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: The basic structure, principle, and operating mode of photon counting and pixelated X-ray detectors

What is photon counting and pixelated X-ray detector (HPC)?
Photon counting and pixelated X-ray detector is a new type of detector that combines CMOS technology and photon counting technology. It achieves direct and efficient detection of hard X-rays whi

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The basic structure, principle, and operating mode of photon counting and pixelated X-ray detectors (HPC)

What is photon counting and pixelated X-ray detector (HPC)?
Photon counting and pixelated X-ray detector is a new type of detector that combines CMOS technology and photon counting technology. It achieves direct and efficient detection of hard X-rays while maintaining a high frame rate.
Basic structure, working principle, and three working modes of HPC
Basic structure:

The mixed pixel photon counter can generally be divided into two layers:

Pixelated sensor layer

Read out chip

The sensor and readout chip are assembled using an inverted method, which involves bonding metal balls together. Each solder ball corresponds to a pixel, which is achieved by pixelization through solder balls. Each pixel corresponds to a processing unit of the readout chip.

Working principle:

Each pixel is equivalent to a detector, and its readout electronics include an amplifier, threshold comparator, counter, setting, and readout circuit.

Photons hit the sensor, generating electron hole pairs. Electrons or holes are collected by adjacent pixel electrodes, amplified by an amplifier (usually a charge sensitive amplifier), and sent to a comparator. If the signal amplitude exceeds the given threshold (external input), the pixel counter increases by a count, and this mode is called Counting Mode.

The animation provided by Wikipedia vividly illustrates this process, which can be accessed by clicking on the image link.

The number of electron hole pairs is proportional to the energy of the incident photon, so the amplitude of the signal pulse output by the amplifier is also proportional to the energy of the photon. Therefore, the threshold level of the comparator is directly related to the photon energy. By changing the comparator threshold from small to large (or from large to small) and conducting multiple measurements, photon flow rates at different energy levels can be obtained. By using mathematical methods, photon flow rates and energy spectra of different energies can be obtained.

In fact, the output signal of the amplifier has other information besides amplitude information. Due to the inherent characteristics of electronic circuits, it takes a certain amount of time (called peak time) for a pulse to reach its maximum value (peak), and it also takes a certain amount of time (called descent time) to return from peak to (baseline) 0. The higher the peak value, the longer the time to reach and decrease, and the peak value is related to energy. That is to say, the duration of signal amplitude above the threshold is related to photon energy, and this duration is called TOT (Time Over Threshold). If this duration can be measured, photon energy information can also be obtained. This mode is called TOT mode.

Some applications need to know the time of arrival of photons, which requires setting a certain time benchmark and measuring the time interval between photon arrival time and this benchmark. There are two ways to use time benchmarks: based on the measurement start time (Timepix 3 chip), or based on the measurement end time (Timepix chip). The arrival time can be set as the timing of the signal pulse (rising or falling edge) valve. This mode is called TOA mode.
In order to digitize, in order to measure TOT and TOA information with certain accuracy, it is necessary to introduce a high stability and high-frequency clock. By measuring the clock pulse count values corresponding to TOT and TOA, digitization can be achieved.

The following figure is a schematic diagram of pulse counting for three modes:

TOT mode: When the signal rises and the amplitude exceeds the threshold, each clock pulse counter increases once; When the signal falls below the threshold, the counter stops counting. The measurement result is the number of clock pulses corresponding to the accumulated TOT time.

TOA mode: Starting from the first signal pulse amplitude exceeding the threshold, the clock pulse is counted until the measurement is completed, ignoring any subsequent signal pulses whose amplitude exceeds the threshold. The measurement result is the time interval between the moment when the amplitude of the first signal pulse exceeds the threshold and the end of the measurement.

Counting mode: When the rising edge of the signal exceeds the threshold once, the counter increases by 1. This mode does not require clock pulses and can achieve multiple measurement methods on the same chip area, as seen in the Medipix 3 chip.
The measurement time in TOT mode and TOA mode is relatively short.The data-driven readout method of Timepix 3 can meet the needs of this application.

In addition to signal processing (amplification, comparison, and counting), each pixel also needs a signal readout circuit, parameter setting, and holding circuit. Measuring TOT and TOA also requires a high-frequency clock signal processing circuit, so each readout unit of the readout chip requires a larger chip area. Due to manufacturing process limitations, the pixel size of the mixed pixel photon counter is larger than that of CCD: 55 μ m x 55 μ m. In addition, due to the complexity of the readout circuit, the readout chip cannot be made very large, usually 256 x 256, but multiple pixels can be achieved through splicing technology.

