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Investigation of system-induced background radiation using a 0–160 keV high-purity germanium detector
Ultramicroscopy 34 (1990) 229-236 North-Holland
229
Investigation of system-induced background radiation using a 0-160 keV high-purity germanium detector R. S c h m i d t a n d M. F e l l e r - K n i e p m e i e r Institut ]'dr Metallforschung Technische Universitiit Berlin, BH18, Strasse des 17. Juni 135, W-IO00 Berlin 12, Germany Received 23 August 1990
System-induced background radiation using a high-purity germanium (HPGe) detector coupled to a 300 keV TEM has been studied. It could be shown that the appearance of high-energy characteristic tungsten K lines in the system's bremsstrahlung was due to insufficient thickness of the shielding parts of the detector's collimator.
1. Introduction For use in analytical electron microscopy, high-purity germanium (HPGe) detectors are available now as an alternative to the known Si(-Li) solid state detectors. Their resolution is
improved by about 7% [1]; they can analyse X-rays up to 160 keV and they are less sensitive to thermal stress (deicing). However, the following experiments with a Tracor HPGe-detector coupled to a 300 kV TEM (Philips CM30) show that the new option of
TRK~i ........ i ........................ ~........................ i ................. TI
i
TF::I~a2
!
................... ~*~~
..I{._JLi
i
............... ! ........................ i ........................ ! ........................ i ....................... ! ...................
i
£,
....................
i,li~......li!~-...'-~...-i ......... i ..... !~.~.,.~.I..!...,I,..... i..... ~,~
....... i ........................ i........................ ,
i Hi
"'.ii:i"
................
i; VFS = ~56
5i.260 1351
Fig. 1. WKaradiation in a Ni-2.89wt%Tasin~ephase Mloy. 0304-3991/90/$03.50 © 1990 - Elsevier Science Publishers B.V. (North-Holland)
71.740
230
R. Schmidt, M. ~l~r-Kni@meier / ~stem-induced background radiation
................................................................................................................................................................................... i iiiiiii~iii~i~i~i~iiiiiiii~i!i'~.~:.:~
i
~
i ............
i :ii: iii!?:i!i iii ,:i!!i! 0.000
' v ' F ' _=,
=
:32
:-:1 9 7 - 0
188
Fig. 2. S p e c t r u m of b r e m s s t r a h l u n g with c h a r a c t e r i s t i c W K lines.
analysing high-energy X-rays requires a better protection of the detector against background radiation in this energy range. The insufficient thickness of the detector's collimator system causes a self-induced tungsten signal, which makes a quantitative analysis less exact.
2. Experiment In fig. 1 tungsten radiation is visible in a N i 2.89wt%Ta single phase alloy. Comparing the W K with the Ta K intensities the tungsten content in the alloy would be 1.9 wt%.
i . . .
(3 0 0 0 t 80
VFS Fig. 3. S p e c t r u m o f p u r e t u n g s t e n .
= 819.'-':'
81
920
R. Schmidt,M. Feller-Kniepmeier/ System-inducedbackgroundradiation
/
/
~
COLLIMATO
B ~
PLANE
]
OF
. . I
COND.:9:
I
i O
~l"i"~"~ax
~
~/7~ LOWERPOLEPIECE
COND.5:
/
4 / ' / / / / / I
"/
[POSITION I OF VALVE
o-as~
l
231
~
D-40~4t,f
7 //
t.-
I I I
Fig. 4. Experimental set-up (configurations of the detector, schematically).
A spectrum of the bremsstrahlung without any sample is shown in fig. 2. It was acquired with a 100 /~m condensor aperture, an emission current of 3.3 ~tA and with spot size 4 for 277 s. In
addition to the bremsstrahlung, the characteristic W n series without L and M series are detected. The spectrum of pure W is shown in fig. 3. Because of the decreasing efficiency of the detec-
Table 1 Conditions used in the experiment and system response, no specimen in object plane Conditions used
System response
No.
