Multi-pixel Hybrid Photodiode Research Articles

Recent developments for the readout of a matrix of scintillators by means of a multi-pixel HPD

We have been investigating the possibility to readout a large number of crystals (more than 400) by means of WaveLength Shifting (WLS) square section fibers and multi-pixel Hybrid PhotoDiodes (HPDs) for PET applications. However, due to the small light yield, the detection efficiency for 511keV photons could be too low. To overcome this limitation, we have recently proposed to shape the surface of each finger crystal like an “half-pipe” to achieve a better scintillator—WLS fiber coupling. In this paper, we present the results of the light yield experimental measurements on a test 5×5 “half-pipe” YAP:Ce crystal matrix and a comparison with square fibers—standard matrix configuration.In view of future availability of large-area HPD, we have also assembled a small detector head using a test 4×4 YAP:Ce crystal matrix directly coupled to a 61-pixel HPD. Experimental measurements on the imaging capabilities are also presented.

Investigating the energy resolution of arrays of small scintillation crystals

Arrays of small scintillation crystals are being used increasingly for high-resolution imaging applications in nuclear medicine. Although the degree of pixellation now available is high for some scintillation materials, this spatial resolution is often achieved at the expense of degraded energy resolution due to the lower, and more variable, light-collection efficiency. The energy resolution of a detector is, however, especially important in nuclear medicine, where events that have been scattered in the body need to be rejected efficiently. The light output from a range of CsI(Tl) arrays was measured in terms of the number of photoelectrons; detected, using a hybrid photodiode (HPD). This data, used in conjunction with the measured energy resolution and an estimate of the intrinsic energy resolution of CsI(Tl), were used to assess the magnitudes of the various contributions to the overall energy resolution of these detectors. This information suggested that there is an opportunity to improve their energy resolution by carefully choosing the geometry and reflector material to minimize the variance in the light collection. If the nature of this variance is understood sufficiently well, there may be an opportunity to apply a postprocessing technique similar to that that has dramatically improved the performance of other standard scintillation detectors. This possibility depends on the use of either the uniform quantum efficiency of the multipixel HPD photocathodes or a monolithic array of PIN diodes. This would ensure that no additional or indeterminate variance in the light collection is introduced. Measurements made using fine BGO arrays for higher energy applications and columnar grown CsI(Tl) for X-ray imaging will also be presented.

A compact readout system for multi-pixel hybrid photodiodes

The excellent properties of the recently developed multi-pixel hybrid photodiodes (M-HPD) will be easier to exploit following the development of the new hybrid read-out circuits described in this paper. This system will enable all of the required read-out functions to be accommodated on a single board into which the M-HPD is plugged. The design and performance of a versatile system is described in which a trigger-signal, derived from the common-side of the silicon anode in the M-HPD, is used to trigger the readout of the 61-anode pixels in the M-HPD. The multi-channel amplifier section is based on the use of a new, commercial VLSI chip, whilst the read-out sequencer uses a chip of our own design. The common anode signal is processed by a fast amplifier and discriminator to provide a trigger signal when a single event is detected. In the prototype version, the serial analogue output data-stream is processed using a PC-mounted, high speed ADC. Results obtained using the new read-out system in a compact gamma-camera and with a small muon tracking-chamber demonstrate the low-noise performance of the system. The application of this read-out system in other position-sensitive or multi-anode photomultiplier tube applications are also described.

Results obtained using a 61-pixel Hybrid Photodiode scintillation camera

In this paper, the performance characteristics of a small prototype gamma-camera system based on the use of the recently developed Multi-pixel Hybrid Photodiode (M-HPD) are described. A compact read-out system has been developed to record the signals from all 61 pixels in the device. Single photo-electrons can be detected in each pixel. This provides an excellent means for calibrating the signal detected in each pixel in terms of the number of photo-electrons. The position-resolution of the detector has been measured as a function of the number of photo-electrons by using a finely collimated beam of light. The spatial resolution (FWHM) was measured to be 0.5 mm for a signal level of 100 photo-electrons. The M-HPD has also been used in a miniature Anger-camera system to locate the position of interaction of gamma-ray photons in a scintillator. The light-pool generated by each event in a continuous scintillation crystal is spread in the 2mm thick entrance window of the M-HPD so that the photo-electron cloud is detected by a small cluster of anode pixels. The light-spread information has been measured and is compared with optical Monte Carlo simulations.

The effects of neutron irradiation on multi-pixel hybrid photodiode tubes

The effects of neutron irradiation on a multi-pixel hybrid photodiode tube (HPD) were studied. The HPD underwent an integrated fluence of 10 13 neutrons cm 2 from a 252 Cf source. Increased leakage currents were observed during this irradiation, consistent with that observed from bare silicon detectors. The HPD was observed to be fully operational, with acceptable leakage currents, at the fluences expected during the lifetime of the CMS experiment. After the full dosage, small intrapixel inhomogeneities were observed in the HPD's response. The origins of these inhomogeneities and methods to mitigate the radiation damage were investigated.

The detection of minimum ionising particles with scintillating fibres using multi-pixel hybrid photodiodes

Recent measurements of the performance of the newly available multi-pixel Hybrid Photodiode (M-HPD) have demonstrated their particular value in the detection of very low light-level signals in the visible region. The single and multiple photo-electron response characteristics of these devices is unmatched by any other room-temperature device. This characteristic, coupled with their speed of response and the availability of an internally-generated trigger signal when one or more of the pixels detect an event, makes them particularly interesting as possible photo-detectors for fast plastic scintillators and, in particular, as detectors for reading out scintillating fibres. The results of tests made when Minimum Ionising Particles (MIPs) pass through single and multi-clad plastic scintillating fibres have confirmed the usefulness of these devices in particle-tracking applications. The technique used to read-out 61 channels of data is described along with a way to view as many as 2000 fibres with just two 61-pixel M-HPDs. Finally, an outline concept for a new high energy gamma-ray telescope is described. This could use multiple planes of scintillation fibres read-out by M-HPDs.

Ring-imaging Cherenkov detectors for LHC-B

The particle identification system proposed for the LHC-B experiment is presented. It consists of ring-imaging Cherenkov detectors with three radiator materials, including the novel use of aerogel in an imaging detector. High performance photodetectors are under development, utilizing multipixel hybrid photodiodes. Significant π K separation will be possible for isolated tracks with momenta between 1 and 150 GeV/ c. The first studies of pattern recognition in regions of high track density are described.