At present, there are two main types of readout chips: Medipix series and Timepix series. The former is mainly used for counting, while the latter focuses on time measurement.

Different energy ranges and radiation types can use different semiconductor materials, such as CdTe CdZnTe, GaAs, etc.

The basic structure, principle, and operating mode of photon counting and pixelated X-ray detectors (HPC)
What is photon counting and pixelated X-ray detector (HPC)?
Photon counting and pixelated X-ray detector is a new type of detector that combines CMOS technology and photon counting technology. It achieves direct and efficient detection of hard X-rays while maintaining a high frame rate.
Basic structure, working principle, and three working modes of HPC
Basic structure:

The mixed pixel photon counter can generally be divided into two layers:

Pixelated sensor layer

Read out chip

The sensor and readout chip are assembled using an inverted method, which involves bonding metal balls together. Each solder ball corresponds to a pixel, which is achieved by pixelization through solder balls. Each pixel corresponds to a processing unit of the readout chip.

Working principle:

Each pixel is equivalent to a detector, and its readout electronics include an amplifier, threshold comparator, counter, setting, and readout circuit.

Photons hit the sensor, generating electron hole pairs. Electrons or holes are collected by adjacent pixel electrodes, amplified by an amplifier (usually a charge sensitive amplifier), and sent to a comparator. If the signal amplitude exceeds the given threshold (external input), the pixel counter increases by a count, and this mode is called Counting Mode.

The animation provided by Wikipedia vividly illustrates this process, which can be accessed by clicking on the image link.

The number of electron hole pairs is proportional to the energy of the incident photon, so the amplitude of the signal pulse output by the amplifier is also proportional to the energy of the photon. Therefore, the threshold level of the comparator is directly related to the photon energy. By changing the comparator threshold from small to large (or from large to small) and conducting multiple measurements, photon flow rates at different energy levels can be obtained. By using mathematical methods, photon flow rates and energy spectra of different energies can be obtained.

In fact, the output signal of the amplifier has other information besides amplitude information. Due to the inherent characteristics of electronic circuits, it takes a certain amount of time (called peak time) for a pulse to reach its maximum value (peak), and it also takes a certain amount of time (called descent time) to return from peak to (baseline) 0. The higher the peak value, the longer the time to reach and decrease, and the peak value is related to energy. That is to say, the duration of signal amplitude above the threshold is related to photon energy, and this duration is called TOT (Time Over Threshold). If this duration can be measured, photon energy information can also be obtained. This mode is called TOT mode.

Some applications need to know the time of arrival of photons, which requires setting a certain time benchmark and measuring the time interval between photon arrival time and this benchmark. There are two ways to use time benchmarks: based on the measurement start time (Timepix 3 chip), or based on the measurement end time (Timepix chip). The arrival time can be set as the timing of the signal pulse (rising or falling edge) valve. This mode is called TOA mode.

In order to digitize, in order to measure TOT and TOA information with certain accuracy, it is necessary to introduce a high stability and high-frequency clock. By measuring the clock pulse count values corresponding to TOT and TOA, digitization can be achieved.

TOT mode: When the signal rises and the amplitude exceeds the threshold, each clock pulse counter increases once; When the signal falls below the threshold, the counter stops counting. The measurement result is the number of clock pulses corresponding to the accumulated TOT time.

TOA mode: Starting from the first signal pulse amplitude exceeding the threshold, the clock pulse is counted until the measurement is completed, ignoring any subsequent signal pulses whose amplitude exceeds the threshold. The measurement result is the time interval between the moment when the amplitude of the first signal pulse exceeds the threshold and the end of the measurement.

Counting mode: When the rising edge of the signal exceeds the threshold once, the counter increases by 1. This mode does not require clock pulses and can achieve multiple measurement methods on the same chip area, as seen in the Medipix 3 chip.
The measurement time in TOT mode and TOA mode is relatively short.

The data-driven readout method of Timepix 3 can meet the needs of this application.

In addition to signal processing (amplification, comparison, and counting), each pixel also needs a signal readout circuit, parameter setting, and holding circuit. Measuring TOT and TOA also requires a high-frequency clock signal processing circuit, so each readout unit of the readout chip requires a larger chip area. Due to manufacturing process limitations, the pixel size of the mixed pixel photon counter is larger than that of CCD: 55 μ m x 55 μ m. In addition, due to the complexity of the readout circuit, the readout chip cannot be made very large, usually 256 x 256, but multiple pixels can be achieved through splicing technology.

At present, there are two main types of readout chips: Medipix series and Timepix series. The former is mainly used for counting, while the latter focuses on time measurement.

Different energy ranges and radiation types can use different semiconductor materials, such as CdTe CdZnTe, GaAs, etc.

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