d (ram)
SP
OA
Valve
1
236
1
Very decentered
Closed
W K line detectable
2
200
1
Decentered
Open
Pt K line detectable; signal(W K ) / n o i s e ratio increasing
3 4
60 40
1 3
Centered Centered
Open Open
Increasing S / N ; detection of Pt no longer possible Increasing Pt K/WK ratio; in the energy range 0 - 1 0 keV Pt L line, Ir(OA), Cu, Co and Fe (TEM) can be measured
5
30
3
Centered
Open
Further increase of Pt K/WK ratio; decreasing Pt K / P t L ratio
6
20
5
Centered
Open
Increasing Pt K / P t L ratio
7
18
5
Centered
Open
Increasing Pt K / P t L ratio
8
16
5
Centered
Open
Decreasing Pt K/WK ratio; Pt L lines no longer visible
9
14
5
Centered
Open
Increasing Pt K / W K ratio
d = distance Ge crystal-optical axis; SP = spot size (current density decreasing with increasing size); OA = objective aperture.
R. Schmidt, M. Feller-Kniepmeier / System-induced background radiation
232
iiiiiii!iiiiiiiii!iiiiiiiii!iiiiiiiiiii!ii' .........................................................
i_.~i..........i_Li ..........i_i_i_L_i_i ..........i,J~_i ..........iH,_i,.3~.~,iii~.,,i .......... .........
........ i.;
iliiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii~~ ....i-61---i--i-+-i--~i--~i,-~i--i--i!!!!!!,+;-~!-~!3;H3!--!-!i~!K~!!!!!!!!
iiiiiiiiiiiiiiiiiiiiiiiiiiiiil
iiiiiiiiiil
"¢'FS
0. 0 0 0
=
128
81
-~20
184
Fig. 5. Spectrum for condition 1 of table 1 (d = 236 mm, fig. 10).
tor with increasing energy the I(W K)/I(WL) ratio is only 0.089 after background subtraction (for comparison: I(Ni K ) / I ( N i L ) ~ 1.3). In order to find out the origin of the W K radiation in the spectrum of the bremsstrahlung the characteristic Pt radiation of the inserted objective aperture was generated without any sam-
...............................................................
ple. In addition, the distance d of the Ge crystal to the optical axis of the microscope was varied from retracted ( d = 2 3 6 mm) to the inserted standard operation position (d = 14 mm). The experimental set-up is shown in fig. 4. By using a suitable choice of the spot size (SP) and different centering of the inserted objective aperture (OA),
r-.~.-.-.~-.-~.---.~
.
.
.
.
.
..... i-i~i.............................. iii! iiiiiiiiiiii!iiiiii~p~+~i~
iii!!iiiiiiiiiii!!iiiii!iii!ip~+~
"...
.....
...............
.......... ?i__?.i.~.
...i...i...!..i...i...i..i...i...i..i. i i i i i i i i i i ii~4::iS~ii iiP~4K~.~,~ FE#K ~#LI i!i!iiiiiii!!i~iiii!iiiiiiiiiliiiiiii ,, ~ ~, ~, ~ , ~ ,., ~ . ~
0000
IV'F --~' =
128
111
Fig. 6. Spectrum for condition 4 of table l ( d = 40 ram, fig. 10).
233
R. Schmidt, M. Feller-Kniepmeier / System-induced background radiation
~'T~g~,i !: i ii ! i i i i !i ii i iii i! ! !i il i ! i i i i i: iil iil iii ii]
•..
•i:i !:ii-i ' ."i
0 ~0
VFS
: 25~
8i.92~
iii
Fig. 7. Spectrum for condition 5 of table 1 (d = 30 mm, fig. 10).
standard operation conditions of about 40% dead time were achieved. The different conditions used and the system response are listed in table 1. The corresponding spectra are shown in figs. 5-9. The intensity ratios I(W K)/noise, I(Pt K)/I(WK) and I(PtK)/I(PtL) are plotted in fig. 10 as function of the distance d of the crystal to the optical axis of the microscope.
