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Electron capture ratios in the decay of 105Ag
4.D
I
Nuclear Physics AI04 (1967) 692--696; (~) North-Holland Pubhshing Co., Amsterdam Not to be reproduced by photoprmt or microfilm without written permtsslon from the pubhsher
ELECTRON CAPTURE RATIOS IN THE DECAY OF lOSAg G. S C H U L Z and K. Z I E G L E R
Zwettes Physikahsches Institut der Universttiit Heidelberg Received 21 August 1967
Abstract: The ground state decay of X°SAg was investigated with NaI(TI) scintillation crystals containing 1°SAg as a constituent o f the crystal lattice. The PL/PK electron capture ratios were measured for the transitions to the 1088 keV and to the 344 keV levels of t°~Pd. The results are: (PL/PK)~osa -- 0.152 '--0.002 and (PL/PK)~4 : 0.128 -- 0.003. F r o m these data we derived a mass difference between the ground states of the X°SAg and the ~°~Pd atom of 1300_).~0° keV and an exchange correction of 1.10J:0.05. E
R A D I O A C T I V I T Y 1°SAg; measured I x. Deduced PL/PK, Q# exchange correction.
1. Introduction The nucleus t°SAg decays only by electron capture 1). Several authors investigated the decay scheme 2-6). The results have been compiled by the Nuclear Data Group 7). There are four branches of electron capture decay and 38 7-transitions. Although there exist many data concerning the y-transitions, the electron capture decay of 1°SAg and the mass difference between 1°SAg and l°SPd have not been measured until now. It has been shown 8-1o) that the formulae normally used for electron capture ratios 11) have to be modified, since the electrons in different atomic shells are indistinguishable. This modification is called exchange correction. Its magnitude was calculated by Bahcall 1o). For atomic number less than about 40 his results are justified by experiments lo). But, very few experimental data for atomic numbers larger than 40 exist 12). 2. Technique of measurement and apparatus We mixed 1°SAg with the NaI-TlI raw material. From this mixture we grew NaI(TI) scintillation crystals using a slightly modified Stockbarger method 13,14). Such doped crystals can be applied for the measurement of electron capture ratios only if mixed crystals are formed between NaI and the respective radioactive element. Otherwise, the fluorescence quanta and the Auger electrons following an electron capture decay are not detected quantitatively because there is absorbing material between the source and the sensitive detector volume 14). One of the authors stated a chemical and a physical criterion for mixed-crystal formation 14). Applying these criteria, we could assure that silver forms mixed-crystals with NaI(TI). 692
l~Ag DECAY
693
In spite of the great variety of "/-quanta, we could separate capture transitions to the 1088 keV and to the 344 keV level by using a coincidence technique described earlier 12). To demonstrate this, that part of the decay scheme 7) which is essential for our measurement is depicted in fig. 1. In a first series of measurements coincidences were required between L- and K-capture events in the doped crystal and 1088 keV -/-quanta absorbed in a 10.2 cm diam. x 10.2 cm high NaI(TI) crystal t. In this way, we selected electron capture decays to the 1088 keV level of l°SPd. In a second series of measurements the large NaI(TI) crystal was replaced by a 1.9 cm 3 Ge(Li) crystal. The energy resolution of this detector was good enough to separate the photo-absorption peak of the 344 keV -/-quanta from all other photoabsorption peaks. Therefore, we could select capture transitions to the 344 keV level by requiring coincidences between 344 keV -/-quanta totally absorbed in the Ge(Li) crystal and L- as well as K-capture events in the doped crystal.
1/2-
3/2-
I 1
105Ag ~
40d
/////
.
('1/2)+ 344 2"~°/°t i / ~ I 36% 5]2 + f f 105pd stable Fig. l. Decay scheme of 1°SAg.It is simplified by omitting all transitions which are not essential for this work. The dashed hne stands for three different transitions. 3. Results A spectrum of L- and K-capture transitions is shown in fig. 2. The energy resolutions meet standard values for NaI(T1). Within the limits of error the peaks are situated at the L- and K-binding energies of the daughter nucleus l°SPd, respectively. The measured intensity ratio B/A does not equal the PL/PK electron capture ratio because of three distortions. (i) Capture decays to the 344 keV level cannot be separated from captme transitions to the 1088 keV level which result in the population of the 344 keV level. According to the decay scheme 7), this intensity is 4 % of the intensity of 344 keV -/-quanta. (ii) The pulse-height spectrum of the Ge(Li) crystal at 344 keV consists of the photo-absorption peak as well as Compton continua o f -/-quanta with higher energies. This background is one tenth of the photo-absorption peak. About 80 % of it is caused by 7-quanta following electron capture decays to the 1088 keV level, whereas 20 % follow capture transitions to the 651 keV level. t Harshaw Chemie N.V., De Meern, Netherlands.
