GAMMA SPECTROSCOPY 

OVERVIEW

Th Id ifi i f G E i iThe Identification o

f

Gamma 

E

mitti

Radionuclides

6/1/2011
Contents

General

Direct gamma Spec Analysis of RadionuclidesNon-Gamma Emitting RadionuclidesNon-Gamma Emitting Radionuclides – Secular EquilibriumNon-Gamma Emitting Radionuclides – Scaling FactorSpectroscopy vs. SpectrometryAdvantages of Gamma SpecDisadvantages of Gamma Spec

g p

Components of a Gamma Spectroscopy System

The Most Basic ComponentsDetector TypesTwo “Functions” of the DetectorMultichannel AnalyzerTwo “Functions” of the MCAMajor Components of the MCAAmplifierAnalog to Digital Convertor
Contents

Pulse Height Analysis

General

The Spectrum

GeneralThe Big PictureReal SpectraResolution

Energy Calibration

GeneralPeak CentroidEnergy Calibration Curve

Identifying Unknown Gamma Emitting Nuclides

GeneralGamma LibraryUse of ComputerCommon Problems
Contents

Appendix

Dead TimeGain ShiftsUpper and Lower level DiscriminatorsSingle Channel Analyzer
General
• Radionuclides must emit gamma rays to be analy

directly by gamma spectroscopy

• Gamma rays are like fingerprints: they have spec

ener

gies that can be used to identify the radioact

GeneralDirect Gamma Spec Analysis of Radionuclides 

g y

material. 

• The next two slides show a somewhat random se

of common gamma emitting radionuclides.

• Many of these nuclides emit gamma rays at man

different energies. Only the most important gammare shown.
Radionuclide Gamma Ray Energy (keV) Gamma 

Na-22

1274.5 1

511.0 (annihilation) 1

K-40 1460.8 0

Cr-51 320.1 0

C 57 122 1 0 

GeneralDirect Gamma Spec Analysis of Radionuclides 

Co-57 122.1

0.

Fe-59

1099.2 0.1291.6 0.

Co-60

1173.2 1

1332.5 1

Zn-65 1115.5 0.

Ga-67

93.3 0.

300.2 0

Mo-99 739.6 0.
General

Direct Gamma Spec Analysis of Radionuclides 

Radionuclide Gamma Ray Energy (keV) Gamma 

Tc-99m 140.5 0

Cd-109 (Ag-109m) 88.0 0.

I-131 364.5 0

Ba-133 356.0 0

Cs-134

604.7 0.795.8 0.

Cs-137 (Ba-137m) 661.6 0

Ir-192 316.5 0.

Tl-201 167.4 0

Ra-226 186.2 0

U-235

143.8 0185.7 0

Am-241 59.5 0
• Radionuclides that do not emit gamma rays, or do 

very small percentage of their decays (e.g., H-3, C32, Sr-90, Pu-239) cannot be identified or quantifidirectly by gamma spectroscopy. 

In most cases they must be analyzed by some ot

GeneralNon-Gamma Emitting Radionuclides 

In most cases, they must be analyzed by some ot

means, e.g., radiochemistry. 

• However, if a ratio can be established between a 

emitting nuclide and a non-gamma emitter, the latbe analyzed indirectly by gamma spec. 
• If a non-gamma emitter and a gamma emitting ra

are in secular equilibrium, their activities are equaratio).

• Secular equilibrium involves a long-lived parent (

the non gamma emitter) and a short lived decay p

GeneralNon-Gamma Emitting Radionuclides – Secular E

the non

-gamma emitter) and

a short

-lived decay p

(usually the gamma emitter).

• There can be several reasons why secular equilib

might not exist, hence everyone involved in the amust agree with the assumption that the gamma eand non-gamma emitter are in equilibrium.
Example 1.

U-238, which does not emit gamma rays, can b

quantified by measuring the activity of one of its lived gamma emitting decay products: Th-234 o

234m

GeneralNon-Gamma Emitting Radionuclides – Secular E

. 

U-238 (4.5 x 109 a)  Th-234 (24 d)  Pa-234m 

Th-234 emits 63 keV gammas in 3.7% of its decaPa-234m emits 1001 keV gammas in 0.84% of it
Example 2. 

