Determination of Mass Attenuation Coefficients of Gamma Rays in Zinc

Abstract

This study presents experimental method in order to determine the mass attenuation coefficient of a Zinc with different energy for ²² Na and ¹³⁷ Cs sources. It is compared with two theoretical methods a graphic method and a transmission method. The method proposes to realize a numerical absorption calibration curve to process experimental results. The gamma-ray mass attenuation coefficients of Zinc absorber were determined experimentally at different gamma-ray energies and different Zinc thicknesses in order to investigate how the number of gamma photons and their energies affect the calculation of mass attenuation coefficients of the Zinc. The mass attenuation coefficients ( µ _m ) for Zinc (Cu) were measured in the γ-ray energy ²² Na, ¹³⁷ Cs as sources. The measured values are compared with the theoretical values obtained by WINXCOM software.

Keywords

Mass attenuation coefficient, Transmission method, Gamma-rays, Detector

Introduction

Gamma rays play an important role in vast fields like in industry, medicine, military agriculture, research, space science, etc. It also plays an important role in many interdisciplinary fields. In all these fields γ -ray sources are used. These sources may be weak (low activity µ Ci) or strong (with activity may be as large as thousands of Ci). In areas where humans are likely to encounter gamma rays, it is often necessary to provide proper shielding to reduce the risk of exposure to gamma rays. Various types of shields have been developed using high density concrete, lead brills etc. The gamma ray attenuation of these materials has been widely studies and attenuation data is available such as the National institute of Standards and Technology (NIST) WINXCOM software.

Composite materials such as lead wool, high density metals dispersed in organic polymers etc are finding more use as shielding materials. The attenuation of γ -rays occurs through the interaction with matter. The degree to which gamma radiation is attenuated is dependent upon the energy of incident radiation, the atomic number and density of shielding materials and thickness of the shielding composite materials may offer additional benefits like chemical resistance, physical durability, etc., but due to non-availability of composite materials, in the present work, attenuation of gamma rays has been studies natural Zinc. In this work, we evaluate the gamma-ray as a function of energy, for 0.511, 0.662, 1.275 MeV, by using ¹³⁷ Cs and ²² Na radioactive sources [1].

In the present work, gamma radiation mass absorption coefficients of Zinc using ²² Na and ¹³⁷ Cs gamma sources are measured and reported in this work. These measurements were performed using the 1” × 1” NaI (Tl) scintillation detector coupled to photo multiplier tube, preamplifier and amplifier. The signals from amplifiers were fed to the computer based multichannel analyzer (MCA) gamma ray spectrometry system for pulse height analysis [2]. The knowledge of gamma ray interaction mass attenuation coefficient of a material provides not only better understanding of the degradation of intensity but also a basis for safe handling of what might be admittedly harmful material.

The accurate determination of mass attenuation coefficients is essential before a given material is used in as gamma ray shield. Different other workers have calculated attenuation coefficients in different categories such as building materials, alloys, marbles, glasses, biological materials and other composite materials. Half value layer (HVL) is another useful parameter for understanding the interaction of gamma ray. Half value layer (HVL) determines the thickness required to reduce the intensity to a safe level. It also determines the effectiveness of a material to be used for shielding radiation. Many workers have studied HVL for shielding characteristics of materials [3]. When selecting the best shielding material for a particular source of radiation, it is important to understand the mechanisms through which the gamma radiation is attenuated. As discussed earlier the most important factors that determine the relative importance of the mechanisms through which gamma radiation is attenuated are: (1) The energy of the gamma radiation and (2) The atomic number of the element(s) from which the shielding is constructed. Three of most important mechanisms for gamma radiation are photoelectric absorption, Compton scattering and pair production [4].

The reduction of the intensity of a γ-ray beam as it traverses matter is attenuation. The reduction may be caused by absorption or by deflection (scatter) of photons from the beam and can be affected by different factors such as beam energy and atomic number of the absorber [5]. Several factors affect attenuation. Some are related to the γ-ray beam or radiation and the others to properties of the matter through which the radiation is passing. Some of them are factors affecting attenuation, effect of atomic number, effect of thickness effect of density and effect of gamma-ray energy [6].

Experimental measurement of mass attenuation coefficients of a material in our country is not practiced well. This may be because of lack of necessary instruments and radiation sources. Research on the measurement of mass attenuation coefficients is necessary designing radiation shield from the available local materials. As a first beginning on this area of research in Mattu University in this research Review, an attempt was made to experimentally determine the mass attenuation coefficient of Zinc. Gamma ray attenuation coefficients can also be used for thickness determination of these foils.

To explain the attenuation coefficients of gamma rays in Zinc for ¹³⁷ Cs and ²² Na sources and to compare the experimental result of mass attenuation coefficients with theoretical (NIST.WINXCOM).

