Spectroscopic properties and radiation damage investigation of a diamond based Schottky diode for ion-beam therapy microdosimetry

Abstract

In this work, a detailed analysis of the properties of a novel microdosimeter based on a synthetic single crystal diamond is reported. Focused ion microbeams were used to investigate the device spectropscopic properties as well as the induced radiation damage effects. A diamond based Schottky diode was fabricated by chemical vapor deposition with a very thin detecting region, about 400 nm thick (approximately 1.4 μm water equivalent thickness), corresponding to the typical size in microdosimetric measurements. A 200 × 200 μm2 square metallic contact was patterned on the diamond surface by standard photolithography to define the sensitive area. Experimental measurements were carried out at the Ruder Boškovic′ Institute microbeam facility using 4 MeV carbon and 5 MeV silicon ions. Ion beam induced charge maps were employed to characterize the microdosimeter response in terms of its charge collection properties. A stable response with no evidence of polarization or memory effects was observed up to the maximum investigated ion beam flux of about 1.7 × 109 ions·cm−2·s−1. A homogeneity of the response about 6% was found over the sensitive region with a well-defined confinement of the response within the active area. Tests of the radiation damage effect were performed by selectively irradiating small areas of the device with different ion fluences, up to about 1012 ions/cm2. An exponential decrease of the charge collection efficiency was observed with a characteristic decay constant of about 4.8 MGy and 1 MGy for C and Si ions, respectively. The experimental data were analyzed by means of GEANT4 Monte Carlo simulations. A direct correlation between the diamond damaging effect and the Non Ionizing Energy Loss (NIEL) fraction was found. In particular, an exponential decay of the charge collection efficiency with an exponential decay as a function of NIEL is observed, with a characteristic constant of about 9.3 kGy-NIEL for both carbon and silicon ions.

Experimental determination of the PTW 60019 microDiamond dosimeter active area and volume

“…The best fit parameters to the experimental curves were derived according to the procedure described in Ref. 23. The resulting average active volume thickness is (1.16±0.05) µm.…”

“…In particular, the α-particle energy detected by such diamond detectors was studied as a function of the incidence angle between the beam direction and the normal to the detector surface. 23,25 The penetration depth of α-particle in diamond is 13.54 µm, which is definitively higher than the nominal thickness of an MD, which is 1 µm. A fraction of the energy of the impinging α-particles is thus collected by the diamond detector in normal incidence irradiation conditions.…”

Microdosimetric characterization of clinical carbon‐ion beams using synthetic diamond detectors and spectral conversion methods

“…The full width at half maximum of the peaks can be correlated to the contributions to the variance due to the energy-loss straggling, the escape of delta rays, and the experimental factors including electronic noise and non-uniformities of detector response. 19 Increasing the depths, the particles progressively approach the Bragg peak, where small changes of particle energies result in large changes lineal energies giving rise to broader distributions. Furthermore, the carbon-ion fragmentation increases and, correspondingly, the spectra show a contribution from lighter ions appearing at medium lineal energies.…”

“…To this purpose, synthetic single crystal diamond diodes developed at Rome "Tor Vergata" University laboratories and commercialized by PTW-Freiburg as microDiamond type 60019, show very good dosimetric properties for clinical high-energy proton and carbon radiation therapy beam dosimetry. [14][15][16][17] Recently, microdosimeters, based on a similar architecture of microDiamond, featured with very small dimension, good spectroscopic properties and high radiation tolerance have been also proposed [18][19][20] in the framework of a collaboration between "Tor Vergata" University and EBG MedAustron. In addition, diamond microdosimeters based on different architectures have been also developed and investigated by other research groups.…”

Accurate spectra for high energy ions by advanced time-of-flight diamond-detector schemes in experiments with high energy and intensity lasers

“…50-100 µm) modules could give high energy resolution and nice sensitivity to high-energy protons, at expenses of a larger number of modules in the stack. In addition, using thin diamond layers allows for a high radiation hardness of the whole diamond detector 42,43 .…”