Th-232, which does not emit gamma rays, can b

quantified by measuring the activity of its short-lgamma emitting decay product Ac-228. 

GeneralNon-Gamma Emitting Radionuclides – Secular E

Th-232 (1.4 x 1010 a)  Ra-228 (5.75 a)  Ac-2

Ac-228 emits a 911 keV gamma in 28% of its deThe long time required for Ac-228 to grow into sequilibrium from purified thorium-232 makes the assumption of equilibrium a little problematic.
Example 3.

Ra-226, which emits a 186 keV gamma that is i

to distinguish from the 186 keV gamma ray of Ube quantified by measuring the activity of its sho

gamma emitting decay product Pb-214. 

GeneralNon-Gamma Emitting Radionuclides – Secular E

g g yp

Ra-226 (1600 a)  Rn-222 (3.8 d)  Po-218 (3 min)  Pb-2

Pb-214 emits a 352 keV gamma in 37% of its d

Equilibrium is reached very quickly (ca. 1 monthpotential concern is the possible escape of the Rfrom the sample matrix.
• There are times when it is acceptable to make a l

accurate determination of the non-gamma emitter e.g., assaying low level waste.In this case, considerable uncertainty might be to

in the ratio (aka scaling factor) between the non

-g

GeneralNon-Gamma Emitting Radionuclides – Scaling F

in the ratio (aka scaling factor) between the non

-g

emitter and the gamma emitter. The ratio just has to be “good enough.”

• For example, the ratio might be derived from the k

activities of the two nuclides that were used in a gprocess or facility. 
• If the gamma emitter and non-gamma emitter are 

produced by the same process, it might be reason

assume ratio between them. For example:

Radionuclides produced by fission such as the g

GeneralNon-Gamma Emitting Radionuclides – Scaling F

Radionuclides produced by fission such as the gemitter Cs-137 and Sr-90. Radionuclides produced by neutron activation sthe gamma emitter Co-60 and Fe-55.

• A somewhat different example: There is often a u

ratio between the gamma emitter Am-241 and PuAm-241 is a decay product of Pu-241 which is oftpresent along with Pu-239.
• A distinction is not always made between these t

• Often the term gamma spec is used to cover both• When a distinction is made:

GeneralSpectroscopy vs. Spectrometry

Gamma spectroscopy refers to the process of uenergies of gamma rays to identify radionuclideGamma Spectrometry refers to the process of unumber of emitted gamma rays to quantify the the radionuclides.
• Less expensive when compared to radiochemistr

• Fast• Multinuclide analysis. All the gamma emitters can 

analyzed at once.

GeneralAdvantages of Gamma Spec (vs. radiochemistry

analyzed at once.A radiochemical analysis will be for one element oU-234, U-235 and U-238)

• Non-destructive• In some cases can be performed at a distance (re

in the field without the need for a sample.
• Often less sensitive than radiochemistry, i.e., the 

radiochemical analyses are usually lower than for spectroscopy

• Usually requires larger sample masses than radio

GeneralDisadvantages of Gamma Spec (vs. radiochemi
Components of a 

Gamma Spectroscopy SysteGamma Spectroscopy Syste
Components of a Gamma Spectroscopy 

The Most Basic Components1. Detector (and high voltage power supply)2. Multichannel analyzer (MCA)

Multichanne

Analyzer

(MCA)

Detector

High Voltage 

Power Supply
Components of a Gamma Spectroscopy 

Detector Types• The most common (not the only) detectors in gam

spectroscopy systems:

- Sodium Iodide (NaI)- Lanthanum Bromide (LaBr)

Lanthanum Bromide (LaBr)

- High Purity Germanium (HPGe)

• Of these, the HPGe is easily the best.• A LaBr detector is generally preferable to a NaI d
Components of a Gamma Spectroscopy 

Two “Functions” of the Detector1. A pulse is produced for each gamma ray intera

detector.A pulse is a short-term change in the voltage.

