Semiconductor Detect

1/6/2010
Band Theor

y of Solids
When radiation energy is deposited in a semicondu

detector, electron-hole pairs are created. Under the influence of an applied electric field, the hthe electrons move to the cathode and anode respe

Band Theory of SolidsGeneral

the electrons move to the cathode and anode respe

The result is an electronic pulse. The process is analogous to the formation and colleion pairs in the gas of an ionization detector. Indeedsemiconductor detectors are the solid equivalent of ionization chamber.
Band Theory of Solids

The energy levels of electrons in an atom are refershells.

L shellM shell

y

e

K shell

Energ

e

e

e

e
Band Theory of Solids

When atoms are brought close to each other, the hshell occupied by electrons, the valence shell, split

e

e

e

e
Band Theory of Solids

The greater the number of atoms, the greater the s

e

e

e

e

e

ee

e
Band Theory of Solids

When many atoms combine to form a solid, the vashell has split so many times that it forms a continuenergies, i.e., a band.

e e Band

e
Band Theory of Solids

The highest energy band occupied by electrons is valence band.

e e Valence Band
Band Theory of Solids

The empty energy band above the valence band iconduction band.

Valence Band

Conduction Band

e e
CONDUCTION BAND

Band Theory of Solids

The range of energies between the valence band aconduction band is called the band gap or forbidde

VALENCE BAND e e e e e e e e e 

BAND

GAP
CONDUCTION BAND

Band Theory of SolidsIn a pure material, electrons cannot possess energband gap. However, if there are impurities or defects, electronlocations can possess energies in the band gap.

e e e e e e e e e 

CONDUCTION BAND

VALENCE BAND

BAND

GAP

Impurity/defect 
Band Theory of Solids

Insulator

The valence band is full. As such, the electrons a

immobile and the material does not conduct elec

Conductor

The alence band is not f ll As s ch the electroThe valence band is not

f

ull. As

s

uch, the electro

mobile and the material conducts electricity.

Semiconductor

The valence band is full. As such, the electrons aimmobile and the material does not conduct elec
Band Theory of Solids

The difference between a semiconductor and an inthe width of the band gap: Semiconductor

Band gap < 5 electron voltsBand gap of 1 to 2 eV is typical

Insulator

Band gap > 5 electron volts 
Silicon and Germaniu

C t l St tCrys

t

al Struc

ture
Silicon and Germanium Crystal Stru

GeneralCarbon, silicon and germanium are group IV elemeThey have four unpaired electrons that can participcovalent bonds. As such, the atoms in a silicon or g

t l d i di d h d l tti

crys

t

al are arrange

d in a diamon

d-s

hape

d lattice.

The figure on the next slide shows the arrangemenin a germanium crystal. Each germanium atom participates in four covalenand each bond is represented by a pair of electron
Ge Ge Ge

ee- e e - - e- ee- e- e- e- 

e 

- e- e- e- e- eSilicon and Germanium Crystal Stru

General

Ge Ge Ge

e

Ge Ge

ee- e- e- e e - - e- e e- e- e- e- e- 

These electrons have energies in the valence ban
Silicon and Germanium Crystal Stru

GeneralIn the next slide, two electrodes applied to opposite the crystal: 

a positively charged anode on the left 

a negatively charged cathode on the right

The bottom half of the slide illustrates the distributielectrons in the valence and conduction bands.All the electrons are attracted towards the anode bthe electric potential, but they cannot move becausvalence band is full.
+

Ge Ge Ge

eGe Ge

ee- ee- e- e- e- e- e- e- e- e- ee- e- e- e- 

e 

- e- e- e- e- e- 

Cathode

+

Ge

Anode

Cathode

Conduction Band

e- e- Valence Band e- 
Pulse Formation
Pulse Formation

The following sequence of slides illustrates the forma pulse in a semiconductor detector. The first slide shows the initial position of the electThe second slide shows an electron being promoteconduction band because of the absorption of radienergy. This creates a “hole” or vacancy in the valband.In the third slide, the promoted and now mobile elemoves to the anode.The electrons in the valence band are now mobile it is no longer full.
Pulse Formation

Drawn by the electric field between the anode and one of the electrons moves towards the left to fill thThis creates a hole at the at the original site of the that moved to the left. The net effect is that the “hole” moves to the cathoThe subsequent series of slides show this process repeating itself until the hole reaches the cathode.
+

Ge Ge Ge

eGe Ge

ee- ee- e- e- e e - - e- e- e- e- e- e- ee- e- e- e- 

e 

- e- e- e- e- e- e- e- 

e- e- e- e- e- 

+ Ge Ge Ge 
Ge Ge Ge

eGe Ge

ee- ee- e- e- e e - - e- e- e- e- e- e- ee- e- e- e- 

e 

- e- e- e- e- e- e- e- 

+ 

+ e- e- e- e- 

Ge Ge Ge e- + 
Ge Ge Ge

eGe Ge

ee- ee- e- e- e e - - e- e- e- e- e- e- ee- e- e- e- 

+ e- e- e- e- e- e- e- 

+ 

+ e- e- e- e- 

Ge Ge Ge + 
Ge Ge Ge

eGe Ge

ee- ee- e- e- e e - - e- e- e- e- e- e- ee- e- e- e- 

+ e- e- e- e- e- e- e- 

+ 

e- + e- e- e- 

Ge Ge Ge + 
Ge Ge Ge

eGe Ge

ee- ee- e- e- e e - - e- e- e- e- e- e- ee- e- e- e- 

+ e- e- e- e- e- e- e- 

+ 

e- e- - + e- 

Ge Ge Ge + 
Ge Ge Ge

eGe Ge

ee- ee- e- e- e e - - e- e- e- e- e-e- ee- e- e- e- 

+ e- e- e- e- e- e- e- 

+ 

e- e- e- e- + 

Ge Ge Ge + 
Ge Ge Ge

eGe Ge

ee- ee- e- e- e e - - e- e- e- e- e-e- ee- e- e- e- 

e- e- e- e- e- e- e- 

+ e- 

e- e- e- e- e- Ge Ge Ge + 
Pulse Formation

Energy Required to Produce Electron-Hole PairThe average energy absorbed by the detector for eelectron-hole pair created is approximately three tiwidth of the band gap. This depends on the materdetector is made of. Typical values are as follows.

A 

verage ene

produce 

Detector Temperature electron-ho

Germanium detector 80 oK 2.95 eV

Silicon detector 295 oK 3.62 eV

Silicon detector 80 oK 3.72 eV
Types of 

Semiconductor MateriaSemiconductor Materia
Types of Semiconductor Material

GeneralEven the purest crystal of silicon of germanium consome impurities. Germanium and silicon are classified according to p yp p y y p redominant type of im

purit

y that the

y

possess.

