CN102665370A - Field emission x-ray generating apparatus - Google Patents

Abstract

A FIELD EMISSION X-RAY tube (10) comprises a cold cathode (12) as an electronic emission element, a tens of kilovolt negative voltage from a DC power supply (21) is applied to the cold cathode (12) through a high voltage cable (22), at the periphery of the DC power supply (21) there is provided a first current control resistor (31), and further at the periphery of the cold cathode (12) there is provided a second current control resistor (41). Even though an overcurrent is generated due to inductance L and capacitance C parasitic in the high voltage cable (22) and flows into the cold cathode (12), the second current control resistor (41) can be used for preventing the overcurrent from flowing into the cold cathode (12) without any change, in order to seek for a long service life of the cold cathode (12), thereby dispensing with special electronic emission elements and obtaining a FIELD EMISSION X-RAY GENERATING APPARATUS with a long service life.

Description

Field emission type X-ray generator

technical field

The present invention relates to a field emission type X-ray generating device.

Background technique

In recent years, for example, in the manufacturing process of flat panel displays, a technique of irradiating soft X-rays (weak X-rays: a soft X-ray) to generate ions and using them to remove static electricity has been used. As a device for generating soft X-rays, a filament-type X-ray generator has been used until now, but the filament-type X-ray generator has a problem of high power consumption.

Therefore, recently, an X-ray tube (field emission type X-ray tube) using a field emission type electron source can suppress power consumption low because it can emit electrons at room temperature, so it is an alternative to the existing filament method. It is highly anticipated as an alternative to X-ray tubes. However, when a high voltage of several kV to several tens of kV is applied to the field emission X-ray tube, there is a problem that the electron emission characteristics deteriorate with the lapse of the lighting time. Therefore, in applications (for example, for static elimination) that require at least a lifetime of several thousand hours or more, devices using field emission X-ray tubes have not actually been put into practical use.

In this regard, as a technique for securing a long life of a field emission type X-ray generator, a technique of short-circuiting an emitter electrode and a gate electrode with a wiring having a predetermined resistance has been proposed (Patent Document 1).

Patent Document 1: Japanese Unexamined Patent Publication No. 2008-53241

Contents of the invention

However, the technology described in Patent Document 1 aims at preventing discharge breakdown due to potential generation due to electrostatic charging, and is configured as a device that connects a power supply and a field emission X-ray tube with a high-voltage cable as described later. Under the circumstances, the countermeasures against the overcurrent generated in the high-voltage cable, especially the sudden abnormal current that tends to occur at the start of lighting are not sufficient.

The present invention has been made in view of this situation, and an object of the present invention is to provide a long-life field emission type X-ray generator in order to solve the above-mentioned problems.

The inventors have made in-depth studies and found that when a high voltage voltage of several kV to several tens of kV is applied to the X-ray tube, the major reason for the deterioration of the electron emission characteristics is that the electron emission characteristic of the field emission type X-ray tube The generation of overcurrent caused by the current fluctuation of the emission source.

When this situation is described based on the drawings, as shown in FIG. 2 , in the case of a device for eliminating static electricity, for example, the field emission type X-ray tube 101 is configured to be connected to the field from a DC power supply 103 via a high-voltage cable 102. A voltage is applied to the electron emission elements in the emission type X-ray tube 101 . The high-voltage cable 102 has so-called parasitic inductance L and capacitance C that are parasitic due to the length of the cable. In addition, it is generally considered that an overcurrent prevention resistor 104 for preventing overcurrent is provided at a location near the DC power supply 103 in the high voltage cable 102 .

When the field emission type X-ray tube 101 is turned on in this state, it is known that the fluctuation of the electron emission amount of the field emission type X-ray tube 101 is larger than that of the filament type X-ray tube, and the above-mentioned The parasitic inductance L and capacitance C of L cause a high voltage to be generated in the high voltage cable 102 such as secondary. This secondary generated voltage does not pass through the overcurrent prevention resistor 104. As a result, the device configuration shown in FIG. 2 is finally equivalent to the circuit shown in FIG. 3 . Therefore, the overcurrent prevention function by the overcurrent prevention resistor 104 does not work, the electron emission amount of the electron source further increases, and the emitter (electron emission element) deteriorates due to abnormal discharge.

