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X-ray spectra of highly ionised iron and nickel atoms
Volume 40A, number 1
X-RAY
PHYSICS LETTERS
SPECTRA
OF HIGHLY
IONISED
B. S. FRAENKEL Laboratory
IRON AND NICKEL
19 June 1972
ATOMS
and J. L. SCHWOB *
of X-rays and Far Ultra-Violet Spectroscopy.
The Racah Institute of Physics, The Hebrew University, Jerusalem, Israel
Received 27 April 1972
Sharp X-ray spectra of extremely high ionised Fe and Ni were obtained by vacuum sparks, with a curved crystal X-ray spectrometer. Fe XXVI and Ni XXVII were reached.
Highly ionized iron and nickel X-ray spectra have been obtained by high power vacuum sparks. The highest degrees of ionization obtained were 25 in iron and 26 in nickel. A good reproducibility of the spark was achieved through a systematic investigation of the influence of the geometric and electric parameters on the discharge. The X-ray spectra were obtained wjth a curved crystal X-ray spectrometer, used in the Cauchois mounting. By use of a focussing spectrometer instead of a plane crystal spectrometer, the fluctuation in location of the X-ray source did not cause a broadening of X-ray lines. The high spectral resolution achieved here, of about 0.0004 A [l] , constitutes a substantial improvement relative to previous observations of the Fe spectrum in solar flares [2-51 and in laboratory plasmas [6, 71 . This allowed us to resolve new lines and to determine their widths. A device similar to the one used by Elton and Lie [7] was used to produce the vacuum spark. 16 kV, 15 IF and 80 nH are the electrical parameters of the discharge, resulting in a peak current of about 2 X105 A. Good concentric alignment yields a point of hot and dense plasma appearing about 1 mm below the anode tip, for a very short time, when the current is near its first maximum. Similar concentrated hot plasmas were first observed in sparks by Cohen et al. [6]. This phenomenon is considered to result from a strong pinch effect. It was possible to achieve good reproducibility of the hot point, in part due to the * On leave of absence from the Laboratoire des Hautes Pressions, C.N.R.S.-92 Bellevue (France).
Fig. 1. Pinhole X-ray photograph of a single spark.
geometric arrangement of the electrodes, and particularly due to the sharp form of the anode, and to the choice of the electrical parameters; it should be mentioned that we found an optimal value for the inductance of the discharge circuit (80 nH in our case). The hot plasma point is seen on an X-ray pinhole photograph in fig. 1, obtained through a beryllium window by a single spark. Two features should be noted on the picture. The edge of the X-ray spot is sharp and this sharpness indicates that the dimensions of the hot point are very small (the diameter of the 83
Volume
40A, number
PHYSICS
1
1972
b)
a)
I 1.750
I 1800
vt
I lB50
1.900
Fig. 2. Microphotometer
1.950
recording
spot itself is determined by the dimension of the pinhole which is 300 pm). Geometrical considerations set the diameter of the hot point at less than 4 pm. The intensity of the X-ray emitted by the plasma point is much stronger than the X-ray radiation from the anode metal. This stronger intensity of the plasma emission serves as a crude indication of the quality of the hot point. Fig. 2a shows a microphotometer recording of the Fe X-ray spectrum obtained with 1000 sparks. A series of lines appears at the short wavelength side of the Ko,,, lines. This series of lines are emitted by the hot point of plasma, and not by the anode, as may be verified by space resolution. This is achieved by shadow techniques, using a tungsten wire located between source and crystal. These lines or groups of lines represent ls--2p optical and inner-shell transitions of Fe atoms at different degrees of ionization. Seven of the lines were identified by Lie and Elton [7] as the main ls-2p transitions of Fe XXV-Fe XIX. Some of these lines were seen in the features of the spectra of the solar flares [4, 51. We were able to obtain the ls-2p main line of Fe XVIII at 1.9267 A, in agreement with the prediction of House [8]. This was possible due to the resolution achieved with the Cauchois spectrometer, while this line was masked by the Ko lines in the plane crystal spectrometer experiment of Lie and 84
19 June
LETTERS
of the X-ray spectra:
1.650
moo
Is50
1500
woo
a) Fe; b) Ni.
Elton. We observe many additional new weak lines belonging to the eight groups of lines emitted by the various ions. These definitely observed lines are indicated in fig. 2a by marks above them. The list of wavelengths involved will be published elsewhere. The K/I line also displays a satellite structure (fig. 2a). Spatial resolution shows that these new lines are emitted by the plasma point. These as yet unidentified lines represent inner-shell transitions between the M and K shells in atoms of various degrees of ioniza-
Fe
1~.
do_
Ni
I
I
I
I
I’I
I
I
1’I’l’f’I
ly
30 -
60
Ni
20-K&
1
10 -
40
WOO-
9060-
. .
