Instrumentation & Methods: Gamma Spectroscopy

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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 Ra 222 Rn + 4 He + energy

2 Review of Radioactive Modes of Decay, Cont. Properties of Alpha Decay Alpha particle and progeny (recoil nucleus) have welldefined energies spectroscopy based on alpha-particle energies is possible Counts Energy (MeV) 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 charge Beta particle, antineutrino, and recoil nucleus have a continuous range of energies no spectroscopy of elements is possible Often followed by emission of gamma Review of Radioactive Modes of Decay, cont. Cl-36 Counts Ar-36 Energy (MeV) Beta Emission from Cl-36. From G. F. Knoll, Radiation Detection and Measurement, 3rd Ed., (2000). 2

3 Review of Radioactive Modes of Decay, Cont. Properties of Positron decay No change in mass number of progeny Progeny loses 1 nuclear charge Positron, neutrino, and recoil nucleus have a continuous range of energies no spectroscopy of elements is possible Positron is an anti-particle of an electron Review of Radioactive Modes of Decay, Cont. Properties of Positron decay When the positron comes in contact with an electron, the particles are annihilated Two 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 decay Electron Capture Neutron deficient isotopes Electron is captured by the nucleus from an outer electron shell Vacancy left from captured electron is filled in by electrons from higher energy shells X-rays are released in the process 3

4 Review of Radioactive Modes of Decay, Cont. Other modes of decay Auger electrons Excitation of the atom resulting in the ejection of an outer electron Internal conversion electrons Excitation of the nucleus resulting in the ejection of an outer electron Bremsstrahlung Braking radiation Photon emitted by a charged particle as it slows down Adds to the continuum Review of Radioactive Modes of Decay, Cont. Gamma Emission No change in mass, protons, or neutrons Excess excitation energy is given off as electromagnetic radiation, usually following alpha or beta decay Gamma emissions are high-energy, short-wave-length Source: 4

5 Review of Radioactive Modes of Decay, Cont. Gamma Emission Decay Schemes KEY PE Photoelectric absorption CS Compton scattering PP Pair production γ gamma-ray e - Electron e + Positron γ Source γ e - γ γ γ e γ 511 γ Pb X Ray CS e - γ CS e - PE e - Pb Shielding γ e - CS PP e e - - e γ 511 γ Pb Shielding Gamma Spectrum Features Source: Practical Gamma-Ray Spectrometry, Gilmore & Hemingway 5

6 Resolution Basic Electronic Schematic Gamma Spectroscopy Low Voltage Supply Detector Preamplifier Amplifier Multichannel Analyzer (MCA) Detector Bias Supply Configurations of Ge Detectors Electrical contact True coaxial Closed-end coaxial n+ contact Holes Holes Electrons Electrons + p+ contact p-type coaxial, -type n-type coaxial, v-type 6

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8 Nature of Semi-conductors Good conductors are atoms with less than four valence electrons atoms with only 1 valance electron are the best conductors examples copper silver gold Nature of Semi-conductors, Cont. Good insulators are atoms with more than four valence electrons atoms with 8 valance electron are the best insulators examples noble gases 8

9 Nature of Semi-conductors, Cont. Semiconductors are made of atoms with four valence electrons they are neither good conductors nor good insulators examples germanium silicon Nature of Semi-conductors, Cont. Energy Band Diagram CONDUCTION BAND CONDUCTION BAND CONDUCTION BAND FORBIDDEN BAND FORBIDDEN BAND VALENCE BAND VALENCE BAND VALENCE BAND Insulator Semiconductor Conductor Nature of Semi-conductors, Cont. Covalent bonds are formed in semiconductors the atoms are arranged in definite crystalline structure the arrangement is repeated throughout the material each atom is covalently bonded to 4 other atoms 9

10 Nature of Semi-conductors, cont. Pure Semi-conductor Each atom has 8 shared electrons there 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 carriers a pure semi conductor has few carriers of either type more carriers lead to more current doping 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 doping The impurities are either trivalent or pentavalent trivalent examples indium, gallium, boron pentavalent examples arsenic, phosphorus, antimony 10

