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Rubidium metal target development for large scale 82Sr production: LA-UR-14-22338
Full text linkStrontium-82 (t1/2 = 25.5 d) is one of the medical isotopes produced on a large scale at the Isotope Production Facility (IPF) of the Los Alamos National Laboratory (LANL), employing a high intensity 100 MeV proton beam and RbCl targets. A constant increase in the 82Sr demand over the last decade combined with an established thermal limit of molten RbCl salt targets [1,2] has challenged the IPF’s world leading production capacity in recent years and necessitated the consideration of low-melting point (39.3 °C) Rb metal targets. Metal targets are used at other facilities [3–5] and offer obvious production rate advantages due to a higher relative density of Rb target atoms and a higher expected thermal performance of molten metal. One major disadvantage is the known violent reaction of molten Rb with cooling water and the potential for facility damage following a catastrophic target failure. This represents a significant risk, given the high beam intensities used routinely at IPF. In order to assess this risk, a target failure experiment was conducted at the LANL firing site using a mockup target station. Subsequent fabrication, irradiation and processing of two prototype targets showed a target thermal performance consistent with thermal modeling predictions and yields in agreement with predictions based on IAEA recommended cross sections [6].
Target failure test: The target failure test bed (FIG. 1) was constructed to represent a near replica of the IPF target station, incorporating its most important features. One of the most vulnerable components in the assembly is the Inconel beam window (FIG. 2) which forms the only barrier between the target cooling water and the beam line vacuum. The test bed also mimicked relevant IPF operational parameters seeking to simulate the target environment during irradiation, such as typical cooling water flow velocities around the target surfaces. While the aggressive thermal effects of the beam heating could not be simulated directly, heated cooling water (45 °C) ensured that the rubidium target material remained molten during the failure test. A worst case catastrophic target failure event was initiated by uncovering an oversized predrilled pinhole (1 mm Φ) to abruptly expose the molten target material to fast flowing cooling water.
Prototype target irradiations: Two prototype Rb metal target containers were fabricated by machining Inconel 625 parts and by EB welding. The target containers were filled with molten Rb metal under an inert argon atmosphere. Follow-ing appropriate QA inspections, the prototype targets were irradiated in the medium energy slot of a standard IPF target stack using beam currents up to 230 µA. After irradiation the targets were transported to the LANL hot cell facili-ty for processing and for 82Sr yield verification.
During the target failure test, cooling water conductivity and pressure excursions in the target chamber were continuously monitored and recorded at a rate of 1 kHz. Video footage taken of the beam window and the pinhole area combined with the recorded data indicated an aggressive reaction between the Rb metal and the cooling water, but did not reveal a violent explosion that could seriously damage the beam window. These observations, together with thermal model predictions, provided the necessary confidence to fabricate and fill prototype targets for irradiation at production-scale beam currents. X-ray imaging of filled targets (FIG. 3) shows a need for tighter control over the target fill level. One prototype target was first subjected to lower intensity (< 150 µA) beams before the second was irradiated at production level (230 µA) beams. During irradiation, monitoring of cooling water conductivity indicated no container breach or leak and, as anticipated given the model predictions, the post irradiation target inspection showed no sign of imminent thermal failure (see FIG. 4). Subsequent chemical processing of the targets followed an established procedure that was slightly modified to accommodate the larger target mass. TABLE 1 shows that post chemistry 82Sr yields agree to within 2 % of the in-target production rates expected on the basis of IAEA recommended cross sections. The table also compares 82Sr yields from the Rb metal targets against yields routinely obtained from RbCl targets, showing an increase in yield of almost 50 %
Proton Beam Energy Characterization
Full text linkIntroduction
The Los Alamos Isotope Production Facility (IPF) is actively engaged in the development of isotope production technologies that can utilize its 100 MeV proton beam. Characterization of the proton beam energy and current is vital for optimizing isotope production and accurately conducting research at the IPF.
Motivation
In order to monitor beam intensity during research irradiations, aluminum foils are interspersed in experimental stacks. A theoretical yield of 22Na from 27Al(p,x)22Na reactions is cal-culated using MCNP6 (Monte Carlo N-Particle), TRIM (Transport of Ions in Matter), and Andersen & Ziegler (A&Z) [1] computational models. For some recent experiments, experimentally measured activities did not match computational predictions. This discrepancy motivated further experimental investigations including a direct time-of-flight measurement of the proton beam energy upstream of the target stack. The Isotope Production Program now tracks the beam energy and current by a complement of experimental and computational methods described below.