694
G. SCI-IULZ A N D K. Z I E G L E R
Since the (PLIPK)651 r a t i o d o e s n o t d e v i a t e m u c h f r o m the (PL/PK)344 ratio, t h e r e s p e c t i v e c o n t r i b u t i o n c a n be neglected. (iii) T h e K - c a p t u r e t r a n s i t i o n s are r e g i s t e r e d as L - e v e n t s if a f o l l o w i n g P d K , X - q u a n t u m escapes f r o m the d o p e d crystal. A s t h e
~4 CO
o
o 3 _1:5 o ¢
2
~.~
c -,E, C 1-
5 3 °/o
oval
_X
.
0
I 5
0
i 10
k_ -
i I 15 20 e n e r g y (keV)
I 25
I 30
Fig. 2. Spectrum of L- and K-capture transitions to the 344 keV level of l°~Pd as measured with a
doped NaI(TI) c~'stal. i
0.89 -~ -
0.88 087 ~'086 +
~084 < O83 C.82 0
5 10 15 s u r f a c e / v o l t , me (cm -I)
20
J
Fig. 3. Extrapolation of the intensity ratio A/(A+B) to a surface/volume ratio of zero. By this procedure escape effects disturbing the determination of PL/PK capture rauos are eliminated.
l°SAg DECAY
695
mean free path of Pd K X-rays in NaI is 0.02 cm, this escape effect is restricted to the surface region of the doped crystal. The distortions (i) and (ii) were corrected by using the measured (PL/PK)1 o88 ratio and the contribution of capture transitions to the I088 keV level in the measurement of the (PL/PK)344 ratio. The correction amounts to 2.3 %. Distortion (iii) was eliminated by determining the intensity ratios B/A of several crystals with different sizes and extrapolating to an infinitely large crystal with vanishing surface effects. As pointed out by Leutz et al. 15), this is done most accurately by plotting the quotient A/(A + B ) versus the surface/volume ratio of the doped crystal and extrapolating it to a surface/volume ratio of zero, which corresponds to an infinitely large crystal. The extrapolation procedure is shown in fig. 3. The result is:
(PL/PK)344 =
0.128
+_0.003.
The spectrum of L- and K-capture transitions to the 1088 keV level is similar to the spectrum depicted in fig. 2. There is no error similar to distortions (i) and (ii) of the preceding measurement. But, escape effects also occur. They were eliminated in the same way as the escape effects disturbing the determination of the (PL/PK)344 ratio. The extrapolation procedure is illustrated also in fig. 3. It results in
(PL/PK)t088
=
0.152_0.002.
4. Discussion
The PL/P K capture ratios can be applied in order to evaluate transition energies and exchange corrections. The correlation between these quantities is given by the formula lo, it)
,,,._ PK
\RK]
\Q--Er--IEKI]
where RK, L is the "large" component of the Dirac electron radial wave function, Q the mass difference between the ground states, Er the energy of the y-transition used for the coincidence measurements described above, EK, L the electron binding energy in the K, L-shell, S wK the matrix element factor and X Lm the exchange correction. Wapstra et al. 16) give (RL/RK) 2. We assume that their value is exact. The capture decay to the 1088 keV level is allowed 7). The corresponding matrix element factor S L/K equals it) one. The capture decay to the 344 keV level is first forbidden 7). As its l o g f t value is of normal magnitude, S L/K does not deviate much from one 1¢), and we take S L/K = i.00+0.03. With these assumptions we can derive the mass difference Q and the exchange correction X L/K by inserting both PL/PK ratios into eq. (1). The results are Q = 1300_3o +50 keV, xL]K = 1.10-t-0.05 Mattauch et al. 18), using beta decay systematics, estimated Q = 1300+_1000 keV,
696
G. S C I I U L Z A N D K. Z I E G L E R
which agrees with the measured value. The exchange corrections has been calculated by Bahcall 10). H e derived for silver, X L/K = 1.07. Provided the limits o f er r o r are considered, there is no c o n t r a d i c t i o n to o u r value. W e are indebted to Professor O. Haxel for his stimulating interest and encouragement. In particular we wish to t h a n k Professor U. H a u s e r and Dipl. Phys. E. K i i h n for p r o v i d i n g us with the G e ( L i ) crystal. T h e B u n d e s m i n i s t e r i u m fiJr Wissenschaftliche F o r s c h u n g supported this research with e q u i p m e n t .
