of 1.1 MeV on impact. The Si(Li) X-ray detector views the target at 135° to the direction of beam propagation. The X-ray ®lter preceding the detector is made of 100 lm Mylar, with a central 1 mm bore. This study diered from the work on O incorporation [3] in two ways. First, PIXE was used throughout for determining the areal density of Cl. Hence there is no direct information on the depth distribution of Cl in the polymer ®lms. Second, the ®lms were bombarded and stored under standard PIXE conditions, i.e., in the He ambient. Further analysis after exposure to air was not carried out.
633
3. Results and discussion Fig. 1 shows a comparison of PIXE spectra of two neat cellulose acetate ®lms from the same batch (Sartorius SM 11107, pore size 0.2 lm). The thick line relates to a measurement prior to cleaning with CCl4 . The spectrum is dominated by a continuous background which is attributed to bremsstrahlung generated by knock-on electrons. A number of characteristic X-ray lines due to impurities in the ®lm are easily identi®ed, such as S, Cl, K, Ca, Ti and Fe. The actual amount of these impurities varies from sample to sample and across a sample by up to several 10%. High levels of Fe occasionally observed are presumably due to external contamination introduced during handling of the ®lms. After the CCl4 cleaning procedure described in Section 1, another set of polymer samples was introduced into the sample chamber. The spectrum of cellulose acetate observed after storage in the He-back®lled chamber for about 2 h is plotted in Fig. 1 as a thin line with solid circles. Compared to the ÔcleanÕ spectrum, the (background-corrected) Cl signal has increased by more than two orders of magnitude. Undoubtedly, the enhanced Cl signal must be attributed to adsorption of CCl4 , or residues thereof, still present in the analysis chamber at that stage of outgassing.
Fig. 1. PIXE spectra of cellulose acetate, either clean or exposed for several hours to CCl4 at a partial pressure of about 5 10ÿ4 mbar.
634
N. Menzel, K. Wittmaack / Nucl. Instr. and Meth. in Phys. Res. B 166±167 (2000) 632±636
Samples of other polymers mounted on the same target holder also revealed contamination with Cl. The amount of Cl present on/in the different ®lms diered by up to about a factor of 5, being smallest for cellulose nitrate (Sartorius SM 11304-47, pore size 0.8 lm), intermediate for polycarbonate (Millipore HTTP 03700, pore size 0.4 lm) and largest for cellulose acetate. Apparently, the sticking coecient of the contaminant gas on the dierent polymer materials is not the same. In view of these results it was clearly desirable to know the partial pressure of Cl containing molecules in the He gas. For this purpose we made use of the results of a prior study devoted to neon gas phase analysis by PIXE and backscattering. PIXE spectra observed after introducing neon into the evacuated analysis chamber through a leak valve are shown in Fig. 2. The proton beam was passing through a large hole in the sample holder and only the X-rays emitted due to the interaction of protons with the low-pressure gas were viewed by the detector. The spectrum is dominated by the characteristic Ne-Ka line at 0.85 keV. The observed Ar-Ka; b doublet at 2.96 and 3.19 keV is attributed to argon present in the residual air (with the standard pump con®guration, the minimum pressure in the analysis chamber is only about 10ÿ4 mbar). The Al line was found to be due to spurious
Fig. 2. PIXE spectra of Ne introduced into the evacuated analysis chamber at two dierent pressures.
X-ray emission originating from the Al±Mg window. Meanwhile, the Al contamination in the spectrum has been removed by shielding with a graphite tube. In this work, however, the Al turned out to be useful in that it served to normalise the Ne spectra (with He back®lling, the primary ion ¯uence is controlled by backscattering of protons from He atoms [4]; this procedure is not applicable in vacuum). PIXE spectra of the Cl-contaminated He gas are shown in Fig. 3, for dierent times after use of CCl4 . It should be noted that due to the presence of the 12 lm Be window of the detector and the additional X-ray ®lter the detection sensitivity for Ne is lower by an extrapolated factor of about 20 compared to Cl (see [2]). Hence the partial pressure of Cl relating to the upper spectrum in Fig. 3 (thin line and solid circles) is estimated to be 5 10ÿ4 mbar. For the lower spectrum (thick line) the pressure was a factor of about 20 lower. The eect of proton bombardment on the uptake of Cl was explored 3 days after CCl4 exposure, i.e. at the lower Cl partial pressure of 2 10ÿ5 mbar. Fig. 4 shows four spectra out of a total of 13 measured at the same position (A) on cellulose acetate, with about 3.5 min between two consecutive spectra (2 min per measurement, 1.5 min to save data and to initiate the next spectrum).
Fig. 3. PIXE spectra of the chlorine gas component in the He back®lling gas of the analysis chamber, at two dierent levels of the chlorine partial pressure.
N. Menzel, K. Wittmaack / Nucl. Instr. and Meth. in Phys. Res. B 166±167 (2000) 632±636
Fig. 4. Expanded PIXE spectra of cellulose acetate after repeated exposures to the probing proton beam.
