الفهرس 
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Abstract
Up to now the acceleration of charged particles in most cases is based on the radio-frequency technology. Another approach is to employ ultrahigh collective fields of plasmas produced by ultra high-power lasers. Using various schemes, these plasma fields can be used to accelerate electrons, protons, and heavier ions. Laser plasma accelerators attract a lot of attention because they are compact sources of extremely short duration electron bunches, intrinsically synchronized with the pump laser.
Laser plasma accelerator is regarded as a lab scale machine that can delivers GeV class electrons. When such electrons impinging heavy metallic targets such as Tungsten, Tantalum and Lead high energy bremsstrahlung photons and photo-neutrons will be produced.
The current study evaluates bremsstrahlung photons and photo-neutron production through MCNP modeling of 1 GeV and 150 MeV electron beam modalities produced from Laser plasma accelerator. The electron driven bremsstrahlung photons source of high mean energy is expected to be used for photo-transmutation of long lived radioactive waste while the electron driven neutron source is expected to be coupled with short (lab scale - few meters) time of flight facility (TOF) for elemental and isotopic micro assay via neutron resonance spectroscopy. The 150 MeV electron beam is considered for comparison due to reasons related to the photo-nuclear library in Monte Carlo MCNP code. MCNP modeling was accomplished in five stages by using various target specifications and replicating the geometry of the accelerator source and the investigated targets.
In the first stage, the e-γ targets are simulated along with the relevant parameters for bremsstrahlung photons:
a) The optimized thickness for tungsten (W), Tantalum (Ta) and Lead (Pb) respectively.
b) The angular distribution of bremsstrahlung photons.
c) The yield of bremsstrahlung photons.
d) The fluence of bremsstrahlung photons.
e) The energy distribution.
f) The photons mean energy.
The above parameters can help in choosing the appropriate target material, its dimensions and the maximum outcome of bremsstrahlung photons. In addition, the angular distribution can help in shielding design around the target. In conclusion, for 1 GeV and 150 MeV electron beam, MCNP results showed that tungsten produces bremsstrahlung photons higher than Tantalum and Lead for the same thickness. from the angular distribution, the maximum photon fluence is produced in the forward direction which is at center and it drops radially as the angle increases. The maximum bremsstrahlung photons mean energy is found to be 26 MeV. The mean energy of the produced bremsstrahlung photons is almost higher than the Giant Dipole Resonance (GDR) regions for the majority of isotopes. This enables the electron driven photon source to be used for photo-transmutation of long lived radioactive waste and other photo-nuclear researches.
In the second stage, the γ-n targets are simulated along with the relevant parameters for photo-neutron production:
a) The ideal thickness for tungsten, Tantalum and Lead.
b) The angular distribution photo-neutrons.
c) The photo-neutron yield.
d) The photo-neutron fluence.
e) The energy distribution of photo-neutron.
f) The mean energy and nuclear temperature for each target material.
The above parameters can help in choosing the appropriate target material, its dimensions and the maximum outcome of photo-neutrons. In addition, the angular distribution can help in shielding design around the target as well as the configuration of the time of flight facility. In conclusion, for 1 GeV and 150 MeV electron beam, MCNP results showed that tungsten produces photo-neutrons higher than Tantalum and Lead for the same thickness. from the angular distribution of photo-neutrons, the produced fluence decreases smoothly at small angles and drops gradually at large angles. As the incident electron energy and/or target thickness increases, the neutron fluence is shown less isotropic. The forward-peaked angular distributions enable the configuration of the time of flight facility to be linearly with the electron driven neutron source. Tantalum is regarded as the best target for photo-neutron production since it emits neutrons with low mean energy and requires small thickness of moderators. The energy distribution of photo-neutrons appears to be independent from the target density and thickness as well as the incident electron energy and depends only on the nuclear temperature T of target material.
In the third stage, calculations of transmission and backscattering coefficients, backward emission of bremsstrahlung photons and backward emission of photo-neutrons for 1 GeV and 150 MeV electron beam impinging on tungsten, Tantalum and Lead respectively have been carried out. That can help in safety improvement of the accelerator. In conclusion, for 1 GeV and 150 MeV electron beam, MCNP results showed that the electrons backscattered coefficient (B) is directly proportional to the atomic charge Z and inversely to the energy of the incident electrons. As the target thickness increases, the backward emission of bremsstrahlung photon increases up to the optimal thickness then saturates in the same manner of forward bremsstrahlung photon. Also the backward emission of photo-neutrons that comes from the nature of isotropic production increases in the same way as the forward emission. Those backward emissions lead to high radiation level in the undesirable direction.
In the fourth stage, calculations have been carried out on multi-layered Tantalum target for 1 GeV and 150 MeV electron beam to determine the relevant parameters for bremsstrahlung photons and photo-neutron production:
a) The yield.
b) The fluence.
c) The mean energy.
d) Transmission and backscattering coefficients.
e) Backward emission of bremsstrahlung photons and photo-neutrons.
The results are compared with that for bulk target. The above parameters can help in choosing the appropriate target configuration for both of bremsstrahlung photons and photo-neutrons production as well as in improving the safe design of the target. In conclusion, MCNP results showed that as the tantalum layer thickness decreases, more the electrons get transmit from the target and the backscattered electron shows a slight decrease. The forward bremsstrahlung photons emission shows small increase while the backward bremsstrahlung emission shows a slight decrease. Also as the tantalum layer thickness decreases, the backward photo-neutrons emission decreases as same as the forward photo-neutrons emission. By applying the multi-layer configurations will decrease the radiation levels in undesirable directions.
Finally, in the fifth stage, calculations of energy deposition for 1 GeV and 150 MeV electron beam impinging on tungsten, Tantalum and Lead respectively have been carried out. The energy deposition in multi-layered Tantalum target for 1 GeV and 150 MeV electron beam will also considered and then compared with that for bulk Tantalum target. The energy deposition can help in choosing the appropriate target material and cooling systems of the metallic target. In conclusion, for 1 GeV and 150 MeV electron beam, MCNP results showed that the energy deposition increases as both target density and incident electron energy increase. For multi-layered Tantalum target, the energy deposition show small decrease as the layer thickness decreases. Such arrangement of multi-layered target will deliver an increase in heat dissipation by increasing the whole target surface area. This helps in efficient cooling of the target assembly.