CINCH NucWik

User Tools

Site Tools

You are here: NucWik » Textbooks » NRC Textbook - Basics of Nuclear Physics and of radiation detection and measurement » 8. Measurement of radionuclides

Sidebar

CINCH project

NRC Network

CINCH Hub

textbook:nrctextbook:chapter8

Differences

This shows you the differences between two versions of the page.

Link to this comparison view

Both sides previous revision Previous revision
Next revision 		Previous revision
textbook:nrctextbook:chapter8 [2025-04-16 15:00]
Merja Herzig 		textbook:nrctextbook:chapter8 [2025-09-01 13:46] (current)
Merja Herzig
Line 6: Line 6:
 ### ###
 There are two principal means to measure radionuclide activities: radiometric and mass  There are two principal means to measure radionuclide activities: radiometric and mass 
-spectrometric. Radiometric methods are based on detection and measurement of radiation emitted by radionuclides whereas in mass spectrometric methods number of atoms are counted. The results of these methods, [[textbook:nrctextbook:chapter6#activity|activity]] (//A//) in case of radiometry and number of radioactive atoms (//N//) in mass spectrometry, can be can be converted to each other by the radioactive decay law equation+spectrometric. Radiometric methods are based on detection and measurement of radiation emitted by radionuclides whereas in [[textbook:nrctextbook:chapter8#mass_spectrometry|mass spectrometric methods]] number of atoms are counted. The results of these methods, [[textbook:nrctextbook:chapter6#activity|activity]] (//A//) in case of radiometry and number of radioactive atoms (//N//) in mass spectrometry, can be can be converted to each other by the radioactive decay law equation
 ### ###
    
Line 61: Line 61:
 ;;# ;;#
  
-{{:textbook:nrctextbook:effect_of_counting_geometry_on_radiation_detection_fig_8_1.png?400|}} 
  
 +{{:textbook:nrctextbook:effect_of_counting_geometry_on_radiation_detection_of_a_point_source_light.png?200 |}}
 Figure VIII.1. Effect of counting geometry on radiation detection of a point source. Figure VIII.1. Effect of counting geometry on radiation detection of a point source.
  
 ### ###
-In practice the situation is more complicated since the sources are seldom point sources. As a rule the geometry factor is the higher the closer is the source to the detector. To improve geometry in [[textbook:nrctextbook:chapter9|gamma spectrometry]] well-type detectors, instead of planar, are used. In these the source is placed inside a hole in the detector and a larger fraction of gamma rays are thus detected. The best geometry in obtained in [[textbook:nrctextbook:chapter12|liquid scintillation counting]] where the [[textbook:nrctextbook:chapter4|radionuclide]] is uniformly distributed in liquid [[textbook:nrctextbook:chapter12#scintillator_molecule|scintillation cocktail]] and in principal all [[textbook:nrctextbook:chapter5#beta_particle|beta]] and [[textbook:nrctextbook:chapter5#alpha_particle|balpha particles]] can lead to formation of light pulses when exciting [[textbook:nrctextbook:chapter12#scintillator_molecule|scintillator molecules]] are surrounding them in all directions.+In practice the situation is more complicated since the sources are seldom point sources. As a rule the geometry factor is the higher the closer is the source to the detector. To improve geometry in [[textbook:nrctextbook:chapter9|gamma spectrometry]] well-type detectors, instead of planar, are used. In these the source is placed inside a hole in the detector and a larger fraction of gamma rays are thus detected. The best geometry in obtained in [[textbook:nrctextbook:chapter12|liquid scintillation counting]] where the [[textbook:nrctextbook:chapter4|radionuclide]] is uniformly distributed in liquid [[textbook:nrctextbook:chapter12#scintillator_molecule|scintillation cocktail]] and in principal all [[textbook:nrctextbook:chapter5#beta_particle|beta]] and [[textbook:nrctextbook:chapter5#alpha_particle|alpha particles]] can lead to formation of light pulses when exciting [[textbook:nrctextbook:chapter12#scintillator_molecule|scintillator molecules]] are surrounding them in all directions.
 ### ###
  