As a result of repeated bombardment all impurity signals increase. The very large enhancement seen for Cl is attributed to continuous incorporation from the gas phase, with an incorporation rate that increases with increasing damage in the ®lm (see below). For impurities other than Cl one observes a rapid initial increase in signal. Thereafter, the signals tend to become constant, as shown in more detail in Fig. 5. The initial enhancement for these impurities is dicult to explain. One might speculate that there is some impurity transport to the surface. It is also likely that the eect is related to the porous structure of the ®lm. The eect of bombardment time and exposure time can be distinguished on the basis of the results of Fig. 6 which shows two second spectra (#2), one recorded at position A and the other one at position B on the same sample. The dierence between the two is that spectrum #2 at A was measured right after the ®rst spectrum (as discussed with reference to Fig. 4) whereas #2 at B was taken 47 min after #1, with no additional bombardment in between. Clearly the Cl content is higher after a longer period of exposure to the contaminated gas, but the dierence in Cl content is much smaller than the dierence in time. The importance of proton bombardment is also evident from the fact that, for about the same time of exposure since spectrum #1, the Cl content is 4.5 times higher with 10 consecutive periods of bombardment than
635
Fig. 5. Time (¯uence) dependence of impurity peak heights measured after repeated exposure to the proton beam at position A. The superscript B denotes peak heights measured at another position B on the same sample, with beam interruption for about 45 min.
without, as illustrated more clearly in Fig. 5 (full and open circles, respectively). In an attempt to evaluate aspects of the incorporation mechanism, the Cl signal have been corrected for the background and the initial Cl level. Corrected signals are presented in Fig. 7 as a function of bombardment time, which is directly proportional to the bombardment ¯uence. The straight line through the data points re¯ects a
Fig. 6. Expanded PIXE spectra for the second run at positions A and B, after dierent beam-o periods.
636
N. Menzel, K. Wittmaack / Nucl. Instr. and Meth. in Phys. Res. B 166±167 (2000) 632±636
ration rate per spectrum. After a proton ¯uence of 1:3 1015 cmÿ2 the areal density of incorporated Cl corresponds to 1:5 1016 Cl atoms/cm2 . This high number implies that Cl atoms must be distributed over a signi®cant depth. At a proton ¯uence of 1:3 1015 cmÿ2 , the dierential incorporation rate is 3.3 times higher than at 1 1014 cmÿ2 . Apparently, the damage responsible for Cl uptake has not saturated at a ¯uence of 1:3 1015 cmÿ2 . 4. Conclusion Fig. 7. Background-corrected peak heights versus time (or bombardment ¯uence).
power dependence, i.e. the Cl content N increase with time t as N / tp , with p 1:37 0:03. This non-integer p-value indicates a complex incorporation mechanism, i.e., the Cl content measured in the evaluated experiment apparently depended on more than one variable. In fact, the results presented above provide clear evidence that Cl uptake increases with increasing bombardment ¯uence as well as with exposure time. In the experiment of Fig. 7, these two parameters were not independent of each other. Hence the details of the incorporation mechanism cannot be determined. Based on the results of a previous calibration exercise [2], we can use the present data to derive incorporation rates as a function of bombardment ¯uence and exposure time, for the particular experiment under study. The proton ¯uence per spectrum was calibrated to be 1 1014 cmÿ2 . The areal density of Cl incorporated during the ®rst spectrum at this ¯uence was 2 1014 cmÿ2 , i.e. about two Cl atoms were incorporated within 2 min as a result of the damage produced by one incident proton. After an additional 45 min of exposure to the contaminated gas, the areal density increased to 6 1014 cmÿ2 , corresponding to three Cl atoms incorporated by the impact of one proton. It can be expected that further exposure of the slightly damaged sample to the gas would result in even higher Cl uptake. Extended bombardment, on the other hand, enhances the dierential incorpo-
The accidental contamination of our PIXE analysis chamber with Cl has been utilised to explore adsorption and incorporation of this species in polymer ®lms, notably in cellulose acetate. It turned out that the proton bombardment generates active sites in the ®lm which can eciently bond or trap Cl atoms or Cl containing molecules. The amount of Cl actually incorporated was found to depend on the bombardment ¯uence as well as on the availability of Cl, i.e. the respective partial pressure. The transport mechanism into the bulk of the ®lm may be another factor controlling the ÔspeedÕ of Cl uptake. Last but not least, bombardment-induced charging of the sample surface(s) may have an eect on the sticking coecient of the Cl carrying molecules. Combining the present results with those previously obtained for O we conclude that incorporation of ÔreactiveÕ gases into damaged polymer ®lms is a common phenomenon that may well be worth further studies.
References [1] H. Halder, N. Menzel, B. Hietel, K. Wittmaack, Nucl. Instr. and Meth. B 150 (1999) 90. [2] N. Menzel, B. Hietel, M. Leirer, W. Szymczak, K. Wittmaack, Nucl. Instr. and Meth. B 150 (1999) 96. [3] B. Hietel, K. Wittmaack, International Symposium on Materials Science Applications of Ion Beam Techniques, Seeheim, Germany, 1996. [4] B. Hietel, N. Menzel, K. Wittmaack, Nucl. Instr. and Meth. B 109/110 (1996) 139.