Delta ray

A delta ray is sometimes used to describe any recoil particle that causes secondary ionization. The term was coined by J.J. Thomson. It is entirely unrelated to the family of subatomic particles named delta baryon. The term is rarely used today.

Characteristics
A delta ray is characterized by very fast electrons produced in quantity by alpha particles or other fast energetic charged particles knocking orbiting electrons out of atoms. Collectively, these electrons are defined as delta radiation when they have sufficient energy to ionize further atoms through subsequent interactions on their own. Delta rays appear as branches in the main track of a cloud chamber. These branches will appear nearer the start of the track of a heavy charged particle, where more energy is imparted to the ionized electrons.

Delta rays in particle accelerators
Otherwise called a knock-on electron, the term "delta ray" is also used in high energy physics to describe single electrons in particle accelerators that are exhibiting characteristic deceleration. In a bubble chamber, electrons will lose their energy more quickly than other particles through Bremsstrahlung and will create a spiral track due to their small mass and the magnetic field. The Bremsstrahlung rate is proportional to the square of the acceleration of the electron.

References in popular culture
In science fiction, a "delta ray" is often used to describe an undefined, fictional type of ray or radiation, sometimes as a hazard, sometimes used in energy weapons.