Neutron Activation Analysis

1
Neutron Activation Analysis

• A Lecture for the International Workshop on
• Nuclear Science and Education
• By
• Nader M. A. Mohamed
• Atomic Energy Authority, ETRR-2
• Cairo, Egypt
• Email nmahmoud_at_etrr2-aea.org.eg

2
What is Neutron Activation Analysis (NAA)?

• NAA is a method for qualitative and quantitative
  determination of elements based on the
  measurement of characteristic radiation from
  radionuclides formed directly or indirectly by
  neutron irradiation of the material.

3
NAA Method
4
NAA Categories

• According to type of emitted ?-ray measured
• If the Prompt ?-ray is the measured radiation
• Prompt ? -ray neutron activation
  analysis (PGNAA)
• The measurements take place during irradiation.
• If Delayed ?-ray is the measured radiation.
• Delayed ? -ray neutron activation analysis
  (DGNAA)
• The measurements take place after a certain
  decay period.
• (DGNAA) is more common.

5
I. PGNAA

• The PGNAA technique is generally performed by
  using a beam of neutrons extracted through a
  reactor beam port.
• detectors are placed very close to the sample
  compensating for much of the loss in sensitivity
  due to flux.
• The PGNAA technique is most applicable to
  elements with extremely high neutron capture
  cross-sections (B, Cd, Sm, and Gd) elements
  which decay too rapidly to be measured by DGNAA
  elements that produce only stable isotopes or
  elements with weak decay gamma-ray intensities.

6
II. DGNAA

• DGNAA (sometimes called conventional NAA) is
  useful for the vast majority of elements that
  produce radioactive nuclides.
• The technique is flexible with respect to time
  such that the sensitivity for a long-lived
  radionuclide that suffers from an interference by
  a shorter-lived radionuclide can be improved by
  waiting for the short-lived radionuclide to
  decay.
• This selectivity is a key advantage of DGNAA
  over other analytical methods.

7
Instrumental vs. Radiochemical NAA

• It is generally possible to simultaneously
  measure more than thirty elements in most sample
  types without chemical processing.
• The application of purely instrumental
  procedures is commonly called instrumental
  neutron activation analysis (INAA) and is one of
  NAA's most important advantages over other
  analytical techniques.
• If chemical separations are done to samples
  after irradiation to remove interferences or to
  concentrate the radioisotope of interest, the
  technique is called radiochemical neutron
  activation analysis (RNAA).

8
NAA procedure

• Sampling
• Pre-irradiation sample treatment (such as
  cleaning, drying or ashing, pre-concentration of
  elements of interest or elimination of
  interfering elements, sub-sampling and packing)
• Irradiation (and prompt gamma-ray counting in
  PGNAA)
• Radiochemical separation (only in RNAA)
• Radioactivity measurement
• Elemental concentration calculation
• Critical evaluation of results and preparation of
  the NAA report.

9
Irradiation

• There are several types of neutron sources
  reactors, accelerators, and radioisotopic neutron
  emitters.
• Nuclear reactors with their high fluxes of
  neutrons offer the highest available
  sensitivities for most elements.
• Most neutron energy distributions are quite broad
  and consist of three principal components
  (thermal, epithermal, and fast).

10
Neutron Energy Distribution
11
I. Thermal Flux

• The thermal neutron component consists of
  low-energy neutrons (energies below 0.5 eV) in
  thermal equilibrium with atoms in the reactor's
  moderator.
• At room temperature, the energy spectrum of
  thermal neutrons is best described by a
  Maxwell-Boltzmann distribution with a mean energy
  of 0.025 eV and a most probable velocity of 2200
  m/s.
• In most reactor irradiation positions, 90-95 of
  the neutrons that bombard a sample are thermal
  neutrons.

12
II. Epithermal Flux

• The epithermal neutron component consists of
  neutrons (energies from 0.5 eV to about 0.5 MeV)
  which have been only partially moderated.
• A cadmium foil 1 mm thick absorbs all thermal
  neutrons but will allow epithermal and fast
  neutrons above 0.5 eV in energy to pass through.
• In a typical unshielded reactor irradiation
  position, the epithermal neutron flux represents
  about 2 the total neutron flux.
• Both thermal and epithermal neutrons induce
  (n,gamma) reactions on target nuclei.
• An NAA technique that employs only epithermal
  neutrons to induce (n,gamma) reactions by
  irradiating the samples being analyzed inside
  either cadmium or boron shields is called
  epithermal neutron activation analysis (ENAA).