They are classified as either: n-type

p-type
Types of Semiconductor Material

N-type N–type germanium or silicon has an excess of donimpurities (more donor impurities than acceptor imDonor impurities are usually group V elements sucphosphorous. Being group V elements they have five unpaired vaelectrons.Positioned in the lattice of a germanium or silicon conly four of these electrons can participate in covaThe impurity thereby "donates" an extra electron ththe material with an extra negative charge.
Ge Ge 

e- e- e e - - e- ee- e- e- e- 

e 

- e- e- e- e- eTypes of Semiconductor Material N-type Donated t

Ge Ge 

e

P Ge

ee- e- e- e- e-e- e e- e- e- e- e- 

e- extra electron
Types of Semiconductor Material

P-type P–type germanium or silicon has an excess of accimpurities. Acceptor impurities are usually group III elements, boron. Being group III elements they have three unpaired electrons.Positioned in the lattice of a germanium or silicon ctheir electrons can only complete three covalent baThe impurity thereby adds a hole/vacancy thus leamaterial with an extra positive charge.
Ge Ge 

e- e e - - e- ee- e- e- e- 

e 

- e- e- e- e- eTypes of Semiconductor Material P-type Positive h l

Ge Ge 

+

B Ge

ee- e- e- e e - -e- e e- e- e- e- e- 

hole
Effect of Impurities on 

Pulse FormationPulse Formation
Effect of Impurities on Pulse Formation

GeneralImpurities interfere in the charge collection processIf an electron moving through the conduction band the anode becomes trapped at an acceptor impuritnot contribute to the pulse. For this to happen, the only has to remain at the impurity until the pulse hacollected (which only takes 0.1 usec or so). The same thing applies to the trapping of holes by impurities. The result is a deterioration of detector resolution. 
Effect of Impurities on Pulse Formation

GeneralIt is preferable that the impurities form "shallow traretain the electrons or holes for short periods of timCommonly encountered impurities such as boron aphosphorous produce this sort of phosphorous produce this sort of

"innocuous innocuous

" shal

shal

Overall, the charge collection process in a semiconextremely efficient, only one in one thousand chargis lost because of trapping. Nevertheless, the active volume of a semiconductoshould be effectively free of impurities.
Detector Resolution
Detector Resolution

GeneralThe resolution of a detector refers to the width of thon the spectrum. The narrower the peaks, the bettresolution. A detector with good resolution will generate two pA detector with good resolution will generate two p

radiations of similar, but different, energies (e.g., 5keV gamma rays). A detector with poor resolution unable to resolve the two radiations and only produpeak instead of two. Detector resolution depends on the energy of the rwell as the type of detector. The lower the radiatiothe narrower the peaks.
Detector Resolution

Full Width Half MaximumThe conventional way to describe the resolution of a semiconductor detector is to specify the full width of a peak in keV at one-half its 

FWHM

maximum height (FWHM). 

For alpha detectors, the FWHM of the Am-241 peak usually specified. For high purity germanium detectors, the FWHM of keV peak of Co-60 is used.
Detector Resolution

Resolution and the Number of Charge CarriersThe greater the number of charge carriers produceenergy deposited in a detector, the better the resolIn a semiconductor detector, the charge carriers arelectron-hole pairs. The relevant charge carriers in a scintillator are the photoelectrons ejected from the photocathode of th(not the electron hole pairs in the scintillator). Charge carriers are also referred to as information because their number carries information about the energy.
Detector Resolution

Resolution and the Number of Charge CarriersImagine six particles of radiation deposit exactly thamount of energy in the detector (e.g., 1 MeV). Alsexactly the same number of charge carriers are proand collected. Six pulses of the same size are prod

Time

Pe

Pulse Size

Number of Pulses

Pulse S

Pulses
Detector Resolution

Resolution and the Number of Charge CarriersSince the six pulses were exactly the same size, thsorted into the same channel by the ADC. The peanarrow as possible.This doesn’t occur in the real world. The major restatistical nature of the processes by which the radstatistical nature of the processes by which the rad

absorbed energy produces the charge carriers.Even if exactly the same energy were deposited in detector six times, the number of charge carriers apulse sizes would vary. This variation is proportional to the square root of taverage number of charge carriers. 
Detector Resolution

Resolution and the Number of Charge CarriersIf N is the average number of charge carriers per pstatistical nature of the process is such that 68% opulses are produced by:
Detector Resolution

Resolution and the Number of Charge CarriersIf the average number of charge carriers per pulse 68% of the pulses are produced by 100 " 10 charg

There is a large fluctuation in pulse size, i.e., " 10peak on the spectrum is wide:

Time

Peak

Pulse Size

Number of Pulses

Pulse S

Pulses
Detector Resolution

Resolution and the Number of Charge CarriersHowever, if the average number of charge carriers is 10,000, 68% of the pulses are produced by 10,0

charge carriers. There is a smaller relative fluctuapulse size, i.e., " 1%. The peak on the spectrum i

Time

Peak

Pulse Size

Number of Pulses

Pulse S

Pulses
Detector Resolution

Resolution and the Number of Charge CarriersIn a NaI detector system, the variation in pulse size primarily due to the statistical variation in the numbphotoelectrons produced at the photocathode of thTo free one photoelectron from the PMT’s photoca1000 to 2000 eV of gamma ray energy must be abthe NaI crystal. As such, the deposition of 1 MeV of gamma ray enNaI detector would produce 750 or so charge carri
Detector Resolution

Resolution and the Number of Charge CarriersIn a germanium semiconductor detector, the averato produce a charge carrier (electron-hole pair) is approximately 3 eV. The deposition of 1 MeV in a gdetector produces approximately 300,000 charge cAs such the relative peak width for NaI and germanAs such the relative peak width for NaI and german

detectors is:NaI peaks are approximately twenty times the widtpeaks obtained with a germanium detector.
Detector Resolution

Resolution and the Reproducible Collection of CarriersThe more reproducible the collection of the charge the better the system's resolution.You can't collect 90% of the charge carriers one timthe next, and get two pulses of identical size.Reproducible collection of the charge carriers requuniform distribution within the detector volume of thimpurities that trap the holes and electrons. 
Detector Resolution

Resolution and the Reproducible Collection of CarriersReproducible collection of the charge carriers also that the strength of the electric field be as uniform possible throughout the volume of the detector. For this reason, large detector crystals often have edges rounded off (bulletized). In the old days, detcrystals were usually right angle cylinders and the field would be distorted near the corners.
Detector Resolution

The Fano Factor and Detector ResolutionIf, as we have implied, the uncertainty in the numbcharge carriers produced in a germanium detector depends on Poisson statistics, the following equatigive the smallest peak width (FWHM) possible for ray of energy E (in keV):

FWHM (keV) = 0.128 √E

This would mean that the best peak width for the CkeV gamma would be: 

FWHM (keV) = 0.128 √1332 = 4.67 keV

In reality, the FWHM is much smaller! As a rule, thfrom 1.6 to 2.0 keV or so.
Detector Resolution

The Fano Factor and Detector ResolutionThe fact that reality is better than predicted by theo

quantified by the Fano factor (F).The Fano factor is the observed real world variationumber of charge carriers divided by that predicted 

Poisson statistics Poisson statistics.The smaller the Fano factor, the better the real worresolution, i.e., the narrower the peaks.Published Fano factors for germanium detector oftfrom 0.058 to 0.108. 

