11

Lecture 5Lecture 5

Overview on the Analytical Procedures Overview on the Analytical Procedures (())

Moncef BenmansourMoncef BenmansourCNESTEN, Rabat MoroccoCNESTEN, Rabat Morocco

IAEA Regional Training Course

Sediment Core Dating Techniques - RAF/7/008 Project

CNESTEN, Rabat, 05-09 July 2010IAEA CNESTE

N

22

ContentsContents

Radionuclides and radiationsRadionuclides and radiations

Basis of Gamma spectrometryBasis of Gamma spectrometry

Hyper Germanium detectorsHyper Germanium detectors

Detection calibrationDetection calibration

Activity calculation, uncertainty, detection limitActivity calculation, uncertainty, detection limit

Correction factorsCorrection factors

Comparative measurements: Comparative measurements: 137137Cs, Cs, 210210Pb, Pb, 226226Ra, Ra, 241241AmAm

33

Radionuclides and RadiationsRadionuclides and Radiations

Natural Radionuclides Natural Radionuclides

– Cosmogenic RadionuclidesCosmogenic Radionuclides

1414C, C, 33H, H, 2222Na, Na, 77Be….Be….

– Primordial Radionuclides (Singly) Primordial Radionuclides (Singly)

4040K, K, 8787Rb, Rb, 5050V, V, 144144Nd...Nd...

– Primordial Radionuclides ( Natural series)Primordial Radionuclides ( Natural series)

238238U, U, 235235U, U, 232232Th seriesTh series

44

SERIE 238U

SERIE 232Th

SERIE 235U

Np

U 238U

4,49E9A 234U 2,48E5a

235U 7,13E8a

Pa 234Pa 1,18m

231Pa 3,25E4a

Th 234Th

24,1j 230Th 7,5E4a

232Th 1,39E10a

228Th 1,9a

231Th 25,6h

227Th 18,6j

Ac 228Ac

6,13h 227Ac

22,0a

Ra 226Ra

1622s 228Ra

6,7a 224Ra

3,64j 223Ra

11,1j

Fr

Rn 222Rn

3,825j 220Rn

54,5s 219Rn

3,92s

At

Po 218Po

3,05m 214Po 1,6E-4s

210Po 138,4j

216Po 0,158s

212Po 3,0E-7s

215Po 1,83E-3s

Bi 214Bi 19,7m

210Bi 5j

212Bi 60,5m

211Bi 2,16m

Pb 214Pb

26,8m 210Pb

21,4a 206Pb STABLE

212Pb 10,6h

208Pb STABLE

211Pb 36,1m

207Pb STABLE

Tl 210Tl

1,3m 206Tl 4,19m

208Tl 3,1m

207Tl 4,79m

55

Radionuclides and RadiationsRadionuclides and Radiations

Man made Man made RadionuclidesRadionuclides

– Fissions ProductsFissions Products

137137Cs, Cs, 9090Sr, Sr, 8989Sr, Sr, 131131I, I, 9999TcTc

– Activation ProductsActivation Products

239239Pu, Pu, 240240Pu, Pu, 241241Pu, Pu, 241241Am, Am, 242242Cm,Cm,

6060Co, Co, 6565Zn, Zn, 5454Mn, Mn, 5555FeFe……

- Nuclear Weapons testing

- Chernobyl Accident

- Discharges from reprocessing

66

Radionuclides and RadiationsRadionuclides and Radiations

Alpha particlesAlpha particles ( () helium) helium

Beta particles (Beta particles ( e- and e e- and e++

Electronic CaptureElectronic Capture

Gamma rays (Gamma rays (): Photons): Photons– 137137Cs, Cs, 210210Pb, Pb, 241241Am,…Am,…

Ee

Eg

77

Gamma-Matter Interaction: Gamma-Matter Interaction: Interaction processusInteraction processus

Photoelectric effect Compton Pair production

lc EEE

e-

)1(1 2

0

20

CmECm

EEc

20

)cos1(1

'

