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EP-4740235-A1 - DISTURBANCE DETECTION

EP4740235A1EP 4740235 A1EP4740235 A1EP 4740235A1EP-4740235-A1

Abstract

A method for detecting disturbances in an electron-impact X-ray source is disclosed. The method comprises controlling an electron beam to be directed towards and form a spot at an intended target position on a stationary target, the electron beam thereby forming a spot on the target at an actual target position, the target comprising a first edge between regions of different electron backscatter probabilities; measuring a first series of values indicative of an electric current absorbed in the target as a function of time with the electron beam spot directed towards the intended target position and overlapping the first edge, wherein the intended target position is maintained stationary when measuring the first series of values; and calculating a first quality measure indicative of a disturbance in a relative position of the intended target position and the actual target position based on the first series of values. An alternative method in which the electron beam is scanned over the edge is also disclosed, as well as an X-ray source configured to perform the methods.

Inventors

  • Rosén, Robert

Assignees

  • Excillum AB

Dates

Publication Date
20260513
Application Date
20240619

Claims (12)

  1. 1. A method for detecting disturbance in an electron-impact X-ray source, comprising: controlling an electron beam to be directed towards and form a spot at an intended target position on a stationary target, the electron beam thereby forming a spot on the target at an actual target position, the target comprising a first edge between regions of different electron backscatter probabilities; measuring a first series of values indicative of an electric current absorbed in the target as a function of time with the electron beam spot directed towards the intended target position and overlapping the first edge, wherein the intended target position is maintained stationary when measuring the first series of values; and calculating a first quality measure indicative of a first deviation between the intended target position and the actual target position based on the first series of values.
  2. 2. The method of claim 1, wherein the target comprises a second edge between regions of different electron backscatter probabilities, the first edge and the second edge being non-parallel, the method further comprising: measuring a second series of values indicative of an electric current absorbed in the target as a function of time with the electron beam spot directed towards a second intended target position and overlapping the second edge; calculating a second quality measure indicative of a second deviation between the second intended target position and the actual target position based on the second series of values.
  3. 3. A method for detecting disturbance in an electron-impact X-ray source, comprising: controlling an electron beam to be directed towards and form a spot at an intended target position on a stationary target, the electron beam thereby forming a spot on the target at an actual target position, the target comprising a first edge between regions of different electron backscatter probabilities; measuring a first series of values indicative of an electric current absorbed in the target as a function of time, wherein the intended target position is repeatedly scanned over the first edge when measuring the first series of values; and calculating a first quality measure indicative of a deviation between the intended target position and the actual target position based on the first series of values by: determining, for each of a plurality of scans and based on the values measured during each respective scan, an edge position at which the intended target position coincides with the first edge; and calculating a difference between the edge position determined for each respective scan from an average edge position for the plurality of scans.
  4. 4. The method of claim 3, wherein the intended target position is repeatedly scanned also over a second edge when measuring the first series of values, and wherein the first quality measure is further calculated by: determining, for each of the plurality of scans and based on the values measured during each respective scan, a second edge position at which the intended target position coincides with the second edge; and calculating a difference between the second edge position determined for each respective scan from an average second edge position for the plurality of scans.
  5. 5. The method of any one of the preceding claims, wherein the first edge is a straight line.
  6. 6. The method of claim 2 or claim 4, wherein the first edge and the second edge are portions of a common circle.
  7. 7. The method of any one of the preceding claims, further comprising: extracting a frequency spectrum from the first series of values; and determining a type of disturbance in the X-ray source based on the frequency spectrum.
  8. 8. The method of any one of the preceding claims, further comprising controlling the electron beam to reduce the detected disturbance.
  9. 9. An electron-impact X-ray source, comprising: an electron beam generator arranged to provide an electron beam; a stationary target comprising a first edge between regions of different electron backscatter probabilities; an electron optic system arranged to focus and position the electron beam; and a controller configured to control the X-ray source to perform the method of any one of the preceding claims.
  10. 10. The electron-impact X-ray source of claim 9, wherein the or each edge is a straight line or a portion of a circle.
  11. 11. The electron-impact X-ray source of claim 9 or 10, wherein the stationary target comprises a substrate and a target layer arranged on top of the substrate, the target layer being configured to generate X-ray radiation by interaction with the electron beam; wherein the or each edge separates a first region where the substrate is exposed and a second region comprising the target layer.
  12. 12. The electron-impact X-ray source of claim 11, wherein the substrate comprises beryllium or a carbon material such as diamond; and the target layer comprises a material selected from tungsten, rhenium, molybdenum, vanadium, and niobium.

