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Chemical techniques used in connection with β-spectroscopic work on rare earth elements
N U C L E A R I N S T R U M E N T S 2 (1958) 39-43; N O R T H - H O L L A N D P U B L I S H I N G CO. - A M S T E R D A M
CHEMICAL TECHNIQUES USED IN CONNECTION WITH fl-SPECTROSCOPIC WORK ON RARE EARTH ELEMENTS P. G R E G E R S
H A N S E N * AND R. K. S F I E L I N E t t
Institute/or Theoretical Physics, University o] Copenhagen R e c e i v e d t2 O c t o b e r 1957
Cyclotron t a r g e t
p r e p a r a t i o n , c h e m i c a l s e p a r a t i o n b y ion e x c h a n g e c h r o m a t o g r a p h y , a n d source p r e p a r a t i o n are
described.
1. Introduction Individually, the specific methods described in this paper have been used previously. It is therefore intended here to describe their collective application in beta spectroscopy with emphasis on the range of their applicability and usefulness rather than the specific details of the individual methods. In general, procedures for the preparation of sources for /5-ray spectroscopy should fulfill two conditions, viz. : (1) The resulting source must be sufficiently thin for the energy region to be studied. (2) The period of time needed for the preparation of the source must not be long as compared with the half-life involved. Since these two conditions, in the case of short half-lives, will have a tendency to counteract each other, the appropriate method will often have to be a compromise. A sequence of procedures that has been found applicable for work on rare earth activities with half-lives greater than two hours follows.
(A) Preparation of a cyclotron target of Eu203. (B) Separation of Tb from Eu b y ion exchange chromatography. (C) Source preparation by microcolumn method. A relatively small column was used for the chromatographic separation since the time required for the separation decreases with the size of the ion exchange column. This, in turn, limited the amount of material that could be bombarded. A general requirement in the preparation of sources is the necessity of high purity. Therefore all reagents used were made up from double distilled cation exchanged water. Ammonia and hydrochloric acid were prepared by bringing their respective gases into contact with water. Stock solutions of lactic ~cid or x-hydroxyisobutyric acid were purified b y passing the solution through a Dowex 50 column on the hydrogen form. All reagents were stored in polyethylene bottles. 2.1. T A R G E T P R E P A R A T I O N
2. Experimental Procedure Most of the spectroscopic work was carried out on cyclotron produced activities. The chemistry involved in this problem, can be considered under the following headings illustrated b y Tb-activity produced from separated Eu-isotopes by (~, n) reactions:
In order to obtain as high a specific activity as possible, it was desirable not to bombard more than the 5 mg of Eu which could be t Permanent address: Danish Atomic Energy Commission, R i s o , Roskilde, D e n m a r k . i t G u g g e n h e i m a n d F u l b r i g h t r e s e a r c h scholar on leave of a b s e n c e f r o m F l o r i d a S t a t e U n i v e r s i t y . Fla. 39
40
P. G R E G E R S
HANSEN
tolerated in the ion exchange step (see § 2.2). F u r t h e r m o r e , the targets h a d to be heat resistant and to some e x t e n t shock resistant so t h a t b o m b a r d m e n t s could be carried out in the internal b e a m of the Copenhagen cyclotron without a n y covering of the target. The m e t h o d used for this purpose was a modification of t h a t originally used b y Glover and Borrell 1) for the preparation of thin films of p l u t o n i u m and uranium. 5 mg E u 2 Q were dissolved in conc. H N Q and h e a t e d u n d e r an infrared lamp to form Eu(NO3) 3. The Eu(NO~) 3 was dissolved in approx. 2 cc of acetone, and a small a m o u n t of Zapon lacquer was added. A gold plate, 30 x 15 × 1 m m 3, was used as target backing. The solution was transferred to the gold plate first b y means of a brush to define the target area, subsequently dropwise from a small s e p a r a t o r y funnel. After each 1-3 drops, the acetone was allowed time to e v a p o r a t e and the gold plate was t h e n h e a t e d slowly in the small flame of a Bunsen b u r n e r until only Eu203 was left. The layer of Eu203 finally obtained in this way adhered well to the target and was r a t h e r uniform. B o m b a r d m e n t s were carried out with the gold plate held to the cyclotron target holder b y means of screws. 2.2. T h e i o n exchange columns u s e d w e r e of the t y p e described b y Thompson, H a r v e y , Choppin and Seaborg ~-).The Dowex 50 resin had a settling rate in water of 2-4 mm/min. Spherical and colloidal types of resin were applied. T h e y give a b o u t the same separation characteristics, the latter, however, seems less convenient since the dropping rate is more difficult to control. The resin bed was 2 m m in diameter and approximately 7 cm long. The free column volume was 5 drops. A glass jacket t h r o u g h which trichloroethylene v a p o r was passed maintained the t e m p e r a t u r e at 87°C. e - h y d r o x y isobutyric acid 3) was used as the eluant, except for a few cases where it was replaced b y lactic acid. The columns were usually operated with a w a t e r head, 0.5 to 3 m, obtained b y means of a m e r c u r y pressure system.