CmE

EE

202 CmEEE ee

2m0C2 = 1,02 MeV

88

Gamma-Matter InteractionGamma-Matter Interaction

Attenuation of Attenuation of

– II(x) = I (0) e(x) = I (0) e--xx

I(0) II(x)(x)

x

Attenuation Attenuation coefficientcoefficient

cmcm-1-1 or cm or cm22/g/g

99

Gamma attenuationGamma attenuation

Lead Aluminium

1010

Gamma spectrometry: General BasisGamma spectrometry: General Basis

Interaction of Interaction of photons with the detectorphotons with the detector Production of electric pulses : Amplitude proportionnel Production of electric pulses : Amplitude proportionnel

to photon energy emitted by the sourceto photon energy emitted by the source Whole information contained in a gamma spectrum Whole information contained in a gamma spectrum

( gamma energy, & activity)( gamma energy, & activity)

1111

Hyperpur Germanium DetectorsHyperpur Germanium Detectors

Semiconductor diodes having a p-i-n structureSemiconductor diodes having a p-i-n structure

Intrinsic (I) region is sensitive to ionizing radiation, Intrinsic (I) region is sensitive to ionizing radiation, particularly x rays and particularly x rays and rays rays

Under reverse bias, an electric field extends across Under reverse bias, an electric field extends across the intrinsic or depleted region.the intrinsic or depleted region.

When photons interact with the material charge When photons interact with the material charge

carriers (holes and electrons) are produced and are carriers (holes and electrons) are produced and are

swept by the electric field to the p and n electrodes.swept by the electric field to the p and n electrodes.

1212

Hyperpur Germanium DetectorsHyperpur Germanium DetectorsRelative efficiency, energy resolution, energy range, peak/compton ratio

1313

Hyperpur Germanium DetectorsHyperpur Germanium Detectors

Detector

Preamplifier Amplifier ADC

Multiplexer

High voltage supply

MCA

NIM Module (low voltage)

rays

Detector

Preamplifier Amplifier ADC

Multiplexer

High voltage supply

MCA

NIM Module (low voltage)

rays

1414

Electronic parmetersElectronic parmeters

Power Supply: H.VPower Supply: H.V

Amplifier Amplifier – Gain : Coarse and FineGain : Coarse and Fine– Shaping timeShaping time– Zero poleZero pole

Parameters of MCAParameters of MCA

1515

Hyperpur Germanium DetectorsHyperpur Germanium Detectors

1616

Energy CalibrationEnergy Calibration

Energy – Canal RelationEnergy – Canal Relation

– Two sources ( Two sources ( 137137Cs, Cs, 6060Co)Co)

– Multi-gamma sources Multi-gamma sources

661 keV 1173 keV

1332 keV

1836 keV

1717

Energy CalibrationEnergy Calibration

1818

Efficiency calibrationEfficiency calibration

Full – energy –peak efficiency: Full – energy –peak efficiency: (E)(E)

(E) (E) == N(E) N(E) /R/R– N (E): count rate in the peak corresponding to the N (E): count rate in the peak corresponding to the

Energy Energy EE– R: rate at which photons of Energy R: rate at which photons of Energy EE are emitted from are emitted from

the sourcethe source

R R = A.I= A.I– A :Source ActivityA :Source Activity

– II: : Gamma ( Gamma ( ) ray emission probability ) ray emission probability

1919

Efficiency calibrationEfficiency calibration

(E) (E) depends on: depends on: – Source dimension and source –detector distanceSource dimension and source –detector distance

– Dimensions of the detector housing and of the sensitive and Dimensions of the detector housing and of the sensitive and insensitive zones of the detectorinsensitive zones of the detector

– Elementary composition and density of all materials Elementary composition and density of all materials traversed by the photonstraversed by the photons

– Photon attenuation coefficients of these materialsPhoton attenuation coefficients of these materials

