An electrochemical method for the preparation of 63Ni source for the calibration of thermoluminescence dosimeter (TLD)

ARTICLE IN PRESS Applied Radiation and Isotopes 67 (2009) 1042–1049

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An electrochemical method for the preparation of 63Ni source for the calibration of thermoluminescence dosimeter (TLD) Manoj Kumar a, J. Udhayakumar a, Shyamala S. Gandhi a, A.K. Satpati b, Ashutosh Dash a,, Meera Venkatesh a a b

Radiopharmaceuticals Division Bhabha Atomic Research Center, Trombay, Mumbai 400 085, India Analytical Chemistry Division Bhabha Atomic Research Center, Trombay, Mumbai 400 085, India

a r t i c l e in f o

a b s t r a c t

Article history: Received 16 July 2008 Received in revised form 19 December 2008 Accepted 20 January 2009

A novel electrochemical approach for preparation of 63Ni sources for their application as check-light source for the calibration of thermo luminescence dosimeters (TLD) is described here. Required amount of 63Ni on a copper substrate could be deposited by optimizing the experimental parameters such as current density, time of deposition, pH of the electrolyte and nickel ion concentration in the bath. 63Ni sources of strength 3.7 MBq could be prepared by electrodeposition at constant current on the copper matrix. Quality assurance tests to ensure nonleachability, uniform distribution of activity and stability of the sources that are necessary before application were performed. & 2009 Elsevier Ltd. All rights reserved.

Keywords: 63 Ni check-light sources Thermo luminescence dosimeter (TLD) calibration Electrodeposition Current density Autoradiography

1. Introduction Thermoluminescence dosimeters (TLD) are commonly used to monitor and quantitate the radiation exposure of a person working in radiation environment. These TLD readers need to be calibrated frequently to ensure that the measurements are accurate. For calibration of such thermoluminescence (TL) based dosimeter, a standard radioactive source which can provide a steady beta radiation flux, with reasonably long shelf life and is safe to handle, is necessary. 63Ni, a b-emitter with a half-life of 100 years is one such radionuclide which satisfies most of these requirements. 63Ni decays without emission of X- or g-rays and is safe to handle. It has been reported that 63Ni-coupled plastic based check-light sources give satisfactory performance (Nagpal et al., 1995) in the calibration of TL-based dosimetric system. At the request of Radiation Standards and Instrumentation Division of our institute, a large number of radioactive 63Ni- sources were fabricated by us. Development of various methods for the preparation of radioactive sources has gained momentum over the past two decades (Denecke et al., 2000; de Sanoit et al., 2004; Robinson,

 Corresponding author. Tel.: +91 22 25595372; fax: +91 22 25505151.

E-mail address: [email protected] (A. Dash). 0969-8043/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2009.01.078

1966; Shanyu et al.,1996; Tsoupko-Sitnikov et al., 2002; Udhayakumar et al., 2008; Was et al., 1993; Zhang et al., 2006). When a large number of sources of uniform activity and negligible leaching rates are required, a quick, inexpensive, easy, reliable and precise method is to be chosen. One of the methods for the preparation of radioactive source is electrodeposition (Hummrich et al., 2008; Joshi and Roy, 1974; Lee et al., 2006; Zhang and Hafeli, 2004).This method has several advantages over the other methods of producing radioactive sources because it is relatively easy, technically simple, fast and reproducible. These appealing advantages have led to the current surge in interest for the preparation of radioactive sources, by electrodeposition. Our attempt was to explore the possibility of depositing radioactive 63Ni electrochemically on a copper substrate in a predicted way. A great deal of research has been devoted on the electrochemical deposition of nickel (Hackensack, 1978; Paunovic and Schlesinger, 1998; Subramanian et al., 2005). The most common nickel plating bath is the sulfate bath known as the Watts bath. We have used ‘‘Modified Watts’ electrolyte bath solution (Ibrahim and Magdy, 2006; Watson, 1989) in which the deposition of nickel can be carried out at low Ni2+ concentrations. Although we used HNO3 as modified Watts bath (Ibrahim and Magdy, 2006), we further modified the electrolyte and carried out the deposition in dil H2SO4 medium. Careful optimization of various electrochemical parameters such as bath composition, pH, current density and plating time enabled us to prepare a large