30
-
TMO”” .“O”IIE LIEL ELTOll C..S.“, nl,*,“,.nl.“,.
20 10
7060lB50
1500
D fiI1lllII 3 5
Fig. 3. Variation as a function
YIY!d 7 9 11 13 15 STAGE OF IONltATiON
90 17
19
21
23
25
27
of the wavelength for the ls-2p transition of the degree of ionization for Fe and Ni.
Volume 40A, number 1
PHYSICS LETTERS
tion, probably not as high as the formerly discussed lines. A similar spectrum was obtained for Ni (fig. 2b), which has not been observed before. In the Ni spectrum we observed the isoelectronic sequences from He I (Ni XXVII) to F I (Ni XX). Here the intensity of the helium-like Ni line is weaker than the lithiumlike multiplet of Ni, while the Lyman (Ydoublet of hydrogen-like Ni was not observed at all. The 1s-2p or Kcu-like transitions are gaining energy with the increasing degree of ionization, as result of the decrease of screening by the outer electrons. This gain becomes substantial with the stripping of the L shell. Fig. 3 compares Hartree-Fock calculations of House [8] to the experimental results of Lie and Elton [7] for Fe, and to our results for Fe and Ni. The experimental points represent an average wavelength of every group of lines belonging to a certain ionization.
This work was facilitated Foundation.
19 June 1972
in part by the Ford
References 111 J. L. Schwab and B. S. Fraenkel, Third Symposium on U.V. and X-ray Spectroscopy of Astrophysical and Laboratory Plasmas, Utrecht, August 1971. Space Science Reviews, to be published. 121 G. Fritz, R. W. Krephn, J. F. Meekins, A. E. Unzicker and H. Friedman, Astrophys. J. 148 (1967) L 133. I31 J. F. Meekins, R. W. Kreplin, T. A. Chubb and H. Friedman, Science 162 (1968) 891. 141 W. M. Neupert, W. Gates, M. Swartz and R. Young, Astrophys. J. 149 (1967) L 79. (51 W. M. Neupert and M. Swartz, Astrophys. J. 160 (1970) L 189. [‘31 L. Cohen, U. Feldman, M. Swartz and 1. H. Underwood, J. Opt. Sot. Am. 58 (1968) 843. [71 T. N. Lie and R. C. Elton, Phys. Rev. A3 (1971) 865. 181 L. L. House, Astrophys. J. Suppl. 18 (1969) 21.
We gratefully acknowledge fruitful discussions with Dr. Elton and Dr. Lie of the N.R.L.
85
X-RAY
PHYSICS LETTERS
SPECTRA
OF HIGHLY
IONISED
B. S. FRAENKEL Laboratory
IRON AND NICKEL
19 June 1972
ATOMS
and J. L. SCHWOB *
of X-rays and Far Ultra-Violet Spectroscopy.
The Racah Institute of Physics, The Hebrew University, Jerusalem, Israel
Received 27 April 1972
Sharp X-ray spectra of extremely high ionised Fe and Ni were obtained by vacuum sparks, with a curved crystal X-ray spectrometer. Fe XXVI and Ni XXVII were reached.
Highly ionized iron and nickel X-ray spectra have been obtained by high power vacuum sparks. The highest degrees of ionization obtained were 25 in iron and 26 in nickel. A good reproducibility of the spark was achieved through a systematic investigation of the influence of the geometric and electric parameters on the discharge. The X-ray spectra were obtained wjth a curved crystal X-ray spectrometer, used in the Cauchois mounting. By use of a focussing spectrometer instead of a plane crystal spectrometer, the fluctuation in location of the X-ray source did not cause a broadening of X-ray lines. The high spectral resolution achieved here, of about 0.0004 A [l] , constitutes a substantial improvement relative to previous observations of the Fe spectrum in solar flares [2-51 and in laboratory plasmas [6, 71 . This allowed us to resolve new lines and to determine their widths. A device similar to the one used by Elton and Lie [7] was used to produce the vacuum spark. 16 kV, 15 IF and 80 nH are the electrical parameters of the discharge, resulting in a peak current of about 2 X105 A. Good concentric alignment yields a point of hot and dense plasma appearing about 1 mm below the anode tip, for a very short time, when the current is near its first maximum. Similar concentrated hot plasmas were first observed in sparks by Cohen et al. [6]. This phenomenon is considered to result from a strong pinch effect. It was possible to achieve good reproducibility of the hot point, in part due to the * On leave of absence from the Laboratoire des Hautes Pressions, C.N.R.S.-92 Bellevue (France).
