Threshold energy

The threshold energy or energy threshold is the minimum energy that must be supplied to a physical system in order to trigger a certain reaction. The fact that there are threshold energies is an expression of the law of conservation of energy .

The term threshold energy is mainly used in nuclear physics and particle physics . Corresponding terms are, for example, ionization energy in atomic physics or activation energy in chemistry .

Nuclear physics

For many nuclear reactions , the collision partners must “bring” a threshold energy as kinetic energy , because z. B.

• the Coulombwall has to be overcome,
• a minimum excitation state must be reached
• or the overall reaction is endothermic .

Below the threshold energy, the cross section is zero, the reaction does not take place. Only the tunnel effect can "soften" the sharp threshold a little.

Particle physics

With high-energy particle collisions, new particles can be created. If the total energy of the two colliding particles (sum of their rest energies plus kinetic energy of their relative motion) is greater than the rest energy of a possible new particle, there is a non-zero cross section for its formation; the threshold energy is therefore equal to this rest energy. If the input energy is smaller, the new particle can only be formed virtually , to put it clearly: it only exists for such a short time that the energy-time uncertainty relation “allows” the violation of the conservation of energy, and then decays again. This process can also be seen in the measurement results, for example the directional distribution of the particles after the collision.

literature

• K. Bethge, G. Walter, B. Wiedemann: Nuclear Physics: An Introduction , 2nd Edition, Springer 2001, ISBN 978-3-540-41444-5
• H. Hilscher: Elementary Particle Physics . Vieweg 1996, ISBN 978-3-322-85004-1