DETECTORS

This article was downloaded by: [Ataturk University] On: 13 March 2014, At: 13:26 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK

Instrumentation Science & Technology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/list20

X-RAY FLUORESCENCE ESCAPE PEAKS IN Ge(Li) DETECTORS a

Ridvan Durak & Yüksel Özdemir

a

a

Atatürk Üniversitesi , Fen-Edebiyat Fakültesi, Fizik Bölümü, Erzurum, 25240, Turkey Published online: 16 Aug 2006.

To cite this article: Ridvan Durak & Yüksel Özdemir (2001) X-RAY FLUORESCENCE ESCAPE PEAKS IN Ge(Li) DETECTORS, Instrumentation Science & Technology, 29:3, 185-192, DOI: 10.1081/CI-100103465 To link to this article: http://dx.doi.org/10.1081/CI-100103465

PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions

INSTRUMENTATION SCIENCE & TECHNOLOGY, 29(3), 185–192 (2001)

Downloaded by [Ataturk University] at 13:26 13 March 2014

X-RAY FLUORESCENCE ESCAPE PEAKS IN Ge(Li) DETECTORS

Ridvan Durak* and Yüksel Özdemir Atatürk Üniversitesi, Fen-Edebiyat Fakültesi, Fizik Bölümü, 25240 Erzurum, Turkey

ABSTRACT The escape peak-to-photopeak intensity ratios and escape fractions of characteristic K X-rays, as a function of incident radiation energy in the range 15.75-50 keV and collimator diameter (or area seen by the crystal ), are measured for a Ge(Li) detector. The experimental results are compared with theoretical predictions.

INTRODUCTION Escape peaks result from the escape of characteristic X-rays of any shell of the detector material, e.g., K-shell, from the detector after photoelectric absorption of the impinging X-ray photon near the absorption edge regions of the detector. Because of this process, the energy deposited in the detector by the incoming X-ray is diminished with the energy of the K photon of the detector material. Thus, escape peaks appear in the low energy side with respect to the parent peak by the value of the characteristic X-ray energy of the detector material; they are often confused with fluorescence peaks of the sample or other scattered peaks. In the analysis of γ and X-ray spectra, determination of accurate detector response function and photopeak efficiency, corrections due to escaping of detector characteristic X-rays must be taken into account. Several studies on the CdTe, *Corresponding author. 185 Copyright © 2001 by Marcel Dekker, Inc.

www.dekker.com

186

DURAK AND ÖZDEMIR

HgI2, Si(Li), Ge(Li), NaI escape peaks have been performed.1-7 In this study, incident radiation energy and region area seen by the crystal dependence of fluorescence escape peak-to-photopeak intensity ratios and escape fractions of characteristic K X-rays were measured for the Ge(Li) detector. The experimental results are compared with theoretical predictions.

Downloaded by [Ataturk University] at 13:26 13 March 2014

EXPERIMENTAL The experimental arrangement and the geometry employed for the measurements is shown in Figure 1. In this arrangement, the excitation annular-source is either 57Co (100 mCi) or 241Am (100 mCi), depending on the element to be measured. Fluorescence X-rays were detected with a Ge(Li) detector (active diameter 10 mm, sensitive depth 5 mm, Be window thickness 0.13 mm) with an energy resolution of 190 eV at 5.9 keV. High purity (99.9 %) circular disc samples of thickness from 16 to 102 mg cm-2 were used for the preparation of the targets. All spectra contained 105 total counts in the Kα line in order to obtain good statistical definition of the escape peaks. The photopeak energies are given in Tables 1 and 2, together with the energies of the main escape peaks and the measured and theoretical values of escape peak-to-photopeak intensity ratios kα and kβ. The photopeak areas were cor-

Figure 1.

The source and Ge(Li) detector geometry.

Downloaded by [Ataturk University] at 13:26 13 March 2014

X-RAY FLUORESCENCE ESCAPE PEAKS

Figure 2.

