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Thermal separation is defined as a mass transfer process driven by molecular forces. The process involves the heat transfer between two phases with different composition. In general, thermal separation occurs when heat is generated in the system additionally to the already existing phases. In a second phase the mass is transferred in the system (adsorption) and at the end of this step the separation is completed. The thermal separation can be achieved in temperature or concentration gradient function of system configuration [1]. Thermo-chromatography is a process in which the separation occurs in gase-ous phase. By passing a heated gas through a column a thermal gradient is created with a continuously decreasing temperature along the column. The separation occurs based on the different volatilization temperatures, the less volatile species will condense on the column walls at the higher temperatures and the highly volatile compounds will condense at lower temperatures. Parameters like temperature, carrier flow rate, column geometry and length have impact on the absorption of the compound on the column material affecting the separation efficiency. The thermal separation has been used for separation of Molybdenum (Mo) and Technetium (Tc) by either sublimation in the case of 94mTc {2,3,4] or dry distillation in the case of 99mTc from neutron irradiated MoO3 [5]. The thermal separation process has been used in the development of a new type of Mo/Tc generators starting from the MoO3 as target material for production of 99mTc in linear accelerators [6]. Dry distillation has become a standard procedure for separation of radioiodine from tellurium targets [7]. The present paper describes the thermal separation of a three component system (Cu/Mo/Tc) used as a target in the production of 99mTc through the 100Mo(p,2n) reaction. Material and Methods The separation method involves the use of oxygen as a carrier gas and oxidation agent. The method is based on the different volatilization temperatures of Tc formed oxides and the MoO3 formed in the system during the oxidation. In the presence of oxygen the existing Tc is oxidized to its anhydride as Tc2O7 (b.p. 319 ⁰C; m.p. 110.9 ⁰C) following the reaction: 4Tc + 7O2 →2Tc2O7 The T2O7 has a saturated vapor pressure of 310 ⁰C whilst Mo is completely oxidized to MoO3 having a sublimation temperature at 750 ⁰C. The initial experimental setup comprised a quartz tube (6 mm internal diameter, 40 cm long) which is introduced into a horizontal tube furnace (model 55035A, Lindberg). The left end of the quartz tube is connected to a pure oxygen supply which flows through the separation tube at a rate of 10 mL/min. The other end of the tube is opened to the atmosphere and protected with quartz wool. The quartz tube is heated over a length of 23 cm at a temperature of 850 ⁰C. The heated carrier gas is flowing on the tube length and the temperature gradient is created along the tube from 850 ⁰C to room temperature. During the process, the oxygen carries out the Tc oxides to a lower temperature and Tc2O7 is deposited in the cooler region of the tube in a similar manner as described by Tachimory [5]. The temperature gradient is calibrated by meas-uring the temperature inside the tube at each centimeter along its length (FIG. 1). The radioactivity counting is performed by scan-ning the tube along its length every 2 centimeters by using a detection system shown in figure 2. The system comprises a GM tube coupled to a computer controlled linear actuator (Velmex Unislide). The tube is placed at a distance of approximately 25 mm from the collimator of GM. Preliminary testing using Mo powder Prior to testing the three component separation, a reference test was performed by using 120 mg of natural Mo powder (Alpha Aesar, 99.9 %) soaked with 50 MBq NaTcO4 (Cardinal Health, radiochemical purity >95 %). After evaporation the dried powder was introduced into a quartz tube (6 mm ID, 40 mm long) and heated up to 850 ⁰C in the presence of oxygen flowing at a rate of 10 mL/min. Three component separation The targets prepared for the production of 99mTc by a cyclotron were comprised of copper (Cu) (C101, oxygen free) support having a Mo layer deposited on the surface in an elliptical form as described in literature [8,9]. About 60 to 250 mg of Mo (99.9%, Alpha Aesar) was deposited on the target surface. 