Electron accelerator-based production of molybdenum-99: Bremsstrahlung and photoneutron generation from molybdenum vs. tungsten

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• 作者：A. Tsechanski^a ; A.F. Bielajew^b ; ; J.P. Archambault^c ; E. Mainegra-Hing^c
• 关键词：Mo99 production ; Target/radiator design ; Medical isotopes
• 刊名：Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms
• 出版年：2016
• 出版时间：1 January 2016
• 年：2016
• 卷：366
• 期：Complete
• 页码：124-139
• 全文大小：1476 K

文摘

A new “one-stage” approach for production of ⁹⁹Mo and other radioisotopes by means of an electron linear accelerator is described. It is based on using a molybdenum target both as a bremsstrahlung converter and as a radioisotope producing target for the production of ⁹⁹Mo via the photoneutron reaction ¹⁰⁰Mo(γ,n)⁹⁹Mo. Bremsstrahlung characteristics, such as bremsstrahlung efficiency, angular distribution, and energy deposition for molybdenum targets were obtained by means of the EGSnrc Monte Carlo simulation code system. As a result of our simulations, it is concluded that a 60 MeV electron beam incident on a thick Mo target will have greater bremsstrahlung efficiency than the same thickness (in units of r₀) W target, for target thickness z > 1.84r₀, where r₀ is the electron range. A 50 MeV electron beam incident on a Mo target will result in greater bremsstrahlung efficiency than the same thickness W target (in units of r₀) for target thickness case: z ⩾ 2.0r₀. It is shown for the one-stage approach with thicknesses of (1.84–2.0)r₀, that the ⁹⁹Mo-production bremsstrahlung efficiency of a molybdenum target is greater by ∼100% at 30 MeV and by ∼70% at 60 MeV compared to the values for tungsten of the same thickness (in units of the appropriate r₀) in the traditional two-stage approach (W converter and separate ⁹⁹Mo producing target). This advantage of the one-stage approach arises from the fact that the bremsstrahlung produced is attenuated only once from attenuation in the molybdenum converter/target. In the traditional, two-stage approach, the bremsstrahlung generated in the W-converter/target is attenuated both in the converter in the ⁹⁹Mo-producing molybdenum target.

The photoneutron production yield of molybdenum and tantalum (as a substitute for tungsten) target was calculated by means of the MCNP5 transport code. On the basis of these data, the specific activity for the one-stage approach of three enriched ¹⁰⁰Mo-targets of a 2 cm diameter and thicknesses of 1, 2, and 3 radiation lengths (RL) were calculated to be: 19.54 Ci/g, 23.05 Ci/g, and 21.23 Ci/g, respectively. These results were compared with the evaluation presented in by Diamond et al. [19] for the same diameter and thickness ¹⁰⁰Mo-targets in a two-stage approach. The comparison demonstrates that for all thicknesses under consideration, the specific activities at equilibrium in the one-stage approach are substantially greater than those evaluated in Diamond et al. [19] for the two-stage approach. More specifically, the specific activities at equilibrium in the Mo converter/target approach are greater than those in the standard W converter/Mo target approach at ¹⁰⁰Mo target thicknesses of 1RL, 2RL, and 3RL by 28.9%, 82.5%, and 80.1%, respectively.