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

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




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