Motohiro Suyama

Characterization of the QUartz Photon Intensifying Detector (QUPID) for Noble Liquid Detectors

Dark Matter and Double Beta Decay experiments require extremely low radioactivity within the detector materials. For this purpose, the University of California, Los Angeles and Hamamatsu Photonics have developed the QUartz Photon Intensifying Detector (QUPID), an ultra-low background photodetector based on the Hybrid Avalanche Photo Diode (HAPD) and entirely made of ultraclean synthetic fused silica. In this work we present the basic concept of the QUPID and the testing measurements on QUPIDs from the first production line. Screening of radioactivity at the Gator facility in the Laboratori Nazionali del Gran Sasso has shown that the QUPIDs safely fulfill the low radioactive contamination requirements for the next generation zero background experiments set by Monte Carlo simulations. The quantum efficiency of the QUPID at room temperature is > 30% at the xenon scintillation wavelength. At low temperatures, the QUPID shows a leakage current less than 1 nA and a global gain of 10^5. In these conditions, the photocathode and the anode show > 95% linearity up to 1 uA for the cathode and 3 mA for the anode. The photocathode and collection efficiency are uniform to 80% over the entire surface. In parallel with single photon counting capabilities, the QUPIDs have a good timing response: 1.8 +/- 0.1 ns rise time, 2.5 +/- 0.2 ns fall time, 4.20 +/- 0.05 ns pulse width, and 160 +/- 30 ps transit time spread. The QUPIDs have also been tested in a liquid xenon environment, and scintillation light from 57Co and 210Po radioactive sources were observed.

High Speed HPD for Photon Counting

We have succeeded in the development of a high-speed Hybrid Photodetector (HPD), by using a reduced electron lens and a newly developed avalanche diode (AD) with very low capacitance. The HPD shows fast time response of less than 500 ps in both rise and fall times, and good timing resolution of 26 ps (one sigma) for single photons at full illumination on a photocathode 8 mm in diameter. Limiting factors of the timing resolution were further investigated, and were found to be the transit time difference of electrons in vacuum along the photocathode and the jitter of the electronics. The timing resolution is 15 ps for illumination of less than 5 mm on the central part of the photocathode, which includes the jitter of 13 ps from the electronics. This resolution is the worlds highest for HPDs, and matches the resolution of MCP-PMTs. Both a GaAsP photocathode having close to 50% quantum efficiency in the visible and a bialkali photocathode having 34% QE in the UV were fabricated in view of making this device available for various applications. In this paper, we report the results of our evaluation, including discussion about the limiting factors of the timing resolution for the new HPD.

A compact hybrid photodetector (HPD)

In order to be utilized in such application fields as high energy physics or medical imaging, where a huge number of photodetectors are assembled in designated small area, the worlds smallest hybrid photodetector (HPD), the compact HPD, has been developed. The overall diameter and the length of the tube are 16 mm and 15 mm, respectively. The effective photocathode area is 8 mm in diameter. At applied voltage of -8 kV to the photocathode, the electron multiplication gain of a PD incorporated HPD (PD-HPD) is 1,600, and that of an APD (APD-HPD) is 65,000. In the pulse height distribution measurement, photoelectron peaks up to 6 photoelectrons are clearly distinguishable with the APD-HPD. Experiments established that there was no degradation of gain in magnetic fields up to 1.5 T, an important performance characteristic of the compact HPD for application in high energy physics.

Femtosecond streak tube

A magnetic focusing streak tube has been developed aiming at femtosecond time resolution. In focusing of photoelectron beam swept very fast, an unique focusing method has been used in order to improve the defocusing of the beam induced under such sweeping conditions. The limiting time resolution better than 500fs has been experimentally obtained.

Electron bombardment CCD tube

For low light level imaging application, a proximity focused electron bombardment CCD (EB-CCD) tube has been developed. In the tube, electrons emitted from the multi-alkali (S-20) photocathode in response to incident light are accelerated by the electric field and bombarded the specially processed CCD which is sensitive to electrons. The electron bombardment gain is 600 at applied voltage of -8kV to the photocathode. Single photon counting operation is possible, because the gain is larger than the readout noise and the dark noise of the CCD. The spatial resolution is better than 360 TV lines, which is the theoretical limit of the full frame transfer CCD (FFT-CCD) of 512 by 512 pixels. No major degradation of either the photocathode sensitivity or the incorporated CCD was observed during the operation for a few tens hours. The life of the EB-CCD tube is being under evaluation. Keywords: Electron tube, Photocathode, Image intensifier, Electron-bombardment, CCD, Low light level imaging

