The comparative method as the one used in this thesis needs a sample to be iiTadiated with a comparator in the same conditions. The comparator could be a selected certified reference material (CRM) that has a matrix almost the same as the sample. However, the lai'ge uncertainties of concenti*ations of elements in CRM could introduce eiTor and limit the analytical accuracy. Fuitheimore, it might become difficult to analyse unexpected elements, which ai*e not certified in the standard unless one use multiple standards. Macroactivity of some elements in a standard may make it difficult to ensure identical iiTadiation and coimting conditions for standard and samples. Therefore, application of MES (instead of standard reference material) as a comparator is reported to be more convenient
MES are prepai-ed by pipetting a known amount of element in solution on to a filter paper or other suitable medium such as Polyethylene (PE) foil. They are then allowed to dry at room temperature. The mixing of element in one MES will depend on the half-lives and other nucleai' chai'acteristics of each activation product. Caiefiil measuies ai*e taken not to mix elements that may result into interferences. For instance, Fe and Mn or P and A1 aie usually not put together. Under the same experimental conditions, the total activity of the MES is prepared to be close to the activity of the measured sample. That is, the MES are prepared in such away that they produce similar dead time with the sample. Moreover, the chemical properties of the elements are also taken into consideration when preparing MES. For example, Ag and Cl are never mixed as they result into insoluble precipitate of AgCl.
In this thesis tlnee sets of MES were used in the short irradiation of food and hair samples. The first set consisted of Na, Cl, Mn, Br, Sr, the second consisted of Mg,
Ca, Ti, Cu, Zn, Dy and the third set consisted of Al, K, V, I, Ba, U. Together the three sets allowed tire detemiination of 17 elements normally obtained from short irradiation.
3,2,5.3 Spectral background
Fig 3.13 illnsti'ates a typical INAA spectrum of food sample collected for 10 min by CANBERRA-Genie-2000™ software programme after irradiation of 2 miir using a flux of 3x10^^ n cm'^ s'^ and 10 min decay time.
Fig 3.13: A typical spectrum of rice sample collected for 10 min.
The backgroimd oir which the full energy photopeaks are standing is mainly due to the interaction of y-rays with the detector. The main contributioir to the background iir INAA is from:
• Detector Compton scattering. The Comptoir scattering events in a detector is the main producer of the coirtinuum background in INAA. When a y-ray of energy between 150 keV - 9 MeV interacts with the Ge detector it may imdergo Compton scattering to produce a continuiirg backgroimd at low energies of < 250 keV.
• Back scattering. The y-rays before entering a detector might be Compton scattered by the suiTounding materials to produce a broad peak of
backscattered radiation at low energies. However, Pb shielding of the detector normally reduces this contribution.
• Natural radioactivity. Contribution from the natural radioactivity decreases with detector shielding.
• P" particles. I s o t o p e s p r o d u c e d in t h e a c t iv a t io n o f s a m p le s u s u a l l y d e c a y b y e m is s i o n o f P" p a r t ic le s w h i c h w h e n in t e r a c tin g w i t h s u iT o m id in g m a t e r ia ls e m it b r e m s s t r a h lim g r a d ia tio n .
3.2.5.4 Interference
In INAA for biological samples, there ar e two main interferences which if not corrected might introduce eiTor to the determined elemental concentrations.
a. Interference from primary reaction
Fast neutrons in a reactor may result into (n, p) and (n, a) reaction that might produce the same isotopes as those produced from (n, y) reaction of thermal neutron.
For example, ^^Al may be produced by the following three reactions:
^^Al + n ---► [^^Al] ► ^^Al + y 28gi^n --- ► [29si] ► 28^1+ p
^^P + n ► [^^P] ► ^^Al + a
The effect of this type of interference is a function of the relative concentrations of the isotopes in a sample as well as the ratio of fast/thermal flux within the reactor. The primary reaction interference normally occurs when the interfering element has much higher concentration than the measured element. In this thesis correction was carried out for the measured elemental concentration whenever the concentration of the interfering element was >1% A standard containing known amounts of the interfering element and no or very low concentration of rneasined element is prepared and irradiated. Since the contribution from the
measured element to the produced isotope is negligible, then the contribution of the interfering element is calculated in terms of a ratio. This ratio is used to normalise the concentration of the interfering element in the samples with the two elements.
The difference between the concentration of the measured element and the normalised concentration of the interfering element is the correct value of the measured element. The correcting ratios of different elements used in this thesis are shown in Table 3.1.
Table 3.1: The interference from the primary reactions and their corresponding correction factors (pg/g per 1% of interfering element). The flux ratio of thermal to fast neutron = 10%.
Primary reaction interference Correction factor
(n, a) ^A1 10.43
(n, p) ^A I 32.18
^Si (n, a) 1.11
"W l(n,p) ""Mg 106.1
""A l(n ,a )"V a 1.75
"'"Mg(n,p)"^Na 3.307
b. Spectral interference
This takes place when two isotopes emit y-rays of the same, or nearly the same energy. For instance, both ^"^Cu and ^"^Na produce 511 keV peak. Thus this peak produced when a sample containing the two isotopes is counted will have contributions from both isotopes. CoiTection for this interference follows the same procedures as those explained for primary reaction interference.
Another example of spectral interference is from umesolved close peak lines.
A peak from ^^Mn (846.7 keV) may interfere with a peak fi'om ^^Mg (843.8 keV)
because their energies are too close to be properly resolved by Ge detectors of normal resolution. Since both isotopes produce more than one peak line, to avoid error, other peak lines for each isotope ai*e used even if they have lower intensity than the interfered peaks.