Absolute fluence rates of soft X-rays using a double ion chamber

Journal of Electron Spectroscopy and Related Phenomena 101–103 (1999) 33–37

Absolute fluence rates of soft X-rays using a double ion chamber Norio Saito*, Isao H. Suzuki Electrotechnical Laboratory, 1 -1 -4, Umezono, Tsukuba, Ibaraki 305 -8568, Japan

Abstract Absolute fluence rates of soft X-rays have been measured using a multi-electrode ion chamber. The measured ion currents approached a constant value when the gas pressure became lower than about 0.1 Pa, showing that secondary ionization does not occur in the chamber under the low-pressure. A fitting calculation with consideration of secondary ionization by ejected electrons has reproduced plotting curves of experimental ion currents against gas density in the region of 10 22 to 10 3 Pa. The fluence rates were obtained from 50 to 1100 eV: e.g. the absolute fluence rate at the photon energy of 900 eV was 4.2310 9 photons / s with an Al filter under the storage ring condition of 750 MeV and 100 mA.  1999 Elsevier Science B.V. All rights reserved. Keywords: Soft X-rays; Absolute intensity measurement; Ion chamber

1. Introduction Synchrotron radiation (SR), including infrared radiation through X-rays, is a powerful tool in regions of fundamental and applied sciences. Microfabrication and characterization of different materials and microanalyses of biological samples have been investigated using soft X-rays of laser-plasma and SR [1,2]. Two types of experiments are being carried out. The first type needs a high photon resolution and intensity, which plays a significant role for a variety of researches. The other requires absolute intensity of the photon beam; examples for these researches are quantitative characterization of irradiation effects and impurity analysis of several elements. In these fields, one efficient method is to use the characteristics of SR of which the intensity and the angular distribution are calculable in the region of ultraviolet radiation to X-rays [3–8]. However, the synchrotron

*Corresponding author.

radiation has a wide energy spectrum and, thus it is usually necessary to employ some focusing mirrors and a monochromator. Since these elements have unknown efficiencies on photon transfer, it is difficult to determine the intensity and the purity of the photon beam at the experimental point. Therefore, it is necessary to develop a technique for measuring the absolute intensity of the soft X-ray beam. In the present study, a double ion chamber has been closely examined, with which the absolute intensity of the soft X-ray beam is obtained in aid of an index constant of rare gases on multiple photoionization.

2. Method for absolute measurement The method for measuring the absolute intensity of the soft X-rays is here discussed, bearing in mind that for vacuum ultraviolet radiation [9,10]. The photon intensity in front of the first electrode, I1 , that in front of the second electrode, I2 , and that at the end of the second electrode, I3 , are given as follows.

0368-2048 / 99 / $ – see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S0368-2048( 98 )00468-X

34

N. Saito, I.H. Suzuki / Journal of Electron Spectroscopy and Related Phenomena 101 – 103 (1999) 33 – 37

I1 5 I0 exp(2ds p), I2 5 I1 exp(2Ls p), I3 5 I2 exp(2Ls P)

(1)

In these equations, I0 , s, p, d, and L denote the incident photon intensity, the absorption cross section, the gas density, the length of the insensitive region in front of the first electrode, and the electrode length, respectively. Then, the ion currents collected with the first (i 1 ) and the second (i 2 ) electrodes are obtained with the following equations. i 1 5 en(I1 2 I2 ) 5 enI0 exp(2ds p)(1 2 exp(2Ls p)),

(2a)

i 2 5 en(I2 2 I3 )

I0 5 i 1 exp(ds P) /en(1 2 exp(2Ls p))

5 enI0 exp(2ds p) exp(2Ls p)(1 2 exp(2Ls p)), (2b) where e denotes the elementary charge and n is the number of electrons totally produced from a photon in the chamber. From Eqs. (2a) and (b), the incident photon intensity is expressed as I0 5 (i 1 )2 exp(ds p) /en(i 1 2 i 2 ).

(3)

In order to obtain the photon intensity, it is necessary to estimate the total number of electrons, which is the summation of the initial ionization and the secondary ionization effects in the chamber, being expressed with Eq. (4). n 5 g 1 d ( p),

as the gas pressure decreases, and the g -values of Ne, Ar, Kr, and Xe have been measured in these years [11]. Using these g -values, the absolute intensity measurement in the soft X-ray region becomes possible under the condition of a low gas pressure. As the gas density becomes low, the difference in the currents between the two electrodes becomes very small. This effect enlarges an uncertainty in measured values of the photon intensity. In the low density, Eq. (3) can not be applied for deriving the accurate fluence rate. Therefore, the following equation is applied for the low density, which is obtained directly from Eq. (2a).

