How a detector works?

LCLS/SSRL users meeting – detector workshop How a detector works ? Sven Herrmann Overview •  Sensor •  x-ray detection: direct & indirect •  silico...
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LCLS/SSRL users meeting – detector workshop

How a detector works ? Sven Herrmann

Overview •  Sensor •  x-ray detection: direct & indirect •  silicon as x-ray detector and the photodiode •  THE CCD •  sidewards depletion and the SDD •  Electronics •  charge readout – aka “the amplifier” •  speed, noise and power compromise •  Information Generation Rate and Figure of Merit •  Camera •  Hybrid Pixel Detectors •  photon counting vs integrating imagers •  a word on crosstalk and scaling

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X-ray cameras : direct and indirect detection

direct conversion detector uses photoeffect in silicon to detect x-rays

indirect conversion detector uses scintillation to convert x-rays into “visible” light and then detect this light with a silicon sensor

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X-ray cameras : direct and indirect detection

x-ray photon

x-ray photon

visible light

electronic signal electronic signal 4

direct conversion in silicon

photoeffect Compton

scattering

•  •  • 

below ca. 40keV photoeffect dominates in Silicon Silicon is an indirect semiconductor Silicon bandgap: 1.11eV

pair production

1.11eV

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M.N. Mazziotta, Electron–hole pair creation energy and Fano factor temperature dependence in silicon NIMA 584 (2008) R. Hartmann, The quantum efficiency of pn-detectors from the near infrared to the soft X-ray region, NIMA 439 (2000)

electron hole pair creation energy and Fano factor

140K

300K

1 electron per 3.65eV

1 photon = 1 electron

for x-rays there is a variance in the numbers of electrons generated:

F= 1 pure Poisson process

​"↓$ =√⁠'​ (↓$  

F= typ 0.117 for Silicon

5.9keV (Fe55) → 1616 e- mean → 13.7 e- sigma → 50.2eV sigma → 118eV FWHM /3.65

*3.65

*2.35

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Yibin Bai et al, Teledyne Imaging Sensors: Silicon CMOS imaging technologies for x-ray, UV, visible and near infrared, SPIE Vol. 7021 (2008)

Photon absorption depth in Silicon wafer thickness

typical CCD epi standard implant shallow implant

Beer-Lambert law:

)(+)=​$↑−  ​1//   + 

at the absorption depth only 37% of the photons are not yet absorbed 7

drift speed and charge cloud •  • 

initial charge cloud generated by the photoelectric delta electron is very small