Real World FWHM (keV) = 0.128 √(E x F)
Semiconductor 

Temperature ofTemperature of

Operation
Temperature of Operation

GeneralAt room temperature, large numbers of electrons in semiconductor might have sufficient thermal energinto the conduction band and generate thermionic This deteriorates the detector resolution.Operating the detector at reduced temperatures (eliquid nitrogen) prevents this, but it places limitationdesign and operation of the system.
Temperature of Operation

GeneralThe greater the width of the band gap, the more susemiconductor material is for operation at room temFor this reason, silicon is better suited to room temoperation than germanium. The tradeoff with a larggap is that fewer charge carriers are produced per energy deposited - this means poorer resolution. Reducing a detector’s physical size also improves performance at room temperature because the detcontain fewer electrons. Fewer electrons means lethermionic noise.
Temperature of Operation

GeneralRoom temperature detectors are most suitable for:

• applications where physically small detectors 

required ( e.g., wound probe)

• al

p p p py ha 

particle detection and s

pectroscopy

• beta particle detection and spectroscopy• low energy photon (e.g., x-ray) spectroscopy 

efficiency is not required

In general, they are unsuitable for spectroscopy invhigh energy photons since this application requires 

large detectors.
Temperature of Operation

GeneralVarious semiconductor materials have been investare better suited than silicon and germanium for rotemperature spectroscopy of photons. 

Z Density (g/cm

3)

Band 

Ga

p (eV)

Energy per 

electron-hole 

i

Temp.

(E

K)

(g ) p( ) pair ( ) 

Silicon 14 2.33 1.12 3.61 300

Germanium 32 5.32 0.74 2.98 80

CdTe 48, 52 6.06 1.47 4.43 300 

CZT 48, 30, 52 5.86 1.57 4.6 300 

HgI

2 80, 53 6.30 2.13 4.22 300 
Temperature of Operation

CdTe and CdZnTe (CZT) DetectorsCZT and CdTe are the most important room tempesemiconductors for the gamma/x-ray detection and spectroscopy. Typical detector crystals range from 1 - 10 mm in slimits their detection efficienc especiall for high elimits their detection efficiency, especially for high e

photons. Larger sizes aren’t employed because hoin these materials is so significant that a complete of charge carriers can only be accomplished over a CZT crystals tend to be larger than CdTe crystals asuited to spectroscopy (as opposed to simple dete
Temperature of Operation

CdTe and CdZnTe (CZT) DetectorsWith regard to detection efficiency, their high atomcompensate to some extent for the their small sizeCdTe detectors are best suited to the detection low photons (e.g., x-rays) and applications where pulse analysis is not required. One common application of CZT and CdTe detectomedical imaging of low energy gamma rays and x-applications include astrophysics and homeland se
Temperature of Operation

CdTe and CdZnTe (CZT) DetectorsBelow is a CZT spectrum of Am-241. CdTe spectra have more pronounced tails on the low energy side peaks.
Three Types of 

Semiconductor

Detectors
Types of Semiconductor Detectors

Three Types of Semiconductor Detectors

1. Diode Detectors - P-N Junction detector

- PIN Detectors

2. Lithium Drifted Detectors - SiLi

- GeLi

3. High Purity Germanium Detectors
Types of Semiconductor Detectors

1. Diode DetectorsDiode detectors for alpha or beta spectroscopy are silicon wafers (sometimes germanium). The active the detector, a region of relative purity, is variously the depletion region. The thickness of the depletion region is so small thThe thickness of the depletion region is so small th

charged particles (e.g., alpha particles) can deposisignificant fraction of their energy in the detector. When operated in the pulse mode, diode detectorsuitable for use with charged particles. When they operated in the current mode, they can be used to the dose due to x-rays and gamma rays as well as particles.
Types of Semiconductor Detectors

1. Diode DetectorsThe major types of diode detectors include:

• P-N junction Diode Detectors

- surface barrier detectors

- passivated ion implanted planar (PIP) d

• PIN Diode Detectors 
Types of Semiconductor Detectors

2. Lithium Drifted DetectorsLithium drifted detectors are made from p-type silicgermanium in which the acceptor impurities are cofor by lithium atoms. The active volume of these detectors is large enouth t b it bl f them 

to 

be suit

able 

for x-ray and/ 

or gamma ray spe

Two types of lithium drifted detectors:

• lithium drifted silicon (SiLi, pronounced “silly”)• lithium drifted germanium (GeLi, pronounced “j
Types of Semiconductor Detectors

3. High Purity Germanium DetectorsHigh purity germanium detectors (HPGe) are made extremely pure p-type or n–type germanium. The impurities, although present, have been reduca low a concentration that their presence is inconsThe active volume of these detectors is large enouthem to be suitable for gamma ray spectroscopy.
Types of Semiconductor Detectors

4. Primary Applications of Semiconductor DeteDiode Detectors

P-N Junction detectors - alpha and/or beta 

- alpha spectrosco

PIN detectors -

p y hoton dosimetr

y

Lithium Drifted Detectors

GeLi detectors - gamma spectrosSiLi detectors - x-ray spectroscop

High Purity Germanium Detectors - gamma spect
Diode Semiconductor 

Detectors
Diode Semiconductor Detectors

GeneralDiodes only permit current to flow in one direction. They are almost always made of silicon.One region of the diode consists of p-type silicon aother region consists of n-type silicon. Even though they might have an extra hole or unpaelectron, the atoms of the impurities in p-type and materials are electrically neutral.
Diode Semiconductor Detectors

GeneralThere are two general types of diode detectors:

P-N diodes 

For charged particle detection and spectrosc

PIN diodes 

For gamma/x-ray detection or spectroscopy 
P-N Junction 

Diode DetectorsDiode Detectors
P-N Junction Diode Detectors

GeneralP-n junction diodes are primarily used for charged detection and spectroscopy.They are more commonly employed for detecting ameasuring alpha particles than beta particles. measuring alpha particles than beta particles.They are also used as dosimeters for photons in thradiology,
P-N Junction Diode Detectors

Diode JunctionThe junction is the region where n–type and p-type meet. It is often referred to as an p-n junction.

