Pile-up Pulse Recognition Method Based on Ballistic Deficit Shape Feature
In X-ray fluorescence spectroscopy analysis,high intensity X-ray is usually used to irradiate the sample for obtaining stable energy spectrum quickly,which also leads to serious pulse pile-up,count rate loss and energy resolution degradation.It is effective for improving energy spectrum performance to recognize and reject pile-up pulses.Fast shaping discrimination technology(FSDT)and pulse feature time discrimi-nation technology(FTDT)are popular methods for identifying pile-up pulses.It is impossible for FSDT to identify pile-up pules with intervals shorter than the fast pulse resolution time.The measured feature time is subject to pulse-height variation and ADC sampling frequency,which limits FTDT detection capability.In this paper,a pulse shape discrimination method was proposed.An original digital pulse was shaped to a triangular pulse and a trapezoidal pulse.The amplitude ratio,which is also known as time-invariant pulse-shape signature(TIPS),is a pulse shape feature for recognizing pile-up pulse.It is not necessary to measure the pulse arrival time.TIPS is not depend-ent on ADC sampling frequency and pulse amplitude,but only on ballistic deficit and noise fluctuations.The triangular pulse shaper is a bandpass filter that can suppress high-frequency noise,which can reduce TIPS broadening.The trapezoidal pulse shaping algorithm can be immune to ballistic deficit,while the triangular pulse shaping algo-rithm cannot.Generally,there are higher TIPS value in pile-up pules because of ballistic deficit.A threshold can be used to determine whether the pulse is pile-up.The TIPS value of some piled pulses is less than that of non-piled pulses.If the threshold is set relatively low,many non-piled pulses will be misjudged.Conversely,a lot of pile-up pulses will be missed.The loss rate of the non-piled pulses can be determined by its TIPS distribution.A copper sample is used for the test experiment.When the tube current is set to 3.9 μA,the measured count rate is only 6 500 s 1 in the fast shaper so that the pile-up can be almost ignored.The measured TIPS distribution can be regarded as the TIPS distribution of non-piled pulses.When the tube current is set to 1 mA,the measured count rate reaches 1.22×106 s-1 in the fast shaper.When α is the same,the accuracy,recall rate and F1 score are all improved by using the proposed method to identify pile-up pulses.For example,when α is 15％,the accuracy,recall rate and Fl score are increased to 73.55％,78.75％and 76.06％respectively.The peak-to-total ratio of the Kα peak of Cu is increased to 76.6％.The proposed method can effectively identify pile-up pulses.
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