Line 86: Line 86:
  
  
-{{:textbook:nrctextbook:observed_count_rate_as_a_function_of_count_rate_with_deadtime_fig_7_2.png|}}+{{:textbook:nrctextbook:observed_count_rate_r_as_a_function_of_count_rate_with_dead_time.png?400 |}}
  
 Figure VIII.2. Observed count rate (R) as a function of count rate taking into account 10 µs dead-time of the detector. Figure VIII.2. Observed count rate (R) as a function of count rate taking into account 10 µs dead-time of the detector.
Line 138: Line 138:
  
  
-{{:textbook:nrctextbook:gamma_spec_of_137cs_solid_scintillation_fig_8_3.png|}}+{{:textbook:nrctextbook:gamma_spectrum_of_137cs_measured_with_a_solid_scintillation_detector.png?400 |}}
  
 Figure VIII.3. Gamma spectrum of <sup>137</sup>Cs measured with a solid scintillation detector. Figure VIII.3. Gamma spectrum of <sup>137</sup>Cs measured with a solid scintillation detector.
Line 171: Line 171:
   * A single channel analyzer (SCA) counts only pulses at a defined height range. As described above, selection of pulse height range is accomplished with voltage discriminators, lower and upper. In addition, there is a pulse counter that sums all pulses coming to the discriminator window. For example, single channel can be set to count only pulses with heights between 50 mV and 150 mV, i.e. pulses that would go channels 50-150 in the multichannel analyzer, presuming same settings. Single-channel analyzer is used to measure only one [[textbook:nrctextbook:chapter4|radionuclide]] at the time. The discriminators are set by measuring the spectrum of the desired radionuclide by using a narrow discriminator window at increasing mV range. Plotting the counts at increasing mV results in the formation of an energy spectrum. The measurement window is set by measuring the spectrum of the desired radionuclide and selecting from the spectrum the lower and upper discriminator voltage values so that the pulses from the [[textbook:nrctextbook:chapter9#photopeak|photopeak]] is between them. Single channel mode is typically used in gamma counters with [[textbook:nrctextbook:chapter9#solid_scintillators|solid scintillation detectors]].   * A single channel analyzer (SCA) counts only pulses at a defined height range. As described above, selection of pulse height range is accomplished with voltage discriminators, lower and upper. In addition, there is a pulse counter that sums all pulses coming to the discriminator window. For example, single channel can be set to count only pulses with heights between 50 mV and 150 mV, i.e. pulses that would go channels 50-150 in the multichannel analyzer, presuming same settings. Single-channel analyzer is used to measure only one [[textbook:nrctextbook:chapter4|radionuclide]] at the time. The discriminators are set by measuring the spectrum of the desired radionuclide by using a narrow discriminator window at increasing mV range. Plotting the counts at increasing mV results in the formation of an energy spectrum. The measurement window is set by measuring the spectrum of the desired radionuclide and selecting from the spectrum the lower and upper discriminator voltage values so that the pulses from the [[textbook:nrctextbook:chapter9#photopeak|photopeak]] is between them. Single channel mode is typically used in gamma counters with [[textbook:nrctextbook:chapter9#solid_scintillators|solid scintillation detectors]].
  
-{{:textbook:nrctextbook:components_and_scheme_of_radiation_measurement_equipment_systems_fig_8_4.png|}}+{{:textbook:nrctextbook:components_and_scheme_of_radiation_measurement_equipment_systems.png|}}
  
 Figure VIII.4. Components and scheme of radiation measurement equipment systems. PMT is [[textbook:nrctextbook:chapter9#photomultiplier_tube|photomultiplier tube]]. Figure VIII.4. Components and scheme of radiation measurement equipment systems. PMT is [[textbook:nrctextbook:chapter9#photomultiplier_tube|photomultiplier tube]].
  