+++++ ----- +++++ --

p-type n-type

Junction

Despite the implication of the positive and negative type and n-type silicon consist of electrically neutra

+

+

+

+

+

+ + + + ++ + + + ++ + + + ++ + + + ++ + + + +

-------------------------

+

+

+

+

+

+ + + + ++ + + + ++ + + + ++ + + + ++ + + + +

----------
P-N Junction Diode Detectors

Diode JunctionThe junction is created by doping one side of the sacceptor impurities and the other side with donor imDiffusion causes some of the unpaired electrons of impurities in the n-type silicon to move across the jinto the p into the

p

-type material and combine with the holes 

type material and combine with the holes

donor impurities that those electrons came from arelectrically neutral – they now have a positive charSimilarly, holes from some of the acceptor impuritietype material “diffuse” across the junction to combidonor electrons on the n-type side of the junction. Tthose acceptor impurities with a negative charge.
P-N Junction Diode Detectors

Diode JunctionThe combination of electrons and holes near the jucreates a non-conductive region almost devoid of fcarriers. This is known as the depletion region. It sthe active volume of the detector.There is a positive charge associated with the n There is

a positive charge associated with the

n

-ty

ty

side of the depletion region and a negative charge type silicon side of the depletion region.This creates a “natural” electric potential across the region of approximately one volt.In the following figure, any electron flow in the depregion (current) would be from left to right.
P-N Junction Diode Detectors

Diode Junction

+ + + + ++ + + + ++ + + + ++ + + + +

--------------------

Depletion Region

+ + + + ++ + + + +

----------

This side of the depletion region has a negative charge because electrons moved here from the right side of the junction to fill the holes. 

This side of the dregion has a posbecause electronfrom here to the the junction to fil
P-N Junction Diode Detectors

Reverse Bias

+++++++++++++++

---------------

Larger Depletion Region

Cathode 

(

)

+++++++++++++++

---------------

Applying a “reverse bias” increases the size of the dregion. This means that the electrical contact on thmaterial is given a positive potential (anode) with rethe potential on the p-type material (cathode. 

(

-

) 
P-N Junction Diode Detectors

Reverse Bias

++++++++++++++++++++

+++++

-------------------------

Cathode 

(-)

P N 

++++++++++ -----

A reverse bias can be imagined to "pull" the holes aelectrons associated with the impurities closer to thand anode respectively - this increases the width of depletion region. P-N diode detectors are unique in dependence of the active detector volume on the avoltage. 
P-N Junction Diode Detectors

Reverse BiasThe electric field strength across the depletion layer as high as 10

7 V/m! 

This produces a rapid collection of charge carriers wturn, permits the detector to operate at very high co

Increasing the applied high voltage increases the ththe depletion region. A thicker depletion (insulating) reduces the detector capacitance and a lower capacmeans less noise.However, increasing the operating voltage beyond a point can cause a breakdown of the detector. 
P-N Junction Diode Detectors

Depletion DepthsDepletion depths of 50 - 100 

μm are common in dio

detectors designed for alpha detection. Most beta particles would only deposit a small fractitotal energy in such a small depletion depth. Junctid t t bl f b t t i d d

e

tec

tors capable o

f

b

e

ta spec

troscopy require 

dep

depths on the order of 1000 

μm.
P-N Junction Diode Detectors

Detector Materials and ConstructionDiode detectors are usually made of silicon. The low number of silicon (compared with germanium) reducresponse to background gamma rays. An advantage charged particle detector.The detector electrodes are usually thin deposits ofThe detector electrodes are usually thin deposits of

aluminum. Gold is sometimes used for the facing electrode beccan be deposited in extremely thin layers, an advanalpha particle detection. However, a gold electrode so thin that cleaning the detector face is impossible touch can effectively destroy a $2000 investment. Dwith thin gold electrodes can be light sensitive.
P-N Junction Diode Detectors

Detector Materials and ConstructionTypical detector areas range from 50 mm

2 to 2000 m

The larger the surface area, the higher the counting efficiency. 

H th l th f th t th However, the larger the surface area, the grea

ter th

capacitance and electronic noise. The greater the npoorer the resolution.
P-N Junction Diode Detectors

Operating ConditionsDiode detectors are usually operated at room tempReducing the temperature can reduce noise and prosome improvement in resolution, but it is the detectocapacitance that ultimately determines resolution. For alpha spectroscopy it is preferable but not alwaFor alpha spectroscopy, it is preferable, but not alwa

possible, to operate in a vacuum. If the alpha particles lose some of their energy in air reaching the detector, the alpha peaks become wideshifted to the left on the spectrum. Alpha spectroscopy system should develop a reprodvacuum each time a sample is counted. 
P-N Junction Diode Detectors

Leakage Current Even in the absence of ionizing radiation, a small cuflow across a semiconductor when a high voltage is Minimizing this “leakage current” improves detector Leakage current involves either: bulk flow through t

leakage across the 

Bulk flow of current through the material can be reduse of blocking (non-injecting) contacts which do noa source of charge carriers to replace those collecteopposite electrode when the high voltage is initially 
P-N Junction Diode Detectors

Leakage Current Leakage current across the detector surface can be by manufacturing techniques that ensure clean surfLeakage current is a particular problem when the ancathode are in close proximity, e.g., the edges of diodetectors

. 

Modifying the manner in which the electrodes are deon the silicon/germanium can affect the leakage cur

cathode

anode

catho

anod

more 

leakage

less 

leakage
P-N Junction Diode Detectors

Efficiency and ResolutionThe intrinsic efficiency for charged particles is 100%particle incident upon the detector window produces The absolute efficiency (the fraction of the emitted aparticles that are detected) is lower. The absolute efficiency depends on: 

• detector (window) area• the detector-source distance 
P-N Junction Diode Detectors

Efficiency and ResolutionThere is a trade-off between efficiency and resolutio

• Larger detector areas increase the absolute effic

the resolution is worse because of the noise caularger detector capacitance. 

• Smaller detector-source distances increase the e

but decrease the resolution.At large source-detector distances, all the alphas 

detector at close to 90

o. This means that they a

the same amount of dead layer on the detector shave the same energy when they reach the detesensitive volume. The peak on the alpha spectrnarrow.
P-N Junction Diode Detectors

Efficiency and Resolution

On the other hand, if the detector-source distance 

the alphas are incident upon the detector at a widangles: from 90

o to nearly 180

o. This means that 

traverse different thicknesses of the inert layer on detector surface. 

