Diffusion-based separation methods: Dry distillation of zinc, cadmium and mercury isotopes from irradiated targets

~

Appl. Radiat. lsot. Vol.48, No. 5, pp. 565-569, 1997

Pergamon

PII: S0969-8043(90)00339-9

~ 1997ElsevierScienceLtd. All rights reserved Printed in Great Britain 0969-8043/97 $17.00+ 0.00

Diffusion-based Separation Methods" Dry Distillation of Zinc, Cadmium and Mercury Isotopes from Irradiated Targets V L A D I M I R T O L M A C H E V ~':, H A N S L U N D Q V I S T .2 a n d LARS EINARSSON 3 qnstitute of General and Nuclear Physics, Russian Research Centre "Kurchatov Institute', Moscow, Russia, ZDepartment Biomedical Radiation Research, Uppsala University, Uppsala, Sweden, and ~The Svedberg Laboratory, Uppsala University, Uppsala, Sweden (Received for publication 24 December 1996)

Diffusion-based separation methods allow the extraction of produced radionuclides with a low loss of target material, which is of special importance when enriched target material is used. We present a simple, non-destructive and rapid method to separate radioactive isotopes of liB group elements (zinc, cadmium and mercury) from IB group metal targets irradiated with protons. Irradiated target foils were heated to a temperature 20'~Cbelow the melting point of the target material. During these conditions at least 90% of the desired radioactivity was evaporated with negligible loss of target material. Separation time was 15 min for mercury, 60 min for cadmium and 120 min for zinc. :C 1997 Elsevier Science Ltd

Introduction The PET-technique is mainly associated with the use of the short-lived radionuclides "C, 13N, 150 and ~SF. However, for several reasons there is an increasing interest to use low energy cyclotrons at PET-centers for the production of other biogenic and non-biogenic radionuclides (Philpott et al., 1995; Herzog el al., 1993; Jansen et al., 1994). Positron emitters with longer half-lives can extend the physiological time window of PET, labeling techniques using chelators can be applied and physiological studies of several essential trace elements may be possible. A pre-requisite is that it should be possible to produce those radio nuclides in useful quantities (for one or two patient studies) with the accelerators available at PET-centers. The production costs (target material, equipment and personnel) should be of the same order as the production cost of commonly used radionuclides like ~SF. In order to produce these radionuclides with high radionuclidic purity often enriched target material has to be used. Proton energy in the range of 12-16 MeV means fairly thin targets (200-300 lam), but target costs may still be high. Low-scale production and low production cost therefore necessitate the re-use of expensive enriched target material. Standard wet separation techniques are in most cases not applicable due to losses in the recovery and due to the need of qualified personnel. In this *Present address for correspondence: BiomedicalRadiation Sciences, Box 535, S-75121 Uppsala, Sweden.

context thermal diffusion-based radionuclide separation methods seem to be attractive. Produced radionuclides are rapidly removed from the bulk of the target. Expensive enriched targets can be re-used several times without working-up and the technique allows a high degree of automation. We are therefore studying the pre-requisites for applying such methods (Lundqvist et al., 1995; Tolmachev and Lundqvist, 1996; L6vqvist et al., 1996). Heat diffusion of the produced radionuclide plays an important role in a number of radiochemical separation methods such as ISOL, dry distillation or volatilization, and surface etching. All these methods work close to the melting point of the target, when the diffusion coefficients in solid state are in the same order as in liquid. A produced radionuclide can be considered to be dissolved in the target material. The surface etching technique seems to be applicable when the melting point of the produced element is lower than that of the target material. In this case the solved radionuclides tend to concentrate on the target surface during heating but do not evaporate. In many cases it is then possible to remove the produced nuclide from the surface by rinsing with very diluted acid (Lundqvist et al., 1995; Tolmachev and Lundqvist, 1996). The losses of target material under such conditions are ~ 1% or less. Dry distillation, or volatilization, has been extensively applied in the production of halogen radioisotopes, like 7SBr(Kovacs et al., 1985; Vaalburg et al., 1985), ~23I(Van den Bosch et al., 1977) and mAt

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Vladimir Tolmachev et al.

