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Nonlinearity at the K-absorption-edge in the Xe-filled gas proportional counter
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH
Nuclear Instruments and Methods m Physics Research A 336 (1993) 301-303 North-Holland
Section A
Nonlinearity at the K-absorption-edge in the Xe-filled gas proportional counter H. Tsuneini a, K. Hayashida and S. Ueno b
a,
K. Torii
a,
K. Tamura
a,
E. Miyata
a,
H. Murakami
a
Department of Earth and Space Science, Faculty of Science, Osaka University, Machikaneyama-cho 1-1, Toyonaka, Osaka 560, Japan b Department of Physics, Faculty of Science, Kyoto University, Sakyo-ku, Kyoto 606-01, Japan
Received 3 May 1993 and in revised form 13 July 1993
We measured the linearity of a Xe-filled gas proportional counter in the energy range between 10 and 40 keV at the synchrotron radiation facility . A clear gap at the XeK-edge energy (34.5 keV) can be seen . We found that the equivalent width of this gap is about 170± 10 eV. In order to understand this value, we discussed the mean ionization energy during the X-ray photo absorption process.
1. Introduction
2. Experiment and results
Gas proportional counters are widely used in X-ray astronomy satellites [1,2]. They are easy to use and can have a large effective area with a broad bandpass . With an X-ray mirror, the effective area of an X-ray detector can be increased for energies below and not exceeding 10 keV. Higher energy range can be covered with a scintillator whose energy resolution is inferior to that of the gas proportional counter. A Xe-filled gas proportional counter covers the intermediate energy range up to several tens of keV where it has enough detection efficiency . In the spectroscopic analysis, it is important for detectors to have good linearity between the incident X-ray energy and the output pulse height . In general, a gas-filled counter has good linearity over wide energy range except around the absorption edges of the counter's gas. When we observe the continuum X-ray spectrum, an emission line feature around the absorption edge energy can be seen . In the gas flow counters using P-10 gas, Jahoda and McCammon [3] reported the nonlinearity at the Ar L-edge (0 .25 keV) that has an equivalent width (EW) about 30 eV . Koyama et al . [4] reported the nonlinearity of the Xe L-edge (4 .8 keV) in the Xe-filled gas scintillation proportional counter that has an EW about 50 eV . Recently we measured the nonlinearity of the Xe K-edge (34.5 keV) using the synchrotron radiation facility and report its results below.
We have carried out an experiment to measure the nonlinearity effect at the XeK-edge in a Xe-filled gas proportional counter. The detector we used is a backup counter for the ASM experiment installed on the Ginga satellite. It contains Xe (752 mmHg) and CO Z (23 mmHg). The effective gas chamber size in our experiment is 47 x 270 x 29 mm3 (width x length x depth) . The details of the counter are described in ref. [5]. We used a synchrotron radiation facility in the Photon Factory of the Institute of High Energy Physics, Tsukuba, Japan. The beam line we used is BL-14C that is equipped with a double crystal monochromator that can cover the energy range between 10 and 40 keV with linearly polarized X-rays . The high voltage supply to the ASM counter was set to be 1950 V through the whole experiment . After the ASM counter, we used a main amplifier (Ortec 571, gain X 100, shaping time 2 lts) and a pulse height analyzer (Canberra S-20). The offset value of this system is checked by using a mercury pulser . In order to avoid complications associated with the proportional counter's anode wire, we used a pinhole (diameter - 1 mm) to position the incident X-ray flux about 12 mm away from the wire . The maximum intensity from the monochromator was too strong to be measured in our system . Therefore, we rotated the two crystals independently to obtain a moderate intensity. We fixed the second crystal at the Bragg condition
0168-9002/93/$06 .00 C 1993 - Elsevier Science Publishers B.V . All rights reserved
302
H Tsunenu et al / Nonlinearity of the Xe K-edge gas proportional counter
1380 1370 1360 1350 1340 1330
5
1000 1500 2000 2500 PHA channel Fig. 1 . An example of the pulse height distribution obtained with the Xe-filled gas proportional counter at an incident X-ray energy of 40 keV.
