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Preparation of thin layers of 237Np for absolute counting and fission detectors
N U C L E A R I N S T R U M E N T S AND METHODS 66 (1968) 1 3 7 - I 4 0 ; © N O R T H - H O L L A N D P U B L I S H I N G CO.
PREPARATION OF THIN LAYERS OF
237Np F O R
ABSOLUTE
COUNTING AND FISSION DETECTORS W. PARKER* and M. COLONOMOS Qulmica Nuclear, Instituto Venezolano de lnvestigaciones Cientificas, Caracas, Venezuela
Received 18 June 1968 A review of electrodeposition procedures for the preparation of 237Np is given and two new procedures reported making use of "molecular plating". High voltage - low current yield determinations were carried-out in the region 500-1 700 V dc with a deposition current constant at 1.0 mA. The suitability of aluminium, titanium, copper and stainless steel as cathode material was also investigated.
it was found that using stainless steel as a cathode and a voltage of 1 700 V for a i0 min period resulted in a yield of 100 K 2a7Np, providing that the total solids in solution were within the limits of 20 ¢tg/cm2 to 500/~g/cm 2. The quality of the layers produced by the described method make them very suitable for alpha-spectrometry and for the fabrication of fission detectors.
1. Introduction
use in absolute alpha-counting and alpha-spectrometry, one is principally interested in obtaining a quantitative yield and uniform deposit. However, in the case of fission detector layers it must be remembered that both the layer of fissionable material and the cathode will be activated. Thus, it is desirable that the cathode material should produce as little radiative interference as possible. This last restricts the use of most of the more c o m m o n materials and leaves us with a fairly small choice, e.g., aluminium and titaniumT). The work to be described is concerned with the fulfillment of the following requirements: a. Samples for absolute alpha-counting and alphaspectrometry should be prepared on thin cathode (backing) materials as a thin uniform layer. For assay work the m e t h o d should be quantitative. b. Fission layers should be deposited on cathode materials such as aluminium or titanium. c. The procedure should be non-hazardous, simple and reproducible. In order to achieve the above the electrodeposition m e t h o d "molecular plating "s ) has been applied. With this particular method alcohol plating solution and high voltage are made use of. Almost any material can be employed as a cathode and the yield is usually quantitative. Two procedures will be described, one a co-deposition m e t h o d suitable for alpha-spectrometry samples and the other, intended for preparation of carrier-free layers as fission detectors.
Thin uniform layers of 237Np are required for use as neutron detectors and for alpha counting. While methods such as v a c u u m deposition, electrospray, etc. have been made use of, they are generally considered both hazardous and wasteful. The following discussion will concern itself only with electrodeposition. Previous work on this subject has been carried out by several researchers and a short review will be given here. Graves, in his classical Techniques of the Los A l a m o s Project 1) described an electrolytic m e t h o d making use of lithium fluoride electrolyte, a current density of 5 m A / c m 2 and a deposition time of 180 rain. Under these conditions a yield of 8 0 - 9 5 ~ , representing 10 /~g 237Np, could be deposited on a platinum cathode. K o z) reported the quantitative deposition of 237Np f r o m a solution of 0.2M HC104 and 0.15M N H 4 C O O H , current density 80 m A / c m 2 and deposition time of 60 min. In this case, however, only 0.5 pg o f material was in solution. For the purpose of plating trace a m o u n t s of material, Mitchell 3) reports the use of a NH4C1-HC1 solution. A high current density of 2000 m A / c m z permitted a deposition time of some 15 min but a yield better than 94% was unobtainable. The a m o u n t of material deposited is not given by this author. Samartseva 4) gives two procedures, both acidic with high current density and claims quantitative yields. However, in a previous investigation one of the authors was unable to reproduce these resultsS). The formic a c i d - a m m o n i u m formate system has been studied by Barnett et al. 6) using thin stainless steel cathodes. Unfortunately, no other data is given. When considering the requirements for samples for * IAEA expert.
