E. Angeli

Feasibility of X-ray interstitial radiosurgery based on plasma focus device

Abstract A narrow beam of relativistic electrons (REB) generated during a pinch implosion (shot) in a Mather-like plasma focus (PF) device and impinging on a high Z target is considered as X-ray brachytherapy source. The main feature of the REB consists of a narrow back warded small divergence beam with an initial cross section of few mm. The apparatus includes a PF device of about 20 cm diameter and 30 cm length. Attached to the housing that contains the inner anode in the PF, an electron pipe is at first realised by several collimations which define a target spot of about 5 mm when the target is posed at 20 cm far from the pinch region. With the actual configuration, this means a practical length of the X-ray probe of about 10 cm. First experimental results with a tungsten target show that the energy spectra of the photons consist mainly of characteristic L-X rays of mean energy around 10 keV superimposed to a continuous low intensity Bremsstrahlung which photon energy extend up to several tens of keV. A selective K shell ionisation assessment performed by using target from Fe to Cd indicates a mean energy of the REB in the range 35–40 keV. The dose rate delivered in a LiF dosimeter at 10 mm far from the X-ray source has been estimated around 4.5 Gy/shot. The obtained dose rate greater than 108 Gy/s and the relatively high repetition rate of the PF devices makes this proposal suitable for interstitial radiosurgery.

Fragmentation of cobalt layers in Co/Cu multilayers monitored by magnetic and magnetoresistive measurements

We have monitored the structural evolution of Co(tCo)/Cu(4×tCo) multilayers when tCo ranges from 12 to 2 A. The investigation has been performed by studying their magnetization and giant magnetoresistance, since these properties are complementary in providing information about the structure of the magnetic species into the samples. In particular, in the intermediate range of thickness, we observed no correspondence between magnetic and magnetoresistive behavior. Finally, at sufficiently low thickness, the samples exhibit noninteracting superparamagnetic features. This kind of evolution has been ascribed to the progressive fragmentation of Co layers.

Role of particle size distribution on the temperature dependence of coercive field in sputtered Co/Cu granular films

The temperature dependence of coercive field and of the ratio between the remanent and saturation magnetization of granular Co/Cu films grown by sequential sputtering has been studied with magneto-optic Kerr effect measurements in the temperature range 8.5–60 K. The observed temperature dependence of coercive field does not fit any of the plausible Tx laws commonly used to describe systems of single-domain ferromagnetic particles. We worked out a generalized model, which also includes temperature effects related to nonuniform single-domain particle size distribution. The model predictions account well for the observed temperature behavior of both coercive field and ratio between the remanent and saturation magnetization.

Transport properties and magnetic disorder/order transition in FexAg100−x films

Abstract We have studied the magnetic disorder/order transition in Fe x Ag 100− x films, with x varying from 10 to 30, focusing our attention on the interplay between transport and magnetic properties. The samples have been deposited by DC magnetron co-sputtering and analyzed by magneto-optic Kerr effect and magnetoresistance measurements, with external magnetic field applied both in and out of the film plane. Magnetization and magnetoresistive results indicate that for low Fe content ( x x ⩽30) the effect of local magnetic ordering emerges and the films can no longer be considered as granular. The presence of magnetic coherence on different length scales in this regime is discussed and related to coalescence of magnetic particles and clusters formation, with increasing Fe concentration above 20%. This value appears as a critical iron content around which the magnetic disorder/order transition occurs.

GMR as a function of temperature in FeAg granular samples: the effect of magnetic interactions

We have deposited various FeAg granular systems having different iron content and we have investigated their giant magnetoresistive properties as a function of temperature. In a previous work we observed that, at room temperature, with increasing Fe content samples behaviour changes from superparamagnetic to ferromagnetic-like, and 20% is the concentration that separates these two regimes. In correspondence with this value, the dependence of giant magnetoresistance intensity as a function of iron concentration shows a maximum. With decreasing temperature, we observed that the position of this maximum shifts progressively towards lower iron concentrations. This behaviour has been discussed in terms of the presence of magnetic interparticle interactions. An analysis of the dependence of giant magnetoresistance as a function of applied field at the various temperatures is also 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.

Coercive field vs. temperature in Fe/Ag nanogranular films

We investigate the temperature dependence of the coercive field of a nanogranular co-sputtered Fe/Ag film. The sample is superparamagnetic at room temperature and displays a 3% giant magnetoresistance. As temperature decreases from 40 to 9 K, hysteresis appears in magnetization loops. The coercive field dependence on temperature turns out to be different from the power law expected for single-domain monodispersed ferromagnetic particle ensembles. The results are analyzed in terms of a model taking into account the effect of magnetic grain size distribution on coercive field. The agreement between experiment and the model is excellent. The possibility to gain information concerning width and symmetry of the particles volume distribution from the study of coercivitys scaling with temperature is discussed.

GMR effect across the transition from multilayer to granular structure

We have studied the evolution of giant magnetoresistance in magnetic multilayers obtained by alternately growing thin cobalt and copper layers having proportional thickness. Co nominal layers thickness was progressively reduced from 12 A down to 2 A. This decrease produces a fragmentation of the Co layers and the evolution of samples magnetic properties indicates a transition from multilayer to granular behavior. Magnetic and magnetoresistive data of the granular phase have been analyzed as a function of Co thickness in order to determine particle size distribution.

Spin-dependent transport in granular films with mixed length-scale magnetic coherence

Abstract We investigated the magnetoresistance effect in presence of mixed length-scale magnetic coherence in systems displaying coexistence of ferromagnetic- and superparamagnetic-like behaviors. In spite of their different microstructure, all the samples investigated show magnetoresistance curves vs. sample magnetization with a characteristic feature: the magnetoresistance remains unchanged (viz., equal to its value for M =0) over a wide range of magnetization values. The results are interpreted considering the interplay between the different magnetic ordering scales probed by the spin-dependent scattering and the magnetization reorientation processes and the presence in the films of magnetic coherence at different length-scales. From the comparison of the results with a phenomenological model, developed for dealing with local variations of magnetic ordering, we determined the field evolution of the ratio between the characteristic length-scale for magnetic coherence and spin diffusion length in the different samples.