Experimental Setup

The experiment comprises of two detectors; a near detector located at Fermilab, Illinois and a far detector situated at the Sanford Underground Research Facility, South Dakota which will observe the neutrinos produced at Fermilab.

An intense beam of trillions of neutrinos will be fired over a distance of 1,300 km through the Earth’s crust from a production facility near Fermilab, which gives ample distance for the neutrino flavours to oscillate. These neutrinos then reach a large volume of liquefied argon, a noble gas, at the far detector in South Dakota.

Map illustrating the distance between the production facility near Fermilab and the far detector at Stanford.

How does the far detector work?

Tracks of electrons and positrons in the Big European Bubble Chamber (BEBC) at CERN.

The far detector consists of four 10 kton liquid-argon time projection chambers  (LAr-TPCs) which are very large with each chamber being approximately 62 × 15 × 14 m. Using LAr-TPCs allows 3D bubble-chamber-like imaging of neutrino interactions (or proton decay) in the vast volume of the detector.

LAr-TPCs are the main component of the far detector. They are a type of Time Projection Chamber (which refers to the “TPC” in Lar-TPC). It is a type of particle detector that uses a combination of electric and magnetic fields together with a volume of gas or liquid that can be ionised to perform a 3-D reconstruction of particle trajectories or interactions. The LAr-TPCs consist of a steel frame and layers of thin wires in different orientations. The basic principle is that of an ionisation chamber, in which incident charged particles ionise (where a charged particle “knocks out” one or more electrons of atoms in a medium, creating ions) a volume of gas/liquid where the resultant positive ions move towards a cathode (the wires) and the dissociated electrons move towards the anode (the frame). This generates an ionization current which can be measured by an electrometer circuit, which is capable of measuring very small output currents in the regions of femtoamperes (a billionth of a millionth of an ampere) to picoamperes (one millionth of one millionth of an ampere). 

Argon’s full outer shell configuration

The medium to be ionised chosen to fill for the LAr-TPCs is liquid argon (where the “LAr” part comes from) as it is a noble gas, meaning it has closed electron shells. This means that the dissociated electrons will not re-join atoms in the medium as they travel towards the anode as there are no free positions for them in the electron shells. Argon also releases a number of scintillation photons (also known as light) when it is interacted with by a charged particle and these photons are proportional to the energy deposited in the argon by the interacting particle. Liquid argon is relatively cheap, making filling the large APA structures more economically feasible. Additionally, liquid argon is approximately 1000x more dense than the gas used in the first TPC design (read more about the origin of the TPC here), which results in an increased chance of particles interacting within the detector by a factor of 1000x. As the particles are so unfathomably small, this is very important to consider otherwise we wouldn’t observe any interactions, as the particles would miss each other!

This design of a ionisation chamber initially appears to be flawed however as the essential ingredient, a charged particle, seems to be missing – neutrinos aren’t charged! However neutrinos interact with some forms of matter and create charged particles such as muons, and it is these that can be detected. Therefore neutrinos are inherently elusive as they can only be inferred as opposed to being directly detected. The diagram below shows a simplified image of how the detector at LHC in Switzerland is not able to directly detect neutrinos although it is able to detect other particles.

Charged particles – electrons, protons and muons – leave traces through ionisation. The energy of neutrons is measured indirectly: neutrons transfer their energy to protons, and these protons are then detected. Neutrinos leave no trace in the detector.

How does the near detector work?

The near detector, which is being designed by collaborators in India, is comprised of a straw tube tracking detector and an electromagnetic calorimeter placed inside of a 0.4 T dipole magnet (400x stronger than the average fridge magnet!) which enables the tracking of particles.

Diagram of the near detector. Components will be surrounded by a dipole magnet, shown here in green.

A straw tube tracking detector is a type of gaseous ionisation detector and the basic principle of detection is the same as the ionisation chamber described above. It is composed of a hollow tube with a wire running alone the centre and a gas which becomes ionised when a particle passes through. Similar to above, a potential difference is maintained between the wire and the walls of the tube, in order to attract the ions and electrons in different directions. This difference in charge produces a current, which indicates that a particle entered the detector medium.

An electromagnetic calorimeter is an experimental piece of apparatus used in order to measure the energy of particles entering the detector. It will enable scientists to further study the behaviour and properties of these particles.

How exactly are neutrinos detected in LAr-TPCs?

Are there other neutrino detectors?

Yes! There are several neutrino detectors around the world, take a look at the image below to find out more about them.

Different neutrino detectors around the world.