Long-lived standards for the efficiency calibration of Ge(Li) detectors

NUCLEAR INSTRUMENTS AND METHODS 99 (I972) 333-337; © NORTH-HOLLAND PUBLISHING CO.

L O N G - L I V E D STANDARDS F O R T H E E F F I C I E N C Y C A L I B R A T I O N OF Ge(Li) D E T E C T O R S * I. AHMAD and M. WAHLGREN Chem&try Div&ion, Argonne National Laboratory, Argonne, Illinois 60439, U.S.A.

Received 30 September 1971 The intensities of the ~-rays and K X-rays associated with the decay of 249Cf, 243Am, 239Np and 243Cm were measured with respect to IAEA standards. Because of their long half-lives and

ease of standardization by accurate alpha counting, these nuclides are recommended as standards for absolute efficiency calibration of Ge(Li) detectors.

1. Introduction

2. Experimental techniques

The set of standards most commonly used for the determination of photopeak efficiency of Ge(Li) detectors in the low energy region (below 500 keV) consists of 24JAm (432.9 y), 57Co (271.6 d), 2°3Hg (46.8 d) and 11aSn (115.2 d). Because of their short half-lives 57Co, 2°3Hg and 1135n are not suitable for long-term use. Recently the use of 133Ba (7.8 y) and 75Se (120.4 d) as calibration standards has been proposedL2). However, these sources can only be used for relative efficiency measurements. In the present paper we propose the use of 249Cf (:352 y) and 243Am (7.95 x 103 y) for the absolute efficiency calibration of Ge(Li) detectors. The nuclide 249Cf decays to levels in 245Cm by ~-particle emission, which then de-excites by 252.7, 333.3 and 388.1 keV ~-rays and Cm K X-rays. Two prominent ~-rays of energy 43.53 and 74.67 keV follow the c~-decay of 2'~3Am. The daughter 239Np (2.35 d), which is present in equilibrium with 24aAm, decays through 106.14, 209.76, 228.20, 277.62, 285.47, 315.91 and 334.33 keV 7-rays and Pu K X-rays. The nuclides 249Cf and 243Am are produced in high flux reactors by the United States Atomic Energy Commission. The 249Cf sample can be obtained with an isotopic purity of > 9 9 % . The 243Am source material usually contains small amounts of 241A m and is available with an isotopic purity of > 9 7 % . In the present paper we also report measurements on 2¢3Cm ~:-rays. This nuclide is produced in reactors along with other Cm isotopes. Thus a Cm sample without any isotopic separation can only be used for relative efficiency measurements.

2.1. SOURCEPREPARATION Samples of 249Cf, 24aAm and 243Cm which were available as a part of the heavy element production program, were chemically purified by the usual ionexchange techniques3"4). The purified material was then used to prepare thin samples. Samples of 243Cm and 243Am were made in the Argonne Isotope Separator and the samples of 249Cf were prepared by vacuum volatilization. Two samples of each nuclide were prepared and used for the present measurements. The source backing for the samples was either a 200 pg/cm 2 A1 or 15 mg/cm 2 A1. The areas of the source deposit were between 0.5 cm 2 and 1 cm 2.

* Work performed under the auspices of the U.S. Atomic Energy Commission. 333

2.2. ALPHA-PARTICLECOUNTING The a-particles associated with the decay of the samples were counted in a low geometry counter using a parallel plate ionization chamber for the detection of a-particles. The counting geometry of the system was calculated from the measured diameter of the aperture (20.0106 + 0.0002 mm) and the distance of the aperture from the source (229.880+0.003 mm). The geometry was calculated for each source size. The error due to the source deposit being off axis was estimated to be < 0.2%. The s-particle spectrum of each sample was measured with a 6 m m diameter semiconductor detector to determine the isotopic purity of the sample. Only one 249Cf source was found to have any impurity; it contained 2.3% of 25°Cf by activity. 2.3. GAMMA-RAYSPECTROSCOPY The absolute effciencies of two co-axial Ge(Li) detectors (active volume 4 c m 3 and 25 cm 3) were measured with standards supplied by IAEA, Vienna. Two sets of standards were obtained, one made on January 1, 1969 and the other on January 1, 1970. All

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EFFICIENCY CALIBRATION OF Ge(Li) DETECTORS m e a s u r e m e n t s were m a d e in the m o n t h o f J a n u a r y , 1970 in o r d e r to a v o i d decay corrections to one set o f standards. T h e efficiencies o f the two detectors were m e a s u r e d by placing the s t a n d a r d s in a fixed, r e p r o ducible position, 10.5 cm a w a y f r o m the d e t e c t o r end cap. T h e efficiencies o b t a i n e d with the two sets o f s t a n d a r d s agreed within 1%. T h e accuracy o f these s t a n d a r d s was further checked b y using an 241Am source m a d e in this l a b o r a t o r y . The s a m p l e was p r e p a r e d by v a c u u m volatilization, a n d its disinteg r a t i o n rate was m e a s u r e d with the low g e o m e t r y counter. The efficiencies o f the 25 cm 3 Ge(Li) spectrom e t e r at 59.54 keV as m e a s u r e d with the two I A E A s t a n d a r d s a n d o u r s t a n d a r d were f o u n d to be 0.593%, 0.587% a n d 0.596%, respectively. T h e excellent agreem e n t between the three values shows the overall consistency o f o u r measurements. The a b s o l u t e efficiency curves o f the two Ge(Li) detectors are shown in fig. 1. A l s o included in the curves are relative efficiency p o i n t s o b t a i n e d with a ~6°Tb sourceS). 3. R e s u l t s