187

An X-ray spectrum of gadolinium taken with a Ge(Li) detector.

rected for self-absorption and photopeak efficiency of the present system. Experimental intensity ratios are determined by (1)

where N(Ki)esc and N(Ki)ph correspond to the net counts under the escape and photopeak, respectively, T(Ki)ph is the self-absorbtion correction factor of target material, given by Table 1.

Measured and Calculated Values of the Escape Peak to Photopeak Ratios k for Kα Lines

Kα Photo Peak (keV)a (%)

Kα Escape Peak (keV)

Measured k (%)

Calculated k

Zr (15.75) Mo (17.44) Ag (22.10) In (24.14) Sn (25.19) Ba (32.06) Ce (34.56) Gd (42.75) Dy (45.71) Er (48.80) Yb (52.01)

Zr Kα-Ge Kα (5.87) Mo Kα-Ge Kα (7.57) Ag Kα-Ge Kα (12.23) In Kα-Ge Kα (14.26) Sn Kα-Ge Kα (15.31) Ba Kα-Ge Kα (22.18) Ce Kα-Ge Kα (24.69) Gd Kα-Ge Kα (32.87) Dy Kα-Ge Kα (35.83) Er Kα-Ge Kα (38.92) Yb Kα-Ge Kα (42.14)

13.05±0.91 10.93±0.78 6.51±0.35 5.50±0.26 6.70±0.33 2.60±0.13 2.40±0.14 1.70±0.11 1.29±0.08 1.14±0.06 1.08±0.05

12.10 10.40 6.87 5.76 5.30 3.00 2.54 1.50 1.24 1.04 0.88

a

Ref.12

188

Downloaded by [Ataturk University] at 13:26 13 March 2014

Table 2.

DURAK AND ÖZDEMIR Measured and Calculated Values of the Escape Peak to Photopeak Ratios k for Kβ Lines

Kβ Photo Peak (keV)b (%)

Kβ Escape Peak (keV)

Measured k (%)

Calculated k

Zr (17.70) Mo (19.65) Ag (25.00) In (27.35) Sn (28.57) Ba (36.53) Ce (39.43) Gd (48.92) Dy (52.35) Er (55.93) Yb (59.65)

Zr Kβ-Ge Kβ (6.72) Mo Kβ-Ge Kβ (8.67) Ag Kβ-Ge Kβ (14.02) In Kβ-Ge Kβ (16.37) Sn Kβ-Ge Kβ (17.59) Ba Kβ-Ge Kβ (25.55) Ce Kβ-Ge Kβ (28.45) Gd Kβ-Ge Kβ (37.94) Dy Kβ-Ge Kβ (41.37) Er Kβ-Ge Kβ (44.95) Yb Kβ-Ge Kβ (48.67)

12.14±0.60 10.51±0.46 6.12±0.25 5.97±0.31 5.27±0.32 2.60±0.22 2.40±0.08 1.16±0.13 -a -

11.80 10.08 6.54 5.45 4.98 2.80 2.32 1.33 1.12 0.94 0.79

a

The escape peaks of the elements couldn’t be measured due to poor statistics in the regions. Ref.12

b

(2) where µex and µe are total mass absorption coefficients of sample for the exciting radiation and the emitted characteristic X-rays, respectively.8 The incident and emission angles with respect to the sample normal, θ ex and θe, were set to 45° and 0° and m is the mass of the target in g cm-2. ε(Ki)ph the detector efficiency values for K X-rays. Measured and calculated k intensity ratios are listed in Tables 1 and 2 and also shown graphically in Figures 3a and 3b. For photons normally incident on the detector of infinite plane the fraction of the emitted Ge K X-rays that escape from the detector (The mean free path of the characteristic K-shell X-rays in Ge is ~0.05 mm, so that escape is probable only for X-rays produced very near a surface (entrance face). Therefore, escape of the X-rays through the sides and back face is negligible, and the detector can be considered a infinite plane.) can be calculated using the equation9

(3)

where µ and µGe are the mass attenuation coefficients of germanium for impinging radiation and for Ge K X-rays (EKα=9.88 keV , EKβ=10.99 keV), respectively, ωK is the K-shell fluorescence yield of germanium,10 r is the K absorption jump ratio of germanium and calculated by11

189

Downloaded by [Ataturk University] at 13:26 13 March 2014

X-RAY FLUORESCENCE ESCAPE PEAKS

Figure 3.

