Patent Description:
Typically, ultraviolet (UV) sensitive detectors may be used to detect flames or sparks. The detectors may typically be used for detecting the flames in indoor spaces, e.g. buildings, or outdoor spaces. Typically, the flames are emitting UV radiation at a wavelength band between <NUM> and <NUM> nanometers. For example, lamps used for lighting, do not emit UV radiation at said wavelength band due to a strong UV absorption in their materials, such as glass. This enables that the UV radiation from the flames may be detected by means of the flame detectors in indoor spaces. Moreover, because the ozone layer in the atmosphere absorbs the UV radiation from the sun at said wavelength band, the UV radiation from the flames may also be detected by means of the flame detectors in outdoor spaces (e.g. forest fires, etc.).

For example, the flame detectors may be implemented as proportional counters comprising a cylindrical shaped photocathode tube and an anode wire travelling substantially along the longitudinal axis of the photocathode tube, which may be filled by a gas.

In order to be able to detect the flame as far away as possible, the flame detector should be sensitive to the UV wavelength band radiation and as insensitive as possible for radiation at longer wavelengths which dominates daylight light. The sensitivity of the flame detector may typically be limited by a background radiation caused by the daylight and pulses caused by the cosmic radiation.

Thus, there is a need for developing solutions in order to improve at least partly sensitivity of the flame detectors.

A patent application <CIT> discloses a flame detector for detection of the presence of a flame or spark in front of the detector comprising a UV sensitive photocathode and an anode.

A patent application <CIT> discloses a photo-sensitive tube employed as a detector for automatic fire alarm systems.

An objective of the invention is to present an ultraviolet flame detector. Another objective of the invention is to provide the ultraviolet flame detector with an improved sensitivity.

The objectives of the invention are reached by an ultraviolet flame detector as defined by the respective independent claim.

According to a first aspect, an ultraviolet flame detector is provided, wherein the ultraviolet flame detector comprises: a cylindrical housing having an opening at a top end of the housing, a window structure arranged to cover the opening of the housing, a photocathode arranged to a bottom end of the housing so that the photocathode is facing inside the housing, and an anode wire arranged between the window structure and the photocathode, wherein the anode wire is configured to travel transversally across the housing, and wherein the ultraviolet flame detector is filled with a gas.

The gas may be a mixture of the following gases: argon (Ar), isobutane (iC<NUM>H<NUM>), and hydrogen gas (H<NUM>).

Alternatively or in addition, the material of the photocathode may be cesium iodide (Csl) or any other solar blind material.

Alternatively or in addition, the inner surfaces of the housing may be coated with a metal having a work function of at least <NUM> eV.

The metal coating may be gold, wherein the work function of the gold may be from <NUM> to <NUM> eV.

Alternatively or in addition, the ultraviolet flame detector may further comprise a wire mesh arranged under the window structure and configured to protect one or more components of the flame detector from electromagnetic interferences.

The wires of the wire mesh may be coated with a metal having a work function of at least <NUM> eV, wherein the metal coating may be gold.

Alternatively or in addition, the housing may comprise two opposing through holes arranged to a longitudinal side wall of the housing for the anode wire.

Alternatively or in addition, the anode wire may be arranged at a predetermined distance (D) from the photocathode.

Alternatively or in addition, the material of the window structure may be one of fused silica, sapphire, calcium fluoride, or magnesium fluoride.

Alternatively or in addition, the window structure may comprise an interference filter.

Alternatively or in addition, the anode wire may be configured to be positively biased, wherein the anode wire is electrically connectable to a preamplifier via a coupling capacitor.

Alternatively, the photocathode may be configured to be negatively biased, wherein the anode wire is directly electrically connectable to a preamplifier.

<FIG> illustrates schematically an example of an ultraviolet (UV) flame detector, i.e. a solar blind UV detector, <NUM> according to the invention. <FIG> illustrates a cross section of the UV flame detector <NUM>. <FIG> illustrates schematically a top view of the UV flame detector <NUM> of <FIG>. The flame detector <NUM> comprises a housing <NUM>, a window structure <NUM>, a photocathode <NUM>, and an anode wire <NUM> arranged between the window structure <NUM> and the photocathode <NUM>. The housing <NUM> is substantially cylindrical.