 {{anchor:energy_resolution}}  {{anchor:energy_resolution}} 
 +{{anchor:fwhm_full_width_at_half_maximum}} 
 ===== 8.4. Energy resolution ===== ===== 8.4. Energy resolution =====
  
Line 184: Line 185:
  
  
-{{:textbook:nrctextbook:energy_resolution_of_spectrum_peak_fig_7_5.png?400|}}+{{:textbook:nrctextbook:energy_resolution_of_spectrum_peak.png?400|}}
  
 Figure VIII.5. Energy resolution of spectrum peak. Figure VIII.5. Energy resolution of spectrum peak.
Line 235: Line 236:
   * [[textbook:nrctextbook:chapter11#semiconductor_detectors_alpha|semiconductor alpha detector(s)]] for the measurement of alpha emitters separated from various samples   * [[textbook:nrctextbook:chapter11#semiconductor_detectors_alpha|semiconductor alpha detector(s)]] for the measurement of alpha emitters separated from various samples
   *[[textbook:nrctextbook:chapter10| gas ionization detector(s)]] to measure low activity beta emitters separated from various samples   *[[textbook:nrctextbook:chapter10| gas ionization detector(s)]] to measure low activity beta emitters separated from various samples
 +
 +{{anchor:mass_spectrometry}}
  
 ===== 8.6. Measurement of radionuclides with mass spectrometry ===== ===== 8.6. Measurement of radionuclides with mass spectrometry =====
  
 ### ###
-An alternative for radiometric methods for the determination of radionuclide activities is mass spectrometry. Most often inductively-coupled mass spectrometry (ICP-MS) is today used for this purpose. As mentioned, mass spectrometer counts atoms instead of radiation. This makes mass spectrometry particularly suitable for the measurement of long-lived radionuclides for which the detection limit of mass spectrometry is below those obtained by radiometric methods. For example,  +An alternative for radiometric methods for the determination of radionuclide [[textbook:nrctextbook:chapter6#activity|activities]] is mass spectrometry. Most often inductively-coupled mass spectrometry (ICP-MS) is today used for this purpose. As mentioned, mass spectrometer counts atoms instead of radiation. This makes mass spectrometry particularly suitable for the measurement of [[textbook:nrctextbook:chapter6#long_lived_radionuclides|long-lived radionuclides]] for which the detection limit of mass spectrometry is below those obtained by radiometric methods. For example, for <sup>99</sup>Tc (t½ = 211000 y) the detection limit is at best 1 mBq for a [[textbook:nrctextbook:chapter10|gas ionization detector]] and clearly higher in [[textbook:nrctextbook:chapter12|liquid scintillation counting]]. 1 mBq means that there are about four decays in an hour but this [[textbook:nrctextbook:chapter6#activity|activity]] of <sup>99</sup>Tc corresponds to about 100 million atoms. This amount of atoms can be easily detected and counted by mass spectrometry and even as low as a 0.001 mBq detection limit can be  
-for <sup>99</sup>Tc (t½ = 211000 y) the detection limit is at best 1 mBq for a gas ionization detector and clearly higher in liquid scintillation counting. 1 mBq means that there are about four decays in an hour but this activity of <sup>99</sup>Tc corresponds to about 100 million atoms. This amount of atoms can be easily detected and counted by mass spectrometry and even as low as a 0.001 mBq detection limit can be  +achieved by ICP-MS. In principle all radionuclides with [[textbook:nrctextbook:chapter6#half_life|half-lives]] longer than 100 years can be measured by ICP-MS. However, for the radionuclides with half-lives round this limit, radiometric methods are still more sensitive and provide with more accurate results.
-achieved by ICP-MS. In principle all radionuclides with half-lives longer than 100 years can be measured by ICP-MS. However, for the radionuclides with half-lives round this limit, radiometric  methods are still more sensitive and provide with more accurate results.+
 ### ###
  