3. Discussion
The disappearance of L and M fines of the heavy elements W and Pt under condition 9 (fig. 10) can be explained by the energy dependence of mass absorption in tungsten, which is used as the detector's collimator material. Assuming that background radiation generates the characteristic
iiii!iiiii!iiiiiiiiiiiiiiiiiiiiiii!iiiiiii iiii!!iiii!!!iiiiiiii!iiiiiiii!iiiiiiiiiii
i i TiTiiiiiiiii++!+iiiiiiiiiJiiJili] ...................
.000
iF'~ i i i
i ......................................................................................
VFS
= 128
122 Fig. 8. Spectrum for condition 7 of table 1 (d = 1 8 m m , fig.I0).
~:i . 9 2 0
R. Schmidt, M. Feller-Kniepmeier / System-induced background radiation
234
! .....
. ..... ~'~- -~- L:~1
........... i ......... i ......... !.../,. k J _ .::, J _ .:: ..5...:: ..~ ............. :: j_/. .....
i
"
•
0000
VFS
:
=
.<
.
~4
81
920
125
Fig. 9. S p e c t r u m for c o n d i t i o n 9 of table 1 ( d = 14 mm, fig. 10).
X-rays of tungsten on the outer surface of the collimator, their decrease in intensity is given by [2]:
pt}.
I = I 0 exp{ [ - f t / o ( A , B)]
(1)
Here tx/p(A, B) is the mass absorption coefficient of characteristic X-rays of element A in the absorbing material B with density P and thickness t. The different absorption of A1K~, WL~I and WK~1 radiation in tungsten is shown in fig. 11. Given a
...-%.
\
±0.00
\ / , I (PTK)/I (WK)
8.00
0 H I'-IT >I-I--I (/3 W Z H
~.00
i
4.00
2.00
~ •
0.00 0.
' ' O0
J
, 2~.
O0
,
~ I , , 50. O0
,
r I , , , , ] , , , , ] , , , , I , , , , I , , , , i , , , , I , , 75. O0 100. O0 ~25. O0 ~LSO. O0 t7~. O0 200. O0 =>25. O0
DISTANCE
O
(MM)
Fig. 10. I n t e n s i t y ratios l ( W K ) / n o i s e , I(PtK)/I(WK) a n d I(PtK)/I(PtL) as function of the d i s t a n c e d of the crystal to the optical axis of the microscope.
R. Schmidt, M. Feller-Kniepmeier / System-induced background radiation
~.00.O0F SO.O0
\ -°-°° i- \
I- \ oo.oo \ ,o.oo \ oo.oo 0 . O0
~ . O0
THICKNESS
O
'10.00
(MICRONS)
Fig. 11. Decrease of intensity of WK,~I,WE,,] and A1K,,1X-rays in tungsten with layer thickness t.
layer thickness t of 10 btm the decrease is about 5% for WK~1 radiation and 95% for the WL~1 intensity. The A1K~] intensity is completely absorbed after 2/zm penetration into tungsten. From eq. (1) the minimum collimator thickness trnin can be determined for a given I / I o ratio. Using this ratio as the detectable minimum mass fraction (MMF), table 2 summarizes tmin for different Xrays and MMF's. Mass absorption coefficients were taken from Zschornack [3]. In today's thin film microanalysis a M M F of 0.1-0.01 wt% is attainable [4]. In order to keep the absorption quality of the collimator constant using W K lines instead of W E lines for the analysis the
235
thickness of the W collimator has to be increased by a factor 40 or even 60 in the case of rhenium (Re) X-rays (table 2). The alternative use of other absorber materials (gold or lead instead of tungsten) does not improve absorption characteristics. Up to now the thickness of the collimator is still dimensioned in such a way that low-energy radiation is absorbed. High-energy radiation of heavy elements, however, can pass the collimator and be detected by the crystal. This is affirmed by the absence of W E radiation in any of the experiments above. A tungsten source in the column of the microscope can be excluded from the experiment under condition 1 (closed valve). The Pt radiation generated from the objective aperture is completely absorbed under condition 9 except for the K lines. Because this Pt source is positioned outside of the detector system and below the normal object plane, Pt k lines can be observed under conditions 4, 5, 6 and 7(18 < d < 40 mm, fig. 10). Changing the geometrical condition of the experiment by varying the distance d, the intensity ratio P t K / P t L is changing also: With maximum solid angle ~max given by the X-ray source and crystal area (condition 5, fig. 4), absorption of the Pt L and Pt M lines by the system as well as the ratio Pt K/Pt L have a minimum. At the same time the P t K / W K ratio has a maximum ( d = 25mm, fig. 10). With ~2 minimized, absorption has a maximum and only the high-energy lines can be detected (condition 1 and 9, d < 18 mm, d > 40 mm, fig. 10).
