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Phosphor screens
Applications
- Electron Microscopy
- MCP Detector Systems
- X-ray detection
- UV and Electron Detection
- Beam profile analysis
Generally speaking phosphor screens are used for the visualisation of particles or energetic photons. The most typical applications are in the fields of electron microscopy, X-ray imaging, synchrotron radiation, neutron & ion imaging.
As a manufacturer of image intensifiers and low light cameras, PROXITRONIC also offers phosphor screens which allow the two-dimensional, visual detection of electrons, charged particles, x-rays and UV-radiation.
The phosphor coating is prepared by sedimentation of phosphor grains on a substrate (plain glass, quartz, fibre optic block or taper, CCD chip, prism, metal) usually provided by our customers. Our sedimentation process leads to a very homogeneous layer structure. Please note that the coatings are very sensitive to mechanic stress, e. g. a light touch with a finger immediately destroys the surface. Nevertheless, the removal of dust or small particles is possible by simply blowing them off with a jet of dry air or nitrogen.
To guarantee a homogeneous layer structure, at least 3 to 4 layers of phosphor grains are necessary. The minimum average grain size available is 1 µm. Bigger grains and an increased layer thickness do improve the quantum efficiency while simultaneously efficiency is reduced.
The resolution limit RL is defined as the spatial frequency resulting in a modulation transfer function MTF with a value of 3%. RL is measured in linepairs/mm (lp/mm) and can be approximated as
RL = 500/D
with D as the thickness of the phosphor layer in µm. The maximum thickness of the phosphor coating is limited technically to ca. 100 µm.
The screens are stable under vacuum conditions and can resist temperatures up to 400°C.
As a standard, coatings can be deposited on substrates up to 250 mm. Larger screen sizes are possible on request.
Features
The screens can be deposited on a wide variety of substrates: glass, metal plate, fiber-optic plate or taper, prism, membrane, CCD/CMOS, …
- P43, P46, P47, P11 and other scintillators
- ITO, chromium, aluminium, water glass coating
- segmentation of the coating area possible
- 5 mm - 250 mm diagonal, bigger on request
- on any plain and rigid surface (metal, glass, plastic)
- coating of special forms like prisms, tapers and others possible
- coating of all line and area CCD/CMOS elements without protection window
- coating thickness up to 100 µm
To guarantee a homogeneous layer structure, the screen thickness should be nominally at least 3 to 4 layers of phosphor grains. The minimum average grain size available is 1 µm. The grain size actually used depends on the overall screen thickness to be applied. The optimum layer thickness will be based on the type of particles / radiation and their energy. Furthermore considerations like efficiency vs. resolution power will utimately guide the definition of the layer tickness.
Phosphor types
Three criteria allow the choice of the most suitable phosphor type:
- conversion efficiency
- emission spectrum
- luminescence decay time
| type | composition | emission | decay | |
|---|---|---|---|---|
| P43 | Gd2O2S:Tb | 540 nm | green | 1 ms |
| P46 | Y3Al5O12:Ce | 530 nm | green | 300 ns |
| P47 | Y2SiO5:Ce,Tb | 400 nm | blue | 100 ns |
Efficiency does not only depend on the type of phosphor but also on parameters like grain size, layer thickness, aluminium reflection layer and special manufacturing parameters like thermal processing. It can be reduced by the presence of alkali ions.
| Type | efficiency (lm/µA) | efficiency (W/mA) | efficiency (ph/el) | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 6 kV | 10 kV | 12 kV | 15 kV | 6 kV | 10 kV | 12 kV | 15 kV | 6 kV | 10 kV | 12 kV | 15 kV | |
| P43 | 0,24 | 0,43 | 0,54 | 0,71 | 0,43 | 0,77 | 0,97 | 1,28 | 185 | 330 | 420 | 550 |
| P46 | 0,08 | 0,15 | 0,19 | 0,25 | 0,22 | 0,39 | 0,49 | 0,65 | 90 | 160 | 200 | 265 |
| P47 | 0,06 | 0,11 | 0,14 | 0,18 | 0,62 | 1,35 | 1,71 | 2,24 | 212 | 380 | 480 | 630 |
Efficiency of phosphor screens in proximity focus image intensifier diodes exposed to 6 keV, 10 keV, 12 keV and 15 keV electrons (screen thickness ca 4-5 µm, grain size ca. 1 µm, aluminium reflection layer). Please note that these screens are sedimented on fiber optic plates, which means that with plain glass the efficiency will be even 40% higher.
Options
ITO underlayer
PROXITRONIC offers the conductive underlay coating of substrates with indium tin oxide (ITO) to reduce electrostatic effects on the screen caused by charged particles. Standard and special conductive parameters can be achieved.
To allow direct electric contact to a phosphor screen, a chromium ring of width up to 100 mm diameter can be sputtered on the outer area of the substrate. This is needed in the combination with ITO coating to reach position independent brightness of the screen fluorescence caused by charged particles.
Cr ring (diameter <100 nm)
To allow direct electric contact to a phosphor screen, a chromium ring of width up to 100 mm diameter can be sputtered on the outer area of the substrate. This is needed in the combination with ITO coating to reach position independent brightness of the screen fluorescence caused by charged particles.
Al reflection overlayer
To increase light efficiency by up to 100 %, it is advantageous for most applications to seal the phosphor coating with an aluminium layer on top of it. As a standard, a 40 nm to 50 nm coating is recommended, but on special request the thickness can be varied between 5 nm and 130 nm. The aluminium is a conductive overlay and cuts off light background in the system. The brightness is increased when used with electrons of more than 4 keV. In case of use with electrons below 4 keV no increase is achieved.
Water glass
To improve the mechanic stability of a phosphor screen, water glass can be added during the sedimentation process. A screen manufactured this way can resist a light finger touch. An efficiency decrease of 30% to 40% results from this manufacturing process as the density of the phosphor grains in the matrix is reduced.
Warranty & storage
The phosphor screens can withstand an electric field of up to 6 kV/mm and can be used under vacuum conditions. The phosphor and Al-layer are very sensitive to any kind of mechanic stress. Even a light touch with a finger can damage the coating. Therefore, we cannot extend our warranty to defects turning up during the operation of our phosphor screens. Prevent phosphor screens from light because it can lead to damages. We recommend to store phosphor screens in darkness under vacuum conditions or in dried nitrogen.