Intensity of escape peak with respect to that of the photopeak.

rK + 1.754×101 – 6.608×10⫺1Z + 1.427×10⫺2Z2 ⫺ 1.1×10⫺4Z3

(4)

The measured f values are compared with the values calculated by using ωK= 0.5464 and r = 7.4024 in Table 3 and also plotted in Figures 4 a, b. To explain escape peak-to-photopeak intensity ratios, k and escape fractions f dependence on collimation, cylindrical lead collimators with 2-9 mm diameter were used in front of the detector. The results are presented in Table 4 and shown in Figure 5.

DURAK AND ÖZDEMIR

Downloaded by [Ataturk University] at 13:26 13 March 2014

190

Figure 4.

Energy dependence of the escape fractions.

CONCLUSIONS The errors in the experimental escape peak-to-photopeak intensity ratios and escape probabilities of characteristic K X-rays are estimated to be 4-13% and 415% except for Sn and Yb, respectively. This error arises from uncertainties in the various parameters used to calculate the k and f, including errors due to peak area evaluation, the detector efficiency, target thickness measurements, and absorption correction factor. From Tables 1 and 2, it can be observed that the present data are

Table 3.

Element

Downloaded by [Ataturk University] at 13:26 13 March 2014

Zr Mo Ag In Sn Ba Ce Gd Dy Er Yb

Measured and Calculated Values of the Escape Fraction f for Kα and Kβ Lines Kα Escape Fraction fα (%) Experimental Calculated 11.54±0.36 9.85±0.34 6.11±0.24 5.21±0.20 6.27±0.25 2.53±0.11 2.34±0.09 1.67±0.07 1.27±0.08 1.13±0.08 1.07±0.07

10.80 9.42 6.43 5.45 5.02 3.00 2.50 1.46 1.22 1.03 0.87

Kβ Escape Fraction fβ (%) Experimental Calculated 10.82±0.43 9.51±0.36 5.77±0.26 5.63±0.26 5.00±0.25 2.53±0.18 2.34±0.13 1.14±0.15 -

10.56 9.16 6.14 5.17 4.74 2.73 2.27 1.32 1.11 0.93 0.80

Table 4. The Escape Peak to Photopeak Intensity Ratios and Escape Fractions of Characteristic K X-rays for Collimator Diameters Diameter (mm) 2 3 4 5 6 7 8 9

kα (%)

kβ (%)

fα (%)

fβ (%)

2.87±0.14 2.74±0.11 2.68±0.09 2.19±0.13 2.28±0.09 2.49±0.11 2.22±0.06 2.28±0.08

2.53±0.10 2.41±0.09 1.75±0.06 1.67±0.06 1.70±0.08 1.39±0.06 1.96±0.06 2.04±0.06

2.79±0.07 2.67±0.13 2.61±0.10 2.14±0.11 2.23±0.08 2.43±0.07 2.18±0.08 2.23±0.08

2.46±0.09 2.35±0.10 1.72±0.06 1.64±0.07 1.67±0.06 1.37±0.05 1.93±0.07 2.00±0.06

Figure 5. The experimental values of the escape/photopeak ratio and the escape fraction as a function of area seen by the detector.

192

DURAK AND ÖZDEMIR

Downloaded by [Ataturk University] at 13:26 13 March 2014

in reasonable agreement, within the experimental uncertainties, with the theoretical predictions. k and f decrease with increasing incident X-ray energy and atomic number. The k and f ratios are