Detectors Use in Atomic Absorption Spectroscopy | Phototube detector | Photomultiplier Tube

Detectors Use in Atomic Absorption Spectroscopy

A detector is a device that is used to measure the intensity of transmitted light. Detectors also measure the variation in the wavelength region.

Phototube

A Phototube (Photocell) is commonly used in the Ultraviolet and visible regions. This consists of Photo emissive cathodes and an anode. A high voltage is applied between the anode and cathode. When a photon enters the window of the tube and strikes the cathode, an electron is emitted and attracted to the anode. It causes current to flow that can be amplified. This amplified current is measured and we get the results on a computer screen.

The response of Photo emissive material is wavelength-dependent and different phototubes are available for different regions of the spectrum.

Photomultiplier tube

Following is a photo-emissive device in which the absorption of photons results in the emission of electrons. It has specified Components of the Photomultiplier Tube by which electron emits like a cascade.

It is divided into three main components:

Photocathode

A thin entry window is made of material in which the valence electrons are weakly bound and have a high cross-section for converting photons to electrons via the photoelectric effect. For example, Cs₃Sb (cesium-antimony) may be used.

Scintillators

 Scintillators are a material e.g.: (NaI) that absorbs a photon of a given energy and then undergoes fluorescence, re-emitting a photon of lower energy.

Dynodes

It increases the number of electrons produced by photons. The series of dynodes made of material of relatively low work function Operated at ever-increasing potential (e.g. ~100-200 V between dynodes)

Working:

ü Ionizing radiation enters the scintillator and interacts with the scintillator material, causing electrons to be raised to an excited state.

ü Charged particles have tracked the path of the particle itself. Gamma rays energy is converted to energetic electrons via either the photoelectric effect, Compton scattering, or pair production.

ü De-excitation of excited atoms of the scintillator material and rapid emission of a photon in the visible light range, quantity is proportional to the energy deposited by the ionizing particle.

ü Light created in the scintillator strikes the photocathode releasing at most one Photoelectron per photon. By using voltage potential group of primary electrons is electrostatically accelerated, focused, and strikes the first dynode with enough energy to release additional electrons. These electrons are further focused toward the second dynode additionally and so on. These entire electrons are attracted by the anode and result in the flow of current which can be amplified and can be measured.