2. Experimental Neptunium-237 having a concentration o f 6.47 mg/ml, representing 17/~Ci/ml in approximately normal H N O a solution was obtained from the Oak Ridge
137
138
W. P A R K E R AND M. COLONOMOS
National Laboratory and served as mother solution for the experiments. A 200 /~1 aliquot was diluted with 10 ml of H 2 0 in a vaccine bottle and calibrated by evaporating to dryness ten 5/d aliquots of the solution on stainless steel counting planchets. The planchets were counted in a proportional gas-flow counter. A carrier solution of natural uranium in the form of uranyl nitrate was prepared with a concentration of 50 mg/ml in HzO. Using the carrier solution the 237Np was diluted six times and calibrated again as described above. In the case of the stock solution for the preparation of the fission layers 200 #1 of the 237Np mother solution were added to 50 ml of chromatography grade isopropanol (IPA) and calibrated in the same manner as above. In both cases the electrodepositions were carried out using a teflon/brass cell described previouslyg). 2.1. PLATING PROCEDURE The plating procedure for the preparation of the counting samples and fission layers consisted of the following: A prepared cathode was inserted in the massive brass base of the cell and the cell assembled, 15 ml of IPA were poured into the cell and a 5 /d aliquot of the lequired stock solution added. A platinum wire 1.0 m m dia. and in the form of a flat spiral was inserted into the cell and the anode/cathode distance adjusted to 15 mm. The plating solution was electrically stirred for five minutes by the anode prior to the application of the plating voltage and consequently stirred for the duration of the plating period. 2.2. CATHODE MATERIALS Cathode materials employed were stainless steel, for the spectrometery, and aluminium, copper and titanium for the fission layers. Each material was subject to a particular cleaning procedure which, for the sake of completeness, will be described here. Stainless steel planchets were polished using a commercial abrasive washing powder taken up on soft tissue paper followed TABLE 1
Effect of cathode material on deposition yield: aluminium, titanium, copper and stainless steel. Cathode Yield
~%) A1 Ti Cu St. steel
81 75 81 58
TABLE 2
High voltage dependence for molecular plating: 500-1 700 V, deposition time 10 rain, stainless steel cathode. hv (V) o~ /o 500 600 700 800 900 1000 ll00 1 200 1300 1400 1500 1600 1700
17 20 28 32 40 47 58 61 69 78 83 95 100
by rinsing with water and alcohol. Thin aluminium foils and titanium were washed with acetone and alcohol and the oxide surface of the copper foils was removed by means of 3N HNO3 and rinsed with water and alcohol. In order to determine the suitability of a given cathode material the optimum conditions were established for stainless steel and these conditions applied to the other materials in question. Table 1 gives the results of these determinations. 2.3. VOLTAGE DEPENDENCE
The rate of deposition as a function of voltage was determined in the region 500 to 1700 V dc, keeping the deposition current constant by means of suitable separation of the anode/cathode distance. Thus in the present experiments a constant current of 1.0 mA was maintained regardless of increase in voltage. It should be pointed out that high current density is accompanied by the evolution of large quantities of hydrogen gas at the cathode which is frequently the cause of nonuniform deposits. Therefore the present method is superior in so much that plating currents in the order of 1 mA are employed. Beginning at 500 V, the voltage was increased in steps of 100 V, five depositions been made to establish the yield with each increase. The time of deposition chosen for these experiments was ten minutes which for the lowest voltage employed gave a yield of 17%. Stainless steel was used in the voltage determinations the results of which can be seen in table 2. 3. Results and discussion As has always been the case when preparing samples
PREPARATION
OF
by means of molecular plating, the deposits leave nothing to be desired in the way of uniformity, yield and reproducibility. The deposited layer has a hard glassy appearance and shows interference colours. In fig. 1, can be seen a spectrum of 2aVNp, and its contaminants 239/24°pu and 24lAin together with the uranium carrier ~3sU. Unfortunately, in this case the resolution has been limited by the measurement set-up rather than the sample which had a total thickness of some 40 #g/cm 2. The uniformity of an average sample as measured using a G M counter and 1.0 m m dia. plexiglass collimator was found to be better than 1%, tor a sample having a diameter of 15 ram. As far as the carrier technique is concerned, as little as 20/~g of carrier can be employed, the amount of 23VNp being independent of this amount but being, in its turn limited to 500 /~g/cm 2 which is the known saturation value for this particular plating method9). It has been demonstrated earlier that self-supporting films can be fabricated using a mercury-copper cathode ~°) and this will almost certainly be the case
THIN
139
LAYERS
with Z3VNp providing that a thickness not less than 50/tg/cm 2 is required. As can be seen from table 1, the yield increases with decreasing atomic number of the cathode material. This could be explained by the formation of different oxide layers on the cathode surface during the plating process. It will be obvious from table 2, that the higher the deposition voltage the shorter the time required for a given deposition. Thus, a voltage of 500 V for 10 rain will give a yield of 17% while 1 700 V for the same time results in a yield of 100%. Above this last value the deposition time must be shortened and for most purposes this is not practical for a time shorter than 5 min. Also at such high voltages the plating solution becomes very hot with an explosive fire risk and it is therefore necessary to employ a water cooled plating cell. 4. Conclusions
Using the described technique ideal samples can be
Np - 2 3 7 > > > > >
t-- i',-. t'--- CO CO
~ ~ 4 ,~ 4
10 4
1
>>
:~:~
10 3
Pu-259/240
/
Pu-238 A m - 241
ooooco Oh 'G
t~
az u')
O
10 2
L~
k_)
aZ
I01
I0 0
I
4.5
5.0
Fig. 1. Spectrum of 2~TNpand its contaminants together with the uramum carrier 2asU.