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number of sources with high reproducibility over a long time span. We report here the successful demonstration of a novel, simple and convenient electrodeposition technique for the preparation of 63Ni sources for their application as check-light source in the calibration of thermo luminescence dosimeter (TLD).

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2. Materials Nickel-63 as nickel (II) chloride of specific activity 518 MBq (14 mCi)/mg) was procured from M/s Amersham Biotech Pharmacia, UK. Ni salt used in the experiments both in the form of radioactive and non-radioactive form is of 499% purity. Reagents such as boric acid, nickel sulfate, sulfuric acid, ammonium hydroxide as well as water were of spectroscopic grade and procured from BDH (India). Copper plate and platinum metal as a foil of high purity with material testing certificates, were procured from M/s Hindustan Platinum Ltd, Mumbai, India. All electrochemical data were acquired from a 1–28 V, 0–2.5A regulated power supply of Aplab Limited (Model No. L1282), India. Beta Radiation Survey Meter (PRM131A) of PLA Electro Appliance Pvt Ltd, India was used to measure the radiation dose on the surface of the source. The liquid scintillation counter used for the radioactivity measurement was Model: Tricarb 2100TR-Packard Instrument Co, USA. The liquid scintillation used was Aquasafe 300 Plus of M/s Zinsser Analytic GmbH which is suitable for low to medium ionic strength aqueous samples with a high counting efficiency. This scintillation cocktail AQUASAFE 300 Plus is capable of holding up to 3 mL of aqueous sample per 10 mL of the cocktail. Deposition and stripping experiments were carried out using Eco chemie potentiostat, AUTOLAB-100 run by GPES software using three-electrode geometry.

3. Experimental 3.1. Preparation of the electrolyte About 20.81MBq (562.5 micro curies) of 63NiCl2 was pipetted out from original stock solution volume of 0.1 mL in a beaker. To this non-radioactive cold Ni carrier as NiSO4  7H2O corresponding to 0.8975 mg of Ni was then added. 1 mL of Conc HNO3 was added to it. Generally, 10–15% excess of 63Ni activity was taken in the electrolyte bath to compensate for the activity due to change in deposition efficiency, if any. The resulting solution was evaporated to dryness and taken up in 125 mL of 0.01 M H2SO4. The volume of radioactive 63Ni solution added was 100 mL, containing 20.81 MBq (562.5 micro curies) of specific activity 518 MBq (14 mCi)/mg Ni. This amounts to 0.04 mg of Ni. The total Ni content was 0.9375 mg which was present in a total electrolyte volume of 125 mL. Thus the Ni concentration in the electrolyte was 0.0075 mg/mL or 7.5 mg/mL. The bath includes boric acid at a concentration of 30 mg/mL. The nickel feed stock; sulfuric acid and water were high purity grade materials. The pH of the bath was adjusted by the addition of 0.1 M H2SO4 or 0.1 M NH4OH. The electrolyte was purged with pure argon (Ar) gas, to remove dissolved gases. 3.2. Preparation of the electrodes The copper cathode was shaped in the form of a series of circular disc joined with each other by a narrow strip in between as shown in the schematic diagram in Fig. 1. Typically five discs were present in each cathode. The copper cathodes used in this work were hand made, machined from a sheet of extremely pure

2 mm

10 mm ( ) Fig. 1. Schematic diagram of the copper cathode for

63

Ni deposition.