Especially inner surfaces of the housing <NUM> form a substantially cylindrical chamber inside the housing <NUM>. The housing <NUM> has first end 101a, e.g. a top end of the housing <NUM>, and a second end 101b, e.g. a bottom end of the housing <NUM>. The housing <NUM> has an opening <NUM> at the first end 101a of the housing <NUM>. <FIG> illustrates an example of the housing <NUM> of the UV flame detector <NUM> according to the invention. <FIG> illustrates a cross section of the housing <NUM>. The window structure <NUM> is arranged to cover the opening <NUM> of the housing <NUM>. In other words, the window structure <NUM> is attached to the housing <NUM> at a region around the opening <NUM> of the housing <NUM>. The photocathode <NUM> is arranged to the second end 101b of the housing <NUM> so that the photocathode <NUM> is facing inside the housing <NUM>. The anode wire <NUM> is configured to travel substantially transversally across the housing <NUM>, i.e. in a direction substantially perpendicular to a longitudinal axis of the housing <NUM>. The UV flame detector <NUM> is filled with a gas.

The UV flame detector <NUM> according to the invention may be used for detecting flames or sparks. Although, hereinafter throughout the application the detection of the flames is discussed, all the same applies also for the detection of the sparks. Typically, the flames are emitting UV radiation, i.e. UV light, at a wavelength band between <NUM> and <NUM> nanometers. The UV flame detector <NUM> according to the invention <NUM> is sensitive to UV radiation at a solar blind UV wavelength band, i.e. UV wavelengths below <NUM> nanometers. The UV flame detector <NUM> according to the invention <NUM> is especially sensitive to the UV radiation emitted by the flames. The UV flame detector <NUM> according to the invention is capable to detect the flames indoors and/or outdoors. The operation of the UV flame detector <NUM> according to the invention may be implemented as a gas-filled proportional counter configured to detect the flames. The UV radiation emitted by the flames penetrates through, i.e. passes through, the window structure <NUM> and reaches the photocathode <NUM>. The anode wire <NUM> is biased in relation to the photocathode <NUM> to create an electric field inside the UV flame detector <NUM>. Because of the created electric field, photoelectrons detaching from the photocathode <NUM> drift towards the anode wire <NUM> and positive ions drift from the anode wire <NUM> towards the photocathode <NUM>. Near the anode wire <NUM> the electric field is high, i.e. the strength of the electric field is large, causing amplification of a signal via a gas amplification. The signal may be induced to a preamplifier <NUM> (for sake of clarity not shown in <FIG>) electrically connected to the anode wire <NUM>, when positive ions drift from the anode wire <NUM> to the photocathode <NUM>. The gas amplification must be higher than <NUM> to resolve it from background noise of the electronics of the UV flame detector <NUM> and lower than <NUM> so that the gas amplification stays in the proportional mode and not reach a Geiger mode, where amplitude information will be lost. In the Geiger mode the gas amplification saturates causing that a signal induced by a single photoelectron cannot be distinguished from a signal induced by background radiation, e.g. a cosmic background radiation caused by thousand(s) of electrons. When the gas amplification is in the proportional mode, i.e. in a linear mode, the signal induced by the single photoelectron may be distinguished from the signal induced by the cosmic background radiation.

A diameter of the cylindrical housing <NUM> may be e.g. from <NUM> to <NUM> millimeters. Preferably, the diameter of the cylindrical housing <NUM> may be e.g. from <NUM> to <NUM> millimeters. The opening <NUM> of the housing <NUM>, which is covered by the window structure <NUM>, may have a diameter smaller than or equal to the diameter of the housing <NUM>. In the example illustrated in <FIG> the diameter of the opening <NUM> of the housing <NUM> is smaller than the diameter of the housing <NUM>, but the invention is not limited to this. The diameter of the radiation window <NUM> may be depend on the diameter of the opening <NUM> of the housing <NUM> and thus also on the diameter of the housing <NUM>. Thus, also the diameter of the window structure <NUM> may be smaller than or equal to the diameter of the housing <NUM>. The diameter of the window structure <NUM> may preferably be as large as possible. The diameter of the window structure <NUM> may be e.g. from <NUM> to <NUM> millimeters depending on the diameter of the opening <NUM> of the housing <NUM> and/or the diameter of the housing <NUM>. According to a non-limiting example, the diameter of the housing <NUM> may be <NUM> millimeters and the diameter of the window structure <NUM> may be <NUM> millimeters.