A hybrid photodetector (HPD) with a III-V photocathode

A hybrid photodetector (HPD) combined with a III-V photocathode is considered to be an ideal photodetector, because the quantum efficiency (QE) is high, the collection efficiency is 100%, and the excess noise factor is negligible. As the first step, an HPD combined with a GaAs photocathode or a blue-enhanced GaAs (BE-GaAs) photocathode has been developed. Presently, the QE is approximately 20% from 550 nm to 850 nm, which is much higher than that of a conventional multi-alkali (S-20) photocathode. The 100% collection efficiency calculated by the computer simulation is supported by the test result of the flat uniformity. Considering the application in high energy physics using a wavelength shifter, energy resolutions of both the BE-GaAs-HPD and the conventional S20-HPD were measured using 650 nm LED light pulse. The BE-GaAs-HPD shows a resolution of 17%, finer than 34% of the S20-HPD. The experimental results are consistent with the theoretical estimation. The QE of the HPD is especially important, because the electron multiplication process is noiseless.

Development of a multi-pixel hybrid photodetector with high quantum efficiency and gain

A hybrid photo-detector (HPD) consisting of a photocathode and a multi-pixel avalanche diode (MP-AD) has been developed for these years. One of the features of the HPD is a good energy resolution, and it was shown in our previous study that the further improvement would be possible by reducing the fluctuation of charge loss in the dead layer at the entrance of the MP-AD. In this paper, we report on the result of the improvement with the newly developed HPD whose MP-AD encapsulated has a thinner dead layer than before. It is demonstrated that the new HPD has a much better energy resolution which enables the clear counting up to nine photoelectrons. Other features like a uniformity among the pixels or a photocathode sensitivity of the HPD are also presented.A hybrid photodetector (HPD) consisting of a photocathode and a multipixel avalanche diode (MP-AD) was developed a few years ago. Our previous studies showed that its inherent potential for high resolution photon counting could be further enhanced by reducing fluctuations in charge loss in the dead layer at the entrance of the MP-AD. In this paper, we report on the improvement with the newly developed HPD whose encapsulated MP-AD has a thinner dead layer than before. It is demonstrated that the new HPD has much better energy resolution, which enables clearer counting up to nine photoelectrons. Further enhancement of the photocathode sensitivity of the HPD is also discussed.

Fundamental investigation of vacuum PD tube [photodiode]

As a fundamental study of photodiodes (PDs) for electron bombardment, two types of PDs have been experimentally investigated to be applied in electron tubes. A PD bombarded from the front surface (FB-PD), where a pn junction of planar structure existed, was evaluated to measure the fast response characteristics such as a 2.1 ns rise time. However, more than an order of magnitude increase of dark current was also confirmed after a long term stability test of 1000 hours. On the other hand, a PD bombarded by electrons from the rear surface (RB-PD) showed no increase of dark current after the stability test and a fast rise time of 2.7 ns. However, it was clarified that the rise time of RB-PD depended on the applied voltage to the PD, and an applied voltage of 200 V was necessary to achieve such a fast response. Since it was a much higher voltage than expected, some modifications may be necessary to achieve a fast response with a lower applied voltage, considering the yield of the PDs. Comparisons of the two types of PDs for some other characteristics are discussed. Preliminary test results of an electron bombardment APD are also discussed. >

Fundamental Investigation Of Vacuum PD Tube

As a fundamental study of photodiodes (PDs) for electron bombardment, two types of PDs have been experimentally investigated to be applied in electron tubes. A PD bombarded from the front surface (FB-PD), where pn junction of planer structure existed, was evaluated to measure fast response characteristics such as 2.1ns in rise time, however, more than an order of magnitude increase of dark current was also confirmed after a long term stability test of 1,000 hours. On the other hand, a PD bombarded by electrons from the rear surface (RB-PD) showed no increase of dark current after the stability test and fast rise time of 2.7ns. However, it was clarified that the rise time of RB-PD depended on applied voltage to the PD, and applied voltage of 200 V was necessary to achieve such fast response. Since it was a much higher voltage than expected, some modifications may be necessary to achieve fast response with lower applied voltage, considering the yield of the PDs. Comparison of two types of PDs on some other characteristics are discussed. Preliminary test results of an electron bombardment APD are also discussed.

Analysis Of Space Charge Effects In The Femtosecond Streak Tube

The space charge effects in the magnetic focusing femtosecond streak tube have been analyzed by some experiments and computer simulations. The computer simulations have been very precisely performed, especially with respect to obtaining the transient and three dimensional solution. The results have shown that the accumulation of space charge effects between the photocathode and the deflection electrode induces the broadening of the emitted photoelectron packet along the tube axis and it seriously degrades the temporal resolution at high intensity light level in femtosecond range. The detail mechanism of space charge temporal broadening has been clarified. The improvement method of the broadening is referred, too.