Induced photonuclear interaction by Rhodotron-TT MeV electron beam

Transcription

1 PRAMANA c Indian Academy of Sciences Vol. 78, No. 2 journal of February 2012 physics pp Induced photonuclear interaction by Rhodotron-TT MeV electron beam FARSHID TABBAKH 1,, MOJTABA MOSTAJAB ALDAAVATI 2, MAHDIEH SADAT HOSEYNI 2 and KHADIJEH REZAEE EBRAHIM SARAEE 3 1 Institute of Nuclear Science and Technology, P.O. Box , Tehran, Iran 2 Department of Physics, Faculty of Science, University of Isfahan, Isfahan , Iran 3 Department of Nuclear Engineering, Faculty of Modern Sciences and Technologies, University of Isfahan, Isfahan , Iran Corresponding author. ftabbakh@aeoi.org.ir MS received 18 February 2011; revised 11 August 2011; accepted 26 August 2011 Abstract. In this paper the photonuclear interaction induced by 10 MeV electron beam generating high-intensity neutrons is studied. Since the results depend on the target material, the calculations are performed for Pb, Ta and W targets which have high Z, in a simple geometry. MCNPX code has been used to simulate the whole process. Also, the results of photon generation has been compared with the experimental results to evaluate the reliability of the calculation. The results show that the obtained neutron flux can reach up to n/cm 2 /s with average energies of 0.9 MeV, 0.4 MeV and 0.9 MeV for these three elements respectively with the maximum heat deposited as 3000 W/c 3, 4500 W/c 3 and 6000 W/c 3. Keywords. Photonuclear interaction; Rhodotron; electron beam; photoneutron. PACS Nos Dz; Lx; Cz 1. Introduction The electron accelerators can be considered as neutron generators with some specifications such as higher intensity, lower cost and much smaller size compared to the nuclear reactors but with lower flux in thermal spectra. On the other hand, isotope production method via (γ, n) reaction has more production yield [1] due to the wide range of neutron energy beside the research reactors in which, the neutron energy widely is in the thermal range. In this paper, we study the results related to the neutron generation by 10 MeV electron beam. Since most of the previous works for producing neutrons were performed using highenergy electron beams [2 8], in the present work we use 10 MeV electrons with a current DOI: /s y; epublication: 30 January

2 Farshid Tabbakh et al Table 1. Photonuclear threshold energy and the isotope abundances of Pb, Ta and W. Isotopes Threshold energy (γ, n) (MeV) Abundance (%) Pb Pb Pb Ta W W W W of 10 ma in a continuous beam impinging the target to generate the neutrons. The desired beam is obtained from Rhodotron TT-200. The MCNPX(2.4.0) [9] code used to simulate the photonuclear interaction is prepared from the processed data provided by Los Alamos National Laboratory (LANL) and the Chinese Nuclear Data Center (CNDC) which are submitted to the IAEA Coordinated Research Project (CRP) on photonuclear data [10,11] in ENDF-6 format and suitable for MCNPX Monte Carlo codes. They are released as two libraries, lanl01u and cndc01u, respectively. The photonuclear cross-sections for tantalum and lead are extracted from lanl01u library and for tungsten from cndc01u library including the XSDIR file modification for the corresponding elements. The photonuclear threshold energies and abundances of the related isotopes for these elements are presented in table 1 [12]. As shown in this table, lead, tantalum and tungsten elements are selected because their threshold energy of less than 10 MeV and higher Z increase the Bremsstrahlung interaction. Though, there are other isotopes such as 65 Cu, 61 Ni, 57 Fe, 53 Cr and 49 Ti with threshold energies less than 10 MeV, they should be ignored due to their very small abundances. In the following sections, we shall study the photoneutron yield and flux corresponding to the target materials and the geometry (target thickness) including the energy distribution and the heat deposited in the targets. 2. Calculation and results MCNPX calculates the electron-to-photon conversion via Bremsstrahlung interaction as well as the photon-to-neutron conversion. The target is cylindrical with 5 cm radius and variable height (will be taken as the target thickness) along the z-axis and the beam impinges on one of its ends as shown in figure 1. The results are obtained under the variance reduction methods of weight windows energy (WWE), weight windows parameter (WWP) and cell-based weight windows bound (WWN1). Another variance reduction method in this work is using the tally F5 with errors less than 5% instead of using surface or cell tallies F1, F2 and F4. But as we 258 Pramana J. Phys., Vol. 78, No. 2, February 2012