70 MBq of Tc (Cardinal Health) as NaTcO4 (> 99 % radiochemical purity) was deposited on the Mo insert to mimic the conditions created during proton irradiation. The Tc spike was evaporated to dryness and the Cu/Mo/Tc target was then introduced into the experimental setup. The process was allowed to continue for 20 min. The experiment was carried out by inserting the target plates in a quartz tube (CanSci, Canada) of similar design to those described by Fonslet for the separation of radio-iodine from TeO2 targets [7]. The quartz tube can be seen in FIG. 2 and illustrated with dimensions indicated in FIG. 3. Separation of in-situ cyclotron produced Tc by irradiation of Mo targets with a proton beam. A third set of experiments have been performed for in-situ generated Tc by irradiation of circular targets containing approximately 60 mg Mo deposited on a copper support. The targets were irradiated for 30 min with a proton beam with the energy of 15 MeV and a current of 50 µA. The separation was performed using similar experimental conditions as previously described. The quartz tube was scanned in length by using a RadioTLC scanning system calibrated for 99mTc and 99Mo isotopes. After the thermal separation was completed 99mTc was recovered as NaTcO4 by selectively washing the quartz tube with 1 M NaOH (Fisher) solution. The presence of Mo in the NaTcO4 solution was verified by a colorimetric strip test (EM-Quant Mo test kit, Millipore). The presence of copper was qualitatively analyzed by adding a few drops of concentrated NH4OH (Fisher) solution and checking the formation of Schweitzer reagent. Results Thermal separation of Tc-Mo powder After 20 min the deposition of MoO3 was ob-served as yellow crystals in the region of tem-perature of 770 ⁰C, which is in accordance with the results reported in the literature [5]. The activity of 99mTc was detected at about 5 cm from the exit of the tube furnace in a temperature range starting with 310 ⁰C and ending at 46 ⁰C (FIG. 4).

Introduction Alternative methods for producing the medical imaging isotope 99mTc are actively being developed around the world in anticipation of the imminent shutdown of the National Research Universal (NRU) reactor in Chalk River, Ontario, Canada and the high flux reactor (HFR) in Petten, Holland that together currently produce up to 80 % of the world’s supply through fission. The most promising alternative methods involve accelerators that focus Bremsstrahlung radiation or protons on metallic targets comprised of 100Mo and a supporting material used to conduct heat away during irradiation. As an example, the reaction 100Mo(p,2n)99mTc provides a direct route that can be incorporated into routine production in regional nuclear medicine centers that possess medical cyclotrons for production of other isotopes, such as those used for Positron Emission Tomography (PET). The targets used to produce 99mTc are subject to a number of operational constraints. They must withstand the temperatures generated by the irradiation and be fashioned to accommodate temperature gradients from in situ cooling. The targets must be resilient, which means they cannot disintegrate during irradiation or post processing, because of the radioactive nature of the products. Yet, the targets must be easily post-processed to separate the 99mTc. In addition, the method used to manufacture the targets must not be wasteful of the 100Mo, because of its cost (~$2/mg). Any manufacturing process should be able to function remotely in a shielded space to accommodate the possibility of radioactive recycled target feedstock. There are a number of methods that have been proposed for large-scale target manufacturing including electrophoretic deposition, pressing and sinter-ing, electroplating and carburization [1]. How to develop these methods for routine production is an active business [2,3]. From the industrial perspective, plasma spraying showed promising results initially [4], but the process became very expensive requiring customized equipment in order to reduce losses because of overspray,which also required a large inventory of expen-sive feedstock. In this paper we report the ex-perimental validation of an industrial process for production of targets comprising a Mo layer and a copper support. Materials and methods Target Design Targets have been manufactured for irradiation at 15 MeV. Two targets are shown in FIG. 1: one as-manufactured and another after irradiation; no visible changes were observed following irradiation. The supporting circular copper (C101) disks have diameters of 24 mm and thickness of 1.6 mm. The molybdenum in the center of the target is fully dense with thickness 230 μm determined from SEM cross-sections.Targets have also been manufactured for