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169Yb and 110Agm as efficiency calibration standards for Ge(Li) detectors
NUCLEAR INSTRUMENTS AND METHODS
to7 (I973) I97-I98; ©
NORTH-HOLLAND PUBLISHING CO.
169yb A N D n ° A g ~ AS E F F I C I E N C Y C A L I B R A T I O N S T A N D A R D S F O R
Ge(Li) D E T E C T O R S N. LAVI Isotopes Applications Department, Soreq Nuclear Research Centre, Yavne, Israel
Received 21 June 1972 and in revised form 3 October 1972 Accurately measured energies and relative intensities of the gamma- and X-rays emitted in the decay of 169yb and 11°Agm are recommended for use as calibration standards for Ge(Li) detectors. The usual m e t h o d of calibrating a detector for energy a n d intensity m e a s u r e m e n t s of p h o t o n s is to use calibrated sources such as: 24~Am, 2°3Hg, 57C0 a n d aX3Sn in the 50-400 keV range a n d t13Sn, 22Na, 137Cs, 54Mn, 88y and 6°C0 in the 400-1800 keV range. 169yb was f o u n d to be a suitable source for calibrating Ge(Li) detectors in the former range a n d ta°Agm in the latter range. The 169yb nucleus decays via electron capture (EC) with a half-life of 32 d ~) to excited levels of the ~69Tm nucleus. The d e p o p u l a t i o n of the 169Tm levels to the g r o u n d state occurs by nine g a m m a transitions. I n
addition to the g a m m a rays, two relatively intense K~ a n d Ka X-rays of 169Tm are emitted following the EC process. The ll°Agm nucleus decays in two ways: 1.3% goes t h r o u g h isomeric transition a n d 98.7% decays 2) by //-emission with a half-life of 253 d t) to excited levels of ~ ° C d . The d e p o p u l a t i o n of the ~ ° C d levels to the g r o u n d state occurs by m a n y g a m m a transitions, of which a b o u t 15 are of interest in the present work. The relative g a m m a ray intensities of 169yb a n d tl°Agm were measured with several calibrated Ge(Li) detectors. The absolute p h o t o p e a k efficiency curves a n d
TABLE I Energies and relative intensities in the decay of 32 d t69yb and 253 d 11°Agm.The results are in good agreement with those obtained by Wakatl). The errors quoted are one standard deviation.
Photon energy (keV)
169yb Relative intensitya
50.5 4-0.20 Kc¢ 57.5 4-0.20KB 63.124-0.38 93.644-0.16 109.884-0.28 118.284-0.23 130.47 4-0.16 177.144-0.21 198.024-0.09 261.11 4-0.21 307.774-0.28
409 4-24 b 106 4- 6.31b 125.374-6 . 5 8 5.074- 0.16 49.094- 1 . 7 0 3.71 4- 0 . 1 1 29.73 4- 0.87 62.084- 2.02 100 3.704- 0 . 0 9 2 27.974- 0 . 9 0
Photon energy (keV)
110Agm Relative intensity e
446.604-0.20 620.184-0.10 657.704-0.11 677.57 686.714-0.20 706.784-0.20 744.33 4-0.09 763.81 4 - 0 . 1 8 818.254-0.20 884.224-0.08 937.424-0.10 1383.85 4-0.20 1475.424-0.16 1504.65 4 - 0 . 2 9 1561.924-0.20
3.574-0.71 2.794-0.06 100 11.935:0.41 7.25-t-0.33 17.15=1:0.85 4.43 4-0.13 23.734-0.72 7.81 4-0.39 80.28 4-4.01 37.31 4- 1.42 28.26 4- 1.42 4.444-0.16 15.194-0.49 1.404-0.09
a The intensity 198 keY is taken as 100 units. b The intensity ratio Ka/K B is 3.8584-0.38 in agreement with Lederer et al.Z). e The intensity 657.7 keV is taken as 100 units. 197
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Fig. 1. Comparison of the relative photopeak efficiency of Ge(Li) detectors using a 169yb source. Source point distance: 5 cm. (~ 9.75 cm3 true coaxial detector; • 30 cm3 true coaxial detector; [] 60 crn3 true coaxial detector. the energy calibration for these spectra were p r e p a r e d by counting absolutely calibrated standards*. A 4096 channel analyzer (Packard) was used. The energy resolution (fwhm) o f the 60 cm 3 d e t e c t o r t was 1.6 keV for 122 keV 57Co g a m m a rays and 2.6 keV for 1332 keV 6°Co y-rays. The measurements were m a d e at a distance o f 5 cm from the surface o f the detectors. The g a m m a r a y energies and relative intensities o f 169yb and al0Agm are listed in table 1. W i t h the aid o f these values, a Ge(Li) detector can be calibrated with an accuracy o f 0.1-0.2 keV, using the least squares fit method. The relative efficiency o f the detector m a y be f o u n d to an accuracy o f better than 6 - 7 % a n d the absolute efficiency with an accuracy o f better than 10-12%. In the low energy region the efficiency decreases. This m a y be explained by the high a b s o r p tion in the insensitive layer o f the detector a n d the a l u m i n i u m capsule. Three examples o f the use o f 169yb as a calibration s t a n d a r d for high resolution Ge(Li) detectors are given in fig. I. The p h o t o p e a k efficiency o f the 198 keV g a m m a peak is taken as 1000 units. It is seen that for g a m m a energies below 198 keV, the 9.75 cm 3 true coaxial detector is m o r e sensitive than the other two * Supplied by I.A.E.A., Vienna, Austria. t Supplied by Seforad Ltd., Emek Hayarden, Israel.
