A DEPFET pixel Bioscope for the use in autoradiography

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Abstract

The DEPFET structure consists of a field effect transistor integrated on high-resistivity silicon, which can be used as a radiation detector. Due to several features (e.g. very low noise at room temperature, information storage capability and a thin, homogeneous entrance window), the DEPFET concept is useful for various applications. In order to apply a DEPFET pixel detector in autoradiography, 64×64 matrices with a pixel size of 50μm×50μm were built. Using several ASIC chips for the readout control and signal processing, a complete sensor system allows a row-by-row detector readout with almost continuous sensitivity. First results on the device homogenity, the quantum efficiency and the very promising noise performance are presented.

Introduction

Autoradiography is one of the main methods used in the fields of cell biology and physiology, giving insight in the structure and dynamics of cells. Extensive descriptions of the method can be found elsewhere [1]; here we will only recall the main issues.

In autoradiography the distribution of radioactive markers inside one cell or a whole agglomeration of cells is examined. Depending on the specific goal of the examination, there are various requirements on the implementation of the experiment.

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    A homogeneous response and a small number of erratic hits are of course basic requests for a good detector.

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    In order to get a good spatial resolution, it is necessary to detect the outgoing radiation as close to the radioactive emitter as possible. This requires a small range of the outcoming radiation. Due to the high stopping power which most detector materials show for low-energy electrons, often beta ray emitters are chosen as markers. An especially good choice is 3H with an end point energy of 18.6 keV (mean energy 5.7 keV). In silicon, the emitted electrons have a mean range of about 1μm. Hydrogen atoms also offer a second advantage: They are abound in organic material and can therefore be easily introduced without changing the properties of the biological tissue. A small range of the incoming radiation puts a remarkable challenge on the construction of detectors. They must have very thin entrance windows and they must show an excellent noise behaviour in order to be sensitive to the small signals generated by low-energetic radiation.

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    If a detector also offers energy resolution, interesting studies can be done using different markers at the same time, each of them characterizing different structural or metabolic attributes.

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    Although the dynamic behaviour of a sample can be studied with films using several similar tissues, it is much better to use a real-time detector and one identical tissue. This is especially true for in vivo studies. Working with living material on the other hand sets very narrow limits to the environment variables (e.g. temperature, atmosphere, etc.).

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    Last but not least, the experimental setup should be small and easy to handle.

Silicon detectors offer several advantages compared to other devices used in autoradiography [1]. Most important of all is the possibility to take fast images allowing to examine dynamic processes. In the following we will show, how a silicon pixel detector based on the DEPFET structure can be used as a device for in vivo β-autoradiography with 3H.

Section snippets

DEPFET pixel detectors

The Depleted Field Effect Transistor (DEPFET) structure consists of a field effect transistor integrated on high-resistivity silicon, which can be used as a radiation detector (Fig. 1). Using the sidewards-depletion method [2], a potential minimum for electrons is created in the fully depleted substrate underneath a field effect transistor. Signal charges are produced by a particle or photon impinging on the unstructured, thin rear-side diode. Whereas the holes will disappear in the rear

Setup of the DEPFET pixel Bioscope

In order to build up a first system applicable in the field of autoradiography, 64×64 matrices with a pixel size of 50μm×50μm were produced. The rear side of the detector chip, on which the samples are placed, is composed of a shallow p+ implantation (quite similar to that used in [3]), 30 nm of SiO2 and a thin protective nitride layer (100 nm) on the surface. This results in an insensitive region of ∼0.15μm thickness forming the uniform entrance window. Using the relation given by Fitting [4]

First measurement results

First results showing the capability to detect tritium were already presented in a previous paper [7]. Here we will show some recent results on the noise performance and the detector homogeneity.

The noise performance of a DEPFET pixel was measured by doing an energy calibration with the help of the Mn-Kα line and the Mn-Kβ line of an 55Fe source (see Fig. 4). Determining the width of the noise peak leads to an ENC of 9.5±0.1e at T=300K and a shaping time of 10μs. Taking into account the

Conclusion

Spectroscopic measurements on a DEPFET pixel show an extremely low noise at room temperature. The value of 146 eV FWHM for the Mn-Kα line is to our knowledge the best result obtained so far at room temperature (T=300K).

Examinations of the detector homogeneity reveal that the current variations within a matrix are very small and to a large extent are due to supply line resistances. Introducing a second metal layer, which was not available, when the detectors were produced, will effectively

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Work supported by the Deutsche Forschungsgemeinschaft (DFG) under contract WE 976/2-1 and by the Ministerium für Wissenschaft und Forschung des Landes Nordrhein–Westfalen under contract IV A5-10600198.

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