Effect of collimator size and absorber thickness on ... - Springer Link

PRAMANA — journal of

c Indian Academy of Sciences

physics

Vol. 53, No. 5 November 1999 pp. 851–855

Effect of collimator size and absorber thickness on gamma ray attenuation measurements for bakelite and perspex GURDEEP S SIDHU, KARAMJIT SINGH, PARJIT S SINGH and GURMEL S MUDAHAR Physics Department, Punjabi University, Patiala 147 002, India Author for correspondence MS received 26 April 1999; revised 23 July 1999 Abstract. The mass attenuation coefficient (m ) of 662 keV gamma rays have been measured in the extended media of bakelite and perspex under different collimation conditions. The increase in attenuation coefficient is seen with increase in sample thickness as well as with collimator size due to the contribution of multiple scattered photons in the uncollided beam of 662 keV gamma rays. Keywords. Attenuation coefficient; narrow beam geometry; multiple-scattered photons; half acceptance angle. PACS Nos 32.80.Cy; 32.80.Wr

1. Introduction The attenuation of gamma radiations through composite materials is of wide interest for industrial, medical and agricultural studies. There is an important parameter for characterizing the penetration and diffusion of gamma rays in the medium which is called attenuation coefficient. This parameter mainly depends on the photon energy and nature of the medium. Several workers have conducted the systematic studies of attenuation coefficients using narrow beam geometry [1–3] from time to time. When the transmitted beam is collimated properly i.e., when narrow beam geometry is made, then, the multiple scattered photons are prevented from reaching the detector and so are not measured. But as the collimator size and sample thickness are increased, the probability of multiple scattered photons to reach the detector increases. So, along with the uncollided photons the multiple scattered photons are also measured. This is because firstly with increase in sample thickness, more number of scattered photons are generated and secondly with increase in collimator size (half acceptance angle) the detector is exposed more to the scattered radiations. In this direction Gopal and Sanjeevaiah [4] have studied the effect of sample thickness on the measured attenuation coefficients of elements, C, Al, Cu, Sn and Pb for 662 keV gamma rays and reported that measured attenuation coefficient () remains fairly constant for values of t < 1 where t is the sample thickness. Similarly, Varier et al [5] have 851

Gurdeep S Sidhu et al also studied the effect of transverse and longitudinal thickness on the measured attenuation coefficient of Cu and Hg and have reported a correlation effect in the measurements due to absorber thickness and its dimensions in transverse direction. So far as composite materials are concerned, the effect of sample thickness on the measured mass attenuation coefficient of perspex and bakelite have been investigated for medium energy gamma rays in our previous studies [6]. In all these above investigations it is reported that attenuation coefficient can be measured accurately only for the sample thickness less than or equal to one mean free path. As the accuracy in the measurements of attenuation coefficient is very important, an attempt has been made in the present paper to study the simultaneous effect of sample thickness and collimator size (half acceptance angle) on the measured mass attenuation coefficient for bakelite and perspex materials.

2. Experimental details The experimental setup used for the present measurements is shown in figure 1. The distance between source and detector was kept fixed at 72 cm. In this arrangement there is a provision to increase the absorber thickness up to about 5 mfp and the half angle of acceptance of the detector from 2 0 to 240 . Bakelite and perspex materials were used as absorbers. Gamma rays of 662 keV were obtained from an 8 mCi source of 137 Cs radioactive isotope. To minimize health hazards and background radiations, the radioactive source was kept in a lead container well shielded from all sides. A NaI(Tl) scintillator detector (4.5 cm diameter and 5.1 cm thick) along with spectrometer assembly and a computerized MCA card was used for detection and recording of transmitted spectra. To prevent the background radiations from reaching the detector, it was housed in a lead container. To stop the fluorescence X-rays ( 75 keV) of lead from reaching the detector, the lead shield was lined on the inside with brass and aluminum sheets. To vary the thickness of the absorber between source and detector square blocks of bakelite and perspex were used. Whole assembly was placed in the center of the room to avoid any contribution of scattered photons from the walls. Care was taken to avoid any shifts in the peak due to environmental changes. The photon spectra of 137 Cs gamma rays were recorded by increasing the thickness of the absorber in various steps between source and detector keeping the collimator size (half acceptance angle) fixed. The whole experiment was repeated by changing the size of collimator from 9 mm (diameter) to 50 mm. With the change in size of collimator, the half acceptance angle of the detector gets changed. The relation between collimator size and the half angle of acceptance are as given in the table.

