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K electron capture ratio in the decay of 125I
t
4.D
I I
Nuclear Physics 89 (1966) 561--564; (~) North-Holland Publishing Co., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher
MEASUREMENT OF THE L + M + . . . / K
ELECTRON CAPTURE RATIO
IN THE DECAY OF 1~5I K. M. S M I T H and G. M. LEWIS
Department of Natural Philosophy, University of Glasgo w, Scotland Received 27 June 1966 Abstract: Using the "well-type" internal source scintillation spectrometer technique, a value o f 0.253±0.005 was obtained for the L ÷ M + . . . / K electron capture ratio in the decay of x2~i to the 35.4 keV excited state o f 12~Te. This corresponds to an atomic mass difference o f (153 J: 5) keV between the ground states o f 125I and a2~Te. Less than 1% o f capture transitions were f o u n d to go directly to the ground state o f 125Te.
El
R A D I O A C T I V I T Y 125I; measured LMN-capture/K-capture; deduced Q.
1. Introduction
The recent development of the theory of electron capture to include a full description of atomic effects has led to improved agreement between theoretical predictions of L/K capture ratios and the results of precise experimental measurements of these ratios 1). The theory therefore permits the use of experimental values of capture ratios in the determination of transition energies in electron capture processes, especially for energies comparable with the K-shell binding energies of the decaying isotopes and for the heavier elements 2). Although the application of scintillation spectrometry to measurements of electron capture ratios is generally limited to high-Z elements by the energy resolution of the thallium-activated sodium iodide scintillator normally used and by photomultiplier "'noise", the method has become more powerful in the last few years as a result of the continuing improvement in photomultiplier tubes and the extended use of the internal source scintillation technique. The present paper describes an application of this technique to a measurement of the L + M + . . . / K capture ratio in the decay of 125I. The results reported here were obtained during a series of experiments designed to evaluate the "well-type" internal source scintillation counter method 3), which eliminates the effects of X-ray escape from radioactive scintillators. Similar results have been obtained by Leutz and Ziegler'~), who have applied an experimentally determined escape correction to the L + M + . . o/K capture ratio which they obtained from internal source scintillators of various sizes. 561
562
~C. M. SMITH AND G. M. LEWIS
According to the spin-parity assignments given in the decay scheme s) of fig. 1, the electron capture decay of 125I to the 35.4 keV excited state of 12STe is an allowed transition, (A J = l, no), and the electron capture ratios are therefore independent of the nuclear matrix elements in the transition. Investigations of the L + M + . . . / K capture ratio by Friedlander and Orr 6) and by der Mateosian 7) have indicated that the transition energy is comparable with the K-shell binding energy. The precise determination of the capture ratio may be used to determine a more accurate value of the transition energy. Were the value of this transition energy determined independently, on the other hand, for example from a measurement of the (p, n) reaction threshold, the L + M + . . . / K capture ratio could be used to evaluate the M/L capture ratio, which is difficult to measure directly because of the low energy of the characteristic M-shell radiations. ~+
12sI
( 60.25d )
E.C y ~"2 4- 1.6n5¢c
~+
/
l
i25T~
3S.4 k¢V
o
Fig. 1. Decay scheme of ~-%I.