34 .5 35 35 .5 Energy (keV) Fig. 3. The same as fig. 2 for the energy range between 33 .5 and 35 .5 keV. The statistical uncertainty for each point is within the dot.
while the first one at slightly off the Bragg condition. In this way, we controlled the intensity to around 1000-2000 counts/s . The relative energy scale could be controlled with a precision better than a few eV by referring to the relative rotation angle of the second crystal. The absolute energy calibration was done at several absorption edges by using foils made of various metals . Fig. 1 shows a typical pulse height distribution obtained at an energy of 40 keV. The main peak around 1500 channel can be well fitted with two Gaussians and a constant . They represent the main peak at 40 keV and the XeL escape peak 35 .9 keV. XeK . and Kß escape peaks are also seen around 400 and 250 channel, respectively. There are two weak spectral features due to contamination sources. One is Pb L line blend from the lead of the counter shield appearing around 550 channel. The other is Ba K line blend from barium contained in the walls of the experimental chamber appearing around 1250 channel. In the analysis, we employed a model function to fit the main peak that contains two Gaussian functions and a constant . One of the Gaussians represents the main peak while the other one represents the Xe L escape peak whose center position is lower by about 4.1 keV than that of the main peak . The constant probably comes from the Comptonization by the higher order X-rays in the
beam . All the data we obtained can be well fitted with this model. Fig. 2 shows a linearity between the incident X-ray energy and the center position of the main peak for the energy range up to 40 keV. The data point at 5 .9 keV comes from SS Fe . The other data points come from X-rays produced in the synchrotron radiation facility . We measured the pulse height distribution sparsely over the whole energy range and densely around the Xe K-edge energy . Fig. 3 shows the close-up around the Xe K-edge (34.5 keV) . Data points show that there is a clear gap at the Xe K-edge energy . We fitted the above and below data points with straight lines and obtained the gap 170 ± 10 eV .
500
2000
c
1500
a°. 1000 y 500 01 0
10
20 30 40 50 Energy (keV) Fig. 2. The relation between the incident X-ray energy and the main peak position up to 40 keV.
34
3. Discussion In the photoabsorption of X-rays, a photoelectron with some kinetic energy is produced while the Xe atom is left with a vacancy in some shell. After that, an Auger electron or a fluorescence X-ray will be produced from the photoabsorbed Xe atom . Then, it will evolve from the excited state to a stable state. In this way, the photoabsorbed Xe atom will pass through various ionization states ejecting several electrons. The original photoelectron will produce electrons through collisions with neutral atoms which will become singly ionized. Therefore, the ionized Xe atoms can be divided into two groups. One is the group of singly ionized atoms each of which produces one electron through the collision with energetic electrons. Most of the primary electrons are produced in this process. Therefore, the mean ionization energy through this process is almost equal to that of the so-called "mean energy per ion pair" which is about 22 eV for Xe gas. The other is that of multiply ionized atoms each of which produces several electrons through photoabsorption. We assume that the mean energy required to produce one electron through the collision is independent of the kinetic
H. Tsunemi et al. / Nonlinearity of the Xe K-edge gas proportional counter energy of electrons. On the contrary, the mean energy required to produce one electron from the photoabsorbed atom depends on the final charge state of the atom . More energy is required to further ionize atoms of initially higher ionization states . The relative abundance in ionization level of Xe ions that are formed as the consequence of a vacancy in the K shell is different from that of a vacancy in the L shell . We calculated the average charges using the data in ref. [6]. When a sudden vacancy in the K shell occurs in the Xe atom, it will cause the mean ionization level to be about +8 .2 electrons consuming 529 eV . A sudden vacancy in the L shell will cause to be about +7.8 electrons consuming 460 eV . For X-rays just above the K-edge energy, the occurrence probability of the photoabsorption in the L shell is about 10% of that in the K shell . When a sudden vacancy in the K shell of a Xe atom, 87% of the time a fluorescence X-ray will be produced while the rest of the time an Auger electron will be produced. We calculate the reabsorption probability of the fluorescence X-ray to be about 10% when taking into account the geometry of the ASM counter . In this way, the absorbed X-rays just above the Xe K-edge energy can be divided into two groups . One is the group that leaves one atom with a sudden vacancy in the K shell while the other is the group that leaves two atoms ionized : one with a vacancy in the K shell due to the original X-ray and one with an L shell vacancy due to the fluorescence X-ray from the first atom . The occurrence probabilities of these two groups are almost equal from the previous argument . On the contrary, the absorbed X-rays just below the Xe K-edge energy will leave one atom with a vacancy in the L shell . As a first approximation, we can expect that the X-rays just above the Xe K-edge will leave one more highly ionized ion than that just below the K-edge . Therefore, we can calculate the nonlinearity at the K-edge to be about 200 eV which is consistent with our result . In the LAC [7] on the Ginga satellite, they used an array of the Xe-filled gas proportional counters whose geometrical area is about 4000 cm2 . The effective area just above the Xe K-edge is about 250 cm 2 , since the Xe gas pressure is about 0 .4 atm . The standard observation energy range is between 2 and 37 keV . Therefore, there is almost no effect on the data analysis by taking into account the nonlinearity effect at the Xe K-edge energy . They sometimes functioned the LAC in the energy range up to 60 keV . However, they did not take into account the nonlinearity effect at the
303
Xe K-edge probably due to the fact that it had relatively small effective area around that energy . They only employed the Xe L-edge effect on the counter . Some future satellite, like the XTE satellite [8], will use Xe-filled gas proportional counters with bigger effective area and a wider energy range . In that case, they will need to take into account this effect in the data analysis.