5.5 E~ EMeV]
140
W. P A R K E R AND M. COLONOMOS
p r e p a r e d q u a n t i t a t i v e l y for a l p h a - s p e c t r o m e t r y a n d c a r r i e r - f r e e layers for use in fission c h a m b e r s . I n t h e l a t t e r i n s t a n c e , w h e r e t h e a m o u n t o f m a t e r i a l in solut i o n is 20 p g / c m 2 o r m o r e , t h e m e t h o d is also q u a n t i tative. P r o v i d i n g t h a t t h e p r e c a u t i o n s o u t l i n e d a b o v e are o b s e r v e d , s h o r t d e p o s i t i o n s can be c a r r i e d - o u t o n t o a n y c o n d u c t i n g surface o f a n y g e o m e t r y .
References 1) R. W. Dodson, A. C. Graves, L. Helmoltz, D. L. Hufford, R. M. Potter and J. G. Povelites, in Miscellaneous physical and chemical techniques of the Los Alamos Project (ed. A. C. Graves and D. K. Froman; McGraw Hill Book Co., Ltd.).
2) 3) 4) ~)
R. Ko, HW 32673 (1954). R. Mitchell, Anal. Chem. 32 (1960) 326. L. Samartseva, Atomic Energy 9, no. 4 (1960). W. Parker, M. DeCro6s and K. Sevier, Nucl. Instr. and Meth. 7 (1960). ~) G. A. Barnett, J. Crosby and D. J. Ferret, Proc. Seminar on Preparation and standardisation of isotopic targets and foils, AERE, Harwell (1965). 7) R. Hardell and S. Nilsson, Nucleonics 20 (.1962). 8) W. Parker and R. Falk, Nucl. Instr. and Meth. 16 (1962). ,0) W. Parker, Dissertation thesis (Isotope Laboratory, Institute of Physics, Chalmers University of Technology, Gothenburg, Sweden). 10) W. Parker and W. Gullhomer, ref. ~).
PREPARATION OF THIN LAYERS OF
237Np F O R
ABSOLUTE
COUNTING AND FISSION DETECTORS W. PARKER* and M. COLONOMOS Qulmica Nuclear, Instituto Venezolano de lnvestigaciones Cientificas, Caracas, Venezuela
Received 18 June 1968 A review of electrodeposition procedures for the preparation of 237Np is given and two new procedures reported making use of "molecular plating". High voltage - low current yield determinations were carried-out in the region 500-1 700 V dc with a deposition current constant at 1.0 mA. The suitability of aluminium, titanium, copper and stainless steel as cathode material was also investigated.
it was found that using stainless steel as a cathode and a voltage of 1 700 V for a i0 min period resulted in a yield of 100 K 2a7Np, providing that the total solids in solution were within the limits of 20 ¢tg/cm2 to 500/~g/cm 2. The quality of the layers produced by the described method make them very suitable for alpha-spectrometry and for the fabrication of fission detectors.
1. Introduction
use in absolute alpha-counting and alpha-spectrometry, one is principally interested in obtaining a quantitative yield and uniform deposit. However, in the case of fission detector layers it must be remembered that both the layer of fissionable material and the cathode will be activated. Thus, it is desirable that the cathode material should produce as little radiative interference as possible. This last restricts the use of most of the more c o m m o n materials and leaves us with a fairly small choice, e.g., aluminium and titaniumT). The work to be described is concerned with the fulfillment of the following requirements: a. Samples for absolute alpha-counting and alphaspectrometry should be prepared on thin cathode (backing) materials as a thin uniform layer. For assay work the m e t h o d should be quantitative. b. Fission layers should be deposited on cathode materials such as aluminium or titanium. c. The procedure should be non-hazardous, simple and reproducible. In order to achieve the above the electrodeposition m e t h o d "molecular plating "s ) has been applied. With this particular method alcohol plating solution and high voltage are made use of. Almost any material can be employed as a cathode and the yield is usually quantitative. Two procedures will be described, one a co-deposition m e t h o d suitable for alpha-spectrometry samples and the other, intended for preparation of carrier-free layers as fission detectors.
