I am planning to make one of my characters in my novel have to have an ability to create positron beams as an offensive ability. I wonder what the effects on regular matter like a wall or a body of human being will be.
Edit: will the human body be completely obliterated due to annihilation or will a cadaver be intact? I know that only electrons will cancel out.
The answer depends on the intensity of the beam and its energy. As L.Dutch's answer has correctly note, each of the positrons in the beam will eventually annihilate an electron, emitting a pair of gamma rays. But that not the most important damage it will do.
The main mode of damage happens because the positrons in the beam are charged particles which strongly interact electromagnetically with the electrons in the air, first dumping their kinetic energy into them and then annihilating them. So a positron beam in air will ionize the air, forming a column of plasma the length of the beam. Since the plasma interacts even more strongly and you, basically, get a lightening bolt. (Note: this is without the annihilation bit happening at all! An electron beam will worm pretty much the same way. This is why electron-beam welding is done in a vacuum.) See this for some more discussion.
If you make the beam less energetic so there's less kinetic energy to be dumped into the air, the beam just annihilates in a shorter distance. If you make it intense enough to just cut a hole right through the air, you turn your lightening bolt into a coruscating beam of ravening energy(1).
Shooting an electron beam is destructive enough. Shooting positrons adds to the fun by annihilating electrons, too, but this does surprisingly little additional damage compared with the effect of the beam's electromagnetic interaction. The energy of annihilation goes entirely into a pair of gamma rays. The gamma rays are mostly absorbed by nearby matter, but the damage they do is small compared with the damage done by the charged particle beam itself. (The gamma rays produced are 500 KeV gammas, and they dump their energy over tens of centimeters of flesh. The incoming positrons have more than 1 MeV of kinetic energy and dump that energy into a narrow line through the body where the beam passes. The effects are much more devastating. ...Which is not to say that the gamma is harmless, it's just that you're so dead from the energy dumped by the electromagnetic interactions you don't care.)
(It's worth noting that a beam of positrons is not stable for two reasons. First, since positrons, all being positively charged, repel one another. No matter how narrow the bean is when it is created, it will widen. If the positrons are fast-moving, though, the beam can get further before falling apart. (This is why the particle beam weapons projects that survived the longest were neutral beam weapons.) Secondly, interactions with the plasma it creates increases the instabilities.)
So the bottom line is that if you have a sufficiently intense beam of positrons to get to the target, you're shooting something like a lightening bolt and (in air) you can't avoid the damage due to the purely electromagnetic effects being far greater than that due to the electron annihilation.
Note the "in air" part: In a vacuum, it's a different matter entirely and the physics of the beam are quite different.
(1): See the works of E. E. Smith, PhD.
Positron is the antiparticle of electron. Same mass, opposite charge.
When a positron gets close enough to an electron, electron-positron annihilation occurs.
Electron–positron annihilation occurs when an electron (e−) and a positron (e+) collide. The result of the collision at low energies is the annihilation of the electron and positron, and the creation of gamma ray photons: $e^−+ e^+→ γ+ γ$. In the most common case, two photons are created, each with energy equal to the rest energy of the electron or positron (0.511 MeV). If either the electron or positron, or both, have appreciable kinetic energies, other heavier particles can also be produced (such as D mesons or B mesons), since there is enough kinetic energy in the relative velocities to provide the rest energies of those particles. Alternatively, it is possible to produce photons and other light particles, but they will emerge with higher kinetic energies.
Summarizing, the positron beam will induce a flash of gamma rays in whatever it goes through.
Gamma rays are nasty, how nasty depends on the intensity of the flux. At low fluxes it will just kill living cells, at high fluxes it will convert to plasma every medium through which it propagates, including the atmosphere, becoming visible from space.
Positrons are used for medical imaging (in PET, Positron Emission Tomography, the patient is given a radioactive sugar, which emits positrons), at low enough fluxes to be less dangerous than the sickness being investigated.
It depends.
If a positron beam hits a human body or part of it the effect will vary from harmless and not detectable to instant vaporization and plasmization depending on the strength of the beam.
As L. Dutch mentioned in his answer, a flux of positrons can be thin enough to do no noticeable damage to a human body, and yet dense enough to be used to image the interior of a human body, in PET scans.
And as Mark Olson said in his answer, atmosphere consists of molecules of gas and particles of dust, etc. and is not empty space. A positron beam weapon used to attack in a space battle will not interact with the matter which is not there in empty space, but a positron beam weapon used within a dense, breathable, atmosphere will interact with that atmosphere for better or for worse depending on the point of view.
Therefore, the answer is "it depends". It depends on the intensity of the positron flux in the beam, on the density of any atmosphere the beam may be fired in, the distance to the target, any defenses the target and the shooter may have, and other factors, that together determine where the effect of the positron beam weapon is on the spectrum between "totally harmless" and "causes total destruction to the target and goes on to destroy stuff behind it in collateral damage that cause both sides to lose everything".