2. Experimental Procedure
The 125I source used in the present measurements was obtained from the Radiochemical Centre, Amersham, in the form of NaT in N/100 N a O H solution. Using a small amount of this solution and approximately 10 g of Harshaw NaI(T1) chippings, a radioactive NaT(T1) scintillator was grown by the Bridgman 8) method. A small cubic crystal of side ~ 5 mm was cut from this scintillator and mounted in the well of a Harshaw NaI(TI) well-type crystal. With a lid of similar material covering the well, the radioactive crystal was surrounded on all sides by a thickness of inactive scintillator of not less than 3 mm. Under these conditions, the escape of KX-rays from the assembly was less than 0.05 %, and so could be neglected. The well-type scintillator assembly, immersed in liquid paraffin and surrounded by a 0.002 cm aluminized mylar reflector, was mounted in a glass container which was optically coupled to an E.M.I. 9514S low-noise photomultlplier. Following amplification by a non-overloading amplifier (NE 5202) the signals from this tube were analysed by a CDC 100-channel analyser. 3. Results and Discussion
The total disintegration rate fi'om the radioactive crystal was about 5000 per rain. A typical pulse-height distribution obtained from this crystal is shown in fig. 2. As a
563
ELECTRON CAPTURE IN 125I
result of the 100 ~ detection efficiency of the scintillator assembly for the gamma-rays, X-rays and internal conversion and Auger electrons emitted in the decay of ]2sI, the pulse-height spectrum consists of two peaks at 67 keV and 39 keV, respectively. The former is due to summation of the effectively simultaneous 35.4 keV de-excitation energy of the daughter nucleus with the K-shell radiation (either X-rays or Auger electrons), signalling a K capture event. The lower energy peak is made up of contributions from summing peaks associated with electron capture from one of the L-, M- or higher shells followed by the nuclear gamma-ray or internal conversion electron (the resolution of the scintillation counter is not high enough to permit the separation o f these different components). 12ooc 67 kcV
6 IO00C ..1 i,1 Z Z
,¢
I
80O0
U rr i,i Q_
~000
LD t-Z
O
4000
U
2000
0
i
0
IO
I
20
30
40
CHANNEL
•
50
60
70
I
8O
NUMBER
Fig. 2. Pulse-height distribution from radioactive NaI(TI) crystal containing 12~I.
The purity of the source is indicated by the presence in the pulse-height spectrum of only the two peaks expected. From the number of events observed in the region of the L-shell binding energy at 4 keV, the intensity of the electron capture transitions between the ground states of 12~I and lZSTe is certainly less than 1%. Since corrections for X-ray escape from the scintillator were negligible, the L + M + . . . / K electron capture ratio was calculated from the ratio of the areas of the two peaks in fig. 2, after subtraction of a small background, (less than 1%). The separation of the contributions from the two peaks at the region of overlap was effected by assuming symmetrical shapes for these peaks. The value of the capture ratio obtained from a series of measurements was then L + M +.../K
= 0.253_+ 0.005.
564
K. M. SMITH AND G. M. LEWIS
The error limit quoted in this result is due largely to a possible uncertainty in the fitting of the two peaks as described above. The statistical uncertainty is only a small fraction of 1%. The value obtained for the capture ratio is in excellent agreement with the value of 0.2543 +_0.0027 obtained by Leutz and Ziegler 4), who adopt a different approach to the problem of X-ray escape from radioactive scintillators. Using the theoretical wave function ratios given by Wapstra et al. 9), the capture ratio of 0.253 +0.005 corresponds to a mass difference of 153 + 5 keV between the ground states of ~25i and ~25Te" References 1) 2) 3) 4) 5) 6) 7) 8) 9)
J. N. Bahcall, Phys. Rev. 129 (1963) 2683; 131 (1963) 1756; 132 (1963) 362 B. L. Robinson and R. W. Fink, Revs. Mod. Phys. 32 (1960) 117 B. R. Joshi, G. M. Lewis and K. M. Smith, Nucl. Instr. 24 (1963) 77 H. Leutz and K. Ziegler, Nuclear Physics 50 (1964) 648 Nuclear Data Cards, National Academy of Sciences, National Research Council, Washington D.C. G. Friedlander and W. C. Orr, Phys. Rev. 84 (1951) 484 E. der Mateosian, Phys. Rev. 92 (1953) 938 P. W. Bridgman, Proc. A m . Acad. Arts Sci. 60 (1925) 350 A. H. Wapstra, G. J. Nijgh and R. van Liesbout, Nuclear spectroscopy tables (North-Holland Publ. Co., A m s t e r d a m , 1959)
4.D
I I
Nuclear Physics 89 (1966) 561--564; (~) North-Holland Publishing Co., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher
MEASUREMENT OF THE L + M + . . . / K
ELECTRON CAPTURE RATIO
IN THE DECAY OF 1~5I K. M. S M I T H and G. M. LEWIS
Department of Natural Philosophy, University of Glasgo w, Scotland Received 27 June 1966 Abstract: Using the "well-type" internal source scintillation spectrometer technique, a value o f 0.253±0.005 was obtained for the L ÷ M + . . . / K electron capture ratio in the decay of x2~i to the 35.4 keV excited state o f 12~Te. This corresponds to an atomic mass difference o f (153 J: 5) keV between the ground states o f 125I and a2~Te. Less than 1% o f capture transitions were f o u n d to go directly to the ground state o f 125Te.
