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A non-radioactive sample can sometime become radioactive   after   bombardment with particle or gamma   radiations. Activation   Analysis:	 An analytical technique in which radiation   is  induced by   bombardment Photon Activation Analysis:  The method which utilizes   gamma  rays	 for  bombardment is   PAA.

Neutron Activation Analysis : The method which utilizes   neutron  for bombardment is   NAA -lack of charge and the mass of a neutron allows   efficient penetration and energetic transfer to the   nucleus.

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What is  Neutron Activation Analysis   (NAA)?

Powerful analytical technique to analyze the sample i.e., to identify the elements present in the sample both qualitatively and quantitatively.  Principle is based on converting various elements of the sample to radioactive isotopes by irradiating with neutrons in a nuclear reactor by neutron capture.

Radioactive isotopes so formed decay according to their characteristic half lives varying from seconds to years emitting gamma radiations with specific energies .   Examples are Natural Sodium  23 Na, natural copper  63 Cu through (n,  γ ) reactions.

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Characteristic   γ  rays emitted subsequently measured with semi conductor  γ -ray spectrometers, to identify the source of these elements. Technique based on measurement of prompt  γ -radiations subsequent to neutron induced nuclear reactions, the technique is also called neutron induced prompt  γ  activation analysis (PGAA or PGNAA) .

NAA useful method for simultaneous determination 25-30 major, minor and trace elements in geological, environmental and biological samples in ppb-ppm range with or without chemical separation.

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NAA sensitive technique for multi-elemental analysis of various elements from various fields of scientific or technical interest.

Combined with computerized high resolution  γ -ray spectrometry, it offers  multi-elemental routine analysis.

Compared to well developed atomic analytical methods like PIXE and XRF, its still preserving its role because it is non destructive and instantaneous.

Favourable characteristics are negligible matrix effect, excellent selectivity and high sensitivity.

Sensitivities are superior to those attainable by other methods (ppb) for many elements and applications.

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Special capabilities of IBA methods like RBS, ERD etc. give them ability to provide information about surface composition and elemental distribution yet NAA considered to be more mature and most widely applied.

Standards prepared from certified solutions of known concentration of elements of interest.

Certified solutions may be purchased from different suppliers across the world.

Orivini  and  Speziali  (2001) reviewed applicability and limitations up to year 2000..

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NAA  was  discovered in 1936  by  George Charles de  Hevesy   (Hungary)  and Hilde Levi   (Denmark). While studying the action of slow neutrons on rare earth elements, it was found that the element ( Dy ) became highly radioactive after exposure to neutrons.

Hevesy   and Levi   then detected  Dy  in Y matrix through this method.

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Principle is based on nuclear reaction, specifically neutron capture and subsequent  γ -emission through beta decay, called (n,  γ ) reaction .

The sample  is  bombarded  with  neutrons,  neutrons interact with target nucleus through non elastic collision. Compound nucleus (in an excited state) is then formed, the excitation energy of which due to binding energy of the neutron with the nucleus. Compound nucleus almost instantaneously de-excite in to more stable configuration through emission of one or more characteristics prompt  γ -rays. In many cases, the new configuration yields a radioactive nucleus which also de-excite (decays) by emission of one or more characteristics delayed  γ -rays (at much slower rate) according to unique half life (fraction of seconds to several years).

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Probability of  Nuclear Reactions  during  Neutron   Bombardment

Probability	 α	 cross-sectional area  of  target   (σ) The  net  rate (r) at  which  a target reacts  to  produce radioactive atoms  while  being  bombarded on all sides  with  neutron is given   by

Rate  of  decay  of   Formed   radionuclide Rate  of  formation  of   radionuclide Φ  = neutron  flux  (particles/cm 2 .s),  N  =  no  of nuclei  in  the sample,  N*  = the  no  of radionuclide  formed by  the  reaction during  time  t ,  σ  = cross-section area of target (cm 2 ),  λ =  decay  constant   (s -1 )

The no. of radionuclide   ( N* )  present at the end of   the irradiation is  given   by:

The activity  (A)  of  radionuclide  produced when the bombardment   is stopped is  given   by:

This  equation  shows   that   Activity  ( A ) increased as   bombardment time  ( t )  increases

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Prompt-gamma neutron activation analysis (PGAA ):  N uclear decay products (gamma  rays   or particles)  are  measured during neutron   irradiation PGNAA is characterized by short irradiation times and short decay times, often in the   order   of seconds and   minutes. PGNAA is generally applied   to: elements  with  extremely high neutron capture cross-sections (B, Cd, Sm, 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.

