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1 Instrumentation & Methods: Gamma Spectroscopy Lynn West Wisconsin State Lab of Hygiene Instrumentation – Gamma Spectroscopy/Alpha Spectroscopy Quick review of Radioactive Decay (as it relates to σ & γ spectroscopy) Interaction of Gamma Rays with matter Basic electronics Configurations Semi-conductors Resolution Spectroscopy Calibration/Efficiency Coincidence summing Sample Preparation Daily instrument checks Review of Radioactive Modes of Decay Properties of Alpha Decay Progeny loses of 4 AMU. Progeny loses 2 nuclear charges Often followed by emission of gamma 226 88 Ra 222 86 Rn + 4 2 He + energy

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Instrumentation & Methods: Gamma Spectroscopy

Lynn West

Wisconsin State Lab of Hygiene

Instrumentation –Gamma Spectroscopy/Alpha Spectroscopy

Quick review of Radioactive Decay (as it relates to σ & γ spectroscopy)Interaction of Gamma Rays with matterBasic electronicsConfigurationsSemi-conductorsResolutionSpectroscopyCalibration/EfficiencyCoincidence summingSample PreparationDaily instrument checks

Review of Radioactive Modes of Decay

Properties of Alpha DecayProgeny loses of 4 AMU.Progeny loses 2 nuclear chargesOften followed by emission of gamma

226

88Ra 22286

Rn + 42He + energy

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Review of Radioactive Modes of Decay, Cont.

Properties of Alpha Decay

Alpha particle and progeny (recoil nucleus) have well-defined energiesspectroscopy based on alpha-particle energies is possible

Energy (MeV)C

ount

s4.5 5.5

Alpha spectrum at the theoretical limit of energy resolution

Review of Radioactive Modes of Decay, Cont.

Properties of beta (negatron) decay

No change in mass number of progeny.Progeny gains 1 nuclear chargeBeta particle, antineutrino, and recoil nucleus have a continuous range of energiesno spectroscopy of elements is possibleOften followed by emission of gamma

Review of Radioactive Modes of Decay, cont.

Cou

nts

Ar-36

Cl-36

Energy (MeV)

Beta Emission from Cl-36.

From G. F. Knoll,Radiation Detection and Measurement, 3rd Ed., (2000).

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Review of Radioactive Modes of Decay, Cont.

Properties of Positron decayNo change in mass number of progenyProgeny loses 1 nuclear chargePositron, neutrino, and recoil nucleus have a continuous range of energiesno spectroscopy of elements is possible Positron is an anti-particle of an electron

Review of Radioactive Modes of Decay, Cont.

Properties of Positron decayWhen the positron comes in contact with an electron, the particles are annihilatedTwo photons are created each with an energy of 511 keV (the rest mass of an electron)The annihilation peak is a typical feature of a spectrum

Review of Radioactive Modes of Decay, Cont.

Other modes of decayElectron Capture

Neutron deficient isotopesElectron is captured by the nucleus from an outer electron shellVacancy left from captured electron is filled in by electrons from higher energy shellsX-rays are released in the process

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Review of Radioactive Modes of Decay, Cont.

Other modes of decayAuger electrons

Excitation of the atom resulting in the ejection of an outer electron

Internal conversion electronsExcitation of the nucleus resulting in the ejection of an outer electron

Bremsstrahlung“Braking” radiationPhoton emitted by a charged particle as it slows downAdds to the continuum

Review of Radioactive Modes of Decay, Cont.

Gamma EmissionNo change in mass, protons, or neutronsExcess excitation energy is given off as electromagnetic radiation, usually following alpha or beta decayGamma emissions are high-energy, short-wave-length

Source:http://lasp.colorado.edu

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Review of Radioactive Modes of Decay, Cont.

Gamma Emission Decay Schemes

Pb S

hield

ing

Pb S

hield

ing

e-

e+

511 γ

511 γ

γ

γ

Pb X Raye-

PE

e-

e-

CS

γ γ

Source

γ

γ

γe+ 511 γ

511 γ

e-

e-

e-PP

CS

CS

KEYPE Photoelectric absorptionCS Compton scatteringPP Pair productionγ gamma-raye- Electrone+ Positron

Gamma Spectrum Features

Source: Practical Gamma-Ray Spectrometry, Gilmore & Hemingway

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Resolution

Basic Electronic Schematic – Gamma Spectroscopy

Detector Bias Supply

Detector PreamplifierMultichannel Analyzer (MCA)Amplifier

Low Voltage Supply

Configurations of Ge Detectors

Electrical contact

True coaxial Closed-end coaxial

Holes

Electrons

+

Holes

Electrons

p-type coaxial, ∏-type

n-type coaxial, v-type

p+ contact

n+ contact

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Nature of Semi-conductors

Good conductors are atoms with less than four valence electronsatoms with only 1 valance electron are the best conductorsexamples

coppersilvergold

Nature of Semi-conductors, Cont.

