Basics of Avalanche Photodiode (APD)
Photodetector is a small single semiconductor unit unlike a photomultiplier tube (PMT) which consists of a variety of devices. In a single photodetector, impurities are doped into a silicon wafer, and a p-n junction is formed between the p-type and the n-type region. A region about the p-n junction is a depletion region with a high reverse-biased electric field, where incident photons are initially detected. An incident photon is absorbed in the depletion region, generating an electron-hole pair. Each of these two carriers drifts in opposite directions, and will be read out as a signal at electrodes. Photodetectors have advantages over photomultiplier tubes due to high quantum efficiency and improved spectral resolution. Recently, one type of photodetectors called photodiode has been widely used.
Comparison of Readout Devices
Photodiodes have high quantum efficiency, small size, and low operating voltage, whereas main concern is that they are vulnerable to electronic noise due to lack of signal multiplication mechanism. Although PMTs are large and need high operating voltage, they have high multiplication gain, so that PMTs can detect weak light signals. Avalanche photodiodes (APDs) were developed with the combined benefits of photodiodes and PMTs. In 1990’s they were used as the light detectors. For the purpose of radiation detection, large area APDs were developed. Currently two types of APDs are popular. One is used for direct X-ray detection, and the other is coupled with a scintillator crystal to detect X-ray. Our team focuses on the study of the latter type of APDs.
To employ an APD as a scintillation detector, the following basic characteristics should be taken into account: (1) multiplication gain (M), (2) capacitance (C) and (3) dark current (Id). Precise measuring of these parameters leads to the better understanding of operation characteristics of APDs and enables better performance of the detector. Without theses pre-measurements, however, many difficulties, such as inability to detect signals, would face users of APDs and other photodetectors. Pre-measurements would enable users to attribute the cause of the inability to an APD, scintillator, or experimental system. In the diagram below, spectra detecting X-ray with an APD (Red) and PMT (Black) coupled with a scintillator are plotted. At the first sight, the energy resolution of a peak in a spectrum with the APD is better than that with the PMT.
Evaluation of Noise
Measuring the abobe-mentioned basic parameters allows noise evaluation. The dark noise is separated into 2 components; one is dark-current shot noise, and the other is thermal noise (or capacitance noise). In the figure below, the noise drops as the bias voltage increases, but it does not seem to decline when M value exceeds 100. Thus, from the point of view in the circuit noise, the present sample of APD coupled with scintillator cannot be expected to accomplish better performance with M exceeding 100.
Evaluation of Excess Noise Factor
The excess noise factor (F) is a noise component unique to an APD. F is defined as the multiplication gain fluctuation. In the figure below, F value measured against M is plotted, and the result is compared with McIntyre model, where F depends on M and effective ionization coefficient ratio (keff). keff is described using the electron and hole ionization coefficients, which denote alpha and beta, respectively.
Our team precisely evaluated the voltage dependence of capacitance of APDs and succeeded in modeling C-V characteristics. In the equation below, Vlim means n-layer becomes fully depleted and depletion region will grow in n-layer when the APD is operated smaller than Vlim.
Evaluation of Intrinsic Energy Resolution of Scintillators
Our team established the method to evaluate the intrinsic energy resolution of scintillators through precise measurement of Id, C and F. The relation between the intrinsic energy resolution and the energy resolution of peaks in spectra measured with APDs are described below, with this method Prof. Moszynski and co-workers (Soltan Institute for Nuclear Studies in Swierk, Poland) evaluated a variety of samples. However, this technique needs very low temperature conditions, this why only a small number of groups can conduct experiments with their method. Our study on basic properties of APDs has allowed establishing the way to evaluate the intrinsic energy resolution at room temperature. With our method, we evaluated the intrinsic energy resolution of BGO scintillator, and it is consistent with the results obtained by Prof. Moszynski within 1sigma error bars. It will be announced at IEEE conference in 2007.
Vision for Future Application
Currently, it was attempted to develop a multi-pixel APD as an imaging device like PMTs. The only study announced in the field of imaging APDs is the Position Sensitive APD (PSAPD) developed by Radiation Monitoring Devices, Inc. (RMD, Inc.). This device is the beveled-edge type APD, which needs about 2,000 V to operate, but only 4 channels are needed to read out signal. It enables to fabricate a small electrical system. This type of APD would allow medical imaging apparatus such as a MRI-PET, which is currently unavailable. Our team is now negotiating with RMD, Inc for PSAPDs. In addition, we also work together with Dr. Kataoka group at the University of Tokyo’s Institute of Technology for development of a multi-pixel APD, which will be combined with our scintillator as an imaging device. The figure below shows an image taken with PSAPD by Dr. Shar et al., quoted from their paper (IEEE Nucl. Trans. Sci, vol 49, 2002).