2. The greater the energy deposited in the detecto

larger the pulse.
Source

320 keV 1173 keV

How many puthe detector p

Components of a Gamma Spectroscopy Two “Functions” of the Detector

Detector

662 keV

320 keV 1173 keV

1332 keV

How many disizes would tproduce (assgamma ray edeposits all itthe detector) 
Source

Components of a Gamma Spectroscopy Two “Functions” of the Detector

Seven pulses of four different sizes

Liquid 

Nitrogen

Dewar

HPGe

Detector M

Pulses
Components of a Gamma Spectroscopy 

Multichannel Analyzer• The MCA contains most of the system’s electroni• All of the MCA components (and possibly the det

well) might be housed in a single stand-alone uncases, this might be a portable hand-held device

• In the laboratory, the memory, display and analys

functions of the MCA are usually handled by a co

The rest of the MCA’s electronic components mighoused in a single “box” connected to the compuIn other cases, the MCA electronics might consisseveral modules arranged in a NIM bin.
Components of a Gamma Spectroscopy 

Multichannel Analyzer

Stand-alone MCA (old system)

MCA’s hardware ocomputer circuit b(old system desig
Common laboratory 

setup.MCA electronics consists of modules Components of a Gamma Spectroscopy Multichannel Analyzer

HPGe Detecto

in external NIM Bin.Computer stores, displays and analyzes the spectra.

NIM Bin with Modules
Components of a Gamma Spectroscopy 

Multichannel Analyzer

In-situ gamma spec. Identifying and quantifying gamma emitters in soil.

Portable hand-held spectroscopy system internal NaI detecto
Components of a Gamma Spectroscopy 

Two “Functions” of the MCA1. Count the pulses from the detector.

The number of pulses can be related to the activ

radionuclides in the sample (gamma spectromet

2. Measure the size of the pulses (pulse height an

The height of the pulses can be related to the ethe gamma rays. This is used to identify the radin the sample.
• Amplifier

• ADC• MemoryComponents of a Gamma Spectroscopy Major Components of the MCA• Display
• The amplifier increases the size of the pulses by 

called the “gain.”

• The gain determines the range of gamma ray ene

are seen on the spectrum. 

Components of a Gamma Spectroscopy Amplifier

For example, a particular gain might result in a spviewing gamma rays of 20 to 2000 keV.If higher energy gamma rays must be seen, the glowered. The gain might be increased if only low energies interest.
• The amplifier also changes the pulse shape. It sh

the long tails on the pulses coming from the prea

and rounds off their leading edge.In most cases the resulting amplifier output pulse 

Ga ssian

Components of a Gamma Spectroscopy Amplifier

Ga

ussian.

• This makes it easier to measure the height of the 

• The amplifier also filters out electronic noise (ran

fluctuations in the baseline voltage.
Information is of two types:

• Analog - Analog information has an infinite and 

continuous variety of values. 

Output pulses from the amplifier are analog

Components of a Gamma Spectroscopy Analog to Digital Convertor

Output pulses from the amplifier are analog.

• Digital - Digital information has discrete values 

binary data). It is easier to store and manipulate digital data.Output pulses from the ADC are digital.
• Pulse conversion is the process of turning the an

pulses into digital pulses.

• As it converts the pulses, the ADC sorts them into 

size ranges called channels.

Components of a Gamma Spectroscopy Analog to Digital Convertor• The total number of channels (size categories) is 

as the “conversion gain.” 

• Typical conversion gains: 

NaI detectors: 256, 512, 1024 HPGe Detectors: 4096, 8K, 16K
Pulse Height Analysis
• The different types of ADCs measure the pulse h

different ways.

• We will imagine that the ADC measures the size 

pulses with a ruler.

Pulse Height AnalysisGeneral• The conversion gain is 10.• Looking at the following figure, how many pulses 

sorted into each of the ten channels?
89

10

r

Pulse Height AnalysisGeneral

Source

12345678

Channel Number

Pulses

Liquid 

Nitrogen

Dewar

HPGe

Detector
Channel Number Number of Pulses

123

Pulse Height AnalysisGeneral

456789

10
Channel Number Number of Pulses

1 02 23 0

Pulse Height AnalysisGeneral

4 35 06 07 18 09 1

10 0
The Spectrum
• The results of the ADC’s analysis of the pulse siz

stored in a memory.

• The contents of the memory are displayed on a L

monitor or CRT.

The SpectrumGeneral• The display, known as the spectrum, shows the n

pulses (counts) as a function of pulse size (channnumber). 
of Counts 345 Spectrum

The SpectrumGeneral

Channel Number

1 2 3 4 5 6 7 8 9 

Number o

123
ber of Counts

2345

Source

The SpectrumThe Big Picture

Pulses

Am

A

Me

Num1

0

1 2 

Liquid 

Nitrogen

Dewar

HPGe

Detector
• In reality, a spectrum will have hundreds to thous

channels rather than ten.