Table 2 Minimum collimator thickness tmin as function of X-ray energy and absorbing material for a given minimum mass fraction (MMF) Emitter
Energy (keV)
Absorber
#/O (cmZ/g)
train (/~m) MMF 0.1
A1K~I We,,1 WK,~I ReK/~I WK,,1 WK~,I
1.487 8.398 59.318 69.310 59.318 59.318
W W W W Pb Au
1920 154 3.8 2.57 5.06 4.63
0.6 7.8 314.6 465.2 235.5 257.4
0.01 1.2 15.5 629.2 930.4 471.1 514.8
0.001 1.9 23.3 943.8 1395.6 706.6 772.2
236
R. Schmidt, M. Feller-Kniepmeier / System-induced background radiation
4. Conclusion
References
Following the discussion of current problems in X-ray microanalysis [4,5] this paper gives an example for system-induced background radiation using a H P G e detector for the analysis of characteristic X-rays in the energy range of 0 - 8 0 keV. In order to keep the absorption qualities of the detector's collimator when analysing high-energy X-rays, a significant increase in thickness of the shielding parts of the collimator is necessary.
[1] C.E. Lyman, D.B. Williams and J.I. Goldstein, Ultramicroscopy 28 (1989) 137. [2] K.F.J. Heinrich, Electron Beam X-ray Microanalysis (VNR Company, New York, 1981). [3] G. Zschornack, Atomdaten fiir die R6ntgenspektralanalyse (Springer, Berlin, 1989). [41 J.I. Goldstein, D.B. Williams and C.E. Lyman, Ultramicroscopy 28 (1989) 56. [5] J.N. Chapman, Ultramicroscopy 28 (1989) 76.
229
Investigation of system-induced background radiation using a 0-160 keV high-purity germanium detector R. S c h m i d t a n d M. F e l l e r - K n i e p m e i e r Institut ]'dr Metallforschung Technische Universitiit Berlin, BH18, Strasse des 17. Juni 135, W-IO00 Berlin 12, Germany Received 23 August 1990
System-induced background radiation using a high-purity germanium (HPGe) detector coupled to a 300 keV TEM has been studied. It could be shown that the appearance of high-energy characteristic tungsten K lines in the system's bremsstrahlung was due to insufficient thickness of the shielding parts of the detector's collimator.
1. Introduction For use in analytical electron microscopy, high-purity germanium (HPGe) detectors are available now as an alternative to the known Si(-Li) solid state detectors. Their resolution is
improved by about 7% [1]; they can analyse X-rays up to 160 keV and they are less sensitive to thermal stress (deicing). However, the following experiments with a Tracor HPGe-detector coupled to a 300 kV TEM (Philips CM30) show that the new option of
TRK~i ........ i ........................ ~........................ i ................. TI
i
TF::I~a2
!
................... ~*~~
..I{._JLi
i
............... ! ........................ i ........................ ! ........................ i ....................... ! ...................
i
£,
....................
i,li~......li!~-...'-~...-i ......... i ..... !~.~.,.~.I..!...,I,..... i..... ~,~
....... i ........................ i........................ ,
i Hi
"'.ii:i"
................
i; VFS = ~56
5i.260 1351
Fig. 1. WKaradiation in a Ni-2.89wt%Tasin~ephase Mloy. 0304-3991/90/$03.50 © 1990 - Elsevier Science Publishers B.V. (North-Holland)
71.740
230
R. Schmidt, M. ~l~r-Kni@meier / ~stem-induced background radiation
................................................................................................................................................................................... i iiiiiii~iii~i~i~i~iiiiiiii~i!i'~.~:.:~
i
~
i ............
i :ii: iii!?:i!i iii ,:i!!i! 0.000
' v ' F ' _=,
=
:32
:-:1 9 7 - 0
188
Fig. 2. S p e c t r u m of b r e m s s t r a h l u n g with c h a r a c t e r i s t i c W K lines.
analysing high-energy X-rays requires a better protection of the detector against background radiation in this energy range. The insufficient thickness of the detector's collimator system causes a self-induced tungsten signal, which makes a quantitative analysis less exact.
