One solution is to create a primitive type Iaea40 covering the 40 bits of information and manually implementing the interface needed to reinterpret it as a Particle (with which you do the processing). So, from memory-mapped Matrix{Iaea40} the getters and setters would do the conversion to and from Particle i.e. reinterpret(Particle, Iaea40). I never tried such an approach so this is somewhat theoretical but it should work 
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