L X-ray fluorescence cross sections experimentally determined for elements with 45<Z<50 at 9 keV

https://doi.org/10.1016/j.apradiso.2011.12.039Get rights and content

Abstract

Experimental determination of L fluorescence cross-sections for elements with 45<Z<50 have been determined at 9 keV using Synchrotron radiation. This work is part of an investigation we did at low energies in the same group of elements. The individual L X-ray photons, Ll, , I, II, Ι and ΙΙ produced in the target were measured using a Si(Li) detector. The experimental set-up provided a low background by using linearly polarized monoenergetic photon beam, improving the signal-to-noise ratio. The experimental cross sections obtained in this work were compared with data calculated using coefficients from Scofield (1973), Krause et al., (1978), Krause (1979) and Scofield and Puri et al. (1993, 1995)

Highlights

► Experimental data of L fluorescence cross-sections with 45<Z<50 were determined at 9 keV by Synchrotron radiation. ► The cross sections Ll, , I, II, Ι and ΙΙ obtained, were compared with calculated data. ► The Hypermet function was used to fit the data, because it considers a tail on the left side of the peak. ► The tail is relevant when a small peak has another one on the right side with a big area.

Introduction

The accurate measurement of X-ray parameters like fluorescence yields and cross-sections are very important to understand the ionization of atoms and the atomic processes that are needed in several fields such as material science and medical physics.

Due to the different experimental set-ups used and the differences in the data analysis there are still some discrepancies observed in the experimental and theoretical cross-sections reported by many authors.

Although many L X-ray fluorescence cross-sections were reported for a wide range of elements and excitation energies (Rao et al., 1995, Rao et al., 1996, Barrea and Bonzi, 2001a, Barrea and Bonzi, 2001b, Barrea and Bonzi, 2001c, there are still several elements and energy ranges that have not yet been measured.

This work is part of a systematic investigation on elements with 45<Z<50, which is carried out at different energies.

It is important to notice as well that most of the measurements were done using an X-ray tube with a secondary target that provides a quasi monoenergetic excitation beam. In these cases, the scattering radiation from the secondary target produces an extra contribution that should be considered. Additionally, the results are only reported considering the L lines in groups referred to as Ll, , , and .

Discrepancies between the theoretical cross sections calculated using different Coster-Kronig coefficients and fluorescence yields from Krause's et al. (1978), Krause's (1979), Chen et al., 1980, Chen et al., 1981 and Puri et al., 1993, Puri et al., 1995 tables, are found. Krause's tabulations are compilations of semi-empirical fitted values while Chen's and Puri et al.'s are theoretical values based on ab initio relativistic Dirac-Hartree-Slater calculations.

In this work we measure L X-ray cross-sections with monoenergetic excitation photon beam at 9 keV for elements with atomic number in the range of 45<Z<50. The cross-sections of elements with atomic numbers between 45<Z<50 were reported only as total cross-sections without reference to any particular characteristic line. Here we report cross-sections for each spectral line or group of lines, depending on the resolution of the detector used.

The experimental determinations of L X-rays fluorescence cross-sections at 9 keV were carried out using a Si(Li) solid-state detector that can resolve individual component lines of the spectral emission of the elements observed. The experimental cross sections were grouped considering the transitions scheme, the energy of the emission lines and the detector's resolution.

In our experiment, the geometry factor and efficiency of the detector were determined by measuring the emission of K-lines of Ca, Cl, Ti and Fe using expression given by Jaklevic and Giauque (1993). The measured values were compared with the values obtained by Monte Carlo simulations using the package of subroutines named PENELOPE-2008 (Salvat et al. (2008)).

In general, the cross section values obtained experimentally in this work are comparable and show the same trend with those calculated values using Scofield, 1974, Scofield, 1973 and Krause's et al., (1978), Krause's (1979) coefficients, except for the and Ll lines. In particular, the Cd I experimental value is about 50% smaller than the calculated values.

