Radiation Detection and Measurement II IRAD 2731

 What is a semiconductor?  Types of semiconductors  Why is it different than scintillators

 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

ScintillatorSolid State Band Gap 10eV <3eV Valence band Conduction band

 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

 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

 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

 Surface barrier detector  PIPS  Silicon detectors  Gemanium detectors

 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

-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

 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

 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

 Designed for highly penetrating charged particle Up to 3 MeV Betas, 30 MeV protons, 140 MeV Alpha

 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

 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

 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

 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

 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

 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

 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