References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17) 18)
J. R. Gum and M. L. Pool, Phys. Rev. 80 (1950) 315 T. Suter et al., Ark. Fys. 20 (1961) 431 R. Bhattacharayya, Nuovo C~m. 24 (1962) 1000 R. W. Hayward and D. D. Hoppes, Bull. Am. Phys. Soc. 1 (1956) 42 J. Y. Mei, C. M. Huddlcston and A. C. G. Mitchell, Phys. Rcv. 79 (1950) 1010 M. Raethcr, Z. Phys. 150 (1958) 38 Nuclear Data Sheets, NRC 61-4-16 (National Academy of Science ° National Research Council, Washington, D.C., 1961) P. Benoist-Gueutal, Compt. Rend. 230 (1950) 624 S. Odlot and R. Daudel, J. de Phys. 17 (1956) 60 J. N. Bahcall, Phys. Rev. 132 (1963) 362 H. Brysk and M. E. Rose, Revs. Mod. Phys. 30 (1958) 1169 G. Schulz, Nuclear Physics AI01 (1967) 177 A. Smakula, Emkristalle (Springer-Verlag, Berlin, 1962) G. Schulz, Nucl. Instr. 53 (1967) 320 H. Leutz, G. Schulz and H. Wenninger, Nuclear Physics 75 (1966) 81 A. H. Wapstra, G. J. Nljgh and R. van Lieshout, Nuclear spectroscopy tables (North-Holland Publ. Co., Amsterdam, 1959) G. Schulz, thesis, Heidelberg (1966) J. H. E. Mattauch, W. Thiele and A. H. Wapstra, Nuclear Physics 67 (1965) 73
I
Nuclear Physics AI04 (1967) 692--696; (~) North-Holland Pubhshing Co., Amsterdam Not to be reproduced by photoprmt or microfilm without written permtsslon from the pubhsher
ELECTRON CAPTURE RATIOS IN THE DECAY OF lOSAg G. S C H U L Z and K. Z I E G L E R
Zwettes Physikahsches Institut der Universttiit Heidelberg Received 21 August 1967
Abstract: The ground state decay of X°SAg was investigated with NaI(TI) scintillation crystals containing 1°SAg as a constituent o f the crystal lattice. The PL/PK electron capture ratios were measured for the transitions to the 1088 keV and to the 344 keV levels of t°~Pd. The results are: (PL/PK)~osa -- 0.152 '--0.002 and (PL/PK)~4 : 0.128 -- 0.003. F r o m these data we derived a mass difference between the ground states of the X°SAg and the ~°~Pd atom of 1300_).~0° keV and an exchange correction of 1.10J:0.05. E
R A D I O A C T I V I T Y 1°SAg; measured I x. Deduced PL/PK, Q# exchange correction.
