A Prototype Of Electronics System For Low-Energy Positron Lifetime Spectroscopy

Positron annihilation lifetime spectroscopy(PALS) is a sensitive probe of vacancy type defects in materials.Low-energy positron beam is accepted to be a useful tool for surface studies,such as nondestructive depth profiling of defects in surfaces and interfaces.There are two practical methods to obtain the start signal from a slow positron beam:(1) detecting the secondary electrons generated on the implantation of the positron into the sample as the start signal;(2) pulsing the positron beam by means of an appropriate radio-frequency fields and using the pulsing signal of the beam pulsing system as the start signal.The former method,although easier to implement,suffers usually from poor resolution due to the spread in secondary electron velocities.The best time resolution was 350 ps.The former technique gives good results(high counting efficiency and time resolution) but its complexity of installation and operation make it available in only a few laboratories.The time resolution was 135～350 ps.In order to break the limitations of the internal research for materials science and reach international advanced level,an apparatus of low-energy positron lifetime spectroscopy(LEPLS) based on beam pulsing method has recently been established in the University of Science and Technology of China(USTC),and the beam pulsing system consists of a reflection type chopper,a pre-buncher,a main-buncher,and a beam-pulsing electronics system.The design goal is to compress the DC beam into pulse duration of 150～200ps(FWHM) at the target and obtain time resolution with better than 200ps for lifetime measurement of LEPLS.According to the principle of LEPLS and referring to the similar apparatuses in the University of Helsinki and the Beijing High Energy Institute,the prototype electronics system is in two electronics subsystems:beam pulsing subsystem and time measurement subsystem.The beam pulsing subsystem generates three beam pulsing signals to chop and buncher the beam,and one "start" signal for lifetime measurement.The time measurement subsystem measures the time interval of the "start" and the "stop" signal from PMT.In order to achieve the design goals of LEPLS,the requirements for electronics system are as follows:1.Chopper signal is a 50 MHz pulse signal with amplitude greater than 5 V, edge time less than 2ns and pulse width about 7ns;pre-buncher signal is a 50MHz sine signal with amplitude greater than 2 V;main-buncher signal is a 200MHz sine signal with amplitude greater than 2 V.2.Phase jitter between three signals is less than 60ps.3.The step of phase adjust between three signals is less than 50ps.4.RMS of time measurement in range of 20ns is better than 64ps.According to the requirements above,the difficult points of the electronics system design are how to obtain the signal with fast edge-time,maintain the low jitter between three signals,and achieve good resolution of time interval measurement.Aiming at the design requirements and difficult points,the main technical features of this system are:generating three required signals from one source signal and using techniques of frequency synthesis,clock multi phase,precise delay and broadband pulse amplification to meet requirements of the waveform,frequency and phase relationship for the three beam pulsing signals,and using dedicated TDC IC for time interval measurement.Most key characters in this thesis are listed as follows:1.The method of generating three required signals from one source signal ensures to obtain certain phase between three signals at the electronics system power-up every time.2.Using the technique of broadband pulse amplification to obtain signal with fast edge-time,and add small additional jitter to the signal meantime.3.The precise phase adjust is achieved by using the technique of precise gate delay.4.A TDC-GPX chip with high precision for time interval measurement is used to maintain the good time resolution.5.A large capacity and high performance FPGA(Field Programmable Gate Array) device,by taking the advantages of its embedded DSP cores and other abundant resources,provides a platform to implement digital nuclear signals processing in hardware,and implement time measurement in leading edge timing with charge correction which is used to compare with time measurement in constant fraction timing.6.The first prototype of electronics system for LEPLS internally,based on IC,is self-designed and achieved.The core function of the electronics system is integrated in two NIM modules,which avoids using a large number of discrete devices with the risk for compatibility,and avoids the shortcoming that the integrated system constructed by the foreign commercial modules is not easy to transform in accordance with the experimental conditions.Presently,the prototype of electronics system has been produced.After a series of electronics test,the electronics system prototype is proved to meet the electronics requirements.Afterwards the in-filed test with LEPLS will be implemented.