11 n-type Semiconductor An impurity with 5 valence electrons (group V) will form 4 covalent bonds with the atoms of the semiconductor One electron is left over & loosely held by the atom This type of impurity is known as donor impurities. There are more negative carriers n-type Semiconductor Donor electron forbidden band Donor electron Energy level CONDUCTION BAND VALENCE BAND Valence electron forbidden band p-type semiconductors An impurity with 3 valence electrons (group III) will form 3 covalent bonds with the atoms of the semiconductor The absence of the fourth electron leaves a hole This type of impurity is known as acceptor impurities. There are more positive carriers 11

12 p-type Semiconductor, cont. Acceptor hole forbidden band Acceptor hole Energy level CONDUCTION BAND VALENCE BAND Valence electron forbidden band Depletion Zone V p-type n-type In the depletion zone the charge carriers have canceled each other out voltage is developed across the depletion zone due to the charge separation V c Depletion zone 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 12

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 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 value Larger detectors suffer more from TCS than do smaller detectors TCS can be expected whenever samples contain nuclides with complicated decay schemes 13

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 14

15 Coincidence Summing Random Coincidence Summing Also known as pile-up Two or more gamma rays being detected at nearly the same time Counts are lost from the full-energy peaks in the spectrum Affected by count rate Pile-up rejection circuitry reduces problem Sample Preparation Acidify water samples Note: 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 count Linearity check Resolution check Additionally, a long background cout is needed for backgound subtraction 15

16 Instrumentation & Methods: Gamma Emitting Radionuclides USEPA Jeff Brenner Minnesota Department of Health EPA Method Gamma Emitting Radionuclides Gamma Emitting Radionuclides γ EPA Method What we ll cover Scope of the method Summary of the method Calibration Determining energy calibration Determining efficiency calibration Determining system background Quality control Interferences Application Calculations Activity 16

17 EPA Method Scope The method is applicable for analyzing water samples Measurement 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 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 Gamma Emitting Radionuclides Summary Sample aliquots are counted long enough to meet the required sensitivity. 17

18 EPA Method Gamma Emitting Radionuclides Summary EPA Method Gamma Emitting Radionuclides Summary EPA Method 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 energy Medium energy High energy Example (Sb-125, Eu154, and Eu-155) 18

19 EPA Method Gamma Emitting Radionuclides Summary Eu Eu Eu Sb Eu Sb Sb Eu Sb Sb Eu Eu Eu Eu Eu Eu Eu Eu-154 EPA Method 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 Gamma Emitting Radionuclides Summary Eu Eu Sb Sb Sb Sb Eu Eu Eu

20 EPA Method Calibrations Gamma Emitting Radionuclides FWHM used to monitor peak shape Smaller tolerance for low energy Greater tolerance for high energy Document a few FWHM to determine instrument drift EPA Method Gamma Emitting Radionuclides Summary Eu Eu Eu Sb Eu Sb Sb Eu Sb Sb Eu Eu Eu Eu Eu Eu Eu Eu-154 EPA Method Gamma Emitting Radionuclides Summary 20

21 EPA Method (Determine System Background) Contribution of the background must be measured Measure under the same conditions, counting mode, as the samples Background determination is performed every time the liquid nitrogen is filled EPA Method (Batch Quality Control) Instrument efficiency check Analyzed daily Control chart Establish action limits Low background check Analyzed weekly Control chart Establish action limits Analytical Batch Sample Duplicates at a 10% frequency Sample Spikes at a 5% frequency Control chart Establish action limits EPA Method Interferences 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 HNO 3 to water sample container to lessen the problem of radionuclides adsorbing to the container 21

22 EPA Method Application The limits set forth in PL , 40 CFR recommend that in the case of manmade 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 radioactivity Calculations are performed by the instrument software. Gamma (pci/l) = Where: C 2.22 * BEV 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 22