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Solid State Detectors

Solid State Detectors. Radiation Detection and Measurement II IRAD 2731. Agenda. What is a semiconductor? Types of semiconductors Why is it different than scintillators. Solid State Detectors. Semiconductor – has electrical conductivity between metals and insulators

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Solid State Detectors

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  1. Solid State Detectors Radiation Detection and Measurement II IRAD 2731

  2. Agenda • What is a semiconductor? • Types of semiconductors • Why is it different than scintillators

  3. Solid State Detectors • Semiconductor – has electrical conductivity between metals and insulators • Pure- pure Si or Ge crystals are used to generate signal • Small band gap • Creates hole/electron pair • Numbers of electrons produced is proportional to energy deposited in crystal

  4. Solid State Detectors Conduction band Band Gap 10eV <3eV Valence band Scintillator Solid State

  5. Semiconductors • N-type- material is doped with a “donor impurity” which has a loosely attached electron • This generates free electrons easier than pure Si cause electrons are in different energy state • Si has 4 electrons ,As or P, are used at doped material, have 5 electrons

  6. Semiconductors • P-type-material is doped with an “acceptor impurity” which has a need for an extra electron • This generates ”holes” easier than pure Si • Si has 4 electrons, AL or B, are used at doped material, have 3 electrons

  7. P-N junctions

  8. Semiconductors • When semiconductors are exposed to radiation the electrical properties change • Intrinsic- material has been doped with both n and p type impurities • Doping with both material aligns the holes on one side and the electrons on the other • Appling reverse bias increases the hole/electron area • This forms a depletion layer, active volume of the detector

  9. Solid state • Surface barrier detector • PIPS • Silicon detectors • Gemanium detectors

  10. Surface barrier • In pure Si and Ge and natural current exists that excludes holes/electrons close to the surface • P-type material is electroplated onto the surface of a n-type Si surface, usually gold • With reverse bias applied this creates a depletion layer • Thin dead layer, very little energy loss of charged particles

  11. Surface Barrier -Very good resolution , better than p-n junction detectors Depletion layer is not as thick (best for low energy particles) -Light sensitive (2-4eV) -Very low background -Electronic noise -Very fragile- can not touch surface

  12. Surface Barrier

  13. PIPS • Passivated implanted planar silicon • Photo diode • Measures signals as photo current so can be very sensitive • Low noise • Needs to be shielded from visible light • Alpha/beta detection • More rugged that SSB, lower leakage current, window material is thinner

  14. PIPS

  15. Silicone • Most common semiconductor • Used to detect heavy charged particles • Alpha spectroscopy • Good energy resolution • SiLi detectors (used for gamma spect) have to be cold all the time • Prevent the movement of Li inside the Si crystal • BUT not for charged particles

  16. SiLi charged particle detectors • Designed for highly penetrating charged particle Up to 3 MeV Betas, 30 MeV protons, 140 MeV Alpha

  17. Germanium • Used to be doped with Li top get larger depletion zone • Have to keep cold all the time • Easier to get high purity Ge than Si cause of melting temp • GeLi has been replaced with HPGe • HPGe detectors can be warmed to room temp when not in use

  18. Applications • Planar • Slab of detector • Limited in size • Coaxial • Can have either n or p type coaxial detectors • Larger active volume of detector • Large dead layer does not affect most gamma rays

  19. Parts • Cryostat- container that holds liquid Nitrogen (or other cold liquid) • A method of transmitting this to the detector (usually a copper cold finger) • Can have several orientations • Detector capsule- consisting of the detector and electronics housed in protective endcap

  20. Solid State Detectors

  21. HPGe • Band gap is only 0.7 ev • Thermal noise will generate tremendous leakage current leading to noise • Will need to be cold (LN) to operate • Decrease in movement of the atoms in the crystal will decrease thermal noise

  22. HPGe • Have smaller band gap get more pieces of info from each radiation event • More events better statistics • Energy resolution depends on • Statistical spread in number of charged carriers • Variations ion charged collect ion efficiencies • Electronic noise • HPGe have better resolution than scintillators • which means that you can see gamma peaks that are closer together than in the scintillation crystals

  23. HPGe • NaI detectors are more efficient than HPGe • HPGe detectors have better resolution than Na I • BUT have some large HPGe detectors that are more efficient than their NaI counterparts • More expensive than NaI crystals • NaI gamma spectroscopy system about 10K • Same efficiency HPGe system about $100K

  24. NaI and Ge detectors

  25. New Materials • CZT -cadmium-zinc-telluride crystals • Operates at room temperature • Good energy resolution better than NaI but not as good as HPGe • Hard to grow • High density • LaBr • Similar characteristics

  26. QUESTIONS?

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