radiation detection and measurement ii irad 2731
TRANSCRIPT
Solid State DetectorsRadiation Detection and Measurement II
IRAD 2731
What is a semiconductor? Types of semiconductors Why is it different than scintillators
Agenda
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
Solid State Detectors
Solid State Detectors
Scintillator Solid 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
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
Semiconductors
P-N junctions
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
Semiconductors
Surface barrier detector PIPS Silicon detectors Gemanium detectors
Solid state
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
Surface barrier
Surface Barrier
-Very good resolution , better than p-n junction detectorsDepletion 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
Surface Barrier
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
PIPS
PIPS
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
Silicone
SiLi charged particle detectors
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
Germanium
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
Applications
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
Parts
Solid State Detectors
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
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
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
HPGe
NaI and Ge detectors
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
New Materials
QUESTIONS?