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In this paper, the dispersion relation of the relativistic travelling-wave tube (RTWT) and relativistic backward-wave oscillator (RBWO) filled with plasma is obtained in the presence of an annular electron beam and solved by an exact numerical solution. The interaction of the beam with the waveguide modes is clearly identified and beam line splitting into the slow and fast waves is observed. In addition, the growth rates of each structure are investigated. The results show that the effect of plasma on the temporal growth rate of the TM 01 mode in the RBWO is negligible and the higher modes are not affected. Conversely, the spatial growth rate in the RTWT increases significantly with the plasma density.

Build-up factor (BF) is one of the radiation interaction parameters, which is important in radiology and power sector to mitigate the radiation risks as low as possible. In this paper, the BF values for water and soft tissue have been computed for gamma rays within the energy range 0.2–2 MeV for penetration depths up to 10 mean free path (mfp), using MCNP simulation code by introducing a co-centric multilayer model. The results were compared with previously published data and standards. The employed model improved prediction of the BFs at 7 mfp and 10 mfp for 0.2 and 2 MeV compared to slab geometry model of MCNP. Besides, for the most part, the BFs calculated by simulation had a good agreement (<10%) with those of the ANSI/ANS-6.4.3 standard. There was a maximum difference at 10 mfp and 0.5 MeV and the minimum difference was in exact agreement. The uncertainties for the simulation were always below 5%, which is a partial explanation of the differences between the BFs given by the ANS standard and simulation. There were some improvements in prediction of BF values with reference to the mentioned standard, mostly for lower energies. The maximum improvement was 2%. It was also observed that the soft tissue (4 component) and water behave as the equivalent materials compared to soft tissue (ICRU-44) and A-150 plastic. It is concluded that the proposed model has such a potency for further extension of investigation for higher photon energies and penetration depths, to be used for calculation of the BFs.

In this study, a model of MCNP based on Monte Carlo method was developed to calculate the mass attenuation coefficients of 15 gel dosimeters, namely LMD1, LMD2, PAG, nPAG, nMAG, VIPAR, PABIG, MAGAT, MAGAS, PAGAT, PRESAGE, BANG-1, BANG-2, BANG-3 and MAGIC at 10-1332.5 keV photon energies. In order to validate the model, the obtained results of simulation work were compared with data of XCOM. It was observed that the results were in high accordance with data of XCOM program. Accordingly, the model can be used in dosimetry, medicine and industry for various polymers and materials. Furthermore, on one hand, parameters of radiation transport in developing new gel dosimeters and materials can be calculated and assessed using the proposed model before clinical application and on the other hand, the data obtained by simulation can be used to analyze the behavior of gel dosimeters in photon radiation fields. In this regard, among 15 gel dosimeters studied here, LMD1 and PRESAGE showed the best and worst performance in comparison with other gel dosimeters, respectively. Other gel dosimeters had a poor performance, especially in energy range of 10-50 keV. Their relative mass attenuation coefficient difference with the value of water was less than 1.4%. K: Dosimetry concepts and apparatus; Interaction of radiation with matter; Models and simulations; Detector modelling and simulations I (interaction of radiation with matter, interaction of photons with matter, interaction of hadrons with matter, etc) 1Corresponding author.