Federico Rocchi

Feasibility analysis of a Plasma Focus neutron source for BNCT treatment of transplanted human liver

Abstract Boron Neutron Capture Therapy preliminary treatments on transplanted human liver have been recently conducted at Pavia University. The need of high fluences of thermal neutrons imposed the use of the available thermal channel of a TRIGA reactor properly modified for this application. We analyse the possibility of using the Plasma Focus (PF) machine as a pulsed neutron source for this medical application instead of a nuclear reactor. Thermalization of the fast (2.45 MeV for D–D reactions) neutrons produced by the PF is gained with a paraffin or polyethylene moderator which contains both the neutron source and the irradiation chamber. The design parameters of a PF optimized for such an application are discussed, as well as other considerations on the advantages that this machine can bring to this kind of cancer therapy.

EBT2 dosimetry of x-rays produced by the electron beam from a Plasma Focus for medical applications

The electron beam emitted from the back of Plasma Focus devices is being studied as a radiation source for intraoperative radiation therapy applications. A Plasma Focus device is being developed to this aim, to be utilized as an x-ray source. The electron beam is driven to impinge on 50 μm brass foil, where conversion x-rays are generated. Measurements with gafchromic film are performed to analyse the attenuation of the x-rays beam and to predict the dose given to the culture cell in radiobiological experiments to follow.

A range-based method to calibrate a magnetic spectrometer measuring the energy spectrum of the backward electron beam of a plasma focus

The electron beam emitted from the back of plasma focus devices is being studied as a radiation source for intraoperative radiation therapy applications. A plasma focus device is being developed for this purpose, and there is a need for characterizing its electron beam, particularly, insofar as the energy spectrum is concerned. The instrument used is a magnetic spectrometer. To calibrate this spectrometer, a procedure relying on the energy-range relation in Mylar® has been devised and applied. By measuring the transmission through increasing thicknesses of the material, electron energies could be assessed and compared to the spectrometer readings. Thus, the original calibration of the instrument has been extended to higher energies and also to better accuracy. Methods and results are presented.

Radiation protection of PFMA-1, a plasma focus for medical applications.

A plasma focus is being developed for breeding short-lived radionuclides. The different radiation protection issues and concerns posed by the machine once in operation are analysed and discussed. Activation is shown to be totally negligible and likewise neutron emission is found to pose no concern at all. The only source of radiation risk is found to rest in the radionuclides produced, 18F and 15 O, generating a peak exposure of 1.114 Sv y(-1) at the distance of closest approach of 2.5 m. Shielding to protect against this hazard is calculated to be 5 cm Pb or 54 cm concrete for the operation area and 5.5 cm Pb for the transportation flask.

The heating of plasma focus electrodes

Plasma focus (PF) technology development today is strictly related to the possibility of a high frequency repetitive working regime. One of the more relevant obstacles to this goal is the heating of structural components due to direct interaction with plasma. In this paper, temperature decay measurements of the inner electrode of a 7 kJ Mather type PF are presented. Data from several series of shots at different bank energies are analysed and compared with theoretical and numerical models. Two possible scale laws are derived from the experimental data to correlate thermal deposition with bank energy. It is found that a fraction of about 10% of total energy is released to the inner electrode. Finally, after some considerations about the cooling and heating mechanisms, an analysis on maximum temperature sustained by materials is presented.

Dispersion relations in weakly degenerate plasmas

From a quantum mechanical point of view, electrons in laser produced plasmas can be regarded as weakly degenerate. For instance, for a plasma with electron density of 1022 cm−3 and electron temperature of 1 eV, Sommerfelds parameter is between 1 and 2. Under these conditions the usual dispersion relations for waves in plasmas need be corrected to account for degeneracy. In the present work, starting from the transport equation with a simplified version of the Boltzmann–Uehling–Uhlenbeck collision kernel the propagation of waves impinging on a plasma with weakly degenerate electrons is investigated and dispersion relations accounting for degeneracy are derived. These dispersion relations give the classical ones in the limit for Sommerfelds parameter approaching zero. A shift of the wavenumber value and a non-collisional damping due to degeneracy effects are predicted which render a weakly degenerate plasma more opaque to radiation than a non-degenerate one.

