CHAPTER RESULTS AND DISCUSSION ON EFFECT OF COLLIMATOR SIZE ON THE ATTENUATION OF GAMMA RAY IN PURE ELEMENTS 5.1 INTRODUCTION The increasing use of isotopes in the field of medicine and biology has demands a detailed knowledge of attenuation coefficient of elemental solids over a wide energy range. In the recent years the environment involves nuclear radiation. Mankind has added to this by production of power and by application of artificial radiation to medical and other uses. With the increasing use of radioactive materials, it is becoming important to know with some accuracy, how the interaction of gamma rays with matter varies with gamma ray energy and atomic number [1]. In addition to usual materials a search for the other non-radioactive materials, which can be used as a shielding materials, is used. By determining linear attenuation coefficient, mass attenuation coefficient, of elements with varying thickness were use to study. The experimental values of linear attenuation coefficient, mass attenuation coefficient, total photon interaction cross-section and effects of collimator diameter of the materials and their values gives us a idea about the materials which is good for the attenuation of gamma rays. The mass attenuation coefficient measures the probability of all interactions between gamma rays and atomic nuclei; it depends on the incident photon energy and the nature of the absorbing material. There have been a significant number 112
of theoretical calculations of mass attenuation coefficients. Tabulation most widely used by different scientific communities are those of Scoffield [2], Saloman, Hubbel, and Scoffiled [3], Henke et al.[4,5], Ceragh et al.[6] and Chantler [7,8]. There are a significant number of publications in the literature on experimental determination of mass attenuation coefficient of various elements, compounds and mixtures [9]. Most of the time experimental data are compared with the theoretical tabulation used by Berger and Hubble in X-com [10]. The experimental and theoretical data did not agreed due to the different counting setup. A number of factors could cause the discrepancies between published mass attenuation coefficients. One of these factors is the lack of proper collimation incident and transmitted beam [11]. Improper detector collimation can cause systematic errors due to the scattering effect since the scattered photon can be counted as part of the transmitted beam. However a qualitative analysis of the detector collimation effect on mass attenuation coefficient determination is lacking required being determined. In order to study the effect of detector collimation on mass attenuation coefficient, one requires several radiation energies. The attenuation of gamma rays in elements has been studied for variable energy and using narrow beam geometry. The study of attenuation coefficient of several elements can throw light on shielding properties and to find the density of materials. Taking into considerations the importance of gamma ray attenuation we have carried out systematic investigations of linear attenuation and mass attenuation coefficient of several elements such as Lead, Gold, Silver, Zinc, Nickel and Magnesium for variable energy (0.360 MeV 1.33 MeV) using 113
narrow beam geometry technique. The effect of beam collimation has been studied for elemental samples in the form of thin uniform thicknesses from higher atomic number to lower atomic number values under investigations. The results obtained on linear attenuation and mass attenuation coefficient of Lead, Gold, Silver, Zinc, Nickel and Magnesium are presented in this chapter. In the present work, we report our results on linear, mass attenuation coefficient, total atomic cross-section and effects of collimator diameter were studied. The results are verified with Hubble s X-Com Program which is well agreed with experimental data for collimator diameter 0.3cm under investigation. The present work is useful for shielding applications. 5.2 EXPERIMENTAL DETAILS The effects of beam collimation on linear attenuation coefficient and mass attenuation coefficient and total photon interaction cross-section of Lead, Gold, Silver, Zinc, Nickel and Magnesium for varying energy have been measured using narrow beam geometry technique. The schematic experimental set-up of narrow beam geometry used in the present measurement is shown in previous chapter. The counting setup consists of source holder, incident and transmitted beam collimators of Lead, which are well aligned as already explained in previous chapter. To minimize radiation exposure and background radiation contribution, the radioactive source was kept in a Lead source holder and placed subsequently in the Lead castle which is well shielded from all sides [12]. 114
