Gamma Spectrum Featur

02/04/2011
Contents

Introduction

Pulse SizeOrigin of PhotonsA Perfect WorldThe Real WorldThe Fundamental Issue

Photons Come Directly from the Source

Ph t l t i Eff t Ph t kPhotoelectric Effect - PhotopeakPhotoelectric Effect – X-ray Escape PeakCompton Scattering – Compton ContinuumPair Production – Double and Single Escape Peaks

Photons Interactions in the Shield

GeneralPhotoelectric Effect – X-raysPhotoelectric Effect – Graded ShieldCompton Scattering – Backscatter PeakPair Production – Annihilation Peak
Contents

Coincidence Summing

General – Sum PeakTwo Types of CoincidenceTwo Problems with Coincidence SummingTrue CoincidenceRandom Coincidence

Bremsstrahlung

G lGeneral

Shielded Spectra

GeneralPotential Changes to SpectrumHeavily Shielded SpectrumShielded Ra-226 SpectrumImproving a Shielded SpectrumSelf-shielded Sample

Summary
Introduction

9/23/09
Introduction

Pulse Size• The size of a pulse (determined by the energy abs

the detector) depends on the:

- photon energy

- type of interaction in the detector: Photoele

Compton Pair prod
Introduction

Origin of Photons• The photons that interact in the detector are:

- coming directly from the source being analyze- secondary radiation generated in materials ot

than the detector (especially the shield)

- background radiation (e.g., K-40’s 1461 keV g
Introduction

A Perfect World• In a perfect world, pulses that contribute to the spe

would only be due to photons:

- coming directly from the source being analyze

(do not interact before reaching the detector)(do not interact before reaching the detector)

- interacting in the detector so that all their ener

absorbed (primarily via the photoelectric effect

- interact singly, i.e. two photons should not inte

detector at the same time (in coincidence)

• That this does not happen in the real world is indic

the next two slides. 
Shield Source

Introduction

The Real World

PE CS PP

Detector

PE
Introduction

The Real World

Source Source Two photons intera

coincidence (at the time) and produce pulse.

Detector
Introduction

Fundamental Issue• What impact do the preceding phenomena have o

spectrum?
Photons Coming Directly from 

Source Interact in the DetecSource Interact in the Detec
• If you ruled the world, the only thing you would allo

Photons coming directly from the source interact i

detector via the photoelectric effect so that all thei

deposited in the detector. 

Photons Come Directly from SourcePhotoelectric Effect - Photopeak

If this happens repeatedly, a number of pulses of ssize will be produced.

• The result on the spectrum is the photopeak (aka 

peak or total absorption peak)
Photo

Source

Photons Come Directly from Source

Photoelectric Effect - Photopeak

E

Detector 

PE

eAuger electron

or

x-ray

Energy

Gamma-ray photon deof its energy (E) in detResult: Photopeak
• In the photoelectric effect, most of the photon’s en

given to the photoelectron. The latter only travels distance (e.g., less than a mm) before it gives up energy to the detector.

Photons Come Directly from SourcePhotoelectric Effect - Photopeak

• In addition, somewhat less energy is carried off by 

or auger electron. In most situations, this energy is deposited in the either the x-ray or Auger electron are completely a
• However, if the photoelectric interaction was near 

detector surface and the x-ray heads in the right d

the x-rays might escape the detector.

• If this happens repeatedly, what is known as an X-

Photons Come Directly from SourcePhotoelectric Effect – X-ray Escape Peak

peak appears on the spectrum to the left of the ph

• X-ray escape peaks are most likely with:

- low energy gamma rays (e.g., less than 100 - small detectors. 
x-ray

Source 

X-ray escape 

peak

Photons Come Directly from SourcePhotoelectric Effect – X-ray Escape Peak

E

Detector

PE

eEnergy The energy deposited 

detector is the gamma energy (E) minus energResult: x-ray escape p
• The energy of the x-ray escape peak energy woul

gamma ray minus the x-ray energy.

• With a NaI detector, the x-ray escape peak would 

30 keV below the photopeak.

Photons Come Directly from SourcePhotoelectric Effect – X-ray Escape Peak• With a HPGe detector, the x-ray escape peak is 

approximately 10 keV below the photopeak.

• Keep in mind: The energy deposited in the detecto

energy that goes in (the photon energy) minus the that leaves the detector.
• Now we consider the situation where the photon in

the detector (once) via Compton scattering and leadetector.

• The energy deposited in the detector varies accor

angle of scatter

Photons Come Directly from SourceCompton Scattering – Compton Continuum

angle of scatter. 