According to an example embodiment of the invention, the inner surfaces of the housing <NUM> may be coated with a metal having a work function of at least <NUM> eV. This eliminates or at least reduces background radiation caused by daylight penetrated through, i.e. passed through, the window structure <NUM> and hit to the inner surfaces of the housing <NUM>, e.g. the longitudinal side wall of the housing <NUM>. Before the daylight hits to the inner surfaces of the housing <NUM>, it may have been scattered from the photocathode <NUM>. Preferably, the metal coating may be gold. The work function of the gold may be from <NUM> to <NUM> eV. The material of the housing <NUM> itself may be for example, but is no limited to, stainless steel.

The material of the window structure <NUM> may be selected so that the window structure <NUM> is transparent to the UV radiation, especially UV radiation at the wavelength band between <NUM> and <NUM> nanometers, to enable the UV radiation emitted by the flames to enter inside the detector <NUM> and to reach the photocathode <NUM>. The material of the window structure <NUM> may be e.g. one of fused silica, sapphire, calcium fluoride, or magnesium fluoride. The mentioned materials enable that the window structure <NUM> is transparent to the UV radiation emitted by the flames.

Alternatively or in addition, the material of the photocathode <NUM> may be selected so that the photocathode <NUM> is sensitive to the UV radiation emitted by the flames, i.e. the UV radiation at the wavelength band between <NUM> and <NUM> nanometers. The material of the photocathode <NUM> may be e.g. cesium iodide (Csl) or any other solar blind material. These materials enable that the photocathode <NUM> is sensitive to the UV radiation emitted by the flames. The photocathode <NUM> may be implemented as a coating on a surface of the second end 101b of the housing <NUM> facing inside the housing <NUM> as illustrated in the example of <FIG>. Alternatively, the photocathode <NUM> may form the second end 101b of the housing <NUM>, i.e. the bottom end of the housing <NUM>. In other words, the second end 101b of the housing <NUM> may itself act as the photocathode <NUM>.

According to an example embodiment of the invention, the gas with which the UV flame detector <NUM> is filled may be a gas mixture of argon (Ar), isobutane (iC<NUM>H<NUM>), and hydrogen gas (H<NUM>), i.e. the gas mixture of Ar + iC<NUM>H<NUM> + H<NUM>. Preferably, the UV flame detector <NUM> may be filled with the gas mixture of Ar + (<NUM>-<NUM>%)iC<NUM>H<NUM> + (<NUM>-<NUM>%)H<NUM>. Alternatively, the gas may be e.g. a gas mixture of argon (Ar) and carbon dioxide (CO<NUM>) or any other suitable gas. By filling the UV flame detector <NUM> with the gas mixture of Ar + iC<NUM>H<NUM> + H<NUM> enables that the UV flame detector <NUM> expires more slowly, i.e. a lifetime of the UV flame detector <NUM> filled with the gas mixture of Ar + iC<NUM>H<NUM> + H<NUM> may be over an order of magnitude longer than a lifetime of the UV flame detector <NUM> filled e.g. with the gas mixture of Ar + iC<NUM>H<NUM> without H<NUM>. Moreover, the gas mixture of Ar + iC<NUM>H<NUM> + H<NUM> is radiation-resistant and enables substantially low high voltage (HV) for the gas amplification. The mixture of Ar + iC<NUM>H<NUM> is so called Penning mixture. In the gas amplification process the argon atom either ionizes or excites. The ionization energy of the isobutane is lower than the excitation energy of the argon. Thus, the excited argon atoms ionize the isobutane (so called Penning process). Because of this more powerful ionization process, the needed HV for the gas amplification may be substantially low, i.e. lower in comparison to other gas mixtures, e.g. with the gas mixture of Ar + CO<NUM> higher HV is needed.