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Collimator size (mm)

Half angle of acceptance (minutes)

9 16 20 25 33 50

2 3 5 9 14 24

Pramana – J. Phys., Vol. 53, No. 5, November 1999

Attenuation measurements for bakelite and perspex

Figure 1. Experimental arrangement (not to scale).

The transmitted spectra were recorded for sufficient time for the precision and accuracy of the results. The background counts recorded for the same time were subtracted from each spectrum. The change in half acceptance angle of the detector of incident radiations enable us to study the contribution of multiple scattered photons in the recorded spectra, because with increase in half acceptance angle, more scattered radiations reach the detector.

3. Results and discussion With the increase in medium thickness, the probability of multiple-scattered photons to be added in the uncollided beam of 662 keV gamma rays increases. Moreover, when the half acceptance angle is increased, the probability of counting of more number of single or multiple scattered photons along with the beam of 662 keV gamma rays increases due to larger exposure of the detector. So the contribution of multiple-scattered photons leads to the variation in the attenuation coefficient due to changes in thickness of medium and collimator size (half acceptance angle). This means that there is definitely some kind of correlation between these two effects i.e., absorber thickness and half acceptance angle due to which attenuation coefficient gets affected. The results of the attenuation coefficient measurements for bakelite and perspex are plotted in figures 2 and 3 respectively. From these results, it is seen that at a larger absorber thickness (greater than 1 mfp), the attenuation coefficient increases with increase in absorber thickness at different half acceptance angles. Further the increase in attenuation coefficient is larger for greater half acceptance angle with increase in thickness, which is due to the contribution of multiple-scattered photons as has already been discussed in the above paragraph. From figure 2, it is seen that for a large half acceptance angle of the detector i.e., 24 0 , the contribution of multiple scattered photons is more even around one mean free path thickness of bakelite material. This leads to an increase in the value of m above one mean free path thickness of the absorber. But the multiple scattering effect is reduced when the half acceptance angle is decreased with the decrease in diameter of the collimator. Due to this m is unaffected due to multiple scattering even at large


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Figure 2. Plot of 662 keV gamma ray mass attenuation coefficient of bakelite as a function of absorber thickness for various half acceptance angles of the detector.

Figure 3. Plot of 662 keV gamma ray mass attenuation coefficient of perspex as a function of absorber thickness for various half acceptance angles of the detector.

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Attenuation measurements for bakelite and perspex absorber thickness. For half acceptance angle of 2 3 0 i.e., for a very fine beam of radiations, the effect of multiple scattered photons is reduced to the limits of errors in the measurements of m which is expected to be about 2% in composite materials. As can be seen from figure 3, similar variations in attenuation coefficient are observed in the extended medium of perspex with different collimations as in case of bakelite. From the above discussion, it can be concluded that the effect of multiple scattered photons in the measurements of attenuation coefficient of composite materials like bakelite and perspex can be minimized by using a well collimated narrow beam geometry. So, the effect of multiple scattered photons can be neglected even upto a large absorber thickness by reducing collimator size i.e., half acceptance angle of the detector, to a very fine level. Finally, it is concluded that with an optimum half acceptance angle for a given absorber thickness, the attenuation coefficient can be measured more accurately for their better use in different practical fields.

References [1] [2] [3] [4] [5] [6]

S A Colgate, Phys. Rev. 87, 592 (1952) M Wiedenbeck, Phys. Rev. 126, 1009 (1962) A L Conner, H F Atwater, E H Plassmann and J H McCrary, Phys. Rev. A1, 539 (1970) S Gopal and B Sanjeevaiah, Nucl. Instrum. Methods 107, 221 (1973) K M Varier, S N Kunju and K Madhusudanan, Phys. Rev. 33, 2378 (1986) M Singh and G S Mudahar, Indian J. Phys. A67, 79 (1993)


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