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Energy dependence of proton-induced L X-ray production cross-section for W
Available online at www.sciencedirect.com
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 266 (2008) 1203–1205 www.elsevier.com/locate/nimb
Energy dependence of proton-induced L X-ray production cross-section for W Harsh Mohan *, Arvind Kumar Jain Department of Physics, M.L.N. College, Yamuna Nagar 135001, Haryana, India Received 20 September 2007; received in revised form 29 November 2007 Available online 8 January 2008
Abstract The energy dependence of proton-induced L X-ray production cross-section for W is studied in the energy range of 260–400 keV, with an interval of 20 keV. The measurement is made by observing the X-ray emission, with the help of HPGe detector coupled with ORTEC multichannel analyzer. The present results are critically examined with prevailing theory (ECPSSR) in this energy regime. The role of these studies in understanding the analytical spectra has been highlighted. Ó 2008 Elsevier B.V. All rights reserved. PACS: 29.30.Kv; 32.70.Fw; 39.30.+w; 81.70.Jb Keywords: X-rays; Proton induced X-ray emission; Cross-section; ECPSSR theory
1. Introduction The ionization of inner-shell by energetic proton is a fundamental problem in ion beam analysis. In recent years, this has received considerable attention [1]. The information regarding the experimental values of L X-ray production cross-section, at various proton energies, is important due to its wide use in areas of basic and applied Physics. In the case of L-shell ionization, most of the experiments have been conducted with bombarding ions having energies >500 keV [2]. Thus, there is not only a lack of data but there also exist large discrepancies between the experimental measurements and theoretical calculations [3]. The present work is a part of a series of measurements directed towards the study of X-ray production by proton bombardment [4–6]. The high speed collisions between bombarding ion and target atoms cause ionization of the target atoms due to Coulomb interactions. This subsequently results in the emission of characteristic X-rays. A
*
Corresponding author. Tel.: +91 1732 233768; fax: +91 1732 225560. E-mail address: [email protected] (H. Mohan).
0168-583X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.12.102
review of literature [7–9] indicates that the measurement for tungsten in the low energy region is carried out by Petukhov et al. [10]. They have measured the total L-shell Xray production cross-sections in proton energy range from 70 to 500 keV. Although the L X-ray production cross-section for medium and high Z elements, are small at these low proton energies, yet analytical work such as PIXE studies can only be done if accurate experimental L Xray production cross-section data is available. Another dimension which has resulted in perpetuating these studies is their role in understanding the chemical environment effects in analytical spectra [11]. It is presently well known that L-shell X-ray yields relative to the La, transition depends on the irradiation ion beam energy and the chemical species being irradiated. It gives rise to the possibility that dependence persists for transitions to the same subshell, where the ionization process is expected to play no role. Nevertheless, its use in application is quiet recent and needs more development [12]. Thus, in this work, Xray production cross-section for L‘, La, Lb and Lc are measured in the proton energy range from 260 to 400 keV at the interval of 20 keV. We have also obtained line intensity ratio Lb/La, Lc/La and L‘/La for each ion beam energy.
1204
H. Mohan, A.K. Jain / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 1203–1205
2. Experimental procedures and data analysis
3. Results and discussion
The AN – 400 Van de Graaff accelerator at Nuclear Science Laboratories, Punjabi University, Patiala, is used to yield a proton beam of continuously variable energy up to 400 keV and beam current up to 50 lA. The accelerated particles are passed through a 15° bending magnet to ensure that the beam which strikes the target is isotopically pure. The beam energy is calibrated using the 15F(p, ac)16O resonance at 340.5 keV. Thick targets are mounted at 45° to the beam direction. The high purity Germanium (HPGe) detector placed at right angle to the beam, is used to detect the L X-rays. The H+ beam current on the target is measured by current integrator Elcor-A309F, which has high sensitivity, accuracy, low drift and an internal calibrating source. A guard ring at 300 V is placed in front of the target to prevent secondary electron emission from the target. The beam current employed is 10 nA to 1 lA, so as to obtain X-ray yields compatible with the input characteristics of detecting electronics and thus avoiding the problem of dead time. The L X-ray spectra from the targets are recorded with the help of ORTEC multichannel analyzer. A typical spectrum for W is shown in Fig. 1 at 400 keV proton energy. Spectra are analyzed with the same input model, in order to prevent the numerical fitting errors during the data analysis of all spectra. The high intensity M X-rays of W are reduced using a thick Mylar foil in front of the detector. To ensure the precision of the relative intensities, the experiment is repeated for each beam energy and spectra with good statistic for W-La are collected. This spectrum consists of four peaks of L X-ray groups corresponding to L‘, La, Lb and Lc which are well separated from each other. Their production cross-sections rLx ðEÞ are derived at various energies as described in our earlier work [6].
