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Measurement of L X-ray intensity ratios in tantalum by proton and Si-ion impact
.__ __ !!B
Nuclear Instruments and Methods in Physics Research B
111(1996) 22-26
-
NIOMI B
Beam Interactions with Materials&Atoms
ELSEVIER
Measurement of L X-ray intensity ratios in tantalum by proton and Si-ion impact J.S. Braich a, D.P. Goyal a, A. Mandal b, B.B. Dhal ‘, B.P. Singh d, H.C. Padhi ‘, C.S. Khurana a, H.R. Verma av* a Department of Physics, Punjabi Universily, Patiala 147 002, India b Nuclear Science Centre, JNU (New) Campus, N. Delhi II0 067, India ’ In&ate of Physics, Sachivalya Marg, Bhubaneswar 751 005, India ’ All India Council for Technical Education, Delhi 110 002, India Received 26 July 1995; revised form rc_,;eived 2 November
1995
Abstract ?-he L W1,4,6. LP2,15,3. I-Y,, LY,,,,~ and Lx.4, X-ray intensities relative to the L CL,caused by the impact of protons of energy 1 to 4.6 MeV and Si-ions of 70 to 98 MeV on Ta targets, have been measured. The results show that the intensity ratios drop significantly for all transitions except Ly ~,&LcY with %-ions of the same energy/amu as compared to those of protons. The experimental results have been compared with those based on the ECPSSR theoretical values. From the energy shift and change in the intensity ratios of various transitions caused by Si-ion impact, the number of outer shell vacancies in the M, N and O-shells simultaneous to that of L-shell have been estimated.
1. Introduction The inner-shell ionization has been the subject of extensive study during the last few decades. A review of the measurements list a large number of K and L X-ray cross-section data for various targets using protons [l] and a-particles [2]. With the advent of high energy heavy ion accelerators and Si(Li) X-ray detectors, the experimentalists have continued their interest in such studies. This has further given rise to some new phenomenon deserving further study, e.g. multiple ionization and its effect on the atomic parameters affecting the X-ray production. From the theoretical view point, the direct ionization (DI) of a target electron to the continuum has been considered as a principle mechanism of ion-atom collision for systems with Z, << Z, and vt >> v,,; where Z, and Z, are the atomic number for the projectile and target respectively and ur and uz,_ are incident ion and target L-shell electron velocities. Several direct ionization (DI) theories viz. plane-wave born approximation (PWBA) [3] and ECPSSR [4] have been formulated to have an understanding of the inner-shell ionization in the ion-atom collision. the ECPSSR theory is an extension beyond the first order Born approximation (PWBA) and accounts for energy loss
* Corresponding
author. Fax +91
175 822881.
and Coulomb deflection of the projectile as well as for perturbed stationary state and relativistic effect of innershell electrons. The experimental work of Chang et al. [5] gives the measurement of the Ta Lr-, L,,- and L,,,-subshell ionization cross sections for proton energies ranging from 1.0 to 5.5 MeV and a-particle energies from 1.0 to 11 .O MeV/amu. They compared the data with simple model calculations. They further discussed the necessity of including some small but significant additional effects in the plane wave Born approximation treatment particularly wave functions more reliable than the normally assumed hydrogenie forms. The notable study by heavy ion impact includes that of Li et al. [6] using carbon and oxygen ions in the energy range 0.5 to 6 MeV/amu, that of Uchai et al. [7] with Ag-ion (1 MeV/amu) impact and that of O’Kelley et al. [S] for Ar-ions in the range of 0.9 to 2.6 MeV/amu. Li et al. [6] measured the L-subshell ionization crosssection ratios and found large deviations from the theoretical predictions below 2 MeV/amu. They further observed that the deviation becomes more pronounced for higher projectile charge. Uchai et al. [7] measured the energy shifts and the changes in the intensity ratios of various L X-ray lines and hence estimated the existence of outer shell spectator vacancies in the h+, N- and O-shells. They further showed that the observed L X-ray intensity ratios are consistent with the Scofield rates if the rates calculated
0168-583X/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved SSDI 0168-583X(95)01299-0
J.S. Braich et al./Nucl.
Instr. and Meth. in Phys. Res. B Ill (1996) 22-26
for a single vacancy are multiplied by a statistical scaling factor. O’Kelley et al. [8] measured the variations of the (Lcu + Lp, + LL) X-ray production cross-sections due to Ar-ion impact of energy 36 to 103 MeV and found the experimental results between those based on the ECPSSR and PWBA theories. The significant discrepancies between the data and ECPSSR theory were attributed primarily to the influence of multiple ionization on the X-ray emission probabilities. Accurate determination of the L-shell X-ray production cross sections and their intensity ratios of an element, with different projectiles, is important because of their wide use in the fields of atomic, molecular and radiation physics and in non-destructive elemental analysis of materials. As the X-ray relative intensities can be determined with greater accuracy than the absolute X-ray production cross-sections, we present in this paper the relative L-shell X-ray intensities in Ta using 70 to 98 MeV Si6+- and Sisc-ions and with a proton beam of energy range 1 to 4.6 MeV/amu. The measured results have been compared with the theoretical calculations of ECPSSR theory. The number of outer shell vacancies caused by the heavy ion impact has been estimated from the change in intensity ratios and energies of various lines vis-a-vis the proton impact of the same energy.
