PHOTOMULTIPLIER TUBE BASIC INFORMATION AND TUTORIALS

BASIC INFORMATION ON PHOTOMULTIPLIER (PM) TUBE USED IN PET

Photomultiplier Tube - What you need to know about.

A photomultiplier (PM) tube is needed to convert the light photons produced in the detector as a result of g-ray interaction to an electrical pulse. The PM tube is a vacuum glass tube containing a photocathode at one end, 10 dynodes in the middle, and an anode at the other end.

The photocathode is usually an alloy of cesium and antimony that releases electrons after absorption of light photons.

The PM tube is fixed on to the detector by optical grease or optical light pipes.
A high voltage of ~1000 volts is applied between the photocathode and the anode, with about 100 - volt increments between the dynodes. When light photons from the detector strike the photocathode of the PM tube, electrons are emitted, which are accelerated toward the next closest dynode by the voltage difference between the dynodes.

Approximately 1 to 3 electrons are emitted per 7 to 10 light photons. Each of these electrons is again accelerated toward the next dynode and then more electrons are emitted.

The process of multiplication continues until the last dynode is reached and a pulse of electrons is produced, which is then attracted toward the anode.
The pulse is then delivered to the preamplifier. Next, it is amplified by an amplifier to a detectable pulse, which is then analyzed for its size by the pulse height analyzer, and finally delivered to a recorder or computer for storage or to a monitor for display.

A photomultiplier tube showing the photocathode at one end, several dynodes inside, and an anode at the other end.

COMPTON SCATTERING PROCESS BASIC INFORMATION AND TUTORIALS FOR CHEMISTRY

WHAT IS COMPTON SCATTERING PROCESS?

Compton Scattering Process What You Need To Know About It

Compton Scattering Process: In a Compton scattering process, a g radiation with somewhat higher energy interacts with an outer shell electron of the absorber atom transferring only part of its energy to the electron and ejecting it.

The ejected electron is called the Compton electron and carries a part of the g-ray energy minus its binding energy in the shell, i.e., E¢g - EB, where E¢g is the partial energy of the original g ray.The remaining energy of the g ray will appear as a scattered photon.

Thus, in Compton scattering, a scattered photon and a Compton electron are produced.The scattered photon may again encounter a photoelectric process or another Compton scattering process, or leave the absorber without interaction.

As the energy of the g radiation increases, the photoelectric process decreases and the Compton scattering process increases, but the latter also decreases with photon energy above 1.0MeV or so.The probability of Compton scattering is independent of the atomic number Z of the absorber.

The Compton scattering process in which a g ray transfers only a part of its energy to an electron in a shell and is itself scattered with reduced energy. The electron is ejected from the shell with energy,E g - EB, where E¢g is the partial energy transferred by the g ray and EB is the binding energy of the electron in the shell.

The remaining g-ray energy appears as a scattered photon.