The transmitted photon beam is detected by a NaI (Tl) scintillation gamma photo spectrometer. The optimum voltage of 900 Volt was chosen to provide good resolution characteristic for the isotopes used. The detector was calibrated for various photon energies using radioactive sources detailed discussed in previous chapter Table 4.1. The photon transmission measurements were done under a narrow beam counting geometry employing high resolution scintillation detector. The NaI(Tl) detector was used in the present work. The NaI(Tl) scintillation detector used in the present work is 4.5 cm in diameter and 5.0 cm thick and is supplied by Nucleonix Enterprises, India. The experimental set-up consists mainly of two collimators of each diameter 0.2 cm, 0.3 cm, 0.4 cm and 0.5 cm, well aligned by LASER beam so as to provide a scatter free collimated photon beam. To establish the optimum collimation condition, the gamma ray spectra of Cs 137 source were taken with the incident and transmitted collimated beam and were found to be identical and had unchanged energy resolution characteristic. The photon spectra thus taken establish that the energy of transmission photon did not change appreciably due to scattered or fluorescent radiation emanating from the collimators. A provision was made midway between the collimators to introduce absorbers which were in the form of thin uniform foils. The entire system was arranged vertically over the NaI (Tl) detector, ensuring that the central axis of incident and transmitted collimator are coaxial. The source holder was kept over the collimator so as to allow, a narrow well collimated photon beam from the collimator incident normally on the thin absorber. The gamma spectrums from each source 115
of photon energy 0.360 MeV to 1.33 MeV were recorded on the single channel analyzer pre set to record counts under the full energy absorption peak. The transmitted photons from the absorbers were accumulated for a set time so as to provide statistical variation within one percent. For absorption study of gamma ray, thin and uniform foils with high purity of Lead, Gold, Silver, Zinc, Nickel and Manganese procured locally were used in the present study. 5.3 RESULTS AND DISCUSSION 5.3.1 ELECTRON DENSITY It is well known that the attenuation coefficient is proportional to the electron density P, which is proportional to the bulk density of the absorbing material. However, for a given material, the ratio of the electron density to the bulk density is constant equivalent to Z/A, which is independent of bulk density ρ. In the present study the electron density P was calculated using the following relation, P = Z.ρ/A 5.1 where, P- Electron density Z- Atomic number A- Atomic mass number ρ- Bulk density The value of electron density obtained using above relation is found almost constant for moderate elements i.e. Silver, Zinc, Nickel, and it decreases in Magnesium however for heavier elements like Gold and Lead, the electron density is not constant. The values of electron density obtained using above equation for Lead, Gold, Silver, Zinc, Nickel and 116
Magnesium are given in table 5.1. Table 5.1 also shows the values of areal density of each element (Pb, Au, Ag, Zn, Ni and Mg) along with their atomic number and density. 5.3.2 EFFECT OF BEAM COLLIMATION ON ATTENUATION COEFFICIENT The coherent and incoherent photon scattering in the photon energy range dominates within the effect of collimator size on the experimental measurement of attenuation coefficient need to be studied; hence it was decided to study this effect. Accordingly collimators of varying diameter 0.2 cm to 0.5 cm in steps of 0.1 cm were fabricated from Lead and were used in the present work. The linear attenuation coefficients of various elements at different energies under investigation were measured for different collimator sizes. The photon spectra was recorded for sources emitting gamma ray in the energy range of 0.360 MeV 1.33 MeV and the values of linear attenuation coefficient were obtained as described earlier. The results of measurements have been summarized in the form of attenuation coefficient as a function of collimator size, for 0.360 MeV photons energy and are given in table 5.2. The variations of linear attenuation coefficient plotted against collimator size are shown graphically in Fig.5.1. The gamma ray attenuation coefficient values shown in table 5.2 as a function of collimator diameter, increased as expected due to multiple scattering within the collimator. The results of these measurements are in accordance with the theoretical consideration. The effect of collimator diameter on mass attenuation coefficient of Lead, Gold, Silver, Zinc, Nickel and Magnesium has been studied in 117
the present work. The values of mass attenuation coefficient for different elements as a function of collimator diameter are given in table 5.3. The variation of mass attenuation coefficient with collimator diameter for the elements under investigation is depicted in Fig 5.2. It can be observed from this figure that mass attenuation coefficient increases with collimator diameter. The similar behaviour is observed for all the elements under investigation. The increase in collimator diameter affects the narrow beam geometry which leads to increase in mass attenuation coefficient of each element. The dependence of total photon cross section on the collimator diameter has also been studied in the present work. Table 5.4 represents the values of total photon cross section as a function of