Small pulses are produced when the angle of scatsmall (e.g., 10 degrees).The largest pulses are produced when the angle o180 degrees. Even then, the pulses are smaller ththat contribute to the photopeak.
• The result on the spectrum is the Compton continu

range of pulses to the left of the photopeak that cano useful information.

• The largest pulses in the Compton continuum form 

Compton edge

Photons Come Directly from SourceCompton Scattering – Compton Continuum

Compton edge.

They are produced when the angle of the scattere

ray is 180 degrees.Photons scattered at 180 degrees frequently have close to 200 keV. As such, the Compton edge is usually 200 keV to the photopeak.
Source Compton 

Continuum

E

Compton 

Edge

Photons Come Directly from SourceCompton Scattering – Compton Continuum

Detector

eEnergy The energy deposited in the de

incident photon energy (E) minof scattered gamma ray (E’).Result: Compton continuum

CS E’
Photons Come Directly from Source

Compton Scattering – Compton Continuum

• Each photopeak has its own Compton Continuum• The lower the photon energy:

- The closer the Compton edge is to the pho

- The smaller the Compton continuum is with 

to the photopeak

• The exact position of the Compton edge can be 

somewhat uncertain because some of the pulses edge are due to photons scattering more than onc
Source

E E2 1

E1

Photons Come Directly from SourceCompton Scattering – Compton ContinuumDetector

eEnergy The Compton continuum fro

energy gamma ray (E1) can 

Compton continuum of a higamma (E2). This increases 

difficulties experienced at lo

CS

CS

e-
• Pair production is only possible for photons with e

greater than 1022 keV.

• If the gamma rays interact in the detector via pair 

so that both 511 keV annihilation photons escape 

detector the energy deposited in the detector is th

Photons Come Directly from SourcePair Production – Double and Single Escape Pea

detector, the energy deposited in the detector is th

gamma ray energy minus 1022 keV.

• If this happens often enough, a double escape (DE

appears on the spectrum.

• The double escape peak is 1022 keV below the ph

energy.
Source

DE

E

Photons Come Directly from SourcePair Production – Double and Single Escape Pea

Detector

Energy

The energy deposited detector is the incidenray energy (E) minus Result: double escap

PP

511 keV

511 keV
• If the gamma rays interact in the detector via pair 

production so that one 511 keV annihilation photoescapes the detector, the energy deposited is the original gamma ray energy minus 511 keV.

If this happens often eno gh a single escape (S

Photons Come Directly from SourcePair Production – Double and Single Escape Pea

• If this happens often eno

ugh, 

a single escape (S

appears on the spectrum.

• The single escape peak is 511 keV below the pho

energy.
Source SE

E

511 keV

Photons Come Directly from Source

Pair Production – Double and Single Escape Pea

Detector

Energy

The energy deposited in dethe incident gamma ray enminus 511 keV.Result: single escape pea

PE

ePP
• If the gamma rays is of very high energy, it might 

seen on the spectrum even though the escape pemight be seen!

Photons Come Directly from SourcePair Production – Double and Single Escape Pea

SE

Energy

DE
Photons Interact in the Shie

and Produce Secondary 

Radiation that Interacts in th

Detector
• Photons from the source (or background) can inte

materials other than the detector. 

• Most such interactions near the detector are in the 

The latter is relatively massive and has a high ato

n mber

Photons Interactions in the ShieldGeneral

n

umber. 

• These interactions generate different types of seco

radiation that the detector will respond to.
• Following photoelectric interactions in the shield, 

characteristic x-rays can be produced. 

• Their energies are characteristic of the element in 

interaction took place.

Photons Interactions in the ShieldPhotoelectric Effect – Characteristic X-rays• Depending on the design of the shield, the peaks 

x-rays might be seen on the spectrum.

• In the case of a lead shield, these peaks would be 

85 and 87 keV.

• Of these, the 75 keV peak is the largest and possi

only one that might be seen. 
Shield Pb x-rays

Source

Photons Interactions in the ShieldPhotoelectric Effect – Characteristic X-rays

Detector

Energy

If these x-rays penetdetector housing, thedeposit all their enerdetector.Result: lead x-ray pe

PE

Pb x-rays
• The lead x-ray peaks can be eliminated by the use 

graded shield.

• A graded shield has a thin sheet (ca. 1 mm) of cad

a material with a similar atomic number lining the 

Photons Interactions in the ShieldPhotoelectric Effect – Graded Shield

Inside the cadmium is a thin sheet of copper.

• The lead x-rays from the shield are stopped by the 

cadmium.In the process, cadmium x-rays are produced. Theare stopped by the copper.The resulting copper x-rays are of so low an energthey do not get through the detector housing.
Source

Photons Interactions in the Shield

Photoelectric Effect – Graded Shield

Detector

Crystal

Pb x-r

Copper Cadmium

PE

Cu x-rays Cd x-rays

PE

DetectorHousing
• Some of the gamma rays that undergo Compton s

in the shield will be scattered back to the detector.