  
 ### ###
-The components of an ICP-MS are presented in Figure VIII.6. The sample solution is introduced into the system by a nebulizer which turns the solution into a fine mist (aerosol). This is transferred with argon flow into the torch where plasma is created with the help of radiofrequency. Plasma atomizes the sample, ionizes the atoms and the ions are directed into a mass analyzer for the separation of ions based on their mass to charge ratio (//m/z//).+The components of an ICP-MS are presented in [[textbook:nrctextbook:chapter8#figure_86|Figure VIII.6]]. The sample solution is introduced into the system by a nebulizer which turns the solution into a fine mist (aerosol). This is transferred with argon flow into the torch where plasma is created with the help of radiofrequency. Plasma atomizes the sample, ionizes the atoms and the ions are directed into a mass analyzer for the separation of ions based on their mass to charge ratio (//m/z//).
 ### ###
 +{{anchor:figure_86}}
  
 {{:textbook:nrctextbook:components_of_an_icpms_system_fig_8_6.png|}} {{:textbook:nrctextbook:components_of_an_icpms_system_fig_8_6.png|}}
Line 256: Line 258:
  
 ### ###
-The mass analyzer is either quadrupole or double focusing system. The former is smaller, cheaper and easier to operate. The latter, however, is much more sensitive yielding to lower detection limits and to more accurate isotopic ratios. A quadrupole consists of four metallic rods aligned in a parallel diamond pattern. By placing a direct current field on one pair of opposite rods and a radio frequency field on the other pair, an ion of a selected mass and charge ratio (//m/z//) is allowed to pass +The mass analyzer is either quadrupole or double focusing system. The former is smaller, cheaper and easier to operate. The latter, however, is much more sensitive yielding to lower detection limits and to more accurate [[textbook:nrctextbook:chapter2#isotope|isotopic ratios]]. A quadrupole consists of four metallic rods aligned in a parallel diamond pattern. By placing a direct current field on one pair of opposite rods and a radio frequency field on the other pair, an ion of a selected mass and charge ratio (//m/z//) is allowed to pass 
 through the rods to the detector while the others are forced out of this path. By varying the combinations of voltages and frequency, an array of different //m/z// ratio ions can be scanned in a very short time. The high-resolution double focusing system in turn consists of an electromagnet and an electrostatic analyzer in series. After mass separation the ions are detected and counted. through the rods to the detector while the others are forced out of this path. By varying the combinations of voltages and frequency, an array of different //m/z// ratio ions can be scanned in a very short time. The high-resolution double focusing system in turn consists of an electromagnet and an electrostatic analyzer in series. After mass separation the ions are detected and counted.
 ### ###
Line 262: Line 264:
  
