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Study on evolution of gases from Mylar under ion irradiation
Nuclear Instruments and Methods in Physics Research B 146 (1998) 504±508
Study on evolution of gases from Mylar under ion irradiation D.K. Avasthi a
a,*
, J.P. Singh a, A. Biswas b, S.K. Bose
b
Nuclear Science Centre, Aruna Asaf Ali Marg, P. O. Box 10502, New Delhi-110067, India b Physics Department, Banaras Hindu University, Varanasi-221005, India
Abstract A swift heavy ion produces a zone of damage area along its path through polymers. The dominant gases hydrogen, carbon monoxide and ethylene released under 180 MeV Ag ion irradiation of Mylar i.e. polyethylene terephthalate (PET) were studied. The evolved gases were monitored as a function of ion ¯uence by quadrupole mass analyzer (QMA). The logarithmic decrease in the release of gaseous products provide an estimate of track diameter. It also indicates two zones of varying damages within the ion track. Ó 1998 Elsevier Science B.V. All rights reserved. PACS: 6180 J Keywords: Electronic energy loss; Ion track; Quadrupole mass analyzer
1. Introduction Swift heavy ions (SHI) provide a unique way of materials modi®cation by inducing such a high degree of localized electronic excitation, which otherwise is not possible by any other means. Its eect on the materials depends mainly on the electronic energy loss of the ion in material and ion ¯uence. The ions lose their energy as they pass through the material. The energy lost is either spent in displacing atoms (of the sample) by elastic collisions or it is spent in exciting or ionizing the atoms via inelastic collisions. The former is the dominant process at low energies. The inelastic collisions dominate at higher energies (MeV).
* Corresponding author. Tel.: +91 11 6893955; fax: +91 11 6893666; e-mail: [email protected]
There is growing interest in nanopores and micropores in polymers generated by swift heavy ions due to vast variety of applications [1,2]. These pores are made by controlled chemical etching of ion irradiated thin polymer foil. The micro/nanostructure fabrication in the chemically etched pores depend largely on the spatial parameters of latent ion tracks. The energetic ion creates damage along its path due to its large electronic energy deposition via inelastic collisions. The diameter of the damaged area along the ion path referred as track diameter is a quantity of interest for the understanding of basic ion insulator interaction. There have been a few attempts to measure the track diameters by scanning force microscopy and other state of the art surface morphology probing equipments. We present here a novel approach to determine the track diameters in polymers by online measurement of H loss in polymer during ion
0168-583X/98/$ ± see front matter Ó 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 0 4 6 4 - 9
D.K. Avasthi et al. / Nucl. Instr. and Meth. in Phys. Res. B 146 (1998) 504±508
505
irradiation. Mittal et al. [3] in a recent work, estimated the ion track radius by on-line H release measurement using elastic recoil detection (ERD) technique [4,5]. The present on-line measurement of H release under ion impingement carried out by quadrupole mass analyzer. This work reports on the study of evolution of gases from polymer as a result of large electronic excitation produced by Ag ion irradiation. Interesting inferences are drawn from the data on evolution of gases. 2. Experimental details Ag14 ions of 180 MeV from 15 MV Pelletron accelerator [6] at NSC [7] were used to irradiate thin polymer ®lms of mylar procured from Goodfellow (UK). The ion current was 0.03 particle nano Ampere in an area of about 0.06 cm2 . The polymer foil of 13 lm thickness was mounted on a metal ladder, which was electrically insulated by the chamber. The range (as estimated by TRIM [8]) of the incident ion was more than the thickness of the polymer foil. The pressure in the chamber was 10ÿ8 Torr before ion impingement on the sample. The ion ¯uence was measured by integrating the ion charge on the sample ladder, which was insulated from the chamber. The inaccuracy in the ion ¯uence measurement can be up to 40%, due to uncertainty in ion beam area, integrated charge etc. The schematic of experimental set up is shown in Fig. 1.