A N D R. K. S H E L I N E
Prior to use, a column was conditioned with the eluant for a few hours in order to bring it to the a m m o n i a form. Liquid above the resin was t h e n removed and a few drops of water were added and pushed down into the column. Subsequently the irradiated europium dissolved in a small excess of hydrochloric acid was added and pushed down. After a n o t h e r few drops of water, the elution could start. E x p e r i m e n t s were carried out in order to determine the a m o u n t of material t h a t could be handled on this t y p e of column. Previous work b y Nervik ~) had stated t h a t a carrier-free rare earth could be separated from massive a m o u n t s of rare earth of lower atomic n u m b e r as long as there were no excessive massive amounts of a rare earth of higher atomic n u m b e r present. Pile irradiated europium and terbium were used as t r a c e r s - - a b o u t 1 #g of each being applied in each run t o g e t h e r with varying a m o u n t s of stable europium. The elution curves are shown in fig. 1. It is seen t h a t good separations of Tb are obtained with up to 5 mg of Eu203. If neighboring rare earths were to be separated, this limit should be somewhat lower. For separations where the desired activity comes out after the bulk amount, 400 ttg will p r o b a b l y be the upper limit. These results are seen to be in accordance with the general rule given b y Nervik. The flow rate used (1.1 cm/min) was selected in accordance with ref. 3). An increase in flow rate to 2.7 cm/min almost doubled the band widths. F r o m fig. 1 it is evident t h a t a high degree of separation is obtained in all cases. When, however, it is the purpose to prepare sources for fi-ray spectroscopy, r a t h e r severe demands are imposed upon the procedure. Assuming a source area of 0.03 cm 2, the fraction of the 5 mg target material t h a t can be tolerated in the 1) K. M. Glover a n d P. Borrell, J. Nuclear E n e r g y 1 (1955) 214. 2) S. G. T h o m p s o n , B. C-. H a r v e y , G. R. C h o p p i n a n d G. T. Seaborg, J. A m . Chem. Soc. 76 (1954) 6229. 3) G. R. C h o p p i n a n d R. I. Silva, J. Inorg. a n d Nucl. Chem. 3 (1956) 153. 4) -W. E. Nervik, J. P h y s . C h e m . 59 (1955) 690.
CHEMICAL
(3,
10~
TECHNIQUES
41
Tb r
Eu
~
ELI
Tb
no r-u2% add~]
0.04 mg
r
t
Eu.,O3added
o10 I-
10
16' I0'
L__ Tb
5O
150
F_..u
Tb 04 mg Eu203added
1
10 z
t0
50 10-' 103
I
100 I
150
1,50
iO0 I
I
I
I
I
Smg Eu,O3added Eu
5O
100
150
I
I
I
I
12 mg Eu,O~added Tb
1o' Eu
t0
5O
10
I
100
150
200
250
I
I
I
I
300 I
drop no-~ Flow rate : 1.1 c m / m i n .
]~ig. l. S e p a r a t i o n of E u a n d Tb. E l u a n t : 0.20 M a m m o n i u m a - h y d r o x y i s o b u t y r a t e ; 0.11 M a - h y d r o x y i s o b u t y r i c acid.
