The Scintillation Mechanism in Thallium Doped Alkali Halides

In the scintillation mechanism for alkali iodides doped with thallous iodide: 1. Most of the luminescence, and possibly all of it, is from recombination and therefore energy transfer is predominantly from charge transport to thallium. 2. There are two modes of recombination at thallium which differ in the sequence of charge trapping at Tl+. The electron may be trapped before a mobile hole recombines with it, or the hole may be trapped before recombination with a free electron. 3. Thermal hole diffusion is an important process at room temperature and determines which mode a recombination event follows. 4. Both modes of recombination are active. 5. There is non-thermal hole trapping which is a significant part of the scintillator mechanism.     

Dependence of the NaI(Tl) Pulse Shape on Thallium Concentration and Temperature

The pulse shape of ?-induced photopeak scintillations in NaI(Tl) has been measured as a function of temperature for several standard commercial crystals and for crystals containing known thallium concentrations. A detailed analysis of the data indicates that the first two microseconds of the pulse is characterized by at least nine separate decay processes. Four of these are attributed to one radiative and three metastable states associated with a thallous ion. Two fast processes are attributed to emission characteristic of pure NaI while two others are associated with high thallium concentration. Energy transport is suggested as the ninth process.     

CsI(Tl)晶体探测WIMP实验中反冲核与电子信号的甄别(英文)

研究了CsI(Tl)晶体探测器中反冲核信号与电子信号的甄别能力。利用参数“tbar”来描述反冲核信号与电子信号两种脉冲波形的特性,并以此来甄别两种信号。对品质因子K的研究表明,脉冲甄别方法的甄别能力随反冲核信号与电子信号能量的降低而减弱。通过这一脉冲甄别方法,对于CsI(Tl)晶体探测器而言,可以区别反冲核信号与电子信号的最低的阈是25个光电子。     

Development of a Double Layered Scintillator for Separating and Detecting Low Energy Protons and Electrons

A scintillator system consisting of a thin (5,000 ? - 15,000 ?) CsI(Tl) layer evaporated onto a plastic scintillator (NE-102) has been developed for the purpose of distinguishing low energy protons from electrons and measuring the energy of each species. Evaporations in a high vacuum (10-8 Torr) produced layers of CsI(Tl) that scintillate with an efficiency comparable to optimally doped bulk material, If the CsI(Tl) layer thickness is 15,000 ?, it stops protons with energies below 170 keV and electrons with energies below 18 keV. Thus, protons with energies between about 25 and 250 keV can be distinguished from electrons with energies above 18 keV by examining the shape of the light pulse generated in the dual scintillator. Results obtained with protons and electron beams will be presented.     

CsI(Tl)晶体探测WIMP实验中反冲核与电子信号的甄别

研究了CsI(Tl)晶体探测器中反冲核信号与电子信号的甄别能力.利用参数"tbar"来描述反冲核信号与电子信号两种脉冲波形的特性,并以此来甄别两种信号.对品质因子K的研究表明,脉冲甄别方法的甄别能力随反冲核信号与电子信号能量的降低而减弱.通过这一脉冲甄别方法,对于CsI(Tl)晶体探测器而言,可以区别反冲核信号与电子信号的最低的阈是25个光电子.     

Radiation-Induced Charge Transport and Charge Buildup in SiO2 Films at Low Temperatures

Studies of the temporal, temperature, and electricfield dependences of radiation-induced charge transport have been performed for radiation-hardened SiO2 films. At room temperature for high applied fields, nearly all electrons and holes generated in the oxide by a pulse of ionizing radiation (5-keV electrons) drift to the interfaces, whereas at low temperatures only electrons contribute to observed transport for relatively low fields. Below ~130°K at high fields, field-induced emission of trapped holes occurs, giving rise to collection within seconds of a significant fraction of the total number of holes generated. The present hole transport data are accounted for quite well in terms of a multiple-trapping model with a spread in trap levels ranging from ~0.3 to ~0.5 eV from the valence band. Comparison with the stochastic hopping transport model is made and that model is found to be less satisfactory in explaining these data. Charge buildup was examined in a Co60 environment and it is demonstrated that oxides exhibiting radiation tolerance at room temperature display severe radiation-induced changes at 77°K. It is also demonstrated that low-temperature charge buildup problems can be alleviated either by employing an ion-implanted oxide or by applying a relatively high field to the oxide during irradiation.     