Monte Carlo Simulation of the Conversion X-Rays from the Electron Beam of PFMA-3

PFMA‐3, a dense Plasma Focus device, is being optimized as an X‐ray generator. X‐rays are obtained from the conversion of the electron beam emitted in the backward direction and driven to impinge on a 50 μm brass foil. Monte Carlo simulations of the X‐ray emission have been conducted with MCNPX. The electron spectrum had been determined experimentally and is used in the present work as input to the simulations. Dose to the brass foil has been determined both from simulations and from measurements with a thermographic camera, and the two results are found in excellent agreement, thus validating further the electron spectrum assumed as well as the simulation set‐up. X‐ray emission has been predicted both from bremsstrahlung and from characteristic lines. The spectrum has been found to be comprised of two components of which the one at higher energy, 30÷70 keV, is most useful for IORT applications. The results are necessary to estimate penetration in and dose to Standard Human Tissue.

PFMA-1: A 1-Hz, 150-kJ Pulsed Power System for Plasma Focus Generation

PFMA-1 is a 150 kJ, 40 nH Mather-type plasma focus designed for repetition frequencies up to 1 Hz and dedicated to the neutron-free endogenous production of 18F. PFMA-1 utilizes 32 capacitors, 11.1 μF each, charged in parallel at 30 kV. Snowplow calculations predict a 1.5 MA peak current and a 2.5 μs quarter-period for this device. To achieve low inductance and high peak current, one spark gap and four coaxial cables are employed per capacitor. Field-distortion, swing-cascade spark gaps were developed by R. E. Beverly III & Associates for this application (model SG-183). These switches mount directly onto the capacitor’s modified Scyllac-type bushing and are enclosed by a low-inductance grounded shield. The rated peak current is 160 kA, the maximum charge transferred is ≈ 0.4 C/shot, the self-inductance is 26 nH, and the breakdown jitter is ≈ 1 ns at 30 kV and 60 kPa dry air. The switches are cooled by a combination of deionized water, SF6 and dry air. The estimated electrode lifetime is ≈ 0.1 Mshots. Special B-dot probes within each switch canister monitor the discharge of every capacitor. Simultaneous triggering of all capacitors is accomplished by a 32-channel PFL generator. Some experimental results and waveforms obtained during preliminary testing of the facility are presented and compared with design values.

Preliminary tests and performance estimate of PFMA-1, the first prototype of a plasma focus device for SLR production

PFMA-1 (Plasma Focus for Medical Applications - 1) is the first prototype of a new application of the plasma focus technology conceived to produce short lived radioisotopes (SLR) for medical PET-TAC imaging applications. The device is a 30 kV, 150 kJ, 40 nH Mather-type plasma focus designed for repetition frequencies up to 1 Hz and dedicated to the neutron- free endogenous production of 18F by 3He-16O reactions. This paper describes some preliminary tests related to the pressure optimization of the gases used in PFMA-1, and presents some final considerations about performance improvements.

Approximate averaged Gaunt factors for free-free Bremsstrahlung emission from a weakly degenerate plasma{

The aim of the paper is the analysis of some aspects of the theoretical models used for the description of the Bremsstrahlung emission from non-Maxwellian plasmas, more precisely when quantum weak degeneracy effects come into play. Referring to the standard Sommerfeld Parameter ρ as a measure of the degree of degeneracy, we considered some reference physical environments, as Inertial Confinement Fusion experiments or the first stage of LIBS, characterized by high densities (even higher than solid state values) and relatively low temperatures (tens of eV). We investigated the relevance of the first order in ρ analytical corrections into the Born and Kramers approximations for the Gaunt factors calculations due to the evaluated non-Maxwellian electron distribution function for the weakly degenerate plasma. The shown results suggest that the degeneracy effects produce up to a 20% correction to the classical values for the Gaunt factors that should be taken into account to properly validate Bremsstrahlung spectra resulting from experiments.