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Ratio of single K-shell ionization cross section to double K-shell ionization cross section in heavy-ion-atom collisions
Volume 75A, number 6
PHYSICS LETTERS
18 February 1980
RATIO OF SINGLE K-SHELL IONIZATION CROSS SECTION TO
DOUBLE K-SHELL IONIZATION CROSS SECTION IN HEAVY-ION-ATOM COLLISIONS Y. AWAYA, T. KATOU, H. KUMAGAI, T. TONUMA, Y. TENDOW, K. IZUMO, A. HASHIZUME, T. TAKAHASHI and T. HAMADA The Institute of Physical and Chemical Research, Hirosawa, Wako-shL Saitama 351, Japan Received 27 September 1979 Revised manuscript received 7 December 1979 The spectra of Ti Ka X-rays induced by He, C, N and O ions were measured with the use of a Braggcrystal spectrometer. The ratios of the integrated hypersateMte to diagram-plus-satellite X-ray yields were obtained. The previouslyobtained data on Cr, Fe and Ni Ks X rays induced by N ions were also analysed. It is found that the double K-shell vacancy production cross section is proportional to Z14.
Since the hypersatellites of Ca Ka X-rays induced by 30 MeV O ion bombardment were observed by Richard et al. [1 ], some work has been done on the ion-excited hypersatellites of target atoms. Olsen and Moore [2] studied the relative X-ray production cross sections for Ca Ka satellite and hypersatellite transitions produced by 24.0 to 48.0 MeV O ion bombardment of thick targets and found qualitative agreement between experimental data and calculations with the binary encounter approximation (BEA). K~twatsura et al. [3] have studied the hypersatellite X-ray spectra of Be, B and O atoms induced by H and He ions and found that the double K-shell ionization cross section O2K depends on Z 4, where Z 1 is the atomic number of the projectile. They also observed that O2K for Be and O has a maximum at E/Xu K = 1, where E is the incident energy of the projectile, u K the K-shell binding energy of the target atom and X the mass ratio of the projectile to the electron [4,5]. These results show that the double Kshell ionization is caused mainly by the direct Coulomb interaction. The present authors have measured the Ks and K~ X-rays of A1, Ti, Cr, Fe and Ni induced by 84 MeV N ions [6] and obtained the energies of each satellite and hypersatellite. They also studied the variation of the satellite X-ray distribution with the atomic number of the target atom Z 2. In this work, the ratio of (r2K to the single K-shell 478
ionization cross section o K of Ti atoms induced by 6 MeV/amu He, C, N and O ions has been obtained to investigate the Z 1-dependence of the ionization cross sections, and that induced by C and N ions in the energy range of 61 MeV to 108 MeV to investigate their dependence on the incident energy. The previously obtained data [6] were also analysed from this point of view. The He 2+, C4+, N4+ (N s+ for 108 MeV) and 05+ ions were accelerated by the cyclotron of the Institute of Physical and Chemical Research. The energies of the C ions were 61, 72 and 81 MeV and those of the N ions were 66, 71, 84, 95 and 108 MeV. The Ti target was a self-supporting metallic foil. Its thickness was about 3 tam except for the case of the 95 MeV N ion bombardment, where a foil of 4 tam was used. The target was placed at an inclination of 60 ° with respect to the beam. Ti K X-rays emitted at 60 ° with respect to the beam were analysed with an automatically controlled step-scanning Bragg crystal spectrometer employing a flat LiF(200) crystal and Soller slits. The detection of the X-rays was made with a scintillation counter which was placed outside the vacuum chamber of the spectrometer through a 25 tam Mylar film in most cases. When the X-ray yield was not enough owing to the low beam intensity, a side-window gas-flow proportional counter placed in the vacuum chamber was used. The efficiency of the
Volume 75A, number 6
PHYSICS LETTERS
spectrometer utilizing the proportional counter is about ten times larger than that of the spectrometer utilizing the scintillation counter for Ti K X-rays, whereas the resolution of the former is lower than that of the latter. The integrated beam current was kept constant at each angle. Spectra of Ti Ka X-rays induced by 24 MeV He ions, 72 MeV C ions, 84 MeV N ions and 96 MeV O ions, that is, with the same velocities corresponding to a kinetic energy of 6 MeV/amu, are shown in fig. 1. The symbol K m Ln denotes the m K-shell and n L-
x103_
5.2
i
5.0 i
i
E (keV) 4.8 i
i
4.6 ;
4.4
t
g o ¢D
= -
xlO
18 February 1980