2. Experiment In fig. 1 tungsten radiation is visible in a N i 2.89wt%Ta single phase alloy. Comparing the W K with the Ta K intensities the tungsten content in the alloy would be 1.9 wt%.
i . . .
(3 0 0 0 t 80
VFS Fig. 3. S p e c t r u m o f p u r e t u n g s t e n .
= 819.'-':'
81
920
R. Schmidt,M. Feller-Kniepmeier/ System-inducedbackgroundradiation
/
/
~
COLLIMATO
B ~
PLANE
]
OF
. . I
COND.:9:
I
i O
~l"i"~"~ax
~
~/7~ LOWERPOLEPIECE
COND.5:
/
4 / ' / / / / / I
"/
[POSITION I OF VALVE
o-as~
l
231
~
D-40~4t,f
7 //
t.-
I I I
Fig. 4. Experimental set-up (configurations of the detector, schematically).
A spectrum of the bremsstrahlung without any sample is shown in fig. 2. It was acquired with a 100 /~m condensor aperture, an emission current of 3.3 ~tA and with spot size 4 for 277 s. In
addition to the bremsstrahlung, the characteristic W n series without L and M series are detected. The spectrum of pure W is shown in fig. 3. Because of the decreasing efficiency of the detec-
Table 1 Conditions used in the experiment and system response, no specimen in object plane Conditions used
System response
No.
d (ram)
SP
OA
Valve
1
236
1
Very decentered
Closed
W K line detectable
2
200
1
Decentered
Open
Pt K line detectable; signal(W K ) / n o i s e ratio increasing
3 4
60 40
1 3
Centered Centered
Open Open
Increasing S / N ; detection of Pt no longer possible Increasing Pt K/WK ratio; in the energy range 0 - 1 0 keV Pt L line, Ir(OA), Cu, Co and Fe (TEM) can be measured
5
30
3
Centered
Open
Further increase of Pt K/WK ratio; decreasing Pt K / P t L ratio
6
20
5
Centered
Open
Increasing Pt K / P t L ratio
7
18
5
Centered
Open
Increasing Pt K / P t L ratio
8
16
5
Centered
Open
Decreasing Pt K/WK ratio; Pt L lines no longer visible
9
14
5
Centered
Open
Increasing Pt K / W K ratio
d = distance Ge crystal-optical axis; SP = spot size (current density decreasing with increasing size); OA = objective aperture.
R. Schmidt, M. Feller-Kniepmeier / System-induced background radiation
232
iiiiiii!iiiiiiiii!iiiiiiiii!iiiiiiiiiii!ii' .........................................................
i_.~i..........i_Li ..........i_i_i_L_i_i ..........i,J~_i ..........iH,_i,.3~.~,iii~.,,i .......... .........
........ i.;
iliiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii~~ ....i-61---i--i-+-i--~i--~i,-~i--i--i!!!!!!,+;-~!-~!3;H3!--!-!i~!K~!!!!!!!!
iiiiiiiiiiiiiiiiiiiiiiiiiiiiil
iiiiiiiiiil
"¢'FS
0. 0 0 0
=
128
81
-~20
184
Fig. 5. Spectrum for condition 1 of table 1 (d = 236 mm, fig. 10).
tor with increasing energy the I(W K)/I(WL) ratio is only 0.089 after background subtraction (for comparison: I(Ni K ) / I ( N i L ) ~ 1.3). In order to find out the origin of the W K radiation in the spectrum of the bremsstrahlung the characteristic Pt radiation of the inserted objective aperture was generated without any sam-
...............................................................
ple. In addition, the distance d of the Ge crystal to the optical axis of the microscope was varied from retracted ( d = 2 3 6 mm) to the inserted standard operation position (d = 14 mm). The experimental set-up is shown in fig. 4. By using a suitable choice of the spot size (SP) and different centering of the inserted objective aperture (OA),
r-.~.-.-.~-.-~.---.~
.