The y-ray spectra o f 249Cf, 24SAm a n d 243Cm samples were m e a s u r e d by p l a c i n g t h e m at exactly the same g e o m e t r y as the I A E A s t a n d a r d s . The spectra were m e a s u r e d with b o t h Ge(Li) detectors a n d analyzed b o t h b y a c o m p u t e r p r o g r a m a n d also b y h a n d plotting. T h e spectra m e a s u r e d with the 25 cm a Ge(Li) s p e c t r o m e t e r are shown in figs. 2-4. The intensities o f the y-rays in terms o f p h o t o n s per 100 a-decays are given in tables 1-3. As 239Np was in equilibrium with the parent,

335

243Am, the n u m b e r o f e-disintegration p e r m i n u t e was used to calculate the intensities o f 239Np y-rays. The energies o f 243Am, 239Np a n d 243Cm y-rays a n d Pu K X-rays, given in the figures a n d tables are t a k e n f r o m refs. 6-9. The 249 C f y-ray energies shown in fig. 2 a n d table 1 are those m e a s u r e d in the present work. The e r r o r in the intensity includes all sources o f error: e r r o r in e-counting, e r r o r in p h o t o p e a k area, e r r o r in TABLE 2 24SAm-239Np y-rays.

Energy (keV)

Intensity (Photons per 100 g-decays)

43.531 74.673 99.536 103.750 106.14 117.1 120.6 181.71 209.76 228.20 254.41 272.87 277.62 285.47 315.91 334.33

5.5 4-0.3 66.0 4- 3.0 14.5 -t-0.6 22.2 -I-0.8 27.8 5:0.9 8.9 4-0.4 2.77 +0.1 0.075 5:0.008 3.42 5:0.10 11.4 5:0.3 0.11 4-0.01 0.08 5:0.01 14.5 5:0.4 0.76 4-0.02 1.52 4-0.05 1.95 4-0.07

Remarks

Pu K~2 Pu K~I Pu Ks1 + 117.8

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e4SCm F-rays. TABLE 1 249Cf y-rays.

Energy (keV)

54.84-0.1 92.54-0.1 104.65:0.1 109.34-0.1 123.44-0.2 127.14-0.2 240.8 4-0.2 252.74-0.1 266.65:0.1 295.7 4-0.2 321.3 5:0.2 333.34-0.1 388.14-0.1

Intensity (Photons per 100 ~-decays)

0.1974-0.009 0.34 4-0.01 2.36 4-0.08 3.72 5:0.12 1.40 4-0.06 0.49 5:0.03 0.22 4-0.02 2.59 4-0.08 0.72 4-0.03 0.143 4-0.009 0.062 4- 0.006 14.6 4-0.5 66.0 4-2.0

Energy (keV)

Intensity (Photons per 100 a-decays)

57.26 99.536 103.750 117.1 120.6 209.76 228.20 254.41 272.87 277.62 285.47 311.7 315.91 322.3 334.33

0.14 4-0.01 13.5 4-0.5 20.8 4- 0.8 7.6 5:0.3 2.6 +0.1 3.30 4-0.10 10.6 4- 0.3 0.11 +0.01 0.08 4-0.01 14.0 4-0.4 0.73 +0.02 0.017 4-0.002 0.0184-0.002 0.007 4-0.001 0.024 4-0.002

Remarks

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EFFICIENCY CALIBRATION OF Ge(Li) DETECTORS tlhe s t a n d a r d itself, a n d the e r r o r in the e x t r a p o l a t i o n o f the absolute efficiency,

4. Comments In the case o f 243Am-239Np y-ray spectrum, care slhould be t a k e n to avoid the s u m m i n g o f the 106.14 keV y-ray with the Pu K X - r a y s a n d the 209.76 a n d 228.20 keV y-rays. W h e n the source is placed close to the detector, the c o n t r i b u t i o n o f these sum p e a k s to the 209.76, 228.20, 315.91 a n d 334.33 keV p e a k s should be t a k e n into account. A l s o there is a w e a k 226.4 keV y-ray in the decay o f 239Np. W i t h the resolution o f o u r Ge(Li) s p e c t r o m e t e r s this p e a k is n o t resolved f r o m the 228.20 keV peak. Hence, the intensity o f the 228.2 keV-y ray in table 2 represents the sum o f the individual-y rays. The a u t h o r s wish to t h a n k J. L e r n e r for the isotopic

337

s e p a r a t i o n o f A m a n d C m samples a n d W. C. Bentley for the assistance in the a - c o u n t i n g o f the samples.

References 1) y. Gurfinkel and N. Notea, Nucl. Instr. and Meth. 57 (1967) 173. 8) T. S. Nagpal and R. E. Gaucher, Nucl. Instr. and Meth. 89 (1970) 311. 8) G. R. Choppin, B. G. Harvey and S. G. Thompson, J. Inorg. Nucl. Chem. 2 (1956) 66. 4) L. Phillips and R. Gatti, unpublished data referred to in U.S. Atomic Energy Commission NAS-NS 3031 (1960) p. 13. 5) T. Yamazaki and J. M. Hollander, Nucl. Phys. 84 (1966) 505. 6) K. J. Blinowska, P. G. Hansen, H. L. Nielsen, G. Schult and K. Wien, Nucl. Phys. 55 (1964) 331. 7) G.T. Ewan, J.S. Geiger, R.L. Graham and D.R. MacKenzie, Phys. Rev. 116 (1959) 950. 8) B. P. Maier, Z. Physik 184 (1965) 143. 9) G. C. Nelson, B. G. Saunders and W. John, Phys. Rev. 188 (1969) 4.