Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J47/00—Tubes for determining the presence, intensity, density or energy of radiation or particles
  • H01J47/02—Ionisation chambers

Definitions

• the present invention relates to a gas-filled photon location detector.
• This detector which can more simply be called a "gas photodetector", applies to obtaining inexpensive photomultipliers, capable of covering large areas.
• the detector object of the invention associated with heavy scintillators, also finds applications in medicine (in particular in radiography, in radiotherapy and in mammography).
• the invention can also be used in the field of high energy physics, to detect the light of scintillating crystal calorimeters or the ultraviolet light of Cerenkov radiation.
• Photon localization detectors are already known, with gas filling. On this subject, we will consult for example the document [1] which, like the other documents cited below, is mentioned at the end of this description.
• the object of the present invention is to remedy the above drawbacks.
• this detector comprising: - a gas enclosure, and
• this detector being characterized in that the external face of the cathode, face opposite to that which is opposite the anode, is covered with '' a photoionizable layer, maintained at the same potential as this cathode and capable of supplying electrons under the impact of photons, the anode being intended to be brought to a sufficiently high potential with respect to the cathode to create, in space between this latter and the anode, an electric field allowing the attraction of almost all the electrons in this space, through the holes, then the multiplication of these electrons by an avalanche process.
• the distance D between the cathode and the anode does not exceed 200 ⁇ m.
• the intensity of the electric field intended to be created in the amplification space is at least equal to 30 kV / cm. This makes it possible to detect high fluxes of photons by ensuring a rapid collection of the ions produced during the avalanche.
• the thickness E of the cathode is less than D / 10.
• the holes of the cathode have a regular distribution of pitch P, the size T of these holes being less than D and greater than the thickness E of the cathode and the pitch P being greater than T and less than the distance D
• pitch P being greater than T and less than the distance D
• the elementary anodes are electrically conductive tracks parallel to each other.
• the elementary anodes are electrically conductive elements forming a two-dimensional network.
• the size of the elementary anodes is substantially equal to or less than the pitch P of the cathode holes. This achieves a very high spatial resolution.
• electrically insulating spacers which are linear or, preferably, point-like, can be used.
• Figure 1 is a schematic sectional view of a particular embodiment of the detector object of the invention, and • Figure 2 schematically illustrates the operation of this detector.
• the detector according to the invention which is schematically represented in FIG. 1, comprises a gas enclosure 2 which is filled, at atmospheric pressure (around 10 5 Pa) or at low pressure (below 10 3 Pa), d '' a suitable gas mixture, allowing the amplification of electrons by an avalanche process.
• This gaseous mixture can remain confined in the enclosure (provided that the latter does not pollute this mixture) or circulate through a purifier (not shown) via pipes 4.
• This enclosure is closed in a sealed manner by a window 6 which is transparent to visible or ultraviolet photons 8 which it is desired to detect.
• the detector of FIG. 1 also comprises an electrically insulating plate 10 of very good flatness, on which elementary anodes 12 are formed which may be metal tracks parallel or metallic elements which one can call "pixels" and which form a two-dimensional network on the plate (see also document [2]).
• the set of tracks or pixels constitutes the anode 14 of the detector.
• the size of the tracks (width of the tracks) or of the pixels (dimensions of these pixels) as well as the pitch of these tracks or of these pixels can vary according to the spatial precision desired for the detector.
• Each track or each pixel is grounded for practical electronics.
• these tracks or these pixels are connected to suitable electronic means 16 provided for amplifying and then processing the electrical signals coming from these tracks or these pixels.
• the detector of FIG. 1 also comprises, facing window 6, a cathode 18 constituted by a metal sheet pierced with holes 20, this sheet thus forming a grid.
• holes 20 can be obtained by drilling the sheet by means of a laser or by electroforming or by another type of etching.
• the thickness E of the grid 18 is less than 10 ⁇ m
• the holes have a size T of between 10 ⁇ m and 35 ⁇ m and are spaced apart by a pitch P of the order of 50 ⁇ m.
• the anode 14 and the grid 18 are held parallel to each other by means of electrically insulating spacers 22 which rest on the anode.
• the distance D between this anode and the cathode is of the order of 100 ⁇ m.
• the spacers 22 may be linear elements such as for example quartz fibers of diameter D (see document [2] on this subject) or point elements of height D which are formed by etching a layer of photosensitive resin on the surface of tracks or metallic pixels (see document [3] on this subject).
• the distance between two adjacent linear or point elements is an increasing function of D and this distance is for example of the order of 2 mm for a distance D of the order of 100 ⁇ m.