El
R A D I O A C T I V I T Y 125I; measured LMN-capture/K-capture; deduced Q.
1. Introduction
The recent development of the theory of electron capture to include a full description of atomic effects has led to improved agreement between theoretical predictions of L/K capture ratios and the results of precise experimental measurements of these ratios 1). The theory therefore permits the use of experimental values of capture ratios in the determination of transition energies in electron capture processes, especially for energies comparable with the K-shell binding energies of the decaying isotopes and for the heavier elements 2). Although the application of scintillation spectrometry to measurements of electron capture ratios is generally limited to high-Z elements by the energy resolution of the thallium-activated sodium iodide scintillator normally used and by photomultiplier "'noise", the method has become more powerful in the last few years as a result of the continuing improvement in photomultiplier tubes and the extended use of the internal source scintillation technique. The present paper describes an application of this technique to a measurement of the L + M + . . . / K capture ratio in the decay of 125I. The results reported here were obtained during a series of experiments designed to evaluate the "well-type" internal source scintillation counter method 3), which eliminates the effects of X-ray escape from radioactive scintillators. Similar results have been obtained by Leutz and Ziegler'~), who have applied an experimentally determined escape correction to the L + M + . . o/K capture ratio which they obtained from internal source scintillators of various sizes. 561
562
~C. M. SMITH AND G. M. LEWIS
According to the spin-parity assignments given in the decay scheme s) of fig. 1, the electron capture decay of 125I to the 35.4 keV excited state of 12STe is an allowed transition, (A J = l, no), and the electron capture ratios are therefore independent of the nuclear matrix elements in the transition. Investigations of the L + M + . . . / K capture ratio by Friedlander and Orr 6) and by der Mateosian 7) have indicated that the transition energy is comparable with the K-shell binding energy. The precise determination of the capture ratio may be used to determine a more accurate value of the transition energy. Were the value of this transition energy determined independently, on the other hand, for example from a measurement of the (p, n) reaction threshold, the L + M + . . . / K capture ratio could be used to evaluate the M/L capture ratio, which is difficult to measure directly because of the low energy of the characteristic M-shell radiations. ~+
12sI
( 60.25d )
E.C y ~"2 4- 1.6n5¢c
~+
/
l
i25T~
3S.4 k¢V
o
Fig. 1. Decay scheme of ~-%I.