Material and Methods
A stacked-foil activation technique, utilizing aluminum monitor foils [2] in conjunction with a direct time-of-flight measurement helps define the current and energy of the proton beam. Theoretical yields of 22Na activity generated in the Al monitor foils are compared with experimental measurements. Additionally, MCNP, TRIM, and A&Z computational simulations are compared with one another and with experimental data.
Experimental Approach
Thin foils (0.254mm) of high purity aluminum are encapsulated in kapton tape and stacked with Tb foils in between aluminum degraders. Following irradiation, the Al foils are assayed using γ-spectroscopy on calibrated HPGe detectors in the Chemistry Division countroom at LANL. We use the well-characterized 27Al(p,x)22Na energy dependent production cross section [3] to calculate a predicted yield of 22Na in each foil. Details of the experimental activity determination and associated uncertainties have been addressed previously [4]. The nominally stated beam parameters are 100 MeV and 100–120 nA for the foil stack irradiation experiments. Time-of-flight measurements performed in the month of January 2014 revealed beam energy of 99.1 ± 0.5 MeV.
Computational Simulations
Andersen & Zeigler (A&Z) is a deterministic method and also the simplest of the three com-putational methods considered. While the mean energy degradation can be calculated using the A&Z formalism, the beam current attenuation cannot. Consequentially, A&Z will also lack the ability to account for a broadening in the beam energy that a stochastic method affords. Additionally, A&Z does not account for nuclear recoil or contributions from secondary interactions. TRIM uses a stochastic based method to calculate the stopping range of incident particles applying Bethe-Block formalisms. TRIM, like A&Z, does not include contributions from nuclear recoil or contributions from secondary interactions. Computationally, TRIM is a very expensive code to run. TRIM is able to calculate a broadening in the energy of the beam; however, beam attenuation predictions are much less reliable. TRIM determines the overall beam attenuation in the whole stack to be less than one percent, whereas 7–10 % is expected. MCNP6 is arguably the most sophisticated approach to modeling the physics of the experiment. It also uses a stochastic procedure for calculation, adopting the Cascade-Exciton Model (CEM03) to track particles. The physics card is enabled in the MCNP input to track light ion recoils. Contributions from neutron and proton secondary particle interactions are included, although their contribution is minimal. For both MCNP and TRIM, the proton beam is simulated as a pencil beam. To find the current, an F4 volumetric tally of proton flux from MCNP simulation is matched to the experimental current for the first foil in the stack. Subsequent foil currents are calculated relative to the first foil based on MCNP predictions for beam attenuation. The equation used for calculating the current from the experi-mental activity is [5]:
where:
is the cross section for the process, [mbarns]
is the atomic mass of the target [amu]
is the is the number of product nuclei pre-sent at End-of-Bombardment
is the average beam current, [μA]
is the density of the target material, [g/cc]
is the target thickness, [cm]
is the decay constant, [s−1]
is the irradiation time, [s]
For each foil in the experimental stack, we also have a statistically driven broadening of the incident energy. The beam energy is modeled as a Gaussian distribution, with the tallies for each energy bin determining the parameters of the fit. TABLE 1 and FIG. 3 summarize the mean energy and standard deviation of the energy for each aluminum monitor foil. To address the energy distribution, we calculate an effective or weighted cross-section. It is especially important to account for energy broadening in regions where the associated excitation function varies rapidly. In the excitation function, we see a strong variation in the energy range from 30–65 MeV, the energy region cov-ered by the last 3 foils in the stack. Cross section weighting also accounts for the mean energy variation within each foil. The excitation function will overlay the Gaussian shaped flux distribution, giving rise to a lateral distribution where incrementally weighted values of the cross section are determined by the flux tally of the corresponding energy bin. With the effective cross section and the current at each of the foils, it is straight-forward to calculate the number of 22Na atoms created and the activity of each foil using the previously stated equation.