• In reality, there will usually be much more than on

three counts in a 

given channel.

The SpectrumReal Spectra• The next slide shows two side-by-side real spect

That on the left was produced by a NaI detector won the right was generated using a high purity gedetector.
The Spectrum

Real Spectra

The most important features on a spectrum are the A single peak is produced by many pulses of similar Ideally, each gamma ray energy produces a peak.
The Spectrum

Real Spectra

The higher the energy of the gamma ray, the farther right the peak appears on the spectrum.A peak should be perfectly symmetrical unless it oveanother peak (a doublet).
• The narrower the peaks, the better the resolution

• Narrower peaks (better resolution) means a grea

to distinguish gamma rays of similar energies.

The SpectrumResolution• HPGe detectors have much better resolution tha

detectors. 

• The resolution of LaBr detectors is better than tha

detectors, but poorer than that of HPGe detectors

• On a given spectrum, the peaks get broader as t

energies get higher. In other words, peaks get brwe move to the right. 
The Spectrum

Resolution

The peaks on the NaI spectrum are approximately 2wider than those on the HPGe spectrum.
• The resolution of a scintillator (e.g., NaI or LaBr

for the 662 keV gamma ray of Cs-137

The SpectrumResolution

1/2

E1 

E2 - E1 is the full width half maximum 

(FWHM)E

0 is 662 keV

Typical NaI resolution: 7-8%Typical LaBr resolution: 2.7-3%

1/21/2
• The resolution of a HPGe detector is 

usually specified as the FWHM (in keV) for the 1332.5 keV peak of Co-60.

FWHM = E

2 - E1

The SpectrumResolution

1/2

• Typical resolutions are 1.7 – 2.0 keV

E1 

1/21/2
• In a gamma spectroscopy laboratory, it is commo

measure the detector resolution each morning.

• A decrease in resolution (broadening of the peaks

usually the first indication that a detector’s perform

deteriorating

The SpectrumResolution

.
Energy Calibration
• The first step in gamma spectroscopy is to perform 

energy calibration.This involves determining the relationship betweeenergy of a gamma ray and the centroid channel 

of the peak produced by that gamma ray

Energy CalibrationGeneral

of the peak produced by that gamma ray.

• To perform an energy calibration, we count source

emit gamma rays of known energy, e.g., Cr-51 (32Cs-137 (662 keV) and Co-60 (1173 and 1333 keV

• We then determine the centroid channel numbers 

resulting peaks. The energy calibration curve is thgamma ray energy as a function of channel numb
• Each peak on the spectrum spans many channels

• Nevertheless we must select one channel to repre

peak location: the channel of the peak centroid.

Th t id i th i i ti l li th t di i

Energy CalibrationPeak Centroid

• The cen

troid is the imaginary vertical line th

a

t di

vi

peak down the middle.

• Doing this by eye can be tricky because the peak

be ragged. It is especially difficult when the peaks narrow.
Energy Calibration

Peak Centroid

When the cursor is positioned in the peak centroid the peak area to the left of the cursor is the same apeak area to the right of the cursor. 
Energy Calibration

Energy Calibration Curve

) in keV 1000 1500 Intercept (E0) in keV

Channel Number (X)

0 500 1000 1500 2000 2

Energy (E)

500 Slope (m) in keV/c
• While the curve is nice to look at, we really want a

equation that relates the gamma ray energy (E) to centroid channel number (X). 

• This can be done with either a linear or quadratic 

i

Energy CalibrationEnergy Calibration Curve

express on:

E = m X + E

0

or

E = a X

2 + b X + E

0
Identifying Unknown Gamm

E itti N lidE

mitting 

Nuclides
• Unknown radionuclides are identified by comparin

energies attributed to the peaks on the spectrum wenergies of gamma rays known to be emitted by vradionuclides.

Identifying Unknown Gamma Emitting NGeneral

• The gamma ray energies emitted by different radio

are found in various gamma ray “catalogs,” the “libof gamma spectroscopy software, etc.

