• V Kumar

      Articles written in Pramana – Journal of Physics

    • Neutron spallation source and the Dubna Cascade Code

      V Kumar H Kumawat Uttam Goel V S Barashenkov

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      Neutron multiplicity per incident proton,n/p, in collision of high energy proton beam with voluminous Pb and W targets has been estimated from the Dubna Cascade Code and compared with the available experimental data for the purpose of benchmarking of the code. Contributions of various atomic and nuclear processes for heat production and isotopic yield of secondary nuclei are also estimated to assess the heat and radioactivity conditions of the targets. Results obtained from the code show excellent agreement with the experimental data at beam energy,E < 12 GeV and differ maximum up to 25% at higher energy

    • Foreword

      V Kumar

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    • Measurement of neutron-induced activation cross-sections using spallation source at JINR and neutronic validation of the Dubna code

      Manish Sharma V Kumar H Kumawat J Adam V S Barashenkov S Ganesan S Golovatiouk S K Gupta S Kailas M I Krivopustov H S Palsania V Pronskikh V M Tsoupko-Sitnikov N Vladimirova H Westmeier W Westmeier

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      A beam of 1 GeV proton coming from Dubna Nuclotron colliding with a lead target surrounded by 6 cm paraffin produces spallation neutrons. A Th-foil was kept on lead target (neutron spallation source) in a direct stream of neutrons for activation and other samples of 197Au, 209Bi, 59Co, 115In and 181Ta were irradiated by moderated beam of neutrons passing through 6 cm paraffin moderator. The gamma spectra of irradiated samples were analyzed using gamma spectrometry and DEIMOS software to measure the neutron cross-section. For this purpose neutron fluence at the positions of samples is also estimated using PREPRO software. The results of cross-sections for reactions 232Th($n, \gamma$), 232Th($n, 2n$), 197Au($n, \gamma$), 197Au($n, \alpha$), 197Au($n, xn$), 59Co($n, \alpha$), 59Co($n, xn$), 181Ta($n, \gamma$) and 181Ta($n, xn$) are given in this paper. Neutronics validation of the Dubna Cascade Code is also done using cross-section data by other experiments.

    • Role of $(n, xn)$ reactions in ADS, IAEA-benchmark and the Dubna Cascade Code

      V Kumar Harphool Kumawat Manish Sharma

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      Dubna Cascade Code (version-2004) has been used for the Monte Carlo simulation of the 1500 MW$_{t}$ accelerator driven sub-critical system (ADS) with 233U + 232Th fuel using the IAEA benchmark. Neutron spectrum, cross-section of $(n, xn)$ reactions, isotopic yield, heat spectra etc. are simulated. Many of these results that help in understanding the IAEA benchmark are presented. It is revealed that the code predicts the proton beam current required for the 1500 MW$_{t}$ ADS for $K_{\text{eff}} = 0.98$ to be 11.6 mA. Radial distribution of heat is fairly in agreement with other codes like the EA-MC and it needs nearly 1% less enrichment than given by other codes. This may be because the code takes care of the role of larger order of the $(n, xn)$ reactions. It is emphasized that there is a strong need to study $(n, xn)$ reactions both theoretically and experimentally for better design.

    • Design, construction and characterization of the compact ultrafast terahertz free-electron laser undulator

      B Biswas V Kumar S Chouksey S Krishnagopal

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      A compact ultrafast terahertz (CUTE) free-electron laser (FEL) is being developed at the Raja Ramanna Centre for Advanced Technology (RRCAT), Indore. The undulator required for the CUTE-FEL has recently been developed. We have designed, built and characterized a variable gap, 5 cm period, 2.5 m long pure permanent magnet undulator in two identical segments. The tolerable error in the magnetic field was 1% in rms, and we have measured it to be 0.7%. The obtained rms phase shake is around 2°. To ensure that the trajectories do not have an exit error in position or angle, corrector coils have been designed. Shimming coils have been applied for both the undulator segments to reduce the amplitude of the betatron oscillations in the vertical trajectory. Details of novel corrector coils and soft iron shims are given and their performance is discussed.

    • Extension of CASCADE.04 to estimate neutron fluence and dose rates and its validation

      H Kumawat V Kumar P Srinivasan

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      Capability to compute neutron dose rate is introduced for the first time in the new version of the CASCADE.04 code. Two different methods, `track length estimator' and `collision estimator' are adapted for the estimation of neutron fluence rate needed to calculate the ambient dose rate. For the validation of the methods, neutron dose rates are experimentally measured at different locations of a 5Ci Am–Be source, shielded in Howitzer-type system and these results are compared with those estimated using (i) modified CASCADE.04.d and (ii) MCNP4A codes and it is found that the agreement is good. The paper presents details of modification and results of the comparative study.

    • Generation of 2.1 𝜇m wavelength from degenerate high gray track resistant potassium titanyl phosphate optical parametric oscillator

      S Verma C Mishra V Kumar M Yadav K C Bahuguna N S Vasan S P Gaba

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      This paper presents the experimental results of degenerate optical parametric generation using a high gray track resistant potassium titanyl phosphate (HGTR KTP) optical parametric oscillator (OPO). An average output power of 7 W at 10 kHz has been achieved that includes both signal and idler powers near degeneracy using 20Waverage power from a 1064 nm Nd:YVO4 pump source corresponding to an optical conversion efficiency of 35%.

    • Elastic constants and Debye temperature of wz-AlN and wz-GaN semiconductors under high pressure from first-principles

      B P Pandey V Kumar Eduardo Menendez Proupin

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      First-principles calculations were performed to study the elastic stiffness constants ($C_{ij}$) and Debye temperature ($\theta_D$) of wurzite (wz) AlN and GaN binary semiconductors at high pressure. The lattice constants were calculated from the optimized structure of these materials. The band gaps were calculated at 𝛤 point using local density approximation (LDA) approach. The unit cell volume, lattice parameters, $c/a$, internal parameter (𝑢), elastic constant ($C_{ij}$), Debye temperature ($\theta_D$), Hubbard parameter (𝑈) and band gap ($E_g$) were studied under different pressures. The bulk modulus ($B_0$), reduced bulk modulus ($B'_0$) and Poisson ratio ($\vee$) were also calculated. The calculated values of these parameters are in fair agreement with the available experimental and reported values.

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