42
P. G R E G E R S
HANSEN
product must be less t h a n 5 × 10 -5, if a /5source better t h a n 10/~g/cm 2 is desired. Therefore, under circumstances where a satisfactory
A N D R. K. S H E L I N E
column indicated t h a t the activity was essentially carrier-free at the end of run two. The final source h a d a thickness of about 10 #g/cm ".
1041 a ~Tb
=
i
o t0~_
b
Tb
\
10'
\
10
10
I 20
I 30
I 40
I 10
I 20
I 30
I 40
I 10
I 20
I 30
I 40
drop no.--*Fig. 2. S e p a r a t i o n of T b 16x f r o m Gd. Flow r a t e : 0.7 c m / m i n . E i u a n t : (a) 0.25 M a m m o n i u m (z-hydroxy i s o b u t y r a t e ; 0.11 M a - h y d r o x y i s o b u t y r i c acid. (b) a n d (c) 0.30 M a m m o n i u m l a c t a t e ; 0.15 M lactic acid. +--+ d r o p s t a k e n o u t for n e x t step in t h e procedure.
separation is likely to be difficult to obtain, it is generally safer and less time-consuming to perform two successive separations t h a n to improve the conditions for a separation in one step. This was demonstrated in the preparation of a Tb 161 source produced from a 14 days pile irradiation of 2 mg of Gd 16° (isotope separated, but containing some Gd~8). The first separation was carried out with e-hydroxy isobutyric acid as the eluant (fig. 2a). The elution curve showed a strong overlap between the Tb and Gd peaks. Two successive separations with lactic acid as the eluant were then performed (figs. 2b and 2c). The first one of these gave a small and well defined peak of Gd activity. The final separation showed no trace of Gd activity. Furthermore, a very pronounced tailing due to adsorption of activity on the glass on top of the
Fig. 3 shows the three L-lines of the 25.6 keV transition in Tb 1~1, as measured from this source with two gaps of an orange spectrometer 5) (max. resolution 0.8%). 2.3. SOURCE
PREPARATION
The sources were prepared by a microcolumn technique~).The drops were made slightly acid and pushed through a Dowex 50 column with a I-3 #l resin bed. After washing with water the activity was taken out with l M ammonia lactate and I #1 fractions collected on 150 #g/cm ~ aluminium foil. After drying and igniting the foil, the sources were ready for mounting. 5) O. B. Nielsen a n d O. K o f o e d - H a l l s e n , Mat. Fys. Medd. Dan. Vid. Selsk 29 (1955) no. 6. s) S. B j o r n h o l m , O. B. Nielsen a n d R. K. Sheline, N a t u r e 178 (1956) 1110.
CHEMICAL TECHNIQUES
3. Conclusion Using the procedures mentioned above, sources suited for most fl-spectroscopic work have been prepared. The time elapsed from the end of the
Crn
I
I
43
to perform two separations where an activity is produced by a (d, n) reaction. Work has so far been done on the elements T b - E u , L u - T m and on the neighboring T b - G d and Ho-Dy. I
I
I
15
L~I 10i
J I
I
I
I
I
200
210
220
230
I
240mA
Fig. 3. LI (16.5 keY), LII (17.0 keY) and LIII (17.6 keV) conversion lines of the 25.6 keV E1 transition in Tb lel measured in a fl-ray spectrometer.
bombardment to the beginning of the source mounting has usually been 6-8 hours. However, this time probably can be much reduced in ca ses where time is a decisive factor. The ultimate choice of conditions under different circumstances will depend on parameters such as halflife, amount of activity available, and desired source quality. Any quantitative guide can hardly be given. On the basis of work done at this Institute, the procedure recommended is to rely on a single separation in case of rare earth activities produced by (~, n) reactions, but
4. Acknowledgements The hospitality and ideal working conditions at Professor Niels Bohr's Institute are gratefully acknowledged. P. G. H. wishes to express his gratitude to the Danish Atomic Energy Commission, and especially to Dr. C. F. Jacobsen, for having given him the opportunity to participate in this work. The authors also wish to acknowledge the assistance of Mr. G S0rensen and of the cyclotron group. Thanks are due Mr. O. B. Nielsen and Mr. S. Bj~ rnholm for their interest and helpful suggestions.