 ### ###
-Some radionuclides, such as uranium, can be measured from natural waters directly with ICP-MS without chemical separation of interfering elements. Most radionuclides, however, need to be separated into a pure form prior to measurement. The separation requirements may essentially differ from those used in radiochemical separations for radiometric measurements. For example, if plutonium is measured by alpha spectrometry 1% of uranium activity in the counting source does not result in a large error. In mass, however, this 1% activity means about 2000-times excess of <sup>238</sup>U  +Some [[textbook:nrctextbook:chapter4|radionuclides]], such as uranium, can be measured from natural waters directly with ICP-MS without chemical separation of interfering elements. Most radionuclides, however, need to be separated into a pure form prior to measurement. The separation requirements may essentially differ from those used in radiochemical separations for radiometric measurements. For example, if plutonium is measured by [[textbook:nrctextbook:chapter11|alpha spectrometry]] 1% of uranium [[textbook:nrctextbook:chapter6#activity|activity]] in the counting source does not result in a large error. In mass, however, this 1% activity means about 2000-times excess of <sup>238</sup>U compared to <sup>239</sup>Pu which would prevent the measurement of plutonium. In mass spectrometry there are three types of interferences that need to be taken into account when measuring radionuclides. First, the [[textbook:nrctextbook:chapter2#isobar|isobars]] with approximately same mass cause interference, for example<sup> 129</sup>Xe in <sup>129</sup>I measurement and <sup>135</sup>Ba in <sup>135</sup>Cs measurement. Second, in the plasma there are not only single atoms formed but also polyatomic ions such as <sup>204</sup>Hg<sup>35</sup>Cl or <sup>238</sup>UH which interfere with the measurement of <sup>239</sup>Pu having approximately the same mass. Even though only a small fraction of the total elemental concentrations forms these polyatomic ions <sup>204</sup>Hg<sup>35</sup>Cl or <sup>238</sup>UH their concentrations are 
-compared to <sup>239</sup>Pu which would prevent the measurement of plutonium. In mass spectrometry there are three types of interferences that need to be taken into account when measuring radionuclides. First, the isobars with approximately same mass cause interference, for example<sup> 129</sup>Xe in <sup>129</sup>I measurement and <sup>135</sup>Ba in <sup>135</sup>Cs measurement. Second, in the plasma there are not only single atoms formed but also polyatomic ions such as <sup>204</sup>Hg<sup>35</sup>Cl or <sup>238</sup>UH which interfere with the measurement of <sup>239</sup>Pu having approximately the same mass. Even though only a small fraction of the total elemental concentrations forms these polyatomic ions <sup>204</sup>Hg<sup>35</sup>Cl or <sup>238</sup>UH their concentrations are +
 nevertheless much higher than that of plutonium due to the greater abundances of the polyatomic ion forming elements, in this case Hg, Cl and U. Therefore, chemical separations are needed to enable measurement of radionuclides at very low concentrations. Third type of interference comes from broadening the neighbor mass peaks at higher concentration. Due to this, for example, <sup>238</sup>U at  nevertheless much higher than that of plutonium due to the greater abundances of the polyatomic ion forming elements, in this case Hg, Cl and U. Therefore, chemical separations are needed to enable measurement of radionuclides at very low concentrations. Third type of interference comes from broadening the neighbor mass peaks at higher concentration. Due to this, for example, <sup>238</sup>U at 
 much higher concentrations compared to plutonium causes extra counts to the mass peak of <sup>239</sup>Pu. much higher concentrations compared to plutonium causes extra counts to the mass peak of <sup>239</sup>Pu.
Line 270: Line 271:
  
 ### ###
-ICP-MS is increasingly used for the measurement of long-lived radionuclides, especially actinides. For neptunium it is clearly superior to radiometric methods due to its low specific activity. In plutonium measurement, mass spectrometry also offers a change to measure individually <sup>239</sup>Pu and <sup>240</sup>Pu which cannot be separated from each other in alpha spectrometry. In turn, <sup>238</sup>Pu cannot be +ICP-MS is increasingly used for the measurement of [[textbook:nrctextbook:chapter6#long_lived_radionuclides|long-lived radionuclides]], especially actinides. For neptunium it is clearly superior to radiometric methods due to its low [[textbook:nrctextbook:chapter6#specific_activity|specific activity]]. In plutonium measurement, mass spectrometry also offers a change to measure individually <sup>239</sup>Pu and <sup>240</sup>Pu which cannot be separated from each other in [[textbook:nrctextbook:chapter11|alpha spectrometry]]. In turn, <sup>238</sup>Pu cannot be measured by mass spectrometry due to interference of uranium. Thus, to determine all relevant plutonium isotopes both mass and alpha spectrometry are needed.
-measured by mass spectrometry due to interference of uranium. Thus, to determine all relevant plutonium isotopes both mass and alpha spectrometry are needed.+
 ### ###
  
textbook/nrctextbook/chapter8.1744808404.txt.gz · Last modified: 2025-04-16 15:00 by Merja Herzig

Page Tools

A-CINCH Consortium

email: mst@evalion.cz | tel: +420 224 358 331 | Copyright © 2021 A-CINCH
This project has received funding from the Euratom research and training programme 2019–2020 under grant agreement No. 945301.