Fig. 1. Schematic of the experimental set up.
sixteen expected masses were ®tted in sixteen channels, which were analyzed later to detect evolved species. As soon as the ion beam was incident on the target PET, pressure rise of one order of magnitude was detected for various evolved species. Fig. 2 shows the partial pressures of major residual gases evolved from irradiated PET as a function of time, detected in situ by RGA. The prominent gases observed were C2 H4 and CO at 28 amu, apart from large rate of H2 evolution at 2 amu. RGA distinguishes between CO and C2 H4 by
2.1. Gas evolution measurements by RGA Molecules and radicals emitted from 180 MeV Ag ions irradiated polyethylene terephthalate (PET) were monitored on-line by a residual gas analyzer (RGA). The RGA is basically a quadrupole mass analyzer (model HAL 2/201 from Hiden, UK). The presence of major mass peaks at 2 and 18 amu were detected in the UHV chamber by the analyzer before ion impingement. Data acquisition was done by the analyzer in secondary electron multiplier (SEM) mode with automatic background subtraction of the previous rest gas measurements. For data acquisition, multiple ion detection (MID) option of RGA was selected and
Fig. 2. Partial pressure of dierent gases at dierent ¯uence of the ion beam.
506
D.K. Avasthi et al. / Nucl. Instr. and Meth. in Phys. Res. B 146 (1998) 504±508
3. Results and discussion
where NH / is the H2 gas density at ¯uence /; C1 and C2 are the pre-exponential constant terms and / is the ion ¯uence. q1 is higher release cross section of H2 and q2 is lower release cross section. The latter corresponds to the ion ¯uence, at which the total overlap of ion tracks occurs. The relation for release of other gases can also be written similarly. The initial (higher) slope of the evolved gas density with ¯uence gives the cross section of gas release, which is equivalent to pr2 where r is the track radius. The particular gas density N in molecules/cc can be obtained using the following relation [9]
3.1. Ion track estimation by gas evolution data
N P =1:035 10ÿ19 T ;
the stored standard spectra of these gaseous species based on their secondary ionization peaks. Since the RGA was set in background subtraction mode prior to the irradiation and PET does not have any functional group containing N2 , so the possibility of occurrence of N2 at 28 amu is ruled out. The probability of adsorbed N2 in the sample is also ruled out, as the pre irradiation RGA spectrum did not show the corresponding mass peak.
The passage of ion through polymer causes electronic excitation of constituent atoms, which results in breaking of various bonds. Hydrogen is liberated due to breaking of bonds associated with H. Free H atoms combine with other H atoms to form hydrogen molecule. Being the lightest gaseous molecule having high diusivity, these molecules escape from the polymer causing reduction in H content due to ion irradiation. Thus incident ion along its path releases H atoms. Each ion is effective in much larger area producing a cylindrical zone of damaged polymer. Evolution of gases takes place from these cylindrical zones referred as ion track. The residual gas pressure data recorded during ion impingement on the sample indicated that it is not only H2 but other hydrocarbon gases also formed in ion polymer interaction and these gases diuse out of the sample. Since these gases are evolved from the zone where the incident ion is eecting, it is possible to estimate the cross section of release of dierent gases. The evaluated cross section will give an estimate of ion track diameter. It can be argued that (i) higher hydrogen release takes place from the polymer with higher H concentration, (ii) the nature of loss of H2 with ion ¯uence is exponential and (iii) the partial pressure is directly related to the H2 loss. Based on these facts, one can write the following relation. NH / C1 exp ÿq1 / C2 exp ÿq2 /;
where N is the gas density, P the pressure in Torr of evolved species, and T the temperature in K. Figs. 3±5 show partial pressures of three major evolved gases in logarithmic scale as a function of ion ¯uence, which reveal two separate exponential regions. The intersection of ®rst exponential and second exponential curve corresponds to the ion ¯uence where the total overlap of ion tracks occur. It means that beyond the region of intersection, when an ion is incident on the polymer, it is always on the region were H2 has been earlier released due to any previous incident ion. Therefore the loss of H2 is comparatively smaller. The track radius as estimated from the release cross section of H2 , CO and C2 H4 are given in Table 1 along with the
Fig. 3. Partial pressure of H2 versus ion ¯uence.
D.K. Avasthi et al. / Nucl. Instr. and Meth. in Phys. Res. B 146 (1998) 504±508
Fig. 4. Partial pressure of CO versus ion ¯uence.