• Polarization means 24 are provided for bringing the grid 18 (that is to say the cathode) to a strongly negative voltage with respect to the anode 14 (this voltage depending on the gas mixture used).
• the anode which is thus brought to a high potential with respect to the cathode, constitutes with the latter a detector with parallel electrodes, capable of amplifying electrons by an avalanche process which develops between these electrodes.
• the high voltage is chosen to create in the space A between the anode 14 and the cathode 18, or amplification space, an electric field E A whose intensity is greater than or equal to 50 kV / cm.
• the detector according to the invention of FIG. 1 also comprises a thin layer 26 (having for example a thickness sufficient to allow the absorption of the photons which it is desired to detect, of the order of 1 ⁇ m) of a substance photoionizable.
• This layer 26 is formed, for example by evaporation under vacuum, on the external face of the grid 18, that is to say the face opposite to that which is opposite the tracks or metallic pixels 12, this external face 12 (and therefore layer 26) located next to window 6.
• This layer 26 maintained at the same potential as the cathode 18, constitutes a photocathode intended to convert into electrons the incident photons 8 which we want to detect and which have passed through the window
• the electrons are extracted from photocathode 26 by a photoelectric effect, as in photomultipliers.
• the intensity of the electric field prevailing on the surface of the grid, surface which is external to the amplification space or multiplication space A is high (greater than 1 kV / cm), which is favorable for the extraction of electrons from the photoionizable layer 26.
• FIG. 2 schematically illustrates the operation of the detector of FIG. 1.
• This photon 8 reaches the photoionizable layer 26 and extracts an electron 28
• the photoionizable layer functioning as a reflective photocathode.
• the electron created follows a field line which brings it towards the center of a hole 20 of the cathode 18 and this electron is then transferred into the space A between the anode and the cathode then amplified under the action of the intense electric field E A which prevails in this space.
• the corresponding avalanche 30 has a size of the order of magnitude of the distance D between the anode and the cathode.
• the electrical signals supplied by the elementary anode or the elementary anodes reached by this avalanche 30 are processed in the electronic means 16 to detect and locate the single electron 28 generated at the photoionizable layer 26 and therefore to detect and locate the photon 8 of visible or ultraviolet light.
• the detector of FIGS. 1 and 2 thus makes it possible to detect and locate incident photons as well as the electrons which correspond to these photons and are extracted from the photoionizable layer (which can have a large surface), each photon corresponding to a single electron.
• This detector makes it possible to locate the electrons with a spatial precision better than 100 ⁇ m and with a temporal precision of the order of 1 ns.
• the great advantage of the detector according to the invention is based on three main effects mentioned below.
• the small amplification space offers a very beneficial effect (which is also encountered in the detectors described in documents [2] and [3]): the value of the uniform electric field applied is such that the amplification coefficient (Townsend coefficient) is close to its saturation (inflection point).
• the amplification obtained is not very sensitive to variations in the amplification space (due to mechanical faults) or the pressure of the gas or even the temperature.
• a detector of the kind of that of the invention has been tested in the laboratory; gains of the order of 10 7 have been obtained with a gaseous mixture of helium and 10% isobutane; the distribution of single photoelectrons presents a peak well separated from the background noise, which is a sign of an excellent homogeneity of the amplification and of a small fluctuation of this amplification (depending on the impact position of the photons); the fluctuation of the electron collection time of an avalanche is typically less than 1 ns, hence an excellent temporal definition.
• the use of very fine reading tracks or of reading pixels is likely to lead to an excellent spatial definition (less than 100 ⁇ m), never obtained with conventional photomultipliers.
• the detector of Figures 1 and 2 is intended to detect photons of visible or ultraviolet light.
• the scintillator can be associated with a scintillator of rectangular shape, having a thickness of the order of 1 cm, placed outside this detector, opposite window 6 thereof, to detect X or ⁇ photons, the scintillator, for example BaF 2 , being provided to convert the latter in photons of visible light which the detector of FIGS. 1 and 2 is then capable of detecting.
• the scintillator for example BaF 2
• Other crystals commonly used in physics or in the medical field can be used.

Landscapes

• Measurement Of Radiation (AREA)

Applications Claiming Priority (3)

Publications (1)

Family

ID=9532758

Family Applications (1)

Country Status (3)

Families Citing this family (3)

Family Cites Families (3)

• 1998
  • 1998-11-16 FR FR9814349A patent/FR2786024B1/fr not_active Expired - Fee Related
• 1999
  • 1999-11-15 EP EP99972366A patent/EP1131843A1/de not_active Withdrawn
  • 1999-11-15 WO PCT/FR1999/002796 patent/WO2000030150A1/fr not_active Application Discontinuation

Non-Patent Citations (1)

Also Published As

Similar Documents

Legal Events