LBE Paper

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Liquid Lead Bismuth Target for Positron Production


Need for positrons

Intense positron sources are urgently needed for numerous applications, primarily for positron spectroscopy, which can have a huge impact on chemistry, physics, materials and biological science [1–4]. Additionally, high intensity positron beams are required to carry out gravitation experiments [5, 6] and to do chemistry with antimatter [7]. Finally, efficient positron traps also require an intense source of positrons [8, 9].

The easiest way to produce positron beams is to use e+-emitting sources, such as Na-22, which can have activity as high as 1 MBq. Another possibility is to generate positrons by pair production. In this case, an electron beam is stopped in a converter creating bremsstrahlung γ-rays. Provided that the energy of the primary electron beam is high enough, the generation probability of e-/e+ pairs is sufficiently high. Typically high Z material (such as tungsten) is preferred for positron production and moderation [10, 11]. For 10 MeV beam the optimum tungsten converter thickness is about 1.4 mm, and about 20% of the electron beam energy is converted into the x-rays.

Conventional W target (for a 10 MeV electron beam) – power limits?

Liquid metal targets as an alternative=

If the electron beam power exceeds ~10 kW it is nearly impossible to cool solid metal converters properly. Such power levels require liquid metal converters, for example lead–bismuth eutectic (LBE) containing 45% of lead and 55% of bismuth. Since the liquid metal simultaneously serves as a converter and a coolant, the concerns regarding possible melting of the components of the system (primarily converter channel windows) are minimal. Both lead and bismuth have high atomic numbers and good conversion efficiency. The eutectic has a low melting point (Tmelt = 124 ºC) and quickly solidifies in the case of leakage. Such converters can withstand tens of kW.

Optimum LBE converter thickness was simulated using MCNX and G4Beamline (see Figure 1) and was found to be about 2 mm which corresponded to ~2 x 10-3 e+/e-. Momentum distribution of the positrons and electron after the 2 mm LBE converter were also simulated (see Figure 2).




Beam Power capability of W -vs- LBE

Simple 1D calculations of heat transfer and temperature gradients

If we can back it up with ANSYS that would be great

Impact of windows on positron production

Walls might be needed for liquid LBE – windowless design is tricky

Effect of thin windows (different materials, different thickness) needs to be evaluated - by how much the production rate drops

Impact of LBE flow rate

Maximum LBE flow rate is defined by corrosion and depends on the material of the window (~ 2 m/s)

Limit on the flow rate means limit on the beam current and e+ production rate.



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