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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
2
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).
3
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
4
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
5
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
6
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
7
8
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
9
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
12
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
13
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
14
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
15
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
17
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
20
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
21
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
22
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