1. Introduction The nucleus t°SAg decays only by electron capture 1). Several authors investigated the decay scheme 2-6). The results have been compiled by the Nuclear Data Group 7). There are four branches of electron capture decay and 38 7-transitions. Although there exist many data concerning the y-transitions, the electron capture decay of 1°SAg and the mass difference between 1°SAg and l°SPd have not been measured until now. It has been shown 8-1o) that the formulae normally used for electron capture ratios 11) have to be modified, since the electrons in different atomic shells are indistinguishable. This modification is called exchange correction. Its magnitude was calculated by Bahcall 1o). For atomic number less than about 40 his results are justified by experiments lo). But, very few experimental data for atomic numbers larger than 40 exist 12). 2. Technique of measurement and apparatus We mixed 1°SAg with the NaI-TlI raw material. From this mixture we grew NaI(TI) scintillation crystals using a slightly modified Stockbarger method 13,14). Such doped crystals can be applied for the measurement of electron capture ratios only if mixed crystals are formed between NaI and the respective radioactive element. Otherwise, the fluorescence quanta and the Auger electrons following an electron capture decay are not detected quantitatively because there is absorbing material between the source and the sensitive detector volume 14). One of the authors stated a chemical and a physical criterion for mixed-crystal formation 14). Applying these criteria, we could assure that silver forms mixed-crystals with NaI(TI). 692
l~Ag DECAY
693
In spite of the great variety of "/-quanta, we could separate capture transitions to the 1088 keV and to the 344 keV level by using a coincidence technique described earlier 12). To demonstrate this, that part of the decay scheme 7) which is essential for our measurement is depicted in fig. 1. In a first series of measurements coincidences were required between L- and K-capture events in the doped crystal and 1088 keV -/-quanta absorbed in a 10.2 cm diam. x 10.2 cm high NaI(TI) crystal t. In this way, we selected electron capture decays to the 1088 keV level of l°SPd. In a second series of measurements the large NaI(TI) crystal was replaced by a 1.9 cm 3 Ge(Li) crystal. The energy resolution of this detector was good enough to separate the photo-absorption peak of the 344 keV -/-quanta from all other photoabsorption peaks. Therefore, we could select capture transitions to the 344 keV level by requiring coincidences between 344 keV -/-quanta totally absorbed in the Ge(Li) crystal and L- as well as K-capture events in the doped crystal.
1/2-
3/2-
I 1
105Ag ~
40d
/////
.
('1/2)+ 344 2"~°/°t i / ~ I 36% 5]2 + f f 105pd stable Fig. l. Decay scheme of 1°SAg.It is simplified by omitting all transitions which are not essential for this work. The dashed hne stands for three different transitions. 3. Results A spectrum of L- and K-capture transitions is shown in fig. 2. The energy resolutions meet standard values for NaI(T1). Within the limits of error the peaks are situated at the L- and K-binding energies of the daughter nucleus l°SPd, respectively. The measured intensity ratio B/A does not equal the PL/PK electron capture ratio because of three distortions. (i) Capture decays to the 344 keV level cannot be separated from captme transitions to the 1088 keV level which result in the population of the 344 keV level. According to the decay scheme 7), this intensity is 4 % of the intensity of 344 keV -/-quanta. (ii) The pulse-height spectrum of the Ge(Li) crystal at 344 keV consists of the photo-absorption peak as well as Compton continua o f -/-quanta with higher energies. This background is one tenth of the photo-absorption peak. About 80 % of it is caused by 7-quanta following electron capture decays to the 1088 keV level, whereas 20 % follow capture transitions to the 651 keV level. t Harshaw Chemie N.V., De Meern, Netherlands.
694
G. SCI-IULZ A N D K. Z I E G L E R
Since the (PLIPK)651 r a t i o d o e s n o t d e v i a t e m u c h f r o m the (PL/PK)344 ratio, t h e r e s p e c t i v e c o n t r i b u t i o n c a n be neglected. (iii) T h e K - c a p t u r e t r a n s i t i o n s are r e g i s t e r e d as L - e v e n t s if a f o l l o w i n g P d K , X - q u a n t u m escapes f r o m the d o p e d crystal. A s t h e
~4 CO
o
o 3 _1:5 o ¢
2
~.~
c -,E, C 1-
5 3 °/o
oval
_X
.
0
I 5
0
i 10
k_ -
i I 15 20 e n e r g y (keV)
I 25
I 30
Fig. 2. Spectrum of L- and K-capture transitions to the 344 keV level of l°~Pd as measured with a
doped NaI(TI) c~'stal. i
0.89 -~ -
0.88 087 ~'086 +
~084 < O83 C.82 0
5 10 15 s u r f a c e / v o l t , me (cm -I)
20
J
Fig. 3. Extrapolation of the intensity ratio A/(A+B) to a surface/volume ratio of zero. By this procedure escape effects disturbing the determination of PL/PK capture rauos are eliminated.
l°SAg DECAY
695
mean free path of Pd K X-rays in NaI is 0.02 cm, this escape effect is restricted to the surface region of the doped crystal. The distortions (i) and (ii) were corrected by using the measured (PL/PK)1 o88 ratio and the contribution of capture transitions to the I088 keV level in the measurement of the (PL/PK)344 ratio. The correction amounts to 2.3 %. Distortion (iii) was eliminated by determining the intensity ratios B/A of several crystals with different sizes and extrapolating to an infinitely large crystal with vanishing surface effects. As pointed out by Leutz et al. 15), this is done most accurately by plotting the quotient A/(A + B ) versus the surface/volume ratio of the doped crystal and extrapolating it to a surface/volume ratio of zero, which corresponds to an infinitely large crystal. The extrapolation procedure is shown in fig. 3. The result is:
(PL/PK)344 =
0.128
+_0.003.