As a result, the alphas will have a wide range of ewhen they enter the detector sensitive volume (delayer) and the alpha peak on the spectrum will be 
P-N Junction Diode Detectors

Efficiency and Resolution

th d

anode

p-type Si

n-type Si

depletion 

layer "" 

cath

o

d

e

alpha source

alpha 

"

"

"

Figure on left: large detector-sdistance means poor efficiencresolution.Figure on right: small detectordistance means good efficiencresolution.
P-N Junction Diode Detectors

ResolutionAlpha detector resolutions are usually described by (full width of half the maximum peak height) for the A5.486 MeV peak. Typical values range from 15-50 kThe actual resolution is often poorer than that stated manufacturer One reason might be that the source manufacturer. One reason might be that the source 

counted consists of a thick deposit and the alphas esignificant self-absorption.Resolutions for monoenergetic electrons (e.g., convelectrons) are superior to those for alpha particles senergy is lost in the detector dead layer. The beta rusually measured with a pulser rather than a radioasource. Typical values are between 5 and 20 keV.
P-N Junction Diode Detectors

Alpha Contamination ProblemsAlpha counters of any type are prone to becoming contaminated for several reasons:• Alpha sources are unsealed

Th d d t f l h d h ff

• The 

decay pro

duc

ts o

f alp

ha 

decay can 

have suff

recoil energy to escape the source and strike the the walls of the counting chamber. These alpha dproducts are often radioactive alpha emitters.

• The emission of alpha particles can damage the s

small bits of the source can be carried off with the particles (alpha creep).
P-N Junction Diode Detectors

Alpha Contamination ProblemsTo reduce the potential of contamination:• Leave alpha sources in counter no longer than ne• Fill the chamber with helium instead of using a va

Helium’s low density allows the alphas to reach th

with little energy loss However helium is dense ewith little energy loss. However, helium is dense e

reduce the number of recoil nuclei striking the det

• An alternative to helium is the use of a controlled 

The resolution is not as good as if no air were prethe residual air in the chamber reduces contamina

• Apply a negative potential (e.g., 6 V) on the sourc

to reduce the emission of the positively charged refrom the sample surface.
P-N Junction Diode Detectors

Detector Background

As a detector is used, it inevitably becomes more ancontaminated. The result is an increase in the detecbackground which can make the detector unsuitable level counting applications. Man

y labs o

perate with two sets of detectors. New d

y p

(those with a low background) are used to count low samples. The older detectors (with the higher backgare used to count the hotter samples.If a very low background detector is required, the manufacturer can be requested to forgo the usual Qthat involve exposing the detector to an alpha sourcphrase “see no alpha’ (or SNA) is sometimes used tsuch detectors.
P-N Junction Diode Detectors

Surface Barrier DetectorsManner of fabrication: 

A thin wafer (disk) of n–type silicon is etched with 

vapor under conditions that promote oxidation. The resultin

g thin la

yer of silicon oxide on the sur

g y

the properties of p-type material. After the oxide is removed from all but one surfacsurface with the p-type oxide layer is coated with of gold to serve as the cathode. The other side of the detector is coated with alumserve as the anode.
P-N Junction Diode Detectors

Surface Barrier Detectors

n-type silicon 

2 Surface etched in

n-type silicon 

n-type 

depletio

3 p type ox

p-type Si oxide p-type 

1 ili f

2. Surface etched in

oxidizing atmosphere

3. 

p

-type ox

from all but 

cathode

(gold)

anode

(aluminum)

depletion layer

n-type silicon 

p-type Si oxide

4. Elec

depo

1. silicon wa

fer 
P-N Junction Diode Detectors

Passivated Ion-implanted Planar (PIP) DetectorsThe process of manufacturing a PIP detector begins large area high-purity n–type silicon wafer. Several can be produced from each wafer. Manner of fabrication: 

The wafer is passivated by creating a p-type surfalayer. The oxide is removed on the backside of the deteon the front where the detector windows will be loThe back side of the detector is doped with arsenimpurity, while the front side is doped with boron, acceptor impurity. 
P-N Junction Diode Detectors

Passivated Ion-implanted Planar (PIP) Detectors

Damage caused by the implantation process is reannealing the wafer at 600 degrees centigrade. Aluminum is deposited on the front and back surfThe aluminum is selectively removed to create th

rear electrical contacts. This is done so as to crearelatively large distance between the front and recontacts (electrodes). The result is that the leakagbetween the electrodes is smaller than that of a sbarrier detector where the front and rear electrodto the very edges of the wafer. The aluminum elealso more rugged than the gold electrode of a surbarrier detector.
P-N Junction Diode Detectors

Passivated Ion-implanted Planar (PIP) Detectors

n-type silicon 

2. Surface etched in 

oxidizing atmospher

n-type silicon 

p-type Si oxide

1. silicon wafer 

3. Selective removal 

of p-type Si oxide 

layer. n-type Si
P-N Junction Diode Detectors

Passivated Ion-implanted Planar (PIP) Detectors

4. Ion implantation of 

boron (acceptor) and 

arsenic (donor)

Boron

5. Annealing and 

removal of oxide 

n-type Si 

Arsenic

n-type Si
P-N Junction Diode Detectors

Passivated Ion-implanted Planar (PIP) Detectors

6. Aluminum metalization

Aluminum

7. Aluminum patt

on front 

n-type Si n-type Si
P-N Junction Diode Detectors

Passivated Ion-implanted Planar (PIP) Detectors

8. Division into multiple detectors

Cathode

Anode

n-type Si p-type Si 
P-N Junction Diode Detectors

Passivated Ion-implanted Planar (PIP) Detectors

PIP detectors are very popular because of their eresolution and their ruggedness. The aluminum electrode facing the source is thick permit cleaning (unlike the gold electrode of a sur

b i dt t )barrier 

d

e

tec

tor

).
P-N Junction Diode Detectors

Silicon wafers. Smaller horizontal wafer has oxide layer on surface.

Three alpha P-N diode 
P-N Junction Diode Detectors

PIP detector.
P-N Junction Diode Detectors

Advantages of Silicon Diode Detectors as Dosi• Their small size (compared to gas detectors or s

makes them excellent for measurements in smaand in steep dose gradients.

• The atomic number of silicon is closer to that of 

tissue than is the atomic number of germanium tissue than is the atomic number of germanium. 

larger band gap means less noise at room tempthan with germanium.

• Unlike TLDs, OSL or film, diodes can provide re

measurements as well as measure an integrated 
P-N Junction Diode Detectors

Disadvantages of Silicon Diode Detectors as Do• They can be directionally dependent.• Silicon’s high atomic number relative to air or tis

that silicon dosimeters are energy dependent, i.eover-respond at low energies.

• Their response can be affected by temperature 

an issue if they are in contact with a body). N–tydetectors are more sensitive to temperature than 

• The dose-rate can affect their response, especia

silicon detectors.
P-N Junction Diode Detectors

Disadvantages of Silicon Diode Detectors as Do• Their sensitivity can decrease over time due to t

accumulation of radiation damage in the crystal. reduction in sensitivity might be as much as 10% or so. The result is that it can be necessary to pefrequent verifications of the calibration, especial

dose applications, e.g., therapeutic radiology.