2. Experimental
technique
The experiment with the Si-ion beam was performed at the Nuclear Science centre (NSC), New Delhi, while the one with the proton beam was done at the Institute of Physics (IOP), Bhubaneswar. The NSC houses a 15UD Pelletron accelerator from where the Si6+ and Si*+-ion beams of 70 to 98 MeV were bombarded on a thin target of Ta (40 pg/cm’) on a carbon backing of 10 pg/cm’. The target thickness was determined with the Rutherford backscattering technique and the error in the thickness has been estimated to be + 10%. The 9SDH2 3MV Tandem Pelletron accelerator at IOP was used to bombard protons of 1 MeV to 4.6 MeV on a self supporting thick tantalum foil (10 mg/cm2). The thin target was fixed on a steel ladder having a hole of diameter = 1 cm. The use of a steel ladder has the advantage of proper positioning of the beam on the target as the change in the beam position can be easily detected by the Fe K X-rays produced. For the proper positioning of the target i.e. 4.5” each to the beam direction as well to the X-ray detector, the ladder could be rotated about its own axis and could also be moved vertically up and down. The targets placed in the evacuated (I 10e6 Torr) scattering chamber were positioned at 45” to the beam direction as well as to the Si(Li) detector. The EG & G ORTEC Si(Li) detectors, SLP-10180-S (having a FWHM of 207 eV at 5.9 keV) placed outside the vacuum chamber at a distance of 10 cm from the target at
23
c
z
”,
lo’
;; LL
:
i
104 ,_; ; ’ ,.:
: 3._:
:
:
: : ::
f
:
Jsi-cTaI
p--~;,.Lp----* ; ,.
:
:
:,
10
a.
..“‘.,,
..,, k---LI-----q
:
.
:.:
:
:’
‘._. ‘.
.:’
'..'
.. . . . .
..:
.:
., ..
..
:. .,
'. '. ._
lo’t
....
ZO
BD -
9.0
X-ray
140
11.0
Energy(keV)
12.0
13.0
14.0
---P
Fig. 1. L X-ray spectra of tantalum bombarded by 98 MeV S&ions and 4.6 MeV protons. The X-ray energy scales for these two spectra are different as these have been recorded by two different data acquisition systems having a different amplifier gain and a different FWHM of the Si(Li) detectors.
NSC while SLP-06165-S (having FWHM of 170 eV at 5.9 keV) was placed at a distance of 9.2 cm from the target at IOP, were used to detect the tantalum L X-rays. The energy and efficiency calibration of these detectors were made before and after the experiment as described elsewhere [9,10]. The incident particles were monitored and measured using a surface barrier detector (SBD)‘placed at 30” to the beam direction. The beam current was kept below 10 nA to minimize the dead time and avoid pile up effects and damage to the target. For the purpose of data acquisition and storage, a multichannel analyser (MCA) was used at IOP while a CAMAC system and a VAX computer were used at NSC. The typical L X-ray spectra of Ta bombarded with 98 MeV Si-ions and 4.6 MeV protons are shown in Fig. 1.
3. Data analysis and errors The Ta L X-ray spectra by Si-ion impact and proton impact have been fitted into various components representing LL, L& Lq? LP1,4,6? LP2,15,3T LP5,%9,10T LYI? L-h,3,6 and Ly,, X-ray lines, after subtracting a suitable background. The data were analysed using the least squares fitting procedure by the “NFIT” computer code [ 111.
24
J.S. Braich et al./Nucl. lnstr. and M&h. in Phys. Res. B 111 (1996) 22-26 Due to the small energy separation
between
the Lp,,
LP4 and LP,; LB,, LPI5 and LP,; LP,,,, L&,,, and Ly,, Ly,, and L-y, X-ray lines, the composite peaks of and Ly,,,,, have been fitted in Lh,4,6y Lb,15,3’ LP5,7,9,lO the Ta L X-ray spectra. The uncertainties due to the counting statistics are I 1% for La, Lp1,4,6 and Lp,,,,, G 2-3% for LL, Ly, and LY~,~,~ and = 7% for Ly,,,,. The sources of error contributing to the measured ratios arise mainly from the counting statistics, self absorption in the sample, air absorption and efficiency correction of the detector. The overall errors estimated are 8% for LL/LoL, Ly,/La
and
LY~,,,/L~,
LP,,,5,3/La and LP(,,,/La, 11% for L y,,/L cy.
