US 3450879 A
Description (OCR text may contain errors)
June 17, 1969 J sEPP| 3,450,879
DIELECTRIC-TYPE CHARGED PARTICLE DETECTOR Filed March 5, =l968 k 48 v t 46 ll BEAM I \W 48 4 l5/ DIELECTRIC 22 .L af: a o
33 BEAM 33 INVENTOR. EDWARD J. SEPP/ ATTORNE).
United States Patent U.S. Cl. 25083.3 4 Claims ABSTRACT OF THE DISCLOSURE A charged particle detector comprised of a thin dielectrio in intimate contact with a pair of electrodes across which a voltage is applied to extract a current from the dielectric upon its being irradiated with charged particles that cause the dielectric to become proportionately electrically conductive.
Background of the invention The present invention relates generally to particle detectors, and more particularly, it pertains to a detector having a dielectric element that is irradiated with the particles to be detected.
The invention disclosed herein was made under, or in, the course of Contract No. AT(O4-3)400 with the US. Atomic Energy Commission.
Secondary emission monitors (SEM) are widely used for detecting the current of a charged particle beam. Such devices operate on the principle of secondary emission in the form of electrons being emitted from a foil surface when an energetic charged particle beam impinges on the foil. A high positive bias potential is applied to a second foil that is separated from the first foil. The positive bias voltage causes the electrons to be collected on the second foil. The resultant foil current is proportional to the beam current and therefore may be used to indicate the intensity of the beam current as well as its location. However, to obtain a high degree of spacial resolution with an SEM, it is necessary to arrange the foils in complex patterns. For good spacial resolutions, narrow, closely spaced patterns are required. Frames for holding the foils therefore become correspondingly smaller and complex, and the electrical connections thereto difficult. For very high resolution, the required foil patterns, frames and connections become so intricate as to make an SEM impracticable.
Summary of the invention Briefly, the invention is the discovery that irradiation of a dielectric with charged particles raises a proportionate number of the electrons in the dielectric toa conduction band, and that application of a voltage to the irradiated dielectric through metal electrodes in intimate contact therewith results in an electric current proportional to the intensity of irradiation impinging on the dielectric positioned directly between opposing electrode surfaces. In accordance with the invention, closely It is an object of the invention to extract a current from a dielectric being irradiated with charged particles.
Another object is to detect the location and intensity of a charged particle beam with a high degree of resolution.
Another object is to simplify the construction and mounting of a charged particle beam detector.
Other objects and advantageous features of the invention will be apparent in a description of an embodiment, given by way of example only, to enable one skilled in the art to readily practice the invention, and described hereinafter with reference to the accompanying drawing.
Brief description of the drawing FIGURE 1 is a cross sectional view of a dielectric-type charged particle detector according to the invention.
FIGURE 2 is a cross sectional view of a dielectric-type charged particle detector having dual dielectric detecting elements and one common electrode.
FIGURE 3 is a side view of an electrode pattern for the charged particle detector of FIGURE 1 or FIG- URE 2.
FIGURE 4 is a side view of another possible electrode pattern for the detector of FIGURE 1 or FIGURE 2.
Description of an embodiment Referring to the drawing, there is shown in FIGURE 1 a charged particle detector 11 comprising a dielectric element 13 in intimate contact with a pair of metal electrodes 15 and 16. A voltage source 18, in series with a load resistor 20, is connected across the electrodes. Upon irradiation of the element 13 with charged particles, as for example with an electron beam of a long linear accelerator, a number of electrons in the dielectric element are raised to conduction bands. The number of electrons raised to a conduction band is proportional to the intensity of irradiation of the element 13. The voltage applied across the electrodes 15 and 16 from the source 18 causes a current flow through the dielectric and external circuit that includes the electrodes, source 18 and load resistor 20. An output signal is produced thereby, and it can be displayed on an oscilloscope connected across the resistor. The output signal is proportional to the intensity of irradiation of the portion of dielectric between directly opposing surfaces of the electrodes.
In practice it is found preferable that the electrodes be molecularly bonded to the dielectric to ensure electrical conduction therebetween during irradiation of the dielectric.
Any type of dielectric material or electrode metal should give significant detection currents. The primary criteria for the detector materials is that the dielectric material be resistant to the radiation to which it is exposed, and that the dielectric material and electrodes be sufficiently thin to avoid destructive thermal rises.
One convenient method of constructing the detector 11 (FIGURE 1) is to anodize one face of a sheet of aluminum foil, such foil being generally commercially available. The anodized face is aluminum oxide (Al O which is a dielectric that is resistant to destruction when irradiated with charged particles. The exposed aluminum oxide may then be coated with another metal, thereby forming a detector wherein the aluminum oxide is a dielectric element between two metal electrodes.
The detector 11 (FIGURE 1) can also be made by coating each side of a wafer of dielectric, for example, a wafer of A1 0 with a metal that is a good electrical conductor, using standard techniques for obtaining a good metal-to-ceramic bond.
