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New gas target system for 83Rb production

Introduction Short-lived isomer 83mKr (T½ = 1.83 h) is an ideal calibration source in several low-energy experiments like or KATRIN (determining the neutrino rest mass, monitoring high voltage stability and investigation of the main spectrometer properties) or XENON (detection of the dark matter). The isomer 83mKr is formed by decay of 83Rb (T½ = 86.2 d) that can be produced predominantly via the reaction 84Kr(p,2n)83Rb by irradiation of natKr (57 % abundance of 84Kr). The design and construction of the new gas target for effective production of radionuclide 83Rb as well as target processing will be shortly described. Material and Methods For the target design, we selected the following criteria: minimizing activation of target components; efficient cooling system allowing higher beam currents; easy handling; high life-time of the target chamber (low impact of the irradiation and radionuclide separation process on the target chamber surface and 83Rb recovery). The target consists of three parts: 1. Water cooled aluminium (alloy EN 6082) mechanical interface for easy connection of the target to the beam line. It also serves as a beam collimator (diameter 9 mm). 2. Holder of He-cooled foils (vacuum separation foil – Havar 0.025 mm, target body window – Ti 0.1 mm). 3. Aluminium (alloy EN 6082) water cooled target body with 150mm long cone-shaped target chamber of the volume 27.1 ml. Internal surface of the chamber is nickel-coated. The target filled with natural Kr of purity 0.9999 and absolute pressure 13 bar was irradiated on the external beam of the isochronous cyclotron U-120M of the NPI AS CR. The proton beam energy was set so that it is decreased after deg-radation in the separation foils to 25.6 MeV. Beam energy loss in the natural Kr gas filling is 9.6 MeV. The target was tested up to 25 µA beam current. After irradiation, the target is left for a week to let the short-lived activation products to decay. Then, 83Rb is washed out from the target walls by two portions of freshly prepared de-ionized water, target is rinsed by high-purity ethanol and dried. The two portions of 83Rb aqueous solution are then connected and activity and radionuclidic purity of the product is determined via γ-spectrometry (HPGe detector). Large-distance sample-detector measurements of the target prior and after the separation are used in order to determine recovery of 83Rb. Results and Conclusion The new gas target for routine production of 83Rb was successfully designed, tested and im-plemented for regular 83Rb production. Six-hour irradiation with 15 µA proton beam resulted repeatedly in ca 300 MBq of 83Rb (EOB). Besides 83Rb, we identified in the separated product also 84Rb (T½ = 32.82 d) at levels ca 31 % of the 83Rb activity (EOB) and 86Rb (T½ = 18.631 d) at levels ca 8 % of the 83Rb activity (EOB). Both radionuclidic impurities do not disturb the use of 83Rb, since none of them emanates any radioactive krypton isotope. Moreover, their relative content decreases in time. Rubidium isotopes are recovered from the target almost quantitatively (98–99 %)