To analyze the energy dependence, variation of line ratios Lb/La, Lc/La and L‘/La are plotted in the Fig. 2 as function of proton energy. We choose X-ray line intensity ratios because systematic error is expected to cancel. Further in order to avoid large error in spectrum fitting, we confine ourselves to intensity ratios of the overall L‘, La, Lb and Lc X-ray line group. Theoretical ratios for these line groups are calculated on the basis of the ECPSSR theory of Brandt and Lapicki [13]. This model modifies the plane wave born approximation (PWBA) by taking into account projectiles energy loss (E) and its Coulomb (C) deflection from the straight-line path and by applying a perturbedstationary state (PSS) description of the L-shell electron with the relativistically increased mass (R). The importance of this effect grows when protons are slow enough to ionize the L-shell electron predominantly at impact parameter within the L-shell. The atomic parameters, namely, fluorescence yields and Coster–Kronig transition probabilities, involved in theoretical calculation of X-ray production cross-sections from ionization cross-sections are used [14]. Tabulations for emission rates are provided by Scofield [15] and Campbell and Wang [16] and that of fluorescence yields and Coster–Kronig transition probabilities are given by Krause [17] and Puri et al. [18]. We have employed the 0.69
Lβ / L α
0.67
Expt. Theory
0.65 0.63 0.61 0.59 240
260
280
300
320
340
360
380
400
420
Proton energy(keV)
Expt.
Lγ / Lα
0.113
Theory
0.102
0.091 240
260
280
300
320
340
360
380
400
420
Proton energy(keV)
0.051
Expt. Theory
Ll / Lα
0.049 0.047 0.045 0.043 0.041 240
260
280
300
320
340
360
380
400
Proton energy(keV)
Fig. 1. L X-ray spectrum of W at 400 keV proton energy.
Fig. 2. Line ratios as a function of proton energy.
420
H. Mohan, A.K. Jain / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 1203–1205
that the already existing theories, such as the ECPSSR, require no further refinements but only a better knowledge or choice of the atomic parameters involved. Conclusively, these investigations have yielded a data in the low energy region, which helps in the emergence of better understanding of proton induced X-ray emission phenomenon.
L α (mb)
300.0 250.0
Expt.
200.0
Theory M.A. Reis et al.
150.0 100.0 50.0 0.0 240
260
280
300
320 340 360 Proton energy (keV)
1205
380
400
420
Acknowledgement
Fig. 3. La X-ray production cross-sections.
tabulations of Campbell and Wang [16] for emission rates and Puri et al. [18] for fluorescence yields and Coster–Kronig transition probabilities. From Fig. 2, it is clear that although there is good agreement between theory and experiment at the upper energy end in this regime, but for lower energy side refinements are required in the theoretical calculations, which may be due to the Stark mixing of target atomic 2s and 2p wave functions [19]. Another important observation from Fig. 2 is that the different yield ratios also show dependencies on the incident ion beam energy. There is no explanation to believe that this observation is exclusive of W, making it a very important result both from the point of view of fundamental aspect of the atomic physics as well as for studies in understanding the chemical environment effects in PIXE applications [12]. In Fig. 3, a comparison is made between our measured La X-ray production cross-section and theoretical results, as well as the semi empirical results of Reis et al. [3]. These results show a general agreement between the theory and experiment over the whole range. Although the results of Reis et al. [3] are somewhat higher, this may be attributed to the choice of atomic parameters such as fluorescence yields, Coster–Kronig transition probabilities and emission rates. 4. Conclusions The volume of experimental data in this low energy regime (i.e. <500 keV) is very scare, it is the region which provides a testing ground for the prevailing theories. The present results are critically examined with prevailing theory (ECPSSR) in this energy regime, so that a confident verdict can be drawn about its validity. It is also possible
