Ion Beam Profile Scanner Having Symmetric Detector Surface to Minimize Capacitance Noise

Ion beams are often used in the study of nuclear physics and are difficult to analyze because their intensity levels are low compared to environmental noise. Because of their low intensity levels, it is important to minimize the noise generated by scanners that are used in determining the profile of these beams.

Ion beam profile scanners, such as the one disclosed in U.S. Pat. No. 3,789,298, issued Jan. 29, 1974 to Herb, experience microphonic noise from sources such as the motion of the probe through the beam and pickup from line power. These noise sources have frequencies near the range of typical beam sample rates of 10-30 Hz. For this reason, microphonic noise is difficult to filter out from the desired signal.

In some prior art devices, such as the Herb patent referenced above, the detector is generally a cylinder coaxial with the ion beam and perpendicular with the axis of rotation of the probe. Using this detector construction, it was discovered that microphonic noise may be minimized by using an uncharged cylindrical detector. But, since charged particles are to be detected, using an uncharged detector places a severe limitation on the sensitivity of the detector. On balance, however, the loss in sensitivity was justified by the noise reduction of this design.

In the present invention, a major source of microphonic noise has been recognized and a detector structure has been devised that significantly reduces noise compared to prior art devices and allows a charged detector surface. In accordance with the present invention, the particle beam profile scanner includes a probe constructed of a material that will emit electrons when struck by an particle beam. The probe is passed through the particle beam and, as a result, electrons are emitted by the probe in proportion to the number of particles striking the probe. The emitted electrons are detected by the detector which generates a signal proportional to the number of electrons detected. The surface of the detector is configured and oriented with respect to the moving probe such that the detector offers a substantially symmetric view to the probe.

Since the surface of the detector offers a substantially symmetric view to the probe as it moves, there will be very little or no change in capacitance between the probe and the detector as the probe changes position. Changing capacitance substantially contributes to microphonic noise, and because such changing capacitance is substantially reduced, there will be an increase in the signal to noise ratio of the detector.

One preferred embodiment of the invention provides for increasing the sensitivity of the detector by electrically charging the surface. Since electrons are to be detected, charging the surface of the detector with a positive charge will increase the number of electrons attracted to the surface. Also, since the detector surface is symmetric with respect to the probe, the charge on the plate does not cause a changing capacitance as the probe moves.

The preferred embodiment of the invention also employs a circular metal detector surface that is positioned to offer a substantially symmetric view to the probe as it moves in a circular manner. It is recognized, however, that a circular surface is not the only one that offers a substantially symmetric view to a rotating probe. An extended flat surface of any geometry, if sufficiently large, also appears substantially symmetric to a rotating probe. In such a case, there is no changing capacitance since the non-symmetric edges of the surface are too remote to affect the capacitance felt by the probe. Concave or convex spherical detector surfaces could also offer a substantially symmetric view to a rotating probe as well.

In the preferred embodiment, a preamplifier and the detector surface are mounted in a vacuum within a housing. The preamplifier is mounted very close to the detector surface to decrease the triboelectric noise due to vibrating signal lines. Noise in the preferred ion beam profile scanner is further reduced by providing a grounded shield that is configured to allow passage of the ion beam. The shield minimizes interferences from external sources.

This invention will be more clearly understood from the following Detailed Description of the preferred embodiment and when considered in conjunction with the accompanying drawing in which a side schematic view of the ion beam profile scanner is shown.

The preferred form of the invention is schematically represented in the drawing as scanner 9 in which an ion beam 10 is traveling down an ion beam passage tube 12 and into the ion beam profile scanner housing 14 through an entry port 16. The housing 14 is cylindrical with its axis oriented perpendicular to the ion beam 10 and the entry port 16 and an exit port 18 are oppositely disposed in a cylindrical side wall 20 of the housing 14 to allow passage of the ion beam 10 through the housing 14. A top surface 22 and bottom surface 24 serve to close the housing 14, and a substantial vacuum is maintained in housing 14 by a vacuum pump 26 connected by vacuum tubing 28 and a vacuum connection 30 to the housing 14.

Attached to the bottom surface 24 of housing 14 is motor 32 with a magnetic coupling 34 to provide drive power to shaft 36 in the interior of the housing 14. Motor 32 is provided with a magnetic pickup 38 which sends motor speed information to signal analyzing circuits 40.