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Delayed gamma neutron activation analysis (DGNAA) :  N uclear decay products (gamma   rays  or particles)  are  measured at some time after   irradiation . More common. When one mentions NAA, generally assumed one refers to measurement of delayed  γ -rays. DGNAA is characterized by long irradiation times and long decay times (hours, weeks   or   longer) DG NAA  analyses are often performed over  days,  weeks or even   months. DGNAA is applicable to the vast majority of elements that form artificial radioisotopes. This  improves sensitivity for long-lived radionuclides as it allows short-lived radionuclide to   decay,  effectively eliminating   interference. About 70% elements have properties suitable for measurement by NAA.

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Qualitative Characteristics : 			Energies of emitted  γ -rays (E) 			Half life of the nuclide  ( T 1/2 ) Quantitative Characteristics : 	Intensity (I), which is equal to number of  γ  quanta of Energy E 	measured per unit time. Ahmad (1983) epithermal neutron activation data for  46 Sc,  51 Ti,  51 Cr,  52 V,  56 Mn,  59 Fe,  60 Co,  75 Se,  86 Rb,  95 Zr,  97 Zr,  124 Sb,  131 Ba,  134 Cs,  140 La,  141 Ce,  160 Tb,  181 Hf,  182 Ta, and  198 Au. Prof.  P.N.Trehan  made significant contribution measuring energies and intensities of various  γ  transitions emitted by different radionuclides. Data is being updated from time to time.

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PGNAA performed by irradiating the sample using a beam of neutrons (from the reactor beam port) and simultaneous measurement of the characteristics  γ -rays emitted by various short lived radionuclides (so formed), to determine the concentration of various elements in the sample.  DGNAA more sensitive (for most elements) cannot be used for sample elements which can be analyzed by PGNAA. Average K.E. (100 keV-10MeV) of fast neutrons produced in nuclear reactor can be lowered by passing the beam through a moderator. Collision with atoms of moderator changes these fast neutrons to epithermal  (E 1keV) and thermal (E 0.025eV).

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Application of NAA activation analysis can be enhanced in some way using sub-thermal i.e. cold neutrons ( E ˂10meV), the same can be produced by using cryogenic moderators composed of solid or liquid methane at temperature (20-110K) Guided cold neutron beams provide comparable neutron flux and lower background than thermal neutron beams, the DL of neutron absorption experiments such as PGNAA is decreased . This decrease DL mainly due to decrease in  γ  –background. Flux on the sample irradiated by neutron beam is of the order of one million times lower than that on the samples inside a reactor, the detector can be placed very close to the sample compensating for much of the loss in sensitivity due to flux.

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PGNAA  more 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 stable isotopes only Or Elements with weak  γ -ray intensities DGNAA  (sometimes called conventional NAA) useful for vast majority of elements that produce radioactive nuclides. DGNAA  is flexible with respect to time such that the sensitivity for a long lived radionuclide that suffers from interferences by a shorter lived radionuclide can be improved by waiting for the short lived radionuclide to decay which is a key advantage of DGNAA over other analytical methods.

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NAA makes use of slow neutrons. The epithermal and fast neutrons may also be used however. NAA technique that only employs epithermal neutrons to induce (n,  γ ) reactions by irradiating the samples being analyzed inside either cadmium or boron shields is called epithermal neutron activation analysis (ENAA). NAA technique that only employs nuclear  reactions induced by fast neutrons is called fast neutron activation analysis (FNAA).

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Destructive  ( radiochemical   ) The sample is chemically manipulated after bombardment but before   counting 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) the resulting radioactive sample may be chemically decomposed, and the elements are  chemically separated  (chromatography,  ion-exchange, extraction, electrochemical  separation) Nondestructive  ( instrumental   ) the resulting radioactive sample  is  kept   intact 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.

