1. Neutron Activation Analysis

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 γ-ray measured
If the Prompt γ-ray is the measured radiation
Prompt γ -ray neutron activation analysis (PGNAA)
The measurements take place during irradiation.
If Delayed γ-ray is the measured radiation.
Delayed γ -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 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,α), 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.

26. Advantages of NAA Very low detection limits for 30–40 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.

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

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.

35. Thank you