This deterioration sequentially deteriorates the electron emission performance, so the life of the field emission X-ray tube 101 is shortened. Further, it is also known that a high voltage is generated in the high voltage cable 102 in a very short time especially at the start of lighting due to the aforementioned parasitic inductance L and capacitance C. According to the knowledge of the inventors, there is a possibility of applying a maximum voltage of 110% of the rating of the field emission type X-ray tube 101 .

Therefore, in view of such problems, the field emission type X-ray generating device of the present invention is characterized in that it comprises: an X-ray tube having an electron emission element emitting electrons, and generating X-rays by irradiation of electrons emitted from the electron emission element. X-ray counter cathode, and a window portion that emits X-rays generated at the counter cathode to the outside; and a power supply unit that applies voltage to the X-ray tube through a high-voltage cable, and a first current control is provided near the power supply unit. resistor, the first current control resistor limits the current flowing from the power supply unit through the high-voltage cable, and a second current control resistor is provided near the electron emission element, and the second current control resistor limits the current flowing from the high-voltage cable The current of the electron emission element, the resistance value of the second current control resistor, is smaller than the total resistance value of the resistance value specific to the electron emission element and the resistance value between the electron emission element and the pair of cathodes.

According to the present invention, the first current control resistor for overcurrent prevention provided near the power supply, and the second current control resistor for limiting the current flowing from the high-voltage cable into the electron emission element is provided near the electron emission element. resistance, so even if a high voltage is generated on the high-voltage cable in a very short period of time due to parasitic inductance and capacitance as described above, the second current control resistor can be used to suppress the amount of electron emission from the electron emission element, thereby achieving protection and achieve long life.

Preferably, when the absolute value of the voltage applied to the X-ray tube is Vo, the resistance value R [Ω] of the second current control resistor is R=0.1×Vo˜1000×Vo.

In addition, it is preferable that the second current control resistor is arranged between the electron emission element and the high-voltage cable, and within 2 m from the electron emission element; and between the second current control resistor and the electron emission element The parasitic inductance of the high-voltage cable between them is 2 μH or less, and the capacitance is 200 pF or less.

Still further, it is preferable that the electron emission element used in the present invention is a cold cathode with graphite as the base material.

According to the present invention, it is possible to obtain a long-life field emission type X-ray generator without using a special electron emission element.

Description of drawings

FIG. 1 is an explanatory diagram schematically showing the structure of a field emission type X-ray generator according to an embodiment.

Fig. 2 is an explanatory diagram schematically showing the structure of a field emission type X-ray generator constructed in the prior art.

FIG. 3 is an explanatory diagram showing an equivalent circuit of the field emission X-ray generator of FIG. 2 in use.

Explanation of reference signs

1 field emission type X-ray generating device; 10 field emission type X-ray tube; 11 housing; 12 cold cathode; 13 pairs of cathodes; 14 window; 21 DC power supply; 22 high-voltage cable; 31 The first current control resistor; 41 The second current control resistor.

Detailed ways

Below, when describing the embodiment of the present invention, Fig. 1 schematically shows the overall structure of the field emission type X-ray generator 1 of the embodiment, and the field emission type X-ray tube 10 has: a housing as a vacuum container (case or housing) 11, a cold cathode (cold cathode) 12 as an electron emission element, a cathode 13 and a window 14 for emitting X-rays generated in the housing 11 to the outside.

The housing 11 is made of an insulating material capable of maintaining an airtight interior. For example, made of glass material and insulating material. Inside the casing 11, a cold cathode 12 and a counter cathode 13 are opposed to each other.

The cold cathode 12 as an electron emission element may be made of a conductive thin plate, but may also be made of graphite. In this embodiment, an electronic material (graphite-nano-spines) using graphite as a base material is used. In addition, a raw material mainly composed of carbon, such as carbon-nano-tube, can also be used.

The counter cathode 13 is grounded through the window 14, and the counter cathode 13 and the window 14 are at ground potential. The material of the counter cathode 13 is, for example, a metal material having good conductivity such as tungsten and copper. The window 14 is made of a material having a function of emitting X-rays generated at the cathode 13 to the outside, for example, beryllium having excellent X-ray transmittance.