Results and Conclusion
The general trend in the amount of activity produced follows the shape of the excitation func-tion for the 27Al(p,x)22Na reaction. Small shifts in the incident energy upstream trickle down to produce much more pronounced shifts in the energy range of foils towards the back of the foil stack. The characteristic “rolling over” of the activity seen in the experimental foils indicates that the 6th foil must be in the energy region below 45 MeV, where the peak of the excitation function occurs. Conservatively, computational simulations are able to accurately determine the proton beam’s energy for an energy range from 100 to 50 MeV. As the beam degrades below 50 MeV, computa-tional simulations diverge from experimentally observed energies by over-predicting the energy. This observation has been noted in past studies [6,7] that compare the stacked foil technique with stopping-power based calculations. A complement of experimental and computational predictions allows for energy determinations at several points within target stacks. While this study focuses on an Al-Tb foil stack, the analysis of a similar Al-Th foil stack resulted in the same conclusions. Although we do not have a concurrent time-of-flight energy measurement at the time of the foil stack experiments, it is reasonable to assume that the energy at the time of the stacked foil experiments was also lower than the assumed energy of 100 MeV. Computational simulations developed in this work firmly support this assumption. Various computational models are able to predict with good agreement the energy as a function of depth for complex foil stack geometries. Their predictions diverge as the beam energy distribution broadens and statistical uncertainties propagate. A careful inspection of the codes reveals that these discrepancies likely originate from minute differences between the cross sections and stopping power tables that MCNP and TRIM/A&Z use respectively
High power conical-shaped Niobium targets for reliable [18F-] production and lower [18O] water consumption: High power conical-shaped Niobium targets for reliable [18F-] production and lower [18O] water consumption
Full text linkIntroduction
In order to address the increasing demand for Fluorine-18 and the rising cost per mL of 18O enriched water, IBA developed improvements to their 18F- production systems. For this new design we started from scratch, with the main objectives of reducing the required enriched water volume and improving the cooling of the insert. A better cooling allows increasing the target current and thus the produced activity. Finally, we aimed to reduce the number of parts and improve the design of auxiliary components.
Material and Methods
Six Niobium conical inserts with different target chamber volumes were machined and tested. Only 4 of these were selected to create the new range of IBA 18F− targets shown in TABLE 1. The new Niobium target inserts have a complex shape with drilled channels on the outside of the chamber and a deep channel next to the beam strike area (FIG. 1, green circle) to ensure efficient cooling.
The 18O water inlet lines are now directly inserted in the Niobium body (FIG. 1, blue circle) to improve 18F- quality (no more contact with small o-rings). In operation, a 35µm Havar® target window is used.
All tests were performed using IBA Cyclone® 18 cyclotron. The targets were filled with different volumes of enriched 18O water (enrichment
> 92 %) and irradiated with 18 MeV protons on target with beam currents up to 145 μA for 30 to 150 minutes, while the internal pressure rise of the target was recorded. For each target, a pressure-current curve was plotted and an optimum balance between target water fill volume, pressure and current has been determined, which maximises available activity after two hours, in each case.
Results and Conclusion
Radionuclidic impurities were measured and more than 100 FDG syntheses on various synthesizers confirmed the effectiveness of the new design. Increasing the current up to 145µA in Conical 16, the production reached 18 Ci in 2 hours, single beam, with a target pressure under 43 bar. Today, the use of these new targets for daily commercial production is increasing within the IBA Cyclone® installed base
Modification of the COSTIS-system mounted at the Cyclone 18/9
Full text linkIntroduction
A widely distributed commercially available target station for the irradiation of solid targets is the COSTIS system. The system is specified for beams up to 500 W and is equipped with a front side He-cooling and water cooling on the back side. The target itself has a coin shape with a diameter of 24 mm and thickness of 2 mm. This recommends the system for irradiation of thin targets like foils but it is also useable for irradiation of metal and oxide powders. However the irradiation of powders and granulates is limited due to the dimension of the target capsule. A setup of a capped closed target is hardly achievable. The purpose of this work was the modification of the COSTIS target station for the use of thicker target capsules. This shall enable the more easy and safe handling and irradiation of powdery targets and the use of lockable target capsules.
Material and Methods
The adaption of the COSTIS system for wider targets is easy and fast achievable by the ex-change of the target guiding plate together with the four distance bolts and their bearings. The effort of the replacement of the standard with the modified parts is comparable with COSTIS maintenance including exchange of the window foil and the O-rings. For the target capsule itself different designs were developed and tested. Now various target capsules are available, depending on required energy, handling needs and properties of the target material. Different locking systems can be used, from “click” capsules to screwable systems. Additionally the tightness of the target capsule can be achieved by placement of on O-ring between the lid and capsule body.
