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On the preparation of 57Co Mössbauer sources
NUCLEAR INSTRUMENTS AND METHODS
54 (1967) 105-108; ©
NORTH-HOLLAND PUBLISHING CO.
ON THE PREPARATION OF 57CO MtlSSBAUER SOURCES I. DEZSI and B. MOLNAR
Central Research Institute for Physics, Budapest, Hungary Received 3 April 1967 A method developed for the preparation of 57CO sources appropriate for Mossbauer effect measurements is described. Investigations have been performed to establish the 57CO electrodeposition rate on stainless steel, Cu, Fe and Pd surfaces. The efficiency of the electrodeposition to adequately pretreated surfaces was found to be better than 90%. The total 57CO activity
deposited on the surface can be diffused into the metal. The width of the 14.4 MeV gamma line emitted from the source does not exceed the theoretical value by more than 50%. Using a stainless steel absorber of 21 mg/cm 2 thickness, the intensity of the effect is found to be 50%.
1. Introduction The Mossbauer study of 57Fe requires sources emitting a single, unsplit line. They are usually prepared by diffusing the parent 57CO isotope into a cubic lattice of some diamagnetic material in order to avoid the splitting of the iron levels by internal magnetic and electric effects. Materials chosen for this purpose are usually stainless steel, copper, chromium, less frequently palladium. Most extensively stainless steel is used despite its known drawbacks, namely, the Mossbauer line-width is about three times that of the natural line and the presence of 57Fe as alloying component induces self-resonance absorption. The best results have been obtained to date with chromium which has a linewidth close or equal to that of the natural width and a surface not effected by exposure to air. 57CO is introduced into the host lattice either directly by alloying!), or by a simpler and more frequently used technique, in which it is deposited first on the surface of the chosen metal, then diffused at high temperature into the lattice 2 ). For the latter, electrodeposition proves to be the most successful technique to date 3- 6). As to the optimum conditions for electrolysis and diffusion the different authors are not of the same opinion, also the data reported on the maximum possible efficiency and the minimum possible duration of the electrolysis are discrepant. For the clarification of this problem measurements were performed, as a result of which a convenient and well reproducible method for the preparation of 57CO sources embedded in chromium with remarkably good Mossbauer characteristics has been developed and will be reported here.
much as possible of the available activity onto the host surface. Methods developed for the electrodeposition of macroquantities e.g. in electroplating and electro· analysis are inadequate for the deposition of carrierfree istopes i.e. for quantities which, e.g. for 57CO, mean the removal of about 0.01 Ilg from the solution to the metal surface. Such minute quantities of cobalt can be deposited only if the electrolyte is such that it meets following requirements. - Its pH must be sufficiently high to prevent redissolution of the already deposited quantities. With pH values above 7 the cobalt has to be bound into a complex to inhibit precipitation. - The electrolyte volume must be kept at a minimum to increase the rate of electrodeposition. - It must not give rise in electrolysis to any compound or free radical which might form a compound from which the metallic ion cannot be electrodeposited. For the systematical investigation of the pHdependence, details of which are not reported here, plug
+ ____ _ _ glass
rubber cqp
PI onode
_ electro/vie _
2. Description of the method
2.1.
cell
___C::::=::::~b~=!±==:;
rubber gasket
metal/oil L -_ _ _ _ _ _ _ _ _ _ _ _ _- = : ; t - - - bIosspiate
~llJL
ELECTROLYSIS
Fig. I. The electrolytic cell.
The essential problem in electrolysis is to deposit as
105
106
I. DEZSI AND B. MOLNAR
buffer solutions of various compositions were used, varying their pH, step by step. The optimum results were obtained using an aqueous solution of 2.5 g/IOO ml ammonium citrate with 2.5 g/IOO ml hydrazine hydrate adjusted to pH = 10 by ammonia. This conposition was similar to that reported in 5) but we have found the addition of (NH 4 hS04 to be pointless, even rather decreasing the rate of deposition. As cathode, alternatively, stainless steel, Cr, Fe, Cu and Pd foils were used. The foils were etched after degreasing with I : I hydrochloric acid and water mixture to obtain dull surfaces, since the electrodeposition to polished bright metal surfaces proved to be poor. 0
The electrolytic cell is shown in fig. I. The total volume of the electrolyte was I ml. The electro lysing current was kept at 1.6 rnA to minimize the hydrogen evolution at the cathode. The potential across the cell was 1.1 V. A drop of hydrazine hydrate was added to the sample every It h for reduction of the oxidization products at the anode. The deposition rate of cobalt was checked on about 10 mm 3 samples taken from the electrolyte every hour by a micropipette, measuring the activity with a well-type NaI(TI) scintillation counter. For the activity measurements we used the 123 keY gamma rays. The intensity curves are to be seen in fig. 2, where for comparison we have shown also
;0
fOO
gO ~ .....