7% for LP,,&La, 9% for LY~,~.JL~
and
4. Results and discussion 4.1. Intensity ratios The intensity ratios for various calculated by using the formula:
xffLj -
YXj Ei c,
I
L X-ray
lines were
’
where “i” and “j” refer to different L X-ray lines (a, p, y etc.), Yx is the yield of L X-rays, F is the efficiency of the X-ray detector corrected for the X-my absorption in the mylar window, separating the Si(Li) detector from the target and the air gap between the mylar window and the detector. CL is the correction factor for the respective X-ray line, accounting for the slowing down of the projectile in the target as well as the absorption of the emitted X-rays in the target. The absorption factor has been calculated according to the procedure illustrated by O’Kelley et al. [S]. The variation of L X-ray intensity ratios with projectile energy for different L X-ray lines relative to La are presented in Fig. 2. In the plot, the energy values for proton data are the mean values before and after the impact. The figure shows that the experimentally measured LL/L~ intensity ratio remains constant except at the lowest and highest energies of proton impact and is higher by = 30% than the emission rate ratio given by Scofield [12]. However, for Si-ion impact, the LL/L~ intensity ratio is within = 15% of the theoretical emission rate ratio within the limited energy range. This observation is contrary to the fact that the intensity ratio LL/L~ should remain the same irrespective of the projectile as both these lines LL(L~-M,) and LcY(L~-M& originate from the M-shell and terminate at the L,-subshell, cancelling the effect of M-shell ionization. Although the intensities of various L X-ray transitions increase with increase in the projectile atomic number, having the same energy/amu, it is important to see the
behaviour of the intensity ratios as a function of the projectile atomic number. The present measurements with protons and Si-ions show that the L&JLcY, Lp ,,4,6/La, Ly,,,/La and Ly 1/La intensity ratios decrease with increase in the atomic number of the projectile of the same energy/amu as is clear from Fig. 2. This indicates that although there is an increase in the L X-ray intensities, this increase is not the same for different L X-ray groups i.e. the increase in the intensity is more for the La group as compared to the Lp and Ly groups. This can be attributed to the fact that the increase in the fluorescence yield for L,,,-subshell may be more than for the L,- and L,,-subshells. The presently measured average L-shell X-ray intensity ratios L~/LL, La/LP,,,5, LP,/Ly,, Ly,,,/Ly4,4 along with the previously reported experimental ratios for Ag-ion impact on Ta by Uchai et al. [7] are presented in Table 1. For the purpose of calculating the value of Lol/Lp,,,,, the peak corresponding to the energy of Lp3 has been extracted to find the net yield of L&ls although the energy of Lp3 is very close to Lp,,,,. Similarly the Lp4 has been subtracted from Lp 1,4,6by fitting a peak corresponding to the energy of Lp4. and thus the area of Lp ,,6 has been evaluated. The contribution of Lpq has been taken as negligible compared to LB, similar to Uchai et al. [7] and =Lp,,JLy,. Further, using the X-ray thus LP,/LY, emission rates, the intensity of Ly, has been calculated from Ly,, to get the intensity of Ly,,, from Ly,,,,,. Since these ratios involve the same atomic level, the theoretical results for such ratios do not need the ionization cross sections, fluorescence yield values and Coster-Kronig transition probabilities. The theoretical results are thus the ratios of the X-ray emission rates as given by Scofield [12]. The measured ratio Lcu/Lp,,,, is = 19% higher with Si-ion impact as compared to the experimental value of the same ratio with protons. The value of La/L& ,5 as given by Uchai et al. [7] for Ag-ion impact is substanially higher (= 48%) showing that the ratio increases with increase of the projectile mass. The measured ratio of LOL/LL for proton and Si-ion impact agrees within experimental error while the Uchai et al. [7] value is 13% higher. Our measured ratio Lp,/Ly, for Si-ion impact is 17% higher while the value given by Uchai et al. [7] for Ag-ion impact is 38% higher than the same ratio with proton impact. The measured ratio Ly,,,/Ly,,, for Si-ion impact is 43% lower than the corresponding ratio for proton impact while it is 74% lower in the case of Ag-ion impact
[71. 4.2. Multiple ionization In the present experiment the increase in the energies of various L X-ray lines vis-a-vis standard X-ray energies have been found due to the change in the centroids of various L X-ray peaks. From the changes in the energies of
J.S. Braich et al./Nucl.
Instr. and Meth.
in Phys. Res. B Ill (1996) 22-26
25
O.l6l-
0.90
_
(a)
(b)
0.14-
I aao
0.120.70
0.60
0.50 1 0.60 yc O.lOT
0.50 E ; 0.08.-
:: 'G 0.40 e
: P_ 0.06-
&0.30 'ii c a, (
;j 5 1
E
2________* _ _ _ - -:::-.-_y:._. -
r
’;I_gf5+
; 0.30 .z 2
___---
-
___---
o.zo-
I 0.10-
a5
1.0
1.5 -
0.06 -
2.0
2.5
3.0
3.5
Energy(MeV/amu)
4.0
4.5
--+
w
G$ ___________ ___-._._ ._._._. _._.____ _---_ .:
0.05 -
I 0.5
I
1.0
I
I
1.5 -
Fig. 2. (a) and (b): The Lo line, with protons impact. The solid lines based theoretical values
=*
2.0
2.5
3.0
3s
4.0
4.5
Energy[MeV/arnul-
variation of L X-ray intensity ratios as a function of projectile energy/amu for various L X-ray lines relative to the and Si-ions. (0) denotes the intensity ratios for proton impact’while (e) denotes the intensity ratios for Si-ion through these points have been drawn to guide the eye. The dashed lines and dot-dashed lines denote the ECPSSR for proton and Won impact respectively.
