Bir cevher numunesindeki izotopların ve yarıömürlerinin belirlenmesi

Abstract

Aktivasyon analizlerin çoğu termal nötronlarla Nükleer reaktörlerde yapılır. Bu çalışmada bir cevherden alınan numuneler. İ.T.Ü.Triga 2 Nükleer reaktöründe 1.66 10 nötron cm sn akılık termal nötron akısı ile ışınlanmış, Sistem 100 adı verilen çok kanallı analizör programı ile spektrumları alınmış, Sampo90, analiz programı ile değerlendirilmiştir. Spektrumların değerlendirilmesi için ilgilenilen enerji bölgesinde Co60, Cs137, Ba133 standart kaynakları kullanılarak enerji, şekil ve verim kalibrasyonları yapılmıştır. Ayrıca, Eu152 standart olmayan nokta kaynağı da bu kalibrasyonlarda relatif olarak kullanılmıştır. Çalışmada, bir cevher içerisindeki saniye ve birkaç dakika mertebesindeki radyoizotopların yarıömürleri saptanmıştır. Bu işlem Sistem 100 analizör programı içerisindeki Task menülerinin kullanılmasıyla ışınlamadan sonra sırasıyla önce 15sn'lik sonra 2.8dak. 'lık 8 er spektrumun ardarda alınması ve bunların değerlendirilmesiyle yapılmıştır. Sonuç olarak numunemizde 18.6 sn yarıömre sahip Hf179m ve 3.76 dak.lık V52 izotopları gözlenmiş ve yarıömürleri de hata payı içerisinde literatür değerleri ile uyum içerisinde bulunmuştur.

Among the nuclear techniques used for elemmantal analysis, activation analysis plays a leading role. Fast neutron, photon and charged particle activation analysis are successfully applied in specific cases, but the majority of analysis is performed with thermal neutrons from nuclear reactors. Activation analysis achieves a qualitative analysis of an unknown sample by irradiating the sample and thus producing radioactive nucleids from stable or unstable isotopes present in the sample. The radioactive nucleids can be identified from properties of the radiations they emit: 1. Type of radiation 2. Energy of radiation 3. Intensity of radiation 4. Half life An advatageous feature of photon and neutron activation analysis in comparison to many other analitical methods is a minimum requirement for sample treatment. Thus, compact samples can be analyzed directly and non- destuctively, and the risk of trace element losses, a danger in chemical procedure, is ruled out. For various reasons activation analysis with thermal neutrons, NAA, has been successful. First, reactors are intense neutron sources with thermal neutron flux densities 14 -2 - 1 up to 10 cm s at the usual sample irradiation positions; second, the cross sections for thermal neutron induced reactions, in particular for (n,y) reactions, are in general much higher than those for reactions induced by fast neutrons, photons or charged particles; and third, each activation product can be assigned unambiguosly to a particular sample nucleid. The reaction obtained by thermal neutron for a nucleid is ; X* * n - XM ? Y In this work, natural sample taken from nature were irradiated with the thermal flux of 1.66 1012 nötron cm"2 sn"1 at I.T.U. Nuclear Energy Enstitution Triga 2 Nuclear Reactor and spectra were taken by Task menus of system 100. The main parts of experimental system are detector, multichannel analyzer (System 100), interactive fitting programme for Y raY spectra taken with high resolution solid state detectors (sampo 90). Gamma ray spectra has been taken with a PC based multichannel analyzer (System 100). System 100, using own microprocessors (Intel 80186), works independently from personal computer. So many works can be done by using system 100 such as using screen with groups, energy calibration, live or true time settings peak information options, region of interest processes and adding or subtracting spectrums in groups. The detector that used in this experiment is HpGe coaxial dedector. Some features of the detector are following. Physical Characteristics: Geometry Diameter Length Xtra - extended range closed-end coaxial 51.1 mm 50 mm Distance from window: 5 mm Electrical Characteristics: Depletion Voltage Recommended Bias Voltage Leakage current at recommended bios Preamplifier test voltage at recommended voltage Capasitance at recommended bias 2000 V 3500 V 0.01 nA -0.67V 22pF The most important feature of detector is resolution. The resolution of this detector is as follows. v I 0.8 keV at 22 keV 0.9 keV at 88 keV 1.8 keV at 1.33 MeV Peak Compton Ratio : 48:1 at 1.33 MeV To obtain numerical results from the measured spectra, they should be analyzed and the peaks have to be fitted with a computer programme called sampo 90 which could be run by a PC. Shape, energy and efficiency calibration files have to be created before starting to analyze spectra of our samples. The efficiency data were fitted to the function below. For this purpose, we used a programme that uses Morquardt method. Cafe - exp^-Co^ e"* >^* IdE) The parameters in above equation are as follows. a[l]= 1.1952 a[2]= 0.7952 a[3]= -1.7302 a[4]= 39.9227 a[5]= -0.1454 The efficiency graph is as figure 1. i I I I I I 1 - I I | BVERGY (keV) -i - r- t - i i | Figure 1. Absulate efficiency at 30cm. V I I The Task programme used in getting consecutive 8 spectra for second half lived isotops is as follows. 001 Clear, Group: Full, Data 002 Preset, Group: 1/4, Live Time : 15 sn 003 Acquire, Group: 1/4, Start 004 Preset, Group: 2/4, Live Time : 15 sn 005 Acquire, Group: 2/4, Start 006 Preset, Group: 3/4, Live Time : 15 sn 007 Acquire, Group: 3/4, Start 008 Preset, Group: 4/4, Live Time : 15 sn 009 Acquire, Group: 4/4, Start 010 Save, Group: Full, Data: Spectrum, Device : Disk, SDnnnn. MCA 011 End of Task: If this programme is used with 2 cycle 8 spectrum can be obtained. In figure 2, it can be seen how Hf 179m peak of 214.3 keV is getting smaller during 15 sn periods. TIME (Sn) Figure 2. Reduction of the counts of Hf m peak v i i i CO ?" as 3 O a y^y/ww S*Wh ^^^^^V^v,. I I I I fr I I i t I 1 I I I I' I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I CHANNEL NUMBER Figure 3. The partial gamma spectrum of the sample irradiated 15 sn. Finally, in the natural sample radionucleids which have short life, Hf179m at 214.3 keV, VA at 1434.5 keV, were observed and their half lives, almost 18.6 sn and 3.76 minute, were calculated.