Good insulators are atoms with more than four valence electronsatoms with 8 valance electron are the best insulatorsexamples

noble gases

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Nature of Semi-conductors, Cont.

Semiconductors are made of atoms with four valence electronsthey are neither good conductors nor good insulatorsexamples

germaniumsilicon

Nature of Semi-conductors, Cont.

Energy Band Diagram

VALENCE BANDVALENCE BAND

FORBIDDEN BAND

VALENCE BAND

FORBIDDEN BAND

CONDUCTION BAND CONDUCTION

BANDCONDUCTION

BAND

Insulator Semiconductor Conductor

Nature of Semi-conductors, Cont.

Covalent bonds are formed in semiconductors

the atoms are arranged in definite crystalline structurethe arrangement is repeated throughout the materialeach atom is covalently bonded to 4 other atoms

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Nature of Semi-conductors, cont. Pure Semi-conductor

Each atom has 8 shared electronsthere are no free electrons

or no electrons in the conduction band

however, thermal energy can cause some valence electrons to gain enough energy to move in to the conduction band

this leads to the formation of a “hole”

Nature of Semi-conductors, cont. Pure Semi-conductor

Both holes (+) & free electrons (-) are current carriersa pure semi conductor has few carriers of either typemore carriers lead to more currentdoping is the process used to increase the number of carriers in a semiconductor

Nature of Semi-conductors, cont. Pure Semi-conductor

Impurities can be added during the production of the semiconductor, this is called dopingThe impurities are either trivalent or pentavalenttrivalent examples

indium, gallium, boronpentavalent examples

arsenic, phosphorus, antimony

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n-type Semiconductor

An impurity with 5 valence electrons (group V) will form 4 covalent bonds with the atoms of the semiconductorOne electron is left over & loosely held by the atomThis type of impurity is known as donor impurities. There are more negative carriers

n-type Semiconductor

VALENCE BAND

CONDUCTION BAND

Valence electron forbidden band

Donor electron forbidden band

Donor electron Energy level

p-type semiconductors

An impurity with 3 valence electrons (group III) will form 3 covalent bonds with the atoms of the semiconductorThe absence of the fourth electron leaves a holeThis type of impurity is known as acceptor impurities. There are more positive carriers

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p-type Semiconductor, cont.

VALENCE BAND

CONDUCTION BAND

Valence electron forbidden band

Acceptor hole forbidden band

Acceptor hole Energy level

Depletion Zone

In the depletion zone the charge carriers have canceled each other outvoltage is developed across the depletion zone due to the charge separation

+-

p-type n-type

Vc

Depletion zone

V

++ ++ +

+

+

+

++

+ - ---

- --- -

++++ ++

++

+

+ ++

------

---

-

Calibration/Efficiency

Ideally, calibration sources would be prepared such that a point calibration is performed for each nuclide reported

this is totally impractical for analyzing routine unknown samples

Sources should be prepared to have identical shape and density as the sample

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Calibration/Efficiency

Differences in density are less important than differences in geometry

Newer software packages allow the user to create different efficiencies mathematically

Source strength should not be so great as to cause pile-up

Calibration/Efficiency

The calibration energies should cover the entire range of interest For close to the detector geometries, choose a multi-lined source made from a combination of nuclides which do not suffer from True Coincidence Summing (TCS). See Table 7.2 pg 153 Gilmore, G. and Hemingway, J. 1995. Practical Gamma-Ray Spectrometry. John Wiley & Sons, New York

Coincidence Summing

True Coincidence Summing (TCS) The summing of gamma rays emitted almost simultaneously from the nucleus resulting in a negative bias from the true valueLarger detectors suffer more from TCS than do smaller detectors TCS can be expected whenever samples contain nuclides with complicated decay schemes

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Coincidence Summing

True Coincidence Summing (TCS)TCS can be expected whenever samples contain nuclides with complicated decay schemes The degree of TCS is not dependent on count rate TCS is geometry dependent and is worse for close to the detector geometries

Coincidence Summing

True Coincidence Summing (TCS)TCS is geometry dependent and is worse for close to the detector geometries Summed pulses will not be rejected by the pile-up rejection circuitry because the pulses will not be misshapen For detectors with thin windows X-rays that would normally be absorbed in the end cap may contribute to TCS Well detectors suffer the worst from TCS

Coincidence Summing

True Coincidence Summing (TCS)Newer software packages have systems for reduces this problem

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Coincidence Summing

Random Coincidence Summing Also known as pile-upTwo or more gamma rays being detected at nearly the same timeCounts are lost from the full-energy peaks in the spectrumAffected by count ratePile-up rejection circuitry reduces problem

Sample Preparation

Acidify water samplesNote: Iodine is volatile in acidic solutions

Active material should be distributed evenly throughout the geometry

Samples should be homogenous

Calibration materials should simulate samples (actual or mathematical)

Daily Instrument Checks

Short background countLinearity checkResolution checkAdditionally, a long background coutis needed for backgound subtraction