metal (99.99%) as the presence of impurities inhibits radio nickel electrodeposition, leading to anomalous deposition. Platinum foil of equal surface area was used as anode. The electrode surface was thoroughly cleaned to remove any particulates, oil, grease, or other matter that may have adhered during the previous machining operations or handling. The copper cathodes were mechanically polished using steel wool, rinsed with 5% sulfuric acid followed by deionized water. They were then subsequently cleaned ultrasonically in acetone and chloroform for 5 min and dried under an infra red (I.R.) lamp. 3.3. Electrochemical cell The schematic diagram of the electrochemical cell is shown in Fig. 2. The set up consists of a 200 mL quartz cylinder containing electrolyte solution. The electrodes were fitted 5 mm apart on the acrylic cap, which, along with the electrodes, fitted tightly on the mouth of the quartz cylinder. The acrylic cap helps to maintain a constant distance between cathode and anode. The electrodes were adjusted parallel to each other and then connected to the power supply using small screws which were embedded into the acrylic cap and touched the rod. A provision was made for passing Ar gas through an acrylic tube, which dipped into the electrolysis solution. A small outlet of 2.5 mm (f) was provided in the acrylic cap for venting the gases. One side of the copper substrate as well as the area separating the copper discs (copper strips between two discs) were masked with wax to prevent electrodeposition in these areas. Electrodeposition of Ni2+ on the cathode was achieved by applying a potential difference between the electrodes using a D.C. power supply. Several experiments were carried out, varying the electrodeposition parameters such as current density, time of deposition, pH of the electrolyte and nickel ion concentration in the bath to achieve optimal deposition of Ni. In stripping experiments copper strip was used as the working electrode, glassy carbon rod as the counter and saturated

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D.C. Power Supply Inert Gas Bubbling A

Vent

Platinum Plate 63

Ni Electrolyte

Copper Electrode

Argon Gas Cylinder

Magnetic Stirrer Fig. 2. Electrochemical cell.

calomel (SCE) as the reference electrodes fitted on the VA663 stand.

3.4. Preparation of

63

Ni sources

Electrodeposition of 63Ni was carried out at constant current (current density of about 4–6 mAcm–2) for 2 h at room temperature. Prior to each experiment, the solutions were deoxygenated bubbling of Ar gas for 1 h. After the deposition, the cathode was rinsed in DD water and dried under an I.R. lamp. It was subsequently heated at 2501–300 1C in a furnace for 30 min. Each 63 Ni deposited disc was subsequently cut along the boundary line. Typically 5 discs were obtained each time. The amount of 63Ni deposited on the copper matrix was measured by comparing the dose registered in a beta radiation survey monitor, which was in turn calibrated using a set of sources of 63Ni prepared by evaporation method. Typically, a known amount of 63Ni activity was deposited on the surface of a copper disc of 4 cm2 total area (same geometry as the electrodeposited sources) and allowed to evaporate under infra red lamp. The 63Ni activity on this source was measured as dose at a fixed geometry using a beta radiation survey monitor. Similarly the whole set of process with increasing 63Ni activity was prepared and the dose values in each were measured. A calibration curve of activity Vs dose was plotted and used for calculation of the 63Ni activity in the electrodeposited sources. The activity measurement values were used only as a guideline to follow the progress of electrodeposition on the cathode with time. The assay procedure of each 63Ni beta source adapted consists of two stages for activity confirmation. In the first step, the total electrolyte activity strength was assayed by liquid scintillation counting by drawing suitable electrolyte aliquots before and after the plating. The amount of 63Ni deposited on the matrix was calculated to assess the overall batch deposition efficiency. The amount of activity on a series of five tandem sources was determined by difference in activity divided by 5. Later, in the second step, each source was individually measured for its beta ionization current using precalibrated Ionization Chamber by comparison method.