3 Induced photonuclear interaction by 10 MeV e-beam Figure 1. Target geometry and the beam direction. represent the detector locations. The points along the z-axis shall discuss later, we have used tally F2 (figure 4) to indicate that the flux along the beam direction has the maximum value. The energy deposition in the targets is calculated using tally type 6. The threshold energies presented in table 1 are considered as the cut-off energy to decrease the computation time. The output files provide the statistical information by which, one can study many aspects such as, photon and neutron angular distributions and the reactions which cause neutron, photon and electron creation and loss including their contributions. The output files also illustrate that, almost 97% of the neutrons come from photonuclear reaction and the rests belong to (n, xn) interaction. In the following, we present more information for comparing our results with the experimental measurements or available standard data that can prove the reliability of the results. In a high-z target, the ratio of electron energy lost via Bremsstrahlung interaction to the total energy lost (Bremsstrahlung + collisions) is given by the expression ((de/ρdx) r /((de/ρdx) r + (de/ρdx) c )), where (de/ρdx) r and (de/ρdx) c are mass radiative and mass collision stopping power respectively. According to ref. [13], the corresponding values for 10 MeV incident electrons for the mentioned elements are almost 0.5 and the MCNPX outputs will lead us to the same value by dividing the Bremsstrahlung weight energy by summation of both the Bremsstrahlung and scattering weight energies. In addition, the average energy of the generated neutrons can be calculated by dividing the weight energy produced by total weight produced for photoneutrons in the targets [7]. Therefore, the average energies of neutrons are 0.9 MeV, 0.4 MeV and 0.9 MeV for Pb, Ta and W respectively. For further evaluation of calculation precision, we experimentally have measured the X-ray dose rate from a 0.3 cm thick tantalum sheet as X-ray target which is adjusted under the scanning horn of Rhodotron TT-200 (figure 2). In this method, a GM counter tube, LB 6500, with 20% of accuracy is used which is located 100 cm below the centre of the target sheet and to omit the backscattered photon, the dosimeter is placed in a lead shield with an open aperture to receive the X-rays emitted from only the horn. The measured dose rate is Sv/h and the calculated dose rate under the same experimental condition is Sv/h. The experiment is performed using 20% accuracy detector. Also, measurement accuracy of the target position (5%) and MCNP error (less than 5%) will lead us to something Pramana J. Phys., Vol. 78, No. 2, February

4 Farshid Tabbakh et al Figure 2. Detector positioned 100 cm below the centre of the X-ray target sheet. around 30% accuracy. So the calculated value is in good agreement with the experimental value which means that we can rely on our calculation for photoneutron as it cannot be measured experimentally. Figure 3 shows the yield of photon produced inside the targets via Bremsstrahlung interaction. It indicates an increase in the photon production by increasing the target thickness as a consequence of increasing the interaction and then there are smooth peaks representing the Auger electrons. The vanishing electron interaction and so the photon production are shown as straight lines after the peaks. Figure 4 shows the neutron flux variation across the Ta target end-surface at z = 0.5 cm. The large circle represents the cross-section of the whole target. One can understand that Figure 3. Yield of photons produced via Bremsstrahlung interaction in the targets. 260 Pramana J. Phys., Vol. 78, No. 2, February 2012

5 Induced photonuclear interaction by 10 MeV e-beam Figure 4. Neutron distribution across the target cross-section at z = 0.5 cm. the flux variation decreases along the cylinder radius. In this figure tally F2 as surface tally is used. Figure 5 shows the neutron yield per one incident electron in terms of Pb, Ta and W target thicknesses represented by the solid, dashed and dotted curves respectively. Increasing the target thickness will cause an increase in the photoneutrons yield. As one can see, at the optimal thicknesses of 8 cm, 4 cm and 2 cm, the gradients of the corresponding curves are negligible. In other words, we can conclude that increasing the thickness more than the optimal thickness will not affect the neutron yields. By increasing the thickness, Bremsstrahlung interaction will be more, and so more photons will be there to generate photoneutrons, but after reaching the optimal thickness Figure 5. The neutron yields in terms of the target thickness for Pb, Ta and W elements. Pramana J. Phys., Vol. 78, No. 2, February

