X射线光学与成像04.ppt

1、X射线光学与成像郭金川2653859213530036880第 4讲X射线探测器（ X-ray detector）直接探测：光子 -电子间接探测：光子 -光子 -电子直接探测：光导效应 -光生电子空穴对 -电荷分布 - 读出电荷分布间接探测：荧光效应 -光耦合 -光敏器件 -电子荧光效应 -光电效应 -电子放大 -荧光 -低灵敏度成像器件在 X射线光学与成像领域中的一些关键的探测器性能参量1. 空间分辨率 :If one wants to know positional information regarding X-ray photons, either in one or two dime

2、nsions, then the spatial resolution of the detector should be sufficiently fine that the intensity of the X-ray field is adequately sampled. For one-dimensional position-sensitive detectors, arrays on the order of 1000 pixels or more are desirable; for two-dimensional devices, arrays on the order of

3、 12001200 pixels or more are desirable. Useful pixel size can range from microns to tens of microns or more, depending on the application. Note that, in principle at least, a partially coherent X-ray wave-field carries information over length scales that are as small as the shortest wavelength for w

4、hich the X-ray spectrum is non-negligiblethus there is plenty of room at the bottom regarding the finest desirable pixel size quoted above. （ 费 曼在 1959年 诺奖 演 讲 的 标题 “plenty of room at the bottom”）2. 时间分辨率In order to adequately sample an X-ray beam in the context of time-resolved studies, the tempora

5、l resolution of the detector must be sufficiently fine. As an extreme example, recall that free-electron lasers are anticipated to yield hard-X-ray pulses with durations on the order of hundreds of femtoseconds or less. Rather less demanding （过分要求的） is the question of performing time-resolved experi

6、ments using third-generation facilities, with storage-ring electron bunches emitting pulsed X-rays with characteristic timescales on the order of several tens of picoseconds or more. Much more modest time resolutions are often useful: for example, temporal resolutions on the order of several hundred

7、ths of a second are often adequate for the real-time imaging of biological systems such as breathing lungs and beating hearts, or in obtaining time-resolved far-field diffraction patterns of contracting muscle.3. 光谱分辨率Spectral information, whether or not it be coupled with position information, is o

8、ften useful in the context of coherent X-ray optics. Note that energy sensitivity in a detector may be viewed as a form of monochromatization of the radiation, with the energy filtering being performed by the detector rather than by an optical element such as a crystal or a multi-layer mirror. Ideal

9、ly, the spectral resolution should be at least one or two orders of magnitude finer than the bandwidth of the spectra under study, although useful information can nevertheless often be gained if the spectral resolution does not meet this criterion.4. 量子效率（ Quantum efficiency）Whether or not one seeks

10、 to discriminate individual X-ray photons, it is often desirable that: (i) a single photon should produce a measurable signal in a given detector; (ii) the measured signal should be proportional to the number of photons that impinge upon the detector.If both of these criteria are fulfilled, then the

11、 detector is said to have unit quantum efficiency. Many common X-ray detectors have near-unit quantum efficiency, or some appreciable （可评估的） fraction thereof, when operated under appropriate conditions.5. 探测量子效率（ Detective Quantum efficiency-DQE）The DQE of a detector may be defined as the squared ra

12、tio, of the output signal-to-noise ratio (SNR) to the input SNR, produced by a given detector under specified conditions. Typical detector DQEs, expressed as a percentage, lie in the range from 1% to 10%. The DQE will, in general, depend on a number of factors, including the frequency (or mean frequ

13、ency, as appropriate) of the X-ray photons incident on the detector, together with the intensity of the field incident on the detector.6. 噪声（ Noise）Detector noise, which it is evidently desirable to minimize, is arandom perturbation in the detected X-ray signal which originates from a variety of mec

14、hanisms. For detectors that require a signal to be read out, there is an associated read-out noise. In the absence of any X-ray illumination, detectors will always produce a signal, known as dark noise. Both read-out noise and dark noise may be reduced by cooling the detector, where applicable. Last

15、ly, there is a detector-independent source of noise known as photon shot noise, which is a manifestation of the quantized nature of the electromagnetic field.7. 动态范围（ Dynamic range）The dynamic range of a detector, which may be defined asthe ratio of the largest detectable signal to the smallest dete

16、ctable non-zero signal, should be sufficiently large. Imaging experiments typically have rather modest requirements on dynamic range, with 12-bit information often being adequate. Conversely, diffraction experiments typically demand many decades of dynamic range from a detector.8. 尺寸（ Size）Detector

17、size should be appropriately matched to the application.9. 光子计数率（ Acceptable photon count-rate ）For detectors which count individual X-ray photons, the rate, at which photons pass through the detector, cannot be too high. Sometimes it is necessary to attenuate an X-ray beam, if the photon-counting d

18、etector is not able to cope with the rate at which incident photons are incident upon it.10. 稳定性（ Stability）When two given X-ray inputs are close to identical, differingonly in the commencement time of the temporal interval during which they are detected, and with all other relevant variables being kept fixed (e.g. temperature and pressure), the resulting signal outputs should also be close to identical.