Thin uniform layers of 237Np are required for use as neutron detectors and for alpha counting. While methods such as v a c u u m deposition, electrospray, etc. have been made use of, they are generally considered both hazardous and wasteful. The following discussion will concern itself only with electrodeposition. Previous work on this subject has been carried out by several researchers and a short review will be given here. Graves, in his classical Techniques of the Los A l a m o s Project 1) described an electrolytic m e t h o d making use of lithium fluoride electrolyte, a current density of 5 m A / c m 2 and a deposition time of 180 rain. Under these conditions a yield of 8 0 - 9 5 ~ , representing 10 /~g 237Np, could be deposited on a platinum cathode. K o z) reported the quantitative deposition of 237Np f r o m a solution of 0.2M HC104 and 0.15M N H 4 C O O H , current density 80 m A / c m 2 and deposition time of 60 min. In this case, however, only 0.5 pg o f material was in solution. For the purpose of plating trace a m o u n t s of material, Mitchell 3) reports the use of a NH4C1-HC1 solution. A high current density of 2000 m A / c m z permitted a deposition time of some 15 min but a yield better than 94% was unobtainable. The a m o u n t of material deposited is not given by this author. Samartseva 4) gives two procedures, both acidic with high current density and claims quantitative yields. However, in a previous investigation one of the authors was unable to reproduce these resultsS). The formic a c i d - a m m o n i u m formate system has been studied by Barnett et al. 6) using thin stainless steel cathodes. Unfortunately, no other data is given. When considering the requirements for samples for * IAEA expert.
2. Experimental Neptunium-237 having a concentration o f 6.47 mg/ml, representing 17/~Ci/ml in approximately normal H N O a solution was obtained from the Oak Ridge
137
138
W. P A R K E R AND M. COLONOMOS
National Laboratory and served as mother solution for the experiments. A 200 /~1 aliquot was diluted with 10 ml of H 2 0 in a vaccine bottle and calibrated by evaporating to dryness ten 5/d aliquots of the solution on stainless steel counting planchets. The planchets were counted in a proportional gas-flow counter. A carrier solution of natural uranium in the form of uranyl nitrate was prepared with a concentration of 50 mg/ml in HzO. Using the carrier solution the 237Np was diluted six times and calibrated again as described above. In the case of the stock solution for the preparation of the fission layers 200 #1 of the 237Np mother solution were added to 50 ml of chromatography grade isopropanol (IPA) and calibrated in the same manner as above. In both cases the electrodepositions were carried out using a teflon/brass cell described previouslyg). 2.1. PLATING PROCEDURE The plating procedure for the preparation of the counting samples and fission layers consisted of the following: A prepared cathode was inserted in the massive brass base of the cell and the cell assembled, 15 ml of IPA were poured into the cell and a 5 /d aliquot of the lequired stock solution added. A platinum wire 1.0 m m dia. and in the form of a flat spiral was inserted into the cell and the anode/cathode distance adjusted to 15 mm. The plating solution was electrically stirred for five minutes by the anode prior to the application of the plating voltage and consequently stirred for the duration of the plating period. 2.2. CATHODE MATERIALS Cathode materials employed were stainless steel, for the spectrometery, and aluminium, copper and titanium for the fission layers. Each material was subject to a particular cleaning procedure which, for the sake of completeness, will be described here. Stainless steel planchets were polished using a commercial abrasive washing powder taken up on soft tissue paper followed TABLE 1
Effect of cathode material on deposition yield: aluminium, titanium, copper and stainless steel. Cathode Yield
~%) A1 Ti Cu St. steel
81 75 81 58
TABLE 2
High voltage dependence for molecular plating: 500-1 700 V, deposition time 10 rain, stainless steel cathode. hv (V) o~ /o 500 600 700 800 900 1000 ll00 1 200 1300 1400 1500 1600 1700
17 20 28 32 40 47 58 61 69 78 83 95 100
by rinsing with water and alcohol. Thin aluminium foils and titanium were washed with acetone and alcohol and the oxide surface of the copper foils was removed by means of 3N HNO3 and rinsed with water and alcohol. In order to determine the suitability of a given cathode material the optimum conditions were established for stainless steel and these conditions applied to the other materials in question. Table 1 gives the results of these determinations. 2.3. VOLTAGE DEPENDENCE
