Suppression of bremsstrahlung losses from relativistic plasma with energy cutoff

We study the effects of redistributing superthermal electrons on bremsstrahlung radiation from hot relativistic plasma. We consider thermal and nonthermal distribution of electrons with an energy cutoff in the phase space and explore the impact of the energy cutoff on bremsstrahlung losses. We discover that the redistribution of the superthermal electrons into lower energies reduces radiative losses, which is in contrast to nonrelativistic plasma. Finally, we discuss the possible relevance of our results for open magnetic field line configurations and prospects of the aneutronic fusion based on proton-boron-11 (p-B11) fuel.

Evidence for higher order QED effects in e+ e- pair production at the BNL Relativistic Heavy Ion Collider

A new lowest order QED calculation for BNL Relativistic Heavy-Ion Collider e+ e- pair production has been carried out with a phenomenological treatment of the Coulomb dissociation of the heavy-ion nuclei observed in the STAR ZDC triggers. The lowest order QED result for the experimental acceptance is nearly 2 standard deviations larger than the STAR data. A corresponding higher-order QED calculation is consistent with the data.

Monte Carlo dose calculations in homogeneous media and at interfaces: a comparison between GEPTS, EGSnrc, MCNP, and measurements

Three Monte Carlo photon/electron transport codes (GEPTS, EGSnrc, and MCNP) are bench-marked against dose measurements in homogeneous (both low- and high-Z) media as well as at interfaces. A brief overview on physical models used by each code for photon and electron (positron) transport is given. Absolute calorimetric dose measurements for 0.5 and 1 MeV electron beams incident on homogeneous and multilayer media are compared with the predictions of the three codes. Comparison with dose measurements in two-layer media exposed to a 60Co gamma source is also performed. In addition, comparisons between the codes (including the EGS4 code) are done for (a) 0.05 to 10 MeV electron beams and positron point sources in lead, (b) high-energy photons (10 and 20 MeV) irradiating a multilayer phantom (water/steel/air), and (c) simulation of a 90Sr/90Y brachytherapy source. A good agreement is observed between the calorimetric electron dose measurements and predictions of GEPTS and EGSnrc in both homogeneous and multilayer media. MCNP outputs are found to be dependent on the energy-indexing method (Default/ITS style). This dependence is significant in homogeneous media as well as at interfaces. MCNP(ITS) fits more closely the experimental data than MCNP(DEF), except for the case of Be. At low energy (0.05 and 0.1 MeV), MCNP(ITS) dose distributions in lead show higher maximums in comparison with GEPTS and EGSnrc. EGS4 produces too penetrating electron-dose distributions in high-Z media, especially at low energy (<0.1 MeV). For positrons, differences between GEPTS and EGSnrc are observed in lead because GEPTS distinguishes positrons from electrons for both elastic multiple scattering and bremsstrahlung emission models. For the 60Co source, a quite good agreement between calculations and measurements is observed with regards to the experimental uncertainty. For the other cases (10 and 20 MeV photon sources and the 90Sr/90Y beta source), a good agreement is found between the three codes. In conclusion, differences between GEPTS and EGSnrc results are found to be very small for almost all media and energies studied. MCNP results depend significantly on the electron energy-indexing method.

Review of photon interaction cross section data in the medical and biological context

The probability of a photon (x-ray, gamma-ray, bremsstrahlung, etc) of a given energy E undergoing absorption or scattering when traversing a layer of material Z can be expressed quantitatively in terms of a linear attenuation coefficient mu (cm(-1)). Since mu is dependent on the material's density, rho (g cm(-3)), which can be variable, the quantity usually tabulated is the mass attenuation coefficient mu/rho (cm2 g(-1)) in which the dependence on the density has been removed. Mu/rho, in turn, can be obtained as the sum of the different types of possible interactions of photons with atoms of the material. For photon energies below 1 MeV the major interaction processes to be considered are incoherent (Compton) scattering, coherent (Rayleigh) scattering and atomic photoeffect absorption. Above 1 MeV one must also include nuclear-field pair production and atomic-field (triplet) production, and above 5 MeV one in principle should include photonuclear absorption, although the latter is neglected in data tabulations up to the present time. This review includes a selective history of measurements and theory relating to mu/rho from the turn of the century up to the present time, and is intended to provide a basis for further calculations and critical tabulations of photon cross section data, particularly as required by users in radiation medicine and biology. The mass energy-absorption coefficient mu(en)/rho is also briefly discussed.

The absorption of high-energy photons in matter

Total absorption cross-sections of hydrogen, carbon, copper, silver, lead and uranium have been measured for photons of mean energy 94 MeV. The method was to measure the transmission through an absorber of photons from the Oxford synchrotron, using a biased liquid scintillation counter to select photons of energy near the peak energy of the machine. The experimental data for hydrogen have been used to deduce a value for the cross-section for pair production in the field of the electron of 4.7 ± 0.4 millibarn. The relative cross-sections for the heavier elements have been determined to better than ± 0.2 %, and confirm the most recent calculations of the correction to the Born approximation in the theory of pair production.

Designing, benchmarking, and applying a Monte Carlo electron transport code

The design of a Monte Carlo electron transport code is described, with particular attention being given to the modeling of multiple-scattering, ionization, and bremsstrahlung production. Comparisons of code predictions with experimental data are presented, together with simple radiation therapy applications.

Small-angle multiple scattering and spatial resolution in charged particle tomography

The formulae for the RMS scattering angle given by small-angle multiple-scattering theory are discussed and a correction to the standard Rossi formula is derived. Values of the correction factor are calculated for protons, alpha-particles and heavy ions; these results show that the correction is important for protons and alpha-particles but negligible for heavy ions with mass number 12-40. The values of the RMS scattering angle and the RMS lateral displacement are calculated for a proton beam passing through water. For protons the correction for energy loss in thick targets is even more important.