Table 1 Experimental L X-ray intensity ratios produced by proton and Si-ion bombardment emission rate ratios given by Scofield [12] for a singly ionised atom Intensity ratios
‘~L~-M.,s) L&-M, 1 ‘4L,-M,,,) LP,,&-NV) JAW-b) L?I,(L,-N.+) Lx,&,-%,I LY‘dL,-02.3)
Experimental
on Ta target. Theoretical
results
present data with
Uchai et al. [7] for Ag-ion impact
values in the table arc the Theoretical values
protons
S&ions
17.3 + 2.0
18.4 f 1.7
19.8
21.8
4.3 i 0.5
5.3 * 0.4
8.3
5.7
4.9 + 0.5
5.9 * 0.5
8.0
5.4
6.1 A 0.7
4.2 + 0.5
3.5
6.8
26
J.S. Braich et al. /Nucl. Insir. and Meth. in Phys. Res. B II 1 (1996) 22-26
various L X-ray lines viz. LL, La, Lp,, L-y, and Ly4,$, observed in the present experiment and the standard values of the energy shift as given by Uchai et al. [13], we have calculated the number of spectator vacancies in the M-shell to be 3 to 4 for Ta when bombarded by Si-ions of 70 to 98 MeV. The La(L3-M4,s)/L~(L3-M1) intensity ratios, for which both transitions filling the 2p,,, vacancy originate from the M-shell, are in fair agreement with the corresponding Scofield values for Si-ion impact on Ta, a finding in agreement with a similar observation made earlier [7,14]. The situation is quite different, when we consider branching ratios for transitions originating from different shells. The measured Lp,(L,-M,)/Ly ,(L2-N4) ratios are higher about 9% than the Scofield values. Qualitatively this result implies that more vacancies are present in the N-shell than in the M-shell. The L y,,,(L rN&/L y,,#(L ,-0, s> observed branching ratio is 12% lower than the Scofield theoretical value. This indicates that fewer vacancies are present in the O-shell than in the N-shell suggesting the lower degree of multiple ionization in the O-shell than in the N-shell. The number of vacancies in the N-shell have been calculated using the scaling procedure illustrated by Uchai et al. [7]. According to this, the observed intensity ratios are compared to the Scofield rates corrected for multiple ionization by multiplying it with an appropriate statistical scaling factor (N, - V,>/N, where iV, is the number of electrons in the xth shell of the neutral atom, V, is the number of vacancies in the xth shell (where x = M, N, 0 etc.). For Si-ion impact on Ta, the number of spectator vacancies in the N-shell have been found to be 10 to 11 for V, = 3 to 4. The ratio L?I,,~(L~-N~,~)/LY~,~,(L,-O~,~) suggests the existence of 2 to 3 spectator vacancies in the O-shell. Thus we expect the existence of 2 to 3 O-shell spectator vacancies in addition to 3 to 4 M-shell and 10 to 11 N-shell vacancies for Si-ion (70 to 98 MeV) impact on tantalum.
5. Conclusion In this paper, we have reported the variation of intensity ratios of various L X-ray transitions relative to the La transition in Ta bombarded by 70 to 98 MeV Si-ions and 1.0 to 4.6 MeV protons. The comparison of the intensity ratios found with these two projectiles show that the ratios
drop significantly for all the transitions except Ly,,,,,/Lol with Si-ion impact (of the same energy/amu) as compared to those with protons. From the energy shift and change in the intensity ratios of various transitions caused by Si-ion impact, the number of outer shell vacancies in the M, N and O-shells simultaneous to each vacancy in the L-shell have been estimated to be 3-4, lo-11 and 2-3 respectively.
Acknowledgements The authors thank the operating staff of the Pelletron accelerators at the Nuclear Science Centre Delhi as well as at the Institute of Physics Bhubaneswar. Thanks are also due to Mr. D. Kabiraj for the preparation of the targets. One of the authors (H.R.V.) is indebted to the DST, Ministry of Science and Technology, Govt. of India and the Nuclear Science Centre/UGC for the grant of research projects. J.S.B. and D.P.G. are thankful to DST and UGC for the award of JRF and Teacher fellowship respectively.
References [l] I. Orlic, C.H. Sow and S.M. Tang, Atom. Data Nucl. Data Tables 56 (1994) 159. [2] T.L. Hardt and R.L. Watson, Phys. Rev. A 14 (1976) 137. [3] B.H. Choi, E. Merzbacher and G.S. Khandelwal, Atom. Data Nucl. Data Tables 5 (1973) 291. 141 W. Brandt and G. Lapicki, Phys. Rev. A 287 (1981) 1717. [5] C.N. Chang, J.F. Morgan and S.L. Blatt, Phys. Rev. A 11 (1975) 607. 161 T.K. Li, D.L. Clark and G.W. Greenlees, Phys. Rev. Lett. 37 (1976) 1209. [7] W. Uchai, G. Lapicki, W.T. Milner, S. Raman, P.V. Rao and C.R. Vane, J. Phys. B 18 (1985) L389. [8] G.D. O’Kelley, R.L. Auble, L.D. Hullett Jr., H.J. Kim, W.T. Milner, S. Raman, 0. Shahal, C.R. Vane and J.P. Young, Nucl. Instr. and Meth. B 3 (1984) 78. [9] D.P. Goyal, J.S. Braich, S. Murlithar, B.P. Singh and H.R. Verma, Z. Phys. D 35 (1995) 155. [lo] B.B. Dhal, T. Nandi and H.C. Padhi, Phys. Rev. A 49 (1994) 329. [l 11 H. Nakano, H.R. Verma and Y. Nakayama, unpublished. [ 121 J.H. Scofield, Atom. Data Nucl. Data Tables 14 ( 1974) I121. [13] W. Uchai, C.W. Nestor Jr., S. Raman and C.R. Vane, Atom. Data Nucl. Data Tables 34 ( 1986) 201, [14] L. Sarkadi and T. Mukoyama, J. Phys. B 13 (1980) 2255.
