PIN Photodiode Array Research Articles

PSPMT and photodiode designs of a small scintillation camera for imaging malignant breast tumors

The authors are investigating the concept of utilizing a small gamma ray scintillation camera to help identify and localize malignant breast tumors after a in a mammogram. Excellent sensitivity and specificity for malignant breast tumors has been achieved using conventional nuclear medicine scintillation cameras with certain /sup 99m/Tc-labeled radiopharmaceuticals. However, because of the large size, low image resolution and high cost of these devices, they are not ideal for use in breast imaging in a mammography suite. A dedicated miniaturized camera would allow imaging at angles that are physically impossible with the standard camera. These lateral views would not include the background activity from the heart and liver. In addition, with a potentially higher intrinsic resolution, shorter collimator and if breast compression is applied, a small camera could significantly improve the sensitivity and signal to noise ratio for the scintillation imaging method. The authors are exploring 2 different photodetector technologies for a small prototype camera development. The first uses a position sensitive photomultiplier (PSPMT) as the photodetector, the second, an array of silicon PIN photodiodes (PD). In this report, the authors present imaging results obtained with a NaI(Tl)-PSPMT design, and the design features, expected performance and relevant energy and position measurements obtained for a test CsI(TI)-PD device.

The JET soft x-ray diagnostic systems

A new multicamera soft x-ray detector system has been constructed for the pumped divertor phase of JET to provide improved spatial and temporal resolution. It is based on 12 compact pinhole cameras each of which uses a 35 element PIN photodiode array. The cameras are housed in a secondary vacuum and view the plasma through beryllium windows 250-μm-thick which absorb x rays with energies below ∼2 keV. The front-end electronics, located up to 10 m away from the detectors, sample data at 250 kHz, with a 100 kHz passband. The diodes are operated at zero bias voltage. The system ran successfully during the 1995 phase of JET operation with poloidal mode numbers up to m=5 being successfully reconstructed. Detector noise levels (<1 nA) were small compared to signal statistical noise. Some deterioration in diode performance was observed after exposure to an integrated neutron fluence of ∼1013 n. cm−2. Tests have shown that operating with 3 V reverse bias can offset much of the fall in performance from radiation damage. Using a similar 16 element PIN photodiode, a windowless camera has been developed for installation in the JET primary vacuum, to investigate fast XUV/x-ray events which develop at lower energies near the plasma edge or in the divertor. Finally, a radiation hardened system is being developed for D-T operation.

Semiconductor arrays with multiplexer readout for gamma-ray imaging: results for a 48 × 48 Ge array

We are developing a new kind of gamma-ray imaging device that has sub-millimeter spatial resolution and excellent energy resolution. The device is composed of a slab of semiconductor detector partitioned into an array of detector cells by photolithography and connected to a monolithic circuit chip called a multiplexer (MUX) for readout. Our application is for an ultra-high-resolution SPECT system for functional brain imaging using an injected radiotracer. We report here on results obtained with a Hughes 48 × 48 Ge PIN-photodiode array with MUX readout, originally developed as an infrared focal-plane-array imaging sensor. The device functions as an array of individual gamma-ray detectors with minimal interpixel crosstalk. Linearity of energy response is excellent up to at least 140 keV. The array exhibits excellent energy resolution, ∼ 2 keV at ≤ 140 keV or 1.5% FWHM at 140 keV. The energy resolution is dominated by MUX readout noise and so should improve with MUX optimization for gamma-ray detection. The spatial resolution of the 48 × 48 Ge array is essentially the same as the pixel spacing, 125 μm. The quantum efficiency is limited by the thin Ge detector (0.25 mm), but this approach is readily applicable to thicker Ge detectors and room-temperature semiconductor detectors such as CdTe, HgI 2 and CdZnTe.

A 16-channel charge sensitive amplifier IC for a PIN photodiode array based PET detector module

A 16-channel integrated circuit charge sensitive preamplifier has been developed for a PET detector module that uses an 8/spl times/8 array of PIN photodiodes to identify the crystal of interaction. The amplifier, which is made using 1.2 /spl mu/m CMOS technology, produces an output of 100 mV per 1000 e/sup -/ input, and its noise performance is optimized for 10 pF detector capacitance. The rise time, fall time, bias current through the first FET, and detector current compensation are each controlled by a dc current applied to the 2 mm square chip. The Johnson noise (e/sup -/ fwhm at 1 /spl mu/s peaking time) is 173+13C, where C is the input capacitance in pF, and the shot noise (e/sup -/ fwhm) is 16.4/spl radic/(IT), where I is the detector dark current in pA and T is the amplifier shaping time in /spl mu/s. With this amplifier, the target noise of 300 e/sup -/ fwhm can nearly be met with 300 /spl mu/m depletion thickness photodiodes (3 pF, 200 pA per 3/spl times/3 mm element). >

Monolithic InGaAs photodiode array illuminated through an integrated waveguide

A linear array of InGaAs PIN photodiodes has been fabricated which can be illuminated through a slab waveguide from the opposite side of the InP substrate via total reflection at a V-groove mirror. Absorption losses in the double-heterostructure waveguide are 2.5 dB/cm. The internal quantum efficiency of the photodiodes, including the mirror loss, is 87%, and an optical crosstalk between the diodes better than −40 dB was measured.