3.5. Quality assurance of the sources 3.5.1. Removable activity The radioactive source should be free from loose contamination under normal condition of use and swipe test of the surface is necessary to check the integrity of the radioactive deposit. The sources were tested for absence of loose activity (surface contamination) by swiping the sources using alcohol immersed cotton wool and estimating the radioactivity on the cotton wool. 3.5.2. Leachability The leachability of the 63Ni source was tested as per the method prescribed by the Atomic Energy Regulatory Board, India (AERB Safety Standard no:AERB/SS/3 (Rev.1), 2001). Five numbers of sources were randomly selected. Each source was placed in beaker containing100 mL water at room temperature for 48 h, at the end of which the sources were removed. The radioactivity in the water was concentrated to 0.1 mL by heating and measured in a liquid scintillation counter. 3.5.3. Uniformity of activity distribution Five number of randomly selected electrodeposited 63Ni sources were subjected to autoradiography examination to assess the uniformity of distribution of the activity. A photographic film was wrapped on the source in a dark room. The film gets exposed when kept on the source. The optical density distribution of the exposed film was measured by B/W transmission densitometer and the image densities obtained were scanned for its uniformity. Each source samples were analyzed in a similar method.

4. Results Electrodeposition of radioactive nickel 63Ni on the cathode was achieved by applying a potential difference between the electrodes. In order to obtain a reproducible, uniform deposit of 63Ni, it was essential to maintain a constant distance between cathode and anode in the electrolytic cell. The concentration of Ni2+ used for this purpose was low and the electrolysis had to be performed

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Percentage of Radiation out-put from the source (normalised to maximum out-put)

Percentage of Radiation out-put from the source (normalised to maximum out-put)

100

80

60

40

20

100

80

60

40

20

0

0 1

0

2

3

4

0

5

1

2

4

Fig. 5. Effect of electrodeposition time.

63

Ni.

Percentage of Radiation out-put from the source (normalised to maximum out-put)

Fig. 3. Effect of pH of electrolyte on electro deposition of

Percentage of Radiation out-put from the source (normalised to maximum out-put)

3

Time (hours)

pH of the electrolyte

100

80

60

40

20

100

80

60

40

20

0

0

0

0

2

4

6

8

10

12

2

3

4

5

Fig. 6. Effect of Ni2+ concentration of electrolyte on 63

6

7

8

Nickel content of the bath (microgram/ml)

Current density (mA/cm2) Fig. 4. Effect of current density on electro deposition of

1

63

Ni deposition.

Ni.

in a custom made electrochemical cell. Therefore, optimization of various electrochemical parameters such as pH of the electrobath, current density, time of deposition and chemical concentration of Ni2+ was essential and hence pursued. In order to make the electrolyte free from Cl, Conc HNO3 was added to the feed and heated. Presence of chloride ions may corrode the electrode surface. The resulting solution was evaporated to dryness and then reconstituted with 125 mL of 0.01 M H2SO4. The dependence of pH on the electrodeposition of 63Ni is shown in the Fig. 3. It was observed that hydrogen is evolved as the bath pH decreases and this lowers the deposition of 63Ni. It is seen from the result that 63Ni deposition was best at pH 3 and 4, prompting us to adapt this pH for the preparation of sources. As we were aware that Ni2+ tends to form hydroxide at alkaline pH, we did not carried out experiment at alkaline range. The effects of current density on 63Ni deposition in duration of 2 h at pH3 at a constant Ni concentration (6 mg/mL ) is shown in Fig. 4. Percentage of 63Ni deposition was found to increase initially with increase in current density up to 3 mA cm–2 and remained constant with further increase in current density. About 99% of