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Photon energy (keY)
4bo
Fig. 2. Relative photopeak efficiency of a 60 cm3 Ge(Li) detector using a 110Agm source. Source point distance: 5 cm. detectors. The use o f 169yb as a c a l i b r a t i o n source has the a d v a n t a g e o f avoiding the use o f multiple sources, a n d the errors in g e o m e t r y which could result from interchanging them. A n o t h e r a d v a n t a g e o f the proposed source is the ease o f p r e p a r a t i o n . D u e to the high thermal neutron cross section o f 168yb (5500 b)3), very short i r r a d i a t i o n times are enough to o b t a i n highly efficient sources. A n example o f the use o f 1]°Agm as a calibration s t a n d a r d for Ge(Li) detectors is given in fig. 2. The p h o t o p e a k efficiency o f the 657.7 keV g a m m a peak is t a k e n as 1000 units. A straight line is o b t a i n e d by plotting the relative efficiency against the corres p o n d i n g g a m m a ray energy. The i m p o r t a n c e o f H°Agm as a calibration s t a n d a r d for use with Ge(Li) detectors is evident in view o f its 15 g a m m a rays in the range 450-1600 keV and its c o m p a r a t i v e l y long half-life o f 253 dX). A sum p e a k o f 1541.9 keV (657.7 keV + 884.2 keV) is also emitted. There are very few nuclides with g a m m a rays in this energy range a n d with a half-life longer than a h u n d r e d days. As in the case o f 169yb, the need for multiple sources is avoided and the source is also easily prepared.
References 1) M. A. Wakat, Nucl. Data Tables 8 (1971). 2) C. M. Lederer, J. M. Hollander and I. Perlman, Table o f isotopes (6th ed. J. Wiley, New York, 1967). 3) Nucl. data sheets (Academic Press, New York, 1964) p. 6-4-82.
to7 (I973) I97-I98; ©
NORTH-HOLLAND PUBLISHING CO.
169yb A N D n ° A g ~ AS E F F I C I E N C Y C A L I B R A T I O N S T A N D A R D S F O R
Ge(Li) D E T E C T O R S N. LAVI Isotopes Applications Department, Soreq Nuclear Research Centre, Yavne, Israel
Received 21 June 1972 and in revised form 3 October 1972 Accurately measured energies and relative intensities of the gamma- and X-rays emitted in the decay of 169yb and 11°Agm are recommended for use as calibration standards for Ge(Li) detectors. The usual m e t h o d of calibrating a detector for energy a n d intensity m e a s u r e m e n t s of p h o t o n s is to use calibrated sources such as: 24~Am, 2°3Hg, 57C0 a n d aX3Sn in the 50-400 keV range a n d t13Sn, 22Na, 137Cs, 54Mn, 88y and 6°C0 in the 400-1800 keV range. 169yb was f o u n d to be a suitable source for calibrating Ge(Li) detectors in the former range a n d ta°Agm in the latter range. The 169yb nucleus decays via electron capture (EC) with a half-life of 32 d ~) to excited levels of the ~69Tm nucleus. The d e p o p u l a t i o n of the 169Tm levels to the g r o u n d state occurs by nine g a m m a transitions. I n
addition to the g a m m a rays, two relatively intense K~ a n d Ka X-rays of 169Tm are emitted following the EC process. The ll°Agm nucleus decays in two ways: 1.3% goes t h r o u g h isomeric transition a n d 98.7% decays 2) by //-emission with a half-life of 253 d t) to excited levels of ~ ° C d . The d e p o p u l a t i o n of the ~ ° C d levels to the g r o u n d state occurs by m a n y g a m m a transitions, of which a b o u t 15 are of interest in the present work. The relative g a m m a ray intensities of 169yb a n d tl°Agm were measured with several calibrated Ge(Li) detectors. The absolute p h o t o p e a k efficiency curves a n d
TABLE I Energies and relative intensities in the decay of 32 d t69yb and 253 d 11°Agm.The results are in good agreement with those obtained by Wakatl). The errors quoted are one standard deviation.
