Why Do Positrons Lose Energy Before Annihilating With Electrons?

[SteveDC]

I thought that when positrons and electrons collide they annihilate straight away, giving up energy in forms of photons and other particles. However, I'm reading through some notes and they suggest that the positron loses energy rapidily in collisions with electrons but does not annihilate until it has slowed down sufficiently.

Is this correct, why does the positron lose energy so rapidly in collisions but not annihilate straight away?

Thanks

 

[Orodruin]

Can you provide a reputable source for where you read this? "Some notes" is not very helpful.

 

[SteveDC]

Some notes that I have for a lab module I am doing at University, its just a folder of pre lab notes not like a published source or anything

 

[Orodruin]

I would say it depends completely on the setting. In a particle collider, you would typically smash them together and get all of the energy out. If you have positrons imponging on some material, they may very well be stopped before finding an electron to annihilate with - giving a characteristic photon pair with energy 0.5 MeV for each photon.

 

[SteveDC]

The context given is in the decay of Na22 nuclide and I just don't understand how the positron created could ever collide with an electron without annihilation occurring.

 

[Orodruin]

You are probably thinking of electrons and positrons as small balls that collide if they hit each other. This is not a very accurate description. Electrons and positrons interact through electromagnetic interactions which generally act at a distance. Energy and momentum can be transferred between a positron and a target electron without an annihilation occurring.

 

[mfb]

The probability for an annihilation is much larger at lower energy. If you shoot a positron at low energies (MeV or below) into a material as it typically happens in radioactive decays, the positron usually loses most of its energy before it annihilates.

In colliders, that process cannot happen (there is no time for the positrons to lose their energy) - you get a few interactions at high energy.

 

[snorkack]

The probability for an annihilation is much larger at lower energy. If you shoot a positron at low energies (MeV or below) into a material as it typically happens in radioactive decays, the positron usually loses most of its energy before it annihilates.

In colliders, that process cannot happen (there is no time for the positrons to lose their energy) - you get a few interactions at high energy.

Perhaps it is better to say that it does not have space to happen?

Most positrons that collide in colliders collide elastically, and once they fly away with most of their energy they will meet nothing - they are not interesting, and the few that do annihilate at high energy are examined.
In matter, the positrons (most) that do not annihilate at high speed go on to lose some energy and, still being in matter, collide repeatedly and lose more energy until they do annihilate - usually after they have lost most of their energy.