Results and Conclusion
The wider target body allows the capping of the target material. This enables a wide range of applications. One aspect is the nanoparticle research, where radiolabelling is an excellent tool for in situ online investigations. The chosen design of the target capsule allowed the direct activation of TiO2 nanoparticles. Via the nuclear reaction 48Ti(p,n)48V radiolabelled [48V]TiO2 nanoparticles can be obtained. Another example is the use of recoil effects for radiolabelling of nanoparticles. In this case the kinetic energy of the product of the nuclear reaction 7Li(p,n)7Be is used to implant a radioactive tracer in different nanomaterials like Ag0 – nanoparticles and MWCNT (multi wall carbon nano tubes). In general the irradiation of powders and granulates benefits from the modified design that allows the more flexible adaption to experimental needs
Numerical simulation of a liquid cyclotron target
Full text linkOne of the most common PET isotopes, 18F, is mainly produced in liquid targets. The production yield depends linearly on the proton beam current used. However, for a fixed proton-beam energy increasing the current of the proton beam results in depositing increasing amounts of heat into the enclosed water target chamber and eventually in its failure. Hence, understanding the thermodynamics of a water target chamber could lead to a target optimization, removing the maximum amount of heat to balance the pressure, increasing the yield and guaranteeing the stability and durability of the system. Work in modeling the thermodynamic processes in a liquid target has also been per-formed by other groups [1-3] and others such as Steinbach [4] have performed analytical analyses of thermal behavior
18F− saturation yield in Large Volume cylindrical IBA target
Full text linkIntroduction
In last decade increasing demand for clinical F-18 Fludeoxyglucose requires a greater F-18 fluoride production. From the other side increasing price of enriched O-18 water compel us to find the most effective way of F-18 activity production. One of the possible way, how to optimize and increase yield of F-18, is to increasing target current with retaining the same or less volume of enriched water. Optimization of F-18 production on IBA Large Volume cylindrical target is presented.
Material and Methods
Irradiations of [18O]H2O by 18MeV proton beams with intensities 40–55 μA were performed on CYCLON 18/9, IBA cyclotron and on LV cylindrical IBA target.
Irradiated enriched water was transported to the hot cell using RDS (Radioactive Delivery System) system and was measured in Curriementor 4 Isotope Calibrator made by PTW.
At the beginning it was necessary to satisfy several requirements:
i) target and water cooling.
Using a simple two dimensional equation we can roughly estimate the equilibrium temperature inside the target [1]:
Δt = HT/Ak
where:
Δt = the temperature rise in the target chamber over cooling water temperature
H = is the heat load
T = thickness of metal wall
A = area of metal in contact with target water
k = thermal conductivity
In our case with heat load 720 W (40 μA×18 MeV) is Δt = 78 oC. From the curve of boiling point of water as a function of pressure [2], we can observe t = 212 °C at 20 bar or 243 °C at 35 bar, respectively, which corresponds to max. heat load up to 90–95 µA of target current.
ii) pressure and filling water volume.
Filling water volume was from 2 to 2.15 ml to guarantee stop all beam in water. Also during experiments for safety reasons the operating pressure was limited to 35 bar as the window rupture pressure is > 50 bar for used 0.05 mm Havar foil. In this case increasing target volume with increasing current was provided with longer tube.
Results and Conclusion
The saturated yields of F-18 for 40 µA to 55 µA target currents are given in TABLE 1. No systematic decrease in yields with increasing target current was observed and yields were in line with the 230 ± 10 mCi/µA measured at acceptance test of target.
The [18F]FDG yields from productions using the TRACERlab-Mx module are shown in FIGURE 1. All presented productions of F-18 were prepared with LV target with 55 µA. No decrease in the yield was observed with increasing beam current.