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(Lambrecht and Mirzadeh, 1985). However, only few investigations were associated with the application of this method in the separation of other elements. In the present work we report an experimental feasibility study regarding the separation of liB group elements from target metals belonging to the IB group of the Periodic Table• To our knowledge, dry distillation of 195mHgfrom gold (Guillaume et al., 1982; Bett et al., 1983) is the only reported application among these elements. For PET applications the technique may be used for the separation of two zinc isotopes (62Zn and 63Zn) since enriched targets have to be used to avoid contamination with long lived 65Zn. Material

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o 600

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700

800

900

1000

1100

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Fig. 1• Release of produced radioactivity as a function of temperature. The targets were at each temperature heated during a fixed time (30 min for Zn and Cd, 5 min for Hg).

and Methods

Radioactivity measurement An ultra pure germanium detector on line with a PC-based 8192 channel multichannel analyzer was used to measure gamma spectra and to quantify the amount of produced radioactivity. To monitor the separation process, gamma lines from 65Zn ( l l l 5 k e V ) , 1°9Cd (88keV) and '93Hg (572 keV) were used.

Targets Metal targets with the natural isotopic composition were used to investigate the separation by dry distillation• Foils with a diameter 15 mm and with a thickness of about 100/lm were stacked• The precise thickness of each foil was determined by weight. The stacked foils were encapsulated in an aluminum box and were irradiated with protons at the Gustaf Werner cyclotron (The Svedberg Laboratory, Uppsala, Sweden). Targets were water cooled during irradiation. Total target thickness and proton energy on targets were 1100 mg/cm 2 and 20.4 MeV for copper, 520 mg/cm 2 and 20 MeV for silver, and 1100 mg/cm 2 and 72.6 MeV for gold, respectively. The proton beam current varied between 5 and 101aA. Typical integrated beam currents were 50-70 laAh.

The dependence of the recovery from the target as a function of temperature and heating time was investigated. During the study of dependence on the heating time, the temperature was kept at 20°C below the melting point of the target material. Experimental conditions during these experiments are given in Figs 1 and 2. Repeated irradiation and separations (three cycles) were made in five copper foils to study the reproducibility of the distillation process. Deposition of evaporated radioactivity on the quartz tube wall was measured as a function of the temperature gradient in the oven. After separation, the radioactive parts of the tube were rinsed with diluted acid.

Calculation of diffusion coefficients Diffusion coefficients at different temperatures were calculated from the following formula (Beyer et al., 1988) which describes Fick's second law in the special case of a thin foil. F(%) = 100 -- (800/n 2) 20

• ~ 1/(2n + 1)2 exp( - (2n + 1)2 n:Ot/d:) n=0

Dr); distillation Irradiated foils were placed in a quartz tube (i.d. 20 mm) in the middle of a furnace (EuroTherm, regulated temperature up to 1200°C). A flow of argon gas (60 mL/min) was established in the tube during heating. At the end of the tube the carrier gas was bubbled through a trap with diluted HCI to prevent breakthrough of radioactivity. Before and after the distillation, the weight of the target material was noted. The radioactivity of different parts of the system (ship with target material, quartz tube and acid trap) was measured. The difference in radioactivity of the target material before and after heating, emitted from the radionuclide of interest, was taken to be the yield of the dry distillation process.

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60

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120

Heating time, min

Fig. 2. Release of radioactivity as a function of heating time. The target temperature was kept at 20°C below the melting temperature of the target (1060°C for Zn, 960°C for Cd and 1043°C for Hg).

Dry distillation of zinc, cadmium and mercury isotopes

separated zinc was deposited in an area where the quartz wall temperature varied from 735 to 985°C. This radioactivity was easily removed from the quartz glass by washing. In order to facilitate further ion-exchange purification 2 M hydrochloric acid was used.

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0 0

k 200

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Discussion 600

800 1000 1200

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Fig. 3. Deposition of separated zinc vs temperature in the quartz tube used.

where: F = fractional release of soluate D = diffusion coefficient d = foil thickness and t = diffusion time. The diffusion coefficients were then plotted as a function of temperature according to the Arrhenius equation D = D,, exp( - E,,/RT)

where: E,, = activation energy of the diffusion process.

Results

Keeping the heating time of the targets constant (30 min for Zn and Cd, 5 rain for Hg) but varying the heating temperature gave the results shown in Fig. 1. The amount of radioactivity distilled from the targets increased with temperature. When the temperature was close to the melting point of the target (T/Tm,, > 0.8) results were found to be in good agreement with the predicted exponential temperature dependence of the diffusion coefficient as seen in the Arrhenius plot (Fig. 3). In another experiment the temperature of the target was held constant (20°C below melting point of the target material) but heating time was varied. The amount of radioactivity distilled from the targets increased with heating time as shown in Fig. 2. Repeated irradiation and distillation of the same copper foils did not reveal any statistically significant variation in the separation yield. No detectable losses of target material were observed which is of importance if enriched target material is used. The deposition of the separated radioactivity as a function of the quartz wall temperature in the oven was studied. As an example, radiozinc distribution is given in Fig. 4. It is clearly seen that 90% of the

