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TOTAL RATE IMAGING WITH X-RAYS IN A SCANNING ELECTRON MICROSCOPE
P. Bernsen, L. Reimer
To cite this version:
P. Bernsen, L. Reimer. TOTAL RATE IMAGING WITH X-RAYS IN A SCANNING ELEC- TRON MICROSCOPE. Journal de Physique Colloques, 1984, 45 (C2), pp.C2-297-C2-300.
�10.1051/jphyscol:1984266�. �jpa-00223980�
Colloque C2, supplCment au n02, Tome 45, fivrier 1984 page C2-297
TOTAL RATE IMAGING WITH X-RAYS
IN
A SCANNING ELECTRON MICROSCOPEP . Bernsen and L. Reimer
PhysikaZisches I n s t i t u t , Universitift Mtlnster, Domgkstrasze 75, 0-4400 Mnster, F.R.G.
R6sum6
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Les intensitss du rayonnement X total sont calcu- l&es pour des 616ments purs et cornparses aux valeurs exp6- rimentales obtenues avec un dgtecteur scintillateur. L'ap- plication de ces rssultats aux images X est discut6e.Abstract
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Calculations of the total rate of emitted x-rays are compared with measurements on pure elements using a scin- tillation detector. Application of the total rate imaging with x-rays are discussed.The total rate imaging mode with x-rays (TRIX) records the continuum and characteristic x-ray quanta without energy dispersion and can be used in a scanning electron microscope (SEM) for image recording. First Ingrarn and Shelburne / l / used a Si(Li) detector for x-ray counting and image formation. In comparison to the backscattered electrons (BSE) we expected more depth information for the TRIX-signal. The TRIX-signal does not increase monotonously with atomic number. This can result in contrast reversals compared with the BSE-signal. Therefore, the TRIX- mode allows additional material differentiation and expends the imaging modes in a SEM.
x Dyson 11959)
- -
Compton and Allison (19351 I Green and Cosalett 11968)k
%-'l I\ -
TheorieFig. 1 Fig. 2
Fig. 1
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TRIX-detector with scintillator,lighttube and photomultiplier.(PE
-
primary electrons, PM-
photomultiplier)Fig. 2
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Kramers constant k as a function of the atomic number 2 .Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984266
C2-298 JOURNAL DE PHYSIQUE
Ingram and Shelourne used a conventional Si(Li) detector for x-ray-*
cou ting which has the disadvantgges of a small solid angle R 2 10
-
10
-9
sr and a limitation of 2 10 c.p.s.. Better results are obtainable using a scintillation detector /2/ with a solid angle R = 2.5 sr and an analogue output signal of a phqtomultiplier (Fig. 1). W5 use a plastics scintillator (NE 102A, T a 10 S) with an area of 2 cm and a thick- ness of 3.5 cm to absorb all x-ray quanta. The backscattered electrons are absorbed by a 50 pm plastics foil coated on both sides with a 130 nm A1 film to aasorb also the light emitted from the specimen and by cathodolurninescence in the foil. The foil is placed in front of the scintillator and can be removed for the recording of BSE with the same scintillator.Calculation of the TRIX-signal
The calculated TRIX-intensity (I ) is a superposition of the continuum (IC) and the K-, L- and M-series (Inl) T :
The contribution of the characteristic x-ray quanta is given by
Enl-ionization energy E -energy of characteristic quanta Qnl-ionization cross-section
--
dEX stopping powerpds
wnl-fluorescence yield znl-number of electrons in the shell
E -detector efficiency Q -solid angle of the detector K -backscattering correction f(x)-absorption correction F -continuum fluorescence correction
We integrated (2) numerically using cross-sections of Gryzinski /3/ and the Bethe-formula /4/ for the stopping power with a mean ionization potential fitted by Berger and Seltzer /5/.
The intensity of the continuum is calculated similar to equation ( 2 1 , but additionally integrating over the whole energy range of the x-ray quanta.
Bremsstrahlung cross-sections QB are tabulate$ by Pratt et al. / 6 / . Fig. 2 shows the calculated Kramers constant - compared with experi- mental data / 7
-
9/. Due to the uncertainty03
the experimental data, the calculations should varified by new measurements.Fig. 3a and b shows the calculated and measured signals for pure ele- ments of atomic number Z at primary energies E = 15 and 45 keV. At the primary energy of 45 kev (Fig. 3b) the max?mum is 25 eV/electron.
This fraction of 45 keV contributes to the calculated signal, i.e. on average, an x-ray quantum energy of 25 eV is absorbed by the detector per incident electron.
The experimental data are output signals of the photomultiplier which are proportional to the total energy rate of the absorbed quanta. The first maximum is caused by the K-series. With increasing energy the minima become smoother. The secand maximum, which belongs to the L-
do-'
0 Experiment
I ' I . I ' I . (
0 20 L0 M) 80 100
z-
Fig. 3
-
Intensity of the TRIX-signal as a function of the atomic number Z at a) E. = 15 and b) 45 keV.series shifts to higher Z with increasing energy.
Imaging biological specimens, the main problem is the low contrast because of the small atomic number differences. Therefore the specimen are contrasted with elements of high atomic numbers.
Fig. 4
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Rat kidney with TiFe particles ( E = 35 kev), a) BSE- and b) TRIX-image. 0C2-300 JOURNAL DE PHYSIQUE
Fig. 4 shows the BSE- (Fig. 4a) and TRIX-images (Fig. 4b) of a freeze- dryed rat kidney. An EDX-analysis demonstrated the storage of fly ash of pit coal fired combustion in the cavities. Contrary to the TRIX- image the TiFe particles cannot be identified in the BSE-image because of the superposition of topographic contrast. This example demonstrates the advantage of TRIX when imaging freeze-dryed biological specimens due to the suppression of surface roughness. Measurements and theore- tical estimation show, that the TRIX-signal is generated through a depth of approximate twice the exit depth of the BSE.
References:
/ l /
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Ingram P, Shelburne J D: SEM 1980/II, SEM Inc., AMF 0 ' Hare (1980) 285/2/
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Bernsen P, Reimer L: Electron Microscopy 1982, Vo1.L (1982) 685 /3/-
Gryzinski M: Phys. Rev.138,
A (1965) 336/4/
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Bethe H: Ann. Physik5
(1930) 325/5/
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Berger M J, Seltzer S M: Nat. Academy Science/Nat. Res. Council Publ. 1133, Washington (1964) 205/6/
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Pratt R H, Tseng H K, Lee C M, Kissel Lynn: Atomic Data and Nuclear Tables 20 (1977) 175/7/
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Dyson N A: Proc. Phys. Soc. 73 (1959) 924/8/
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Compton A H, Allison S K: x - g y s in Theory and Experiment, Van Nostrand, New York (1 935) 90/9/