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Instrumentation & Methods: Gamma Emitting Radionuclides USEPA 901.1

Jeff Brenner

Minnesota Department of Health

EPA Method 901.1Gamma Emitting Radionuclides

Gamma Emitting Radionuclides

γ

EPA Method 901.1What we’ll cover

Scope of the method Summary of the method Calibration

Determining energy calibrationDetermining efficiency calibrationDetermining system background

Quality controlInterferencesApplicationCalculations

Activity

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EPA Method 901.1Scope

The method is applicable for analyzing water samplesMeasurement of gamma photons emitted from radionuclides without separating them from the sample matrix.Radionuclides emitting gamma photons with the following energy range of 60 to 2000 keV.

EPA Method 901.1 Gamma Emitting Radionuclides Summary

Water sample is preserved in the field or lab with nitric acid

Homogeneous aliquot of the preserved sample is measured in a calibrated geometry.

EPA Method 901.1 Gamma Emitting Radionuclides Summary

Sample aliquots are counted long enough to meet the required sensitivity.

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EPA Method 901.1 Gamma Emitting Radionuclides Summary

EPA Method 901.1 Gamma Emitting Radionuclides Summary

EPA Method 901.1 Calibrations Gamma Emitting Radionuclides

Library of radionuclide gamma energy spectra is prepared Use known radionuclide concentrations in standard sample geometries to establish energy calibration.Single solution containing a mixture of fission products emitting

Low energyMedium energyHigh energyExample (Sb-125, Eu154, and Eu-155)

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EPA Method 901.1 Gamma Emitting Radionuclides Summary

86.54 Eu-155105.31 Eu-155123.07 Eu-154176.33 Sb-125247.93 Eu-154427.89 Sb-125463.38 Sb-125591.76 Eu-154600.56 Sb-125635.90 Sb-125692.42 Eu-154723.30 Eu-154756.86 Eu-154873.20 Eu-154996.30 Eu-154

1004.76 Eu-1541274.51 Eu-1541596.45 Eu-154

EPA Method 901.1 Gamma Emitting Radionuclides

Counting efficiencies for the various gamma energies are determined from the activity counts of those known standard values.A counting efficiency vs. gamma energy curve is determined for each container geometry and for each detector.

EPA Method 901.1 Gamma Emitting Radionuclides Summary

86.54 Eu-155105.31 Eu-155176.33 Sb-125427.89 Sb-125463.38 Sb-125600.56 Sb-125996.30 Eu-154

1004.76 Eu-1541274.51 Eu-154

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EPA Method 901.1 Calibrations Gamma Emitting Radionuclides

FWHM used to monitor peak shapeSmaller tolerance for low energy Greater tolerance for high energy

Document a few FWHM to determine instrument drift

EPA Method 901.1 Gamma Emitting Radionuclides Summary

86.54 Eu-155105.31 Eu-155123.07 Eu-154176.33 Sb-125247.93 Eu-154427.89 Sb-125463.38 Sb-125591.76 Eu-154600.56 Sb-125635.90 Sb-125692.42 Eu-154723.30 Eu-154756.86 Eu-154873.20 Eu-154996.30 Eu-154

1004.76 Eu-1541274.51 Eu-1541596.45 Eu-154

EPA Method 901.1 Gamma Emitting Radionuclides Summary

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EPA Method 901.1 (Determine System Background)

Contribution of the background must be measured Measure under the same conditions, counting mode, as the samplesBackground determination is performed every time the liquid nitrogen is filled

EPA Method 901.1 (Batch Quality Control)

Instrument efficiency checkAnalyzed dailyControl chartEstablish action limits

Low background checkAnalyzed weeklyControl chartEstablish action limits

Analytical Batch Sample Duplicates at a 10% frequencySample Spikes at a 5% frequency Control chartEstablish action limits

EPA Method 901.1Interferences

Significant interference occurs when counting a sample with a NaI(Tl) detector.

Sample radionuclides emit gamma photons of nearly identical energies.

Sample homogeneity is important to gamma count reproducibility and counting efficiency.

Add HNO3 to water sample container to lessen the problem of radionuclides adsorbing to the container

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EPA Method 901.1Application

The limits set forth in PL 93-523, 40 CFR 34324 recommend that in the case of man-made radionuclides, the limiting concentration is that which will produce an annual dose equivalent to 4 mrem/year.

If several radionuclides are present, the sum of their annual dose equivalent must not exceed 4 mrem/year.

EPA Method 901.1Calculations Gamma radioactivityCalculations are performed by the instrument software.Gamma (pCi/l) = C

2.22 * BEVWhere:

C= Net count rate, cpm, in the peak area above baseline continuum

B= the gamma-ray abundance (gammas/disintegration)

E= detector efficiency (counts/gamma) for the particular photopeak energy

V= volume of sample aliquot analyzed (liters)

2.22= conversion factor from dpm/pCi