The material of the anode wire <NUM> may be e.g. tungsten, i.e. wolfram. Tungsten itself is a strong material. The anode wire <NUM> may be coated with a metal having a work function of at least <NUM> eV, e.g. gold. The coating of the anode wire <NUM> enables that the surface of the anode wire <NUM> maintains stable and does not react with the gas. The anode wire <NUM> may be arranged at a predetermined distance D from the photocathode <NUM> as illustrated in <FIG>. The housing <NUM> may comprise two opposing through holes 110a, 110b arranged to the longitudinal side wall <NUM> of the housing <NUM> for the anode wire <NUM>, i.e. for providing the anode wire <NUM> inside the housing <NUM> to enable the anode wire <NUM> travel transversally across the housing <NUM> at the predetermined distance D from the photocathode <NUM>. A first end of the anode wire <NUM> may pass through one of the two through holes, e.g. a first through hole 110a, and a second end of the anode wire <NUM> may pass through the other one of the two through holes, e.g. a second through hole 110b. An insulating material, e.g. ceramic material, <NUM> may be arranged inside the two through holes 110a, 110b for insulation of the anode wire <NUM> from the housing <NUM>. Alternatively or in addition, a pin 112a, 112b may be arranged to each end of the anode wire <NUM>, i.e. a first pin 112a may be arranged to the first end of the anode wire <NUM> and a second pin 112b may be arranged to the second end of the anode wire <NUM>. The pins 112a, 112b may be arranged at least partially inside the through holes 110a, 110b as illustrated in <FIG>, i.e. the first pin 112a may be arranged at least partially inside the first through hole 110a and the second pin 112b may be arranged at least partially inside the second through hole 110b. The pins 112a, 112n enable providing electrical connections to the anode wire <NUM>, e.g. the electrical connection to the preamplifier <NUM> and/or the electrical connection for biasing the anode wire <NUM>. The anode wire <NUM> may be positively biased, e.g. by means of a positive HV, wherein the preamplifier <NUM> may be electrically connected to the anode wire <NUM> via a coupling capacitor <NUM>. <FIG> illustrates schematically a simple example of biasing the UV flame detector <NUM> according to invention, wherein the anode wire <NUM> is positively biased by a voltage source <NUM>. Alternatively, the photocathode <NUM> may be biased negatively, e.g. by means of a negative HV. This enables that the preamplifier <NUM> may be electrically connected directly to the anode wire <NUM> without the coupling capacitor <NUM>, which in turn reduces input capacitance and substantially also microphonism of the UV flame detector <NUM>. <FIG> illustrates schematically another simple example of biasing the UV flame detector <NUM> according to invention, wherein the photocathode <NUM> is negatively biased by the voltage source <NUM>. The longitudinal side wall <NUM> of the housing <NUM> may have a thickening at a location of the through holes 110a, 110b, i.e. the longitudinal side wall <NUM> of the housing <NUM> may be thicker at the location of the at the through holes 110a, 110b than at the other parts of the longitudinal side wall <NUM> of the housing <NUM> as illustrated in the example of <FIG>. This may improve mechanical robustness, i.e. mechanical strength, of the housing <NUM>. Alternatively, the longitudinal side wall <NUM> of the housing <NUM> may have similar, i.e. consistent, thickness throughout the entire longitudinal side wall <NUM> of the housing <NUM>.

Diameters of the through holes 110a, 110b may have an effect on the electric field inside the housing <NUM> of the UV flame detector <NUM>. Preferably, the diameters of the through holes 110a, 110b may be defined so that the electric field in the longitudinal direction of the anode wire <NUM> is substantially constant, i.e. that substantially uniform electric field may be achieved over an entire surrounding area of the anode wire <NUM>. The substantially constant electric filed in the longitudinal direction of the anode wire <NUM> enables that a photoelectron detaching from any part of the photocathode <NUM> may be at the same position with regard to the gas amplification, which in turn enables that the signal may be amplified at every point of the anode wire <NUM> so that the signal is above the noise of the electronics of the UV flame detector, e.g. the noise of the preamplifier <NUM>. The diameters of the two through holes 110a, 110b may be for example, but are not limited to, from <NUM> millimeters to <NUM> millimeters. Preferably, the diameters of the through holes 110a, 110b may be e.g. <NUM> millimeters.

Alternatively or in addition, a ratio between the predetermined distance D and the diameter of the housing <NUM> may have an effect on the electric field inside the housing <NUM> of the UV flame detector <NUM>. Preferably, the predetermined distance D may be defined so that the ratio between the predetermined distance D and the diameter of the housing <NUM> enables that the electric field in the longitudinal direction of the anode wire <NUM> is substantially constant.