(4)

where g is the number of electrons ejected from the atom which has absorbed a photon and d is the number of electrons secondarily produced through the collision between ambient atoms and electrons, which depends on the gas density in the ion chamber. In the VUV region, the values of g and d are equal to unity and zero, respectively, because the photon energy is low and, thus the ejected electrons have no capability to ionize other atoms. On the contrary, the value of g exceeds unity and significantly depends on the photon energy in the soft X-ray region [11–13]. Moreover, the value of d has not been estimated so far, because this factor considerably varies with changes in experimental conditions and in geometrical configuration of the apparatus. However, the value of d approaches zero

(5)

In this equation, the value s is obtained with the following procedure. The ratio between i 1 and i 2 gives us the value s from Eqs. (2a) and (b). i1 1 s 5 ] ln ] Lp i 2

(6)

The s -value can be obtained in an appropriate gas density, where the current i 1 are not close to i 2 .

3. Experimental Monochromatic soft X-rays were obtained by dispersing synchrotron radiation from the electron storage ring at the Electrotechnical Laboratory. The storage ring was usually operated at 750 MeV, but the energy of the electron beam in the ring was occasionally reduced to 250–600 MeV for suppressing higher order light. Thin foil was used as a filter for decreasing stray light behind the monochromator. The soft X-ray beam passed through a differential pumping stage and entered a double ion chamber of cylindrical shape (see Fig. 1). The window with a thin VYNS foil (Manson type ‘F’ filter) for the incident photon is positioned at 15 mm from the central axis, and electrodes for collection of produced ions are fixed at 22 mm from the axis. The set of the electrodes is composed of six cylinders of 5 mm f. Each electrode plated with Au is electrically separated with Teflon insulators. The ion chamber was evacuated with a turbo-molecular pump of 280

N. Saito, I.H. Suzuki / Journal of Electron Spectroscopy and Related Phenomena 101 – 103 (1999) 33 – 37

35

Fig. 1. Schematics of the measurement system using the multi-electrode ion chamber.

l / s just before measurements, and supplied with a rare gas at about 10 22 to 10 3 Pa in obtaining ion currents. Ion current at each electrode was led to a pico-ammeter and transferred to a personal computer. The gas density was detected with a capacitance manometer (Baratron 690), and controlled with an automatic valve system. The size of the soft X-ray beam was defined using a circular aperture of 1 mm f.

zero. Since the secondary ionization increases with the gas density, the photon intensity increases with increasing gas density.

4. Results and discussion Apparent photon intensities at the photon energies of 70, 200, 600 and 900 eV are plotted as a function of the Ar gas pressure at the applied electrode voltage of 15 V in Fig. 2. The apparent photon intensity has been derived as the intensity calculated from Eq. (5) in assumption with n5g, i.e., I 5 i 1 exp(ds p) /eg (1 2 exp(2Ls p)).

(7)

Data points denote the experimental results. A photon intensity monitor of Au was used during these measurements for the correction of current reduction in the storage ring. Data were normalized with an electron beam current of 100 mA in the storage ring. Real photon intensities have been obtained through extrapolation into the gas density of

Fig. 2. Observed photon intensity as a function of the supplied gas density. The data marks, d, \, h, and m denote the results at photon energies of 900, 600, 200, and 70 eV, respectively. The used gas is Ar.

N. Saito, I.H. Suzuki / Journal of Electron Spectroscopy and Related Phenomena 101 – 103 (1999) 33 – 37

36

The solid curves for the data at 70, 200 and 900 eV in the figure show results calculated using the following equation on the assumption that the photon includes neither stray light nor higher order lights. The calculated apparent photon intensity I has been obtained from Eqs. (3) and (7). n g 5 d ( p) I 5 I0 ] 5 I0 ]]] g g

(8)

The number of the secondary electrons is zero at p50 and approaches a certain value (d0 ) at higher pressures. The number of collisions between electrons and ambient atoms affects the number of the secondary electrons. The number of collisions depends on the mean free path. Therefore, the number of the secondary electrons, d, can be approximated with d ( p) 5 d0 (1 2 exp(2a p)). (9) In Eq. (9), d0 denotes the number of the secondary electrons at a sufficiently high pressure and a indicates the parameter corresponding to the mean free path. The experimental results are reproduced with the fitting calculation using the following equation. g 1 d0 (1 2 exp(2a P)) I 5 I0 ]]]]]]] (10) g

