Neutrino Simulation Game Introduction to the EXO-200 detector and some of the science involved

Neutrinos are very small neutral particles which have very little interaction with ordinary matter. How little interaction you ask? There are 100 million neutrinos created in the sun passing through every square centimeter of your body every second. And over the course of your lifetime maybe 2 will interact with the atoms of your body. How little interaction you ask? Well it would take a light-year thickness of lead to block 63% of these. Compare that to the few feet of concrete to block the radiation in a nuclear power plant and you start to get an idea of how difficult it is for scientists to measure neutrinos.

In the field of particle physics there are two basic routes scientists take. One is high energy physics (HEP), like that found at accelerators where particles are made to go at high energies and then are collided with other particles or sometimes with targets. The Stanford Linear Accelerator (SLAC) and the Large Hadron Collider (LHC) at CERN are both examples of high energy physics in action. These create great energies but are also greatly expensive.

The other option is to do what is called low energy particle physics and EXO-200 is an example. This route is cheaper, easier and quicker for physicists to obtain useful data. Exo-200 follows an exotic natural nuclear decay that could give us a peak into the inner workings of the universe. This unusual nuclear decay is a simultaneous double beta decay in which Xenon-136 will decay into Barium-136 and two electrons. Scientists have observed this double beta decay with the production of two antineutrinos. Xe-136  Ba e- + 2 v ordinary 2-neutrino

The nucleus cracks open two neutrons (down, down, up quarks) and converts these to protons (up, up, down quarks) and spits out two electrons and two antineutrinos. This double beta decay is the s l o w e s t process measured by humans; with a half-life of 2.1 x years. To put that in prospective the universe is “only” 1.4 x years old! The energy (2.448 MeV) of this decay can be calculated by the energy mass equivalence equation (E=mc 2 ) from Einstein. Due to conservation of energy this total energy is spread out over the energy of the neutrinos and the energy of the electrons. Which yields a distribution of energies.

Xe-136  Ba e- exotic zero-neutrino Scientists are looking for an even more rare double beta decay which produces two electrons and no neutrons. They call this “zero neutrino double beta decay” and if they can observe this it will require some major adjustments to the current Standard Model Some of the new science which might result form this research is: 1) A new type of particle currently forbidden by the SM which acts as it’s own antiparticle 2) A new mechanism of the generation of mass 3) Adjustments to conservation laws

The detection chamber Lead shielding wall cryogenics EXO-200 is 2,000 feet below the surface in an old salt mine

EXO-200 has a large chamber filled liquid Xenon and two types of sensors. One kind of sensor is a complex of electrodes which detects the presence and the energy of emitted electrons. The other kind of sensor is a wall of photo-tubes which detect energy emitted as light as particles bounce around in the liquid xenon. The core of the detector thus consists of the source (liquid xenon) and lots of two types of detectors. This is supported by a huge refrigerator, pumps, purification systems and massive amounts of electronics to collect data and monitor all the systems. Around this are four levels of protection from contamination.

View from outside the clean rooms with walls of the salt on the right

cryostat

Cryostat and xenon pumping/ purification system

Lead shield wall

Xenon chamber before placement in cryostat

Xenon chamber before placement in cryostat

Xenon chamber before placement in cryostat Scientists take great measures to prevent contamination

Photo tubes to detect light

Xenon circulation and purification system