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Sample matrix plus standard irradiated for select period of time in the neutron flux. After irradiation and appropriate radioactive decay, energy spectrum is measured by counting the sample with a high resolution gamma detection system (to separate various gamma transitions of close-by energies). Provides highly resolved analysis. Quantitative analysis is provided by element to element comparison of the number of gamma rays emitted per unit time by the unknown sample to the number of gamma rays emitted per unit time by the calibration curve.

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NAA can be applied to most sample matrices without any pretreatment of the sample including; 	Solids such as coal, metals, sediments, ores, tissue, bone and 	synthetic fiber. 	Liquids such as blood, water, manufacturing wastes, oil, gasoline. 	Gases such as argon, chlorine and fluorine 	Suspension and slurries such as sewage sludge, river water or 	foods Elemental concentration is measured in Nano grams, micrograms or milligrams per gram or milliliter of unknown sample.

NAA   procedure  (Experimental)  conti …………..

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General procedure into three steps: preparing and irradiating of the samples, performing measurements and Interpreting the results. Step 1: Involving pulverizing, homogenizing, mass determination, packing and preparation of standards. About 50-200 mg of the sample material packed in high purity polyethylene capsules. Maximum of 10-12 capsules are filled with sample material together with a capsule filled with certified reference standard and an empty capsule (the blank) are sealed together in polyethylene foil. Whole package is packed in an irradiation container and irradiated by shooting it to a position close to the reactors core. Transportation time is 10-15 seconds Both unknown and standard are irradiated (few seconds to few hours) in nuclear reactor to produce prompt gamma radiations, characteristics of the parent isotope.

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Step 2: After irradiation and appropriate radioactive decay (both sample and standard), these are measured on either a well-type or a co-axial large volume  Ge (Li)  γ -ray detector system coupled to a PC based  γ  spectroscopy system. For analysis, the information about the  γ -radiations emitted by various radionuclides must be available at hand . Depending on the elements of interest, a waiting time is applied in order to let the nuclides with shorter half-life times decay. Which gives a lower background and less interference. Samples are usually measured for 1h as goes for the reference material and blank.

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Step 3: Using a computer software, the peaks are fitted, their peak areas are determined to calculate the abundance of various elements present in the sample .

In a full analysis i.e. the prompt as well as delayed gamma emission analysis, the short lived nuclides are determined first. In actual practice the most intense gamma radiations are measured first and then observation about their decrease of intensity is made.

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Several types of neutron sources; Radio isotopic neutron emitters like  239 Pu-Be. Particle accelerators (to produce high flux of neutrons through  ( p,n ), ( d,n ),  (α,n) or other reaction. Nuclear reactors (which are sources of neutrons due to fission reaction.

Nuclear reactors with their high fluxes of neutrons from uranium fission offer the highest available sensitivities for most elements.  Different types of reactors and different positions within a reactor can vary considerably with regard to their neutron energy distribution and fluxes due to the materials used to moderate the primary fission neutrons.

Neutron beam available at the nuclear reactor or accelerator site cannot be used in the filed investigations. Fission neutron sources like  252 Cf or the D-D/D-T fusion neutron sources or radio isotopic neutron sources can be used for filed applications.

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An An  actinide such  as  californium ( 252 Cf) which  emits  neutrons through   spontaneous   fission Neutron   generators Nuclear   reactors An alpha  source such  as radium or  americium, mixed  with beryllium; this   generates neutrons by a (α,12C+n)   reaction

An gamma  source  mixed  with beryllium or deuterium; the  gamma  radiation induces  neutron  emission  from Be or   Deuterium 9 Be + ɤ	  	 8 Be  +	 1 n +  (-Q) 4	4	0

2 H   + ɤ	  	 1 H   +	 1 n +   (-Q) 1	1	0

Q= E emitted for each nucleus of the target during   rxn  Exoergic  reaction: E is produced (Q=   +ve) Endoergic reaction:  E  is absorbed (Q=   -ve)

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Fission Neutron   sources Fusion type D-D/D-T Neutron generators Neutron beam using radioisotopes through  (α, n) and  (n,  γ ) reactions  Neutron beam using Protons, Deuterons or other charged particles from particle accelerators Neutron beam from reactors

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Spontaneous fission of some artificially produced transuranium isotopes can be applied as small neutron source. For example; 252 Cf (half life 2.6 y) undergoes fission, producing 3.76 neutrons of 1.5 MeV per event.  252 Cf  ( α -decay for ~96% and spontaneous emission ~4%) is very strong neutron emitter (one mg emits  2.28 × 10 9  neutrons per second.