4 . Conclusion We measured the nonlinearity effect in a Xe-filled gas proportional counter at the Xe K-edge energy (34 .5 keV) using a synchrotron radiation facility . The nonlinearity effect has an EW about 170 ± 10 eV. This can be understood as the difference of the final mean ionization level of the photoabsorbed atoms above and below the K-edge . This effect should be taken into account when we use the Xe-filled gas proportional counter for wide energy range with large effective area . This value would slightly depend on the counter geometry because of how the fluorescence X-rays are reabsorbed in the counter .
Acknowledgements The authors express their thanks to all the members of the CCD team in Osaka University that assigned the machine time to our experiment. This work is partly supported by the Special Coordination Fund of the Science and Technology Agency . The manuscript is carefully read by K.C . Gendreau in MIT .
References [1] H .V .D. Bradt, T . Ohashi and K .A . Pounds, Rev . Astron . Astrophys . 32 (1992) 392 . [2] G .W . Fraser, X-ray Detectors in Astronomy (Cambridge Univ. Press, 1989) p . 179 . [3] K. Jahoda and D . McCammon, Nucl. Instr . and Meth. A 272 (1988) 800 . [4] K. Koyama et al ., Publ . Astron. Soc. Jpn . 3 6 (1984) 659 . [51 H . Tsunemi, S . Kitamoto, M . Manabe, M. Miyamoto, K . Yamashita and M . Nakagawa, Publ. Astron . Soc . Jpn . 4 1 (1989) 391 . [6] T.A . Carlson, W .E . Hunt and M .O . Krause, Phys . Rev . 151 (1966) 41 . [7] M .J .L . Turner et al ., Publ . Astron . Soc . Jpn . 4 1 (1989) 345 . [8] H .V .D. Bradt et al ., Proc . IAU Colloq . 123 (1990) 89 .
Nuclear Instruments and Methods m Physics Research A 336 (1993) 301-303 North-Holland
Section A
Nonlinearity at the K-absorption-edge in the Xe-filled gas proportional counter H. Tsuneini a, K. Hayashida and S. Ueno b
a,
K. Torii
a,
K. Tamura
a,
E. Miyata
a,
H. Murakami
a
Department of Earth and Space Science, Faculty of Science, Osaka University, Machikaneyama-cho 1-1, Toyonaka, Osaka 560, Japan b Department of Physics, Faculty of Science, Kyoto University, Sakyo-ku, Kyoto 606-01, Japan
Received 3 May 1993 and in revised form 13 July 1993
We measured the linearity of a Xe-filled gas proportional counter in the energy range between 10 and 40 keV at the synchrotron radiation facility . A clear gap at the XeK-edge energy (34.5 keV) can be seen . We found that the equivalent width of this gap is about 170± 10 eV. In order to understand this value, we discussed the mean ionization energy during the X-ray photo absorption process.
1. Introduction
2. Experiment and results
Gas proportional counters are widely used in X-ray astronomy satellites [1,2]. They are easy to use and can have a large effective area with a broad bandpass . With an X-ray mirror, the effective area of an X-ray detector can be increased for energies below and not exceeding 10 keV. Higher energy range can be covered with a scintillator whose energy resolution is inferior to that of the gas proportional counter. A Xe-filled gas proportional counter covers the intermediate energy range up to several tens of keV where it has enough detection efficiency . In the spectroscopic analysis, it is important for detectors to have good linearity between the incident X-ray energy and the output pulse height . In general, a gas-filled counter has good linearity over wide energy range except around the absorption edges of the counter's gas. When we observe the continuum X-ray spectrum, an emission line feature around the absorption edge energy can be seen . In the gas flow counters using P-10 gas, Jahoda and McCammon [3] reported the nonlinearity at the Ar L-edge (0 .25 keV) that has an equivalent width (EW) about 30 eV . Koyama et al . [4] reported the nonlinearity of the Xe L-edge (4 .8 keV) in the Xe-filled gas scintillation proportional counter that has an EW about 50 eV . Recently we measured the nonlinearity of the Xe K-edge (34.5 keV) using the synchrotron radiation facility and report its results below.