We gratefully acknowledge the financial support for this research work from Department of Science and Technology (DST), New Delhi, India under the Project No. SR/ S2/LOP-0018/2006. References [1] B.N. Jones, J.L. Campbell, Nucl. Instr. and Meth. B 258 (2007) 299. [2] M. Goudarzi, F. Shokouhi, M. Lamehi-Rachti, P. Oliaiy, Nucl. Instr. and Meth. B 247 (2006) 217. [3] M.A. Reis, A.P. Jesus, Atom. Data Nucl. Data Table 63 (1996) 1. [4] Harsh Mohan, Parjit S. Singh, D. Singh, H.R. Verma, C.S. Khurana, Nucl. Instr. and Meth. B 26 (1987) 507. [5] Harsh Mohan, Parjit S. Singh, D. Singh, H.R. Verma, C.S. Khurana, Physica 145C (1987) 249. [6] Harsh Mohan, Arvind Kumar Jain, A.N. Tripathi, in: Sixteenth National Conference on Atomic and Molecular Physics, TIFR, Mumbai, Jan. 8–11, 2007 p. 156. [7] A. Kahoul, M. Nekab, Nucl. Instr. and Meth. B 234 (2005) 412. [8] D. Strivay, G. Weber, Nucl. Instr. and Meth. B 190 (2002) 112. [9] Sam J. Cipolla, Brian P. Hill, Nucl. Instr. and Meth. B 241 (2005) 129. [10] V.P. Petukhov, E.A. Romanovskii, H. Kerkow, G. Kreysch, Phys. Status Solidi (a) 60 (1980) 79. [11] M.A. Reis, P.C. Chaves, J.C. Soares, Nucl. Instr. and Meth. B 229 (2005) 413. [12] P.C. Chaves, M.A. Reis, N.P. Barradas, Matjazˇ Kavcˇicˇ, Nucl. Instr. and Meth. B 261 (2007) 121. [13] W. Brandt, G. Lapicki, Phys. Rev. A 23 (1981) 1717. [14] D.A. Close, R.C. Bearse, J.J. Malanify, C.J. Umbarger, Phys. Rev. A 8 (1973) 1873. [15] J.H. Scofield, Atom. Data Nucl. Data Table 14 (1974) 121. [16] J.L. Campbell, J.-X. Wang, Atom. Data Nucl. Data Table 43 (1989) 281. [17] M.O. Krause, J. Phys. Chem. Ref. Data 8 (1979) 307. [18] S. Puri, D. Mehta, B. Chand, Nirmal Singh, P.N. Trehan, X-Ray Spectrom. 22 (1993) 358. [19] H.C. Padhi, B.B. Dhal, T. Nandi, D. Trautmann, J. Phys. B: Atom. Mol. Opt. Phys. 28 (1995) L59.
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 266 (2008) 1203–1205 www.elsevier.com/locate/nimb
Energy dependence of proton-induced L X-ray production cross-section for W Harsh Mohan *, Arvind Kumar Jain Department of Physics, M.L.N. College, Yamuna Nagar 135001, Haryana, India Received 20 September 2007; received in revised form 29 November 2007 Available online 8 January 2008
Abstract The energy dependence of proton-induced L X-ray production cross-section for W is studied in the energy range of 260–400 keV, with an interval of 20 keV. The measurement is made by observing the X-ray emission, with the help of HPGe detector coupled with ORTEC multichannel analyzer. The present results are critically examined with prevailing theory (ECPSSR) in this energy regime. The role of these studies in understanding the analytical spectra has been highlighted. Ó 2008 Elsevier B.V. All rights reserved. PACS: 29.30.Kv; 32.70.Fw; 39.30.+w; 81.70.Jb Keywords: X-rays; Proton induced X-ray emission; Cross-section; ECPSSR theory
1. Introduction The ionization of inner-shell by energetic proton is a fundamental problem in ion beam analysis. In recent years, this has received considerable attention [1]. The information regarding the experimental values of L X-ray production cross-section, at various proton energies, is important due to its wide use in areas of basic and applied Physics. In the case of L-shell ionization, most of the experiments have been conducted with bombarding ions having energies >500 keV [2]. Thus, there is not only a lack of data but there also exist large discrepancies between the experimental measurements and theoretical calculations [3]. The present work is a part of a series of measurements directed towards the study of X-ray production by proton bombardment [4–6]. The high speed collisions between bombarding ion and target atoms cause ionization of the target atoms due to Coulomb interactions. This subsequently results in the emission of characteristic X-rays. A
*
Corresponding author. Tel.: +91 1732 233768; fax: +91 1732 225560. E-mail address: [email protected] (H. Mohan).
