5) Xray detector systems

What are counting detectors

detect and count individual photons; scintillators; gas ionisation; semiconductors

What are integrating detectors

accumulate photon induced signals over a period of time and counting later; imaging plates; charge coupled devices; CMOS active pixel sensors

What are the important properties of an xray detector

cost; weight; size; linearity (I accuracy); dead time; energy resolution; peak resolution (affected by optics)

What are scintillator detectors

γ absorbed by crystal (NaI) producing blue light; light converted to e in photomultiplier and amplified; count voltage pulses

Positives/negatives about scintillator detectors

inefficient, only count 1/10000; poor energy resolution; stable and reliable

What are imaging plate detectors

γ captured by a phosphor eg Eu2+ doped with BaFBr; Eu2+ oxidises to 3+; e trapped in F vacancies; after a while release e by exposure to white light; emits blue light during reduction back to 2+; blue light registered by photomultiplier

Positives/negatives of imaging plate detectors

wide angular range 100deg 2theta; low maintenance; high I; intermediate resolution

What are gas ionisation detectors

chamber of noble gas + small amount of quenching gas; γ absorbed by gas freeing an e; e accelerate by E field towards anode to be counted; e may cause further ionisation amplifying the signal ‘Thomson Avalanche’

What are Geiger Muller tubes

gas ionisation detector; show xray background in counts/s; uses small sealed xray source to verify; scan around diffractometer to check for leaks; wavelength dependent; need conversion tables to calculate radiation dose in milliSieverts

Positives/negatives of Geiger Muller tubes

rugged; cheap; good for measuring doses; large dead time; very poor energy resolution

Wire based proportional detectors

wire based gas ionisation detector; I of diffracted beam measured at many points along a wire simultaneously; high resolution; limited efficiency; low count rates <10^5/s

How do semiconductor based detectors work

xray generates e/h pair in Si or CdTe crystal; E field generates current to record impact

Positives/negatives of semiconductor detectors

effectively maintenance free; v high count rates >10^8/s; v good energy and peak resolution; v efficient count >98% (except high E)

What are silicon strip detectors

1 or 2D array of pn diodes in strips or pixels each acting as a detector; E field sweeps strips producing electrical signal on location of xray/strip interaction