The cold cathode 12 is electrically connected to a DC power source 21 by a high voltage cable 22 . The positive side of the DC power supply 21 is grounded, and can apply a high-voltage negative voltage to the cold cathode 12, such as -9kV˜-16kV.

The voltage and current of the DC power supply 21 are controlled by the control device 23 . The relevant control is performed by feedback control in which the voltage and current are controlled to be A certain value.

Further, in the vicinity of the DC power source 21 in the high voltage cable 22 , a first current control resistor 31 is provided in series with the high voltage cable 22 . The resistance value of the first current control resistor 31 is, for example, 100 kΩ in this embodiment.

In addition, a second current control resistor 41 is provided in series with the high voltage cable 22 in the vicinity of the DC power supply 21 in the high voltage cable 22 . When the absolute value of the voltage applied to the field emission X-ray tube 10 is Vo [V], the resistance value of the second current control resistor 41 is 0.1×Vo to 1000×Vo [Ω].

According to the inventor's knowledge, in order to prevent the above-mentioned overcurrent with high reliability, the resistance value of the second current control resistor 41 should be 0.1Ω or more, preferably 10Ω or more. The higher the value, the more abnormal Discharge prevention function. That is, as described above, in the case where a voltage caused by parasitic inductance is generated along with the current fluctuation in the field emission X-ray tube 10, the higher the resistance value of the second current control resistor 41, the more voltage is applied to the field emission X-ray tube 10. Accordingly, the negative overvoltage of the second current control resistor 41 to the field emission X-ray tube 10 is reduced, and the risk of overcurrent generation is reduced.

On the other hand, the second current control resistor 34 is connected in series. When the field emission type X-ray tube 10 is turned on stably, for example, when it is turned on with 500 μA, the current of the second current control resistor 41 is applied to the second current control resistor 41. Resistance value × voltage of 0.0005A. Then, by applying a voltage of 50v when, for example, the resistance value of the second current control resistor 41 is 100kΩ, as a result, the second current control resistor 41 consumes 50[v]×0.0005[A]=0.025[W] power.

There is no problem with such a level of about 0.025W, but when the power consumed by the second current control resistor 41 is several W or more, by using the field emission type X-ray tube 10, less power is used than the conventional filament type X-ray tube. The advantage of the field emission type X-ray tube that produces the given X-rays is reduced and there is no superiority.

Therefore, as described above, the upper limit of the resistance value of the second current control resistor 41 is set to 1000×Vo [Ω]. In addition, according to the knowledge of the inventors, when the absolute value of the voltage applied to the field emission X-ray tube 10 is Vo [V], a more suitable and practical range is several Vo to hundreds of Vo [Ω].

In this example, the resistance value of the second current control resistor 41 is set to 10 ⁵ Ω. This is smaller than the resistance of the cold cathode 12 (emitter resistance: the total resistance value of the resistance value of the element constituting the cold cathode 12 and the resistance value between the element and the pair of cathodes 13 ). That is, in this embodiment, when the resistance of the cold cathode 12 is 2×10 ⁷ Ω (calculated from the case of measuring 0.5 mA with 10 kV applied), it is preferable that the resistance value of the second current control resistor 41 is in the range of 0.1×10 ⁴ to 1000×10 ⁴ Ω, that is, 10 ³ to 10 ⁷ Ω. This is smaller than the resistance (emitter resistance) of the cold cathode 12 .

In addition, the position where the second current control resistor 41 is arranged in series near the field emission type X-ray tube 10 is preferably within 2 m from the cold cathode 12 . In this case, the farther away from the cold cathode 12, the larger the inductance and capacitance parasitic in the high voltage cable 22, and accordingly the risk of overcurrent caused by these increases. On the other hand, the inventors confirmed by experiments that the desired effect of the present invention can be obtained as long as the distance from the cold cathode 12 is within 2 m. In addition, if the risk is taken into consideration, the second current control resistor 41 may be directly connected to the cold cathode 12, but this requires changing the structure of the cold cathode 12 itself. Therefore, in actual use, it is preferably within 1 to 10 cm from the cold cathode 12 . Therefore, although it also depends on the material of the high-voltage cable 22, it is possible to suppress the parasitic inductance of the high-voltage cable for several tens of kV, which is a copper wire insulated with silicone rubber or the like generally used in this application. It is less than 2μH, and the capacitance suppression is less than 200pF.