Major advantage of this neutron source is that it can be made portable and generate a stable neutron flux. However as the neutron flux is rather low in comparison to a nuclear reactor, its use in NAA is limited to the determination of elements of high activation cross-section which are in major concentrations.

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These type of neutron generators are small accelerators which produce neutrons using the deuterium-tritium or deuterium-deuterium reactions . Neutron output from these generators may be pulsed or continuous. Yield of this source is between 1 × 10 12  and 6 × 10 12  neutrons per second.

At the Japan atomic energy research institute, D-T neutron source has been applied to conduct benchmark experiments on silicon carbide.

Neutron beam using radioisotopes through (α,n) and (γ,n) reactions

Neutron beam can be produced from  (α,n) reaction using α-radioactive sources and Be-target or using photon sources interacting on Be-D 2 O targets.

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Neutron beam using Protons, Deuterons or other charged particles from particle accelerators

Particle accelerators are used to produce neutrons whereby a convenient target material is bombarded by accelerated charged particles and the neutrons are produced in a nuclear reaction. In the most frequently used and commercially available neutron generators, the accelerated deuterons are bombarded on the tritium ( 3 H) target. Nuclear reaction carried out is  3 H( d,n ) 4 He . The neutron flux generated is approximately 10 9  neutrons cm -2  s -1 . 7 Li( p,n ) 7 Be reaction has proved to be useful due to its relatively high yield and very narrow energy spread. To analyze silica and rocks for Si and Al concentrations, the neutron beam such as  28 Si(n,  γ ) 28 Al was produced by a suitable choice of proton energy and target thickness.

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Neutron reactors produce neutrons due to the phenomenon of fission. The flux of neutrons produced by the reactors is of the order of 10 12  - 10 15  n cm -2  s -1 .

Since the thermal neutrons are required for NAA due to their maximum absorption cross-sections, the neutron beam has to be thermalized as the neutron beam from the reactor consists of thermal, epithermal and fast neutrons.

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Thermal  :  0.025   eV-0.5   eV	 (NAA): Most often used  in  NAA Epithermal : 0.5  eV-100  keV  used in Epithermal NAA   (ENAA) Fast:  0.5-25  MeV  used  Fast  NAA   (FNAA) 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.

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Irradiated samples are analyzed ; Scintillation Detectors NaI ( Tl ) Scintillation Detector

Semiconductor detectors Ge (Li) and  HpGe  Semiconductor detectors

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NaI ( Tl ) scintillation detector consists of thallium ( Tl ) activated  NaI  crystal (having a small amount of impurity of  Tl ) which is optically coupled to a photomultiplier. As the  γ -rays interact with the scintillator material (through the process of photoelectric effect, Compton effect and pair production), the optical photons are produced due to fluorescence/phosphorescence.

These light photons are then converted to an electrical pulse by the photomultiplier tube or a photodiode. High atomic number of iodine in  NaI  gives good efficiency for  γ -ray detection. Role of impurity is to produce luminescent  centres .

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NaI ( Tl ) Scintillation  Detect or (Conti……)

Photomultiplier tube consists of a photocathode, a focusing electrode and 10 or more dynodes that multiply the number of electrons striking them several times each. The commercially available  NaI ( Tl ) detector include a high resolution  NaI ( Tl ) crystal, a photomultiplier tube, an internal magnetic/light shield and a chrome plated aluminum housing.

Other inorganic crystals for  γ  detection can be  CsI ( Tl ),  CsI (Na),  LiI ( Eu ), Be 4 Ge 3 O 12  i.e.  Bismith   Germanate  (BGO),  ZnS (Ag), and CdWO 4 .  Energy conversion efficiency for  NaI ( Tl ) is 12%, for  CsI (Na) is 10% and is 5% for  CsI ( Tl ). Since light output is low, energy resolution is also low.