It has been demonstrated that it is possible to produce routinely 250 GBq/2hr (6.8 Ci/2hr) of 18F-Fluoride using LV cylindrical target (operating conditions: 55 µA, 18 MeV, 98% enriched water). As the next step we want to test dual beam – 2×55 µA with two LV targets and expected activity about 500 GBq of 18F-Fluoride in 2 hours is expected
Solid 100Mo target preparation using cold rolling and diffusion bonding
Full text linkIntroduction
100Mo target design is key to commercially viable large scale cyclotron production of 99mTc. The target back plate supporting the 100Mo must be chemically inert to the target dissolution conditions but ideally it should also be able to dissipate the high thermal loads of irradiation, not contaminate target substrate with radionuclidic by-products, and be adequately inexpensive to allow for single use. Aluminum was selected as our target support as it satisfies these requirements.
Our process entails rolling 100Mo powder into a foil of desired thickness, and then diffusion bonding [1] the foil onto an aluminum back plate. The 100Mo targets were designed to be 20×80×0.1 mm to match our TR24 cyclotron’s proton beam profile and energy. Efforts are currently underway to scale up the process to allow for simultaneous production of multiple targets at once.
Material and Methods
The crude enriched 100Mo foil (99.815% enrichment) was made from 100Mo powder using a horizontally mounted rolling mill and an aluminum hopper. The crude foil was rolled repeated-ly, and the space between the rollers gradually reduced until the thickness of the foil was changed from an initial thickness of 0.3 mm to a thickness of 0.1 mm.
The rolled 100Mo foil was annealed under reducing atmosphere and then bonded to the aluminum target plate support under inert atmosphere in a heated press at 500 °C.
Results and Conclusion
By rolling 100Mo foils from powder we were able to produce uniform foils with an average density of > 98 % compared to the maximum theoretical density of 100Mo (n = 5) and thicknesses of roughly 0.1 mm. All foils produced were the desired 20 mm width (i.e. limited by the width of the opening of the hopper) and trimmed to the desired 80 mm length. The annealing process was necessary due to the brittleness of the un-annealed rolled foil and the difference in the thermal expansion coefficients of molybdenum and aluminum which caused un-annealed foils in previous experiments to crack and break off during pressing (n = 10). Surface preparation of the aluminum support plate was also found to play a critical step in the efficiency of the bond, and continuing effort to scale the above de-scribed procedure to mass produce 100Mo tar-gets is ongoing. Targets have undergone preliminary testing to 250 μA
Practical experience implementing the Comecer ALCEO Metal solid targetry system
Full text linkIntroduction
The Comecer ALCEO Metal system is intended to be a comprehensive solid targetry system, capable of all steps necessary to produce copper isotopes (60Cu, 61Cu & 64Cu) from enriched nickel: plating, transfer to/from cyclotron, irradiation, and dissolution/purification. To develop plating and chemistry methods, we plate natural nickel, and irradiate with deuterons to produce 61Cu. This alleviates the need for expensive enriched nickel isotopes, but gives a lower activity yield. We report a few issues with the ALCEO system, and some of our modifications.
Material and Methods
BRIEF DESCRIPTION OF SYSTEM: The ALCEO system uses cylindrical shuttles (dia 28 mm, height 35 mm) comprised of an Al body with a Pt well, onto which the Ni is plated. Shuttles are transferred pneumatically from the hot cell to the irradiation module, on the end of the cyclotron beamline.
The plating and dissolution are both done at the electrochemical cell, located in the hot cell. This cell is connected by capillary tubes to the electrolytic solution reservoir (for plating), or the acid reservoir (for dissolution). These tubes form a recirculation loop, through which the fluid is propelled by an inline micropump throughout plating and dissolution.
The platinum well is 16 mm in diameter, while an O-ring is used to plate only the center (6 mm in diameter). A constant DC voltage is applied.
PLATING: We dissolve natural nickel nitrate (99.999% pure) into an electrolytic solution comprised of deionized water, ammonium hydroxide and ammonium chloride (pH = 9.3). We use 30–100 mg of natNi in a 10mg/mL solution. We have varied the ALCEO electrochemical cell voltage between 2.25–3 VDC, and tried to maintain a low pump flow rate between 1–2 mL/min.
The electrochemical cell uses a fixed metal tube as the anode (~3mm above the plating surface). This tube also delivers the electrolytic plating solution to the Pt surface, forming part of the recirculating loop. The Pt surface is in contact with a gold-plated cathode.
Due to issues discussed below, we have built a custom plating rig for the ALCEO shuttles, which does not use the pump/recirculation loop, but leaves the reservoir of electrolytic solution in place, atop the plating surface. A graphite rod is used for the anode, rotating offcenter ~2mm above the plating surface. We use the same size O-ring to plate only the center 6 mm of the Pt surface, and apply a constant DC voltage.
