J-PARC

Cosmic Ray Shower

In the T2K (Tokai2Kamioka) experiment we begin our science in a chain of death and birth to create the worlds most powerful beam of neutrino particles.

Particle beams created with conventional particles, such as protons, make use of the fact that they have electric charge and can therefore be accelerated by electric fields as well as pinched and manipulated by magnets into a fine stream. But neutrinos have no charge – so how do you create a beam of them? As with many other technologies we copy Nature.

Protons are constantly being accelerated by the Sun and many other cosmic sources, some of them rain down upon the Earths atmosphere. When a proton strikes the atmosphere it gives birth to a shower of new particles; its energy is turned into mass using Einstein’s E=mc2. The majority of the newly born particles are a type called pions.

piplusbubble

Pions are short lived, they die quickly and give birth to yet more particles which rain down upon us from all directions, as they are created all over the Earths atmosphere. Some of these new particles may also die and give birth to yet another generation of brand new particles. The chain of death and birth end when the particles reach the ground, or reach the lightest versions of themselves they can possibly be, E.g. electrons.

The picture to the left was taken with a particle detector called a bubble chamber (*), it shows the death cycle of a positively charged pion (blue). In death the pion gives birth to an antimuon (green) and muon neutrino before the muon then dies, giving birth to a positron (red) and electron neutrino. The neutrinos cannot be seen here as only particles with electric charge can be seen (the coloured lines are drawn on the picture to guide the eye).

In the T2K experiment we copy this process. We accelerate our very own protons, just like the Large Hadron Collider at CERN, and smash them into our own target. We don’t have a jar of atmosphere to smash our protons into but instead use graphite; the same stuff you’ll find in the centre of your pencil. Below is an animation showing the route the protons take to the different facilities at J-PARC. The one I am most interested in is the last stage where the protons go to the neutrino beam-line.

J-PARC Beam

Horn12

To create a beam we don’t want the neutrinos to go in just any direction, so we need to focus them. With no charge we cannot effect the neutrinos directly, but the pions do have charge and in their short lifetime we focus these into a beam.

Surrounding our graphite target and beyond there are a series of three metal horns which look a little like the exhaust of rockets. For short nano-second (a few billionths of a second) periods a huge amount of electricity is pumped through these horns (~300,000Amps! About 100,000 times that of a standard household plug!) to produce a magnetic field which focuses the pions into a beam. Because the pions are travelling at such high (relativistic) speeds the neutrinos and muons, created when they die, carry on in the same direction. So in creating a beam of pions we in turn create a beam of neutrinos and muons too.

The beam of pions transform into muons and neutrinos as they travel through a 110m tunnel filled with Helium. At the end of the tunnel there is a massive block of concrete and metal which absorbs the muons and remaining pions.

The neutrinos continue their journey unfazed, firstly 280m to a collection of particle detectors and then on through the Earths crust to the Super Kamiokande detector 295km to the West.

T2KOverview1

Below is the most recent snapshot of the beam line at J-PARC. Of most interest for the T2K experiment is the one showing the shot number and power of the neutrino beam line (second row left under the heading ‘MR Beam Cycle and Mode’). The blues shot number shows how many bunches of protons have been sent toward the graphite target to create the neutrino beam. The power shows the intensity, or number of protons, delivered in a shot.

(*) Bubble chamber detector: Superheated liquid, heated well above it natural boiling point, is held in a pressurised container to prevent boiling. As a particle with electric charge passes through the liquid it deposits energy, in the form of ionisation where electrons are stripped from their atoms. This extra energy allows small bubbles of gas, boiling, to occur along the path of the particle, which is what can be seen in the photograph.