– Energy-and angle-dependent cross sections of the detector Energy-and angle-dependent cross sections of the detector material for the various photon interactionsmaterial for the various photon interactions

– Information on the electron and positron transport in the Information on the electron and positron transport in the detectorsdetectors

2020

Efficiency calibrationEfficiency calibration

Efficiency calculationEfficiency calculation– Monte Carlo codes, but many constraintsMonte Carlo codes, but many constraints

Uncertainties in the shape and size of the effective or Uncertainties in the shape and size of the effective or sensitive crystal volumesensitive crystal volume

Uncertainties on the photons and electron interaction Uncertainties on the photons and electron interaction probaility and angular distributionsprobaility and angular distributions

Efficiency measurementsEfficiency measurements– Calibration sources: easier and more accurate than Calibration sources: easier and more accurate than

calculation calculation

– (E) VS Energy (keV)(E) VS Energy (keV)

2121

Standard sourcesStandard sources

Liquid multi-gamma sources with certified Liquid multi-gamma sources with certified activities purchased from an international activities purchased from an international providerprovider

Different Marked matrixes prepared by the Different Marked matrixes prepared by the supplier in different geometries supplier in different geometries

Reference Materials: (e.g.. IAEA) Reference Materials: (e.g.. IAEA)

2222

Standard SourcesStandard Sources

Radionucléide Energie (keV) Nombre de

photons pour 100 désintégrations

Activité massique (Bq/g)

Periode ( j )

Am-241 59.537 0.001 35.9 0.4 2000 3 157935.5 296 Cd-109 88.034 0.002 3.65 0.06 9830 3.7 462.6 0.4 Co-57 122.061 0.001 85.68 0.13 463 3 271.77 0.10 Co-57 136.4740.001 10.67 0.13 463 3 271.77 0.10 Ce-139 165.8570.006 79.87 0.06 473 3 137.640 0.023 Cr-51 320.0840.001 9.85 0.09 14800 3 27.703 0.004

Sn-113 391.7020.004 64.89 0.17 2810 3.4 115.08 0.03 Sr-85 514.0090.012 99.29 0.04 1760 3 64.850 0.007

Cs-137 661.6600.003 85.2 0.2 2980 3 11004.75 7.3 Y-88 898.0420.004 94.1 0.5 2770 3 106.62 0.02 Co-60 1173.2380.004 99.89 0.02 2990 3 1923.915 0.365 Co-60 1332.5020.005 99.983 0.001 2990 3 1923.915 0.365 Y-88 1836.0640.013 99.36 0.05 2770 3 106.62 0.02

2323

Efficiency CurvesEfficiency CurvesHPGe, coaxial – P Type: Rel. Eff.30%HPGe, coaxial – P Type: Rel. Eff.30%

0

1

2

3

4

5

6

7

8

0 200 400 600 800 1000 1200 1400 1600 1800 2000

Energy (keV)

Eff

icie

ncy

(%

)

Cyl-20 ml

Cyl-200 ml

Mar-0,5 l

Mar-1,5 l

2424

Efficiency Curves Efficiency Curves P-type and N- type detectorsP-type and N- type detectors

0

1

2

3

4

5

6

7

0 200 400 600 800 1000 1200 1400

Energy (keV)

Eff

icie

ncy

(%

)

P-Type 30%

N-Type 45%

2525

Efficiency fittingEfficiency fitting

-6

-5

-4

-3

-2

-1

0

3 4 5 6 7 8

LN(E)

LN

(Eff

.)

0,001

0,01

0,1

1

10 100 1000 10000

Energy (keV)

Effic

ienc

y (%

)

n

j

jj EEa

00 )/(loglog

2626

Spectral EvaluationSpectral Evaluation

2727

a1, a2, b1 b2

N = Nt - Nb

2828

Spectral EvaluationSpectral Evaluation

Automatic peak search Evaluation of the peak positions in energy Identification of radionuclides by the use of a nuclide library Efficiency-calibration curve Calculation of the net peak area Calculation of activity concentrations in selected units Calculation of the detection limits for specific nuclides

Nt =

1

2

b

aiiN

Nb= ( )2

1

2

1 1

1

b

bii

a

aii NN

)(

)1(

1212

21

bbaa

ab

N = Nt - Nb

where: N = net count or net peak area Nb = background count Ni = counts in the channel i Nt = total counts

a1, a2,b1 and b2 = the number of the respective channel.

