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Of Giant Chips, Particle Physics, and Old Tech Made New

The silicon die on the lab bench was huge, almost four square inches. It had 1664 wire bond connections running down each edge of the chip and into 26 analog to digital converters with 128 channel a piece.  It wasn’t an overblown design for the next multi-core processor, it was a particle detector employed by high energy physicists at the Relativistic Heavy Ion Collider, (RHIC), at Brookhaven National Laboratory, (BNL).  Similar devices have been pressed into service at the Large Hadron Collider, (LHC) home of the Higgs Boson search at CERN.  The specific detector this silicon was going into was the Forward Vertex Silicon Detector, (FVTX), for the PHENIX experiment at RHIC.

Silicon Detector Wedge

Silicon Detector Wedge

For some of the unanswered question of physics, the particle accelerator is the tool of choice.  At RHIC, New Mexico State University’s students, (shown below), were building the FVTX as part of the search for the proton’s missing spin.  When protons were discovered to be made up of three quarks in the 1960’s, it was assumed that the ‘spin’ of the proton was split among its three constituent quarks.  In the late 1980s, what became known as the ‘spin crisis’ began when physicists discovered that the spins of the valence quarks accounted for less than a third of the proton’s total spin.


NMSU team Darshana Perera, Elaine Tennant, Feng Wei, and Abraham Meles

At RHIC, the PHENIX experiment collides beams of high energy protons.  Each beam is accelerated to an energy of over 200 GeV, (billion electron volts).  1 electron volt is the energy an electron gains when it is accelerated through a one volt electric field.   At these energies when two protons collide their constituent quarks and gluons, (the particles that hold quarks together inside the proton) form other exotic particles that fly away from the collision.  By studying the properties of these particles, physicists gain insights about the spin content of the quarks that make up the proton.  There are a few problems though.  First, the temperatures at the collision point, (vertex), are hotter than the surface of the sun.  Any detector put at the vertex will quickly be irreparably damaged.  The second problem is that the initially produced particles quickly decay into other particles.  Quarks combine to make particles called mesons, and the mesons decay into yet other particles amongst them, muons.

A muon is identical in almost every way to an electron.  Muons, however, are roughly two hundred times more massive than electrons.  Compared to the other particles created in the collision, muons are relatively long lived.  Much of the data gathered by the PHENIX experiment is captured by measuring the trajectories of the resultant muons and then inferring what types of initial particles decayed into the muon and what properties they must have had.

Two of the most important pieces of data are the energy of the muon, and how close to the vertex the muon was when it was created.  A large magnet surrounding the outer ends of the experiment is used to bend the path of the muons.  Detectors within the magnet housing produce a track of the muon.  By measuring the curvature of the track, the momentum of the muon can be determined.

Detector Magnets

Detector Magnets

The magnets mentioned above are the green structures at the left and right sides of the picture below.  The FVTX detector is situated around the smaller beam pipe seen in the middle.

The same detectors record the muons location at points along its trajectory.  These locations are used to calculate a track back to the original collision point.  The track is used to determine what type of particle decayed into the muon.  The closer the muon track is to the vertex, the shorter the lifetime of the particle that created it.  Because the detectors within the magnet are meters away from the vertex, when the trajectory of the muon is traced back, a small error in the detected position of the muon at the magnet leads to a large error in the calculated distance of the muon from the vertex.

Reducing this error is where the silicon wedges within the FVTX come in.  The wedges each contain 3328 diodes arranged in a pattern shown below.  The diodes are composed of p doped material on the n doped substrate of the wedge.

Strip Diodes Arranged on the Wedge

Strip Diodes Arranged on the Wedge

The diodes are reverse biased so normally they don’t conduct.  However, when a muon passes through one of these diodes, it gives some of its energy to the electrons within the diode generating a current that can be detected.  The FVTX contains 384 wedges arranged radially around the pipe that contains the particle beam collision as shown below. (The wedges on the viewers side of the pipe are not shown.)

Detector Assembled Around Central Beam Pipe

Detector Assembled Around Central Beam Pipe

By coordinating data obtained from the FVTX, the accuracy of the muon track at distances close to the vertex is greatly enhanced allowing physicists to more accurately determine what kind of particles created the muon.

In 1902, Greenleaf Whittier Pickard invented the crystal radio set allowing listeners to detect/hear radio waves without a battery.  At the heart of the circuit was a primitive diode.  One hundred and ten years later, the diode is still in service as a detector searching for answers to the fundamental questions of physics.

One Response to “Of Giant Chips, Particle Physics, and Old Tech Made New”

  1. The Hodoscope | The Canonical Hamiltonian Says:

    [...] After writing a post about the hodograph of Sir W. R. Hamilton and how it relates along with a few other rather obscure mathematical theorems to angular momentum and planetary orbits, my wife, a PhD physicist, remarked, “Yes, but what about the hodoscope?”.  A little bit of digging revealed that unbeknownst to me she had actually built a hodoscope. [...]

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