shell vacancies in the initial state. For some cases the K~ X-ray spectra were also measured. The symbol t~ or/~ attached to each peak means that the peak belongs to the Ka or K/~ transition. Since the K-absorption edge of the target element, Kab s, lies between the I ~ and KI43 lines, the self-absorption of the KLn/3 lines with n >f 1 becomes much larger than that of the K~ line (and/or Km Lnt~ line). The integrated hypersatellite X-ray intensity relative to the integrated diagram-plus-satellite intensity, ox(K2)/ox(K), was obtained for each case, where ox(K ) and ox(K2 ) are the X-ray production cross section for the single and double K-shell vacancy configuration, respectively. The previously obtained data of Cr, Fe and Ni Kt~ X-rays induced by 84 MeV N ions [6] were also analysed and the corresponding values obtained. The values of ox(K2)/ox(K) were corrected for crystal reflection, detection efficiency and absorption of X rays in the target. The reflectivity of the crystal was determined as the ratio of the peak area of the target element X-rays by photons from an X-ray tube measured around 20 = 0 ° without the crystal and that measured around the Bragg reflection angle of the target element X-rays with the crystal. The measurements were made on the Ka X-rays of seven target elements ranging from Ca to Ni and on the La X-rays of Au. The projectile will be in a charge equilibrium state in most part of the target and more than 90% of the C and N ions and more than 85% of the O ions are considered to be bare by extrapolating the results of ref. [7]. Tawara et al. [8] suggested that the dependence of the ionization cross section on the charge state of the ions disappears as the incident energy increases. So, the difference between N 4+ and N 5+ ions is not taken into account. The effect of energy loss of the projectile in the target is also neglected, since the estimated value is less than 6% of each incident energy. The ratio of the intensity ox(K2)/ox(K) can be related to O2K and o K as t
50
°°I 0
o X (K2)/ox(K) = CO2KO2K/(WK OK + COKe2K) ,
.
'
7'4
I
7'8
'
J2
I
i
2(9 (deg)
Fig. 1. Ti K X-ray spectra excited by 6 MeV/amu He, C, N and O ions.
where ¢0K and co2K are the fluroescence yield for the single and double K-sheU vacancy configuration, respectively, and co]( is the fluorescence yield for a single K shell vacancy resulting from deexcitation of double K-shell vacancies and is approximated as co~: = coKco2K I Then the term coK a2K can be neglected, because the 479
Volume 75A, number 6
PHYSICS LETTERS
measured value o f o x (K 2) is less than four percent of a x ( K ). According to the calculation o f Bhalla and klein for Ne [9], O92K/6OK ~ 1 when the number of L-shell vacancies is the same. It is assumed that this is the case for the present target atoms, since it is found that I ( K L n a)/I(Ka) = I ( K 2 Ln a)/I(K 2a) in the limits of accuracy, where I(K m Lna) is the intensity o f the K m Lna X rays. Then the value o f o x ( K 2 ) / o x ( K ) is approximated by cr2K/OK. The values of o x ( K 2 ) / o x ( K ) are plotted against the reduced parameter E/Xu K in fig. 2. The solid circles marked by atomic symbols were obtained for the corresponding target element bombarded by 84 MeV N ions and those without marks are for bombardment of a Ti target with N ions with the corresponding value o f E. It is found that these two kinds of points, one with decreasing Z 2 and constant E and the other with increasing E and constant Z 2, are connected smoothly. This means that the value of O2K / o K obeys the scaling law predicted by BEA. In this experimental condition, it was concluded that single K-shell ionization is caused mainly by direct Coulomb interaction, so that the BEA prediction is applicable to cr2K. The open circles in fig. 2 show the values of C ion bombardment. When the values o f a x ( K 2 ) / o x ( K ) are
Ni I
Fe I
Ti
Cr I
18 February 1980
/ /
0.04
/
/
0.03
O/'
/
/
/
,I(
/
0.02
/ / / / /
0.01 / /
0.00
/
/
I I
16
I
I
i
36 49 zl2
I
64
Fig. 3. Total K2Lna X-ray intensity relative to total KLnc~ X-ray intensity as a function of Z] for the Ti target. The incident energy of the projectiles with atomic number Z1 is 6 MeV/amu. divided by Z 2, the solid and open circles overlap each other. In fig. 3, the plot of ax(K2)/ox(K) against Z 2 is shown for the Ti target. This result shows that the value Of O2K/OK is proportional to Z12. Since the value of o K is proportional to Z 2 as long as the ionization is caused by the direct Coulomb interaction, this shows that the value o f a2K is proportional to Z 4. The Z 4scaling for double ionization o f the K-shell obtained by Kawatsura et al. [4,5] b y using light ions can be applied to h e a v y - i o n - a t o m collisions at least as long as the direct excitation mechanism prevails.