• Most of the scattered gamma rays seen by the de

scattered at an angle close to 180 degrees. 

Photons Interactions in the ShieldCompton Scattering – Backscatter Peak• The result on the spectrum is a “backscatter peak• Except for very low energy photons, the backscatt

is usually close to 200 keV.

• Unlike a “legitimate” peak, a backscatter peak is n

symmetrical, it has a high energy tail. The latter is scattering at angles less than 180 degrees.
Backscatter 

peak

E

Shield

Source

CS

Photons Interactions in the ShieldCompton Scattering – Backscatter Peak

Detector

Energy

The energy depositedetector is the energscattered gamma raResult: backscatter 

CS

E’
• While pair production is only possible for photons 

energies above 1022 keV, there are often such ph

background (e.g., the 1461 keV K-40 gamma rays

• Following the pair production interactions,, some o

res lting 511 keV annihilation photons ill be direc

Photons Interactions in the ShieldPair Production – Annihilation Peaks

res

ulting 511 keV annihilation photons will be direc

towards the detector.

• The result on the spectrum is an annihilation peak 

keV.
511 keV

Annihilation Peak

E Shield Source

Photons Interactions in the ShieldPair Production – Annihilation Peaks

Detector

Energy

The energy deposidetector is 511 keVResult: Annihilatio

PP

511 keV
Coincidence Summing
• When two or more photons (gamma rays or x-ray

in the detector at the same time, only one discernresults. 

• This is know as coincidence summing.

Coincidence SummingGeneral – Sum Peak• The pulse size reflects the sum of the energies de

by the two photons.

• If both photons interact via the photoelectric effec

happens with sufficient frequency, a summation (sappears on the spectrum. 

• The energy attributed to the sum peak is the sum 

energies of the coincident photons.
Source

Pulse c

to su

Coincidence SummingGeneral – Sum Peak

Detector

PE

e

Energy Two photons deposit all 

energy (E + E) in the decoincidence.Result: Sum peak

PE

e-
Source

Pulse sorted 

photopeak a

sum p

Coincidence SummingGeneral – Sum Peak

Detector

PE

e

Energy CS e- One photon deposited a

energy in the detector buphoton didn’t, it interacteCompton scattering.
• True coincidence

• Random (chance) coincidence

Coincidence SummingTwo Types of Coincidence Summing
• Coincident summing can cause the production of 

more sum peaks.These sum peaks might be misidentified.

• The more serious problem is that summation res

Coincidence SummingTwo Problems with Coincidence Summing

The more serious problem is that summation res

loss of counts in the photopeak.This loss of counts in the photopeak can lead to aunderestimate of the sample activity.
• True coincidence occurs when the two coincident 

rays were emitted during the decay of a single ato

• True coincidence is only possible if a given radion

emits more than one gamma ray and that these p

d d i th d t th ti (i

Coincidence SummingTrue Coincidence

are pro

duce

d in the 

decay a

t the same time (in 

coincidence).

• In the case of Co-60, the 1173.2 and 1332.5 keV 

rays are emitted at the same time. 

• The resulting sum peak would be at 2505.7 keV.
Sourc

Co-6

1173 

Coincidence Summing

True Coincidence

• True coincidence is most likely 

when the sample is immediately adjacent to the detector. 

• The count rate is irrelevant.

Detect

PE

e

e-

• The probability of true 

coincidence can be reduced by moving the sample farther from the detector.This reduces the solid angle as seen on the next slide.
Co-60

Source

Co-60

Source

Co 60

Coincidence SummingTrue Coincidence

Detecto

1173

Detector

Co-60

1332

Co-60

1173
• Counting samples in a well 

detector significantly increases the probability of true coincidence. 

Wh i ll d t t

Coincidence SummingTrue Coincidence

• When using a well

d

e

tec

tor 

to identify radioactive material, the sample should be outside the well.
• Gammas produced by 

positron emitters (e.g., Na22) are prone to true coincidence because there are two 511 photons per 

Coincidence SummingTrue Coincidence

decay.

• A nuclide can be quantified 

inside a well detector if the counting efficiency was determined for that nuclide with the standard source inside the well.
• Random (or chance) coincidence occurs when th

coincident photons were emitted by the decay of 

atoms.

• If a radionuclide emits a single photon per decay 

137) only random coincidence is possible

Coincidence SummingRandom Coincidence

137), only random coincidence is possible. 

• The probability of random coincidence increases 

count rate increases.