507
parison of track diameters of three measurements is valid. The measured track radius varies with dierent evolved gaseous species, and expectedly come largest for H2 evolution as shown in the Table 1. The track radius estimated by CO and C2 H4 are smaller as the release of these gases occur from smaller cross sectional area. The estimated error in the track radius measurement is up to 20% due to uncertainty in the ion ¯uence. Our approach to estimate the track dimension in the present case is unique as the measurements depend on the dynamic on-line results. The results [10] based on FTIR measurements requires irradiation of several samples at dierent ¯uences for IR measurements. Whereas in the present case, measurements can be done by in situ gas evolution data. Other method for track radius determination [11] requires chemical etching and hence the measurement is likely to be over estimated. Fig. 6 shows the eective area of ion track damage due to evolution of H2 , CO and C2 H4 gases. The region within about 6 nm is the maximum aected central region where the gases hydrogen, ethylene and carbon monoxide are evolved. This is the region, where important intra track chemistry proceeds and governs the ion beam induced physical and chemical modi®cations. The lesser damaged zone between 6 and 8 nm of the track corresponds to the release of hydrogen gas only.
Fig. 5. Partial pressure of C2 H4 versus ion ¯uence.
values given by Steckenreiter et al. [10] and Apel et al. [11]. The electronic energy loss Se values in the [10,11] are quite close to the corresponding Se value of the present measurement. So the com-
4. Conclusion When ion beam is incident on the polymer PET, dierent gases (H2 , CO and C2 H4 ) are evolved due to breaking of bonds. We present a
Table 1 Ion track radii based on the release of gases during irradiation as measured in the present experiment along with the previous reported values. The values of the electronic energy loss are also given. Se (keV/nm)
Ion track dimension (radius) in PET (nm) Present measurements
Ref. [10]
Ref. [11]
Present case
Ref. [10]
Ref. [11]
H2 : 4 CO: 2.9 C2 H4 : 2.8
3
7.5
10.22
8.7
10.84
508
D.K. Avasthi et al. / Nucl. Instr. and Meth. in Phys. Res. B 146 (1998) 504±508
central region where the gases hydrogen, ethylene and carbon monooxide are evolved. The outer region of track from 6 to 8 nm is lesser damaged as compared to the inner zone and this corresponds to the release of hydrogen. It seems that the dominant part of intra track chemistry which governs the ion beam induced modi®cation, occurs within the maximum damaged zone of 6 nm. References
Fig. 6. Eective area of ion track damage due to evolution of H2 , CO and C2 H4 gases.
novel technique to estimate ion track dimension in polymers, based on in situ gas evolution data which does not require chemical etching. Dierent cross sections of release of dierent gases indicate that ion beam damage zone varies with the type of gases evolved. Thus each ion produces a damaged cylindrical zone, in which there are two dierent zones having dierent degrees of damage. The region within about 6 nm is the maximum eected
[1] C.R. Martin, Science 266 (1994) 1961. [2] L. Piraux, S. Dubois, S. Demoustier-Champagne, Nucl. Instr. and Meth. B 131 (1997) 1357. [3] V.K. Mittal, S. Lotha, D.K. Avasthi, Radiat. E. Def. Solids, accepted. [4] J. L'Ecuyer, C. Brassard, C. Cardinal, B. Terrault, Nucl. Instr. and Meth. 149 (1978) 271. [5] D.K. Avasthi, D. Kabiraj, A. Bhagwat, G.K. Mehta, V.D. Vankar, S.B. Ogale, Nucl. Instr. and Meth. B 93 (1994) 480. [6] D. Kanjilal, S. Chopra, M.M. Narayanan, I.S. Iyer, V. Jha, R. Joshi, S.K. Datta, Nucl. Instr. and Meth. A 238 (1993) 97. [7] G.K. Mehta, A.P. Patro, Nucl. Instr. and Meth. A 268 (1988) 334. [8] J.F. Ziegler, J.P. Biersack, U. Littmark, Stopping and Ranges of Ions in Matter, Pergamon Press, New York, 1985. [9] H.G. Tompkins, The Fundamentals Of Vacuum Technology, AVS Monograph Series, M-6, 1991, American Vacuum Society, New York, USA. [10] T. Steckenreiter, E. Balanzat, H. Fuess, C. Trautmann, Nucl. Instr. and Meth. B 131 (1997) 159. [11] P. Apel, A. Schulz, R. Spohr, C. Trautmann, V. Vutsadakis, Nucl. Instr. and Meth. B 131 (1997) 55.