6 Farshid Tabbakh et al (2 cm for W, 4 cm for Ta and 8 cm for Pb) there will not be any Bremsstrahlung interaction because the electron population will be vanished in more target depth and we have only photons all around with energy less than the threshold energy. Increasing the thickness will not change the electron population anymore, and so the electron population and consequently the photoneutron yield will remain constant. We can see from figure 5 that the total neutron value in the whole target first increases when the thickness increases and then will remain constant. As illustrated in this figure, a neutron yield of neutron/s will be achieved for a beam current of 10 ma. Figure 6 shows the logarithmic scale of variation of the neutron flux at the centre of the end of the cylinder along the beam direction, in terms of target thickness represented by solid, dashed and dotted lines for Pb, Ta and W respectively. For a current of 10 ma, the maximum fluxes are in the order of n/cm 2 /s and the thickness up to 8 cm will cause the flux to drop to less than n/cm 2 /s. As mentioned before, the more is the deviation from the beam direction, the less will be the flux (illustrated in figure 4). The curves in figure 6 should be started from 0 cm thickness where we shall have photons and so no photoneutrons. When electrons hit the target, at first there are not enough Bremsstrahlung interactions and therefore there are not much photoneutrons. Then it increases very sharply to its maximum value, which is shown in this figure, and then decreases as the thickness increases as explained before. Figure 6 confirms the conclusion of figure 5 which says that at the end of the target there are no extra electron because electron population is vanished, and therefore, no more neutrons will be detected. The total neutron number is constant (yield in figure 5) but the flux at the far points along the target will decrease as electrons cannot reach there. It shows that after such optimal distances no neutron is produced (like shielding aspects). Figures 7a and 7b show the energy distribution of photoneutrons in the targets per one incident electron for 0.1 cm and 6 cm thick targets respectively. The black, red and green curves represent Pb, Ta and W elements respectively. As can be seen, different energies Figure 6. The neutron flux in terms of thickness for Pb, Ta and W target. 262 Pramana J. Phys., Vol. 78, No. 2, February 2012

7 Induced photonuclear interaction by 10 MeV e-beam (a) Figure 7. Energy distribution of photoneutrons in the targets. (a) The target with 0.1 cm thickness and (b) the target with 6 cm thickness. (b) are related to different fluxes in which, when we calculate the average value of each curve from this figure (using the method of integrating the area under each curve) it will give the particular value as mentioned earlier. Here there is nothing to do with the thickness. Both figures 7a and 7b show neutron energy range from 0 to 4 MeV with the maximum distribution at 1 MeV. The neutron energy range can be described by referring to the photonuclear threshold energies given in table 1. In figure 8 the energy deposited inside the impinged elements of Pb, Ta and W are presented which determine the heat generated inside the target. It should be emphasized that, the heat is produced due to the collisions of electrons, photons and neutrons with Figure 8. Energy deposited as a function of thickness. Pramana J. Phys., Vol. 78, No. 2, February

8 Farshid Tabbakh et al the target materials. The MCNPX outputs show that most of the contributions belong to the electrons and then to photons. Therefore, the curves in figure 5 show the electron and photon contributions only. 3. Conclusion In the present work, the neutrons were generated via photonuclear interaction by 10 MeV electron beam inside the specified targets. Photon production also was studied and the related results were evaluated by experimental measurements. It is found that, there is an agreement between the calculation and the experiment. Accordingly, we have obtained the data with sufficient reliability which can be used for conceptual designing of the neutron generator based on the electron-to-neutron conversion system. These results also show the importance of considering the removal of heat from the target in the design stage. Acknowledgements The authors acknowledge the operators of Rhodotron-TT200 and the dosimetry group in Yazd Irradiation and Application Research School for their collaboration in experimental measurements. References [1] N P Dikiy, A N Dovbnya and V L Uvarov, Proceedings of the EPACK-98 Conference (Stockholm, Sweden, 1998) pp [2] G N Kim, H S Kang and J Y Choi, J. Accel. Plasma Res. 3(1), 9 (1998) [3] J Chen et al, ADSS Experiments Workshop (2006) [4] V C Petwal, V K Senecha and K V Subbaiah, Pramana J. Phys. 68(2), 235 (2007) [5] R Moreh, R C Block and Y Danon, Nucl. Instrum. Methods in Phys. Res. A562, 401 (2006) [6] G N Kim, Y S Lee and D Son, Proceeding of the Particle Accelerator Conference (Chicago, 2001) pp [7] M C White, Los Alamos National Laboratory, New Mexico (2001) [8] Y Danon, R C Block and R Testa, ANS National Meeting, Anaheim, CA, NS Transaction 98, 894 (2008) [9] Oak Ridge National Laboratory, MCNPX (2002) [10] IAEA Photonuclear Data, LA-UR (2002) [11] V C Petwal, V K Senecha, K V Subbaiah, H C Soni and S Kotaiah, Pramana J. Phys. 68, 235 (2007) [12] M B Chadwick et al, IAEA-TECDOC-1178 (2000) [13] International Commission on Radiation Units and Measurements (ICRU), ICRU Report 37, Bethesda, MD (1984) 264 Pramana J. Phys., Vol. 78, No. 2, February 2012