While there are good answer as to why it can't be done I'm going to back up and look at the big picture.
Usual types of radiation:
Alpha--very short ranged in atmosphere, it can't meet your range requirement.
Beta--does a bit better but it still can't do anything like 100m.
X-ray & Gamma ray. The big problem here is that we have no means of focusing these. All devices that produce a "beam" of these do so by using an omnidirectional source and shielding off the part that's not going where you want it. That shield is big and heavy, thus making it impossible for one person to carry.
Neutron. Again, a focusing problem. Neutrons lack a handle to grab. Now, it is possible to make a neutron beam but that involves accelerating a charged particle stream and then somehow splitting the neutrons out once the beam is already underway. That's in atom-smasher territory and atom smashers are characterizes by their huge booster rings. (Absolutely necessary--the smaller the booster ring the faster the particles turn and if you make too high an energy particle turn too fast it loses energy. Thus your energies are limited by the size of the booster ring--you're not packing much in a booster ring you can hold in your hands.)
Now for things other than the traditional types of radiation:
Proton beam. See my argument about neutrons. Pretty much the same equipment. I also question whether it can go far enough in atmosphere.
Now, if you'll drop the portable requirement it can be done. There are two means of building an x-ray laser with today's tech.
The simple and small one is utterly out of the question for your scenario. A properly-shaped rod exposed to a sufficient energy flux can laze and produce a death beam. Both x-ray and gamma ray beams are possible. The problem is that we have only one means of generating that energy flux: the detonation of a nuclear device.
The other alternative can only produce an x-ray beam. I don't recall most of the details but it's a free electron laser--and the power source is a sibling of the atom smashers. IIRC your ring is about 1km across.
However, after shooting down your weapon how about an alternative that still doesn't meet your requirements but comes a lot closer:
Lets consider a microwave beam--sibling to the mundane microwave oven. A microwave oven puts out something like 1kw, not a good thing to be exposed to but it's not going get a kill fast enough for combat purposes. Lets beef it up considerably, if your focus and aim is good 100kw should do the job. That should give you a kill of the flesh to the penetration depth of the beam over a target the size of a human head. While the actual dying will take time you have a combat kill as any of their sensory organs pointed your way is gone. (You have to shoot from in front of them to make this work!) 100kw and the focusing dish are way beyond what your soldier can carry but I wouldn't be shocked if this could be built as a dedicated vehicle weapon. Slow firing, though.
(On the other hand, I can see no reason to build it other than for reasons of stealth.)