The data for 600 eV were seriously affected by the higher order light. The solid curve along with the data at 600 eV in Fig. 2 was obtained through a calculation in the consideration of the higher order light with 2.5% of the total intensity. Fig. 3 shows the photon fluence rates of monochromatized soft X-rays under conditions of several operated ring energies and used filters. The fluence rates ranged between 10 7 and 10 10 photons / s / 100 mA. The fluence rates are low compared with those of soft X-ray beam lines in other SR facilities because we have sacrificed the photon intensity to realize a high purity of the incident photon beam. We have compared the fluence rates measured using Ar with those using Ne at the photon energies of 110, 140, and 160 eV. The measured rates agreed within 5% each other. Therefore, the present measurement method is considered to be reliable. The uncertainty in the fluence rates was derived through estimation of uncertainties in used apparata and in measurement procedures [14]. Briefly, the uncertainty at the photon energy of 200 eV was

In the fitting calculation, I0 , d0 and a are adjustable parameters. When the photon includes the stray light and / or higher orders, the fitting procedure becomes slightly complicate. The ion current, i, is expressed with Eq. (11). i5

O e[g 1 d ((1 2 exp(2a p))] k

k

k

k 50

3 Ik exp(2dsk p)(1 2 exp(2Lsk p))

(11)

The terms of k50 show the parameter values for the photon of interest and those with other k-values show the parameters for the stray light or higher orders. From Eqs. (7) and (11), the apparent photon intensity is obtained as follows. I5

S O [g 1 d (1 2 exp(2a p)] k

k

k

k 50

D

3 Ik exp(2dsk P)(1 2 exp(2Lsk p)) 3 exp(ds0 p) /g (1 2 exp(2Ls0 p))

(12)

Fig. 3. Fluence rates of the monochromatized soft X-rays for the several storage ring energies and used filters.

N. Saito, I.H. Suzuki / Journal of Electron Spectroscopy and Related Phenomena 101 – 103 (1999) 33 – 37

estimated as follows. The purity of the photon beam is higher than 99.5%, which gives the uncertainty of 0.5%. The uncertainties from the measurements of the ion current, the electrode length, the density of gas, the cross section, and g -value were estimated to be 3, 1, 2, 2, and 1%, respectively. The combined uncertainty at the photon energy of 200 eV was estimated to be about 4.5%. It is important to compare the present technique with that of another measurement apparatus, such as a cryogenic electrical substitution radiometer, which has provided the uncertainty of about 1% at 1–10 mW [6]. The power of 1 mW at the photon energy of 200 eV corresponds to about 3310 10 photons / s. In the present experiment, the photon intensity has ranged 10 7 –10 9 photons / s. If we measure the intensity using the cryogenic radiometer under the present condition, an uncertainty would become larger than 4.5%. Then the comparison is being planned after improvement in the optical system for monochromatic soft X-rays or accomplishment of the calibration of a secondary standard detector.

5. Conclusion The fluence rate in the soft X-rays of 50 to 1100 eV has been measured using the double ion chamber and the g -value for Ar. Obtained absolute values of the fluence rate were comparable to those expected from the Au monitor. Uncertainties (about 4.5%) are not small and then a decrease in the fluctuation of noise currents is necessary for establishing a standard soft X-ray field with high quality.

37

Acknowledgements The authors wish to thank the staff of the accelerator group at the Electrotechnical Laboratory for their continued operation of the storage ring.

References [1] H. Saisho, Y. Gohshi, Applications of Synchrotron Radiation to Material Analysis, Elsevier Science, Amsterdam, 1996. [2] N. Atoda, in: T. Tomimasu (Ed.), Techniques for Synchrotron Radiation, Kogyo-chosakai, Tokyo, 1990, p. 337, in Japanese. [3] I.H. Suzuki, N. Saito, Jpn. J. Appl. Phys. 25 (1986) 130. [4] R. Krode, J.S. Cable, L.R. Canfield, IEEE Trans. Nucl. Sci. 40 (1993) 1655. ¨ [5] A. Lau-frambs, U. Kroth, H. Ranus, E. Tegeler, G. Ulm, Rev. Sci. Instrum. 66 (1995) 2324. [6] F. Scholze, H. Henneken, P. Kuschnerus, H. Rabus, M. Richter, G. Ulm, J. Synchrotron Rad. 5 (1998) 866. [7] M. Sakurai et al., Rev. Sci. Instrum. 60 (1989) 2089. [8] I.H. Suzuki, N. Saito, Bull. ETL 57 (1993) 34, in Japanese. [9] J.A.R. Samson, Techniques of Vacuum Ultraviolet Spectroscopy, Wiley Interscience, New York, 1967. [10] T. Saito, Res. ETL 960 (1994) (in Japanese). [11] N. Saito, I.H. Suzuki, Int J. Mass Spectrom. Ion Processes 115 (1992) 157. [12] D.M.P. Holland, K. Codling, J.B. West, G.V. Marr, J. Phys. B12 (1979) 2465. [13] N. Saito, I.H. Suzuki, J. Phys. Soc. Jpn. 66 (1997) 1979. [14] N. Saito, I.H. Suzuki, to be published.