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| WU He-xi1, 2，DI Run-jie2，LIU Yu-juan1, 2，XU Hui2，LIU Yi-bao2* |
1. Engineering Research Center of Nuclear Technology Application (East China University of Technology)，Ministry of Education, Nanchang 330013，China
2. School of Nuclear Science and Engineering，East China University of Technology，Nanchang 330013，China |
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Abstract Because the interaction of incident X-ray produced from the X-ray tube and reflection X-ray from sample disk create standing wave in Total-Reflection X-ray Fluorescence (TXRF) spectrometer, the characteristic X-ray produce from sample changes periodically. So a large number of piled-up nuclear pulses will be generated at the time of wave crest. In order to improve the performance of the TXRF spectrometer, the ability to accurately extract amplitudes of piled-up pulses must be strengthened. The falling edge of every pulse from an SDD (silicon drift detector) decays with time constant τrc exponentially. According to the principle of attenuation compensating, a straight-line shaping method is devised, and its transfer formula is deduced for digitized pulses. When the ADC sampling period is equal to T, the method has a stronger ability to remove noise in pulse as the value of attenuation compensation coefficient k increases. The flat-top of shaping results by this method can be approximated to be straight line only when k≥T/τrc. And the width of the rising edge is narrowest when k=T/τrc. In this case, the straight-line shaping method has the best ability to extract amplitudes of piled-up pulses. Compare the result of a trapezoidal shaper at smallest rise time with this method at k=T/τrc, both of their rising edge and height of flat-top are same. For the smooth degree of flat-top area, this method is better than trapezoidal shaper. This method is better than trapezoidal shaper in separating piled-up pulses. Through fitting the rising edge of pulse transformed with this method, the distinguishable narrowest interval between the start positions of two pulses is determined. Magnifications of different amplitudes of pulses are the same by experiment, which shows that this method does not change the energy linearity of the TXRF spectrometer. The start position of the pulse must be determined before the application of this method. So the transfer formula of impulse shaper is deduced. By analyzed shaping results of impulse shaper, an impulse response appears at the start position of one pulse and the value approaches to the amplitude of the pulse. A threshold method is used to locate the start position of pulse accurately. Finally, all transfer formulas are implemented in an FPGA chip. Experiment results illustrate that the straight-line shaping method has a better energy resolution, peak to background ratio and suppression ability of sum-peak than trapezoidal shaper and analog energy spectrum system. It shows that the performance of the TXRF spectrometer is greatly improved by applying the straight-line shaping method.
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Received: 2020-05-31
Accepted: 2020-10-06
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Corresponding Authors:
LIU Yi-bao
E-mail: ybliu@ecut.edu.cn
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