TRANSFER TO/FROM CYCLOTRON: The pneumatic transfer tube is 50 ft long between the hot cell and the cyclotron vault, and has a rise of
14 ft from under the floor to the ceiling of the cyclotron vault.
IRRADIATION: The ALCEO irradiation module holds the plating surface orthogonal to the beam path. The module has a 10-mil-thick (0.010”) Al front foil, supported by a hex-grid. Once we realized the thickness, we replaced this with a 1-mil-thick Al foil, followed by a 1-mil-thick Havar foil.
The foil is cooled by a flow of helium, while the shuttle and grid are cooled by a flow of water. The helium and water are cooled in heat ex-changers by chilled water. As initially plumbed, the chilled water flowed through the heat ex-changers in series, cooling the helium first, then the water. After initial runs, we plumbed the heat exchangers in parallel, teeing the chilled water to the supply of each heat exchanger, and teeing the returns together.
Irradiation is performed with a PETtrace 800 accelerating deuterons to 8.4 MeV on target. The beam current limit is 20 μA for the ALCEO Metal target. A set point of 19 μA is used to avoid the system tripping off.
DISSOLUTION/PURIFICATION: The ALCEO system circulates 5 mL of 6M HCl, while heating the shuttle to 100 ⁰C for 40 min. This solution is loaded onto a column containing 10 g of 200–400 mesh chromatographic resin in chloride form.
A separation is performed yielding three solutions: The column is washed with 40 mL of 6M HCl to obtain the Recovered Nickel Solution, then 20 mL of 4M HCl to obtain the Cobalt Solution, then 10 mL of 0.5M HCl to obtain the Cop-per Product Solution.
Results and Conclusion
PLATING: Using the ALCEO method, the platings obtained had a tendency to mound, (up to 0.75 mm thick for 50 mg) giving a lower density of 3–4 g/cm3. This was attributed to the anode tube being fixed in place over the center of the plating surface. Using the custom rig, almost no mounding was observed, (0.25mm thick for 50 mg) giving a density of 7 g/cm3, closer to nickel’s nominal density of 8.9 g/cm3.
FIGS. 1 and 2 attempt to show the mounding from the ALCEO method, and relative flatness from the custom rig. Both methods give a rough, or “fuzzy” plated surface. FIG. 2 shows that the custom rig exaggerates this “fuzziness”. Using the ALCEO method, a slower pump speed (~1 mL/min) gives a smoother plating surface, but the pump has a tendency to lock up at this lower set point, stalling the plating.
Using the ALCEO method, slight increases in the voltage (2.65 V instead of 2.25 V), can form thin stalagmites of nickel, electrically connecting the anode and the plating surface, ending the chance for a useable plating. Using the custom rig, no stalagmites are seen, adjusting the voltage from 2.3 to 3.0 V. This leads to the potential for a faster plating.
Both the ALCEO method and the custom rig have obtained plating efficiencies of 95 %.
TRANSFER TO/FROM CYCLOTRON: The shuttle typically transfers without issue in < 15 seconds. Once or twice it has remained in the transfer tube, but been retrieved by cycling the com-pressed air/vacuum a couple of times.
IRRADIATION: During initial testing, the temperature of the return water rose rapidly during irradiation. This was attributed to the chilled water already gaining heat from the helium heat exchanger. Once the parallel chilled water plumbing was implemented, the water temperature rose much more slowly.
Initial testing with the 10-mil Al foils gave a very poor activity yield. The 1-mil Al foil ruptured under the 20 psi helium pressure before beam was applied. The 1-mil Havar foil produced 1.57 mCi of 61Cu at EOB, giving an activity yield of 0.308 mCi/μAh (results summarized in TABLE 1). This compares to yields of 1.4 obtained by [1], and 0.29 obtained by [2] for deuterons on natNi.
DISSOLUTION/PURIFICATION: The dissolution in 6M HCl is close to 100% efficient by weight.
After purification, the three solutions were assayed by dose calibrator as summarized in TABLE 2. The purification is very efficient at removing the starting material, and long-lived Co isotopes from the Copper Product Solution, as seen with the 57-hour EOB measurements. However, much of the desired 61Cu is removed as well, with only 32% remaining in the product.