We have carried out an experiment to measure the nonlinearity effect at the XeK-edge in a Xe-filled gas proportional counter. The detector we used is a backup counter for the ASM experiment installed on the Ginga satellite. It contains Xe (752 mmHg) and CO Z (23 mmHg). The effective gas chamber size in our experiment is 47 x 270 x 29 mm3 (width x length x depth) . The details of the counter are described in ref. [5]. We used a synchrotron radiation facility in the Photon Factory of the Institute of High Energy Physics, Tsukuba, Japan. The beam line we used is BL-14C that is equipped with a double crystal monochromator that can cover the energy range between 10 and 40 keV with linearly polarized X-rays . The high voltage supply to the ASM counter was set to be 1950 V through the whole experiment . After the ASM counter, we used a main amplifier (Ortec 571, gain X 100, shaping time 2 lts) and a pulse height analyzer (Canberra S-20). The offset value of this system is checked by using a mercury pulser . In order to avoid complications associated with the proportional counter's anode wire, we used a pinhole (diameter - 1 mm) to position the incident X-ray flux about 12 mm away from the wire . The maximum intensity from the monochromator was too strong to be measured in our system . Therefore, we rotated the two crystals independently to obtain a moderate intensity. We fixed the second crystal at the Bragg condition
0168-9002/93/$06 .00 C 1993 - Elsevier Science Publishers B.V . All rights reserved
302
H Tsunenu et al / Nonlinearity of the Xe K-edge gas proportional counter
1380 1370 1360 1350 1340 1330
5
1000 1500 2000 2500 PHA channel Fig. 1 . An example of the pulse height distribution obtained with the Xe-filled gas proportional counter at an incident X-ray energy of 40 keV.
34 .5 35 35 .5 Energy (keV) Fig. 3. The same as fig. 2 for the energy range between 33 .5 and 35 .5 keV. The statistical uncertainty for each point is within the dot.
while the first one at slightly off the Bragg condition. In this way, we controlled the intensity to around 1000-2000 counts/s . The relative energy scale could be controlled with a precision better than a few eV by referring to the relative rotation angle of the second crystal. The absolute energy calibration was done at several absorption edges by using foils made of various metals . Fig. 1 shows a typical pulse height distribution obtained at an energy of 40 keV. The main peak around 1500 channel can be well fitted with two Gaussians and a constant . They represent the main peak at 40 keV and the XeL escape peak 35 .9 keV. XeK . and Kß escape peaks are also seen around 400 and 250 channel, respectively. There are two weak spectral features due to contamination sources. One is Pb L line blend from the lead of the counter shield appearing around 550 channel. The other is Ba K line blend from barium contained in the walls of the experimental chamber appearing around 1250 channel. In the analysis, we employed a model function to fit the main peak that contains two Gaussian functions and a constant . One of the Gaussians represents the main peak while the other one represents the Xe L escape peak whose center position is lower by about 4.1 keV than that of the main peak . The constant probably comes from the Comptonization by the higher order X-rays in the
beam . All the data we obtained can be well fitted with this model. Fig. 2 shows a linearity between the incident X-ray energy and the center position of the main peak for the energy range up to 40 keV. The data point at 5 .9 keV comes from SS Fe . The other data points come from X-rays produced in the synchrotron radiation facility . We measured the pulse height distribution sparsely over the whole energy range and densely around the Xe K-edge energy . Fig. 3 shows the close-up around the Xe K-edge (34.5 keV) . Data points show that there is a clear gap at the Xe K-edge energy . We fitted the above and below data points with straight lines and obtained the gap 170 ± 10 eV .
500
2000
c
1500
a°. 1000 y 500 01 0
10
20 30 40 50 Energy (keV) Fig. 2. The relation between the incident X-ray energy and the main peak position up to 40 keV.