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Ge (Li) and  HpGe  semiconductor Detectors

Ge (Li) and hyper-pure  Ge  ( HpGe ) detectors are semiconductor detectors. These are P-I-N diodes, in which intrinsic region is created by depletion of charge carriers. When a photon interacts in the intrinsic region, tracks of electron hole pairs are produced (analogous to electron-positive ion pairs in a counting gas.

In the presence of the electric field, these pairs separate and are swept rapidly to their respective collecting electrodes by the electric field.

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Ge (Li) and  HpGe  semiconductor Detectors (Conti…..)

Resultant charge is integrated by a charge sensitive pre-amplifier and converted to a voltage pulse with amplitude proportional to the original photon energy. To keep the leakage current low, the detector must have very few electrically active impurities. For example,  Ge -detectors are made from zone refined crystals that have lower than  10 10  electrically active impurities cm 3 .

The attenuation of  γ -rays in a material is caused by their interaction due to Photoelectric, Compton and pair production effects.  Very pure material are needed to ensure high counting efficiency for high energy photons.

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In intrinsic germanium detectors (called  HPGe  detectors) with impurity concentrations less than 10 10  atoms cm -3  have the advantage of not having to be kept at LN 2  temperature at all times.   HPGe  detector is used for low energy  γ -ray detection .

In LN 2  cooled detectors, the detector element are housed in a clean  vaccum  chamber, which is attached to or inserted in a LN 2  Dewar.  The detector is in thermal contact with the liquid nitrogen, which cools it to around 77  o K  or -200  o C.  At these temperatures, reverse leakage currents are in the range of 10 -9  – 10 -10  amperes.

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Semiconductor detectors provide greatly improved energy resolution over other types of radiation detectors.   Fundamentally resolution advantage can be attributed to the small amount of energy required to produce a charge carrier and the consequent large output signal relative to other detector types for the same incident photon energy.

At 3  eV  per e-h pair, the number of charge carriers produced in  Ge  is about one and two orders of magnitude higher than in gas and scintillations detectors respectively.  While  Ge (Li) detectors are primarily used for  γ -ray spectroscopy in the energy range of 50  keV  to 10 MeV, the pure  Ge  detectors have inherently good response down to very low energies.

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Irradiated samples are analyzed by   gamma-spectrometry

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Instrumentation used to measure  γ -rays from radioactive samples consists of a semiconductor detector, associated electronics and a computer based multichannel analyzer as shown in figure.

Most of the laboratories possesses  HpGe  or  Ge (Li) detectors which are used for the measurement of  γ -rays with energies over the range from about 60  keV  to 3.0 MeV.

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Gamma-ray   Spectroscopy  (Conti………)

The two most performance characteristics when purchasing a detector are its resolution and efficiency. Other characteristics are peak shape, peak to Compton ratio, crystal dimensions or shape and price. The detectors resolution is a measure of its ability to separate closely spaced peaks in a spectrum.

Detector efficiency depends on the energy of the measured radiation, the solid angle between sample and detector crystal and the active volume of the crystal. A large volume detector have higher efficiency. As detector volume increases, the detector resolution however gradually decreases. For most NAA applications, an  HpGe  detector of 15-30% efficiency is adequate.

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Since  HpGe  and  Ge (Li) detectors require liquid nitrogen for cooling, these have the limitations to be used for field study. NaI ( Tl ) detectors do not require cooling but have the limitation of relatively poor resolution. For field applications e.g., the verification of nuclear material, the  CdZnTe  detectors have advantages of better resolution over  NaI ( Tl ) detectors. However since the  CdZnTe  detectors exhibit asymmetric peak shapes  particulary  at higher energies are difficult to use.

For inter-comparison purposes of  γ -ray spectra or for testing the computer software for  γ -analysis,  γ -spectrum are created using  226 Ra i.e., decay chain products of  226 Ra including  222 Rn gas and other isotopes like  214 Pb,  214 Bi,  210 Pb and  210 Bi etc.