The experimental cross-sections are similar compared with both Puri et al., 1993, Puri et al., 1995 and Krause's et al. (1978), Krause's (1979) values. But in the case of the atomic number Z = 50 the II experimental datum is much higher than the calculated values from Puri et al.'s and Krause's coefficients.

Section snippets

The experiment

The measurements were carried out at the X-ray Fluorescence beam line at the National Synchrotron Light Laboratory (LNLS), Campinas, Brazil (Perez et al. (1998)). The most important components of the experimental set-up were the following:

  • Silicon (111) channel-cut double crystal monochromator, which can tune energies of the photon beam between 3 and 30 keV. The energy resolution is 3–4x10−4 between 7 and 10 keV.

  • A Canberra Si(Li) solid state detector, 5 mm thick and 5 mm in diameter, with a

Spectra analysis

The L X-ray fluorescence lines were grouped considering the energy of the emission lines tabulated by Scofield, 1973, Scofield, 1974 and the detector's resolution. This line arrangement was used for the fitting of the L spectrum, where the and compound lines have been noted with a Roman subscript according to the most intense contribution line, with its corresponding atomic transition:

  • Ll=L3M1,

  • Lα=L3M5+L3M4,

  • I=L2M4+L1M2+L1M3+L3N1,

  • II=L3N5+L3O4+L3O5+L3O1+L1M5+L1M4+L3N4,

  • I=L2

Data analysis

The L experimental fluorescence cross sections were determined from the following expression (Rao et al. (1995)):σLie(E0)=ILiI0Gε(ELi)T(E0,ELi)where σeLi(E0), experimental Li fluorescence cross sections of the element observed at the energy E0, with Li=Ll, , I, II, I or LγII; ILi, measured intensity of the Li spectral line; I0(ELi), factor comprising the intensity of the excitation beam I0, the geometry of the experimental arrangement G and the detector's efficiency ε(ELi); E0, energy

Results and discussion

L X-ray cross section values obtained in our fluorescence experiment and the theoretical values calculated by using coefficients given by Scofield, Krause and Puri et al. are shown in Table 2 and Fig. 3(a)–(f).

Puri et al. predicted theoretical Coster-Kronig and fluorescence values using ab initio relativistic calculations, while Krause's values of ωK, ωLi and fij were obtained by fitting experimental and theoretical compiled data. In Krause's tables the theoretical data were calculated for

Conclusions

In this investigation the L X-ray fluorescence cross sections of a group of elements with 45≤Z≤50 were measured using a synchrotron radiation source for monoenergetic beams at 9 keV. The polarization properties of the monoenergetic excitation beam and the high resolution of the detector system allowed reducing the scattered radiation thus obtaining a better signal-to-noise ratio and a better accuracy for the experimental cross sections.

The cross sections of Ll, , I, II, 1 and II lines

Acknowledgments

This work was carried out under grants provided by SeCyT U.N.C. (Argentina). Research partially supported by LNLS—National Synchrotron Light Laboratory, Brazil.

The authors are grateful to Dr. R.T. Mainardi for providing the set of measured samples.

N.B. is grateful to TWAS and CONICET (Argentina) for the fellowship obtained.

References (23)

  • R.A. Barrea et al.

    Nucl. Instrum. and Meth. in Phys. Res. B

    (2001)
  • K. Baur et al.

    Spectrochimica Acta Part B

    (2001)
  • S. Puri et al.

    At. Data and Nucl. Data Tables

    (1995)
  • J.H. Scofield

    At. Data and Nucl. Data Tables

    (1974)
  • AIC Software, 1994. PHOTOCOEF. Grafton,...
  • R.A. Barrea et al.

    X-Ray Spectrom.

    (2001)
  • R.A. Barrea et al.

    Phys. Scr.

    (2001)
  • E.V. Bonzi et al.

    X-Ray Spectrom.

    (2005)
  • M.H. Chen et al.

    Phys. Rev. A

    (1980)
  • M.H. Chen et al.

    Phys. Rev. A

    (1981)
  • J.H. Hubbell et al.

    NISTIR 5632

    (1995)
  • Cited by (0)

    View full text