• If a radionuclide emits more than one gamma ray, 

relative heights (or areas) of the different peaks cathe identification. 
The best gamma ray energy (often known as the ke

with which to identify and quantify a radionuclide sh• Have a high intensity (abundance)

H hi h t i i i tt ti

Identifying Unknown Gamma Emitting NGeneral

•

Have a hi

g

h energy 

to minimize attenuation. 

High energy peaks are on a cleaner portion of the There is higher “background” on the low energy pthe spectrum due to Compton continuum and x-ra

• Be different from the gamma ray energies of com

radionuclides.
SEARCH NUCLIDE

HALFLIFE KEY GAMMA ASSOCIATED GAMMA

1157.01157.5

Sc-44

I-130

3.93 h12.4 h

1157.0

(1.00)

536.1

(0 99)

1499.4 (.009)

668.5 (0.96)

418 0 ( 34)

511.0 739.5 

1157 5 

Identifying Unknown Gamma Emitting NGamma Library

1173.21175.1

Co-60Co-56

5.27 y

78.8 d

(0.99)

1332.5

(1.00)

846.8

(1.00)

418.0 (.34)

1173.2 (1.00)2034.8 (.78)2598.4 (.169)1037.9 (.14)1360.2 (.043)1175.1 (.023)

511.0 (.40)

1157.5 1238.3 1771.3 3253.4 2015.4 

977.4 
Identifying Unknown Gamma Emitting N

Use of Computer
• If multiple gamma rays have similar energies, th

might overlap and not all of them might be recogThis is very common with NaI detectors. The shape of the peak (peak width) might indica

is happenin

g.

Identifying Unknown Gamma Emitting NCommon Problems

pp g

• Some peaks on the spectrum might not be direc

attributable to gamma rays: x-ray peaks

sum peakssingle escape pedouble escape pebackscatter peak
• Not all the gamma ray energies of a radionuclid

in the library might be seen.

• More gamma ray energies might be seen than a

for a given radionuclide.

Identifying Unknown Gamma Emitting NCommon Problems

• More than one radionuclide emits a gamma ray 

close to the energy attributed to the photopeak.
Appendix
• The dead time is the difference between the “rea

and the “live time.” 

Dead time = real time - live time

• The real time is the real-world count time, i.e., the 

db l k

AppendixDead Time

as measure

d

by a cloc

k.

• The live time is the time that the spectroscopy sy

able to process incoming pulses. When we set a time, we are specifying the live time.

• The larger the dead time, the greater the error 

associated with its estimated value.

• In general, it should be kept below 10% or so.
Things that increase the dead time:

• Higher count rates• Longer amplifier shaping times• Higher conversion gains (Wilkinson ADCs)

AppendixDead Time

Higher conversion gains (Wilkinson ADCs)

• Larger pulse sizes (Wilkinson ADCs)
• Gain shifts are undesirable

• They cause the pulse sizes to change (get large

smaller) during the count.

• This causes the peaks to shift to the right or left 

AppendixGain Shifts

This causes the peaks to shift to the right or left 

spectrum. As such, the location (centroid channel number) attributed to these peaks will not reflect the true of the associated gamma rays.

• Gain shifts are larger on the high energy peaks 

lower energy peaks.
Things that might cause a gain shift:

• Unstable high voltage • Poor electrical connections, e.g., corrosion on th

it h i th k t f PMT

AppendixGain Shifts

swit

c

hes or in the soc

k

e

t o

f a PMT.

• Changes in count rates.
• The LLD and ULD are used to set a window of p

sizes that will be analyzed by the ADC.

• Pulses smaller than the LLD are not analyzed – 

often set around 0.2 volts.

AppendixUpper and Lower level Discriminators

• Pulses larger than the ULD are not analyzed - i

usually set at 10 volts.

• Narrowing the window established by the LLD an

ULD can be a useful means to reduce excessive dead time.
• An SCA uses the upper and lower level discrimin

(other names are often used for these) to establiwindow of pulse sizes that will be counted.

• The heights of the counted pulses are not analyz

SCA t d f t ( 

AppendixSingle Channel Analyzer (SCA)

SCAs are no

t use

d

for gamma spec

troscopy 

(an

• As an example, they might be used with a FIDLE

only Am-241 (59.5 keV) gamma rays are counteimproves sensitivity by reducing background.