:s: to ..., ~ C)
-.:
""-
c,..
10 i
60 50 ~O
30
lO
stainless Jteel
I'
\\ 1\ \ \
~
\ 0
\
o
fO 0
'-t--+---t-I----+___+_'~ 1 2 ) ~ J 6 '8 0
-\r--
o
LJ
/0
fOO
\
90 10
\
1O &0
50 ~O
Fe.
Pd
(,
30
ZO fO
o
I
t- I
I
t
f
-f
r
I I t t + "'/-T-
13t56/'
lJ
t --+--,-+ t - ++ - I -f - ~,r-+--- ._. 'IJtJ6/' ZJ
time -
Fig. 2. Time dependence of the relative values of 57CO activity in the electrolytes of ref. metric error in sampling ± 10%.
5)
--
In
hourJ ~
(0), ref.
6) (£::.,)
and the authors ( +). Volu-
THE PREPARATION OF
"l
'-'
c:
"
::l)
.... .... ..., .0 .... 0
to
o~ o
o~
0 ~ \
....
~
"l
co
"
~
\
107
MOSSBAUER SOURCES
000
~~
/
~
0
s
57CO
0
0°
;
--0°:0\ ~
~ I
\
()
:r
U
I·
~
2
1
o
2
velocity
~
in mm/.s!c
~
Fig. 3. Spectrum of iron absorber taken for 57CO Mossbauer source embedded in chromium. The appearance of four lines on the spectrum instead of six is due to that the two extreme lines could not be covered within the low velocity range of the measurement.
the curves we have measured for electrolytes used by other authors. It is wel1 apparent from the figures that under the conditions described above, cobalt could be deposited in a relatively short time on any of the metal surfaces used in the experiment. The most satisfactory results were obtained with the electrolyte containing ammonium citrate free from sulphate. The pH of the electrolyte is left unaltered even by electrolysis continued for 20-30 h. This is due to both the low current and the nearly closed electrolyzer cel1 preventing the ammonia from escaping. Some possible effects on the electrodeposition were also investigated. The rate of electrodeposition was found to be not affected by cobalt concentrations up to a few tenth of Ilg. The activity of the electrolyte did not increase more than 3% in 10 min after finishing the electrolysis. Considering that the electrolytic cell can Surely be dismounted within this time no activity loss due to redissolution will occur. Similarly, the activity loss due to rinsing the plates two times in 2 ml distilled Water did not exceed 3%. A 16 mC source was prepared by this method from about 5 ml nCo-solution·. It was first evaporated not qUite to dryness and washed with twice 0.5 ml electrolyte from the ampoule into the electrolytic cell. The closed device as well as the minimum gas evolution kept the surroundings free from any contamination. After electrolysis for 8 h the activity of the chromium surface was measured under nearly the same geometrical conditions as used to measure the radioactive solution • Philips-Duphar, The Netherlands.
