Scintillation Detectors

Scintillators are materials that produce light
when ionizing radiation passes through them. These materials can be solids or liquids or
gases. Scintillator materials absorb incident gamma
radiation by one of the three mechanisms we discussed before. As the kinetic energy of the reaction electrons
is deposited in the material, it raises the materials electrons to excited states. During the subsequent de-excitation, the scintillator
usually emits a photon in the visible light range. The light emitted from the scintillator is
guided to a photomultiplier tube, where it interacts with a photocathode, releasing electrons. These electrons from the photocathode are
guided with the help of an electric field toward the first dynode. The dynodes are coated with a material that
emits secondary electrons, usually more than one. The secondary electrons from the first dynode
move toward the second, and so on down the dynode string to the anode. More than a million electrons can be created
for each electron starting from the photocathode. Hopefully, many light photons were created
as the photoelectron moves through the scintillator, so that a respectable signal arrives out of
the PM tube. Many different scintillation materials are
used for detectors. Some of the more common are sodium iodide,
cesium iodide, BGO, which is chemist lingo for bismuth germanium oxide, zinc sulfide,
which is the great-granddaddy of all scintillating materials (This was the original material
that Rutherford used to do his scattering experiments.), and lithium iodide, which is
used as a neutron detector. The main reason that photomultiplier tubes
are needed is that we need a large amplification, because the primary signal out of the scintillation
material is low. Unfortunately, this large amplification leads
to poor energy resolution. To see the battle that we’re trying to fight,
let’s pick typical values here. Scintillator efficiencies are on the order
of 13% or less. Scintillator efficiencies are the ratio of
the photon energy to the kinetic energy of the electrons put into the scintillator material. Light losses are in the order of 2/3. It’s a good day when we can steer 1/3 of
the light to the photocathode and the multiplier tube. The quantum efficiency, which is the frequency
in which the light hitting the photocathode is converted back to an electron, is on the
order of 7-20%. These factors taken together lead to values
of 230eV or so per electron fed into the dynode strings of the photomultiplier. If we go calculate the full width half max
of the signal from this ideal detector, we get 29keV, and this is as good as it gets
for cesium-137, which emits a 662keV gamma ray, and, believe me, it’s generally much
worse than this.

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13 Responses

  1. ursula keen says:

    This seems rather detailed. Surpasses my understanding of high school nuclear chemistry

  2. Gazinbali says:

    I was thinking of using a scintillation detector instead of a Geiger counter.. SD needs calibration and a PM tube… think I will give it a miss…

  3. joy alz says:

    thanQ ! that's help me to understand how it's gonna work

  4. Chris Pang says:

    A great boost for my presentation!

  5. Srichakra Manikanta Adike says:

    yes, the potential is divided to each dynode using a dividor circuit. Each dynode has higher voltage than the predecessor.

  6. mameluco Bony says:

    Easy and simple way to explain that complex problem. Thanks

  7. Mohammed Alsubhi says:

    Thank u guys so much

  8. aliselin a says:

    where did he get that white photo behind him ?

  9. mnpd3 says:

    Not ZnS, but ZnS(AG). Zinc Sulfide has to be activated in order to detect ionizing radiation. In its usual form, ZnS is nothing more than a "glow in the dark" compound.

  10. ali riza says:

    Why not translate it into Arabic

  11. Aditya Sahasranshu says:

    so can we increase the efficiency and resolution without too much modification to the crystals?

  12. Hayder Gfg says:

    Thanks sr but what is this material use in detection I can use nanotechnology

  13. Laura Maynard says:

    Great use of animation along with key points in bullet point and a real person speaking.

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