Short communicationA method for obtaining the pure annihilation peak of radioisotope
Introduction
Positron annihilation lifetime spectroscopy (PALS) is a descriptive, sensitive and widely used method in material microanalysis. PALS technique is based on positron's annihilation in the medium to be analyzed. radioactive source is mostly used as a positron emitter (90%) for this technique.
When the positron stops in a medium (in a detector or in a source itself), it annihilates with an electron and gives two 0.511 MeV energy gamma rays simultaneously. A peak at 0.511 MeV channel of a multichannel analyzer (MCA) is observed corresponding to the detection of one of the annihilation photons. In addition, radioisotope emits high-energy (1.274 MeV) gamma rays as can be seen in Fig. 1.
As a result of 1.27 MeV gamma emission of , Compton effect appears in the spectrum. The influence of multiple Compton events generates gamma rays at around 0.5 MeV energy region. Also, not only Compton rays but also X-rays, backscattering and background effects are effective in this region. These gamma rays, which should be eliminated, superimpose on the original annihilation radiation peak, and distort the original spectrum and annihilation lifetime spectrum. For this reason, the 0.511 MeV peak in the spectrum should be undistorted and pure. This pure peak is much more significant for PALS technique.
The aim of the present work is to discriminate the gamma rays generated by Compton effect, backscattering, X-rays and background from the annihilation peak of radioisotope, and to obtain pure annihilation gamma ray spectrum for PALS.
Section snippets
Equipment
Fig. 2 is a schematic block diagram of the spectrometer used in the present work. Ortec 401A BIN, Bicron inch NaI(Tl) inorganic scintillation detector, Ortec 265 photomultiplier base (PMT), Ortec 459 power supply (PS), Ortec 113 preamplifier (PA), Ortec 472 spectroscopy amplifier (AMP), Ortec 427A Delay amplifier (DA), Ortec 420A timing single channel analyzer (SCA), Ortec 552 pulse shape analyzer (PSA), Ortec 437A time to pulse height converter (TPHC) and Ortec Trump 8K multichannel
Results and conclusions
The gamma ray spectrum was collected in the MCA card where it is divided into 2048 channels and then stored in a computer. The gamma ray spectrum of source with a 300 s run time at room temperature of 301 K is shown in Fig. 3.
The circuit in Fig. 2 determines the time difference between the initial rise of an anode pulse and the time at which a bipolar pulse, produced by a shaping circuit, crosses zero. This time difference is sensitive to the proportion of decay components in the light
Conclusions
The discrimination of the gamma rays which arise from the Compton effect of the 1.27 MeV gamma rays, backscattering from the detector surface and surrounding material, X-ray emission and background from the 0.511 MeV annihilation peak of , and the observation of the pure annihilation peak were investigated in the present work. The detection system presented here provided adequate energy resolution and system performance parameters for the pure annihilation peak. With this system, the
Acknowledgements
This work was supported by TUBITAK, the Scientific and Technical Research Council of Turkey under Project no. TBAG-1683(197T087) and by EBILTEM, Center of Science and Technology, Ege University under Project no. 99 BIL 001. The author (C. Celiktas) thanks Dr. S. Selvi for his kind helps and presents his appreciation to Dr. U. Yahsi for providing the radioisotope.
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