Due to the increased numbers of PET facilities world wide low energy accelerators placed at hospital sites are becoming quite common. An increasing interest is seen to utilize such accelerators for the production of non-conventional positron emitters in order to increase the application areas of PET (Philpott et al., 1995; Herzog et al., 1993; Jansen et al., 1994; Lundqvist et al., 1995; Tolmachev and Lundqvist, 1996; L6vqvist et al., 1996). There should, as well, be an interest to use such facilities for the production of short-lived single photon emitters to be used in context with planar gamma cameras and SPECT. Low-energy nuclear reactions with protons and deuterons imply thin targets (foils) but of enriched material in order to avoid contamination of long-lived isotopes. Thermal separation techniques, if they can be applied, are ideal since thin foils allow an efficient and rapid extraction of produced radioactivity and the targets can be reused several times without any appreciable loss of expensive target material. The solution of Fick's second law given by Beyer et al. (1988) seems to describe the target yields in a proper way although the radioactivity was not even distributed (as assumed in their paper) due to the rapid change of cross-section with decreasing proton energy in target. The amount of released radioactivity

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0.0008 0 . 0 0 0 9 0.0010 Inverse absolute temperature ( K-I) Fig. 4. Arrhenius plot for diffusion coefficients of Zn ( x ), Cd (©) and Hg ( + ) . Linear dependence between given variables implies that diffusion is a limiting stage in the dry distillation process.

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from targets was found to be consistent with diffusion laws indicating that diffusion in solid state is the limiting stage of the separation process. Only when the released radioactivity fraction exceeded 80%, a slight deviation of the exponential behavior was seen, which might be explained by the trapping of diffusing atoms due to, e.g. imperfections of crystalline structure. The amount and the character of such 'traps' depend on many factors, e.g. purity of target material and of pre-handling processes, and are difficult to predict in individual cases. Separation yields were not affected when targets were repeatedly irradiated and processed. In this paper we have studied the distillation process in natural material. Enriched targets often have to be made by the users themselves from metallic powder or oxides and one might suspect that this could change the separation conditions. However, our experience in other systems like "°Cd/H°In (Lundqvist et al., 1995) and 76Se/76Br(L6vqvist et al., 1996) is that the yields are increasing in the enriched systems compared with natural targets. The preparation of enriched copper and silver targets should not give any problems and we foresee the same separation patterns in these targets as in the natural targets. An increase in temperature will increase the diffusion coefficient. However, melting of the target will form a droplet and the increased target thickness might actually decrease the evaporation yield. Higher temperatures will also increase target losses due to evaporation of target material. Melting of the target also necessitates a mechanical treatment (rolling) before next irradiation which complicates the handling and will increase radiation dose to personnel. From a practical point of view it is more convenient to work at a temperature as close as possible to, but below the melting point of the target material. The temperatures used in this paper, 20°C below the melting point, was then mainly set by the quality of the temperature control of the oven. As seen in Fick's second law, the separation rate is dependent on the square of the foil thickness. The optimal proton range for the production of ~3Zn is 7-13.5 MeV, which corresponds to a target thickness of 0.25 g/cm 2 (280 ttm). This thickness is somewhat too large to obtain an appropriate separation yield within a reasonable separation time. A double foil or a slanted target with a physical thickness of 100 mm may solve this problem. The separation equipment should be optimized in routine production. It is expected, for example, that a more narrow tube should give a sharper deposition peak which should minimize the rinsing liquid volume. However, this is not a serious problem since the radiometals can be concentrated after the separation with the use of standard ion-exchange technique, which can also serve as an additional purification step.

The following criteria may be stated for a successful application of the distillation or etching techniques in radionuclide production: • The target material should not undergo sublimation at working temperature. • The melting point of the produced element should be lower than that of the target material. • Diffusion of the element produced should be the rate determining step in the separation process. • If the boiling temperature of the element produced is high compared with the working temperature, the etching technique may be applied. • If the boiling temperature of the element produced is low compared with the working temperature, dry distillation may be applied.

Conclusion Thermal diffusion methods, which allow simple and nondestructive separations, are of special interest in low-energy nuclear reactions. Thin targets are applied and isotopically enriched target material is often necessary to use in order to minimize isotopic impurities. Using separation methods, like dry distillation or etching, low-energy accelerators available at PET-centers can in an easy and economic way produce clinically relevant amounts of a variety of short-lived radionuclides for PET and SPECT. Acknowledgements--This work was supported by the

Swedish Medical Research Council (project No. B95-14P09822-04A) and the Medical faculty, Uppsala University.

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