Study on evolution of gases from Mylar under ion irradiation D.K. Avasthi a
a,*
, J.P. Singh a, A. Biswas b, S.K. Bose
b
Nuclear Science Centre, Aruna Asaf Ali Marg, P. O. Box 10502, New Delhi-110067, India b Physics Department, Banaras Hindu University, Varanasi-221005, India
Abstract A swift heavy ion produces a zone of damage area along its path through polymers. The dominant gases hydrogen, carbon monoxide and ethylene released under 180 MeV Ag ion irradiation of Mylar i.e. polyethylene terephthalate (PET) were studied. The evolved gases were monitored as a function of ion ¯uence by quadrupole mass analyzer (QMA). The logarithmic decrease in the release of gaseous products provide an estimate of track diameter. It also indicates two zones of varying damages within the ion track. Ó 1998 Elsevier Science B.V. All rights reserved. PACS: 6180 J Keywords: Electronic energy loss; Ion track; Quadrupole mass analyzer
1. Introduction Swift heavy ions (SHI) provide a unique way of materials modi®cation by inducing such a high degree of localized electronic excitation, which otherwise is not possible by any other means. Its eect on the materials depends mainly on the electronic energy loss of the ion in material and ion ¯uence. The ions lose their energy as they pass through the material. The energy lost is either spent in displacing atoms (of the sample) by elastic collisions or it is spent in exciting or ionizing the atoms via inelastic collisions. The former is the dominant process at low energies. The inelastic collisions dominate at higher energies (MeV).
* Corresponding author. Tel.: +91 11 6893955; fax: +91 11 6893666; e-mail: [email protected]
There is growing interest in nanopores and micropores in polymers generated by swift heavy ions due to vast variety of applications [1,2]. These pores are made by controlled chemical etching of ion irradiated thin polymer foil. The micro/nanostructure fabrication in the chemically etched pores depend largely on the spatial parameters of latent ion tracks. The energetic ion creates damage along its path due to its large electronic energy deposition via inelastic collisions. The diameter of the damaged area along the ion path referred as track diameter is a quantity of interest for the understanding of basic ion insulator interaction. There have been a few attempts to measure the track diameters by scanning force microscopy and other state of the art surface morphology probing equipments. We present here a novel approach to determine the track diameters in polymers by online measurement of H loss in polymer during ion
0168-583X/98/$ ± see front matter Ó 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 0 4 6 4 - 9
D.K. Avasthi et al. / Nucl. Instr. and Meth. in Phys. Res. B 146 (1998) 504±508
505
irradiation. Mittal et al. [3] in a recent work, estimated the ion track radius by on-line H release measurement using elastic recoil detection (ERD) technique [4,5]. The present on-line measurement of H release under ion impingement carried out by quadrupole mass analyzer. This work reports on the study of evolution of gases from polymer as a result of large electronic excitation produced by Ag ion irradiation. Interesting inferences are drawn from the data on evolution of gases. 2. Experimental details Ag14 ions of 180 MeV from 15 MV Pelletron accelerator [6] at NSC [7] were used to irradiate thin polymer ®lms of mylar procured from Goodfellow (UK). The ion current was 0.03 particle nano Ampere in an area of about 0.06 cm2 . The polymer foil of 13 lm thickness was mounted on a metal ladder, which was electrically insulated by the chamber. The range (as estimated by TRIM [8]) of the incident ion was more than the thickness of the polymer foil. The pressure in the chamber was 10ÿ8 Torr before ion impingement on the sample. The ion ¯uence was measured by integrating the ion charge on the sample ladder, which was insulated from the chamber. The inaccuracy in the ion ¯uence measurement can be up to 40%, due to uncertainty in ion beam area, integrated charge etc. The schematic of experimental set up is shown in Fig. 1.
