Field output correction factors and electron fluence perturbation of the microSilicon and microSilicon X detectors.

Objective.The aim of this study was to determine field output correction factorskQclin,Qreffclin,frefand electron fluence perturbation for new PTW unshielded microSilicon and shielded microSilicon X detectors.Approach.kQclin,Qreffclin,freffactors were calculated for 6 and 10 MV with and without flattening filter beams delivered by a TrueBeam STx. Correction factors were determined for field sizes ranging from 0.5 × 0.5 cm2to 3 × 3 cm2using both experimental and numerical methods. To better understand the underlying physics of their response, total electron (+positron) fluence spectra were scored in the sensitive volume considering the various component-dependent perturbations.Main results.The microSilicon and microSilicon X detectors can be used down to the smallest studied field size by applying corrections factors fulfilling the tolerance of 5% recommended by the IAEA TRS483. Electron fluence perturbation in both microSilicon detectors was greater than that in water but to a lesser extent than their predecessors. The main contribution of the overall perturbation of the detectors comes from the materials surrounding their sensitive volume, especially the epoxy in the case of unshielded diodes and the shielding for shielded diodes. This work demonstrated that the decrease in the density of the epoxy for the microSilicon led to a decrease in the electron fluence perturbation.Significance.A real improvement was observed regarding the design of the microSilicon and microSilicon X detectors compared to their predecessors.

Topological Features of Hot Carrier Induced Anisotropic Breakdown on Silicon Diode Surfaces.

Both Gunn and Morozov have reported breakdown paths (tracking) on the surface of germanium under hot carrier conditions. Many silicon and gallium arsenide device failures appear to have been caused by similar breakdown tracks extending between contacts or across junctions. In the present work on silicon, extremely anisotropic tracking has been observed on the surface of long, thin, forward biased, n + - p - p +, silicon diodes. The tracks propagate only in 〈100〉 crystallographic directions, independent of the applied field orientation, the temperature, or the crystal growth direction. For example, on a sample with a {100} surface plane, having the field oriented along a 〈110〉 direction, the tracks propagate along 〈100〉 directions which are 45° away from the applied field. Tracks on {100}-plane, disk shaped diodes, propagate radially from the center (positive biased) ohmic contact region, and mark off the 〈100〉 directions within 2°. Tracking requires both high current densities (~ 5 × 103 A/cm2) and high fields (~ 15 kV/cm), and occurs most readily on the p-region of 10 Ω cm, n + - p - p +, 〈100〉 oriented diodes. Diodes with n + - p - p + structures having n-regions ~ 1 Ω cm, and n + - n - n + structures of ~ 1 Ω cm will also track, but require much higher fields and the 〈100〉 tracking orientation is not clearly defined. Tracking does not occur on diodes having the field oriented along a 〈111〉 direction. Two basic types of tracks are observed. The first resembles a series of tiny explosion craters (~ 10 µ m diam). The second appears to be continuous in nature, even though it is extended by each applied pulse. These tracks may be several micrometers wide and deep. It is also shown that hot-minority-carrier sample explosions are anisotropic and not, as generally assumed, caused by thermal breakdown. Typically a {110} 〈111〉 oriented n + - p - p + sample requires 4 to 6 times more impulse energy to explode than a {100} 〈100〉 oriented sample.