63

Ni deposition was observed beyond 4 mA cm–2 current density. However, it was also observed that at higher current density (4 6 mA cm–2), there was a formation of a black deposit on the cathode which could be the black Ni. Hence we conducted further experiments between 4-6 mA cm–2, to obtain maximum deposit of 63 Ni without blackening. Certain minimum electrodeposition time was required. For example, from Fig. 5, it is seen that at a constant current density of 6 mA cm–2 at pH3, at least 2 h were required to obtain constant radiation out put from the deposited 63Ni on the substrate. The influence of nickel concentration on the radiation out put of the source at a constant current density of 6 mA cm–2 in 2 hours at pH3 is depicted in Fig. 6. As expected, the carrier Ni2+ is seen to influence the deposition rate. At lower concentrations, the deposition rates were not high enough to yield maximum radiation out put from the deposited source after 2 h of deposition. Perhaps on longer deposition time, it would have reached the maximum 63Ni deposition. It was noted that at least 6–8 mg/mL concentration of Ni2+ carrier was necessary to obtain maximum radiation out put in 2 h time.

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0.0

-0.6

-0.9

Current, A

Potential, V (SCE)

-2.0x10-3

-4.0x10-3

-6.0x10-3

-1.2

-8.0x10-3

-1.5 0

5000

10000 Time, s

15000

0

20000

5000

10000

15000

20000

Time, s

Fig. 7. Amperometric deposition of Ni on Cu electrode (with application of 2 mA cm2 current on the working electrode using three electrode geometry).

Fig. 9. Potentiometric deposition of Ni on the Cu electrode surface at a constant potential of 1.0 V (SCE) at the working electrode.

0.40 600

500

Dose rate (mR/hr)

Potential, V (SCE)

0.35

0.30

400

300

200

0.25 100

0

0

5000

10000 Time, s

15000

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

63Ni activity (MBq)

Fig. 8. Amperometric stripping of Ni+2 from its deposit on Cu electrode (with application of 2 mA cm2 current on the working electrode using three electrode geometry).

Fig. 10. Dose rate versus activity relationship.

Experiments were carried out under constant cathodic current of 2 mA/cm2 for 5 h to observe the changes in the potential with deposition of Ni. As seen in Fig. 7, under -2 mA/cm2 current, the potential at the working electrode was nearly constant at around 1.0 V (SCE). This potential did not increase with increase in the deposition time. On the other hand there was a marginal decrease in the discharge potential. Anodic stripping experiments were carried out at constant anodic current of 2 mA/cm2 for 5 h. It is seen from Fig. 8 that the stripping potential remains constant in the range of 0.28 V to 0.3 V at the initial 2 h of stripping, however after 2 h time, the potential increases steeply up to 0.38 V. There is a possibility of the oxidation of the base Cu matrix with the application of anodic stripping current of 2 mA/cm2 for long time. This is supported by the fact that at the end of stripping process, the Cu substrate was found to be oxidized. Electrodeposition experiments were also carried out at a constant potential of 1.0 V for 5 h the result of which are shown in Fig. 9. A steady cathodic current density of 2 mA/cm2 was seen to be maintained with the application of 1.0 V as the deposition potential. This result is in concurrence with the result of constant current deposition under

cathodic current density (2 mA/cm2) where the electrode potential stayed at around 1.0 V. Based on the above series of experiments, an electrolytic bath composition as described in the source preparation method was selected for the preparation of the radioactive source. Since we are interested in the deposition of 63Ni irrespective of its chemical identity, we have not taken efforts to analyze the deposition morphology and its chemical nature. The 63Ni was designed to deliver b-radiation flux of prescribed intensity into the TLD from the surface. In order to correlate the dose rate from the source to the activity of 63Ni, simulated sources with known amount of 63Ni were used. Fig. 10 displays the dose rate observed in the radiation monitor versus the 63 Ni activity deposited by evaporation on the source matrix. The response is seen to be linear and therefore used as a guideline to follow the electrodeposition of 63Ni. The indirect methods of measurements of 63Ni activity by Liquid Scintillation counting of the residual activity in the electrobath was fairly accurate and was used for calculating the total amount of activity deposited on the five sources. Later, each source was

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5.0 4 hr Exposure 8hr Exposure 12hr Exposure