Fig. 1. Schematic of the experimental set up.
sixteen expected masses were ®tted in sixteen channels, which were analyzed later to detect evolved species. As soon as the ion beam was incident on the target PET, pressure rise of one order of magnitude was detected for various evolved species. Fig. 2 shows the partial pressures of major residual gases evolved from irradiated PET as a function of time, detected in situ by RGA. The prominent gases observed were C2 H4 and CO at 28 amu, apart from large rate of H2 evolution at 2 amu. RGA distinguishes between CO and C2 H4 by
2.1. Gas evolution measurements by RGA Molecules and radicals emitted from 180 MeV Ag ions irradiated polyethylene terephthalate (PET) were monitored on-line by a residual gas analyzer (RGA). The RGA is basically a quadrupole mass analyzer (model HAL 2/201 from Hiden, UK). The presence of major mass peaks at 2 and 18 amu were detected in the UHV chamber by the analyzer before ion impingement. Data acquisition was done by the analyzer in secondary electron multiplier (SEM) mode with automatic background subtraction of the previous rest gas measurements. For data acquisition, multiple ion detection (MID) option of RGA was selected and
Fig. 2. Partial pressure of dierent gases at dierent ¯uence of the ion beam.
506
D.K. Avasthi et al. / Nucl. Instr. and Meth. in Phys. Res. B 146 (1998) 504±508
3. Results and discussion
where NH / is the H2 gas density at ¯uence /; C1 and C2 are the pre-exponential constant terms and / is the ion ¯uence. q1 is higher release cross section of H2 and q2 is lower release cross section. The latter corresponds to the ion ¯uence, at which the total overlap of ion tracks occurs. The relation for release of other gases can also be written similarly. The initial (higher) slope of the evolved gas density with ¯uence gives the cross section of gas release, which is equivalent to pr2 where r is the track radius. The particular gas density N in molecules/cc can be obtained using the following relation [9]
3.1. Ion track estimation by gas evolution data
N P =1:035 10ÿ19 T ;
the stored standard spectra of these gaseous species based on their secondary ionization peaks. Since the RGA was set in background subtraction mode prior to the irradiation and PET does not have any functional group containing N2 , so the possibility of occurrence of N2 at 28 amu is ruled out. The probability of adsorbed N2 in the sample is also ruled out, as the pre irradiation RGA spectrum did not show the corresponding mass peak.
The passage of ion through polymer causes electronic excitation of constituent atoms, which results in breaking of various bonds. Hydrogen is liberated due to breaking of bonds associated with H. Free H atoms combine with other H atoms to form hydrogen molecule. Being the lightest gaseous molecule having high diusivity, these molecules escape from the polymer causing reduction in H content due to ion irradiation. Thus incident ion along its path releases H atoms. Each ion is effective in much larger area producing a cylindrical zone of damaged polymer. Evolution of gases takes place from these cylindrical zones referred as ion track. The residual gas pressure data recorded during ion impingement on the sample indicated that it is not only H2 but other hydrocarbon gases also formed in ion polymer interaction and these gases diuse out of the sample. Since these gases are evolved from the zone where the incident ion is eecting, it is possible to estimate the cross section of release of dierent gases. The evaluated cross section will give an estimate of ion track diameter. It can be argued that (i) higher hydrogen release takes place from the polymer with higher H concentration, (ii) the nature of loss of H2 with ion ¯uence is exponential and (iii) the partial pressure is directly related to the H2 loss. Based on these facts, one can write the following relation. NH / C1 exp ÿq1 / C2 exp ÿq2 /;
where N is the gas density, P the pressure in Torr of evolved species, and T the temperature in K. Figs. 3±5 show partial pressures of three major evolved gases in logarithmic scale as a function of ion ¯uence, which reveal two separate exponential regions. The intersection of ®rst exponential and second exponential curve corresponds to the ion ¯uence where the total overlap of ion tracks occur. It means that beyond the region of intersection, when an ion is incident on the polymer, it is always on the region were H2 has been earlier released due to any previous incident ion. Therefore the loss of H2 is comparatively smaller. The track radius as estimated from the release cross section of H2 , CO and C2 H4 are given in Table 1 along with the
Fig. 3. Partial pressure of H2 versus ion ¯uence.
D.K. Avasthi et al. / Nucl. Instr. and Meth. in Phys. Res. B 146 (1998) 504±508
Fig. 4. Partial pressure of CO versus ion ¯uence.