The spectrum of L- and K-capture transitions to the 1088 keV level is similar to the spectrum depicted in fig. 2. There is no error similar to distortions (i) and (ii) of the preceding measurement. But, escape effects also occur. They were eliminated in the same way as the escape effects disturbing the determination of the (PL/PK)344 ratio. The extrapolation procedure is illustrated also in fig. 3. It results in
(PL/PK)t088
=
0.152_0.002.
4. Discussion
The PL/P K capture ratios can be applied in order to evaluate transition energies and exchange corrections. The correlation between these quantities is given by the formula lo, it)
,,,._ PK
\RK]
\Q--Er--IEKI]
where RK, L is the "large" component of the Dirac electron radial wave function, Q the mass difference between the ground states, Er the energy of the y-transition used for the coincidence measurements described above, EK, L the electron binding energy in the K, L-shell, S wK the matrix element factor and X Lm the exchange correction. Wapstra et al. 16) give (RL/RK) 2. We assume that their value is exact. The capture decay to the 1088 keV level is allowed 7). The corresponding matrix element factor S L/K equals it) one. The capture decay to the 344 keV level is first forbidden 7). As its l o g f t value is of normal magnitude, S L/K does not deviate much from one 1¢), and we take S L/K = i.00+0.03. With these assumptions we can derive the mass difference Q and the exchange correction X L/K by inserting both PL/PK ratios into eq. (1). The results are Q = 1300_3o +50 keV, xL]K = 1.10-t-0.05 Mattauch et al. 18), using beta decay systematics, estimated Q = 1300+_1000 keV,
696
G. S C I I U L Z A N D K. Z I E G L E R
which agrees with the measured value. The exchange corrections has been calculated by Bahcall 10). H e derived for silver, X L/K = 1.07. Provided the limits o f er r o r are considered, there is no c o n t r a d i c t i o n to o u r value. W e are indebted to Professor O. Haxel for his stimulating interest and encouragement. In particular we wish to t h a n k Professor U. H a u s e r and Dipl. Phys. E. K i i h n for p r o v i d i n g us with the G e ( L i ) crystal. T h e B u n d e s m i n i s t e r i u m fiJr Wissenschaftliche F o r s c h u n g supported this research with e q u i p m e n t .
References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17) 18)
J. R. Gum and M. L. Pool, Phys. Rev. 80 (1950) 315 T. Suter et al., Ark. Fys. 20 (1961) 431 R. Bhattacharayya, Nuovo C~m. 24 (1962) 1000 R. W. Hayward and D. D. Hoppes, Bull. Am. Phys. Soc. 1 (1956) 42 J. Y. Mei, C. M. Huddlcston and A. C. G. Mitchell, Phys. Rcv. 79 (1950) 1010 M. Raethcr, Z. Phys. 150 (1958) 38 Nuclear Data Sheets, NRC 61-4-16 (National Academy of Science ° National Research Council, Washington, D.C., 1961) P. Benoist-Gueutal, Compt. Rend. 230 (1950) 624 S. Odlot and R. Daudel, J. de Phys. 17 (1956) 60 J. N. Bahcall, Phys. Rev. 132 (1963) 362 H. Brysk and M. E. Rose, Revs. Mod. Phys. 30 (1958) 1169 G. Schulz, Nuclear Physics AI01 (1967) 177 A. Smakula, Emkristalle (Springer-Verlag, Berlin, 1962) G. Schulz, Nucl. Instr. 53 (1967) 320 H. Leutz, G. Schulz and H. Wenninger, Nuclear Physics 75 (1966) 81 A. H. Wapstra, G. J. Nljgh and R. van Lieshout, Nuclear spectroscopy tables (North-Holland Publ. Co., Amsterdam, 1959) G. Schulz, thesis, Heidelberg (1966) J. H. E. Mattauch, W. Thiele and A. H. Wapstra, Nuclear Physics 67 (1965) 73