Nuclear Instruments and Methods in Physics Research B
111(1996) 22-26
-
NIOMI B
Beam Interactions with Materials&Atoms
ELSEVIER
Measurement of L X-ray intensity ratios in tantalum by proton and Si-ion impact J.S. Braich a, D.P. Goyal a, A. Mandal b, B.B. Dhal ‘, B.P. Singh d, H.C. Padhi ‘, C.S. Khurana a, H.R. Verma av* a Department of Physics, Punjabi Universily, Patiala 147 002, India b Nuclear Science Centre, JNU (New) Campus, N. Delhi II0 067, India ’ In&ate of Physics, Sachivalya Marg, Bhubaneswar 751 005, India ’ All India Council for Technical Education, Delhi 110 002, India Received 26 July 1995; revised form rc_,;eived 2 November
1995
Abstract ?-he L W1,4,6. LP2,15,3. I-Y,, LY,,,,~ and Lx.4, X-ray intensities relative to the L CL,caused by the impact of protons of energy 1 to 4.6 MeV and Si-ions of 70 to 98 MeV on Ta targets, have been measured. The results show that the intensity ratios drop significantly for all transitions except Ly ~,&LcY with %-ions of the same energy/amu as compared to those of protons. The experimental results have been compared with those based on the ECPSSR theoretical values. From the energy shift and change in the intensity ratios of various transitions caused by Si-ion impact, the number of outer shell vacancies in the M, N and O-shells simultaneous to that of L-shell have been estimated.
1. Introduction The inner-shell ionization has been the subject of extensive study during the last few decades. A review of the measurements list a large number of K and L X-ray cross-section data for various targets using protons [l] and a-particles [2]. With the advent of high energy heavy ion accelerators and Si(Li) X-ray detectors, the experimentalists have continued their interest in such studies. This has further given rise to some new phenomenon deserving further study, e.g. multiple ionization and its effect on the atomic parameters affecting the X-ray production. From the theoretical view point, the direct ionization (DI) of a target electron to the continuum has been considered as a principle mechanism of ion-atom collision for systems with Z, << Z, and vt >> v,,; where Z, and Z, are the atomic number for the projectile and target respectively and ur and uz,_ are incident ion and target L-shell electron velocities. Several direct ionization (DI) theories viz. plane-wave born approximation (PWBA) [3] and ECPSSR [4] have been formulated to have an understanding of the inner-shell ionization in the ion-atom collision. the ECPSSR theory is an extension beyond the first order Born approximation (PWBA) and accounts for energy loss
* Corresponding
author. Fax +91
175 822881.
and Coulomb deflection of the projectile as well as for perturbed stationary state and relativistic effect of innershell electrons. The experimental work of Chang et al. [5] gives the measurement of the Ta Lr-, L,,- and L,,,-subshell ionization cross sections for proton energies ranging from 1.0 to 5.5 MeV and a-particle energies from 1.0 to 11 .O MeV/amu. They compared the data with simple model calculations. They further discussed the necessity of including some small but significant additional effects in the plane wave Born approximation treatment particularly wave functions more reliable than the normally assumed hydrogenie forms. The notable study by heavy ion impact includes that of Li et al. [6] using carbon and oxygen ions in the energy range 0.5 to 6 MeV/amu, that of Uchai et al. [7] with Ag-ion (1 MeV/amu) impact and that of O’Kelley et al. [S] for Ar-ions in the range of 0.9 to 2.6 MeV/amu. Li et al. [6] measured the L-subshell ionization crosssection ratios and found large deviations from the theoretical predictions below 2 MeV/amu. They further observed that the deviation becomes more pronounced for higher projectile charge. Uchai et al. [7] measured the energy shifts and the changes in the intensity ratios of various L X-ray lines and hence estimated the existence of outer shell spectator vacancies in the h+, N- and O-shells. They further showed that the observed L X-ray intensity ratios are consistent with the Scofield rates if the rates calculated
0168-583X/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved SSDI 0168-583X(95)01299-0
J.S. Braich et al./Nucl.
Instr. and Meth. in Phys. Res. B Ill (1996) 22-26
for a single vacancy are multiplied by a statistical scaling factor. O’Kelley et al. [8] measured the variations of the (Lcu + Lp, + LL) X-ray production cross-sections due to Ar-ion impact of energy 36 to 103 MeV and found the experimental results between those based on the ECPSSR and PWBA theories. The significant discrepancies between the data and ECPSSR theory were attributed primarily to the influence of multiple ionization on the X-ray emission probabilities. Accurate determination of the L-shell X-ray production cross sections and their intensity ratios of an element, with different projectiles, is important because of their wide use in the fields of atomic, molecular and radiation physics and in non-destructive elemental analysis of materials. As the X-ray relative intensities can be determined with greater accuracy than the absolute X-ray production cross-sections, we present in this paper the relative L-shell X-ray intensities in Ta using 70 to 98 MeV Si6+- and Sisc-ions and with a proton beam of energy range 1 to 4.6 MeV/amu. The measured results have been compared with the theoretical calculations of ECPSSR theory. The number of outer shell vacancies caused by the heavy ion impact has been estimated from the change in intensity ratios and energies of various lines vis-a-vis the proton impact of the same energy.