2. Experimental Procedure
The 125I source used in the present measurements was obtained from the Radiochemical Centre, Amersham, in the form of NaT in N/100 N a O H solution. Using a small amount of this solution and approximately 10 g of Harshaw NaI(T1) chippings, a radioactive NaT(T1) scintillator was grown by the Bridgman 8) method. A small cubic crystal of side ~ 5 mm was cut from this scintillator and mounted in the well of a Harshaw NaI(TI) well-type crystal. With a lid of similar material covering the well, the radioactive crystal was surrounded on all sides by a thickness of inactive scintillator of not less than 3 mm. Under these conditions, the escape of KX-rays from the assembly was less than 0.05 %, and so could be neglected. The well-type scintillator assembly, immersed in liquid paraffin and surrounded by a 0.002 cm aluminized mylar reflector, was mounted in a glass container which was optically coupled to an E.M.I. 9514S low-noise photomultlplier. Following amplification by a non-overloading amplifier (NE 5202) the signals from this tube were analysed by a CDC 100-channel analyser. 3. Results and Discussion
The total disintegration rate fi'om the radioactive crystal was about 5000 per rain. A typical pulse-height distribution obtained from this crystal is shown in fig. 2. As a
563
ELECTRON CAPTURE IN 125I
result of the 100 ~ detection efficiency of the scintillator assembly for the gamma-rays, X-rays and internal conversion and Auger electrons emitted in the decay of ]2sI, the pulse-height spectrum consists of two peaks at 67 keV and 39 keV, respectively. The former is due to summation of the effectively simultaneous 35.4 keV de-excitation energy of the daughter nucleus with the K-shell radiation (either X-rays or Auger electrons), signalling a K capture event. The lower energy peak is made up of contributions from summing peaks associated with electron capture from one of the L-, M- or higher shells followed by the nuclear gamma-ray or internal conversion electron (the resolution of the scintillation counter is not high enough to permit the separation o f these different components). 12ooc 67 kcV
6 IO00C ..1 i,1 Z Z
,¢
I
80O0
U rr i,i Q_
~000
LD t-Z
O
4000
U
2000
0
i
0
IO
I
20
30
40
CHANNEL
•
50
60
70
I
8O
NUMBER
Fig. 2. Pulse-height distribution from radioactive NaI(TI) crystal containing 12~I.
The purity of the source is indicated by the presence in the pulse-height spectrum of only the two peaks expected. From the number of events observed in the region of the L-shell binding energy at 4 keV, the intensity of the electron capture transitions between the ground states of 12~I and lZSTe is certainly less than 1%. Since corrections for X-ray escape from the scintillator were negligible, the L + M + . . . / K electron capture ratio was calculated from the ratio of the areas of the two peaks in fig. 2, after subtraction of a small background, (less than 1%). The separation of the contributions from the two peaks at the region of overlap was effected by assuming symmetrical shapes for these peaks. The value of the capture ratio obtained from a series of measurements was then L + M +.../K
= 0.253_+ 0.005.
564
K. M. SMITH AND G. M. LEWIS
The error limit quoted in this result is due largely to a possible uncertainty in the fitting of the two peaks as described above. The statistical uncertainty is only a small fraction of 1%. The value obtained for the capture ratio is in excellent agreement with the value of 0.2543 +_0.0027 obtained by Leutz and Ziegler 4), who adopt a different approach to the problem of X-ray escape from radioactive scintillators. Using the theoretical wave function ratios given by Wapstra et al. 9), the capture ratio of 0.253 +0.005 corresponds to a mass difference of 153 + 5 keV between the ground states of ~25i and ~25Te" References 1) 2) 3) 4) 5) 6) 7) 8) 9)
J. N. Bahcall, Phys. Rev. 129 (1963) 2683; 131 (1963) 1756; 132 (1963) 362 B. L. Robinson and R. W. Fink, Revs. Mod. Phys. 32 (1960) 117 B. R. Joshi, G. M. Lewis and K. M. Smith, Nucl. Instr. 24 (1963) 77 H. Leutz and K. Ziegler, Nuclear Physics 50 (1964) 648 Nuclear Data Cards, National Academy of Sciences, National Research Council, Washington D.C. G. Friedlander and W. C. Orr, Phys. Rev. 84 (1951) 484 E. der Mateosian, Phys. Rev. 92 (1953) 938 P. W. Bridgman, Proc. A m . Acad. Arts Sci. 60 (1925) 350 A. H. Wapstra, G. J. Nijgh and R. van Liesbout, Nuclear spectroscopy tables (North-Holland Publ. Co., A m s t e r d a m , 1959)