• They can be subject to magnetic and rf interfere
P-N Junction Diode Detectors

Dosimeters Used in RadiologySilicon diode dosimeters are primarily used for in-vmeasurements. They are more commonly employed in therapeuticopposed to diagnostic, radiology because of the reopposed to diagnostic, radiology because of the re

for dose verification. Detectors take various configurations including line
P-N Junction Diode Detectors

Dosimeters Used in RadiologyThe intrinsic potential at the p-n junction of a diode approximately 0.7 volts and the depletion layer is a of micrometers. Although a bias would increase se(intrinsic region volume) and reduce leakage curresignal to noise ratio is superior with no bias. For thsignal to noise ratio is superior with no bias. For th

diode dosimeters are usually operated (in the currewith no bias applied. Sensitivity is not a major concbecause of the relatively high dose rates involved.However, due to the high exposures employed in tgradual decrease in sensitivity brought about by radamage to the crystal is of particular concern. This necessitate the use of regular calibrations.
P-N Junction Diode Detectors

Dosimeters Used in RadiologySince silicon diodes are energy dependent, it is dethe calibration energies to be as close as possible energy of the radiation that will be measured. Unfthe energy spectrum depends on a variety of factofield size, focus to detector (surface) distance, and field size, focus to detector (surface) distance, and 

thickness of the phantom or patient. Often designed for a specific type of radiation (e.g.might incorporate a particular thickness of metal to the appropriate buildup) and corrections might havperformed when used for other types of radiation. 
P-N Junction Diode Detectors

Dosimeters Used in RadiologyBecause of their planar construction and encapsuldetectors can experience considerable angular depAs such, it is important that the diodes’ orientation calibration is the same as that during measuremenDiode detectors are somewhat temperature depencan be necessary to calibrate at the same temperawhich the measurements are performed, e.g., skin temperature.
PIN Diode Detectors
PIN Diode Detectors

GeneralPIN diodes consist of a large relatively pure “intrinsthat is sandwiched between a heavily doped p-type and a heavily doped n-type region.

p-type n-type

+ + + + ++ + + + ++ + + + ++ + + + ++ + + + ++ + + + +

------------------------------

Intrinsic Region
PIN Diode Detectors

GeneralThe depletion region extends almost completely acintrinsic region. When reversed biased, the volume depletion layer is not affected by the applied voltag

p-type n-type

+ + + + ++ + + + ++ + + + ++ + + + ++ + + + ++ + + + +

------------------------------

cathode Intrinsic Region

(-)
PIN Diode Detectors

GeneralThe large depletion region gives the PIN detector a larger sensitive volume than the P-N detector.This makes it more suitable for x-ray of gamma ray detection/spectroscopy.For photons above 60 keV, the predominant interathe detector is Compton scattering and the efficienor less independent of the photon energy. Typical iefficiencies are on the order of 1 – 2%
PIN Diode Detectors

Electronic Personnel DosimetersElectronic personnel dosimeters (EPDs or EDs) hainto widespread use, especially in the nuclear powwhere they have almost completely eliminated the direct reading pocket dosimeters. They are used to keep exposures ALARA, control work areas and provide a backup for the informatioby the TLD or film. In the U.S., EDs cannot be used to determine the drecord. At least some nuclear power plants in Greuse EDs for the dose of record however. 
PIN Diode Detectors

Electronic Personnel DosimetersThey indicate the dose rate (e.g., 0.1 to 1000 mremwell as accumulated dose (e.g., 0.1 mrem to 1000 Their alarms can be set to sound (or vibrate) when specified dose or dose rate is exceeded. Their output can be transmitted via radio to a base where the worker’s exposures are continuously moDepending on the design, EPDs might respond to betas, and/or neutrons. Being energy dependent, a EPD contains several diodes each of which has a filter. The relative responses of the different detectois used to estimate and correct for the energy of th
PIN Diode Detectors

Electronic Personnel Dosimeters

PIN Diode Detectors

RTI Electronics’ BARRACUDAThe Barracuda is a multimeter that is used for X-raA somewhat unique feature is its use of a hand-heinterface (Palm). With a single multipurpose PIN didetector, it can measure kVp, irradiation time, doserat d l l t f d h 

te, dose per pulse, pulse rate, waveforms and ha

layer. 

The use of additional detectors make it possible to also measure mA, mAs, light and CT dose.
PIN Diode Detectors

RTI Electronics’ BARRACUDAThe small sensitive area of the detector (3 x 21.1 mmakes it ideal for measurements of narrow x-ray bSince silicon detectors over-respond at low energieMPD is energy compensated. This is accomplished MPD is energy compensated. This is accomplished 

metal filter that preferentially attenuates the low enphotons to which the detector would otherwise oveSince diode detectors are directionally dependent, Barracuda incorporates a built-in position check fe
Lithium Drifted 

Detectors
Lithium Drifted Detectors

GeneralThere are two types of lithium drifted detectors:

• Lithium drifted silicon (SiLi) detectors• Lithium drifted germanium (GeLi) detectors Lithium drifted germanium (GeLi) detectors

Unlike diode semiconductor detectors, lithium driftedetectors can be manufactured in very large sizes cubic centimeters). As such, lithium drifted detectors are primarily desigamma ray or x-ray spectroscopy.
Lithium Drifted Detectors

The Lithium Drifting ProcessLithium drifting is a method employed with p-type sgermanium to "compensate" for the acceptor impuThe entire process might take two weeks or so. A silicon or germanium crystal is exposed to a lithiu60 

oC (lithium atoms are very small atom and poss

unpaired electron). With a reverse bias applied, the lithium atoms migrthrough the crystal. 
Lithium Drifted Detectors

The Lithium Drifting Process

++++++++++++++++++++++++++++++++++++++++++++

Li

Li

Li

Li

Li

Li

Li

Li

Li

Li

+++++++++++++++

+++++++++++++++++++++++++++++++++

Li

Li

Li

Li

Li

Li

Li

Li

+++++++++

Lithium drifting into

silicon or germanium crystal

Compensation 

except for cent

core which wil

the central e
Lithium Drifted Detectors

The Lithium Drifting ProcessThe lithium atoms "locate" and pair up with the accimpurities in the crystal - the lithium atom’s unpaireelectrons "fill" the positively charged holes associathe impurities. This is referred to as compensationThe compensation process is stopped before the e

crystal is compensated. The central core is left as material.The compensated region of the crystal, which behawere pure material, serves as the sensitive volume detector. 
Lithium Drifted Detectors

