Indian J. Phys. 83 (8) 1141-1146 (2009)
Experimental evaluation of multiple Compton backscattering of gamma rays in copper Arvind D Sabharwal*, Manpreet Singh, Bhajan Singh and B S Sandhu Department of Physics, Punjabi University, Patiala-147 002, Punjab, India E-mail :
[email protected]
Abstract : The gamma ray photons continue to soften in energy as the number of scatterings increases in thick target, and results in the generation of singly and multiply scattered events. The number of these multiply scattered events increases with an increase in target thickness and saturates beyond a particular target thickness known as saturation depth. The present experiment is undertaken to study the saturation depth for 279 and 320 keV incident gamma ray photons multiply backscattered from copper targets of varying thickness. The backscattered photons are detected by a Nal(Tl) gamma detector whose pulse-height distribution is converted into a photon spectrum with the help of an inverse matrix approach. To extract the contribution of multiply backscattered photons only, the spectrum of singly scattered photon is reconstructed analytically. We observe that the numbers of multiply scattered events increases with an increase in target thickness and then saturate. The saturation depth is found to be decreasing with increase in incident gamma energy. Keywords : Multiple scattering, intensity distribution, backscattered inelastic peak, saturation thickness (depth). PACS Nos. : 13.60.Fz, 32.80.Cy, 78.70.-g
1. Introduction The aim of nuclear spectroscopy is to measure the energy and intensity distribution of gamma ray photons involved in a particular process. In Compton profile studies, the photons scattered by a target should have undergone only one elastic collision. The probability of photon being scattered several times is significant for a target of finite dimensions both in depth and lateral dimensions, and acts as an interfering noise in Compton profile studies. A correct evaluation of Compton profiles requires an accurate measurement of the intensity and energy distributions of multiply scattered photons. The present experiment is undertaken to study the contribution of multiple scattering of 279 and 320 keV incident gamma ray photons from copper target of various thicknesses in terms of a parameter known as saturation depth (thickness). *Corresponding Author
© 2009 IACS
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Arvind D Sabharwal, Manpreet Singh, Bhajan Singh and B S Sandhu
2. Method of measurements The present experiment is designed to detect the flux of backscattered photons from copper targets of varying thickness. In order to determine the contribution of multiply scattered photons only, the spectrum of singly scattered photons is reconstructed analytically [1]. This singly scattered spectrum is normalized at the backscattered peak of the measured pulse-height distribution. This normalized peak intensity distribution is then divided by peak-to-total ratio of the detector corresponding to peak energy. The resulting spectrum is subtracted from the intensity distribution of scattered photons obtained by applying the inverse response matrix method to the observed pulse-height distribution (scattered spectrum). 3. Experimental set-up In the present measurements 203Hg and 51Cr radioactive sources, of strength of the order of 3.7 × 104 Bq emitting 279 and 320 keV gamma rays respectively, are used. A beam of gamma ray photons from the radioactive source impinges on the rectangular targets of copper having dimensions 80 × 40 mm (Figure 1). The radioactive source
Figure 1. Experimental set-up of present measurements.
touches the front face of copper target and the distance between the target and the gamma detector is kept 75 mm. The Nal(Tl) gamma detector (51B mm × 51 mm) is placed at 180° to the incident beam and the line joining the axes of the radioactive source and Nal(Tl) detector passes through centre of the target. The experimental setup is placed in the centre of a room and at a sufficient height from the foam bed to avoid the scattering from the walls of the room and from the scattering table respectively. The observed energy resolution of the scintillation spectrometer comes to be 12.3% and 8% for 279 and 662 keV gamma ray photons respectively. The experimental data are accumulated on a plug-in multi-channel analyzer (MCA) coupled to a Personnel Computer (PC). The recording time of the spectrum for each thickness of the target is 10 ks with the background for the same time duration. To evaluate the true scattered spectrum for each thickness, the spectra are taken with and without
Experimental evaluation of multiple Compton backscattering of gamma rays in copper
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target in the primary incident beam. The subtraction of background spectrum from the observed target-in spectrum results in events originating from interactions of gamma ray photons with the target. 4. Use of inverse response matrix In the observed spectra, the pulses resulting from the partially absorbed photons get superimposed on the true photon spectra. Increasingly complex and sophisticated methods are being used for the conversion of observed pulse-height distributions into true gamma ray spectra. We have performed this conversion of pulse-height distributions of the Nal(Tl) crystal to a true photon energy spectrum with the help of an inverse response matrix approach [2–4]. 5. Results and discussions A typically observed backscattered spectra for 13 mm thick copper target, corrected for events unrelated to the target (background events), originating from interactions of 279 keV incident photons with copper target is a composite of singly as well as multiply backscattered (curve-a of Figure 2) photons.
Figure 2. An experimentally observed pulse-height distributions (curve-a), corrected for background events, of 279 keV incident photons with copper target of 13 mm thickness for 10 ks time duration. Normalized analytically reconstructed singly scattered full energy peak (curve-b) and resulting calculated histogram (curve-c) of N(E) converting observed pulse-height distribution to a true photon spectrum.
