What is Target Design?

In our industrial x-ray machines an electron accelerator adds energy (speed) to the electrons until, in this case, they are moving at 99.95% the speed of light.  These high energy electrons are then stopped abruptly, typically by running them into a dense material like copper or tungsten.  When they stop, the electrons give up their accumulated energy in the form of x-rays (and heat).

Designing and manufacturing a target is a complex job.  On the design side there is the interaction of nuclear and structural computational codes, while on the manufacturing side the multi-step brazing and welding are challenges.  I have designed a target for 16 MeV electrons and followed the life of the target through 15 units with operational times of up to 12 years.  There have been no failures.

On the design side I was able to improve the fidelity of the thermal and structural analysis by getting the nuclear and structural codes to interact with each other.  By that I mean that in MCNPX, the nuclear code developed by Los Alamos Nat’l Lab, I could set up a tally mesh that directly mapped to a portion of the Finite Element mesh used by the structural model.  By doing this I could have MCNPX output data (heat loads) that simple reformatting could change to structural analysis heat data.  No interpolation or extrapolation necessary since the meshes matched exactly.  In addition, because there was this mesh correspondence, design cycle time was very low, with several design cycles per day possible.

On the manufacturing side, the multi-step brazing and welding are difficult but not unknown quantities.  Procedures and processes exist to perform these tasks.  Proper braze joint mechanical design, care and handling, material compatibility, and overall cleanliness requirements must, of course, be strictly observed.