惰性气体β-γ符合测量系统探测器能量及分辨率刻度

β-γ符合法是全面禁止核试验条约(CTBT)放射性核素核查中惰性气体氙测量的一种重要方法,探测器能量及分辨率刻度是其首要解决的关键技术。本工作详细介绍了β-γ符合测量系统NaI(Tl)闪烁体和塑料闪烁体探测器能量及分辨率刻度的方法和结果,采用γ放射性核素点源刻度NaI(Tl)γ射线能量及分辨率,利用137Cs661.66keVγ射线康普顿散射电子刻度塑料闪烁体β射线能量及分辨率,并与131Xem内转换电子刻度的β射线能量分辨率结果进行了比较。结果表明:用137Cs康普顿散射电子刻度塑料闪烁体β射线能量是一种简便可行的方法,但用其刻度的β射线分辨率比实际的大。     

(α、γ)分辨在测量CsI(Tl)晶体固有放射性本底中的应用

描述了通过CAMAC－微机进行数据采集,用电荷比较法测量CsI（Tl）晶体的（α、γ）分辨性能的方法;同时利用此方法测量了CsI（Tl）晶体固有放射性含量,估算了晶体内^238U和6232Th的含量。     

Luminescence from NaI(Tl) between 5°K and 30°K

The several emission bands of scintillator crystal NaI(TlI) excited by X-irradiation have been studied between 5° and 80°K. It was possible to trap electrons as Tl° and holes as I2- and Tl++ during irradiation and cause recombination thermally or by optical stimulation after irradiation to give the same emissions. The processes responsible for recombination after irradiation are discussed and assignments based on these results made for processes during irradiation.     

Characterization of atom and ion-induced “internal” electron emission by thin film tunnel junctions

Highly excited charge carriers are released when single or multiply charged ions impinge on metal surfaces. While electron emission into the adjacent vacuum phase is well investigated, one has only limited knowledge about the transport of excited electrons or holes into the bulk of a metal. This shortcoming can be reduced by studying the transport of these excited carriers over buried tunnel barriers in thin film metal-insulator-metal devices. The internal barriers can be tuned by a tunnel voltage which makes the device to a balance for excited electrons and holes. With a simple theoretical model we derive the balance function of different tunnel barriers and show their feasibility for the characterization of particle induced electronic excitations on metal surfaces.     

Energy Transfer and the Role of Thallium Dimers in the Nal(Tl) Scintillation Process

The thallium dimer center, which is a pair of thallous ions occupying nearest neighbor cation sites in NaI, is found to have a decay time of less than 7 nanoseconds. Analysis of the emission spectrum of a typical NaI(Tl) scintillation crystal indicates that the dimer centers are responsible for approximately 25% of the scintillation light although only a small part of this is rapid. It is proposed that energy of excitation migrates among the monomers and between monomers and dimers as a result of thermal excitation.     

Numerical simulation program of multicomponent ion beam transport from ECR ion source

In order to research multi-component ion beam transport process and improve transport efficiency, a special simulating program for ECR beam is becoming more and more necessary. We have developed a program written by Visual Basic to be dedicated to numerical simulation of the highly charged ion beam and to optimization of beam dynamics in transport line. In the program the exchange of electrons between highly charged ions and low charged ions or neutral atoms (residual gas in transport line) is taken into account, adopting classical molecular over-barrier model and Monte Carlo method, so the code can easily give the change of charge state distribution along the transmission line. The main advantage of the code is the ability to simultaneously simulate a large quantity of ions with different masses and charge states, and particularly, to simulate the loss of highly charged ions and the increase of low charged ions due to electron exchange in the whole transport process. Some simulations have been done to study the transmission line of LECR3 which is an ECR ion source for highly charged ion beam at IMP. Compared with experimental results, the simulations are considered to be successful.     

Low temperature radioluminescence spectra of CsI:Tl

Four radioluminescence bands associated with thallium at 400, 488, 550 and 593 nm, plus an intrinsic band at 340 nm have been observed in the temperature range 280-25 K in CsI:Tl. The temperature dependence of the emission intensity is obviously different from each band to another. The 488, 550 and 593 nm bands are associated with the decay of different type thallium perturbed V_K-e excitons, respectively, and the 340 nm band is related with the unperturbed H-F excitons. It is suggested that V_K centres are also involved in the emission of the 400 nm photons. The re-absorption of the 488 nm signals and the resulting increase of the 593 nm band have been discussed. At 25 and 290 K, the relative RL intensities between the four thallium associated bands do not change with the X-ray tube voltage from 15 to 40 kV. But the intensity of the 340 nm band increases faster than that of the thallium related bands when the tube voltage increases.     