It is anticipated that other dielectric materials may provide an increased efiiciency of extracted output current for specifically applied source voltages and intensities of irradiation over the efficiency obtained with aluminum oxide. It is expected that higher efliciencies can be obtained with dielectric compounds formulated with elements from the following groups listed in the Periodic Table: Groups I and V, e.g., Na Sb; I and VI, e.g., Cu O; I and VII, e.g., CuBr; II and IV, e.g., Mg Si; II and V, e.g., Cd P II and VI, e.g., CdS; III and V, e.g., AlSb; III and VI, which includes Al O IV and IV, e.g., SiC; IV and VI, e.g., PbS; V and VI, e.g., AsO and VI and VI, e.g., TeO It is also expected that three-element dielectric compounds such as PbCO ZnSiAs CdGeP etc., would give higher efficiencies. Generally, dielectrics with small band gaps, and constructed in such a way (i.e., to be low in impurities and crystal imperfections) as to have long recombination times, are expected to be most eflicient. However, a dielectric so constructed would also be sensitive to radiation so that a compromise is necessary to give a dielectric that has a reasonable etficiency and is relatively insensitive to radiation.
Furthermore, the detector is not critical as to which electrode is presented to the charged particles, and it will operate with only an exposed end of the dielectric element presented to the radiation; the detector is not sensitive to the polarity of the source; and it is found to conduct equally in either direction.
In FIGURE 2 is shown a detector 24 having dual dielectric elements 26 and 27 in contact with opposite faces of a common electrode 29. Outer electrodes 31 and 32 may be molecularly bonded to the dielectric elements by means of a metallic bonding material 33. Equal voltages are applied in parallel across each half of the detector 24 by means of a voltage source 35. Load resistors 37 and 38 are connected in series, respectively, with each half of the detector to provide individual outputs at terminals 40 and 41.
The detector 24 may be conveniently constructed by making the electrode 29 of aluminum foil. The dielectric elements 26 and 27 may be made of aluminum oxide by anodizing both faces of the foil. The metallic bonding material 33 may be a very thin coating of titanium, while the electrodes 31 and 32 may be gold coated over the titanium.
In FIGURES 3 and 4 are shown electrode plates 43 and 44 with representative electrode patterns. The plate 43 is comprised of conducting strips 46 over a dielectric 48, while the plate 44 is comprised of conducting strips 50 and 51 over a dielectric 52. Either or both of the patterns could be coated, for example, over the dielectric element 13 (FIGURE 1). Alternatively, the detector 24 could have the pattern of FIGURE 3 coated over the dielectric element 26 (FIGURE 2) and the pattern of FIGURE 4 coated over the dielectric element 27.
With one pattern coated on the detectors in the manner described, the intensity of a charged particlebeam and its location in one direction can be detected, while with two patterns the beam location in two directions can be detected. Known electronic circuitry and devices may be connected to the strips 46, 50 and 51 and foil electrode for processing and displaying the detected information.
The detectors 11 and 24 can be simply fabricated by masking and vacuum deposition of electrons or by chemically milling or etching the desired patterns by means of well-known printed circuit techniques.
Detectors exemplifying the invention were constructed and tested. One such detector was an aluminum oxide ceramic disc having electrodes formed by vacuum deposition of a very thin coating of gold over titanium. The aluminum oxide was approximately 2 mm. thick, while the deposited electrodes were less than .025 mm. thick. The electrodes on one side of the disc were arranged in a pattern of 2 mm. wide, /2" long parallel strips, and the opposite side was arranged in a like pattern, shifted 90 with respect to the first pattern,
Other detectors similar to detector 24 were constructed with 0.5 mm. thick aluminum foil having both faces anodized to form aluminum oxide dielectric elements 0.018 mm. to 0.1 mm. thick. Gold was vacuum-deposited on both sides of the elements'to form electrodes in patterns substantially as shown in FIGURES 3 and 4. The foil may be first integrally mounted in a frame and then etched while in the frame. This permits attachment of the foil to the frame in such a way as to make the frame a heat sink and thereby give the detector optimum thermal conduction properties. Such a frame is also a convenient means for providing electrical connections to the detector as well, providing a convenient support for mounting the detector.
The constructed detectors were tested with a 5 mev. linear accelerator with peak beam currents ranging from 1 ma. to 30 ma. A potential of 400 volts was placed across opposing electrodes. It was found that with applied potentials of 8000 volts/mm. of aluminum oxide dielectric, there was an extracted detection current between opposing electrodes which ranged from 24% of impinging beam current.
While the embodiment of the invention has been shown and described, further embodiments or combinations of those described herein will be apparent to those skilled in the art without departing from the spirit of the invention or the scope of the appended claims.
1. A charged particle detector, comprising:
first and second metal electrodes, and a first dielectric element in intimate contact with said first and second electrodes,
said element electrically insulating said first electrode from said second electrode,
said element being responsive to impingement of charged particles to cause detectable electrical conduction between said first and second electrodes upon application of an electrical potential thereacross, said element being a ceramic, and said second electrode being a vacuum deposition of a conducting metal over a molecular bonding metal on said element.
2. A charged particle detector according to claim 1, wherein at least one of said electrodes is arranged to have a plurality of electrically independent conducting strips.
3. A charged particle detector according to claim 1, wherein said first electrode is an aluminum foil, and
said element is an anodized coating of aluminum oxide on said foil.
4. A charged particle detector according to claim 1, further including a third metal electrode, and
a second dielectric element in intimate contact with said first and third electrodes,
said second element being responsive to impingement of charged particles to cause detectable electrical conduction between said first and third electrodes upon application of an electrical potential thereacross.
References Cited UNITED STATES PATENTS 2,408,910 10/1946 Burnham 317-230 X 2,694,112 11/1954 McKay 25083.3 X 2,877,371 3/1959 Orthuber et al. 250-71 X 3,293,435 12/1966 Huth 25083.3
ARCHIE R. BORCHELT, Primary Examiner.
D. L. WILLIS, Assistant Examiner.
US. Cl. X.R. 25083