SiLi Detectors

• referred to as "silly" detectors • primarily used for x-ray spectroscopy• must be operated cold (e.g., liquid nitrogen tem

must be operated cold (e.g., liquid nitrogen tem

• can come up to room temperature during stora• less efficient than GeLi detectors for high ener

due to lower Z 
Lithium Drifted Detectors

GeLi Detectors

• referred to as “jelly" detectors • primarily used for gamma-ray spectroscopy• must be kept cold at all times (e.g., during ope

must be kept cold at all times (e.g., during ope

storage) or will have to be re-drifted

• not produced anymore - have been replaced 

purity germanium detectors 
Lithium Drifted Detectors

Why GeLi Detectors Must be Kept ColdIf a GeLi detector comes up to room temperature, atoms will have sufficient thermal energy to migrate from the impurity sites. This is not an issue with SiLi detectors since the atsilicon crystal have a relatively close spacing. The atoms are relatively immobile in a SiLi crystal. However, the atoms in a germanium crystal are moseparated and the lithium atoms can move more frthus become necessary to maintain GeLi detectors nitrogen temperatures to fix the lithium at the impu
Lithium Drifted Detectors

Why GeLi Detectors Must be Kept ColdIf the detector warms up and compensation is lost, detector can be “redrifted” but this process is expethe detector performance may not be quite as good The detector might warm up because of a failure ocryostat (the insulating jacket used to keep the detseals. If a seal fails, the vacuum in the cryostat is loliquid nitrogen in the dewar will be rapidly used up. Although all lithium drifted and high purity detectoroperated at low (e.g., liquid nitrogen) temperatures noise, only the GeLi must always be kept at such temperatures, even during storage.
Lithium Drifted Detectors

GeLi Detectors
High Purity Germanium 

Detectors
High Purity Germanium Detectors

GeneralHigh purity germanium (HPGe) detectors have two advantages over GeLi detectors:

• they can be allowed to warm up to room tempe

during storage 

• they can be produced more quickly than GeLi 

HPGe systems deal with the problem of impurities 

reducing these impurities to an inconsequential levone impurity will be found for every 1012 atoms of g

Despite the low levels of impurities, high purity gerwill still be classified as either n–type or p-type.
High Purity Germanium Detectors

HPGe ProductionA bar of germanium is placed in a quartz trough maon a slight incline. The bottom end of the germanium is heated to the point using an induction heater. The latter is slowly the germanium causing the molten region (glowing the germanium causing the molten region (glowing 

move with it. Impurities in the germanium preferentially migrate molten region. As the induction heater moves alongermanium, the impurities in the germanium move When the process is complete, the impurities are concentrated at the upper end of the germanium b
High Purity Germanium Detectors

HPGe Production cont.The end containing the impurities is removed leavia bar of high purity germanium. Next, the high purity germanium is melted in a quachamber. A rod with a seed crystal of germanium ais lowered into the molten germanium. As the germcrystallizes about the seed, the rod is turned and pthe molten material. The rate at which this is done the size of the crystal. The result is a cylindrical crythe shape of a small loaf of French bread. Finally, the crystal is cut into sections which are shpolished.
High Purity Germanium Detectors

Detector ConstructionHPGe and SiLi detectors are either: planar

coaxial

Planar DetectorsPlanar detectors are 

p y 

rimaril

y intended for low ene

or x-ray spectroscopy in the 1 to several hundred kPlanar detectors generally have better resolution thdetectors below 100 keV or so. They also often hasuperior count rate capabilities.They consist of disks of high purity (usually n–typegermanium or silicon, approximately 1-5 cm in diam0.5- 2.0 cm thick.
High Purity Germanium Detectors

Planar Detectors cont.A very thin (<0.3 micron) ion-implanted electrical coboron (Z = 5) is deposited on the front surface to scathode.A thicker (500 - 1000 micron) lithium diffused contadeposited on the back side of the detector. This sethe anode.To improve transmission of low energy photons, thhousing (cryostat) has a thin (ca. 0.02 - 0.03”) berywindow. In some systems plastic/carbon fiber windused instead of beryllium. 
High Purity Germanium Detectors

Planar Detectors cont.

Be window

Catho

(p con

Cathode 

(p contact)

Anode 

(n contact)

Anode 

(n contact)

Electrode configuration on left has reduced leakaglower capacitance and improved resolution,
High Purity Germanium Detectors

Planar Detectors cont.

Early version of planar GeLi detector. End caberyllium window on right.
High Purity Germanium Detectors

Coaxial DetectorsThe two electrodes of a coaxial detector share the In a "true" coaxial detector (standard configuration detectors) the central electrode runs the length of tMost HPGe detectors are "closed end" s

ystems in 

central electrode extends only part way into the cryclosed end coaxial detector has a higher counting than a true coaxial detector since there is more actgermanium close to the source.Coaxial detectors are usually 4 - 7 cm in diameter cm in height. A hole is "drilled" into one end of the form the surface for the central electrode while the the cylinder serves as the surface for the other con
High Purity Germanium Detectors

Coaxial Detectors

Closed end coaxial HPGe detector crystal on True coaxial GeLi detector crystal on right.
High Purity Germanium Detectors

P-type Coaxial DetectorsIn a “conventional” detector of p-type germanium, telectrode is a 500 - 1000 micron thick deposit of litgiven a positive bias and serves as the anode for tcollection of the electrons. The inner contact the cathode is ion implanted boThe inner contact, the cathode, is ion implanted bo

evaporated gold (<0.3 microns thick). It serves as tfor the collection of the holes.Since most photon interactions occur closer to the than the cathode, the charge collection time is primdetermined by the time it takes for the holes to be These types of detectors are useful from 50 keV to 10 MeV.
High Purity Germanium Detectors

N-type Coaxial DetectorsWith n–type germanium the position of the electrodreversed. In fact, n–type HPGe coaxial detectors asometimes referred to as reverse electrode detectoSince the outer electrode is thinner than that of a “conventional

”

p

-type coaxial detector an n type coaxial detector, an

n

–type d

type d

more suitable for low energy work. As such, the enn–type coaxial detector will have the same type of that found in a planar detector. 
High Purity Germanium Detectors

N-type vs. P-type Coaxial DetectorsP-type detectors are useful from 50 keV to more thwhereas n-type detectors are useful from 10 keV tthan 10 MeV.N-type detectors are more expensive.P-type detectors have slightly better resolution.N-type detectors are more resistant to neutron damwhich can result in low energy tails on the peaks, tdetectors. Fast neutrons create hole trapping centelectrons are the primary charge carrier with n-type 
High Purity Germanium Detectors

N-type vs. P-type Coaxial Detectors

Be window

Al End Cap

N-type Ge P-type Ge

+ A

(n c

- Cathode 

(p contact)

- Cathode 

(p contact)

+ Anode 

(n contact)
High Purity Germanium Detectors

End Cap WindowsDetectors capable of low energy spectroscopy (e.gSiLi , and n-type coaxial HPGe detectors) have a tentrance window on the cryostat end cap just abovdetector crystal.These windows are usually made of beryllium and These windows are usually made of beryllium and 