Photon energy (keV)
169yb Relative intensitya
50.5 4-0.20 Kc¢ 57.5 4-0.20KB 63.124-0.38 93.644-0.16 109.884-0.28 118.284-0.23 130.47 4-0.16 177.144-0.21 198.024-0.09 261.11 4-0.21 307.774-0.28
409 4-24 b 106 4- 6.31b 125.374-6 . 5 8 5.074- 0.16 49.094- 1 . 7 0 3.71 4- 0 . 1 1 29.73 4- 0.87 62.084- 2.02 100 3.704- 0 . 0 9 2 27.974- 0 . 9 0
Photon energy (keV)
110Agm Relative intensity e
446.604-0.20 620.184-0.10 657.704-0.11 677.57 686.714-0.20 706.784-0.20 744.33 4-0.09 763.81 4 - 0 . 1 8 818.254-0.20 884.224-0.08 937.424-0.10 1383.85 4-0.20 1475.424-0.16 1504.65 4 - 0 . 2 9 1561.924-0.20
3.574-0.71 2.794-0.06 100 11.935:0.41 7.25-t-0.33 17.15=1:0.85 4.43 4-0.13 23.734-0.72 7.81 4-0.39 80.28 4-4.01 37.31 4- 1.42 28.26 4- 1.42 4.444-0.16 15.194-0.49 1.404-0.09
a The intensity 198 keY is taken as 100 units. b The intensity ratio Ka/K B is 3.8584-0.38 in agreement with Lederer et al.Z). e The intensity 657.7 keV is taken as 100 units. 197
198
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Photon energy (keV)
Fig. 1. Comparison of the relative photopeak efficiency of Ge(Li) detectors using a 169yb source. Source point distance: 5 cm. (~ 9.75 cm3 true coaxial detector; • 30 cm3 true coaxial detector; [] 60 crn3 true coaxial detector. the energy calibration for these spectra were p r e p a r e d by counting absolutely calibrated standards*. A 4096 channel analyzer (Packard) was used. The energy resolution (fwhm) o f the 60 cm 3 d e t e c t o r t was 1.6 keV for 122 keV 57Co g a m m a rays and 2.6 keV for 1332 keV 6°Co y-rays. The measurements were m a d e at a distance o f 5 cm from the surface o f the detectors. The g a m m a r a y energies and relative intensities o f 169yb and al0Agm are listed in table 1. W i t h the aid o f these values, a Ge(Li) detector can be calibrated with an accuracy o f 0.1-0.2 keV, using the least squares fit method. The relative efficiency o f the detector m a y be f o u n d to an accuracy o f better than 6 - 7 % a n d the absolute efficiency with an accuracy o f better than 10-12%. In the low energy region the efficiency decreases. This m a y be explained by the high a b s o r p tion in the insensitive layer o f the detector a n d the a l u m i n i u m capsule. Three examples o f the use o f 169yb as a calibration s t a n d a r d for high resolution Ge(Li) detectors are given in fig. I. The p h o t o p e a k efficiency o f the 198 keV g a m m a peak is taken as 1000 units. It is seen that for g a m m a energies below 198 keV, the 9.75 cm 3 true coaxial detector is m o r e sensitive than the other two * Supplied by I.A.E.A., Vienna, Austria. t Supplied by Seforad Ltd., Emek Hayarden, Israel.
s6o
Iobo aobo
Photon energy (keY)
4bo
Fig. 2. Relative photopeak efficiency of a 60 cm3 Ge(Li) detector using a 110Agm source. Source point distance: 5 cm. detectors. The use o f 169yb as a c a l i b r a t i o n source has the a d v a n t a g e o f avoiding the use o f multiple sources, a n d the errors in g e o m e t r y which could result from interchanging them. A n o t h e r a d v a n t a g e o f the proposed source is the ease o f p r e p a r a t i o n . D u e to the high thermal neutron cross section o f 168yb (5500 b)3), very short i r r a d i a t i o n times are enough to o b t a i n highly efficient sources. A n example o f the use o f 1]°Agm as a calibration s t a n d a r d for Ge(Li) detectors is given in fig. 2. The p h o t o p e a k efficiency o f the 657.7 keV g a m m a peak is t a k e n as 1000 units. A straight line is o b t a i n e d by plotting the relative efficiency against the corres p o n d i n g g a m m a ray energy. The i m p o r t a n c e o f H°Agm as a calibration s t a n d a r d for use with Ge(Li) detectors is evident in view o f its 15 g a m m a rays in the range 450-1600 keV and its c o m p a r a t i v e l y long half-life o f 253 dX). A sum p e a k o f 1541.9 keV (657.7 keV + 884.2 keV) is also emitted. There are very few nuclides with g a m m a rays in this energy range a n d with a half-life longer than a h u n d r e d days. As in the case o f 169yb, the need for multiple sources is avoided and the source is also easily prepared.
References 1) M. A. Wakat, Nucl. Data Tables 8 (1971). 2) C. M. Lederer, J. M. Hollander and I. Perlman, Table o f isotopes (6th ed. J. Wiley, New York, 1967). 3) Nucl. data sheets (Academic Press, New York, 1964) p. 6-4-82.