before the electrolyis. For measurement a GM counter was used shielded from the 6 and 14 keY gamma rays of 57 Co. It was found that 90010 of the solution activity was deposited by this technique onto the chromium plate. 2.2. DIFFUSION The vacuum diffusion was performed in a quartz tube at llOO°C. This temperature and the time of diffusion was chosen rather arbitrarily in lack of any data on the diffusion coefficient of cobalt in chromium. The temperature of the sample was raised keeping the vacuum pressure by continuous pumping below 2 x 10- 5 mm Hg. When the sample reached the chosen temperature the vacuum pressure amounted to 7 x 10- 6 mm Hg. After 2 h diffusion, about I h was required to cool the sample to room temperature. After this time the dark spot produced by the electrodeposition completely disappeared showing that the deposited layer was diffused into the metal plate. 3. Mossbauer spectra The Mossbauer spectra of the 57CO source were taken with a variable speed spectrometer operated in "amplitude mode". A minimum Iinewidth of 0.25 mm/sec was measured using a preannealed high purity iron absorber (fig. 3). Taking the value of the Iinewidth to be additively composed of those of the source and absorber, the half-width of the line obtained from our source exceeds only by maximum SOOIo the theoretical value of 0.098 mm/sec. The intensity of the Mossbauer effect with 21 mg/cm 2 stainless steel (CSN 17241)
108
I. DEZSI AND B. MOLNAR
absorber was measured as 50% compared with the 35% obtained under the same conditions with a source diffused into stainless steel. 4. Conclusions Experiments with carrierfree S7CO proved the method described above to be of better than 90% efficiency for the deposition of layers suitable for diffusion on Cr, stainless steel, Fe, Cu and Pd plates. The efficiency of the electrolytic separation depends with an appropriately chosen electrolyte only on time. The diffusion of the electrodeposited activity at 10 - 5 mm Hg vacuum pressure does not involve the formation of any surface layer which has to be removed at the expense of activity losses amounting with other methods as high as 50%.
For optimum choice of diffusion temperature and time, it would be necessary to determine the temperature dependence of the diffusion constant of cobalt in various metals of interest. Thanks are due to Mr. A. Balazs for his assistance in the measurements. References 1) G. A. Chackett, K. F. Chackett and B. Singh, J. Inorg. Nucl.
Chern. 14 (1960) 138.
2) J. Bara, A. Z. Hrynkiewicz and I. Stroinski, Kernenergie 7
(1964) 317. D. M. J. Compton and A. H. Schoen, The Mossbauer Effect (Wiley, New York, 1962) p. 76. 4) W. Kerler und W. Neuwirth, Z. Phys. 167 (1962) 176. fi) J. Stephen, Nucl. Instr. and Meth. 26 (1965) 269. 6) A. Mustachi, Nucl. Instr. and Meth. 26 (1964) 219. 3)
54 (1967) 105-108; ©
NORTH-HOLLAND PUBLISHING CO.
ON THE PREPARATION OF 57CO MtlSSBAUER SOURCES I. DEZSI and B. MOLNAR
Central Research Institute for Physics, Budapest, Hungary Received 3 April 1967 A method developed for the preparation of 57CO sources appropriate for Mossbauer effect measurements is described. Investigations have been performed to establish the 57CO electrodeposition rate on stainless steel, Cu, Fe and Pd surfaces. The efficiency of the electrodeposition to adequately pretreated surfaces was found to be better than 90%. The total 57CO activity
deposited on the surface can be diffused into the metal. The width of the 14.4 MeV gamma line emitted from the source does not exceed the theoretical value by more than 50%. Using a stainless steel absorber of 21 mg/cm 2 thickness, the intensity of the effect is found to be 50%.
1. Introduction The Mossbauer study of 57Fe requires sources emitting a single, unsplit line. They are usually prepared by diffusing the parent 57CO isotope into a cubic lattice of some diamagnetic material in order to avoid the splitting of the iron levels by internal magnetic and electric effects. Materials chosen for this purpose are usually stainless steel, copper, chromium, less frequently palladium. Most extensively stainless steel is used despite its known drawbacks, namely, the Mossbauer line-width is about three times that of the natural line and the presence of 57Fe as alloying component induces self-resonance absorption. The best results have been obtained to date with chromium which has a linewidth close or equal to that of the natural width and a surface not effected by exposure to air. 57CO is introduced into the host lattice either directly by alloying!), or by a simpler and more frequently used technique, in which it is deposited first on the surface of the chosen metal, then diffused at high temperature into the lattice 2 ). For the latter, electrodeposition proves to be the most successful technique to date 3- 6). As to the optimum conditions for electrolysis and diffusion the different authors are not of the same opinion, also the data reported on the maximum possible efficiency and the minimum possible duration of the electrolysis are discrepant. For the clarification of this problem measurements were performed, as a result of which a convenient and well reproducible method for the preparation of 57CO sources embedded in chromium with remarkably good Mossbauer characteristics has been developed and will be reported here.
