Radiation Detection and Measurement II

surprisesameSemiconductor

Nov 1, 2013 (3 years and 7 months ago)

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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





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



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

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



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