.
.
.
.
..... i-i~i.............................. iii! iiiiiiiiiiii!iiiiii~p~+~i~
iii!!iiiiiiiiiii!!iiiii!iii!ip~+~
"...
.....
...............
.......... ?i__?.i.~.
...i...i...!..i...i...i..i...i...i..i. i i i i i i i i i i ii~4::iS~ii iiP~4K~.~,~ FE#K ~#LI i!i!iiiiiii!!i~iiii!iiiiiiiiiliiiiiii ,, ~ ~, ~, ~ , ~ ,., ~ . ~
0000
IV'F --~' =
128
111
Fig. 6. Spectrum for condition 4 of table l ( d = 40 ram, fig. 10).
233
R. Schmidt, M. Feller-Kniepmeier / System-induced background radiation
~'T~g~,i !: i ii ! i i i i !i ii i iii i! ! !i il i ! i i i i i: iil iil iii ii]
•..
•i:i !:ii-i ' ."i
0 ~0
VFS
: 25~
8i.92~
iii
Fig. 7. Spectrum for condition 5 of table 1 (d = 30 mm, fig. 10).
standard operation conditions of about 40% dead time were achieved. The different conditions used and the system response are listed in table 1. The corresponding spectra are shown in figs. 5-9. The intensity ratios I(W K)/noise, I(Pt K)/I(WK) and I(PtK)/I(PtL) are plotted in fig. 10 as function of the distance d of the crystal to the optical axis of the microscope.
3. Discussion
The disappearance of L and M fines of the heavy elements W and Pt under condition 9 (fig. 10) can be explained by the energy dependence of mass absorption in tungsten, which is used as the detector's collimator material. Assuming that background radiation generates the characteristic
iiii!iiiii!iiiiiiiiiiiiiiiiiiiiiii!iiiiiii iiii!!iiii!!!iiiiiiii!iiiiiiii!iiiiiiiiiii
i i TiTiiiiiiiii++!+iiiiiiiiiJiiJili] ...................
.000
iF'~ i i i
i ......................................................................................
VFS
= 128
122 Fig. 8. Spectrum for condition 7 of table 1 (d = 1 8 m m , fig.I0).
~:i . 9 2 0
R. Schmidt, M. Feller-Kniepmeier / System-induced background radiation
234
! .....
. ..... ~'~- -~- L:~1
........... i ......... i ......... !.../,. k J _ .::, J _ .:: ..5...:: ..~ ............. :: j_/. .....
i
"
•
0000
VFS
:
=
.<
.
~4
81
920
125
Fig. 9. S p e c t r u m for c o n d i t i o n 9 of table 1 ( d = 14 mm, fig. 10).
X-rays of tungsten on the outer surface of the collimator, their decrease in intensity is given by [2]:
pt}.
I = I 0 exp{ [ - f t / o ( A , B)]
(1)
Here tx/p(A, B) is the mass absorption coefficient of characteristic X-rays of element A in the absorbing material B with density P and thickness t. The different absorption of A1K~, WL~I and WK~1 radiation in tungsten is shown in fig. 11. Given a
...-%.
\
±0.00
\ / , I (PTK)/I (WK)
8.00
0 H I'-IT >I-I--I (/3 W Z H
~.00
i
4.00
2.00
~ •
0.00 0.
' ' O0
J
, 2~.
O0
,
~ I , , 50. O0
,
r I , , , , ] , , , , ] , , , , I , , , , I , , , , i , , , , I , , 75. O0 100. O0 ~25. O0 ~LSO. O0 t7~. O0 200. O0 =>25. O0
DISTANCE
O
(MM)
Fig. 10. I n t e n s i t y ratios l ( W K ) / n o i s e , I(PtK)/I(WK) a n d I(PtK)/I(PtL) as function of the d i s t a n c e d of the crystal to the optical axis of the microscope.