I
References
0.03
{ 2~
"~ 0.02
{ • N - ions o C-ions
0.01
I
I
I
I
I
0.4
0.5
0.6
0.7
0.8
E ~ .~oK
Fig. 2. Total K2Lne~ X-ray intensity relative to total KLnct X-ray intensity versus E/huK. Solid circles marked by atomic symbols: corresponding target element was bombarded with 84 MeV N ions; other solid circles: Ti target was bombarded with N ions of energy E; open circles: Ti target was bombarded with C ions of energy E.
480
[ 1] P. Richard, W. Hodge and C.F. Moore, Phys. Rev. Lett. 29 (1972) 393. [2] D.K. Olsen and C.F. Moore, Phys. Rev. Lett. 33 (1974) 194. [3] K. Kawatsura, K. Ozawa, F. Fujimoto and M. Terasawa, Phys. Lett 64A (1977) 282. [4] F. Fujimoto, K. Kawatsura, K. Ozawa and M. Terasawa, Phys. Lett. 57A (1967) 263. [5] K. Kawatsura, K. Ozawa, F. Fujimoto and M.Terasawa, Phys. Lett. 58A (1976) 446. [6] Y. Awaya et al., Phys. Lett. 61A (1977) 111. [7] J.B. Marison and F.C. Young, eds., Nuclear reaction analysis (North-Holland, 1968). [8] tl. Tawara et al., Phys. Lett. 59A (1976) 14. [9] C.P. Bhalla and M. Hein, Phys. Rev. Lett. 30 (1973) 39. [10] Y. Awaya et al., Phys. Rev. A13 (1976) 992.
PHYSICS LETTERS
18 February 1980
RATIO OF SINGLE K-SHELL IONIZATION CROSS SECTION TO
DOUBLE K-SHELL IONIZATION CROSS SECTION IN HEAVY-ION-ATOM COLLISIONS Y. AWAYA, T. KATOU, H. KUMAGAI, T. TONUMA, Y. TENDOW, K. IZUMO, A. HASHIZUME, T. TAKAHASHI and T. HAMADA The Institute of Physical and Chemical Research, Hirosawa, Wako-shL Saitama 351, Japan Received 27 September 1979 Revised manuscript received 7 December 1979 The spectra of Ti Ka X-rays induced by He, C, N and O ions were measured with the use of a Braggcrystal spectrometer. The ratios of the integrated hypersateMte to diagram-plus-satellite X-ray yields were obtained. The previouslyobtained data on Cr, Fe and Ni Ks X rays induced by N ions were also analysed. It is found that the double K-shell vacancy production cross section is proportional to Z14.
