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Particle Induced X-ray Emission (pixe): a Tool of Qualitative Elemental Analysis for Biological Sample

M. Hasnat Kabir
Dept. of Information and Communication Engineering, University of Rajshahi, Rajshahi 6205, Bangladesh

Abstract/ Executive Summary:

The facility of ion beam laboratory at Kochi University of Technology (KUT) has been extended by installing the Particle Induced X-ray Emission (PIXE) technique, in order to provide qualitative and quantitative elemental analysis and in-air micro-PIXE analysis. This paper is a description of PIXE setup and its application in biological sample for qualitative elemental analysis. The energy calibration of the system shows linearity. The minimum detection limit indicates that the system has good detection limit. The homogeneity shows uniformity of the sample itself and the internal standard within the sample. A 4 MeV He++ ion beam was used to analyze shellfish samples. Analyzing samples, it was found that the lower Z elements as well as some trace elements were detected.

Keywords: PIXE, LOD, Qualitative analysis

Location: Dept. of Information and Communication Engineering, University of Rajshahi, Rajshahi

Start Date:

End Date:

Program Area: Farm Mechanization

Non-commodity: Performance

Objectives:

1. To provide qualitative and quantitative elemental analysis and in-air micro-PIXE analysis.

Methodology:

Instruments setup - A Nissin high voltage accelerator (NT1700S) with a 1.7 MV (Maximum voltage) has been setup at the beginning of ion beam laboratory at KUT. Target chamber is made by steeliness steel. The inner diameter of the target chamber was 50 cm. A computer control sample holder made by aluminium was placed inside the chamber which can tilt 45o in y-axis whereas 180o in x-axis. Sample holder was monitored by PC through a stepping motor. The diameter of beam spot on the target was about 1 mm with a roughly circular shape, defined by a graphite collimator. A camera was placed around 10 cm before to the beam entry in the analysis chamber to control the beam alignment and its focusing. The vacuum pressure could reach 10-6 torr inside the target chamber using a turbo-molecular pump. In order to accurate measurement, secondary electron was suppressed by ring shape suppressor electrode shown in figure 1 which was placed in front of target with a biased voltage of -250 V . The distance between target and Si(Li) detector may vary from 15 to 50 mm. The collected charge was measured by current integration from the sample holder which was served as a Faraday cup, electrically isolated from the analysis chamber. RÖNTEC XFlash 2001, a Silicon Drift Detector (SDD) type detector was fixed at 135^o referring to the beam direction in the target chamber for detecting X-ray emission from targets. The active area and the silicon thickness of the detector are 10 mm2 and 0.3 mm, respectively. A polymer coated beryllium window of 8 μm thickness is equipped with the detector. A zirconium ring with an aperture of 3.4 mm serves as a collimator in the detector. The detector was connected to a high resolution pulse processing unit with a single cable. The ORTEC 572 amplifier was coupled to the pulse processing unit to amplify the signal from the detector. This amplifier was used for its low noise, wide-gain range and selectable shaping networks. Finally, the output of the ORTEC 572 amplifier was then connected to a computer via an analog to digital converter (ADC) and a multi-channel analyzer (MCA). Acquisition system setup - The data acquisition system should have the facility to improve multiple-parameter data-acquisition, on-line data visualization and monitoring which enable a wide range of ion-beam experiments. To achieve this goal, system was configured into two ways for data acquisition system: a front-end computer with MCAWIN software system for monitor and real-time data-acquisition, and a back-end system for data collection from the detector via processing unit, pre-amplifier and multi-channel analyzer (MCA). The transportation of data from the back-end to the front-end system is controlled by the front-end system. Energy Calibration - If it is considered a pulse-height spectrum containing ith peaks for the ions of energies Ei, then the centroids (Yi) of the peak can be expressed as Y_i = X_a + X_bE_t, where Xa and Xb are constants. The centroid of a specific peak may be shifted due to high X-rays count rates, the linearity should be maintained by adjusting the value of parameters. In the pulse processor systems, the amount of peak shifting is very small, near about 1 eV at a rate of 100000 counts per second. Using equation (1), the energy calibration was done for the system shown in figure 3. It clearly shows the linearity of energy calibration as a function of channel of the desired system. The energy resolution of any detector is usually denoted as the full width at half-maximum (FWHM) of the Gaussian. The energy resolution of XFlash 2001 detector is 139 eV @ manganese Kα line, this is a general practice for energy calibration which is easily acquired from standard thin radionuclide sources [8]. The choice of manganese Kα1 line due to the small separation of 11 eV between the Kα1 and Kβ1 line, which is negligible for the system rather than the large separation of 173 eV for choice of silver. The K X-ray line of manganese is shown with indication of background level. This PIXE spectrum was routinely used to calibrate the spectroscopy system. It can be seen from this figure that the system was calibrated as 20 eV per channel, i.e. Mn Kα1 at 295 channels.

Source of Information: Rajshahi University J. of Sci. 39, 01-10 (2011); ISSN 1681-0708

Article Link (Online Journal):

Funding Source:

Total Budget (Tk.) :

Achievement/Findings:

The uniformity between sample itself and internal standard for target preparation was confirmed by homogeneity check. The minimum detection limit of Ca is 0.14 ppm which represents very good sensitivity of the system. Analyzing shellfish, Cl, Ca, Mn and Fe were found as major elements whereas Cu, Zn, Br, Sr and Zr were detected as trace elements. Hence, it can be concluded from the present study that PIXE is a well-suited method for trace element analysis in biological sample.

Knowledge Management: Journal