Absolute spectral characterization of silicon barrier diode: Application to soft X-ray fusion diagnostics at Tore Supra

Abstract

This paper presents an experimental protocol for absolute calibration of photo-detectors. Spectral characterization is achieved by a methodology that unlike the usual line emissions-based method, hinges on the Bremsstrahlung radiation of a Soft X-Ray (SXR) tube only. Although the proposed methodology can be applied virtually to any detector, the application presented in this paper is based on Tore Supra's SXR diagnostics, which uses Silicon Surface Barrier Diodes. The spectral response of these n-p junctions had previously been estimated on a purely empirical basis. This time, a series of second-order effects, like the spatial distribution of the source radiated power or multi-channel analyser non linearity, are taken into account to achieve accurate measurements. Consequently, a parameterised physical model is fitted to experimental results and the existence of an unexpected dead layer (at least 5 μm thick) is evidenced. This contribution also echoes a more general on-going effort in favour of long-term quality of passive radiation measurements on Tokamaks.

Impurity density derivation from bandpass soft x-ray tomography: applicability, perspectives and limitations

“…Obviously, absolute density values can only be obtained provided the SXR detectors are absolutely calibrated and have known spectral responses [21,22].…”

“…This is achieved by computing this ratio for the real spectral response η SBD of Tore Supra SXR diagnostics and computing compensation coefficients that account for the different spectral response, assuming that the level of noise is not dependent on the spectral response used. The experimental spectral response η SBD of the silicon barrier diodes (SBD) used on Tore Supra was determined in laboratory using a methodology described in details in [22]. It is visible (along with η CrCu ) in figure 29(a), where account was also taken of the 50 µm Be foils behind which the diodes lie.…”

“…Technical solutions already exist that could provide the kind of spectral response required for a large number of detectors. Indeed, actively cooled Si p-i-n with multi-channel analyser (MCA) electronics such as described in [22] provide users with a histogram with 150-200 eV width for each bar. Data treatment then allows to selecting only the bars that are inside the interval of interest, thus artificially creating the necessary spectral response.…”