BACKGROUND

[oooi] There are a variety of detectors that rely upon ionization. For example, ionization is used to ionize gas molecules for detecting the presence of a particular gas or substance.

[0002] There are well known ionization sources including Ni63 and Americium 241. Both of these are radioactive. Ni63 is used in ion mobility spectrometry (IMS) and field asymmetric ion mobility (FAIMS) detectors and other applications where high energy, high flux ionization is required. FAIMS, for example, is highly sensitive and selective for toxic and combustible gas detection. Americium 241 is used in smoke detectors, for example.

[0003] The ionization in a detector will ionize air or another gas within a test chamber. The test chamber typically has a metal plate that is maintained at a positive voltage. Molecules within the chamber that have been ionized are accelerated toward the plate. A portion of the ions collide with a collector electrode located between the ionization source and the plate. Ions that collide with the plate generate a current and ions that collide with the collector electrode also generate a current. The ratio of these currents is compared to determine whether a substance of interest is present.

[0004] One drawback associated with known detectors is that they include a radioactive material within the ionization source. Another drawback is that the source of radioactive particles does not provide a consistent or tunable energy level.

[0005] One suggestion for avoiding radioactive materials within an ionization source is to use soft x-rays for ionization. There are challenges associated with realizing an x-ray source for such purposes that fits within the miniaturized electronics requirements for many detector applications.

SUMMARY

[0006] An exemplary ionization device includes a pyroelectric electron accelerator that causes electrons to move away from the accelerator. A silicon target is positioned in a path of the electrons. X-ray radiation results from the electrons colliding with the target. [0007] The various features and advantages of a disclosed example will become apparent to those skilled in the art from the following detailed description. The drawings that accompany the detailed description can be briefly described as follows.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Figure 1 schematically illustrates an example ionization device designed according to an embodiment of this invention.

[0009] Figure 2 schematically illustrates selected portions of an example detector incorporating an ionization device such as the example of Figure 1.

DETAILED DESCRIPTION

[oooio] Figure 1 schematically shows an ionization device 20. A pyroelectric crystal electron accelerator 22 operates as a source of electrons schematically shown at 24. The accelerator 22 includes crystallographic Z surfaces of pyroelectric crystals that cause electron acceleration away from those surfaces. This is a known phenomena that occurs when the temperature of the pyroelectric crystals is changed in a dilute gas environment. When the crystal surface is negatively charged in a vacuum or reduced pressure environment, electrons in the close proximity become accelerated away from the surface as schematically shown at 24 in Figure 1.

[oooii] A silicon target 26 is positioned in a path of the electrons 24. As the electrons collide with the silicon target 26, x-ray radiation schematically shown at 28 results. The x-rays are useful for ionization of a gas near the ionization device 20.

[00012] One feature of the silicon target 26 is that it attenuates the energy of the electrons 24 to produce x-rays having a high enough wavelength to ionize air or another gas for detection purposes. The electrons 24 accelerated from the pyroelectric electron accelerator 22 can have an energy as high as 60 KeV. For ionization purposes within a detector, for example, the x-rays 28 preferably have an energy between 1 and 3 KeV. The silicon target 26 attenuates the energy of the electrons 24 and provides resulting x-ray radiation 28 with an energy within that range. In one example, the x-ray radiation 28 has an energy of 1.5 KeV. The x-rays 28 also have a wavelength high enough to achieve sufficient interaction with gas molecules near the ionization device 20 so that the ionization device 20 is useful within a detector device. [00013] Using a silicon target 26 provides the desired energy level and wavelength that is useful for ionization. If copper were used for the target, for example, the resultant x-ray radiation would have a higher energy level, for example on the order of 8-10 KeV. That energy level is too high for a desired ionization level within many applications. Silicon is the preferred material for the target 26 in the illustrated example to achieve ionization as needed for efficient detector operation.

[00014] The example ionization device 20 includes a housing 30 that establishes a low pressure or vacuum chamber 32 within which a gas is maintained. The low pressure or vacuum conditions within the chamber 32 facilitate movement of the electrons 24 from the accelerator 22 to the silicon target 26. A window 34 is supported by the housing 30 adjacent the target 26. In one example, the window 34 comprises beryllium (Be).

[00015] Achieving a desired amount of electron acceleration occurs by controlling the temperature of the pyroelectric crystals of the accelerator 22. The illustrated example includes a temperature control element 40 associated with the accelerator 22 for establishing a temperature that facilitates electron acceleration toward the silicon target 26. A temperature sensor 42 provides temperature information to a microcontroller 44, which controls a voltage source 46 to achieve desired operation of the temperature control element 40 so that a desired amount of electron acceleration is realized. In one example, the temperature control element 40 comprises a heater.

[00016] Figure 2 schematically shows one example use of an ionization device 20 within a detector 50. The ionization device 20 is associated with a detector chamber 52 that is adjacent to the target 26. Air within the detection chamber 52 is ionized by the x-ray radiation 28 provided by the ionization device 20. Known techniques for detecting current resulting from the ionization are used in this example.

[00017] One example detector 50 is a smoke detector. If smoke is present within the detection chamber 52, that changes the amount of ionization of the air molecules and the associated current. The detector 50 then provides an indication of the presence of smoke in a known manner. Other detectors such as IMS and FAIMS detectors are possible. The compact, low energy, low power ionization device 20 is useful in a variety of circumstances.

[00018] The preceding description is exemplary rather than limiting in nature. Variations and modifications to the disclosed examples may become apparent to those skilled in the art that do not necessarily depart from the essence of this invention. The scope of legal protection given to this invention can only be determined by studying the following claims.