0168-583X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.12.102
review of literature [7–9] indicates that the measurement for tungsten in the low energy region is carried out by Petukhov et al. [10]. They have measured the total L-shell Xray production cross-sections in proton energy range from 70 to 500 keV. Although the L X-ray production cross-section for medium and high Z elements, are small at these low proton energies, yet analytical work such as PIXE studies can only be done if accurate experimental L Xray production cross-section data is available. Another dimension which has resulted in perpetuating these studies is their role in understanding the chemical environment effects in analytical spectra [11]. It is presently well known that L-shell X-ray yields relative to the La, transition depends on the irradiation ion beam energy and the chemical species being irradiated. It gives rise to the possibility that dependence persists for transitions to the same subshell, where the ionization process is expected to play no role. Nevertheless, its use in application is quiet recent and needs more development [12]. Thus, in this work, Xray production cross-section for L‘, La, Lb and Lc are measured in the proton energy range from 260 to 400 keV at the interval of 20 keV. We have also obtained line intensity ratio Lb/La, Lc/La and L‘/La for each ion beam energy.
1204
H. Mohan, A.K. Jain / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 1203–1205
2. Experimental procedures and data analysis
3. Results and discussion
The AN – 400 Van de Graaff accelerator at Nuclear Science Laboratories, Punjabi University, Patiala, is used to yield a proton beam of continuously variable energy up to 400 keV and beam current up to 50 lA. The accelerated particles are passed through a 15° bending magnet to ensure that the beam which strikes the target is isotopically pure. The beam energy is calibrated using the 15F(p, ac)16O resonance at 340.5 keV. Thick targets are mounted at 45° to the beam direction. The high purity Germanium (HPGe) detector placed at right angle to the beam, is used to detect the L X-rays. The H+ beam current on the target is measured by current integrator Elcor-A309F, which has high sensitivity, accuracy, low drift and an internal calibrating source. A guard ring at 300 V is placed in front of the target to prevent secondary electron emission from the target. The beam current employed is 10 nA to 1 lA, so as to obtain X-ray yields compatible with the input characteristics of detecting electronics and thus avoiding the problem of dead time. The L X-ray spectra from the targets are recorded with the help of ORTEC multichannel analyzer. A typical spectrum for W is shown in Fig. 1 at 400 keV proton energy. Spectra are analyzed with the same input model, in order to prevent the numerical fitting errors during the data analysis of all spectra. The high intensity M X-rays of W are reduced using a thick Mylar foil in front of the detector. To ensure the precision of the relative intensities, the experiment is repeated for each beam energy and spectra with good statistic for W-La are collected. This spectrum consists of four peaks of L X-ray groups corresponding to L‘, La, Lb and Lc which are well separated from each other. Their production cross-sections rLx ðEÞ are derived at various energies as described in our earlier work [6].
To analyze the energy dependence, variation of line ratios Lb/La, Lc/La and L‘/La are plotted in the Fig. 2 as function of proton energy. We choose X-ray line intensity ratios because systematic error is expected to cancel. Further in order to avoid large error in spectrum fitting, we confine ourselves to intensity ratios of the overall L‘, La, Lb and Lc X-ray line group. Theoretical ratios for these line groups are calculated on the basis of the ECPSSR theory of Brandt and Lapicki [13]. This model modifies the plane wave born approximation (PWBA) by taking into account projectiles energy loss (E) and its Coulomb (C) deflection from the straight-line path and by applying a perturbedstationary state (PSS) description of the L-shell electron with the relativistically increased mass (R). The importance of this effect grows when protons are slow enough to ionize the L-shell electron predominantly at impact parameter within the L-shell. The atomic parameters, namely, fluorescence yields and Coster–Kronig transition probabilities, involved in theoretical calculation of X-ray production cross-sections from ionization cross-sections are used [14]. Tabulations for emission rates are provided by Scofield [15] and Campbell and Wang [16] and that of fluorescence yields and Coster–Kronig transition probabilities are given by Krause [17] and Puri et al. [18]. We have employed the 0.69
Lβ / L α
0.67
Expt. Theory
0.65 0.63 0.61 0.59 240
260
280
300
320
340
360
380
400
420
Proton energy(keV)
Expt.
Lγ / Lα
0.113
Theory
0.102
0.091 240
260
280
300
320
340
360
380
400
420
Proton energy(keV)
0.051
Expt. Theory
Ll / Lα
0.049 0.047 0.045 0.043 0.041 240
260
280
300
320
340
360
380
400
Proton energy(keV)
Fig. 1. L X-ray spectrum of W at 400 keV proton energy.
Fig. 2. Line ratios as a function of proton energy.
420
H. Mohan, A.K. Jain / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 1203–1205
that the already existing theories, such as the ECPSSR, require no further refinements but only a better knowledge or choice of the atomic parameters involved. Conclusively, these investigations have yielded a data in the low energy region, which helps in the emergence of better understanding of proton induced X-ray emission phenomenon.
