A Low Noise Silicon Detector Preamplifier System for Room Temperature X-Ray Spectroscopy
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A LOW NOISE SILICON DETECTOR PREAMPLIFIER SYSTEM FOR ROOM TEMPERATURE X-RAY SPECTROSCOPY
G. BERTUCCIO, A. PULLIA
Dipartimento di Elettronica e Informazione, Politecnico di Milano, P.zza L. Da Vinci 32, 20133 Milano, Italy.
ABSTRACT
The design and performances of a system for high resolution X-ray spectroscopy are presented. The detector is a low capacitance diode built on high resistivity silicon. The signal preamplification is made by means of an ultra-low noise charge amplifier of new conception. Presently the system exhibits an equivalent noise charge of 61 r.m.s. electrons at 297 K and 32 r.m.s. electrons at 223 K. It is shown how an improvement down to 30 r.m.s. electrons at room temperature is expected employing an integrated transistor on the detector chip.
LINTRODUCTION
Room temperature soft X-ray spectroscopy (1-30 keV) can be suitably performed by means of silicon detectors. The best resolution in the energy measurements either at room temperature or at low temperature can be obtained with low output capacitance detectors, such as small diode detectors and large drift detectors SDCs and CCDs [1,2]. The amplification of the signal charge delivered by the detector can be done through different electronic configurations [3,4]. A particular care is needed in the assembly of the detector-preamplifier system in order to minimize the stray capacitance ofthe electrical connection between the detector output and the charge amplifier input. The choice of high quality dielectrics (teflon, ceramic) for the detector and electronics mounting supports is moreover essential to minimize the noise contributions related to the dielectric losses [5]. The optimum resolution theoretically achievable for a detector-preamplifier system followed by an ideal optimum signal processor can be expressed in terms of Equivalent Noise Charge
(ENO as [3]:
E
r
(2KT
(1)
where Cd is the detector output capacitance including the stray contributions, IL is the detector leakage current,fT is the cut-off frequency of the preamplifier input FET and C1 is its gate capacitance. Mat. Res. Soc. Symp. Proc. Vol. 302. @1993 Materials Research Society
598
Experimental results do not verify eq. (1) not only because of non ideal filtering of the real signal processors [6] but also because of some noise contributions neglected in (1), that is the f-dependent noises [3] and the feedback resistor noise. In this sense some efforts have been recently made both in reducing the f-dependent noise contributions [5] and in taking away the feedback resistor through new charge amplifier schematics [7,8]. It is worth remarking that the best resolution possible with the adopted detector is achievable when the detector output capacitance Cd is not exceeded by the stray capacitance of the detector-electronics connection or by the FET gate capacitance C,. The latter situations would eliminate the good performance of the system. For this purpose it is useful to put the preamplifier input FET very close to the detector and realize the electrical connection by
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