The heaviest and slowest moving particle in nuclear instrumentation is typically the neutron.

Here's why:

* Heavy: Neutrons are neutral particles with a mass slightly larger than a proton.

* Slow: Neutrons are often slowed down in nuclear reactors and other instrumentation using materials like water or graphite. These materials act as moderators, absorbing some of the neutron's energy and reducing its speed.

While other particles like alpha particles and protons can also be used in nuclear instrumentation, neutrons are often considered the most challenging to detect due to their lack of charge and ability to penetrate deeply into materials.