L α (mb)
300.0 250.0
Expt.
200.0
Theory M.A. Reis et al.
150.0 100.0 50.0 0.0 240
260
280
300
320 340 360 Proton energy (keV)
1205
380
400
420
Acknowledgement
Fig. 3. La X-ray production cross-sections.
tabulations of Campbell and Wang [16] for emission rates and Puri et al. [18] for fluorescence yields and Coster–Kronig transition probabilities. From Fig. 2, it is clear that although there is good agreement between theory and experiment at the upper energy end in this regime, but for lower energy side refinements are required in the theoretical calculations, which may be due to the Stark mixing of target atomic 2s and 2p wave functions [19]. Another important observation from Fig. 2 is that the different yield ratios also show dependencies on the incident ion beam energy. There is no explanation to believe that this observation is exclusive of W, making it a very important result both from the point of view of fundamental aspect of the atomic physics as well as for studies in understanding the chemical environment effects in PIXE applications [12]. In Fig. 3, a comparison is made between our measured La X-ray production cross-section and theoretical results, as well as the semi empirical results of Reis et al. [3]. These results show a general agreement between the theory and experiment over the whole range. Although the results of Reis et al. [3] are somewhat higher, this may be attributed to the choice of atomic parameters such as fluorescence yields, Coster–Kronig transition probabilities and emission rates. 4. Conclusions The volume of experimental data in this low energy regime (i.e. <500 keV) is very scare, it is the region which provides a testing ground for the prevailing theories. The present results are critically examined with prevailing theory (ECPSSR) in this energy regime, so that a confident verdict can be drawn about its validity. It is also possible
We gratefully acknowledge the financial support for this research work from Department of Science and Technology (DST), New Delhi, India under the Project No. SR/ S2/LOP-0018/2006. References [1] B.N. Jones, J.L. Campbell, Nucl. Instr. and Meth. B 258 (2007) 299. [2] M. Goudarzi, F. Shokouhi, M. Lamehi-Rachti, P. Oliaiy, Nucl. Instr. and Meth. B 247 (2006) 217. [3] M.A. Reis, A.P. Jesus, Atom. Data Nucl. Data Table 63 (1996) 1. [4] Harsh Mohan, Parjit S. Singh, D. Singh, H.R. Verma, C.S. Khurana, Nucl. Instr. and Meth. B 26 (1987) 507. [5] Harsh Mohan, Parjit S. Singh, D. Singh, H.R. Verma, C.S. Khurana, Physica 145C (1987) 249. [6] Harsh Mohan, Arvind Kumar Jain, A.N. Tripathi, in: Sixteenth National Conference on Atomic and Molecular Physics, TIFR, Mumbai, Jan. 8–11, 2007 p. 156. [7] A. Kahoul, M. Nekab, Nucl. Instr. and Meth. B 234 (2005) 412. [8] D. Strivay, G. Weber, Nucl. Instr. and Meth. B 190 (2002) 112. [9] Sam J. Cipolla, Brian P. Hill, Nucl. Instr. and Meth. B 241 (2005) 129. [10] V.P. Petukhov, E.A. Romanovskii, H. Kerkow, G. Kreysch, Phys. Status Solidi (a) 60 (1980) 79. [11] M.A. Reis, P.C. Chaves, J.C. Soares, Nucl. Instr. and Meth. B 229 (2005) 413. [12] P.C. Chaves, M.A. Reis, N.P. Barradas, Matjazˇ Kavcˇicˇ, Nucl. Instr. and Meth. B 261 (2007) 121. [13] W. Brandt, G. Lapicki, Phys. Rev. A 23 (1981) 1717. [14] D.A. Close, R.C. Bearse, J.J. Malanify, C.J. Umbarger, Phys. Rev. A 8 (1973) 1873. [15] J.H. Scofield, Atom. Data Nucl. Data Table 14 (1974) 121. [16] J.L. Campbell, J.-X. Wang, Atom. Data Nucl. Data Table 43 (1989) 281. [17] M.O. Krause, J. Phys. Chem. Ref. Data 8 (1979) 307. [18] S. Puri, D. Mehta, B. Chand, Nirmal Singh, P.N. Trehan, X-Ray Spectrom. 22 (1993) 358. [19] H.C. Padhi, B.B. Dhal, T. Nandi, D. Trautmann, J. Phys. B: Atom. Mol. Opt. Phys. 28 (1995) L59.