According to an example embodiment of the invention, the UV flame detector <NUM> may alternatively or in addition comprise a wire mesh <NUM> arranged to the first end 101a of the housing <NUM> under, i.e. below, the window structure <NUM>. In other words, the wire mesh <NUM> may be arranged to the first end 101a of the housing next to a lower surface of the window structure <NUM>, i.e. the surface of the window structure <NUM> which is facing inside the housing <NUM>. The wire mesh <NUM> may be arranged to the first end 101a of the housing <NUM> under the window structure <NUM> so that a gap exists between the window structure <NUM> and the wire mesh <NUM>, i.e. between the lower surface of the window structure <NUM> and the wire mesh <NUM>. The gap between the window structure <NUM> and the wire mesh <NUM> may be for example, but is not limited to, less than <NUM> millimeter. Alternatively, the wire mesh <NUM> may be arranged to the first end 101a of the housing <NUM> under the window structure <NUM> so that the wire mesh <NUM> is substantially in contact with the window structure <NUM>, i.e. with the lower surface of the window structure <NUM>. <FIG> illustrates schematically an example of the UV flame detector <NUM> comprising the wire mesh <NUM>. The wire mesh <NUM> may be configured to protect one or more components of the UV flame detector <NUM> from electromagnetic interferences. Especially, the wire mesh <NUM> may be configured to protect the preamplifier electrically connected to the anode wire <NUM> from the electromagnetic interferences by preventing the propagation of the electromagnetic interferences to the preamplifier <NUM>. Moreover, the wire mesh <NUM> may prevent the window structure <NUM> to be charged and by this way the wire mesh <NUM> enables that the electric field inside the UV flame detector <NUM> may be kept stable. At the same time the wire mesh <NUM> allows the desired radiation, i.e. UV radiation emitted by the flames, to pass, i.e. penetrate, through the wire mesh <NUM> so that the desired radiation reaches the photocathode <NUM>. The wire mesh <NUM> may preferably be substantially sparse, i.e. an area formed by openings of the wire mesh <NUM>, i.e. openings between wires of the wire mesh <NUM>, may be at least <NUM> % of the area of the wire mesh <NUM>. According to a non-limiting example of a sparce wire mesh <NUM> the diameter of the wires of the wire mesh <NUM> may be <NUM> micrometers and the openings of the wire mesh <NUM> may be <NUM> millimeter times <NUM> millimeters. The wire mesh <NUM> may be formed e.g. by a plurality of parallel wires. Alternatively, the wire mesh <NUM> may be formed e.g. a plurality of intersecting wires. The wires of the wire mesh <NUM> may be coated with a metal having a work function of at least <NUM> eV, e.g. with gold. The work function of the gold may be from <NUM> to <NUM> eV as discussed above. The coating of the wires of the wire mesh <NUM> eliminates or at least reduces the background radiation as discussed above.

Alternatively or in addition, according to an example embodiment of the invention, the window structure <NUM> may comprise an interference filter <NUM>. In other words, the interference filter <NUM> may be integrated to the window structure <NUM>. <FIG> illustrates schematically an example of the UV flame detector <NUM> comprising the interference filter <NUM>. In the example of <FIG> the wire mesh <NUM> is not illustrated, but the UV flame detector <NUM> may also comprise both the wire mesh <NUM> and the interference filter <NUM>. The interference filter <NUM> may be a multilayer interference filter that may be grown on top of the window structure <NUM> by using the window structure <NUM> as a substrate for the growing of the interference filter <NUM>. The interference filter <NUM> may be grown on top of the window structure <NUM> e.g. by sputtering or by using thin film manufacturing techniques, e.g. atomic layer deposition (ALD). The window structure <NUM> may be attached to the housing <NUM> so that the interference filter <NUM> of the window structure <NUM> may be facing outwards from the UV flame detector <NUM> as illustrated in the example of <FIG>. Alternatively, the window structure <NUM> may be attached to the housing <NUM> so that the interference filter <NUM> of the window structure <NUM> may be facing inside the UV flame detector <NUM>, i.e. inside the housing <NUM>. The integration of the interference filter <NUM> to the radiation window structure <NUM> enables that a separate interference filter is not needed to be used with the UV flame detector <NUM>. In other words, the integrated interference filter <NUM> eliminates the need for a separate interference filter.

The illustrated dimensions in Figures are not to scale and not comparable to each other; they have been selected only for graphical clarity in the drawings.

Claim 1:
An ultraviolet flame detector (<NUM>), characterized in that the ultraviolet flame detector (<NUM>) comprises:
- a cylindrical housing (<NUM>) having an opening (<NUM>) at a top end (101a) of the housing (<NUM>),
- a window structure (<NUM>) arranged to cover the opening (<NUM>) of the housing (<NUM>),
- a photocathode (<NUM>) arranged to a bottom end (101b) of the housing (<NUM>) so that the photocathode (<NUM>) is facing inside the housing (<NUM>), and
- an anode wire (<NUM>) arranged between the window structure (<NUM>) and the photocathode (<NUM>), wherein the anode wire (<NUM>) is configured to travel transversally across the housing (<NUM>),
wherein the ultraviolet flame detector (<NUM>) is filled with a gas.