The lack of nuclide impurities in the Copper Product Solution was confirmed by gamma spectroscopy using a HP-Ge detector
Neutron activation as an independent indicator of expected total yield in the production of 82Sr and 68Ge with 66 MeV protons
Full text linkIntroduction
A method based on neutron activation is being developed to assist in resolving discrepancies between the expected yield and actual yield of radionuclides produced with the vertical-beam target station (VBTS) at iThemba LABS.
The VBTS is routinely employed for multi-Ci batch productions of the radionuclide pairs 22Na/68Ga and 82Sr/68Ga using standardized natMg/natGa and natRb/natGa tandem targets, respectively [1]. The metal-clad target discs are bombarded with a primary beam of 66 MeV protons at an intensity of nominally 250 µA. The encapsulation materials are either Nb (for Mg and Ga) or stainless steel (for Rb) which serve to contain the molten target materials during bombardment and act as a barrier to the high-velocity cooling water which surrounds the targets in a 4π geometry. The natRb/natGa targets are typically bombarded according to a two-week cycle while natMg/natGa targets are bombarded on an ad-hoc basis, depending on a somewhat unpredictable 22Na demand.
A too-large deviation between expected yield and actual yield has at times plagued this programme. These deviations can manifest both as an apparent loss or an apparent gain (relative to the expected yield) by up to about 15% in either direction. The resulting uncertainty of up to 30% (in the worst case) from one production batch to another can be costly and is unacceptable in a large-scale production regimen. This phenomenon is believed to be brought about by two types of problems:
(1) Production losses, e.g. during the radio-chemical separation process or incomplete recovery of activated target material during the decapsulation step.
(2) Incorrect values obtained for the accumulated proton charge.
A problem of type (1) will always result in a loss of yield. A problem of type (2) can manifest as an apparent loss or gain. In an effort to get a handle on this second type of problem, neutron activation of suitable material samples, embedded in a target holder, is being investigated as an independent indicator of the total yield. For this purpose, samples of Co, Mn, Ni and Zn were activated during production runs and Co was found to be the most appropriate. Preliminary results will be presented after first discussing why the determination of the accumulated pro-ton charge is a problem with the VBTS.
Materials and Methods
The VBTS consists of a central region in which a target holder is located during bombardment as well as two half-cylindrical radiation shields which completely surround the target. The shields can be moved away from the central region on dedicated rails, e.g. when repairs or maintenance is required. FIGURE 1 shows the VBTS with the shields moved to the “open” position. As some components of the station are located below the vault floor, with the target position near floor level, it proved difficult to electrically isolate the VBTS as was done for the two horizontal-beam target stations at iThemba LABS [1]. The VBTS does not act as a Faraday cup like the other target stations. Instead, the beam current and accumulated charge is measured by means of a calibrated capacitive probe [1,2].
There appears to be a variation in the response of the capacitive probe, sensitive to the beam microstructure, in particular a dependence on the beam packet length. This problem is not yet fully resolved.
FIGURE 2 (a) shows the beamstop of a VBTS target holder with several Co samples mounted on the outside as well as one each of Ni, Mn and Zn. The samples are small “tablets” with a 10 mm diameter and 1 mm thickness. The reactions of interest are 59Co(n,γ)60Co, 59Co(n,3n)57Co, nat-Ni(n,X)60Co, natNi(n,X)57Co, natZn(n,X)65Zn and 55Mn(n,2n)54Mn. The relevant half-lives are 60Co(5.271 a), 57Co(271.8 d), 65Zn(244.3 d) and 54Mn(312.2 d). The half-life should be long compared to the two-week cycle in order to reduce the dependence on the exact beam history, which is very fragmented over any production period. In this respect, 60Co is considered to be particularly attractive as its long half-life of more than 5 years leads to a negligible effect by the beam history.
Note that the tandem targets, shown in FIGURE 2 (b), are mounted just upstream of the beamstop – in fact, the targets and beamstop form a single unit before being fitted into the target holder.
At the end of bombardment, all samples were assayed for their characteristic γ-emissions using standard off-line γ-ray spectrometry with an HPGe detector connected to a Genie 2000 MCA. Calculations of the neutron fluence density in the central sample volume on the beamstop were also performed using the Monte Carlo radiation transport code MCNPX. For these calculations, the entire VBTS, a Rb/Ga target and the vault walls were included in the model.