Fig. 1. Pinhole X-ray photograph of a single spark.
geometric arrangement of the electrodes, and particularly due to the sharp form of the anode, and to the choice of the electrical parameters; it should be mentioned that we found an optimal value for the inductance of the discharge circuit (80 nH in our case). The hot plasma point is seen on an X-ray pinhole photograph in fig. 1, obtained through a beryllium window by a single spark. Two features should be noted on the picture. The edge of the X-ray spot is sharp and this sharpness indicates that the dimensions of the hot point are very small (the diameter of the 83
Volume
40A, number
PHYSICS
1
1972
b)
a)
I 1.750
I 1800
vt
I lB50
1.900
Fig. 2. Microphotometer
1.950
recording
spot itself is determined by the dimension of the pinhole which is 300 pm). Geometrical considerations set the diameter of the hot point at less than 4 pm. The intensity of the X-ray emitted by the plasma point is much stronger than the X-ray radiation from the anode metal. This stronger intensity of the plasma emission serves as a crude indication of the quality of the hot point. Fig. 2a shows a microphotometer recording of the Fe X-ray spectrum obtained with 1000 sparks. A series of lines appears at the short wavelength side of the Ko,,, lines. This series of lines are emitted by the hot point of plasma, and not by the anode, as may be verified by space resolution. This is achieved by shadow techniques, using a tungsten wire located between source and crystal. These lines or groups of lines represent ls--2p optical and inner-shell transitions of Fe atoms at different degrees of ionization. Seven of the lines were identified by Lie and Elton [7] as the main ls-2p transitions of Fe XXV-Fe XIX. Some of these lines were seen in the features of the spectra of the solar flares [4, 51. We were able to obtain the ls-2p main line of Fe XVIII at 1.9267 A, in agreement with the prediction of House [8]. This was possible due to the resolution achieved with the Cauchois spectrometer, while this line was masked by the Ko lines in the plane crystal spectrometer experiment of Lie and 84
19 June
LETTERS
of the X-ray spectra:
1.650
moo
Is50
1500
woo
a) Fe; b) Ni.
Elton. We observe many additional new weak lines belonging to the eight groups of lines emitted by the various ions. These definitely observed lines are indicated in fig. 2a by marks above them. The list of wavelengths involved will be published elsewhere. The K/I line also displays a satellite structure (fig. 2a). Spatial resolution shows that these new lines are emitted by the plasma point. These as yet unidentified lines represent inner-shell transitions between the M and K shells in atoms of various degrees of ioniza-
Fe
1~.
do_
Ni
I
I
I
I
I’I
I
I
1’I’l’f’I
ly
30 -
60
Ni
20-K&
1
10 -
40
WOO-
9060-
. .
30
-
TMO”” .“O”IIE LIEL ELTOll C..S.“, nl,*,“,.nl.“,.
20 10
7060lB50
1500
D fiI1lllII 3 5
Fig. 3. Variation as a function
YIY!d 7 9 11 13 15 STAGE OF IONltATiON
90 17
19
21
23
25
27
of the wavelength for the ls-2p transition of the degree of ionization for Fe and Ni.
Volume 40A, number 1
PHYSICS LETTERS
tion, probably not as high as the formerly discussed lines. A similar spectrum was obtained for Ni (fig. 2b), which has not been observed before. In the Ni spectrum we observed the isoelectronic sequences from He I (Ni XXVII) to F I (Ni XX). Here the intensity of the helium-like Ni line is weaker than the lithiumlike multiplet of Ni, while the Lyman (Ydoublet of hydrogen-like Ni was not observed at all. The 1s-2p or Kcu-like transitions are gaining energy with the increasing degree of ionization, as result of the decrease of screening by the outer electrons. This gain becomes substantial with the stripping of the L shell. Fig. 3 compares Hartree-Fock calculations of House [8] to the experimental results of Lie and Elton [7] for Fe, and to our results for Fe and Ni. The experimental points represent an average wavelength of every group of lines belonging to a certain ionization.
This work was facilitated Foundation.
19 June 1972
in part by the Ford
References 111 J. L. Schwab and B. S. Fraenkel, Third Symposium on U.V. and X-ray Spectroscopy of Astrophysical and Laboratory Plasmas, Utrecht, August 1971. Space Science Reviews, to be published. 121 G. Fritz, R. W. Krephn, J. F. Meekins, A. E. Unzicker and H. Friedman, Astrophys. J. 148 (1967) L 133. I31 J. F. Meekins, R. W. Kreplin, T. A. Chubb and H. Friedman, Science 162 (1968) 891. 141 W. M. Neupert, W. Gates, M. Swartz and R. Young, Astrophys. J. 149 (1967) L 79. (51 W. M. Neupert and M. Swartz, Astrophys. J. 160 (1970) L 189. [‘31 L. Cohen, U. Feldman, M. Swartz and 1. H. Underwood, J. Opt. Sot. Am. 58 (1968) 843. [71 T. N. Lie and R. C. Elton, Phys. Rev. A3 (1971) 865. 181 L. L. House, Astrophys. J. Suppl. 18 (1969) 21.
We gratefully acknowledge fruitful discussions with Dr. Elton and Dr. Lie of the N.R.L.
85