Walter H. Barkas was Professor of Physics at the University of California, Riverside

University of California, Riverside

The University of California, Riverside, commonly known as UCR or UC Riverside, is a public research university and one of the ten general campuses of the University of California system. UCR is consistently ranked as one of the most ethnically and economically diverse universities in the United...

 from 1965 on. He specialized in the use of nuclear emulsion

Nuclear emulsion

In a Particle and Nuclear physics, a nuclear emulsion plate is a photographic plate with a particularly thick emulsion layer and with a very uniform grain size. Like bubble chambers, cloud chambers, and wire chambers nuclear emulsion plates record the tracks of charged particles passing through...

s, i.e., photographic plate

Photographic plate

Photographic plates preceded photographic film as a means of photography. A light-sensitive emulsion of silver salts was applied to a glass plate. This form of photographic material largely faded from the consumer market in the early years of the 20th century, as more convenient and less fragile...

s having a thick sensitive layer, for purposes of Particle physics

Particle physics

Particle physics is a branch of physics that studies the existence and interactions of particles that are the constituents of what is usually referred to as matter or radiation. In current understanding, particles are excitations of quantum fields and interact following their dynamics...

.

Together with his collaborators, he discovered the difference in range

Range (particle radiation)

In passing through matter, charged particles ionize and thus lose energy in many steps, until their energy is zero. The distance to this point is called the range of the particle...

 between positive and negative meson

Meson

In particle physics, mesons are subatomic particles composed of one quark and one antiquark, bound together by the strong interaction. Because mesons are composed of sub-particles, they have a physical size, with a radius roughly one femtometer: 10−15 m, which is about the size of a proton...

s of the same initial energy, and he ascribed this effect to the difference in stopping power

Stopping power (particle radiation)

In passing through matter, fast charged particles ionize the atoms or molecules which they encounter. Thus, the fast particles gradually lose energy in many small steps. Stopping power is defined as the average energy loss of the particle per unit path length, measured for example in MeV/cm...

 between positively and negatively charged particles. Hence, this difference is called Barkas effect or Barkas-Andersen effect, see Bethe formula

Bethe formula

In nuclear physics and theoretical physics, charged particles moving through matter interact with the electrons of atoms in the material. The interaction excites or ionizes the atoms. This leads to an energy loss of the traveling particle...

.

Literature

• Barkas W.H., Dyer J.N., Heckmann H.H.: "Resolution of the Σ^--mass anomaly", Physical Review Letters 11 (1963) 26

• Sigmund P.: "Particle Penetration and Radiation Effects, General Aspects and Stopping of Swift Point Charges", Springer Series in Solid State Sciences Vol. 151, Springer Berlin Heidelberg (2006)