Description

DISTURBANCE DETECTION Technical field The present disclosure relates to X-ray sources. In particular, the disclosure relates to detection, identification and estimation of disturbances in X-ray sources. Background In an electron-impact X-ray source, X-ray radiation is generated by letting an electron beam impact upon a solid target. Conventionally, the target may comprise a layer of tungsten (W) deposited on a diamond substrate. The quality of the generated X-ray radiation depends, inter alia, on the spot size of the electron beam on the target and on the stability of both the electron beam and the target. Any disturbance in the electron-impact X-ray source will adversely influence the quality of the generated X-ray radiation. Hence, there is a general desire to detect, categorize and/or characterize such disturbances. WO 2019/154994 discloses a method for protecting a liquid-jet X-ray source. A monitoring arrangement is used for collecting data from various parts of the X-ray source in order to obtain a quality measure indicating a performance of the liquid jet. From the quality measure, any malperformance of the liquid jet is identified. The monitoring arrangement comprises an acoustic sensor, an accelerometer, an optic sensor, an electron detector, an X-ray detector and/or an inductive coil arrangement. Solutions for detecting, identifying or estimating disturbances are sought also for solid-target X-ray sources, for example of the transmission type. Summary In brief, the present invention relates to detection of disturbances, such as variations caused by electrical noise and/or mechanical vibrations, that may be present in an electron-impact X-ray source. By sampling an electrical current absorbed in the target (or at least a quantity indicative of the absorbed target current) while directing the electron beam towards an edge between target regions with different electron absorption properties or different electron backscatter probabilities, variations caused by relative movement between the electron beam and the target can be detected by analyzing the sampled electrical current. A type of disturbance may be identified by analyzing, for example, a frequency content of the sampled electrical current. Other characteristics of the absorbed target current that can be analyzed to detect, categorize and/or characterize disturbances include a standard deviation, a peak-to-peak value, an average absolute deviation, or the like. Further, in case of a repeatable disturbance, the electron beam may be controlled to counteract the disturbance. Several types of disturbances may contribute to X-ray spot instabilities. Two different main types of such disturbances are mechanically induced disturbances (e.g., vibrations of internal or external origin) and electrically induced disturbances (e.g., caused by electron-optical systems, variations in emission current, or acceleration voltage supply). The disturbances can be detected by determining a deviation between an intended and an actual position of the electron beam on the target. While an operator of the X-ray source may observe effects arising from such instabilities, e.g., increased noise levels in a captured X-ray image, the operator may not be able to determine which part of the system that is the source of the noise. Indeed, the operator may not even be able to notice the instability at all. For most practical applications the integration time for an X-ray image is long compared to the time scale of the disturbances, in which case the operator may notice this as an apparently larger X-ray spot size than expected according to system settings. In other words, the spot may be smeared by a disturbance of the electron beam spot position. It would therefore be advantageous to be able to monitor the stability of the electron beam spot and detect/identify/characterize disturbances internally within the X-ray source. When an electron-impact X-ray source is operated, the electron beam is typically controlled to be directed towards and form a spot at an intended target position, at which X-ray radiation is to be generated by interaction between the electron beam and the target. However, since disturbances such as mechanical vibrations may be present, the spot is not necessarily formed at the intended target position but at a slightly different actual target position. A mechanical disturbance such as vibrations will then manifest as deviations between the intended target position and the actual target position that may vary over time with the frequencies of the vibrations. It may be noted that the time scale of such vibrations is in many cases short on the time scale of X-ray imaging, i.e. in a resulting X-ray image the spot is perceived as stationary at the intended position and somewhat enlarged. The present invention relies on measuring the absorbed target current (or a quantity indicative of the absorbed target current) with the electron beam directed at an edge separ