• Random coincidence can be reduced by moving t

sample farther away from the detector - this reduccount rate.
Coincidence Summing

Random Coincidence• How many sum peaks 

could result from random coincidence in the case of Co-60?

Source

1332 1173

Co-60

Co-60

Detector 

PE

e

- PE

1173

1173
Source

1332 1173

Co-60

Co-60

Coincidence Summing

Random Coincidence• How many sum peaks 

could result from random coincidence in the case of Co-60?

Detector 

PE

e- PE

1173 1173 Three Their energies would be: 2346.4 (1173.2 + 1173.2) 2505.7 (1173.2 + 1332.5) 2665.0 (1332.5 + 1332.5)
Bremsstrahlung
• High energy beta particles can complicate the spe

producing bremsstrahlung.

• Most of the bremsstrahlung pulses will show up to

left (low energy) end of the spectrum.

BremsstrahlungGeneral• The bremsstrahlung might be produced in the:

- sample (or sample container)- detector housing. This is sometimes preven

counting with a sheet of plastic (ca 1 cm) bsample and detector.
Beta particle

eBremsstrahlung General

Detector

Energy

Bremsstrahlung

A pure bremsstrahlung spsimilar to the spectrum of tube. Bremsstrahlung is with high energy beta emSr-90) in the presence of 
Beta particle

eBremsstrahlung General Bremsstrahlung

Detector 

Bremsstrahlung 

Bremsstrahlung can prodslope on the left side of th

Gamma continuum. 

Ray
Shielded Spectra
• Material between the source and detector can gre

complicate the analysis:

- Low energy photons from the source can be c

attenuated (stopped).

Shielded SpectraGeneral

- A greater percentage of the photons reaching 

detector will have undergone Compton scatte

• This is most likely to be a concern when gamma 

spectroscopy is performed outside of the laboratoFor example, using a hand-held unit to identify ramaterial inside a shipment of scrap steel.
• Low energy peaks are eliminated. 

• All the peaks might be eliminated.• The valley between the Compton edge and the ph

disappears because it is filled with forward scatte

Shielded SpectraPotential Changes to the Spectrum

disappears because it is filled with forward scatte

photons.

• The backscatter peak becomes prominent.
• In some cases, the shielding might be so severe t

peaks are found on the spectrum.

• Determine the energy on the right end of the spec

This is the energy of the highest energy gamma ra

Shielded SpectraHeavily Shielded Spectrum

This is the energy of the highest energy gamma ra

by the source (in this case, the 1332 keV gamma 

1330 keV

Energy
• The next slide shows a shielded Ra-226 spectrum

• The peak for the most penetrating gamma ray (60

obscured because the forward scattering producethe region between the photopeak and where its C

edge should appear

Shielded SpectraShielded Ra-226 Spectrum

edge should appear.

• The low energy photons at 186, 242 and 295 keV 

sufficiently attenuated that no discernible peaks wpresent.

• The only clear peak was at 352 keV. That gamma 

higher intensity and was more penetrating than tho242 and 352 keV. 
609 600

609

186

295

CS

Spectr

352 ke

Shielded SpectraShielded Ra-226 Spectrum

Source

Shield

Detector

352

242

609

CS 550
• When we are faced with badly shielded spectrum 

analyzing a different side of the source! 

• Measure the exposure rates on all sides of the so

Collect the spectrum on the side of the source wh

Shielded SpectraImproving a Shielded Spectrum

Collect the spectrum on the side of the source wh

highest exposure rates were found.

Detector Detector Poor Spectrum 

S
• In some cases, the shielding occurs inside the sa

• This is most likely to be a problem when dealing w

energy photons and large dense samples.

• For example as seen on the next slide trying to dShielded SpectraSelf-shielded Sample

For example, as seen on the next slide, trying to d

the presence of U-238 via the 63 keV gamma ray can be difficult.The 63 keV gamma ray is easily attenuated by thmatrix.
• Due to self shielding the 63 keV peak is not seen 

spectrum to the right. It looks like a purified Ra-22

• You can’t tell whether it is purified radium or the e

uranium series.

Shielded SpectraSelf-shielded Sample

Unshielded NaI gamma spectrum of uranium decay series

Lightly shielded NaI gammof uranium decay series
Summary
Summary

Spectrum Feature Photons from Source Interact in? Type of I

Photopeak Detector Photoele

X-ray Escape Peak Detector (near surface) Photoele

Compton Continuum Detector Compton 

Single and Double Escape Dt t P i P 

g p

Peaks Detector Pair P

Characteristic X-ray Peaks Shield Photoele

Backscatter Peak Shield Compton 

Annihilation Peak (511 keV) Shield Pair P

Summation Peak Detector

Multiple Ph

via Photoel

Coin