2929

Calcul of activityCalcul of activity

.)1(

0

ctcc

tc

eMtIF

etNA

At = activity or concentration of Radionculide in Bq kg-1 at the sampling date N = net peak area = decay constant (Ln2/T1/2), T1/2 for 137Cs is 30.17 year tc = counting time t0 = time difference between sampling and starting the measurement M = masse of the soil sample (kg) = absolute efficiency emission probability Fc =Correction factor

.0

cc

t

MtIF

NeA

General Case General Case If tc << T1/2 If tc << T1/2

3030

Calcul of activityCalcul of activity: Areal activity: Areal activity

S

CMA t

s C = activity of the sub-sample of the whole core (Bq kg-1)

Mt = total mass of the whole core (kg) S = corer area (m2)

i

iTs CMS

Ai

1 iTM Ci = activity of the i th sub-sample (Bq kg-1)

iTM = total mass of the i th (kg)

S = corer area (m2)

iiis HCA Ci = activity of the i th sub-sample (Bq kg-1) bulk density of the i th sub-sample (kg m-3) Hi = depth of the i th sub-sample (m)

3131

UncertaintiesUncertainties

.0

cc

t

MtIF

NeA

A/A)2 = (N/N)2 + ()2 + (I(Fc Fc

)/()(/ btbt NNNNNN

3232

UncertaintiesUncertainties

IAEA TECDOC 1401

3333

BackroundBackround

(i) Internal background

Natural radioactivity (238U, 235U, 232Th series and 40K) coming from the material used to build the detector

Natural radioactivity coming from the auxiliary equipments and shielding

(ii) External background

Natural radioactivity coming from surrounding environment; earth surface, walls, floor, etc.

Radiations coming from 222Rn (gas) daughters Cosmic rays

3434

Detection LimitDetection Limit

The detection limit is a term which expresses the detection capabilities of measurement system under certain conditions. A general accepted expression of the lower limit of detection (LLD) which contains a pre selected risk of 5% of concluding falsely that activity is present and a 95 % degree of confidence for detecting the presence of activity, is calculated as follows:

TI

BLLD

66.4

B = background count rate (counts/s) = absolute efficiency emission probability T = counting time

3535

Detection LimitDetection Limit

When introducing mass (kg) of the sample, the minimum detectable concentration (MDC) also termed Minimum Detection Activity (MDA) in Bq kg-1 can be expressed as:

TMI

BMDC

66.4

3636

Correction factorsCorrection factors

Factor corrections (Fc)Factor corrections (Fc)

– Coïncidence –summing corrections ( two or more Coïncidence –summing corrections ( two or more photons within the resolving time of the photons within the resolving time of the spectrometer). spectrometer).

– Dead –time and pil-up correctionsDead –time and pil-up corrections

– Attenuation correction: self-absorption Attenuation correction: self-absorption attenuationattenuation

3737

Coïncidence –summing corrections Coïncidence –summing corrections (eg. two Radionuclides )(eg. two Radionuclides )

– NN1 1 = A I= A I11 ( 1 – ( 1 – 1212))

– CC1 1 = 1/(1- = 1/(1- 1212))

– NN22 = AI = AI2 2 22 [1 – (I [1 – (I11/I/I22) ) 1212]]

– CC2 2 = 1 / [1—(I = 1 / [1—(I1 1 /I/I22) ) 1212]]

– NN33 = AI = AI3 3 ] ]

– CC3 3 = 1/[1+I= 1/[1+I11 /(I/(I3333))

1

3

E3 (I3) E1(I1)

