dispersive property of a G-M tube

10 10 10 8 C 10 6 10 4 10 2 1 0 250 500 tube voltage 750 HV + In the proportional region a G-M tube has dispersive properties

Solid Radiation Detectors

Advantage: • High density improves the efficiency of radiation absorption.

• Recombination is not a serious problem.

Types: • Scintillation detectors.

• Semiconductors detectors.

Excitation and Recombination in Solids

electron hole

Conduction Band D A Valence Band

Electronic Band Structure Crystal structure defect

Scintillation Conduction Band D A Valence Band

1) Deliberately added impurities enhance trapping of charge carriers and recombination.

2) Excess energy is released in a form of visible radiation.

3) Due to the size of the band gap, the light is not absorbed in the crystal.

Properties of Selected Scintillators scintillator Anthacene p-Terphenyl Stilbene NaI (Tl) ZnS (Ag) Density (g/cm 3 ) 1.24

1.17

1.19

3.67

4.1

Emission (Å) 4400 4100 4100 4100 4500 Decay time (ns) 26 10 8 0.25

9

Sodium iodide activated with thallium may produce one electron-hole pair for every 50 eV of radiation energy.

The intensity of the emitted light is proportional to the energy of the radiation.

scintillator

Scintillation Detector

dynodes anode photocathod e

_

900 – 1500 V

+

output signal

Scintillation detectors are suitable for radiation spectroscopy.

Scintillation Statistics

Monoenergetic radiation results in a response distributed about the most probable value.

FWHM The primary reasons are: • random nature of electron-hole pair formation • variations in energy distribution between light and other recombination mechanisms • uneven efficiency of reaching the PM tube • nonuniform distribution of sensitivity at the photocathode and dynodes.

channel

The overall response of a scintillator detector obeys Poisson statistics, convergent to normal distribution for large numbers.

charge collection in semiconductor detectors

Preferably each generated carrier should be collected by its attracting electrode.

The signal (fraction of collected carriers) depends on both the lifetime and mobility of the carriers.

The equilibrium carrier concentration should be small (low conductivity).

-

i

V +

Detectors of this type must be cooled (with LN) for proper operation.

-

depletion layer semiconductor detectors

depletion layer p V n + Low intrinsic concentration is achieved in the depletion region.

The diode conducts only after electron-hole pairs are generated by the radiation in the depletion region.

surface-barrier semiconductor detectors

Small combined thickness of the n-layer and the gold contact results in very low radiation absorption in these two regions. (Front surface detector.) p V n + output signal

Multi Channel Analyzer (MCA)

Multi channel analyzer counts separately (in a channel) pulses with amplitudes in a specific range.

channel When connected to a dispersive detector, each channel resisters radiation particles with energy in a corresponding energy range.



- spectroscopy

In a magnetic field, charged particle moves along a circular path determined by the magnetic field, mass and charge of the particle, and its kinetic energy (speed) G-M r  mB qv scaler  +