The singly scattered events (curve-b of Figure 2) under the backscattered peak are obtained by reconstructing analytically the singly scattered inelastic peak using the experimental determined parameters like FWHM, photo-peak efficiency of the detector and counts at the backscattered peak. The experimental pulse-height distribution (curve-
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Arvind D Sabharwal, Manpreet Singh, Bhajan Singh and B S Sandhu
a) is converted to a true photon spectrum with the help of an inverse response matrix approach. The solid curve-c is the resulting histogram of number of photons, N(E), as a function of energy, and is called the response function of the scintillation detector. The events under Compton continuum of the resulting histogram accounts for photons of reduced energy originating from multiple interactions in the target and finally escaping in the direction of gamma detector, bremsstrahlung and Rayleigh scattering. The events under the calculated histogram corresponding to energy range from 90–250 keV accounts for singly and multiply scattered radiations (having energy equal to that of singly scattered ones). The numbers of events under analytically reconstructed backscattered peak (curve-b) of Figure 2 are divided by peak-to-total ratio of the Nal(Tl) detector and then their subtraction from the events under the calculated histogram (curve-c) in the specified energy range results in events originating from multiple Compton backscattering but having the same energy as in singly Compton backscattered distribution. These events, when corrected [5] for the intrinsic (crystal) efficiency of the Nal(Tl) detector, iodine escape peak and absorptions of photons in the aluminium window of the gamma detector and the air column present between the target and the detector, provide the emergent flux of multiply backscattered photons from the copper target with energy corresponding to backscattered peak. The variation of multiply scattered events under the backscattered energy peak as a function of target thickness is shown in Figure 3. × 104
Numbers of multiply scattered events
14
Target – Copper Experimental data
12
320 keV
Best-fit curve
10
8 279 keV 6
4
2
0
0
2
4
6 8 10 Target thickness (nm)
12
14
Figure 3. The numbers of multiply scattered events as a function of copper target thickness at 279 and 320 keV incident energy. The measured statistical uncertainties lie within the size of experimental observed data points represented by filled circles.
The number of multiply scattered events increases with increase in target thickness and saturates beyond a particular thickness. The target thickness at which
Experimental evaluation of multiple Compton backscattering of gamma rays in copper
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the intensity of multiply scattered photons saturates is called saturation depth (thickness). The increase in the number of multiply backscattered photons as thickness increases is caused by an increase in the number of scattering centres in the target. The increase in thickness also results in self-absorption of photons coming out of the target. Thus, a stage is reached when thickness of the target becomes sufficient to compensate for above increase and decrease in the intensity of multiply scattered events. The statistical uncertainties in the experimental observed data of numbers of multiply backscattered photons are estimated to be less than 1.5%. The measured values of saturation thickness in mm and mean free path (m.f.p.), for 279 and 320 keV incident gamma ray photons in copper target, are given in columns 4 and 5 respectively of Table 1. The column 3 of the table provides copper target thickness equal to one mean free path at the respective energy. Table 1. Experimentally measured values of saturation depth in copper for different incident photon energies. Incident energy (keV)
Backscattered 1 m.f.p. photon (mm) energy (keV)
Measured saturation thickness in mm in mean free path
279
133.4
36.6
12.0
0.33
320
142.1
37.0
11.8
0.32
6. Conclusion Our present results confirm that for thick targets, there is significant contribution of multiply backscattered radiation emerging from the target, having energy equal to that of singly scattered Compton process. The numbers of these multiply backscattered events increases with increase in target thickness and saturates beyond a particular thickness known as saturation depth (thickness). The saturation depth at backscattering angle of 180° for 279 and 320 keV incident gamma rays are observed for the first time. The saturation depth decreases with increasing energy because the penetration of gamma ray photons increases with increase in incident energy, so the backscattered radiation has to propagate through a large thickness and the flux of multiply backscattered photons having energy equal to the singly backscattered photons reduces. To have better understanding of energy dependence of saturation depth, the work on multiple backscattering is in progress for different incident photon energies of 511 and 662 keV under same geometrical conditions employing targets of different materials. There is also a need to simulate the experiment with the Monte Carlo calculations for better understanding of the energy and intensity distributions of multiply backscattered photons. References [1]
Manpreet Singh, Gurvinderjit Singh, B S Sandhu and Bhajan Singh Appl. Rad. Isot. 64 373 (2006)
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Arvind D Sabharwal, Manpreet Singh, Bhajan Singh and B S Sandhu
[2]
J H Hubbell Rev. Sci. Instr. 29 65 (1958)
[3]
J H Hubbell and N E Scofield IRE Transactions of the Professional Group of Nuclear Sciences NS-5 156 (1958)
[4]
Manpreet Singh, Gurvinderjit Singh, Bhajan Singh and B S Sandhu Phys. Rev. A74 042714 (2006)
[5]
Gulshan Datta, M B Saddi, Bhajan Singh and B S Sandhu Rad. Meas. 42 256 (2007)