~(204)Tl在几种金属和非金属表面上的释放行为研究

Tl作为类113号元素,测定了它在Ni,Pd,Mo,Ta,Zr,Ti,Au,Ag,Cu,Fe等金属表面上和SiO_2及石墨等非金属表面上的释放率和释放动力学,理论上计算了Tl在上述金属表面上的附吸焓并与实验结果进行了比较,讨论了两者之间存在差异的原因以及有关的影响因素。定性地预计了113号元素在上述表面上的释放行为和吸附焓。     

Study on plasma sheath and plasma transport properties in the azimuthator

A physical model of transport in an azimuthator channel with the sheath effect resulting from the interaction between the plasma and insulation wall is established in this paper.Particle in cell simulation is carded out by the model and results show that,besides the transport due to classical and Bohm diffusions,the sheath effect can significantly influences the transport in the channel.As a result,the ion density is larger than the electron density at the exit of azimuthator,and the non-neutral plasma jet is divergent,which is unfavorable for mass separation.Then,in order to improve performance of the azimuthator,a cathode is designed to emit electrons.Experiment results have demonstrated that the auxiliary cathode can obviously compensate the space charge in the plasma.     

Trap distribution of polymeric materials and its effect on surface flashover in vacuum

The surface trap parameter can significantly affect the development of surface flashover in vacuum, but the effective mode and mechanism are not very clear yet. The trap parameters of three polymeric materials were tested and calculated by means of isothermal surface potential decay. The flashover experiment was developed under different applied voltages. The results show a positive correlation between the withstand voltage and the deep trap, i.e., the deeper trap energy level is, the higher flashover voltage is. The dynamics process of charge trapping and detrapping was analyzed based on the charge transport model in dielectrics with a single trap level and two discrete trap levels. The time of charge trapping was compared with that of the discharge development. The results show that the charge trapping time is longer than the flashover development time. The way to influence flashover for a trap is not to decrease the secondary electrons in single discharge development, but to change the electric field distribution on the dielectric surface by charge capture.     

Electron Response of Nal(Tl) and a Comparison with Anthracene and Pilot B

An investigation was made of the relative response of NaI(Tl) to electrons over an energy range of 6 keV to 1 MeV. The light output at 6 keV is quite sensitive to the nature of the exposed surface. The results are given for the two best cleaved NaI(Tl) crystals and compared to the electron response deduced from photon data. There is good agreement above 20 keV, where little sensitivity to changes in surface condition was observed. Similar measurements were made for anthracene and Pilot B. The relative light output per unit energy is shown for all three scintillators. Their merit as low energy electron detectors was found to be significantly affected by the amount of backscattering and the intensity and duration of long lived excitation states arising from ionizing events.     

Spatial charge and compensation method in a whirler

Based on particle-in-cell simulation,we studied the motions of ions and electrons.The results have shown that electrons are bounded by a magnetic field and only a small number of electrons can pass through the whirler channel.The plasma becomes non-neutral when it is emitted from the whirler,and the spatial charge leads to a beam divergence,which is unfavorable for mass separation.In order to compensate the spatial charge,a cathode is designed to transmit electrons and the quasi-neutral plasma beam.Experiment results have demonstrated that the auxiliary cathode can obviously improve the compensation degree of the spatial charge.     

A Large Area High Resolution Cosmic Ray Charge Composition Detector

A detector designed to study the charge composition of the primary cosmic radiation in the range Z = 3 to 30 has been flown on balloons during 1969. The flight is part of a program to study the charge composition in the energy range 10 to 105 GeV. The telescope consists of a four fold charge measurement using two plastic scintillators, a Cerenkov detector and a mosaic CsI(Tl) detector. The intrinsic limitations on charge resolution due to statistical fluctuations in energy loss are approached by correcting the pulse heights for geometrical effects. This is accomplished by calibrating the detector response in flight using a spark chamber to determine the particle trajectories.     

Analysis and Characterization of Pre-Diffusion in Multi-Linear Silicon Drift Detectors

Multi-linear silicon drift detectors (MLSDD) are silicon detectors in which signal electrons are confined within parallel drifting columns at few micrometers from the anodes' surface and transported towards point-like anodes by an electrostatic field. During the initial motion across the thickness the signal electrons spread in all directions to a given width - we call this phenomenon pre-diffusion - until the signal electrons are focused in the drift channels which suppress any further spread into adjacent channels. This work is devoted to the study of the initial motion of the electron cloud when the electrons are being focused toward the minimum of the potential energy. We developed an analytical model to predict the initial charge spread and carried out a detailed measurement campaign to evaluate the impact of the detector operating conditions and of the charge injection level on the initial charge spread. The design of the electron spread during pre-diffusion allows optimization of MLSDD for different applications.