X-ray or gamma ray systems or devices – Source – Electron tube
Patent
1998-01-08
1999-01-05
Church, Craig E.
X-ray or gamma ray systems or devices
Source
Electron tube
378126, H01J 3530
Patent
active
058570080
DESCRIPTION:
BRIEF SUMMARY
DESCRIPTION
The invention relates to equipment of the kind known from U.S. Pat. No. 4,344,013 (Ledley).
The usability of so-called direct and enlarging radiographic equipment, in particular in the fields of material testing and medicine, is described more closely in the contribution "Entwicklung und Perspektiven der medizinischen Vergrosserungsradiographie" by G. Reuther, H. -L. Kronholz and K. B. Huttenbrink in RADIOLOGE, volume 31 (1991), pages 403 to 406. The function of such equipment is based on the radiation-geometric law, according to which a radiation source leads to high-contrast shadow images of high local resolution only when the radiation surface effective for imaging is very small by comparison with the irradiated surface of the object to be imaged, because otherwise each point of the object would be irradiated at different angles, thus from different places of the radiation source, each object point on projection into the image plane would result in shadow casts displaced relative to one another and the result altogether would be a smudged outline of the object which is illustrated enlarged according to its distance from the image plane.
In spite of the improvement in the resolution achievable thereby, items of microfocus X-ray equipment have not been able to gain acceptance so well in practice, in particular in medical diagnosis. This appears to be traced back above all to them being able to operate only with restricted X-ray power, because the very narrow focussing of the electron beam onto the retarding target results in a focus spot (focus) of very small diameter with correspondingly high energy density. This high specific loading rapidly leads to the target, which is usually irradiated at a direction of 10.degree. to 45.degree., experiencing a change, which is disadvantageous for the conversion of the impinging electron beam energy into X-ray energy to be delivered, in its topography with rapid destruction of the retarding layer. Otherwise, the exposure time per X-ray recording would have to be prolonged when X-rays of lower power were to be used, which would, however, contradict the demand for short exposure times in the range of tenths to hundredths of seconds in order to avoid an unnecessarily high beam loading and defocussing due to the movement of the object. However, the smaller the thermal focus spot is on the target anode, the lower also becomes the electrical power which can be received by the small target area before it begins to melt. This behaviour thus contradicts the requirement for higher density of the electron beams impinging on the target for higher power of the X-ray radiation.
An item of microfocus X-ray equipment, which operates already with a target that has begun to melt, is known from the initially mentioned U.S. Pat. No. 4,344,013 (Ledley). In this equipment, the electron beam impinges on an obliquely set target, so that the produced X-radiation is similarly radiated away from the target at an angle. However, in this equipment, it has not been taken into consideration that a rapidly progressing crater formation leads, even before complete burning-through of the target, to the optical axis of the useful radiated X-ray radiation experiencing a shadowing by the crater rim that is swelling up and absorbs the X-ray radiation to a large extent. There results a diffuse X-ray light which cannot be regarded as emanating from a punctiform source. For that reason, equipment of that kind with an oblique setting of the target relative to the incident electron beam has not proved itself.
German preliminary published specification (DE-OS) 34 01 749 A1 (Siemens) concerns X-ray equipment in which the electron beam is deflected constantly and, for example, in meander shape on the retarding material. However, the effective focus spot is thereby enlarged, as a result of which the image sharpness suffers, as described above.
A transmission target, in which the retarding material is arranged on a carrier material, is known from German preliminary published specification (DE-OS) 26 53
REFERENCES:
patent: 4344013 (1982-08-01), Ledley
patent: 4896341 (1990-01-01), Forsyth et al.
"Electron Beam Melting in Microfocus X-Ray Tubes", by Grider et al, J. Phys. D. ppl. Phys 19 (1986) pp. 2281-2292.
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