Lesson 8.1 - Photomultiplier Tubes (PMTs)
Detection of Light
The light emitted by a scintillation material must be detected using some kind of sensitive light detection device. The options include:
- Photo-multiplier Tubes (PMTs)
- Current Mode
- Temperature Sensitivity
- Magnetic Fields
Photomultiplier Tubes (PMTs)
Light (photons) are converted into photoelectrons by absorbing them in a thin photocathode layer inside a (glass) vacuum tube. Most often a photocathode is semi-transparent and usually consists of a thin layer of evaporated Cs, Sb, and K atoms (or a mixture of all three). Each photoelectron is pulled by an electric field towards a dynode and subsequently amplified. In a 10 stage PMT, the net amplification is of the order of 5x105. Each scintillation pulse produces a charge pulse at the anode of the PMT.
Image 8.1 - Photon conversion process in a PMT
Photoelectron Acceleration in a Scintillation Detector
In addition to the above described pulse mode, PMTs can also be operated in current mode in which case the anode current is a measure for the radiation intensity absorbed in the scintillator. This can only be done when the photocathode is at a negative potential.
Tech Note: This allows a user to operate a scintillation detector in high radiation fields. The disadvantage is that all spectroscopic information is lost.
The energy resolution, coincident resolving time and stability of a scintillation detector depend to a great extent upon the type of photomultiplier tube. The selection of a proper type is fundamental to a good detector design. (Contact Berkeley Nucleonics if you need assistance.)
The light conversion efficiency of a photomultiplier cathode is a function of the wavelength; the Quantum Efficiency (Q.E.) is defined as the chance that one photon produces one photoelectron. In the amplification process, one photoelectron produces per dynode step about 3 ‑ 4 secondary electrons. With a 12 stage PMT, a typical gain in the order of 106 can be obtained.
Tech Note: Fig.1 below shows a schematic of a PMT. It should be noted that PMTs are sensitive to magnetic fields; a μ‑metal shield provides adequate protection from the earth magnetic field. For operation in high magnetic fields, special PMTs are available.
Image 8.2 - PMT Schematic
There are a number of PMT dynode structures, each with their typical characteristics. Important PMT parameters are:
- Amplification as a function of voltage
- Dark current
- Pulse rise time
- Physical size
- Gain stability
- Radiological background
Gain, stability and dark current depend on the used dynode materials and are a function of temperature. Pulse rise time depends on the dynode structure. For fast timing applications, so called "linear focused" PMTs are advised.
A very important factor is the sensitivity as a function of the position on the PMT entrance window. A large variation can cause a degradation of the energy resolution of a scintillation detector. This variation can be caused by a change in quantum efficiency of the photocathode or a non-uniform photoelectron collection efficiency from the cathode onto the first dynode. The above effects can be important for both small and large diameter PMTs.
Tech Note: From the scintillation properties table it is clear that each type of scintillator has a different emission spectrum.
It is important for good performance that the emission spectrum of a scintillator is well matched to the quantum efficiency curve (for definition see above) of the PMT. To detect the fast scintillation component of BaF2 for example, it is necessary to use a PMT with a quartz window since glass absorbs all light below 280nm. The figure below shows the quantum efficiency (Q.E.) of a standard PMT with a bi-alkali photocathode. The emission spectrum of the most common scintillator NaI(Tl) is shown too. It can be seen that the overlap is very good. For other scintillation materials such as BGO, the match is less ideal.
Figure 8.1 - Emission Intensity vs Quantum Efficiency
The gain of a PMT is temperature sensitive. The variation in gain, which depends on the photocathode and dynode material, amounts to typically 0.2 - 0.3 % per oC.
Due to their dynode stages, PMTs are usually quite bulky devices although some short versions and miniature types have been developed.
Care must be taken when PMTs are used inside magnetic fields. Although there are PMT types that have a high magnetic field immunity, this effect remains a problem.
The material of a PMT is usually glass. Glass has an intrinsic amount of 40K which contributes to the radiological background of the scintillation detector. 40K emits gamma rays at 1461 keV and β-particles. The face-plate of the PMT can be constructed of special low-K glass. Furthermore, this background can be limited by using light guides absorbing the β-particles and creating a distance between the crystal and the PMT. The above techniques are used in so-called "low background" scintillation detectors.
Pros & Cons of PMTs