R. Schmidt, M. Feller-Kniepmeier / System-induced background radiation
~.00.O0F SO.O0
\ -°-°° i- \
I- \ oo.oo \ ,o.oo \ oo.oo 0 . O0
~ . O0
THICKNESS
O
'10.00
(MICRONS)
Fig. 11. Decrease of intensity of WK,~I,WE,,] and A1K,,1X-rays in tungsten with layer thickness t.
layer thickness t of 10 btm the decrease is about 5% for WK~1 radiation and 95% for the WL~1 intensity. The A1K~] intensity is completely absorbed after 2/zm penetration into tungsten. From eq. (1) the minimum collimator thickness trnin can be determined for a given I / I o ratio. Using this ratio as the detectable minimum mass fraction (MMF), table 2 summarizes tmin for different Xrays and MMF's. Mass absorption coefficients were taken from Zschornack [3]. In today's thin film microanalysis a M M F of 0.1-0.01 wt% is attainable [4]. In order to keep the absorption quality of the collimator constant using W K lines instead of W E lines for the analysis the
235
thickness of the W collimator has to be increased by a factor 40 or even 60 in the case of rhenium (Re) X-rays (table 2). The alternative use of other absorber materials (gold or lead instead of tungsten) does not improve absorption characteristics. Up to now the thickness of the collimator is still dimensioned in such a way that low-energy radiation is absorbed. High-energy radiation of heavy elements, however, can pass the collimator and be detected by the crystal. This is affirmed by the absence of W E radiation in any of the experiments above. A tungsten source in the column of the microscope can be excluded from the experiment under condition 1 (closed valve). The Pt radiation generated from the objective aperture is completely absorbed under condition 9 except for the K lines. Because this Pt source is positioned outside of the detector system and below the normal object plane, Pt k lines can be observed under conditions 4, 5, 6 and 7(18 < d < 40 mm, fig. 10). Changing the geometrical condition of the experiment by varying the distance d, the intensity ratio P t K / P t L is changing also: With maximum solid angle ~max given by the X-ray source and crystal area (condition 5, fig. 4), absorption of the Pt L and Pt M lines by the system as well as the ratio Pt K/Pt L have a minimum. At the same time the P t K / W K ratio has a maximum ( d = 25mm, fig. 10). With ~2 minimized, absorption has a maximum and only the high-energy lines can be detected (condition 1 and 9, d < 18 mm, d > 40 mm, fig. 10).
Table 2 Minimum collimator thickness tmin as function of X-ray energy and absorbing material for a given minimum mass fraction (MMF) Emitter
Energy (keV)
Absorber
#/O (cmZ/g)
train (/~m) MMF 0.1
A1K~I We,,1 WK,~I ReK/~I WK,,1 WK~,I
1.487 8.398 59.318 69.310 59.318 59.318
W W W W Pb Au
1920 154 3.8 2.57 5.06 4.63
0.6 7.8 314.6 465.2 235.5 257.4
0.01 1.2 15.5 629.2 930.4 471.1 514.8
0.001 1.9 23.3 943.8 1395.6 706.6 772.2
236
R. Schmidt, M. Feller-Kniepmeier / System-induced background radiation
4. Conclusion
References
Following the discussion of current problems in X-ray microanalysis [4,5] this paper gives an example for system-induced background radiation using a H P G e detector for the analysis of characteristic X-rays in the energy range of 0 - 8 0 keV. In order to keep the absorption qualities of the detector's collimator when analysing high-energy X-rays, a significant increase in thickness of the shielding parts of the collimator is necessary.
[1] C.E. Lyman, D.B. Williams and J.I. Goldstein, Ultramicroscopy 28 (1989) 137. [2] K.F.J. Heinrich, Electron Beam X-ray Microanalysis (VNR Company, New York, 1981). [3] G. Zschornack, Atomdaten fiir die R6ntgenspektralanalyse (Springer, Berlin, 1989). [41 J.I. Goldstein, D.B. Williams and C.E. Lyman, Ultramicroscopy 28 (1989) 56. [5] J.N. Chapman, Ultramicroscopy 28 (1989) 76.