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Qualitative and Quantitative Analysis by   NAA

Modern instruments generally monitor  the  gamma emission  from the   sample. Stage 1: Bombardment  for  a fixed  t   Stage 2: Decay  for  a certain  time Stage 3: Recording  the  spectrum  (with  a Ge(Li) detector  which  detects the   ɤ  photons  and resolve them into spectral   peaks Qualitative   Analysis : The E  of emitted ɤ-rays is characteristic  of  the change in nuclear energetic  levels  for  a particular   nuclide. Emission occurs  at  discrete energies  which  can be used  to  identify   the emitting   nuclide. Peak Energy  is thus used  for  qualitative   analysis In  other  words,  compare ɤ-rays spectrum  of  sample  with  those  of  known   nuclides  for  qualitative   analysis Quantitative  Analysis :  Peak Areas  is  used  for  qua nti tative   analysis

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The  relative   method The non-relative   method The  Relative   Method : The Sample and element standard(s) are simultaneously bombarded  with  neutron and   later measured under the same counting conditions (sample-to-detector distance, sample size,  composition (if possible)). Normally a single standard is used for quantitative   analysis. The peak areas of the sample and the standard are used to calculate the number of nuclei of  a particular  type  in the   sample. A direct proportionality can be assumed between the peak area and no. of nuclei  when  equal  irradiation times and neutron fluxes are used  (when  sample and standard are simultaneously  bombarded) The relative method promises the highest accuracy  when  the standard and sample   match

Quantitative  Analysis :  Peak Areas  is used  for  qualitative  analysis   Basically there are  two  standardizations methods used in   NAA:

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Non-destructive   analysis The chemical form and physical state of the elements do not  influence the activation and decay process as the vast majority of  samples are transparent to both the probe (neutron) and the  analytical signal (the gamma   ray). Multi-element analytical   technique H, C, O, N,  P,  and Si (matrix –forming elements) hardly form any  radioactive isotopes (so  less  matrix  effect  in   INAA) Suitable even for determination of masses in the order of  10 –6 –10 –9    g and less In certain fields, NAA is irreplaceable (e.g., analysis of solid material  which are difficult to dissolve or when samples have low trace  element concentrations, analysis where high degree of accuracy   is

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Detection  Limits : The  detection limit (DL) represents  the  ability  of  an analytical  method  to  determine   the  minimum	amounts  of  an element reliably  (with  a  specified precision and reproducibility).  In  other  words,  the  DL is  the  lowest   detectable level  of  analyte  distinguishable  from   zero In NAA,  the detection limit relates  to the  ability  of  detecting a gamma-ray  peak in  the  presence  of  interference  from  natural radioactivity and other  radioactivities induced by neutron activation.  A  peak is detected  when  it  is  distinguishable  from the  uncertainty in this noise level. This signal-to-noise  situation makes that  the  DL is related  to the  square root  of  the noise or  background   level. The  detection limit  depends   on: The  amount  of  material  to  be irradiated and  to  be   counted The  neutron   fluxes The  duration  of the  irradiation  time. The total  induced   radioactivity The  duration of  the  counting   time The  detector size, counting   geometry

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Sensitivity and  Detection  limits of   NAA

The  sensitivity obtained  by NAA  is a function of  nuclear parameters  of the  element in question (neutron  cross section,  isotopic abundance,  half-life,   gamma-rays abundance), available  neutron  flux ,  Length  of  irradiation ,   detector efficiency ,  matrix  compositions ,  and  the total  sample   size

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Small sample sizes  (0.1  mL or 0.001   gm) Non-destructive Can  analyze  multiple element   samples Often  doesn’t  need chemical treatment (sample digestion or   dissolution) High  sensitivity,  High accuracy (5%), high precision (0.1%) and low  detection limits in  the  sub-ppm   range Very  low detection limits  for  30-40   elements Lowest  Detection limits can be achieved by optimizing   irradiation parameters  (Energy,  Fluence rate of neutrons, times  of  irradiation,   counting) Significant  matrix  independence as most sample are transparent  to  both  neutron and gamma   rays 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. It  is self-validating:  Two  or more analytical gamma lines  may  be used  for the   determination  of  the same elements,  allowing  a cross-check  f  the   process.

Instrumental Neutron Activation Analysis (INAA)

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