WITCH is a double Penning trap

Penning trap

Penning traps are devices for the storage of charged particles using a homogeneous static magnetic field and a spatially inhomogeneous static electric field. This kind of trap is particularly well suited to precision measurements of properties of ions and stable subatomic particles which have...

 experiment to measure the recoil energy of decaying nuclei

Atomic nucleus

The nucleus is the very dense region consisting of protons and neutrons at the center of an atom. It was discovered in 1911, as a result of Ernest Rutherford's interpretation of the famous 1909 Rutherford experiment performed by Hans Geiger and Ernest Marsden, under the direction of Rutherford. The...

. A spectrometer in combination with a position-sensitive microchannel plate detector

Microchannel plate detector

A micro-channel plate is a planar component used for detection of particles and impinging radiation . It is closely related to an electron multiplier, as both intensify single particles or photons by the multiplication of electrons via secondary emission...

 is used to count ions (with the MCP) while scanning their energy (using the spectrometer). The experiment is located at the ISOLDE Radioactive Ion Beam Facility in CERN

CERN

The European Organization for Nuclear Research , known as CERN , is an international organization whose purpose is to operate the world's largest particle physics laboratory, which is situated in the northwest suburbs of Geneva on the Franco–Swiss border...

. The beam from ISOLDE is bunched by REXTRAP after which it is transferred to the WITCH set-up.

Working principle

The ion bunch

Charged particle beam

A charged particle beam is a spatially localized group of electrically charged particles that have approximately the same velocity . The kinetic energies of the particles are typically measured in keV or MeV, much larger than the energies of particles at ambient temperature...

 from REXTRAP is slowed down in a pulsed drift cavity in the vertical beamline. After this the bunch is injected into the magnetic field

Magnetic field

A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...

. Here the ion cloud is trapped in the first Penning trap

Penning trap

Penning traps are devices for the storage of charged particles using a homogeneous static magnetic field and a spatially inhomogeneous static electric field. This kind of trap is particularly well suited to precision measurements of properties of ions and stable subatomic particles which have...

 (for cooling). Its purpose is to cool down the ions with buffer gas

Buffer gas

A buffer gas is an inert or nonflammable gas. In the Earth's atmosphere, nitrogen acts as a buffer gas. A buffer gas adds pressure to a system and controls the speed of combustion with any oxygen present...

. In this trap, it is possible to apply different excitation modes (dipolar

Dipole

In physics, there are several kinds of dipoles:*An electric dipole is a separation of positive and negative charges. The simplest example of this is a pair of electric charges of equal magnitude but opposite sign, separated by some distance. A permanent electric dipole is called an electret.*A...

, quadrupolar

Quadrupole

A quadrupole or quadrapole is one of a sequence of configurations of—for example—electric charge or current, or gravitational mass that can exist in ideal form, but it is usually just part of a multipole expansion of a more complex structure reflecting various orders of complexity.-Mathematical...

 and octupolar). The ions are then transferred to the second Penning trap (in which decays are studied).

When an ion decays, it can escape the decay trap into the spectrometer

Spectrometer

A spectrometer is an instrument used to measure properties of light over a specific portion of the electromagnetic spectrum, typically used in spectroscopic analysis to identify materials. The variable measured is most often the light's intensity but could also, for instance, be the polarization...

. The spectrometer consists out of a set of electrode

Electrode

An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...

s to create an electrical potential

Electric potential

In classical electromagnetism, the electric potential at a point within a defined space is equal to the electric potential energy at that location divided by the charge there...

 barrier. If the decayed ion's energy is high enough, it will be able to overcome this barrier to be detected at the top of the setup.

Physics goal

The first goal of the experiment is to measure the so-called beta-neutrino correlation. This is indirectly done by measuring the shape of the spectrum

Spectrum

A spectrum is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. The word saw its first scientific use within the field of optics to describe the rainbow of colors in visible light when separated using a prism; it has since been applied by...

 of the recoiling ions. This beta-neutrino correlation can provide, if measured precisely, information about fundamental weak interaction

Weak interaction

Weak interaction , is one of the four fundamental forces of nature, alongside the strong nuclear force, electromagnetism, and gravity. It is responsible for the radioactive decay of subatomic particles and initiates the process known as hydrogen fusion in stars...

 properties. According to the Standard Model

Standard Model

The Standard Model of particle physics is a theory concerning the electromagnetic, weak, and strong nuclear interactions, which mediate the dynamics of the known subatomic particles. Developed throughout the mid to late 20th century, the current formulation was finalized in the mid 1970s upon...

 the weak interaction has a Vector–Axial vector structure. But there are other mathematical possibilities which have not been detected in nature yet. The WITCH experiment will search for scalar currents in the first place. But it is also possible to look for tensor currents.

Other physics goals are the search for heavy neutrinos, measuring Fermi/Gamov-Teller mixing ratios, measuring electron capture probabilities, etc.