0.03” thick. Carbon composite windows are also cThe windows are easily broken. When the low eneof the spectrum is not being analyzed, the window protected with a cap. Increased efficiency for low energy photons increapotential for coincidence losses since x-rays will be that would not normally reach the detector. 
High Purity Germanium Detectors

Detector CryostatThe cryostat is the insulating jacket that keeps the cold. If a dewar is employed to hold liquid nitrogen detector, the dewar might be considered part of the 

Example Cryostat Configurations

 Vertical(dipstick)

Horizontal (dipstick)

Horizonta (integral
High Purity Germanium Detectors

Detector CryostatThe end cap, usually 1mm thick aluminum, is that cryostat surrounding the detector.The tailstock is the dipstick-like tail of the cryostat tinserted into the neck of the dewar.The tailstock almost completely encloses a copper the cold finger) that thermally connects the detectothe liquid nitrogen. Only the last inch or so of the cextends outside the tailstock.Heat is drawn from the detector crystal, through th(a magnesium or copper block) to the copper coolifinally to the liquid nitrogen.
High Purity Germanium Detectors

Detector Cryostat

End CapPedestal

The vacuum in the cnecessary to minimizconductive heat tranloss of vacuum (e.g.failure of the cryostawould result in rapid li id it

Cooling rod

TailstockAdsorber

liquid nitrogen.Since a good vacuum necessary, the tailstocontains a granular a(molecular sieve or acharcoal) as a “gettecollect any residual g
High Purity Germanium Detectors

Detector CryostatLoss of vacuum is a common problem. One reason happen is that the cryostat seals deteriorate over tComponents of the gas in the cryostat might then pthe surface of the detector crystal. This could lead leakage current across the detector surface which leakage current across the detector surface which 

impair the detector resolution.The resolution might be improved by re-evacuating cryostat (possibly a 1-2 day process).In some cases it might be necessary to “bake off” timpurities by heating the cryostat to 100 C (or so) ais being pumped down. 
High Purity Germanium Detectors

Detector CryostatThe cryostat also encloses various electronic compincluding the high voltage filter, the high voltage shand part (the FET) or all of the preamplifier. Cryostats completely enclosing the entire preamplisometimes said to have “slim” or streamed designsAn advantage of the slimline (or streamlined) systesmaller size of the hole required in the shield for thThis helps reduce background.
High Purity Germanium Detectors

Low Background DetectorsMost cryostat jackets are made of aluminum which contain residual traces of Ra-226. Low background might replace aluminum with magnesium. The end caps might be made from carbon fiber maoxygen

-free high conductivity (OFHC) copper Oth

free high conductivity (OFHC) copper. Oth

components might be made from high purity alumiSince the electronic components inside the cryostanaturally occurring radionuclides (e.g., uranium semight be shielded from the detector (e.g., with old Activated charcoal might be used as the adsorber tailstock instead of molecular sieve.
High Purity Germanium Detectors

Detector ResolutionThe resolution of a detector describes the width of photopeaks. The narrower the peak, the greater th

the system to distinguish different gamma rays of senergies. 

The resolution depends on the photon energy: the The resolution depends on the photon energy: the 

gamma ray energy, the narrower the peak.The resolution of a HPGe detector is usually exprethe Full Width at Half Maximum (FWHM) for the 13peak of Co-60. It is expressed in keV. This contraresolution for a scintillation detector which is usualdescribed as a percent relative efficiency for the Cspeak.
High Purity Germanium Detectors

Detector Resolution cont.HPGe detector resolutions at 1332.5 keV are usua1.6 - 2.0 keV range. It is also common to specify the resolution at 122 k57). If the detector is capable of low energy spectroresolution will also be specified at 5 9 keV (Mn x resolution will also be specified at

5.9 keV (Mn

x

-ra

All other things being equal, a p-type germanium dbetter resolution than an n–type detector.The higher the detector efficiency (i.e., the larger it poorer the resolution.
High Purity Germanium Detectors

Detector Resolution cont.The FWHM of a Gaussian shaped peak, the best ttheoretically possible, completely describes its shaIn the real world where perfection is not achieved, provide additional parameters regarding peak shap

• full width at one tenth the maximum height (FW• full width at one fiftieth the maximum height (FW

For a perfect Gaussian shape, the FWTM/FWHM r

be 1.83 and the FWFM/FWHM ratio would be 2.83
High Purity Germanium Detectors

Detector Resolution cont.Detector resolution should be monitored on a daily recording the FWHM of the Co-60 1332.5 keV peaIf a deterioration in the detector resolution is obserbe due to any number of causes. The most commoa bad preamplifier or as a bad preamplifier or, as discussed earlier a loss discussed earlier, 

a loss o

o

the cryostat.To minimize the chance of this happening, the detebe sent back to the manufacturer for maintenance years or so.Another possible reason for poor resolution is that bias is too high. Dropping the voltage by 100 or 20might be all that is necessary. 
High Purity Germanium Detectors

Detector EfficiencyA germanium detector has a lower efficiency than a iodide detector of equal size because the effective number of NaI is higher than that of germanium. However, this does not necessarily mean that the sthe NaI detector is better The peaks in a HPGe spthe NaI detector is better. The peaks in

a HPGe sp

might have fewer counts than those in a NaI spectthey might be more easily detected because the codistributed in far fewer channels.
High Purity Germanium Detectors

Detector EfficiencyThe most common way to describe an HPGE deteefficiency is to specify its efficiency relative to that NaI detector:The HPGe and NaI efficiencies are for the 1332.5 of a Co-60 point source 25 cm away from the deteThe most common detector efficiency currently beimanufactured is 50%.
High Purity Germanium Detectors

Detector EfficiencyRelative efficiency is used to compare one detectoanother, not to quantify radioactive material.The efficiency usually employed to quantify a gammemitting radionuclide indicates the fraction of the gemitted by the source that produce a count in the pemitted by the source that produce

a count in the p

This efficiency depends on the:

• size and shape of the source• source-detector distance• source matrix which affects self absorption• photon energy
High Purity Germanium Detectors

Detector EfficiencyThis curve shows the efficiency (counts per gammafunction of energy for a typical p-type coaxial detec

Efficiency

The curves on the next page show efficiency as a fenergy for a number of different detectors. 

(c/)

Photon Energy (keV)

10 100 1000

High Purity Germanium Detectors

Peak to Compton RatioEach photopeak on a spectrum has an associated continuum. The latter increases the background at energies and this results in poorer counting statistiminimum detectable activities (MDAs).To reflect the To reflect the

"size

" of the Compton continuum ass

of the Compton continuum ass

with a given detector, manufacturers specify the peCompton ratio: ratio of the count at the center of th1332 keV photopeak divided by the average counchannel at the Compton edge between 1040 and 1The larger the peak to Compton ratio, the better. Befficient detectors have higher ratios than smaller d
Example Specification

for HPGe Detectorfor HPGe Detector