WITCH (standing for "Weak Interaction Trap for Charged particles"), or experiment IS433, was a double

Penning trap A Penning trap is a device for the storage of charged particles using a homogeneous axial magnetic field and an inhomogeneous quadrupole electric field. This kind of trap is particularly well suited to precision measurements of properties of i ...

experiment to measure the recoil energy of decaying nuclei. A spectrometer in combination with a position-sensitive microchannel plate detector (MCP) was used to count ions while scanning their energy. The experiment was located at the ISOLDE Radioactive Ion Beam Facility in CERN. The beam from ISOLDE was bunched by REXTRAP after which it was transferred to the WITCH set-up. The WITCH experiment completed its run in 2015, with its successor being the WISArD experiment currently running at ISOLDE.

Physics goal

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

spectrum A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without gaps, across a continuum. The word was first used scientifically in optics to describe the rainbow of colors ...

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

weak interaction In nuclear physics and particle physics, the weak interaction, which is also often called the weak force or weak nuclear force, is one of the four known fundamental interactions, with the others being electromagnetism, the strong interaction ...

properties. According to the Standard Model the weak interaction has a Vector–Axial vector structure, but there are also other mathematical possibilities which have not been detected in nature yet. The WITCH experiment searched for scalar currents in the first place, but. it was also possible to look for tensor currents. Other physics goals were the search for heavy neutrinos, measuring Fermi/Gamov-Teller mixing ratios, measuring electron capture probabilities, etc.

Experimental setup

The ion bunch from REXTRAP is slowed in a pulsed drift cavity in the vertical beamline. After this the bunch is injected into the magnetic field. Here the ion cloud is trapped in the first

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

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. Any inert gas such as helium, neon, or a ...

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

dipolar In physics, a dipole () is an electromagnetic phenomenon which occurs in two ways: *An electric dipole deals with the separation of the positive and negative electric charges found in any electromagnetic system. A simple example of this system i ...

, quadrupolar and octopolar). 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 A spectrometer () is a scientific instrument used to separate and measure spectral components of a physical phenomenon. Spectrometer is a broad term often used to describe instruments that measure a continuous variable of a phenomenon where the ...

. The spectrometer consists out of a set of

electrode An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials d ...

s to create an

electrical potential The electric potential (also called the ''electric field potential'', potential drop, the electrostatic potential) is defined as the amount of work energy needed to move a unit of electric charge from a reference point to the specific point in ...

barrier A barrier or barricade is a physical structure which blocks or impedes something. Barrier may also refer to: Places * Barrier, Kentucky, a community in the United States * Barrier, Voerendaal, a place in the municipality of Voerendaal, Netherl ...

. 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.

References

External links

WITCH Homepage

WITCH experiment record
on INSPIRE-HEP {{CERN Particle experiments CERN experiments