4.5 4.0

Optical densities

3.5 3.0 2.5 2.0 1.5 1.0

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selected heated sources of 3.6–3.8 MBq strength. The results are depicted in Table 1, where it is seen that 108–0.05 Bq counts are found in the swipe, which is well within the specifications laid by the Atomic Energy Regulatory Board, India (AERB ) (AERB/SS/3, 2001) (Limit 185Bq). Following the procedure described above, several batches of sources were prepared. After each electrodeposition, the spent electrolyte was recycled and replenished with appropriate amount of 63Ni. As designed 3.7 MBq (100 mCi) of 63Ni activity could be deposited in each source. About ten sources were selected randomly and their activities assayed. The radioactivity deposited in ten randomly selected sources were (3.6470.007) MBq. Thus this method could be used to prepare sources with 2% variation.

0.5 0.0

5. Discussion 1

2

3

4

5

Source Number Fig. 11. Variation of the optical density of the radiography plates exposed for different time.

Table. 1 Swipe test of radioactive sources. S. no.

Activity of the

1 2 3 4 5

3.88 3.63 3.55 3.92 3.77

(105) (98) (96) (106) (102)

63

Ni source MBq (mCi)

Surface contamination Bq (nCi) 110 104 114 102 110

(2.97) (2.81) (3.08) (2.76) (2.97)

individually measured for its beta ionization current using precalibrated Ionization Chamber. Leaching studies conducted on 5 randomly selected sources activity of 3.55 MBq (96 mCi) to 3.92 MBq (106 mCi)) indicated that (0.2570.01) % of the original activity leached out, which was 25 times more than the AERB stipulated limits(AERB/SS/3, 2001). This gave us information regarding the level of surface contamination of 63Ni source obtained immediately after electrodeposition experiment. Thermal treatment of the sources to retard the leaching of 63Ni has been reported (Zafeiratos et al., 2004; Zhitomirsky, 2004). Hence heating of the sources to fix any loosely bound 63Ni atoms on the surface of the copper substrate then resorted to. Freshly prepared samples (unleached) were heated to various temperatures to study the effect of heating on leachability. Thus after optimal heating, the leachability could be reduced to 0.007%. The heating parameters for the sources were optimized by conducting a series of experiments. Significant reduction in the leach rate of 63Ni from the sources, could be achieved when the sources were heated to 250–300 1C. Typically 5 sources of activity of 3.45 MBq (93 mCi) to 3.82 MBq (103 mCi)) after heating exhibited leachability of (0.00770.001)% of the original activities, which complies with the specifications laid by the Atomic Energy Regulatory Board, India (AERB/SS/3, 2001). The spatial distribution of 63Ni on the matrix was ascertained by autoradiography examination of the active samples. The optical densities (O.D.) of radiographic film after different times of exposure on five different batches are graphically presented in the Fig. 11. It is seen from the results that the distribution of activity on the source was fairly uniform with a variation of 75%. In order to ensure absence of removable activity from the source, swipe test was performed on 5 numbers of randomly