2. Experimental
technique
The experiment with the Si-ion beam was performed at the Nuclear Science centre (NSC), New Delhi, while the one with the proton beam was done at the Institute of Physics (IOP), Bhubaneswar. The NSC houses a 15UD Pelletron accelerator from where the Si6+ and Si*+-ion beams of 70 to 98 MeV were bombarded on a thin target of Ta (40 pg/cm’) on a carbon backing of 10 pg/cm’. The target thickness was determined with the Rutherford backscattering technique and the error in the thickness has been estimated to be + 10%. The 9SDH2 3MV Tandem Pelletron accelerator at IOP was used to bombard protons of 1 MeV to 4.6 MeV on a self supporting thick tantalum foil (10 mg/cm2). The thin target was fixed on a steel ladder having a hole of diameter = 1 cm. The use of a steel ladder has the advantage of proper positioning of the beam on the target as the change in the beam position can be easily detected by the Fe K X-rays produced. For the proper positioning of the target i.e. 4.5” each to the beam direction as well to the X-ray detector, the ladder could be rotated about its own axis and could also be moved vertically up and down. The targets placed in the evacuated (I 10e6 Torr) scattering chamber were positioned at 45” to the beam direction as well as to the Si(Li) detector. The EG & G ORTEC Si(Li) detectors, SLP-10180-S (having a FWHM of 207 eV at 5.9 keV) placed outside the vacuum chamber at a distance of 10 cm from the target at
23
c
z
”,
lo’
;; LL
:
i
104 ,_; ; ’ ,.:
: 3._:
:
:
: : ::
f
:
Jsi-cTaI
p--~;,.Lp----* ; ,.
:
:
:,
10
a.
..“‘.,,
..,, k---LI-----q
:
.
:.:
:
:’
‘._. ‘.
.:’
'..'
.. . . . .
..:
.:
., ..
..
:. .,
'. '. ._
lo’t
....
ZO
BD -
9.0
X-ray
140
11.0
Energy(keV)
12.0
13.0
14.0
---P
Fig. 1. L X-ray spectra of tantalum bombarded by 98 MeV S&ions and 4.6 MeV protons. The X-ray energy scales for these two spectra are different as these have been recorded by two different data acquisition systems having a different amplifier gain and a different FWHM of the Si(Li) detectors.
NSC while SLP-06165-S (having FWHM of 170 eV at 5.9 keV) was placed at a distance of 9.2 cm from the target at IOP, were used to detect the tantalum L X-rays. The energy and efficiency calibration of these detectors were made before and after the experiment as described elsewhere [9,10]. The incident particles were monitored and measured using a surface barrier detector (SBD)‘placed at 30” to the beam direction. The beam current was kept below 10 nA to minimize the dead time and avoid pile up effects and damage to the target. For the purpose of data acquisition and storage, a multichannel analyser (MCA) was used at IOP while a CAMAC system and a VAX computer were used at NSC. The typical L X-ray spectra of Ta bombarded with 98 MeV Si-ions and 4.6 MeV protons are shown in Fig. 1.
3. Data analysis and errors The Ta L X-ray spectra by Si-ion impact and proton impact have been fitted into various components representing LL, L& Lq? LP1,4,6? LP2,15,3T LP5,%9,10T LYI? L-h,3,6 and Ly,, X-ray lines, after subtracting a suitable background. The data were analysed using the least squares fitting procedure by the “NFIT” computer code [ 111.
24
J.S. Braich et al./Nucl. lnstr. and M&h. in Phys. Res. B 111 (1996) 22-26 Due to the small energy separation
between
the Lp,,
LP4 and LP,; LB,, LPI5 and LP,; LP,,,, L&,,, and Ly,, Ly,, and L-y, X-ray lines, the composite peaks of and Ly,,,,, have been fitted in Lh,4,6y Lb,15,3’ LP5,7,9,lO the Ta L X-ray spectra. The uncertainties due to the counting statistics are I 1% for La, Lp1,4,6 and Lp,,,,, G 2-3% for LL, Ly, and LY~,~,~ and = 7% for Ly,,,,. The sources of error contributing to the measured ratios arise mainly from the counting statistics, self absorption in the sample, air absorption and efficiency correction of the detector. The overall errors estimated are 8% for LL/LoL, Ly,/La
and
LY~,,,/L~,
LP,,,5,3/La and LP(,,,/La, 11% for L y,,/L cy.
