Construction and the Physics of a Spark Chamber

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Construction and the Physics of a Spark Chamber

• By Scott Davis
• and
• Joe Seele

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Motivation and Purpose

• To construct an operational spark chamber.
• Explore the techniques previously and to a small
  extent currently used in the field of particle
  and nuclear physics.
• To understand the physics behind ionization of
  gas, development of the spark and detection of
  high energy, charged particles.

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Background

• The first spark chambers were constructed in the
  late 1940s and used extensively through the mid
  1960s in many experiments in nuclear and
  particle physics. We constructed a basic spark
  chamber. There are many methods one can use in
  addition to spark chambers to detect and identify
  particles.

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Background

• A spark chamber consists of a series of parallel
  metal plates of alternating high and low voltages
  with a gas between the plates. In most spark
  chambers there is also a triggering mechanism
  usually in the form of a scintillating material
  and a set of photomultiplier tubes.

Our plates were constructed out of aluminum.
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Physics of Spark Chambers

• When a charged particle travels through a gas it
  causes the gas to ionize forming an ionization
  trail along the path of the particle. When the
  electric field is applied, the liberated
  electrons will drift toward one of the plates.

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Physics of Spark Chambers Electron Avalanches

• When the field is applied to liberated ions in
  the ion trail, the electrons will drift towards
  one of the plates liberating more ions along the
  way. The heavier positive ions essentially do not
  drift in the field. Once a critical density of
  positive ions is reached (enough to form a
  conductive path between the plates) there will be
  a flow of electrons from one of the plates to the
  other which causes the spark.

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Physics of Spark Chambers Radiative Process

• Once the critical density of ions is reached, the
  electrons flow from one of the plates flow across
  the ionization trail causing the outer electrons
  of the gas to move to an excited state where it
  will then decay back to its original state by
  emitting a photon. This is the light we see.

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Physics of Spark Chambers - Gases

• The gas one fills the spark chamber with is very
  important in the detection of the charged
  particles. It affects both the ease of which an
  ionization trail is produced as well as to a
  lesser extent how much radiation is emitted.

We observed that Neon is a better gas than Argon
for the viewing of tracks.
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Scintillators

• From Bethe-Bloch formula, a relativistic charged
  particle loses energy to excite electrons in
  atoms in a material. These electrons radiate
  to return to lower levels.
• There is also energy loss due to Bremsstrahlung,
  but with low density material, this is less of a
  concern.

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Scintillators

• Organic
• Faster time resolution (ns)
• Low density (not good for gamma rays)
• Efficiency falls off quickly with lower energies
• Inorganic
• Slow time resolution (ms)
• Higher intensity light emitted (but over long
  time span)
• High density (good for gamma rays)
• Good energy resolution

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Photomultiplers

• Used to convert a small number of photons to a
  noticeable signal.
• Start with a photocathode with a low work
  function (often Cesium)
• Use a series of dynodes to increase the current
  by IkVn where n is the number of dynode (from
  Child-Langmuir Law).

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Electronics

• Need to Trigger the Spark Chamber
• Use Scintillators
• Need to Distinguish Signal from Noise
• Use Coincidence Logic
• Avoid Interference from Discharge
• Use Pulse Generator to Start a Veto Pulse After
  Discharge

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Electronics
NIM Bin
Scintillator
Logic
And
Discriminators
Veto
HV
Spark Gap
Pulse Generator
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Electronics - NIM

• Discriminators
• Used to Create a better Pulse from Photomultipler
  Tube
• Have on lowest threshold and 10 ns pulse
• Coincidence Logic
• AND two scintillators to cut down on background
• VETO with the pulse from the Pulse Generator
• Pulse Generator
• Generate pulse when signal sent
• Lasts from 10-100 ms (depends on user)
• Used to prevent re-firing from EMF
• Also can prevent resonance firing which looks
  cool!

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Electronics Spark Gaps

• Spark gaps are used for the triggering of high
  voltage, high current sources. They consist of a
  plate at a high voltage placed across from
  another plate at ground. There is also a small
  fiber (usually tungsten) that protrudes slightly
  into the gap.

• When a negative pulse is applied to the fiber
  it causes a space charge disturbance in the
  electric field which causes a current to flow
  across the gap by way of the same avalanche
  theory used in spark chambers.

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Results!

• First test with Argon. Very little activity, even
  with 20 kV (note, were using a different
  scintillator setup, but similar results with
  previous setup) Spark Chamber with Argon

• Add Neon (with a small amount of Argon).
  Wonderful!
• Spark Chamber with Neon

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Possible Improvements

• Gas system needs extensive upgrade. Plumbing in
  some cases had 4-5 connectors between piping.
  Also, flow rate is very difficult because
  regulators are rough.
• Chamber can be opened for cleaning off carbon,
  improving seals, and rethreading some screws.
• A new spark gap would improve performance.
• The EMF from the spark gap causes the NIM
  electronics to fire. Increased distance and/or
  shielding will decrease this rate.
• If the gap is to be used as a demo, then a sealed
  gas system will allow the system to be used
  longer.