CHEMICAL TECHNIQUES USED IN CONNECTION WITH fl-SPECTROSCOPIC WORK ON RARE EARTH ELEMENTS P. G R E G E R S
H A N S E N * AND R. K. S F I E L I N E t t
Institute/or Theoretical Physics, University o] Copenhagen R e c e i v e d t2 O c t o b e r 1957
Cyclotron t a r g e t
p r e p a r a t i o n , c h e m i c a l s e p a r a t i o n b y ion e x c h a n g e c h r o m a t o g r a p h y , a n d source p r e p a r a t i o n are
described.
1. Introduction Individually, the specific methods described in this paper have been used previously. It is therefore intended here to describe their collective application in beta spectroscopy with emphasis on the range of their applicability and usefulness rather than the specific details of the individual methods. In general, procedures for the preparation of sources for /5-ray spectroscopy should fulfill two conditions, viz. : (1) The resulting source must be sufficiently thin for the energy region to be studied. (2) The period of time needed for the preparation of the source must not be long as compared with the half-life involved. Since these two conditions, in the case of short half-lives, will have a tendency to counteract each other, the appropriate method will often have to be a compromise. A sequence of procedures that has been found applicable for work on rare earth activities with half-lives greater than two hours follows.
(A) Preparation of a cyclotron target of Eu203. (B) Separation of Tb from Eu b y ion exchange chromatography. (C) Source preparation by microcolumn method. A relatively small column was used for the chromatographic separation since the time required for the separation decreases with the size of the ion exchange column. This, in turn, limited the amount of material that could be bombarded. A general requirement in the preparation of sources is the necessity of high purity. Therefore all reagents used were made up from double distilled cation exchanged water. Ammonia and hydrochloric acid were prepared by bringing their respective gases into contact with water. Stock solutions of lactic ~cid or x-hydroxyisobutyric acid were purified b y passing the solution through a Dowex 50 column on the hydrogen form. All reagents were stored in polyethylene bottles. 2.1. T A R G E T P R E P A R A T I O N
2. Experimental Procedure Most of the spectroscopic work was carried out on cyclotron produced activities. The chemistry involved in this problem, can be considered under the following headings illustrated b y Tb-activity produced from separated Eu-isotopes by (~, n) reactions:
In order to obtain as high a specific activity as possible, it was desirable not to bombard more than the 5 mg of Eu which could be t Permanent address: Danish Atomic Energy Commission, R i s o , Roskilde, D e n m a r k . i t G u g g e n h e i m a n d F u l b r i g h t r e s e a r c h scholar on leave of a b s e n c e f r o m F l o r i d a S t a t e U n i v e r s i t y . Fla. 39
40
P. G R E G E R S
HANSEN
tolerated in the ion exchange step (see § 2.2). F u r t h e r m o r e , the targets h a d to be heat resistant and to some e x t e n t shock resistant so t h a t b o m b a r d m e n t s could be carried out in the internal b e a m of the Copenhagen cyclotron without a n y covering of the target. The m e t h o d used for this purpose was a modification of t h a t originally used b y Glover and Borrell 1) for the preparation of thin films of p l u t o n i u m and uranium. 5 mg E u 2 Q were dissolved in conc. H N Q and h e a t e d u n d e r an infrared lamp to form Eu(NO3) 3. The Eu(NO~) 3 was dissolved in approx. 2 cc of acetone, and a small a m o u n t of Zapon lacquer was added. A gold plate, 30 x 15 × 1 m m 3, was used as target backing. The solution was transferred to the gold plate first b y means of a brush to define the target area, subsequently dropwise from a small s e p a r a t o r y funnel. After each 1-3 drops, the acetone was allowed time to e v a p o r a t e and the gold plate was t h e n h e a t e d slowly in the small flame of a Bunsen b u r n e r until only Eu203 was left. The layer of Eu203 finally obtained in this way adhered well to the target and was r a t h e r uniform. B o m b a r d m e n t s were carried out with the gold plate held to the cyclotron target holder b y means of screws. 2.2. T h e i o n exchange columns u s e d w e r e of the t y p e described b y Thompson, H a r v e y , Choppin and Seaborg ~-).The Dowex 50 resin had a settling rate in water of 2-4 mm/min. Spherical and colloidal types of resin were applied. T h e y give a b o u t the same separation characteristics, the latter, however, seems less convenient since the dropping rate is more difficult to control. The resin bed was 2 m m in diameter and approximately 7 cm long. The free column volume was 5 drops. A glass jacket t h r o u g h which trichloroethylene v a p o r was passed maintained the t e m p e r a t u r e at 87°C. e - h y d r o x y isobutyric acid 3) was used as the eluant, except for a few cases where it was replaced b y lactic acid. The columns were usually operated with a w a t e r head, 0.5 to 3 m, obtained b y means of a m e r c u r y pressure system.