Our primary aim was to develop an easy, reproducible method to prepare large number of 63Ni sources of 3.7 MBq strength with good precision. The merits of electrochemical technique for metallic coating are well established. This approach is currently used to prepare a variety of radioactive sources. In order to get small 63Ni sources of uniform intensity and spatial distribution on a metal substrate, electrochemical technique was considered an ideal way. We hence focused on the electrodeposition of 63Ni on a metallic substrate. 63 Ni is a pure beta emitter, with Emax ¼ 0.067 MeV (100%). As the beta energy is low, self-absorption and attenuation would be significant if the source thickness is appreciable. It was therefore necessary to minimize the total thickness of deposition, and hence the total amount of Ni deposited. Although we desired to use ‘‘no carrier’’ added 63Ni of the highest activity possible, the deposition was poor and too slow. Hence the minimum amount of carrier required for optimum deposition was estimated and used. Since the cell capacity of 125 mL was required for uniform deposition of Ni2+ on the base matrix, this limited the maximum concentration of Ni2+ in the electrolyte solution. Owing to all the above factors, the Ni2+ solution used was of low concentration. In view of the precious nature of 63Ni, all attempts to utilize 63 Ni to its full extent, merits attention and therefore the various electrochemical parameters were optimized. The ability to control the amount of 63Ni deposition, achieve uniform distribution of radioactivity on the metal substrate and achieve minimal leaching of radioactivity from the substrate, are the major advantages of this technology. In the electrochemical deposition process, platinum has been used as anode material as it is inert, stable and does not passivate easily. Copper was chosen as cathode because of its suitable electro potential for deposition of Ni. Standard reduction potential for nickel and copper are 0.23 and +0.337 V respectively (Weast and Astle, 1981). Use of copper cathode material will drive the hydrogen over potential higher, and thus favor nickel deposition. Cleaning of the cathode with acid activates the surface of the cathode and renders it receptive for electrodeposition of 63Ni2+. It is also important that the surface of the electrode be nearly plane, since surface irregularities will lead to non uniformity in deposition and hence in b-radiation flux, during application. It is essential to prepare the electrolyte bath using pure chemicals and chromatographic grade water, since any contaminants such as iron, copper, zinc, and organics if present even in trace quantities, could affect the electrodeposition. Hence the entire reagents used were of 499.9% pure grade. Purging of the electrolyte with argon was essential to remove dissolved gases from the electrobath and to provide an agitating action during electrolysis.

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Addition of HNO3 was necessary to destroy the Cl ion, as it can corrode the electrode, under high current density. HNO3 was added to decompose chloride ions to chlorine gas, and render the solution free from chloride ion. Later, the nitrate solution was evaporated to dryness and then reconstituted with 0.01 M H2SO4. In the electrolyte, 100 mL of radioactive 63Ni solution corresponding to 20.81 MBq (562.5 micro curies) was added (specific activity 518 MBq (14 mCi)/mg Ni). This amounts to 0.04 mg of Ni. Considering the 0.8975 mg of non-radioactive cold Ni carrier added, the total Ni content was 0.9375 mg. Therefore, 0.9375 mg of Ni was present in the total electrolyte volume of 125 mL.. Thus, the Ni concentration in the electrolyte was 0.0075 mg/mL or 7.5 mg/mL. The deposition efficiency was monitored by following the 63Ni activity in the electrolyte which is expected to behave in identical manner as cold Ni. Deposition of 63Ni on a copper cathode takes place as per the reaction shown below: 2Ni2þ þ 2e Ð 2NiðsÞ

(1)

One of the competing reactions for nickel deposition is the formation of H2 gas at the cathode as the standard reduction potential for nickel deposition is 0.23, close to that for hydrogen gas evolution, which is 0.00. Evolution of hydrogen could change the pH of the electrobath due to consumption of protons, as per the following reaction. 2Hþ þ 2e Ð H2ðgÞ

(2)