7% for LP,,&La, 9% for LY~,~.JL~
and
4. Results and discussion 4.1. Intensity ratios The intensity ratios for various calculated by using the formula:
xffLj -
YXj Ei c,
I
L X-ray
lines were
’
where “i” and “j” refer to different L X-ray lines (a, p, y etc.), Yx is the yield of L X-rays, F is the efficiency of the X-ray detector corrected for the X-my absorption in the mylar window, separating the Si(Li) detector from the target and the air gap between the mylar window and the detector. CL is the correction factor for the respective X-ray line, accounting for the slowing down of the projectile in the target as well as the absorption of the emitted X-rays in the target. The absorption factor has been calculated according to the procedure illustrated by O’Kelley et al. [S]. The variation of L X-ray intensity ratios with projectile energy for different L X-ray lines relative to La are presented in Fig. 2. In the plot, the energy values for proton data are the mean values before and after the impact. The figure shows that the experimentally measured LL/L~ intensity ratio remains constant except at the lowest and highest energies of proton impact and is higher by = 30% than the emission rate ratio given by Scofield [12]. However, for Si-ion impact, the LL/L~ intensity ratio is within = 15% of the theoretical emission rate ratio within the limited energy range. This observation is contrary to the fact that the intensity ratio LL/L~ should remain the same irrespective of the projectile as both these lines LL(L~-M,) and LcY(L~-M& originate from the M-shell and terminate at the L,-subshell, cancelling the effect of M-shell ionization. Although the intensities of various L X-ray transitions increase with increase in the projectile atomic number, having the same energy/amu, it is important to see the
behaviour of the intensity ratios as a function of the projectile atomic number. The present measurements with protons and Si-ions show that the L&JLcY, Lp ,,4,6/La, Ly,,,/La and Ly 1/La intensity ratios decrease with increase in the atomic number of the projectile of the same energy/amu as is clear from Fig. 2. This indicates that although there is an increase in the L X-ray intensities, this increase is not the same for different L X-ray groups i.e. the increase in the intensity is more for the La group as compared to the Lp and Ly groups. This can be attributed to the fact that the increase in the fluorescence yield for L,,,-subshell may be more than for the L,- and L,,-subshells. The presently measured average L-shell X-ray intensity ratios L~/LL, La/LP,,,5, LP,/Ly,, Ly,,,/Ly4,4 along with the previously reported experimental ratios for Ag-ion impact on Ta by Uchai et al. [7] are presented in Table 1. For the purpose of calculating the value of Lol/Lp,,,,, the peak corresponding to the energy of Lp3 has been extracted to find the net yield of L&ls although the energy of Lp3 is very close to Lp,,,,. Similarly the Lp4 has been subtracted from Lp 1,4,6by fitting a peak corresponding to the energy of Lp4. and thus the area of Lp ,,6 has been evaluated. The contribution of Lpq has been taken as negligible compared to LB, similar to Uchai et al. [7] and =Lp,,JLy,. Further, using the X-ray thus LP,/LY, emission rates, the intensity of Ly, has been calculated from Ly,, to get the intensity of Ly,,, from Ly,,,,,. Since these ratios involve the same atomic level, the theoretical results for such ratios do not need the ionization cross sections, fluorescence yield values and Coster-Kronig transition probabilities. The theoretical results are thus the ratios of the X-ray emission rates as given by Scofield [12]. The measured ratio Lcu/Lp,,,, is = 19% higher with Si-ion impact as compared to the experimental value of the same ratio with protons. The value of La/L& ,5 as given by Uchai et al. [7] for Ag-ion impact is substanially higher (= 48%) showing that the ratio increases with increase of the projectile mass. The measured ratio of LOL/LL for proton and Si-ion impact agrees within experimental error while the Uchai et al. [7] value is 13% higher. Our measured ratio Lp,/Ly, for Si-ion impact is 17% higher while the value given by Uchai et al. [7] for Ag-ion impact is 38% higher than the same ratio with proton impact. The measured ratio Ly,,,/Ly,,, for Si-ion impact is 43% lower than the corresponding ratio for proton impact while it is 74% lower in the case of Ag-ion impact
[71. 4.2. Multiple ionization In the present experiment the increase in the energies of various L X-ray lines vis-a-vis standard X-ray energies have been found due to the change in the centroids of various L X-ray peaks. From the changes in the energies of
J.S. Braich et al./Nucl.
Instr. and Meth.
in Phys. Res. B Ill (1996) 22-26
25
O.l6l-
0.90
_
(a)
(b)
0.14-
I aao
0.120.70
0.60
0.50 1 0.60 yc O.lOT
0.50 E ; 0.08.-
:: 'G 0.40 e
: P_ 0.06-
&0.30 'ii c a, (
;j 5 1
E
2________* _ _ _ - -:::-.-_y:._. -
r
’;I_gf5+
; 0.30 .z 2
___---
-
___---
o.zo-
I 0.10-
a5
1.0
1.5 -
0.06 -
2.0
2.5
3.0
3.5
Energy(MeV/amu)
4.0
4.5
--+
w
G$ ___________ ___-._._ ._._._. _._.____ _---_ .:
0.05 -
I 0.5
I
1.0
I
I
1.5 -
Fig. 2. (a) and (b): The Lo line, with protons impact. The solid lines based theoretical values
=*
2.0
2.5
3.0
3s
4.0
4.5
Energy[MeV/arnul-
variation of L X-ray intensity ratios as a function of projectile energy/amu for various L X-ray lines relative to the and Si-ions. (0) denotes the intensity ratios for proton impact’while (e) denotes the intensity ratios for Si-ion through these points have been drawn to guide the eye. The dashed lines and dot-dashed lines denote the ECPSSR for proton and Won impact respectively.