much as possible of the available activity onto the host surface. Methods developed for the electrodeposition of macroquantities e.g. in electroplating and electro· analysis are inadequate for the deposition of carrierfree istopes i.e. for quantities which, e.g. for 57CO, mean the removal of about 0.01 Ilg from the solution to the metal surface. Such minute quantities of cobalt can be deposited only if the electrolyte is such that it meets following requirements. - Its pH must be sufficiently high to prevent redissolution of the already deposited quantities. With pH values above 7 the cobalt has to be bound into a complex to inhibit precipitation. - The electrolyte volume must be kept at a minimum to increase the rate of electrodeposition. - It must not give rise in electrolysis to any compound or free radical which might form a compound from which the metallic ion cannot be electrodeposited. For the systematical investigation of the pHdependence, details of which are not reported here, plug
+ ____ _ _ glass
rubber cqp
PI onode
_ electro/vie _
2. Description of the method
2.1.
cell
___C::::=::::~b~=!±==:;
rubber gasket
metal/oil L -_ _ _ _ _ _ _ _ _ _ _ _ _- = : ; t - - - bIosspiate
~llJL
ELECTROLYSIS
Fig. I. The electrolytic cell.
The essential problem in electrolysis is to deposit as
105
106
I. DEZSI AND B. MOLNAR
buffer solutions of various compositions were used, varying their pH, step by step. The optimum results were obtained using an aqueous solution of 2.5 g/IOO ml ammonium citrate with 2.5 g/IOO ml hydrazine hydrate adjusted to pH = 10 by ammonia. This conposition was similar to that reported in 5) but we have found the addition of (NH 4 hS04 to be pointless, even rather decreasing the rate of deposition. As cathode, alternatively, stainless steel, Cr, Fe, Cu and Pd foils were used. The foils were etched after degreasing with I : I hydrochloric acid and water mixture to obtain dull surfaces, since the electrodeposition to polished bright metal surfaces proved to be poor. 0
The electrolytic cell is shown in fig. I. The total volume of the electrolyte was I ml. The electro lysing current was kept at 1.6 rnA to minimize the hydrogen evolution at the cathode. The potential across the cell was 1.1 V. A drop of hydrazine hydrate was added to the sample every It h for reduction of the oxidization products at the anode. The deposition rate of cobalt was checked on about 10 mm 3 samples taken from the electrolyte every hour by a micropipette, measuring the activity with a well-type NaI(TI) scintillation counter. For the activity measurements we used the 123 keY gamma rays. The intensity curves are to be seen in fig. 2, where for comparison we have shown also
;0
fOO
gO ~ .....
:s: to ..., ~ C)
-.:
""-
c,..
10 i
60 50 ~O
30
lO
stainless Jteel
I'
\\ 1\ \ \
~
\ 0
\
o
fO 0
'-t--+---t-I----+___+_'~ 1 2 ) ~ J 6 '8 0
-\r--
o
LJ
/0
fOO
\
90 10
\
1O &0
50 ~O
Fe.
Pd
(,
30
ZO fO
o
I
t- I
I
t
f
-f
r
I I t t + "'/-T-
13t56/'
lJ
t --+--,-+ t - ++ - I -f - ~,r-+--- ._. 'IJtJ6/' ZJ
time -
Fig. 2. Time dependence of the relative values of 57CO activity in the electrolytes of ref. metric error in sampling ± 10%.
5)
--
In
hourJ ~
(0), ref.
6) (£::.,)
and the authors ( +). Volu-
THE PREPARATION OF
"l
'-'
c:
"
::l)
.... .... ..., .0 .... 0
to
o~ o
o~
0 ~ \
....