Since the hypersatellites of Ca Ka X-rays induced by 30 MeV O ion bombardment were observed by Richard et al. [1 ], some work has been done on the ion-excited hypersatellites of target atoms. Olsen and Moore [2] studied the relative X-ray production cross sections for Ca Ka satellite and hypersatellite transitions produced by 24.0 to 48.0 MeV O ion bombardment of thick targets and found qualitative agreement between experimental data and calculations with the binary encounter approximation (BEA). K~twatsura et al. [3] have studied the hypersatellite X-ray spectra of Be, B and O atoms induced by H and He ions and found that the double K-shell ionization cross section O2K depends on Z 4, where Z 1 is the atomic number of the projectile. They also observed that O2K for Be and O has a maximum at E/Xu K = 1, where E is the incident energy of the projectile, u K the K-shell binding energy of the target atom and X the mass ratio of the projectile to the electron [4,5]. These results show that the double Kshell ionization is caused mainly by the direct Coulomb interaction. The present authors have measured the Ks and K~ X-rays of A1, Ti, Cr, Fe and Ni induced by 84 MeV N ions [6] and obtained the energies of each satellite and hypersatellite. They also studied the variation of the satellite X-ray distribution with the atomic number of the target atom Z 2. In this work, the ratio of (r2K to the single K-shell 478
ionization cross section o K of Ti atoms induced by 6 MeV/amu He, C, N and O ions has been obtained to investigate the Z 1-dependence of the ionization cross sections, and that induced by C and N ions in the energy range of 61 MeV to 108 MeV to investigate their dependence on the incident energy. The previously obtained data [6] were also analysed from this point of view. The He 2+, C4+, N4+ (N s+ for 108 MeV) and 05+ ions were accelerated by the cyclotron of the Institute of Physical and Chemical Research. The energies of the C ions were 61, 72 and 81 MeV and those of the N ions were 66, 71, 84, 95 and 108 MeV. The Ti target was a self-supporting metallic foil. Its thickness was about 3 tam except for the case of the 95 MeV N ion bombardment, where a foil of 4 tam was used. The target was placed at an inclination of 60 ° with respect to the beam. Ti K X-rays emitted at 60 ° with respect to the beam were analysed with an automatically controlled step-scanning Bragg crystal spectrometer employing a flat LiF(200) crystal and Soller slits. The detection of the X-rays was made with a scintillation counter which was placed outside the vacuum chamber of the spectrometer through a 25 tam Mylar film in most cases. When the X-ray yield was not enough owing to the low beam intensity, a side-window gas-flow proportional counter placed in the vacuum chamber was used. The efficiency of the
Volume 75A, number 6
PHYSICS LETTERS
spectrometer utilizing the proportional counter is about ten times larger than that of the spectrometer utilizing the scintillation counter for Ti K X-rays, whereas the resolution of the former is lower than that of the latter. The integrated beam current was kept constant at each angle. Spectra of Ti Ka X-rays induced by 24 MeV He ions, 72 MeV C ions, 84 MeV N ions and 96 MeV O ions, that is, with the same velocities corresponding to a kinetic energy of 6 MeV/amu, are shown in fig. 1. The symbol K m Ln denotes the m K-shell and n L-
x103_
5.2
i
5.0 i
i
E (keV) 4.8 i
i
4.6 ;
4.4
t
g o ¢D
= -
xlO
18 February 1980
shell vacancies in the initial state. For some cases the K~ X-ray spectra were also measured. The symbol t~ or/~ attached to each peak means that the peak belongs to the Ka or K/~ transition. Since the K-absorption edge of the target element, Kab s, lies between the I ~ and KI43 lines, the self-absorption of the KLn/3 lines with n >f 1 becomes much larger than that of the K~ line (and/or Km Lnt~ line). The integrated hypersatellite X-ray intensity relative to the integrated diagram-plus-satellite intensity, ox(K2)/ox(K), was obtained for each case, where ox(K ) and ox(K2 ) are the X-ray production cross section for the single and double K-shell vacancy configuration, respectively. The previously obtained data of Cr, Fe and Ni Kt~ X-rays induced by 84 MeV N ions [6] were also analysed and the corresponding values obtained. The values of ox(K2)/ox(K) were corrected for crystal reflection, detection efficiency and absorption of X rays in the target. The reflectivity of the crystal was determined as the ratio of the peak area of the target element X-rays by photons from an X-ray tube measured around 20 = 0 ° without the crystal and that measured around the Bragg reflection angle of the target element X-rays with the crystal. The measurements were made on the Ka X-rays of seven target elements ranging from Ca to Ni and on the La X-rays of Au. The projectile will be in a charge equilibrium state in most part of the target and more than 90% of the C and N ions and more than 85% of the O ions are considered to be bare by extrapolating the results of ref. [7]. Tawara et al. [8] suggested that the dependence of the ionization cross section on the charge state of the ions disappears as the incident energy increases. So, the difference between N 4+ and N 5+ ions is not taken into account. The effect of energy loss of the projectile in the target is also neglected, since the estimated value is less than 6% of each incident energy. The ratio of the intensity ox(K2)/ox(K) can be related to O2K and o K as t
50
°°I 0
o X (K2)/ox(K) = CO2KO2K/(WK OK + COKe2K) ,
.
