Guoqiang Zeng

Reset charge sensitive amplifier for NaI(Tl) gamma-ray spectrometer

The time constant of the output signal of the front-end readout circuit of a traditional gamma-ray spectrometer with a NaI(Tl)+PMT structure is affected by temperature, measurement environment and the signal transmission cable, so it is difficult to get a good resolution spectrum, especially at higher counting rates. In this paper, a reset charge sensitive amplifier (RCSA) is designed for the gamma-ray spectrometer with a NaI(Tl)+PMT structure. The designed RCSA outputs a step signal, thus enabling the acquisition of double-exponential signals with a stable time constant by using the next stage of a CR differentiating circuit. The designed RCSA is mainly composed of a basic amplifying circuit, a reset circuit and a dark current compensation circuit. It provides the output step signal through the integration of the PMT output charge signal. When the amplitude of the step signal exceeds a preset voltage threshold, it triggers the reset circuit to generate a reset pulse (about 5µs pulse width) to reset the output signal. Experimental results demonstrated that the designed RCSA achieves a charge sensitivity of 4.26×10(10)V/C, with a zero capacitance noise of 51.09fC and a noise slope of 1.98fC/pF. Supported by the digital shaping algorithm of the digital multi-channel analyzer (DMCA), it can maintain good energy resolution with high counting rates up to 150kcps and with a temperature range from -19°C to 50°C.

Frequency spectrum analysis for spectrum stabilization in airborne gamma-ray spectrometer.

Abnormal multi-crystal spectral drifts often can be observed when power on the airborne gamma-ray spectrometer. Currently, these spectral drifts of each crystal are generally eliminated through manual adjustment, which is time-consuming and labor-ineffective. To realize this quick automatic spectrum stabilization of multi-crystal, a frequency spectrum analysis method for natural gamma-ray background spectrum is put forward in this paper to replace traditional spectrum stabilization method used characteristic peak. Based on the polynomial fitting of high harmonics in frequency spectrum and gamma-ray spectral drift, it calculates overall spectral drift of natural gamma-ray spectrum and adjusts the gain of spectrometer by this spectral drift value, thus completing quick spectrum stabilization in the power on stage of spectrometer. This method requires no manual intervention and can obtain the overall spectral drift value automatically under no time-domain pre-processing to the natural gamma-ray spectra. The spectral drift value calculated by this method has an absolute error less than five channels (1024 resolution) and a relative error smaller than 0.80%, which can satisfy the quick automatic spectrum stabilization requirement when power on the airborne gamma-ray spectrometer instead of manual operation.

A hybrid method on sourceless sensitivity calculation for airborne gamma-ray spectrometer

The sensitivity calculation of airborne gamma-ray spectrometer (AGS) is usually performed by on-ground or in-flight calibration. However, both methods are cost-ineffective or not permissive, especially for artificial radioisotopes with short half-lives. Alternative to these methods is the Monte Carlo simulation, which has been widely applied over the last few decades. The greatest challenge to the practicability of the Monte Carlo simulation in the AGS calibration is its low computational efficiency for ensuring an acceptable reliability. This article proposes a hybrid numerical method for the sourceless AGS calibration by combining the deterministic point-kernel approach and the Monte Carlo simulation. This method is not only more efficient than the source-based calibration by an empirical method, but also independent of the source availability for on-ground or in-flight calibration. For a given soil test model, AGS sensitivities calculated by this hybrid method agree well with those obtained from the empirical method for the in-flight calibration.

Research and development of a high-performance differential-hybrid charge sensitive preamplifier

A differential-hybrid charge sensitive preamplifier (CSP) was designed by taking a monolithic dual N-Channel Junction Field-effect Transistor (JFET) and a high-speed, low-noise, operational amplifier as the core parts. Input-stage of the circuit employs low-noise differential dual JFET, which ensures high input impedance and low noise. The differential dual transistor makes the quiescent point of the first-stage differential output stable, which is convenient for connecting with the post stage high-speed operational amplifier. Broadband could be amplified by connecting to the double differential dual transistors through the folded cascode-bootstrap. The amplifying circuit which replaces the interstage and post stage discrete components of a traditional CSP with integrated operational amplifier is simpler and more reliable. It simplifies the design of the quiescent point, gives full play to advantages of releasing large open-loop gain, and improves charge-voltage conversion gain stability. Particularly, the charge-voltage conversion gain is larger under a smaller feedback capacitor, thus enabling to gain better signal-noise ratio. The designed CSP was tested, reporting 3.3×1013 V/C charge sensitivity, about 90ns rise time of signals, 35:1 signal-noise ratio to gamma-rays of 137Cs (662keV) and a 0.023 fC/pF noise slope. Gamma-rays of 241Am (59.5keV) were measured by the BPX66 detector and the designed CSP under room temperature, providing 1.97% energy resolution.