E2 (I2)

3838

Dead time and pile-up correctionDead time and pile-up correction

MCA : Real and live timeMCA : Real and live time Pile - up correction rejector Pile - up correction rejector Pulser method:Pulser method:

– NN00 = N f t/N = N f t/Npp NN et Net N00: measured and true number of counts in the : measured and true number of counts in the

peak respectivelypeak respectively NNp: p: number of counts in the pulser peaknumber of counts in the pulser peak F: frequency of the pulserF: frequency of the pulser T: Counting timeT: Counting time

3939

Attenuation correctionAttenuation correction

Attnuation law: Attnuation law: II(x) = I (0) e(x) = I (0) e--dd– : masse attenuation coefficient: masse attenuation coefficient– d : tickness of the sample d : tickness of the sample – : density of the ample: density of the ample

Self-attenuation Facteur: Self-attenuation Facteur: – F (F (dd)= [1-exp(-)= [1-exp(-dd)]/ )]/ dd

Correction Facteur Correction Facteur – CCa a = F (= F (dd))sample sample //F(F(dd))standardstandard

– (E > 100 keV): (E > 100 keV): CCa a depends exclusively on the sample depends exclusively on the sample densitydensity

– (E <100 keV): (E <100 keV): CCa a depends also on the chemical composition depends also on the chemical composition

4040

Attenuation correctionAttenuation correction

Boshkova and Minev , ARI 54 (2001) 777-783

4141

Attenuation correctionAttenuation correction

Can be determinedCan be determined

– Using analytical methodsUsing analytical methods– Using the Monte-Carlo Computation Using the Monte-Carlo Computation

techniquestechniques– ExperimentallyExperimentally

4242

Attenuation correction: Attenuation correction: ExperimentallyExperimentally

Point Source on the top of Point Source on the top of containers:containers:– with unknown sample, standard, with unknown sample, standard,

and airand air

)/(1

)/ln(

0

0, II

IIF

s

ssatt

)/(1

)/ln(

0

0, II

IIF

st

ststatt

statt

sattatt F

FF

,

,

statts F

Cutshall et al., NIM PR A 206 (1983) 309-312

4343

Comparative Comparative measurementsmeasurements

137137Cs, Cs, 210210Pb, Pb, 226226Ra, Ra, 241241AmAm

4444

137137CsCs

4545

210210PbPb

4646

241241AmAm

4747

226226RaRa

4848

Comparison Comparison

Radionuclide

137Cs

210Pb

226Ra

241Am

T1/2 (ans) 30.17 22.3 1600 432 rays (keV) 661.66 46.52 186.21 and of

daughters 59.54

I 85.5 4.05 3.28 35.90

HPGe - Coaxial - BeGe, REGe

- BeGe, REGe - LEGe

- Coaxial - BeGe, REGe

- BeGe, REGe -LEGe - Coaxial

Précautions

- low activity & overlapping with 665 keV of 214Bi

- self absorption correction - Background

-self absorption correction - interference with 235U - Background - 222Rn variation

- self absorption correction

4949

137137Cs & Cs & 210210PbPbSelf-absorptionSelf-absorption

0

2

4

6

8

10

12

0.9 1.1 1.3 1.5

Density(g/cm3)

Eff

icie

ncy

(%

)

46.5 keV

662 keV

Sediment Samples (100 m)

HPGe 45% -N Type -

5050

ConclusionConclusion

Gamma spectrometry: Gamma spectrometry: Direct technique, without Direct technique, without radiochemical separation, but requires some radiochemical separation, but requires some precautions:precautions:

– Selection of suitable HPGe detectorsSelection of suitable HPGe detectors– Selection of suitable standardsSelection of suitable standards– Sample preparation and geometry of countingSample preparation and geometry of counting– Efficiency curveEfficiency curve– BackgroundBackground– FF actor effectsactor effects– All sources of uncertaintyAll sources of uncertainty– Quality Control ProgrammeQuality Control Programme