A N D R. K. S H E L I N E
Prior to use, a column was conditioned with the eluant for a few hours in order to bring it to the a m m o n i a form. Liquid above the resin was t h e n removed and a few drops of water were added and pushed down into the column. Subsequently the irradiated europium dissolved in a small excess of hydrochloric acid was added and pushed down. After a n o t h e r few drops of water, the elution could start. E x p e r i m e n t s were carried out in order to determine the a m o u n t of material t h a t could be handled on this t y p e of column. Previous work b y Nervik ~) had stated t h a t a carrier-free rare earth could be separated from massive a m o u n t s of rare earth of lower atomic n u m b e r as long as there were no excessive massive amounts of a rare earth of higher atomic n u m b e r present. Pile irradiated europium and terbium were used as t r a c e r s - - a b o u t 1 #g of each being applied in each run t o g e t h e r with varying a m o u n t s of stable europium. The elution curves are shown in fig. 1. It is seen t h a t good separations of Tb are obtained with up to 5 mg of Eu203. If neighboring rare earths were to be separated, this limit should be somewhat lower. For separations where the desired activity comes out after the bulk amount, 400 ttg will p r o b a b l y be the upper limit. These results are seen to be in accordance with the general rule given b y Nervik. The flow rate used (1.1 cm/min) was selected in accordance with ref. 3). An increase in flow rate to 2.7 cm/min almost doubled the band widths. F r o m fig. 1 it is evident t h a t a high degree of separation is obtained in all cases. When, however, it is the purpose to prepare sources for fi-ray spectroscopy, r a t h e r severe demands are imposed upon the procedure. Assuming a source area of 0.03 cm 2, the fraction of the 5 mg target material t h a t can be tolerated in the 1) K. M. Glover a n d P. Borrell, J. Nuclear E n e r g y 1 (1955) 214. 2) S. G. T h o m p s o n , B. C-. H a r v e y , G. R. C h o p p i n a n d G. T. Seaborg, J. A m . Chem. Soc. 76 (1954) 6229. 3) G. R. C h o p p i n a n d R. I. Silva, J. Inorg. a n d Nucl. Chem. 3 (1956) 153. 4) -W. E. Nervik, J. P h y s . C h e m . 59 (1955) 690.
CHEMICAL
(3,
10~
TECHNIQUES
41
Tb r
Eu
~
ELI
Tb
no r-u2% add~]
0.04 mg
r
t
Eu.,O3added
o10 I-
10
16' I0'
L__ Tb
5O
150
F_..u
Tb 04 mg Eu203added
1
10 z
t0
50 10-' 103
I
100 I
150
1,50
iO0 I
I
I
I
I
Smg Eu,O3added Eu
5O
100
150
I
I
I
I
12 mg Eu,O~added Tb
1o' Eu
t0
5O
10
I
100
150
200
250
I
I
I
I
300 I
drop no-~ Flow rate : 1.1 c m / m i n .
]~ig. l. S e p a r a t i o n of E u a n d Tb. E l u a n t : 0.20 M a m m o n i u m a - h y d r o x y i s o b u t y r a t e ; 0.11 M a - h y d r o x y i s o b u t y r i c acid.
