International Year of Crystallography

Neutron Activation Analysis

 

OPAL interior shot of the pool
Interior of the OPAL Reactor

 

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Neutron activation analysis (NAA) is a very sensitive method of quantitative multi-elemental analysis. It has the potential to determine concentrations in a sample from ppb to tens of percent, depending on the particular element and bulk matrix composition.

 

Samples are irradiated in the OPAL research reactor. After irradiation the specific activity of each radionuclide in the sample may be determined by measuring the energy and intensity of characteristic gamma rays that are emitted. Radionuclides that have a short half-life (minutes to hours) are best measured after a short irradiation and those with a long half-life (days to years) are irradiated for a longer time.

 

To offer the greatest flexibility to clients and collaborators, both the relative method of standardisation and the k0-method of standardisation have been implemented.

 

The neutron spectrum at the NAA irradiation positions in the OPAL reactor is very highly thermalised, with a thermal to epithermal ratio (f) of greater than 1,500. Many of the interfering reactions produced in other research reactors are avoided in the OPAL facility.
 


Strengths of NAA  

 

  • Minimal sample preparation for solid forms and powders.
  • Large dynamic range of concentrations, from ppm or even ppb to tens of percent.
  • Large dynamic range of sample weights, from a few mg up to several g.
  • Good accuracy with typical uncertainties around 4 to 5% and good precision.
  • The multi-element capability provided by the k0-method means that up to 65 elements can be quantified with just two irradiations.
  • Fully traceable to primary SI-standards. The relative method is recognised as a primary ratio method.
  • Quality in our laboratory is assured through the use of certified reference materials and participation in international intercomparisons at the highest metrological level, such as CCQM key comparisons. 

 


Applications  

 

The following examples are a small selection of the potential applications of NAA.

 

Materials Science
A researcher in photovoltaics wants to confirm that gold doping of silicon wafers was as expected. The very high sensitivity of NAA to gold and the insensitivity to Si means that concentrations down to ppb can be accurately measured.
 

Cultural Heritage & Archaeology
The accuracy, precision and multi-element capabilities of NAA allows researchers to ‘fingerprint’ ochre samples from different sites around Australia using multivariate statistics.

 

The resulting database provides insights into the movement, trade and uses of ochre by Aboriginal peoples.

 

Cosmochemistry
A short irradiation of meteorite chondrules weighing around 2 mg is sufficient for NAA to quantify the major constituents. Because NAA is non-destructive, the chondrules can be returned to the researchers after one month, allowing additional analyses to be performed.

 

Industry Quality Assurance
An iron ore mining company wants to produce reference materials for internal QA/QC. The accuracy of NAA and its ability to quantify ‘difficult’ elements, such as chlorine, is highly valued. NAA provides data that can be used to check alternative analytical methods.

 

Food Security
Trace element analysis of food products are required to confirm their safety and establish their region of origin. NAA’s sensitivity, multi-element capability and ease of sample preparation offer particular advantages for this application.

 

Epidemiology
The level of heavy metals such as arsenic in hair and nails may indicate industrial or environmental exposure to toxins. The ability of NAA to measure small samples ‘as collected’ and the freedom from analytical blanks make it a powerful tool for epidemiological studies.

 


More than 65 elements can be quantified by NAA  

 

EXT_IMG_
More than 65 elements can be quantified by the neutron activation analysis shown in this chart


Short Irradiation
A short irradiation of up to 15 minutes typically allows the concentration of the following elements to be determined:

 

Al, Ar, Ca, Cl, Cu, Dy, I, In, Mg, Mn, Na, Ni, Rh, S, Si, Sn, Ti, V, Y.

 

Long Irradiation
A long irradiation of up to 20 hours enables the analysis of:

 

Ag, As, Au, Ba, Br, Ca, Ce, Cd, Co, Cr, Cs, Er, Eu, Fe, Ga, Ge, Gd, Hf, Hg, Ho, In, Ir, K, La, Lu, Mo, Nb, Nd, Os, Pd, Pr, Pt, Rb, Re, Ru, Sb, Sc, Se, Sm, Sn, Sr, Ta, Tb, Te, Th,Tm, U, W, Yb, Zn, Zr.

 

Reporting times will depend on reactor scheduling but typically reports may be issued within a month of receipt of samples for short irradiations and within two months where long irradiations are required.

 

Typical sample sizes are from 50 to 150 mg for both short and long irradiations. Larger samples may be used if the bulk matrix material does not activate strongly.

 

Reporting times
Reporting times will depend on reactor scheduling but typically reports may be issued within a month of receipt of samples for short irradiations and within two months where long irradiations are required.

 

Typical sample sizes are from 50 to 150 mg for both short and long irradiations. Larger samples may be used if the bulk matrix material does not activate strongly.    


Contact details


To discuss your analytical needs, please contact:

 

Dr John Bennett, Head of the Neutron Activation Group
Phone: (02) 9717 3743


Email: john.bennett@ansto.gov.au
 

Address: Nuclear Operations
             ANSTO
             Locked Bag 2001
             Kirrawee DC
             NSW 2232