'
7'4
I
7'8
'
J2
I
i
2(9 (deg)
Fig. 1. Ti K X-ray spectra excited by 6 MeV/amu He, C, N and O ions.
where ¢0K and co2K are the fluroescence yield for the single and double K-sheU vacancy configuration, respectively, and co]( is the fluorescence yield for a single K shell vacancy resulting from deexcitation of double K-shell vacancies and is approximated as co~: = coKco2K I Then the term coK a2K can be neglected, because the 479
Volume 75A, number 6
PHYSICS LETTERS
measured value o f o x (K 2) is less than four percent of a x ( K ). According to the calculation o f Bhalla and klein for Ne [9], O92K/6OK ~ 1 when the number of L-shell vacancies is the same. It is assumed that this is the case for the present target atoms, since it is found that I ( K L n a)/I(Ka) = I ( K 2 Ln a)/I(K 2a) in the limits of accuracy, where I(K m Lna) is the intensity o f the K m Lna X rays. Then the value o f o x ( K 2 ) / o x ( K ) is approximated by cr2K/OK. The values of o x ( K 2 ) / o x ( K ) are plotted against the reduced parameter E/Xu K in fig. 2. The solid circles marked by atomic symbols were obtained for the corresponding target element bombarded by 84 MeV N ions and those without marks are for bombardment of a Ti target with N ions with the corresponding value o f E. It is found that these two kinds of points, one with decreasing Z 2 and constant E and the other with increasing E and constant Z 2, are connected smoothly. This means that the value of O2K / o K obeys the scaling law predicted by BEA. In this experimental condition, it was concluded that single K-shell ionization is caused mainly by direct Coulomb interaction, so that the BEA prediction is applicable to cr2K. The open circles in fig. 2 show the values of C ion bombardment. When the values o f a x ( K 2 ) / o x ( K ) are
Ni I
Fe I
Ti
Cr I
18 February 1980
/ /
0.04
/
/
0.03
O/'
/
/
/
,I(
/
0.02
/ / / / /
0.01 / /
0.00
/
/
I I
16
I
I
i
36 49 zl2
I
64
Fig. 3. Total K2Lna X-ray intensity relative to total KLnc~ X-ray intensity as a function of Z] for the Ti target. The incident energy of the projectiles with atomic number Z1 is 6 MeV/amu. divided by Z 2, the solid and open circles overlap each other. In fig. 3, the plot of ax(K2)/ox(K) against Z 2 is shown for the Ti target. This result shows that the value Of O2K/OK is proportional to Z12. Since the value of o K is proportional to Z 2 as long as the ionization is caused by the direct Coulomb interaction, this shows that the value o f a2K is proportional to Z 4. The Z 4scaling for double ionization o f the K-shell obtained by Kawatsura et al. [4,5] b y using light ions can be applied to h e a v y - i o n - a t o m collisions at least as long as the direct excitation mechanism prevails.
I
References
0.03
{ 2~
"~ 0.02
{ • N - ions o C-ions
0.01
I
I
I
I
I
0.4
0.5
0.6
0.7
0.8
E ~ .~oK
Fig. 2. Total K2Lne~ X-ray intensity relative to total KLnct X-ray intensity versus E/huK. Solid circles marked by atomic symbols: corresponding target element was bombarded with 84 MeV N ions; other solid circles: Ti target was bombarded with N ions of energy E; open circles: Ti target was bombarded with C ions of energy E.
480
[ 1] P. Richard, W. Hodge and C.F. Moore, Phys. Rev. Lett. 29 (1972) 393. [2] D.K. Olsen and C.F. Moore, Phys. Rev. Lett. 33 (1974) 194. [3] K. Kawatsura, K. Ozawa, F. Fujimoto and M. Terasawa, Phys. Lett 64A (1977) 282. [4] F. Fujimoto, K. Kawatsura, K. Ozawa and M. Terasawa, Phys. Lett. 57A (1967) 263. [5] K. Kawatsura, K. Ozawa, F. Fujimoto and M.Terasawa, Phys. Lett. 58A (1976) 446. [6] Y. Awaya et al., Phys. Lett. 61A (1977) 111. [7] J.B. Marison and F.C. Young, eds., Nuclear reaction analysis (North-Holland, 1968). [8] tl. Tawara et al., Phys. Lett. 59A (1976) 14. [9] C.P. Bhalla and M. Hein, Phys. Rev. Lett. 30 (1973) 39. [10] Y. Awaya et al., Phys. Rev. A13 (1976) 992.