X-RAY SPECTROMETRY, VOL. 22, 349-351 (1993)

Sensitive Method for the Determination of Rare Earth Elements by Radioisotope-Excited XRF Employing a High-Purity Germanium Detector in Optimized Geometry

Madan Lal, H. N. Bajpal,* Daisy Joseph and P. K. Patra Nuclear Physics Division, Bhabha Atomic Research Centre, Bombay-85, India

A close-coupled side-source geometrical configuration is proposed for obtaining a high detection sensitivity for rare earth elements (57 < Z < 69) by radioisotopexcited energy-dispersive x-ray fluorescence spectrometry. In this configuration a disc source of '41Am (100 mCi), a high-purity germanium detector and thin samples of rare earth elements on a Mylar backing are employed in an optimized geometry to achieve detection limits in the range 20-50 ng for these elements in a counting time of 1 h.

INTRODUCTION

Rapid and sensitive methods for the determination of trace impurities of rare earth elements in nuclear fuels are limited,'-' although their determination is becom- ing increasingly important for the characterization of nuclear materials. In x-ray emission methods, excitation of rare earth elements has been carried out either with photon sources6-'0 or with charged particles.' 'J Since rare earth elements occur in natural ores as mixture^'^ of a number of these elements, it is difficult to resolve the complex L x-ray spectra. Recently, using energy- dispersive x-ray fluorescence (EDXRF) detection, some work has been reported**'0,'2 by employing K x-ray spectra excited by either photons or charged particles. By employing K x-ray detection with a Si(Li) detector and excitation of rare earth elements by a disc source of 241Am (100 mCi), detection limits in the range 100-300 ng were obtained'O for the rare earth elements of 57 < Z < 69, which were then found to be most sensi- tive.I4 By employing a high-purity germanium (HPGe) detector and by minimizing various contributions to the background, these detection limits have been further improved to 20-50 ng for the elements of 57 < 2 < 69.

EXPERIMENTAL

The x-ray spectrometer consisted of an HPGe detector of 100 mm2 x 10 mm size, giving an energy resolution of 170 eV for 5.9 keV Mn Ka x-rays. A Si(Li) detector of 30 mm2 x 3 mm size and of the same energy resolution was employed to compare the detection sensitivities of the two types of detectors for rare earth elements. The geometrical configurations of the disc-type side source and annular source of 241Am, each of 100 mCi, are

* Analytical Chemistry Division.

shown in Fig. 1(A) and (B), respectively. A cylindrical shield of 5 mm thickness with a central hole of 6 mm diameter with a collar to shield the detector from direct radiation from the 241Am source was mounted on the end-cap of the detectors. Samples of rare earth mixtures were prepared in solution, transferred by micropipette and dried on a thin Mylar sheet of a sample holder. The distance between the sample and the source-detector configuration was optimized to obtain the best signal- to-background ratio.

RESULTS AND DISCUSSION

The advantage of a side-source geometry over the annular geometry" is the shifts of the Compton- scattered peak and its tailing profile away from the x-ray peaks of the rare-earth elements and the Compton-scattered background intensity is minimized by using a scattering angle of 90" in the side-source geometry. Since L x-rays of materials of high atomic number do not interfere with the K x-ray region of rare earth elements, the scattered intensity is minimized by employing high-Z material such as lead in the collar of the central hole of the shield, and the scatter back- ground from its walls to the detector is reduced. Owing to the superior detection efficiency of the HPGe detec- tor over the Si(Li) detector for K x-rays of rare earth elements, the higher count rate performance of the former provides a better detection sensitivity. Further, by shielding the outer part of the sensitive region of the HPGe detector by 2.0 mm, the incomplete charge col- lection from the peripheral region is reduced, thereby giving less tailing background. These factors contrib- uted to obtaining a better signal-to-background ratio, resulting in improved detection limits for the rare earth elements.

Figure 2 shows x-ray spectra of a thin sample con- taining Ce, Dy and Tb mounted in both annular and

0049-8246/93/050349-03 $06.50 0 1993 by John Wiley & Sons, Ltd.

Received 5 August 1992 Accepted 13 November 1992

350

0

M. LAL, H. N. BAJPAI, D. JOSEPH AND P. K. PATRA

. . 1 -. :..- .C.

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SH IELD ( 6 . 0 m m

thick 1

signals

to pre amp

(2.0m.d)

HP Ge Detector

(100 m m 2 x lomm)

Figure 1 . Schematic diagrams of (A) side-source and (6) annular source geometries.

side-source geometries of the 241Am source for the same counting time of 1 h using the HPGe detector. The superiority of the x-ray spectra with regard to the peak- to-background ratio with the side-source geometry can be clearly seen. Figure 3 shows the x-ray spectra of the same sample obtained in side-source geometry with both the Si(Li) and HPGe detectors in order to compare their performances. Here also the superiority

of the performance of the HPGe over the Si(Li) detector with respect to peak-to-background ratio can be clearly seen, in spite of the fact that the background originating from sources other than the exciting source is greater for the HPGe than the Si(Li) detector.

Figure 4 shows the detection limits obtained for the rare earth elements of 57 < Z < 69 with the HPGe and Si(Li) detectors in a side-source geometry with a

t 1000

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0

2 000

1500

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500

:

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Figure 2. X-ray spectra obtained for a Mylar-backed sample (Ce, Tb, Dy) with the HPGe detector using (A) annular source and (6) side-source geometries.

1500 t 7 1000

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1500 I

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Channel No - Figure 3. X-ray spectra obtained for the same Mylar-backed sample (Ce, Tb, Dy) in side-source geometry using (A) Si(Li) and (6) H PGe detectors.

351 DETERMINATION OF RARE EARTH ELEMENTS BY RADIOISOTOPE-EXCITED XRF

2 Table 1. Detection limits (ng)”

180

160 d S i ( L i ) Detector

B HP Ge Detector

d -I 100 n

= 80 A

6 o t J

58 60 62 6 4 6 6 68 70

ATOMIC NO Figure 4. Detection limits obtained for elements of 57 62 < 69 for Mylar-backed thin samples in the optimum geometry using (A) Si(Li) and (0) HPGe detectors.

counting time of 1 h. Table 1 gives the detection limits for each of the rare earth elements in annular and side- source geometries for the Si(Li) and HPGe detectors with a counting time of 1 h.

Annular geometry Side-source geometry

Si(Li) HPGe Si(Li) HPGe Z Element detector detector detector detector

57 La 58 Ce 60 Nd 62 Sm 63 Eu 65 Tb 66 Dy 67 Ho 69 Tm

38 36 34 36 76 64 67 95

161

27 30 20 25 30 20 28 52 39

77.5 105 110 185 225 345 400 NDb ND

55 57.5 72.5 82.5

52.5 82.5 ND ND

110

a Detection limit = m, where N , = No. of counts in the background and N,=No. of counts under the peak per unit concentration.

ND = not detected.

CONCLUSION

A high detection sensitivity of 20-50 ng for the rare earth elements can be achieved by exciting with a 241Am disc source and an HPGe detector employed in a configuration with a close-coupled side-source geometry. The scattered background can be minimized by employing high-2 materials such as lead in the colli- mator and optimizing the source, sample and detector mounting configuration.

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