Materials

In this research experiment the following instruments has been used. Zinc foils of different thicknesses, Gamma sources ( ²² Na, ¹³⁷ Cs), NaI(Tl) gamma ray detector, Multichannel Analyzer (MCA), High voltage power supply, Measure software installed computer (MEASURE), Desktop Computer.

Method of data analysis

Complete experimental setup used in the present experiment is shown in Figure 1, on the experiment right hand side, NaI (Tl) detector is mounted vertically. Below the detector a well collimated gamma ray source is placed. The collimated is about 0.8 cm in diameter. Next to the detector is a unit which houses high voltage supply and a 4k multichannel analyzer (Figure 1).

Experimental Results and Discussion

Gamma ray spectrum of ¹³⁷ Cs with NaI(Tl) Detector

The gamma ray spectrometry was properly calibrated for energy using one photo peak of ¹³⁷ Cs at 662 KeV. Then first gamma ray spectrum of ¹³⁷ Cs was recorded for 3600 sec without any absorber foil. This spectrum is shown in Figure 2. Then Zinc foil of thickness 0.04 cm was placed between the detector and source. Again, the spectrum was recorded for 3600 sec. Same steps we have for expected for Zinc foils of thickness 0.08 and 0.12 cm. The spectrum recorded with Zinc foil of thickness 0.12 cm is shown in Figure 3. This figure shows the gamma ray spectrum of ¹³⁷ Cs source (Eγ = 0.662 MeV) counted by NaI (Tl) detector coupled to M.C.A without Zinc foil. Figure 2 shows gamma ray spectrum of ¹³⁷ Cs source (Eγ = 0.662 MeV) with Zinc absorber foil of thickness 0.12 cm.

Gamma ray spectrum of ²² Na with NaI(Tl) detector

The gamma ray spectrometry was properly calibrated for energy using two photo peak of ²² Na at 551 KeV and 1274 KeV. Then first gamma ray spectrum of ²² Na was recorded for 3600 sec without any absorber foil. This spectrum is shown in Figure 4.

Then Zinc foil of thickness 0.04 cm was placed between the detector and source. Again the spectrum was recorded for 3600 sec. same steps we have for expected for Zinc foils of thickness 0.08 and 0.12 cm. The spectrum recorded with Zinc foil of thickness 0.12 cm is shown in Figure 5.

Background gamma ray spectrum

As well as detecting radiation emitted from the radioactive source, all detector systems were register events that originate from the surrounding environment. Provided this is relatively constant, measurements of empty sources can be made to quantify the levels of background radiation expected. Before all experiments, background spectra were acquired (Figure 6). The room background was counted with an empty sources holder for 3600 seconds.

Discussion for mass attenuation coefficient

The values of thicknesses in cm, transmitted intensity (I) and transmissions  are expressed for ¹³⁷ Cs (662 KeV) in Table 1, ²² Na (511 KeV) in Table 2 and ²² Na (1274 KeV) (Table 3).

Transmission as a function of thickness in a Zinc sample at three different photon energies of gamma sources ¹³⁷ Cs (662 KeV) and ²² Na (511 KeV, 1274 KeV) are shown in Figure 7.

The plot lines correspond to the best fits to the experimental data. The slope of these graphs gives the value of the linear absorption coefficient.

Theoretical mass attenuation coefficients were determined using WINXCOM software. The comparison of measured mass attenuation coefficients with theoretical values are summarized in Table 4 and plotted as a function of photon energy in Figure 8.

The experimentally measured values are in good agreement with the theoretically calculated values within acceptable experimental errors. It is clearly seen from Figure 8 that the experimental values fall onto the theoretical curve computed by WINXCOM code and show a good agreement within the experimental error, and increasing photon energy causes the decrease of mass attenuation coefficient; the initially mass attenuation coefficient have maximum value, and it drops rapidly with increase in photon energy for Zinc absorber.

Conclusion

The mass attenuation coefficients for Zinc were measured in the gamma energy for ¹³⁷ Cs (0.662 KeV) and ²² Na (0.511 KeV, 1.274 KeV) by using 1’ × 1’ NaI (Tl) scintillation detector. The experimental values were compared with the theoretical data obtained by WINXCOM software. The comparison of the data showed good agreement between the obtained experimental values and theoretical values measured with WINXCOM Software. This shows that the method and experimental setup used by us can be successfully used for determination of linear and mass attenuation coefficients of any material, may be eliminated or composite. The attenuation properties of some Zinc have been evaluated and discussed in terms of mass attenuation coefficients at different gamma ray energies. The attenuation coefficients decrease with the increasing photon energy for these materials (Figure 7). This is due to the different photon absorption mechanism for different photon energies. Mass attenuation coefficients of the studied Zinc were also determined computationally by using WINXCOM code. It is seen that experimental results are in good agreement with the theoretical results [7-22].

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