The rate of deposition as a function of voltage was determined in the region 500 to 1700 V dc, keeping the deposition current constant by means of suitable separation of the anode/cathode distance. Thus in the present experiments a constant current of 1.0 mA was maintained regardless of increase in voltage. It should be pointed out that high current density is accompanied by the evolution of large quantities of hydrogen gas at the cathode which is frequently the cause of nonuniform deposits. Therefore the present method is superior in so much that plating currents in the order of 1 mA are employed. Beginning at 500 V, the voltage was increased in steps of 100 V, five depositions been made to establish the yield with each increase. The time of deposition chosen for these experiments was ten minutes which for the lowest voltage employed gave a yield of 17%. Stainless steel was used in the voltage determinations the results of which can be seen in table 2. 3. Results and discussion As has always been the case when preparing samples
PREPARATION
OF
by means of molecular plating, the deposits leave nothing to be desired in the way of uniformity, yield and reproducibility. The deposited layer has a hard glassy appearance and shows interference colours. In fig. 1, can be seen a spectrum of 2aVNp, and its contaminants 239/24°pu and 24lAin together with the uranium carrier ~3sU. Unfortunately, in this case the resolution has been limited by the measurement set-up rather than the sample which had a total thickness of some 40 #g/cm 2. The uniformity of an average sample as measured using a G M counter and 1.0 m m dia. plexiglass collimator was found to be better than 1%, tor a sample having a diameter of 15 ram. As far as the carrier technique is concerned, as little as 20/~g of carrier can be employed, the amount of 23VNp being independent of this amount but being, in its turn limited to 500 /~g/cm 2 which is the known saturation value for this particular plating method9). It has been demonstrated earlier that self-supporting films can be fabricated using a mercury-copper cathode ~°) and this will almost certainly be the case
THIN
139
LAYERS
with Z3VNp providing that a thickness not less than 50/tg/cm 2 is required. As can be seen from table 1, the yield increases with decreasing atomic number of the cathode material. This could be explained by the formation of different oxide layers on the cathode surface during the plating process. It will be obvious from table 2, that the higher the deposition voltage the shorter the time required for a given deposition. Thus, a voltage of 500 V for 10 rain will give a yield of 17% while 1 700 V for the same time results in a yield of 100%. Above this last value the deposition time must be shortened and for most purposes this is not practical for a time shorter than 5 min. Also at such high voltages the plating solution becomes very hot with an explosive fire risk and it is therefore necessary to employ a water cooled plating cell. 4. Conclusions
Using the described technique ideal samples can be
Np - 2 3 7 > > > > >
t-- i',-. t'--- CO CO
~ ~ 4 ,~ 4
10 4
1
>>
:~:~
10 3
Pu-259/240
/
Pu-238 A m - 241
ooooco Oh 'G
t~
az u')
O
10 2
L~
k_)
aZ
I01
I0 0
I
4.5
5.0
Fig. 1. Spectrum of 2~TNpand its contaminants together with the uramum carrier 2asU.
5.5 E~ EMeV]
140
W. P A R K E R AND M. COLONOMOS
p r e p a r e d q u a n t i t a t i v e l y for a l p h a - s p e c t r o m e t r y a n d c a r r i e r - f r e e layers for use in fission c h a m b e r s . I n t h e l a t t e r i n s t a n c e , w h e r e t h e a m o u n t o f m a t e r i a l in solut i o n is 20 p g / c m 2 o r m o r e , t h e m e t h o d is also q u a n t i tative. P r o v i d i n g t h a t t h e p r e c a u t i o n s o u t l i n e d a b o v e are o b s e r v e d , s h o r t d e p o s i t i o n s can be c a r r i e d - o u t o n t o a n y c o n d u c t i n g surface o f a n y g e o m e t r y .
References 1) R. W. Dodson, A. C. Graves, L. Helmoltz, D. L. Hufford, R. M. Potter and J. G. Povelites, in Miscellaneous physical and chemical techniques of the Los Alamos Project (ed. A. C. Graves and D. K. Froman; McGraw Hill Book Co., Ltd.).
2) 3) 4) ~)
R. Ko, HW 32673 (1954). R. Mitchell, Anal. Chem. 32 (1960) 326. L. Samartseva, Atomic Energy 9, no. 4 (1960). W. Parker, M. DeCro6s and K. Sevier, Nucl. Instr. and Meth. 7 (1960). ~) G. A. Barnett, J. Crosby and D. J. Ferret, Proc. Seminar on Preparation and standardisation of isotopic targets and foils, AERE, Harwell (1965). 7) R. Hardell and S. Nilsson, Nucleonics 20 (.1962). 8) W. Parker and R. Falk, Nucl. Instr. and Meth. 16 (1962). ,0) W. Parker, Dissertation thesis (Isotope Laboratory, Institute of Physics, Chalmers University of Technology, Gothenburg, Sweden). 10) W. Parker and W. Gullhomer, ref. ~).