13
III. Fast Flux

• The fast neutron component of the neutron
  spectrum (energies above 0.5 MeV) consists of the
  primary fission neutrons which still have much of
  their original energy following fission.
• Fast neutrons contribute very little to the
  (n,gamma) reaction, but instead induce nuclear
  reactions where the ejection of one or more
  nuclear particles - (n,p), (n,a), and (n,2n) -
  are prevalent.
• In a typical reactor irradiation position, about
  5 of the total flux consists of fast neutrons.
• An NAA technique that employs nuclear reactions
  induced by fast neutrons is called fast neutron
  activation analysis (FNAA).

14
Radioactivity Measurement

• The instrumentation used to measure gamma rays
  from radioactive samples generally consists of a
  semiconductor detector, associated electronics,
  and a computer-based, multi-channel analyzer
  (MCA/computer).
• Most NAA labs operate one or more hyper pure
  germanium detector (HPGe).

15
Gamma-Spectroscopy System
16
Calibration

• Energy Calibration
• FWHM Calibration
• Efficiency Calibration

17
Gamma-Ray Spectra (Short Irradiation)
18
Gamma-Ray Spectra (Long Irradiation)
19
Gamma-Ray Spectra (Long Irradiation)
20
Elemental Concentration Calculation

• Basically there are two standardizations methods
  used in NAA
• - The relative method
• - The non-relative method.

21
The Relative Method

• Sample and element standards are irradiated
  simultaneously and later measured under the same
  counting conditions.
• The relative method promises the highest accuracy
  when the standard and sample match each other
  well in composition, irradiation and counting
  conditions.

22
The Non-Relative Method

• Multi-element INAA is feasible in the
  non-relative method or single comparator method.

23
Detection Limits

• The detection limit represents the ability of a
  given NAA procedure to determine the minimum
  amounts of an element reliably.
• The detection limit depends on the
• (1)The amount of material to be irradiated and to
  be counted..
• (2) The neutron fluxes.
• (3) The duration of the irradiation time.
• (4) The total induced radioactivity that can be
• (5)The duration of the counting time
• (6) The detector size, counting geometry and
  background shielding.

24
(No Transcript)
25
(No Transcript)
26
Advantages of NAA

• Very low detection limits for 3040 elements,
• Significant matrix independence,
• The possibility of non-destructive analysis
  (instrumental NAA or INAA),
• The use of radiochemical separation to overcome
  interference in complex gamma-ray spectra
  (radiochemical NAA or RNAA),
• An inherent capability for high levels of
  accuracy compared to other trace element analysis
  techniques.

27
(No Transcript)
28
NAA Applications

• Archaeology
• Biomedicine
• Environmental science and related fields
• Geology and geochemistry
• Industrial products
• Nutrition
• Quality assurance of analysis and reference
  materials

29
NAA at ETRR-2

• - Egypt Second Research Reactor (ETRR-2), a 22
  MW light water moderated and cooled open pool
  nuclear reactor.
• - There are many manually loaded sites for
  irradiation of samples for several hours.
• - For analysis that require short lived
  radionuclides to be measured there are two
  computer pneumatic irradiation transfer systems
• - The detectors used to measure gamma rays from
  irradiated samples are two coaxial HPGe detectors
  and a Compton suppression system

30
NAA at ETRR-2 (Con.)

• Instrumental Activation Analysis about 300
  samples (geological, biological, industrial),
  per year.
• Measurements of topaz activities about 1200
  samples (each 1 kg) per year.
• Flux Mapping after core refueling and for the
  reactor facilities.
• Reactor waste analysis.
• Measurements of radioisotopes produced at the
  reactor (100 Co-60 sources in 2005 and 52 sources
  in 2007).
• Participation in research projects
• - Arab Atomic Energy Authority coordinating
  project Study of trace elements in cancerous
  samples
• - AFRA IV-7 Research Reactor Project for
  Socio-economic.
• - The lab participated in three proficiency
  tests (2003, 2005 2007)

31
Samples analyzes by NAA during the last six years
32
(No Transcript)
33
Large Sample-NAA

• The problem of representativeness of the sample
  when dealing with inhomogeneous bulk material
  affects all currently available analysis
  techniques.
• Most techniques do not allow for large samples
  (kg level) because the activating signal or the
  response (or both) cannot penetrate samples of
  that size, or the technique is destructive and
  cannot handle such large amounts.
• NAA, though, has highly penetrating neutrons as
  incoming signal and highly penetrating gamma-rays
  as signal to be detected. This makes NAA (in
  principle) a suitable technique for the analysis
  of such large samples.
• The large sample in INAA could be defined as a
  test portion in which neutron and gamma-ray
  self-attenuation can not be neglected in view of
  the required degree of accuracy. Hence, such a
  large sample would range from a few grams to
  several kilograms and above.

34
References

• IAEA-TECDOC-1215, Use of research reactors for
  neutron activation analysis, 1998.
• http//web.missouri.edu/glascock/naa_over.htm

35
Thank you