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?