~
"l
co
"
~
\
107
MOSSBAUER SOURCES
000
~~
/
~
0
s
57CO
0
0°
;
--0°:0\ ~
~ I
\
()
:r
U
I·
~
2
1
o
2
velocity
~
in mm/.s!c
~
Fig. 3. Spectrum of iron absorber taken for 57CO Mossbauer source embedded in chromium. The appearance of four lines on the spectrum instead of six is due to that the two extreme lines could not be covered within the low velocity range of the measurement.
the curves we have measured for electrolytes used by other authors. It is wel1 apparent from the figures that under the conditions described above, cobalt could be deposited in a relatively short time on any of the metal surfaces used in the experiment. The most satisfactory results were obtained with the electrolyte containing ammonium citrate free from sulphate. The pH of the electrolyte is left unaltered even by electrolysis continued for 20-30 h. This is due to both the low current and the nearly closed electrolyzer cel1 preventing the ammonia from escaping. Some possible effects on the electrodeposition were also investigated. The rate of electrodeposition was found to be not affected by cobalt concentrations up to a few tenth of Ilg. The activity of the electrolyte did not increase more than 3% in 10 min after finishing the electrolysis. Considering that the electrolytic cell can Surely be dismounted within this time no activity loss due to redissolution will occur. Similarly, the activity loss due to rinsing the plates two times in 2 ml distilled Water did not exceed 3%. A 16 mC source was prepared by this method from about 5 ml nCo-solution·. It was first evaporated not qUite to dryness and washed with twice 0.5 ml electrolyte from the ampoule into the electrolytic cell. The closed device as well as the minimum gas evolution kept the surroundings free from any contamination. After electrolysis for 8 h the activity of the chromium surface was measured under nearly the same geometrical conditions as used to measure the radioactive solution • Philips-Duphar, The Netherlands.
before the electrolyis. For measurement a GM counter was used shielded from the 6 and 14 keY gamma rays of 57 Co. It was found that 90010 of the solution activity was deposited by this technique onto the chromium plate. 2.2. DIFFUSION The vacuum diffusion was performed in a quartz tube at llOO°C. This temperature and the time of diffusion was chosen rather arbitrarily in lack of any data on the diffusion coefficient of cobalt in chromium. The temperature of the sample was raised keeping the vacuum pressure by continuous pumping below 2 x 10- 5 mm Hg. When the sample reached the chosen temperature the vacuum pressure amounted to 7 x 10- 6 mm Hg. After 2 h diffusion, about I h was required to cool the sample to room temperature. After this time the dark spot produced by the electrodeposition completely disappeared showing that the deposited layer was diffused into the metal plate. 3. Mossbauer spectra The Mossbauer spectra of the 57CO source were taken with a variable speed spectrometer operated in "amplitude mode". A minimum Iinewidth of 0.25 mm/sec was measured using a preannealed high purity iron absorber (fig. 3). Taking the value of the Iinewidth to be additively composed of those of the source and absorber, the half-width of the line obtained from our source exceeds only by maximum SOOIo the theoretical value of 0.098 mm/sec. The intensity of the Mossbauer effect with 21 mg/cm 2 stainless steel (CSN 17241)
108
I. DEZSI AND B. MOLNAR
absorber was measured as 50% compared with the 35% obtained under the same conditions with a source diffused into stainless steel. 4. Conclusions Experiments with carrierfree S7CO proved the method described above to be of better than 90% efficiency for the deposition of layers suitable for diffusion on Cr, stainless steel, Fe, Cu and Pd plates. The efficiency of the electrolytic separation depends with an appropriately chosen electrolyte only on time. The diffusion of the electrodeposited activity at 10 - 5 mm Hg vacuum pressure does not involve the formation of any surface layer which has to be removed at the expense of activity losses amounting with other methods as high as 50%.
For optimum choice of diffusion temperature and time, it would be necessary to determine the temperature dependence of the diffusion constant of cobalt in various metals of interest. Thanks are due to Mr. A. Balazs for his assistance in the measurements. References 1) G. A. Chackett, K. F. Chackett and B. Singh, J. Inorg. Nucl.
Chern. 14 (1960) 138.
2) J. Bara, A. Z. Hrynkiewicz and I. Stroinski, Kernenergie 7
(1964) 317. D. M. J. Compton and A. H. Schoen, The Mossbauer Effect (Wiley, New York, 1962) p. 76. 4) W. Kerler und W. Neuwirth, Z. Phys. 167 (1962) 176. fi) J. Stephen, Nucl. Instr. and Meth. 26 (1965) 269. 6) A. Mustachi, Nucl. Instr. and Meth. 26 (1964) 219. 3)