Operation of the electrochemical cell above a certain pH value would hinder hydrogen gas formation due to low H+ concentration; facilitate Ni2+ reduction and deposition. However a very high pH in the alkaline range would cause precipitation of Ni2+ as hydroxide. The pH of the electrolytic solution therefore plays a crucial role in the electrodeposition of 63Ni2+ and hence optimized. Boric acid in the electrolyte bath is reported to induce better film formation, and prevent passivation of electrode surface on nickel reduction (Yin and Lin, 1996) and hence we used boric acid in our bath. In order to have minimum attenuation of the beta particles, one would prefer minimum thickness of deposited 63Ni. Our present study has demonstrated that by a judicious optimization of electrochemical parameters, adequate quantities of 63Ni capable of giving required radiation output could be deposited on a copper substrate. As mentioned earlier, 63Ni with specific activity 518 MBq (14 mCi)/mg was used to prepare the beta source samples of 3.7 MBq (100 mCi) by electro-deposition method. Each source of 3.7 MBq (100 mCi) corresponds to 7.1 mg of radioactive 63Ni and the total content of Ni would be 186.6 mg of Ni, including 179.6 micrograms of cold Ni. The source matrix material consists of copper disc of 10 mm dia, i.e., 0.786 sq cm. Therefore, 186.6 mg of Ni on this source would translate to 237.4 mg/cm2 or 0.237 mg/cm2 of Ni. Assuming that Ni is deposited as metallic film, the thickness of this would be 0.000237 cm or 2.37 microns (rNi ¼ 8.9 g/cm3). However, this calculation is only approximate, based on the assumption of uniform deposition of Ni on the substrate. Other compounds of Ni if present in the matrix would alter the calculation. A general indication that the film formed is of few microns thick could be deduced. An ideal radioactive source should contain a homogeneous layer of radioactive material on the metal substrate in order to deliver uniform b-radiation flux on the target. The activity distribution, and thus the homogeneity, was ascertained by auto-radiographic examination of the source using photographic film. For our type of application, it is important to achieve uniform deposition of 63Ni with required b out put. The thickness is not a

major direct concern. The thickness plays a role owing to attenuation and this we desire to have minimum thickness for required source strength. The method described here could result in achieving these aims. Leachability of activity from the sources is hazardous and stringent limits are set for permissible leachability. The leachability of untreated 63Ni sources was quite significant in aqueous solution. However, subsequent thermal treatment at 250–300 1C made it leach resistance. It has been reported that the electrodeposition of nickel takes place through a number of intermediate ˇ a´kova´ et al., 2006) as indicated steps (Kostin et al.,1982; Orin below: Ni2þ þ H2 O Ð NiOHþ þ Hþ

(3)

NiOHþ OH Ð NiðOHÞ2

(4)

The presences of some of the 63Ni activity loosely held on the copper matrix in the form of NiOH+ or Ni (OH)2 is perhaps responsible for high leaching rate. When the electro-deposited 63 Ni sources were heated to 200–300 1C for about 3 min, it would cause the surrounding solution in the electrode to evaporate, leaving the Ni ions trapped in the layers of the deposit and renders it leach resistance. For our purpose, the form in which Ni is deposited is not of importance as the b out put is the main concern. Thermal treatment at 250–300 1C helps in strong adherence of 63Ni on the surface of the copper substrate and results in low leachability. Absence of loose contamination below 185Bq level made the sources safe for use in the applications envisaged. The performances of these sources were evaluated by Radiation Safety System Division of our institute and were found to be effective for carrying out the calibration of thermo luminescence dosimeters. The major advantage of the present method is the simplicity of a single electrolysis step and economy. The platinum electrodes, the electrochemical cell and the power supply are non-recurring investment that can be used repeatedly. The entire electrodeposition process is also amenable for automation and would help in reducing the men rem expenditure. Since its development, more than 100 consignments of 63Ni sources of 3.7 MBq each were prepared and supplied to various users. This has reduced user’s dependence on imported radiation sources, in our country.

6. Conclusion The potential utility of electrochemical method for the preparation of 63Ni sources needed for the calibration of TLD was demonstrated. 63Ni could be electrodeposited on the copper substrate in a single electrolysis step. The deposition process is simple and highly efficient. The average practical deposition yield was 490% in 2–3 h. In comparison to other procedures, the electrochemical method has the advantages that it could be carried out at room temperature, uses relatively inexpensive equipment, and the amount of radioactive deposited can be easily controlled. The mild experimental conditions of this process facilitate safe handling of radioactivity. These sources were very stable, intact and complied with the specifications laid by the Atomic Energy Regulatory Board of India. It is anticipated that there could be a need for other types of radioactive sources for various applications, which could also be made using similar electro deposition technique.

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