507
parison of track diameters of three measurements is valid. The measured track radius varies with dierent evolved gaseous species, and expectedly come largest for H2 evolution as shown in the Table 1. The track radius estimated by CO and C2 H4 are smaller as the release of these gases occur from smaller cross sectional area. The estimated error in the track radius measurement is up to 20% due to uncertainty in the ion ¯uence. Our approach to estimate the track dimension in the present case is unique as the measurements depend on the dynamic on-line results. The results [10] based on FTIR measurements requires irradiation of several samples at dierent ¯uences for IR measurements. Whereas in the present case, measurements can be done by in situ gas evolution data. Other method for track radius determination [11] requires chemical etching and hence the measurement is likely to be over estimated. Fig. 6 shows the eective area of ion track damage due to evolution of H2 , CO and C2 H4 gases. The region within about 6 nm is the maximum aected central region where the gases hydrogen, ethylene and carbon monoxide are evolved. This is the region, where important intra track chemistry proceeds and governs the ion beam induced physical and chemical modi®cations. The lesser damaged zone between 6 and 8 nm of the track corresponds to the release of hydrogen gas only.
Fig. 5. Partial pressure of C2 H4 versus ion ¯uence.
values given by Steckenreiter et al. [10] and Apel et al. [11]. The electronic energy loss Se values in the [10,11] are quite close to the corresponding Se value of the present measurement. So the com-
4. Conclusion When ion beam is incident on the polymer PET, dierent gases (H2 , CO and C2 H4 ) are evolved due to breaking of bonds. We present a
Table 1 Ion track radii based on the release of gases during irradiation as measured in the present experiment along with the previous reported values. The values of the electronic energy loss are also given. Se (keV/nm)
Ion track dimension (radius) in PET (nm) Present measurements
Ref. [10]
Ref. [11]
Present case
Ref. [10]
Ref. [11]
H2 : 4 CO: 2.9 C2 H4 : 2.8
3
7.5
10.22
8.7
10.84
508
D.K. Avasthi et al. / Nucl. Instr. and Meth. in Phys. Res. B 146 (1998) 504±508
central region where the gases hydrogen, ethylene and carbon monooxide are evolved. The outer region of track from 6 to 8 nm is lesser damaged as compared to the inner zone and this corresponds to the release of hydrogen. It seems that the dominant part of intra track chemistry which governs the ion beam induced modi®cation, occurs within the maximum damaged zone of 6 nm. References
Fig. 6. Eective area of ion track damage due to evolution of H2 , CO and C2 H4 gases.
novel technique to estimate ion track dimension in polymers, based on in situ gas evolution data which does not require chemical etching. Dierent cross sections of release of dierent gases indicate that ion beam damage zone varies with the type of gases evolved. Thus each ion produces a damaged cylindrical zone, in which there are two dierent zones having dierent degrees of damage. The region within about 6 nm is the maximum eected
[1] C.R. Martin, Science 266 (1994) 1961. [2] L. Piraux, S. Dubois, S. Demoustier-Champagne, Nucl. Instr. and Meth. B 131 (1997) 1357. [3] V.K. Mittal, S. Lotha, D.K. Avasthi, Radiat. E. Def. Solids, accepted. [4] J. L'Ecuyer, C. Brassard, C. Cardinal, B. Terrault, Nucl. Instr. and Meth. 149 (1978) 271. [5] D.K. Avasthi, D. Kabiraj, A. Bhagwat, G.K. Mehta, V.D. Vankar, S.B. Ogale, Nucl. Instr. and Meth. B 93 (1994) 480. [6] D. Kanjilal, S. Chopra, M.M. Narayanan, I.S. Iyer, V. Jha, R. Joshi, S.K. Datta, Nucl. Instr. and Meth. A 238 (1993) 97. [7] G.K. Mehta, A.P. Patro, Nucl. Instr. and Meth. A 268 (1988) 334. [8] J.F. Ziegler, J.P. Biersack, U. Littmark, Stopping and Ranges of Ions in Matter, Pergamon Press, New York, 1985. [9] H.G. Tompkins, The Fundamentals Of Vacuum Technology, AVS Monograph Series, M-6, 1991, American Vacuum Society, New York, USA. [10] T. Steckenreiter, E. Balanzat, H. Fuess, C. Trautmann, Nucl. Instr. and Meth. B 131 (1997) 159. [11] P. Apel, A. Schulz, R. Spohr, C. Trautmann, V. Vutsadakis, Nucl. Instr. and Meth. B 131 (1997) 55.