Geiger Mode Photodiode Array for Compact Radiation Detection
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Solution Overview
Problem
Conventional radiation detectors, such as those using photomultiplier tubes and solid-state detectors, face issues like bulkiness, magnetic field sensitivity, low quantum efficiency, high voltage requirements, and heat generation, which limit their performance and practicality in medical imaging applications like PET, SPECT, and CT scans.
Innovation Solution
A radiation detector comprising a scintillator coupled with an array of Geiger mode photodiodes, where each photodiode cell is reverse-biased above its breakdown voltage, forming pixels with summed outputs that mimic the signal intensity of PMTs, and utilizing a simple quenching circuit for rapid reset and digital pulse generation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If photomultiplier tubes are used to detect scintillation light photons, then quantum efficiency is improved, but device size increases and magnetic field sensitivity problems occur
Solution Approach 1:
The detector is divided into an array of discrete photodiode cells arranged in pixels, where each cell independently detects photons. This segmentation allows for compact packaging while maintaining high quantum efficiency through the large total active area of the array.
Solution Approach 2:
The patent replaces photomultiplier tubes (which rely on mechanical electron multiplication through dynodes) with solid-state photodiodes operating in Geiger mode. This substitution eliminates the need for magnetic field shielding and reduces detector size while maintaining high gain capability.
2Volume of moving object
If solid-state detectors are used to directly absorb gamma photons, then device size is reduced, but heat generation increases requiring expensive cooling systems
Solution Approach 1:
The patent introduces a scintillation crystal as an intermediary between the gamma photons and the photodiode array. The scintillator converts gamma ray energy into visible light photons, which are then detected by the photodiodes. This indirect detection method reduces heat generation in the semiconductor detector compared to direct gamma absorption.
3Device complexity
If photodiode detectors are used instead of PMTs, then device complexity is reduced, but amplification capability and stability are insufficient
Solution Approach 1:
The patent changes the operating parameters of the photodiodes by reverse-biasing them above their breakdown voltage, causing them to operate in Geiger mode. This parameter change enables single-photon detection with high gain (comparable to PMTs) and improved stability, while maintaining the simplicity of solid-state construction.
4Area of stationary object
If photomultiplier tubes are arranged in an array, then detection coverage is improved, but dead space between tubes increases reducing spatial resolution
Solution Approach 1:
The detector uses an array of discrete photodiode cells with defined boundaries, allowing for tight packaging with minimal dead space between elements. The segmented pixel structure enables precise spatial encoding of gamma ray interaction positions while maximizing the active detection area.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This solution provides improved spatial and energy resolution, reduced heat generation, lower voltage needs, and more compact designs, enhancing image quality and operational simplicity while maintaining high gain capabilities, suitable for various radiation detection applications.
Implementation Method 1
When a gamma photon strikes and is absorbed in the detector crystal, the energy of the gamma ray is converted into a large number of scintillation light photons
Implementation Method 2
an array of photodiode detector cells optically coupled to the scintillator
Implementation Method 3
each of the photodiode cells being reverse-biased above a breakdown voltage thereof
Data Source
AI summary
An imaging radiation detector includes a scintillator coupled to an array of photodiodes operating in Geiger mode. The array is divided into separate detector pixels, each of which is composed of a multiplicity of photodiode cells with their outputs tied together. While each of the cells operates independently in a binary or digital mode, by tying together the outputs of a multiplicity of adjacent photodiode cells forming a single pixel, the sum of the outputs is proportional to the intensity of generated scintillation photons, similar to the output of a PMT. Appropriate quenching circuitry is provided to rapidly reset the photodiodes after scintillation photon detection.