irradi-ation in a general-purpose target holder designed to be attached to all makes of cyclotrons found in regional nuclear medicine centers. The elliptical targets were designed for high-volume production of 99mTc with 15 MeV protons at currents of 400 µA with 15% collimation [4]. The elliptical shape reduces the heat flux associated with high current sources. The cooling channels on the back of the target are designed to with-stand the high temperature generated during Irradiation. A thermal simulation of expected temperatures during irradiation is shown in FIG. 3. The center of the target is expected to reach 260 oC during irradiation. The elliptical targets were formed from a 27 mm C101 copper plate with width 22 mm and length 55 mm. The molybdenum in the center of the target is fully dense with thickness 60 m de-termined from SEM cross-sections. FIG. 4 shows the molybdenum deposition in the center of the target in a form of an ellipse (38×10 mm). Results and Conclusions Circular targets have been produced and suc-cessfully irradiated for up to 5 h with a proton beam with energy 15 MeV and current 50 µA. (FIG. 1). The targets were resilient. Before irradi-ation the targets were subjected to mechanical shock tests and thermal gradients with no ob-servable effect. After irradiation there was no indication of any degradation. The manufacturing process produced 20 consistently reproducible targets within an hour with a molybdenum loss of less than 2 %. After irradiation the targets were chemically processed and the products characterized by Ge-HP gamma spectrometry. Only Tc isotopes were found. No other contami-nants were identified after processing. The de-tails of the separation and purification are de-scribed elsewhere [5]. Circular targets suitable for low-volume produc-tion of 99mTc have been manufactured and test-ed. The targets have been shown to meet the required operation constraints: the targets are resilient withstanding mechanical shock and irradiation conditions; they are readily produced with minimal losses; and post-processing after irradiation for 5 h has been shown to produce 99mTc. Elliptical targets suitable for high-volume pro-duction of 99mTc with high power cyclotrons have been manufactured (FIG. 4). Like the circular targets, the elliptical targets are readily pro-duced with minimal losses and are able to with-stand mechanical shock and thermal gradients; however, they have yet to be irradiated.

International audience; A Markov Chain Monte Carlo model for integrating the observations of inorganic species with a thermodynamic equilibrium model was presented in Part I of this series. Using observations taken at three ground sites, i.e. a residential, industrial and rural site, during the MCMA-2003 campaign in Mexico City, the model is used to analyze the inorganic particle and ammonia data and to predict gas phase concentrations of nitric and hydrochloric acid. In general, the model is able to accurately predict the observed inorganic particle concentrations at all three sites. The agreement between the predicted and observed gas phase ammonia concentration is excellent. The NOz concentration calculated from the NOy, NO and NO2 observations is of limited use in constraining the gas phase nitric acid concentration given the large uncertainties in this measure of nitric acid and additional reactive nitrogen species. Focusing on the acidic period of 9?11 April identified by Salcedo et al. (2006), the model accurately predicts the particle phase observations during this period with the exception of the nitrate predictions after 10:00 a.m. (Central Daylight Time, CDT) on 9 April, where the model underpredicts the observations by, on average, 20%. This period had a low planetary boundary layer, very high particle concentrations, and higher than expected nitrogen dioxide concentrations. For periods when the particle chloride observations are consistently above the detection limit, the model is able to both accurately predict the particle chloride mass concentrations and provide well-constrained HCl (g) concentrations. The availability of gas-phase ammonia observations helps constrain the predicted HCl (g) concentrations. When the particles are aqueous, the most likely concentrations of HCl (g) are in the sub-ppbv range. The most likely predicted concentration of HCl (g) was found to reach concentrations of order 10 ppbv if the particles are dry. Finally, the atmospheric relevance of HCl (g) is discussed in terms of its indicator properties for the possible influence of chlorine-mediated photochemistry in Mexico City.