42
P. G R E G E R S
HANSEN
product must be less t h a n 5 × 10 -5, if a /5source better t h a n 10/~g/cm 2 is desired. Therefore, under circumstances where a satisfactory
A N D R. K. S H E L I N E
column indicated t h a t the activity was essentially carrier-free at the end of run two. The final source h a d a thickness of about 10 #g/cm ".
1041 a ~Tb
=
i
o t0~_
b
Tb
\
10'
\
10
10
I 20
I 30
I 40
I 10
I 20
I 30
I 40
I 10
I 20
I 30
I 40
drop no.--*Fig. 2. S e p a r a t i o n of T b 16x f r o m Gd. Flow r a t e : 0.7 c m / m i n . E i u a n t : (a) 0.25 M a m m o n i u m (z-hydroxy i s o b u t y r a t e ; 0.11 M a - h y d r o x y i s o b u t y r i c acid. (b) a n d (c) 0.30 M a m m o n i u m l a c t a t e ; 0.15 M lactic acid. +--+ d r o p s t a k e n o u t for n e x t step in t h e procedure.
separation is likely to be difficult to obtain, it is generally safer and less time-consuming to perform two successive separations t h a n to improve the conditions for a separation in one step. This was demonstrated in the preparation of a Tb 161 source produced from a 14 days pile irradiation of 2 mg of Gd 16° (isotope separated, but containing some Gd~8). The first separation was carried out with e-hydroxy isobutyric acid as the eluant (fig. 2a). The elution curve showed a strong overlap between the Tb and Gd peaks. Two successive separations with lactic acid as the eluant were then performed (figs. 2b and 2c). The first one of these gave a small and well defined peak of Gd activity. The final separation showed no trace of Gd activity. Furthermore, a very pronounced tailing due to adsorption of activity on the glass on top of the
Fig. 3 shows the three L-lines of the 25.6 keV transition in Tb 1~1, as measured from this source with two gaps of an orange spectrometer 5) (max. resolution 0.8%). 2.3. SOURCE
PREPARATION
The sources were prepared by a microcolumn technique~).The drops were made slightly acid and pushed through a Dowex 50 column with a I-3 #l resin bed. After washing with water the activity was taken out with l M ammonia lactate and I #1 fractions collected on 150 #g/cm ~ aluminium foil. After drying and igniting the foil, the sources were ready for mounting. 5) O. B. Nielsen a n d O. K o f o e d - H a l l s e n , Mat. Fys. Medd. Dan. Vid. Selsk 29 (1955) no. 6. s) S. B j o r n h o l m , O. B. Nielsen a n d R. K. Sheline, N a t u r e 178 (1956) 1110.
CHEMICAL TECHNIQUES
3. Conclusion Using the procedures mentioned above, sources suited for most fl-spectroscopic work have been prepared. The time elapsed from the end of the
Crn
I
I
43
to perform two separations where an activity is produced by a (d, n) reaction. Work has so far been done on the elements T b - E u , L u - T m and on the neighboring T b - G d and Ho-Dy. I
I
I
15
L~I 10i
J I
I
I
I
I
200
210
220
230
I
240mA
Fig. 3. LI (16.5 keY), LII (17.0 keY) and LIII (17.6 keV) conversion lines of the 25.6 keV E1 transition in Tb lel measured in a fl-ray spectrometer.
bombardment to the beginning of the source mounting has usually been 6-8 hours. However, this time probably can be much reduced in ca ses where time is a decisive factor. The ultimate choice of conditions under different circumstances will depend on parameters such as halflife, amount of activity available, and desired source quality. Any quantitative guide can hardly be given. On the basis of work done at this Institute, the procedure recommended is to rely on a single separation in case of rare earth activities produced by (~, n) reactions, but
4. Acknowledgements The hospitality and ideal working conditions at Professor Niels Bohr's Institute are gratefully acknowledged. P. G. H. wishes to express his gratitude to the Danish Atomic Energy Commission, and especially to Dr. C. F. Jacobsen, for having given him the opportunity to participate in this work. The authors also wish to acknowledge the assistance of Mr. G S0rensen and of the cyclotron group. Thanks are due Mr. O. B. Nielsen and Mr. S. Bj~ rnholm for their interest and helpful suggestions.