collimator diameter. As the collimator diameter increases, the total photon crosssection increases as shown in Fig. 5.4. Figure 5.4 depicts the variation of total photon cross section versus collimator diameter. It is evident from figure 5.4 that total photon cross- section increases linearly with collimator diameter for all the elements under investigations. From these results, it is observed that small collimator diameter is more effective for obtaining accurate values of linear and mass attenuation coefficient. It is evident from the experimental data that, the attenuation coefficient increases with the diameter of the collimator. From the present data, it is observed that, the contribution from multiple scattered photons is minimized with collimators of least diameter. The scatter free attenuation values thus, can be obtained with collimator of least size used in the experimental measurements. The accuracy of the 118
measurements depends on beam collimation as is evident from the experimental measurements. The above experimental results on linear attenuation coefficient, mass attenuation coefficient and total photon cross section as a function of collimator diameter has been discussed for 0.360 MeV photon energy. Similar experimental results have been obtained for other photon energy also, which is not mentioned in this work. 5.4 CONCLUSIONS In view of the mounting demand of photon attenuation coefficient of various elemental solids, due to the extensive vital application of radiation, in medical and biological fields in this study, linear attenuation, mass attenuation coefficient and total photon interaction cross-section of most thin uniform elemental solids which are 99.9% pure have been measured at six different energies, in the range of 0.360 MeV to 1.33 MeV in a good geometry setup employing at high purity NaI(Tl) detector. the linear attenuation coefficient and mass attenuation coefficient of various elements with increasing atomic number determined for various collimator size revealed that the best results are obtained for 0.3 cm collimator diameter. 119
REFERENCE [1] Charlotte Meaker Davisson and Robelly D. Evans, Review of modern Physics, 24,(2) (1952). [2] J. H. Scofield. LLNL, Report UCRI-51326 (1973). [3] E. B. Saloman, J. H. Hubbel, J. H. Scofield, Atomic data Nuclear Data tables 38.(1) (1988). [4] B. L. Henke, J. C. Davis, E.C. Gullikon, R. C. C. Perera, LBL Report No. LBL-26259 UC-411. (1988) [5] B. L. Henke, J. C. Davis, E.C. Gullikon, Atomic Data Nuclear Data tables 54. 181-342 (1993). [6] D,C, Ceragh, J.H. Hubbel, In: A.J.C. Wilsons(Ed.), Int. Tables for X-ray crystallography, Kluwer Academic, Dordrecht, 1995, Vol. C sect. 4.2.4 [7] C.T. Chantler, J. Phys.Chem. 22 71-643(1995) [8] C.T. Chantler, J. Phys.Chem. 29 597-1048(2000) [9] Necati Celik, ugur Cevik, Ahmet Celik Nuclear Inst. Amd methods in Phys. Research B 281 8-14 (2012) [10] M.J.Berger, j.h. Hubbel, X-Com: photon cross-section data base NBSIR 87-3597 (1987) [11] D.C. Ceragh, Nucl. Inst. Meth. A 222. 1(1987) [12] Gurdeep S. Sidhu, Karmjit Singh, Parjit Singh and Gurmel S Muduhar, Pramana Journals of Physics, 53, (5) (1999) 851-855. 120
Table 5.1 The variation of atomic number (Z), atomic mass numbers (A), calculated density (ρ cal ), theoretical density (ρ the ) and electron density (P) for different elements. Absorber Atomic number (Z) Atomic mass Number(A) Calculated density (gm/cm 3 ) Theoretical density (gm/cm 3 ) Electron Density (P) Pb 82 207 11.34 11.34 4.492 Au 79 197 19.32 19.32 7.747 Ag 47 108 10.48 10.48 4.607 Zn 30 65 7.13 7.13 3.290 Ni 28 59 8.90 8.90 4.223 Mg 12 24 1.74 1.74 0.870 121
Table 5.2 Effects of collimator diameter on linear absorption coefficient (µ) as a function of energy range (0.360 MeV). Diameter Linear absorption coefficient µ (cm -1 ) (cm) Pb Au Ag Zn Ni Mg 0.2 2.393 3.229 0.939 0.494 0.587 0.122 0.3 3.589 5.593 1.408 0.741 0.881 0.183 0.4 4.786 7.457 1.877 0.989 1.175 0.244 0.5 7.976 9.321 2.347 1.236 1.468 0.305 122
Table 5.3 Effects of collimator diameter on mass absorption coefficient (µ/ρ) as a function of energy range (0.360 MeV) Diameter mass absorption coefficient µ/ρ (cm 2 /gm) (cm) Pb Au Ag Zn Ni Mg 0.2 0.211 0.193 0.089 0.069 0.066 0.070 0.3 0.317 0.289 0.134 0.104 0.099 0.105 0.4 0.422 0.386 0.179 0.138 0.132 0.140 0.5 0.528 0.482 0.223 0.173 0.165 0.175 123
Table 5.4 Effects of collimator diameter on total photon interaction cross-section (σ tot ) as a function of energy range (0.360 MeV). Diameter Total photon interaction cross-section σ tot (barn/atom) (cm) Pb Au Ag Zn Ni Mg 0.2 28.73 25.31 6.976 3.453 3.068 1.398 0.3 43.09 37.97 10.46 5.179 4.602 2.097 0.4 57.45 50.62 13.95 6.906 6.136 2.795 0.5 71.81 63.28 17.44 8.632 7.670 3.494 124
Pb Au Ag Zn Ni Mg 10 9 8 7 µ cm-1 6 5 4 3 2 1 0 0.2 0.25 0.3 0.35 0.4 0.45 0.5 Collimator Diameter cm Fig.5.1 Effects of collimator diameter vs linear attenuation coefficient for photon energy 0.360 MeV. 125
Fig.5.2: Effects of collimator diameter vs mass attenuation coefficient for photon energy 0.360 MeV. 126
Fig. 5.3: Effects of collimator diameter vs total photon interaction crosssection for photon energy 0.360 MeV. 127