34
3. Discussion In the photoabsorption of X-rays, a photoelectron with some kinetic energy is produced while the Xe atom is left with a vacancy in some shell. After that, an Auger electron or a fluorescence X-ray will be produced from the photoabsorbed Xe atom . Then, it will evolve from the excited state to a stable state. In this way, the photoabsorbed Xe atom will pass through various ionization states ejecting several electrons. The original photoelectron will produce electrons through collisions with neutral atoms which will become singly ionized. Therefore, the ionized Xe atoms can be divided into two groups. One is the group of singly ionized atoms each of which produces one electron through the collision with energetic electrons. Most of the primary electrons are produced in this process. Therefore, the mean ionization energy through this process is almost equal to that of the so-called "mean energy per ion pair" which is about 22 eV for Xe gas. The other is that of multiply ionized atoms each of which produces several electrons through photoabsorption. We assume that the mean energy required to produce one electron through the collision is independent of the kinetic
H. Tsunemi et al. / Nonlinearity of the Xe K-edge gas proportional counter energy of electrons. On the contrary, the mean energy required to produce one electron from the photoabsorbed atom depends on the final charge state of the atom . More energy is required to further ionize atoms of initially higher ionization states . The relative abundance in ionization level of Xe ions that are formed as the consequence of a vacancy in the K shell is different from that of a vacancy in the L shell . We calculated the average charges using the data in ref. [6]. When a sudden vacancy in the K shell occurs in the Xe atom, it will cause the mean ionization level to be about +8 .2 electrons consuming 529 eV . A sudden vacancy in the L shell will cause to be about +7.8 electrons consuming 460 eV . For X-rays just above the K-edge energy, the occurrence probability of the photoabsorption in the L shell is about 10% of that in the K shell . When a sudden vacancy in the K shell of a Xe atom, 87% of the time a fluorescence X-ray will be produced while the rest of the time an Auger electron will be produced. We calculate the reabsorption probability of the fluorescence X-ray to be about 10% when taking into account the geometry of the ASM counter . In this way, the absorbed X-rays just above the Xe K-edge energy can be divided into two groups . One is the group that leaves one atom with a sudden vacancy in the K shell while the other is the group that leaves two atoms ionized : one with a vacancy in the K shell due to the original X-ray and one with an L shell vacancy due to the fluorescence X-ray from the first atom . The occurrence probabilities of these two groups are almost equal from the previous argument . On the contrary, the absorbed X-rays just below the Xe K-edge energy will leave one atom with a vacancy in the L shell . As a first approximation, we can expect that the X-rays just above the Xe K-edge will leave one more highly ionized ion than that just below the K-edge . Therefore, we can calculate the nonlinearity at the K-edge to be about 200 eV which is consistent with our result . In the LAC [7] on the Ginga satellite, they used an array of the Xe-filled gas proportional counters whose geometrical area is about 4000 cm2 . The effective area just above the Xe K-edge is about 250 cm 2 , since the Xe gas pressure is about 0 .4 atm . The standard observation energy range is between 2 and 37 keV . Therefore, there is almost no effect on the data analysis by taking into account the nonlinearity effect at the Xe K-edge energy . They sometimes functioned the LAC in the energy range up to 60 keV . However, they did not take into account the nonlinearity effect at the
303
Xe K-edge probably due to the fact that it had relatively small effective area around that energy . They only employed the Xe L-edge effect on the counter . Some future satellite, like the XTE satellite [8], will use Xe-filled gas proportional counters with bigger effective area and a wider energy range . In that case, they will need to take into account this effect in the data analysis.
4 . Conclusion We measured the nonlinearity effect in a Xe-filled gas proportional counter at the Xe K-edge energy (34 .5 keV) using a synchrotron radiation facility . The nonlinearity effect has an EW about 170 ± 10 eV. This can be understood as the difference of the final mean ionization level of the photoabsorbed atoms above and below the K-edge . This effect should be taken into account when we use the Xe-filled gas proportional counter for wide energy range with large effective area . This value would slightly depend on the counter geometry because of how the fluorescence X-rays are reabsorbed in the counter .
Acknowledgements The authors express their thanks to all the members of the CCD team in Osaka University that assigned the machine time to our experiment. This work is partly supported by the Special Coordination Fund of the Science and Technology Agency . The manuscript is carefully read by K.C . Gendreau in MIT .
References [1] H .V .D. Bradt, T . Ohashi and K .A . Pounds, Rev . Astron . Astrophys . 32 (1992) 392 . [2] G .W . Fraser, X-ray Detectors in Astronomy (Cambridge Univ. Press, 1989) p . 179 . [3] K. Jahoda and D . McCammon, Nucl. Instr . and Meth. A 272 (1988) 800 . [4] K. Koyama et al ., Publ . Astron. Soc. Jpn . 3 6 (1984) 659 . [51 H . Tsunemi, S . Kitamoto, M . Manabe, M. Miyamoto, K . Yamashita and M . Nakagawa, Publ. Astron . Soc . Jpn . 4 1 (1989) 391 . [6] T.A . Carlson, W .E . Hunt and M .O . Krause, Phys . Rev . 151 (1966) 41 . [7] M .J .L . Turner et al ., Publ . Astron . Soc . Jpn . 4 1 (1989) 345 . [8] H .V .D. Bradt et al ., Proc . IAU Colloq . 123 (1990) 89 .