INSTITUTE OF NUCLEAR SCIENCE AND TECHNOLOGY
Chemical analysis by nuclear methods
Neutron activation analysis (NAA) is one of the most sensitive and accurate techniques used for qualitative and quantitative analysis of major, minor, and trace elements in samples from almost every conceivable field of scientific or technical interest. For many elements, NAA offers sensitivities that are superior to those possible by any other technique. The accuracy and precision of the technique are such that NAA is still one of the primary analytical methods for producing Standard Reference Materials (SRMs) or Certified Reference Materials (CRMs) worldwide.
The principle involved in NAA consists of first irradiating a sample with neutrons in a nuclear reactor to produce specific radionuclides. After the irradiation, the characteristic gamma rays emitted by the decaying radionuclides are quantitatively measured by a suitable semiconductor radiation detector. The gamma rays detected at a particular energy are usually indicative of a particular radionuclide presence. Data reduction of gamma ray spectra by means of computer then yields the concentrations of various elements in samples being studied.
In the existing facility, sample is activated with neutrons using the BAEC 3 MW TRIGA Mark-II research reactor. A high-resolution high-purity germanium (HPGe) gamma-ray spectrometer is used to detect the delayed gamma rays emitted from the radioactive samples for both qualitative and quantitative analysis. The HPGe gamma-ray counting system is shown below.
|Gamma-ray counting system|
The methodology of NAA involves sample preparation, sequential irradiation and counting, data reduction and analysis and reporting.
The required amount of sample (micro gram to few grams) is placed in a pre-cleaned polyethylene vial and sealed. CRMs / SRMs from NIST, IAEA, NIES, NBS, USG or other sources are prepared in the same manner.
Samples and standards are then irradiated simultaneously with neutrons using the BAEC 3 MW TRIGA Mark-II research reactor. Two irradiation procedures are required for the determination of wide range of elements. The first procedure involves a short irradiation for the determination of Al, Ba, Ca, Cl, Dy, K, Mn, Na, Ti, V etc. depending upon the sample matrix. The second procedure involves long irradiation for the determination of As, Ba, La, Lu, Na, Sm, V, Yb, Ce, Co, Cr, Cs, Eu, Fe, Hf, Rb, Sb, Sc, Ta, Tb, Th, Zn, Zr, etc. also depending upon the sample matrix.
The activities of irradiated samples are measured by HPGe digital gamma-ray spectroscopy system coupled with associated electronics and gamma-ray spectrum analysis software. The HPGe gamma-ray detector is to be calibrated with the standard radioactive sources. Before counting, the irradiated samples were replaced into a new polyethylene bags to avoid radioactive contamination. For the short irradiated samples, the activity is measured as soon as possible after irradiation. The activity of the long-lived radionuclide is measured few days after the end of bombardment (EOB) for allowing decaying the short-lived radionuclides to avoid disturbance. Gamma spectra of both the samples and standards are collected in same geometry. During counting, the dead time is kept below 10% and the counting statistics of peak area is less than 5 %. Each sample is recounted 3-4 times by giving sufficient cooling time interval to avoid the interference of overlapping γ-lines from undesired sources and to be confirmed the interested nuclide by the half-life.
From the measured gamma-ray counts, decay correction factors of both the standards and samples and the known mass of the investigated element in standard, the mass of that element in unknown samples is determined using the well-known relative formula.
The technique is mostly nondestructive, many elements can be determined simultaneously, high accuracy and precision can be obtained, small sample can be analyzed etc. Analysis can be performed on a sample as received. In most cases, neither chemical treatment nor addition of reagent is required to prepare the sample for analysis; thus, contamination from excess sample handling and reagent addition is eliminated.
Because NAA involves measurement of radioactive decay, it is inherently a slow technique, producing radioactive waste.
NAA laboratory provides a service to researchers (from university, research organization, national museum, Department of Environment, etc.), patients (specially arsenocoisis patients), industry and others who may require it. NAA laboratory is capable on multi-elemental analysis of geological, biological, nutritional and health related environmental samples. Particularly, the followings can be mentioned.
- Trace and toxic elements analysis of soil, sediment, industrial effluents such as discharge from tannery, fertilizer-factory, paper mills, etc.
- Trace, toxic and essential elements in human health and nutrition related samples.
- Arsenic in soil, rice, vegetables, hair of arsenic affected patients, etc.
- Industrial products, semiconductor materials, etc. for the determination of impurities.
Using the present NAA facility, the concentration values of the following elements have so far been determined in ppm to ppb level in different sample matrices.
|Sample Matrix||Elements||Rate per Sample Tk|
|Soil, sediment and other geological samples, tobacco, textile effluent, etc.||Short-lived: Ti, Mg, V, Ca, Dy, Mn, Na, K.||2500.00|
|Medium and long-lived: As, Cr, Co, Br, Sb, La, Eu, Yb, Hf, Au, U, Th, Sc, Fe, Se, Rb, Ba, Ce, Sm, Tb.||2000.00|
|Foodstuffs||Short-lived: Ti, Mg, V, Ca, Dy, Mn, Na, K.||3000.00|
|Medium and long-lived: As, Cr, Co, Br, Sb, La, Eu, Yb, Hf, Au, U, Th, Sc, Fe, Se, Rb, Ba, Ce, Sm, Tb.||2500.00|
|Human hair, duckweed, rice, algae, etc.||As, Fe, Mn, Co, Sb, Cr||600.00|
|Beach sand minerals||U, Th, Ti, Cr, Fe, Hf, Zr, Mn, V, Dy|
|Fired sand||Sb, As, Ba, Ce, Co, Cr, Hf, Fe, La, Mn, K, Sm, Sc, Na, Th, U.|
The lower limit of detection is on the order of parts per million to parts per billion depending on the element analyzed and the activity of the bulk sample matrix.
Analysis turn around time
The normal turn around time from receipt of samples to reporting the result of the analysis is four working weeks. However, if short-lived radionuclides can be utilized we can provide a service within one week.
Contact us :
Director INST : Tel : 88-02-7701250
Dr. Syed Mohammod Hossain
Principal Scientific Officer INST, AERE
Ganakbari , Savar
G.P.O. Box-3787, Dhaka-1000
Fax : 88-02-7701337, 88-02-7701620
E-mail: Syed9495@yahoo.com Tel.: 88-02-7788249 (Off.)