Optoelectronic imaging and detection with hybrid photon detector

1. Introduction

The basic concept of Hybrid Photon Detectors (HPDs) is known since 1957 . It was pursued further in the 1960. In recent years, further work on HPDs was started , profiting from improved performance of silicon PIN-diodes. Our new concept of an optoelectronic camera.

2. State of the art

2.1. Hybrid Photomultiplier Tubes (HPMTs) These devices consist of small silicon PIN diodes (anodes) bombarded in the cross-focusing mode (Fig. 1) with accelerated (B15 kV) photoel- ectrons from photocathode’s of much larger areas.

The tiny anode sizes (B2mm diameter)

result in small diode capacitance and low leakage currents. Together with the comparatively large number of electron–hole pairs (278/keV) per absorbed photoelectron, these properties lead to small signal fluctuations.These peculiarities make HPMTs particularly suited for photon counting, which is clearly demonstrated by comparing Fig. 2a (from Ref.[14]) with Fig. 2b (from Ref. [15]). Both figures show photoelectron peak distributions measured with the same experimental arrangement, one (Fig. 2a) taken with an HPMT and the other one (Fig. 2b) with a photomultiplier. Although this photomultiplier (Quantacon 8850)1 was particularly designed for photon counting, it resolved less photoelectron peaks due to its higher statistical fluctuations.HPMTs can also replace photomultipliers in gamma spectroscopy. Besides their small statistical fluctuations.

HPMTs profit in this application from their increased photoelectron collection efficiency. This results from their B15 kV electric acceleration field as compared to the modest voltage (B400V) of photomultipliers, applied between photocathode and their first dynode.

2.2. Imaging Silicon Pixel Array (ISPA) tubes

ISPA-tubes are alternative devices for image intensifiers. They represent a one-stage optoelectronic camera in contrast to the multi stages of image intensifiers needed to achieve the required light amplification. As displayed 

photons from a radiation detector strike the ISPA-photocathode. They generate photoelectrons, which are electro statically accelerated in the proximity focusing mode to 25 keV onto silicon pixels at the ISPA-anode.

It consists of a finely segmented matrix with rectangular or square PIN-diodes, some 50 mm_500 mm or 200 mm2 in size. Each detector pixel is bump bonded to its proper and equally sized electronic pixel (Fig. 3b), which comprises preamplifier, comparator, delay line, coincidence logic, and memory [16]. Their total read-out time amounts to less than 10 ms, far surpassing the several milliseconds of a CCD. The small pixel areas and the direct bonding between detector and electronics reduce considerably the input capacitance of the preamplifier.

This provides low noise levels that are essential for the contrast in optoelectronic images. The contact layer of the pixel chip (Fig. 3a) serves as an electrode for the chip bias voltage. Each time one or more photoelectrons hit the anode chip, they generate a fast (B10 ns) electronic pulse. These analogue signals allow for self-triggering of ISPA                                       fig.3 tubes and enable the suppression of unwanted              background events by setting appropriate energy windows.

ISPA tubes have been successfully applied for particle tracking with scintillating fibres. Track images from square fibre bundles each comprising 1600 hexagonal-shaped 60 mm individual fibres, which have been exposed to a 120 GeV/c negative pion beam [13], are shown in Fig. 4. The particle tracks are composed of micro-vectors [17] with 100 mm (FWHM) residuals, which indicate the two-track resolution.

This exposure confirms that ISPA tubes improve considerably the read-out of scintillating fibres compared with the bulky intensifier chains [18,19] and their CCD-read-out.Self-triggering can be applied to suppress ghost tracks due to showers from gamma pair-conversions and particle multiplicities due to secondary interactions.For their application in beta radiography, thin planar discs of organic beta detectors are mounted in front of ISPA tubes to image their scintillations. To measure the achieved spatial resolution, we placed a brass template with slit patterns (Fig. 5a) between the beta sources and the detector discs.