Cosmic Rays: Two Ways to Detect Them

Cosmic rays: two ways to detect them Cosmic rays are thought to have been produced in the explosions of distant galaxies, in which the galactic core collapses with too much angular momentum to end in a black hole, and instead produces a sort of cosmic tornado in which the spinning magnetic field in the collapsing core acts as a galactic-scale particle accelerator to accelerate protons and nuclei to energies up to 1011 GeV – far beyond anything we could ever reach using a terrestrial accelerator. When the energetic protons reach the upper atmosphere of Earth, they interact with the nuclei of gas atoms to produce showers of strongly interacting particles (hadrons) – pi-mesons, K-mesons, etc. These particles are short-lived: the longest-living one is the , with a rest-frame lifetime of 25 ns and a rest mass of 140 MeV/c2. If a  were traveling at the velocity of light, it would live on average ~25 ns x 3x108 m/s = 7.5 m. The ’s are produced in cosmic-ray showers with an energy spectrum that falls ~exponentially with a scale energy of ~GeV. A 1 GeV  would have a relativistic gamma  = E/mc2 ~7, and would travel a most probable distance of c ~ 50 m. Thus almost all the mesons decay before they reach the surface of the Earth. The mesons typically decay into  leptons, which live 2.2 s, so that they in turn typically do not decay until they reach the surface of the Earth. The flux of cosmic ray muons at sea level is ~0.5 cm-2ster-1s-1. So a detector of ~0.1 m2 would have a total rate of ~ 500  leptons/second traversing it. One starting point for more information about cosmic rays is http://en.wikipedia.org/wiki/Cosmic_ray. There are two ways you could construct a cosmic ray telescope to observe muons reaching the Earth. One is to build a telescope of two Geiger counter tubes, which is a project in our senior lab course. The students this term have finished this experiment, so you could use their setup. You should read about Geiger tubes and their physics in the literature. You will need to design and build a front-end amplifier and a Schmidt trigger to create a logic pulse whenever a signal is produced on each Geiger tube. You would then bring the logic signals developed from two such tubes into an AND circuit, which would produce a HI output only when signals were produced simultaneously in both tubes (a coincidence), signaling traversal of a charged cosmic ray particle.

A second way to construct such a telescope is to use panels of plastic scintillator, which is essentially transparent acrylic plastic in which ~ 5 wt% of naphthalene has been dissolved. The naphthalene is fluorescent: a charged particle traversing the plastic panel can ionize electrons from the ground state, following which an electron falls into the vacancy by emitting a blue/UV photon. The photon is emitted promptly (~ns) so the light signal provides a prompt signal that can be used in coincidence detection. The light is channeled to an avalanche photodiode (APD), which produces an electronic pulse with a modest gain. The data page for the APD is at http://www.rmdinc.com/products/p006.pdf. The pulses from several APDs can be placed in shaped into logic pulses and then placed in coincidence to define a traversing . In either approach, if you construct a three-element stack, with a chunk of matter between layers 2 and 3, you could define a signature for a stopping muon: . If you use that signal as a start signal for a 20 MHz clock (http://www.parallax.com/StoreSearchResults/tabid/768/txtSearch/flexiforce/List/0/SortFiel d/4/ProductID/384/Default.aspx), and use a subsequent signal to stop the clock, you should be able to observe the electron from the decay of each muon via . The time delay between STOP and START should exhibit an exponential decay, with mean lifetime 2.2 s.