In the preferred mode a shield 42 is mounted in the interior of the housing 14. The shield 42 is composed of a cylindrical side wall 44 aligned coaxially with the housing side wall 20. The shield side wall 44 has two openings 46 oppositely disposed to allow complete passage by the ion beam 10, and a top surface 48 and bottom surface 50 serve to close the shield 42. The shield may be grounded with wire 52, connection 54 in side wall 20 of housing 14, and electrical ground 56.

Shaft 36 mentioned above is oriented along the axes of the housing 14 and shield 42. The shaft 36 travels from the magnetic coupling 34 on the bottom surface 24 of the housing 14 through the bottom surface 50 of the shield 42 up to the center of a crossarm 58. Crossarm 58 is perpendicular to shaft 36 and is located just below the path of ion beam 10 through shield 42. In the preferred mode, a curved wire probe 60 is mounted on one end of crossarm 58.

The curved wire probe 60 is configured so that it makes an oblique angle with the axis of the ion beam 10. When the curved wire probe 60 is rotated, it passes through the ion beam 10 twice each revolution. Due to the configuration of the curved wire probe 60, it sweeps across the ion beam 10 at different angles during each pass.

The curved wire probe 60 is constructed of material to emit electrons when struck by the ion beam 10. The total number of electrons emitted by curved wire probe 60 is proportional to the total number of ions in ion beam 10 that strike the curved wire probe 60.

The electrons emitted by the curved wire probe 60 are detected by the detector 62 which generates a signal proportional to the number of electrons detected. The surface 64 of the detector 62 is configured and oriented so that it offers a substantially symmetric view to rotating curved wire probe 60. As the curved wire probe 60 rotates through the ion beam 10 it has an unvarying view of the surface 64 of the detector. In the preferred mode, the surface 64 is a circular metal disc which is positively charged to attract the electrons. The surface 64 is charged by applying a bias voltage to the preamplifer 72 which in turn charges the surface 64.

Placing a charge on surface 64 produces a capacitance between the surface 64 and the curved wire probe 60. If the capacitance changes as the curved wire probe 60 rotates, noise will be introduced into the detection signal. The detector 62 will see a change in capacitance unless the surface 64 shows no change to the curved wire probe 60. With a surface 64 that offers a substantially symmetric view to the curved wire probe 60 there is substantially reduced changing capacitance.

A preamplifier 72 is mounted inside the housing 14 and the shield 42 close to the detector 62 in order to minimize the triboelectric effect noise caused by the vibrations in the connecting wires 74 between the detector 62 and the preamplifier 72 and, thus, one result of mounting the preamplifier 72 in the substantial vacuum of the housing 14 is to decrease the inherent noise of the preamplifier 72. The preamplifier 72 amplifies the signal from the detector 62, and the signal from the preamplifier 72 is sent to signal analyzing circuits and output module 40 through wires 76 and connections 78 through the shield 42 and the housing 14. The magnetic pickup 38 also generates a signal corresponding to the movement and position of the wire probe 60 and this signal is applied to the analyzing circuits and output module 40. The module 40 uses the signals from the preamplifier 72 and the magnetic pickup 38 to determine the profile contour of the ion beam 10 and to generate and display a signal corresponding to the ion beam profile. In the preferred embodiment, module 40 includes analog input amplifiers for receiving both signals, analog to digital converters for converting the signals to digital form, a computer for analyzing the signals and producing a digital profile signal corresponding to the ion beam profile, a video display for graphically displaying the ion beam profile, a magnetic memory for storing raw data and the profile signal, and an output port for outputting data and the profile signal. All of this equipment is well-known in the art and analog equipment may also be substituted into module 40 to perform the same function.

From the foregoing discussion, it will be appreciated that the present invention provides a geometry that reduces noise in an ion beam profile scanner to the point that it is advantageous to charge the electron detector surface 64. In addition, mounting the preamplifier 72 proximate to the detector 62 reduces noise caused by vibrating signal lines and the inherent noise of the preamplifier 72 is reduced by mounting it in the substantial vacuum of housing 14. The further mounting of a shield 42 within housing 14 reduces noise from external sources other than ion beam 10. The net effect is that the present invention greatly reduces the noise inherent in prior art scanners while being able to maintain the sensitivity of a charged detector surface 64.