Table 1 Experimental L X-ray intensity ratios produced by proton and Si-ion bombardment emission rate ratios given by Scofield [12] for a singly ionised atom Intensity ratios
‘~L~-M.,s) L&-M, 1 ‘4L,-M,,,) LP,,&-NV) JAW-b) L?I,(L,-N.+) Lx,&,-%,I LY‘dL,-02.3)
Experimental
on Ta target. Theoretical
results
present data with
Uchai et al. [7] for Ag-ion impact
values in the table arc the Theoretical values
protons
S&ions
17.3 + 2.0
18.4 f 1.7
19.8
21.8
4.3 i 0.5
5.3 * 0.4
8.3
5.7
4.9 + 0.5
5.9 * 0.5
8.0
5.4
6.1 A 0.7
4.2 + 0.5
3.5
6.8
26
J.S. Braich et al. /Nucl. Insir. and Meth. in Phys. Res. B II 1 (1996) 22-26
various L X-ray lines viz. LL, La, Lp,, L-y, and Ly4,$, observed in the present experiment and the standard values of the energy shift as given by Uchai et al. [13], we have calculated the number of spectator vacancies in the M-shell to be 3 to 4 for Ta when bombarded by Si-ions of 70 to 98 MeV. The La(L3-M4,s)/L~(L3-M1) intensity ratios, for which both transitions filling the 2p,,, vacancy originate from the M-shell, are in fair agreement with the corresponding Scofield values for Si-ion impact on Ta, a finding in agreement with a similar observation made earlier [7,14]. The situation is quite different, when we consider branching ratios for transitions originating from different shells. The measured Lp,(L,-M,)/Ly ,(L2-N4) ratios are higher about 9% than the Scofield values. Qualitatively this result implies that more vacancies are present in the N-shell than in the M-shell. The L y,,,(L rN&/L y,,#(L ,-0, s> observed branching ratio is 12% lower than the Scofield theoretical value. This indicates that fewer vacancies are present in the O-shell than in the N-shell suggesting the lower degree of multiple ionization in the O-shell than in the N-shell. The number of vacancies in the N-shell have been calculated using the scaling procedure illustrated by Uchai et al. [7]. According to this, the observed intensity ratios are compared to the Scofield rates corrected for multiple ionization by multiplying it with an appropriate statistical scaling factor (N, - V,>/N, where iV, is the number of electrons in the xth shell of the neutral atom, V, is the number of vacancies in the xth shell (where x = M, N, 0 etc.). For Si-ion impact on Ta, the number of spectator vacancies in the N-shell have been found to be 10 to 11 for V, = 3 to 4. The ratio L?I,,~(L~-N~,~)/LY~,~,(L,-O~,~) suggests the existence of 2 to 3 spectator vacancies in the O-shell. Thus we expect the existence of 2 to 3 O-shell spectator vacancies in addition to 3 to 4 M-shell and 10 to 11 N-shell vacancies for Si-ion (70 to 98 MeV) impact on tantalum.
5. Conclusion In this paper, we have reported the variation of intensity ratios of various L X-ray transitions relative to the La transition in Ta bombarded by 70 to 98 MeV Si-ions and 1.0 to 4.6 MeV protons. The comparison of the intensity ratios found with these two projectiles show that the ratios
drop significantly for all the transitions except Ly,,,,,/Lol with Si-ion impact (of the same energy/amu) as compared to those with protons. From the energy shift and change in the intensity ratios of various transitions caused by Si-ion impact, the number of outer shell vacancies in the M, N and O-shells simultaneous to each vacancy in the L-shell have been estimated to be 3-4, lo-11 and 2-3 respectively.
Acknowledgements The authors thank the operating staff of the Pelletron accelerators at the Nuclear Science Centre Delhi as well as at the Institute of Physics Bhubaneswar. Thanks are also due to Mr. D. Kabiraj for the preparation of the targets. One of the authors (H.R.V.) is indebted to the DST, Ministry of Science and Technology, Govt. of India and the Nuclear Science Centre/UGC for the grant of research projects. J.S.B. and D.P.G. are thankful to DST and UGC for the award of JRF and Teacher fellowship respectively.
References [l] I. Orlic, C.H. Sow and S.M. Tang, Atom. Data Nucl. Data Tables 56 (1994) 159. [2] T.L. Hardt and R.L. Watson, Phys. Rev. A 14 (1976) 137. [3] B.H. Choi, E. Merzbacher and G.S. Khandelwal, Atom. Data Nucl. Data Tables 5 (1973) 291. 141 W. Brandt and G. Lapicki, Phys. Rev. A 287 (1981) 1717. [5] C.N. Chang, J.F. Morgan and S.L. Blatt, Phys. Rev. A 11 (1975) 607. 161 T.K. Li, D.L. Clark and G.W. Greenlees, Phys. Rev. Lett. 37 (1976) 1209. [7] W. Uchai, G. Lapicki, W.T. Milner, S. Raman, P.V. Rao and C.R. Vane, J. Phys. B 18 (1985) L389. [8] G.D. O’Kelley, R.L. Auble, L.D. Hullett Jr., H.J. Kim, W.T. Milner, S. Raman, 0. Shahal, C.R. Vane and J.P. Young, Nucl. Instr. and Meth. B 3 (1984) 78. [9] D.P. Goyal, J.S. Braich, S. Murlithar, B.P. Singh and H.R. Verma, Z. Phys. D 35 (1995) 155. [lo] B.B. Dhal, T. Nandi and H.C. Padhi, Phys. Rev. A 49 (1994) 329. [l 11 H. Nakano, H.R. Verma and Y. Nakayama, unpublished. [ 121 J.H. Scofield, Atom. Data Nucl. Data Tables 14 ( 1974) I121. [13] W. Uchai, C.W. Nestor Jr., S. Raman and C.R. Vane, Atom. Data Nucl. Data Tables 34 ( 1986) 201, [14] L. Sarkadi and T. Mukoyama, J. Phys. B 13 (1980) 2255.