In this embodiment, with the above structure, when a voltage with an absolute value of several tens of kV is applied from the DC power supply 21 through the high-voltage cable 22 to the cold cathode 12 serving as the field emission element of the field emission X-ray tube 10, as shown in FIG. 1, even if the electromotive force caused by the inductance L and capacitance C parasitic in the high voltage cable 22 causes an overcurrent, the second current control resistor 41 can prevent the overcurrent from flowing into the cold cathode 12 as it is. Therefore, the lifetime of the cold cathode 12, that is, the field emission type X-ray tube 10 can be greatly extended compared to conventional ones, for example, a lifetime of several thousand hours or more required for static electricity removal can be ensured.

In addition, by preventing current fluctuations (current fluctuations) and the voltage caused by the electromotive force caused by the interaction of the above-mentioned parasitic inductance L and capacitance C at the start of lighting, it is also possible to prevent the voltage from being applied to the poles at the start of lighting. For example, a pulse (pulse) voltage that flows for a short time.

When the inventors conducted an experiment using the field emission type X-ray generator 1 shown in FIG. 1 , the following results were obtained. That is, under the condition that the voltage applied to the field emission type X-ray tube 10 is -14kV, the resistance value of the second current control resistor 41 is 100kΩ, and the length of the high voltage cable 22 is 15m, the initial stage of lighting and the elapsed time of 1000 hours are measured. The subsequent currents flowing into the cold cathode 12 were all stabilized at 500 μA.

On the other hand, in the device shown in FIG. 2 without the second current control resistor 41, when the investigation was carried out under the same conditions, the current flowing into the cold cathode 12 at the initial stage of lighting was 500 μA, but after 24 hours, it had already decreased. To 200 μA, it can be confirmed that the cold cathode 12 no longer functions as expected after 24 hours.

In addition, in the present embodiment, the electronic material (graphite nanocolumn) using graphite as the base material is used as the cold cathode 12 of the electron emission element, but even without using such an electronic material using graphite, the desired effect of the present invention can be achieved. The effect, that is, a field emission type X-ray generator with better energy efficiency than the filament method and a long life.

In addition, the above-mentioned embodiment is an example configured mainly for static electricity removal, but the present invention is not limited thereto, and can be applied to the same application as the conventional X-ray apparatus of this kind.

Industrial availability

The present invention is particularly useful for field emission type X-ray generators used continuously for a long time.

Claims (4)

1. field emission type X ray generation device is characterized in that possessing:

X-ray tube, have emitting electrons electronic emission element, utilize the target that produces X ray from the irradiation of said electronic emission element electrons emitted and the X ray that will produce at said target to the window portion of external emission; And

Power supply unit applies voltage via high-tension cable for this X-ray tube,

Near said power supply unit, be provided with the 1st current control resistor, the 1st current control resistor limits the electric current that flows through said high-tension cable from this power supply unit,

Near said electronic emission element, be provided with the 2nd current control resistor, the 2nd current control resistor limits the electric current that flows into electronic emission element from said high-tension cable,

The resistance value of said the 2nd current control resistor is littler than the total resistance value of intrinsic resistance value of said electronic emission element and the resistance value between said electronic emission element-said target.

2. field emission type X ray generation device as claimed in claim 1; It is characterized in that; When the absolute value of the voltage that will apply to said X-ray tube is made as Vo; Said the 2nd current control resistor R is R=0.1 * Vo～1000 * Vo, and wherein the unit of voltage Vo is V, and the unit of said the 2nd current control resistor R is Ω.

3. field emission type X ray generation device as claimed in claim 2 is characterized in that said the 2nd current control resistor is located between said electronic emission element and high-tension cable, and in said electronic emission element 2m,

The inductance parasitic at the high-tension cable between said the 2nd current control resistor and the said electronic emission element is below the 2 μ H, and electric capacity is below the 200pF.

4. like each described field emission type X ray generation device of claim 1～3, it is characterized in that said electronic emission element is to be the cold cathode element of basis material with graphite.

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