Sample analysis for PIXE

Samples of rice (n = 71) and maize flour (n = 56) were irradiated by a proton beam of size 3 x 4 pm^, cmrent -200 pA and energy 2.5 MeV. The samples were iiTadiated for 10 minutes scaiming an area of 1.5 x 1.5 mm^ of the pellets. The characteristic x-ray spectra were collected by an 80 imn^ Si(Li) detector of resolution 160 keV at 5.9 keV (with a Be filter of thickness 130 pm) placed at 135° to the beam and 25 cm from the samples. The RBS spectra were collected using a Si(Li) detector placed at 160° and 30 mm fr'om the samples.

A sample of NIST Peach Leaves (SRM 1457) from each plate was used as a compai'ator to the samples analysed together on the same plate to ensure the same analytical conditions. Net aieas from GUPIX of Daii32 package were used to calculate the concentiations of elements in the samples using the comparative method explained in Chapter 3. This set-up allowed the analysis of P, Cl, K, Ca, Cr, Mn, Fe, Cu and Zn. Typical PIXE spectra for rice and maize flour are presented in the Appendix 2.

The MDL for elements found in rice and maize flour determined by PIXE were calculated using equation 3.4 and are presented in Table 4.1. The value were low for elements of Z number between 2 3 - 3 0 (Cr - Zn) and was the lowest for Cu.

Except P and Ca the MDL were slightly higher in rice than in maize flour.

Table 4.1: The MDL (pg/g) of elements determined in rice and maize flour irradiated for 10 minutes with a proton beam of energy 2.5 MeV,

Element

MDL (pg/g)

Rice Maize

flour

P 58 86

Cl 20 26

K 10 12

Ca 13 25

V 3.7 3.5

Mn 3.8 3.6

Fe 2.8 3.0

Cu 2.9 2.0

Zn 3.7 3.5

Cr 3.9 3.6

Br 16 15

M D L = 2Vb

4.4.2 Sample Analysis for INAA

Short iiTadiation INAA of 134 samples (63 samples of rice and 71 samples of maize flour) was carried out in two reactor centres. 34 samples were analysed at the Imperial College Reactor Centie (ICRC) in the U.K while 100 samples were analysed at the Institute of Nuclear Physics (NPI) in Rez, Czech Republic. Under this teclmique, Na, Mg, Al, Cl, K, Ca, V, Mn, Cu, Br and I were determined and are reported.

The two reactor centres used in this thesis applied two different schemes of analysis. At ICRC samples were irradiated with neutrons of thermal and epithermal flux of 2.12 X 10^^ neutrons/cm^s and 1.15 x 10^^ neutrons/cm^s, respectively, for 5 minutes, waited for 2 minutes and cormted for 5 minutes. Wliile at NPI samples were irradiated with neutrons of thermal and epithemial flux of 3.0 x 10^^

neuti'ons/cm^s and 8 x 10^^ neutrons/cm^s, respectively, for 2 minutes waited for 10 minutes and counted for 10 minutes. The neutron flux density was one order of magnitude higher at NPI than at ICRC.

The neutron flux directly influences the number of isotopes produced within a target sample which in turn affects the number of counts produced in the detector.

Equation 3.23 shows the relation between the neutron flux and the net full energy photo-peak area (net counts recorded by the detector). The use of higher neutron flux produces a bigger number of counts which will result into a better statistics and lower MDL values for analysed elements than the use of less neutron flux. For that reason, lower MDL values for elements were expected to be attained when analysed at NPI compared to ICRC.

It was mentioned earlier that the decay time was 2 minutes at ICRC in contrast to 10 minutes at NPI. A short decaying time enables better detection foi- short lived isotopes such as (t >/^ = 3.74 min), which are from tiace elements in biological material. A long decay time (multiple of half life) may reduce the activity of these isotope in such a way that their counts will not be enough to considered as a tme photo-pealc (below MDL). However, a long decay time will also give time for short lived isotopes (such as Al which are found in high concentrations in biological samples) to decay and reduce their activity which will in turn lower the background and the MDL for the long lived isotopes.

At ICRC the concentiations of the elements in the samples were calculated using the K^q method (equation 3.33) which necessitated knowledge of detector efficiency as well as accurate energy calibration of the detector. These were canied out as explained in Chapter 3. The energy calibration was performed on a daily basis before counting the first sample of the day. The background spectra obtained fiom an

empty S2 container inadiated and counted at the same geometry as the sample were used to find the backgiound activities of the elements which were subtmcted from the total activity to give the net activity of the elements in the sample.

At NPI the samples were iiTadiated in the PE rabbits. After irradiation, the capsule was taken out of the rabbit, unfolded fi'om the plastic wrapper and surface washed with water and ethyl alcohol to remove any surface contamination. The samples were counted for 10 min after waiting for 10 min using a well shielded n- type HPGe of relative efficiency 20.8% for ^^Co 1332 keV line. The compositional analysis of the samples was carried out using the comparative method with MES as comparator using equation 3.35. The contributions given by interfering reactions of the (n,p) and (n,a) type were corrected using correction values indicated in Table 3.1.

The corrections were carried out for Na corrected for Al and Mg, for Mg corrected for Al and for Al corrected for P. The concentrations for P in both foods used for correction were values obtained by PDCE analysis in this thesis, where no correction was required since there is no interference.

The MDL of elements determined at both reactor centres were calculated using equation 3.4 (MDL = 2V^b) and are presented in Table 4.2. It can be seen in the table that isotopes with long half lives (^^Na, ^^Mn) and intermediate half-lives (^^Cl and ^^^1) had the expected lower MDL when analysed at NPI than when analysed at ICRC. At the same time the MDL for shor-t lived isotopes (^^Al, and

*^^Cu) were lower when analysed at ICRC with a shorter decaying time than when analysed at NPI.

Table 4.2: The MDL (pg/g) for elements in maize flour determined at ICRC and NPI calculated by equation 3.4

Isotope Half-life

(min) MDL( mg/g)

ICRC NPI

897.54 6 2

"M g 9.46 29 31

^®AI 2.24 0.1 2

38ci 37.24 5 2

" K 741.0 395 155

8.72 42 27

""V 3.74 0.01 0.05

154.8 0.5 0.2

^C u 5.12 1 2

®°Br 17.68 0.2 0.3

128|

24.99 0.8 0.1

M[DL = 2VB

4.5 Results