Results and Conclusion
All samples activated significantly – copious amounts of 60Co were detected in the Co discs after a two-week run.
The neutron fluence density for the case of a 250 µA, 66 MeV proton beam on a natRb/natGa tandem target is shown in FIGURE 3. The dominance of low-energy neutrons is evident, which is in part due to the large amount of paraffin-wax shielding material in close proximity to the target. While reactions such as the (n,2n) and (n,3n) would be sensitive to the more energetic part of the neutron spectrum, the (n,γ) capture reaction benefits from the large low-energy component. This explains the copious amounts of 60Co formed. It was therefore decided to only retain the central Co sample for subsequent bombardments, as shown in FIGURE 4.
The first results are shown in TABLE 1. The accumulated charge as obtained from the capacitive probe (Q), the specific 60Co activity (A) at the end of bombardment (EOB), and their ratio (A/Q) are presented in the table, together with the deviation of individual ratios relative to their average for the case of the Mg/Ga tandem tar-gets only. Note that all samples were counted until the statistical uncertainties were negligible. Any systematic uncertainties are ignored at this stage as they are considered to remain the same from one batch production to another.
For the sake of argument, the average value of the ratio is taken as the expected value. A positive deviation of the A/Q value is then indicative of a too-small value of the accumulated charge obtained from the capacitive probe, leading to a corresponding overproduction. Likewise, a negative value is indicative of a too-large value of the accumulated charge, leading to a corresponding underproduction.
It is certainly true that the data in TABLE 1 are currently very limited. It is envisaged, however, that with time the growing database of values will assist in reducing the uncertainty in determining the accumulated charge and reduce the discrepancies between predicted and actual yields significantly. TABLE 1 illuminates the underlying problem satisfactorily. The four Mg/Ga tandem target bombardments, on identical targetry, were performed successively. The neutron activation correlates well the with actual yields, pointing directly to the current integration as the main source of error.
The method already proves to be useful. An indication of an over or underprediction can be obtained prior to the target processing by recovering and measuring the Co disc. This in-formation can be used to make a decision concerning the present batch production and/or the subsequent one. One can either add beam to the present production target and/or in-crease/reduce the total beam on the subsequent production target to compensate for an expected overproduction or shortfall.
In conclusion, we would like to stress that the capacitive probes show great promise and that better understanding and/or possibly some development of their signal processing algorithm may improve their ability to measure the accumulated charge to the desired accuracy. Segmented capacitive probes used at iThemba LABS and elsewhere for beam position measurement [1,3] are not affected by beam microstructure as only the ratios of the signal strengths on the different sectors are important. In this case, changes in response affect all sec-tors equally and the ratios are unaffected
Charakterisierung der Mikroorganismen im sauren Grubenwasser des ehemaligen Uranbergwerks Königstein
Full text linkBeim Bergbau werden bestehende Ökosysteme in großem Maße beeinflusst. Im ehemaligen Uranbergwerk Königstein (Sachsen) wurde die Umwelt durch den Einsatz von chemischen Säuren zur Lösung des Urans aus dem Erz (Laugung) in Folge der Verschiebung des pH-Wertes zusätzlich belastet. Durch diesen Prozess entstand eine Umgebung, die einen niedrigen pH-Wert und hohe Konzentrationen an gelösten Schwermetall-Ionen aufweist. Die komplexe mikrobielle Lebensgemeinschaft verschob sich daraufhin, indem sich bevorzugt säuretolerante und Schwermetall-tolerante Mikroorganismen durchsetzten. Diese Mikroorganismen wurden durch die Flutung der unter Tage Schächte im Jahr 2010 in ihrer Zusammensetzung erneut beeinflusst.
In dieser Arbeit wurde die mikrobielle Biozönose nach Flutung der unter Tage Schächte des ehemaligen Uranbergwerkes Königstein charakterisiert und mit den Ergebnissen der mikrobiellen Diversität vor dem Flutungsprozess verglichen. Hierfür kam ein breites Spektrum an Methoden zum Einsatz, das klassische mikrobiologische Methoden und molekularbiologische Techniken umfasste. Die Analysen erfolgten dabei über mehrere Jahre hinweg, um die Variabilität der mikrobiellen Population im Grubenwasser planktonisch und im Biofilm zu erfassen