WO2013157975A1

WO2013157975A1 - Radiation detector

Info

Publication number: WO2013157975A1
Application number: PCT/RU2012/000301
Authority: WO
Inventors: Irina Mikhailovna ASTAFIEVA; Oliver Heid; Timothy Hughes
Original assignee: Siemens Aktiengesellschaft
Priority date: 2012-04-18
Filing date: 2012-04-18
Publication date: 2013-10-24

Abstract

A radiation detector (112) in which primary electrons (11) are released into a gas by ionizing radiation and drifted to a collection electrode by means of an electric field, the radiation detector (112) including a gas electron multiplier (101) comprising at least one field condensing area adapted to produce a local electric field amplitude enhancement proper to generate in the gas an electron avalanche from one of the primary electrons (11), the gas electron multiplier (101) operating thus as an amplifier of given gain for the primary electrons (11), the at least one field condensing area comprising a hole (105) arranged in an insulator (103) having a face with a metal cladding (104) facing a cathode (106) of the radiation detector (112), characterized in that the insulator (103) is directly arranged on at least one elementary anode (108) delimiting the hole (105) of the at least one field condensing area so that the at least one field condensing area is arranged as a blind hole (105) in the composite of the at least one elementary anode (108) and the insulator (103) having the metal cladding (104).

Description

RADIATION DETECTOR

DESCRIPTION

The invention relates to a radiation detector in which primary electrons are released into a gas by ionizing radiation and drifted to a collection electrode by means of an electric field, the radiation detector including a gas electron multiplier comprising at least one field condensing area adapted to produce a local electric field amplitude enhancement proper to generate in the gas an electron avalanche from one of the primary electrons, the gas electron multiplier operating thus as an amplifier of given gain for the primary electrons, the at least one field condensing area comprising a hole arranged in an insulator having a face with a metal cladding facing a cathode of the radiation detector.

Radiation detectors utilising the process of releasing primary electrons in a gas by ionizing radiation are well established in the state of art. The primary electrons are accelerated in an electric field so that eventually an avalanche of secondary electrons is generated from each primary electron. The avalanche of secondary electrons is detected at the collection electrode that also acts as an anode for generating the electric field.

EP 0 948 803 Bl describes a radiation detector of the afore-mentioned a type, wherein a so-called gas electron multiplier is introduced between the cathode and anode generating the electric field. The gas electron multiplier acts as a preamplifier for electrons in the gas. The gas electron multiplier comprises a foil made from an insulating dielectric, usually a polyimide. The insulator is cladded on both faces with a metal cladding usually made from copper. Through holes are arranged in the sandwich structure comprising the metal claddings and the insulator. The through holes form field condensing areas when a voltage is applied to the metal claddings. An additional electrical dipole field is generated in the through holes that is used as a local electric field amplitude enhancement suitable to a generate in the gas an electron avalanche from one of the primary electrons. The two metal claddings thus act as electrodes generating the additional electric dipole field increasing the field strength of the electric field in the field condensing area. Field lines of the external electric field generated by the anode and cathode are guided through the through holes. Primary electrons drifting along the field lines generate secondary electrons in the field condensing areas that may be detected when reaching the anode. The gas electron multiplier may be used in combination with a micro strip gas chamber. The field condensing areas are regularly distributed over the insulator and form a matrix of field condensing areas. The insulator with the metal claddings is spaced away from both the anode and the cathode. The gas electron multiplier thus divides the space between the anode and the cathode into a drift region and a detection region. The detection region and the drift region each have a width of approximately three millimetres. The gas electron multiplier comprising the holes for generating the field condensing areas itself may have a width of up to one millimetre.

The anode may be arranged as an array of elementary anodes that may, in particular, be arranged as wires. When connected with a suitable detection means, a spatial resolution of the registered electrons may be achieved.

The radiation detector comprising the gas electron multiplier according to the prior art is illustrated in figures 1 to 3 and described in more detail hereinafter. In particular, disadvantages inherent to the set up known in the prior art will be outlined in more detail.

Although the radiation detector with the gas electron multiplier according to the prior art is capable of generating local electric field enhancements proper to generate avalanches of secondary electrons from a primary electron, unresolved issues include determination of the energy of the primary electron and thus the energy of the incident ionizing radiation and/or limited sensitivity.

It is thus an object of the invention to propose a radiation detector with improved energy resolution.

The object is achieved by a radiation detector of the afore-mentioned type, wherein, according to the invention, the insulator is directly arranged on at least one elementary anode delimiting the hole of the at least one field condensing area so that the at least one field condensing area is arranged as a blind hole in the composite of the at least one elementary anode and the insulator having the metal cladding.

Experiments have shown that a major problem of radiation detectors with gas electron multiplier arranged according to the prior art, i. e. detectors having two faces with metal claddings that are spaced away from both the cathode and the anode, originates from the fact, that a substantial part of the secondary electrons of the avalanche drift back to the positively charged metal cladding of the gas electron multiplier and never reach the elementary anode.

Thus, a part of the electron avalanche containing information about the energy of the primary electron and the incident ionizing radiation is lost. As a consequence, the total charge contained in the avalanche of secondary electrons generated from the primary electron may not be determined and thus the energy resolution of the radiation detector is limited.

Furthermore, as it is not known which fraction of the avalanche electrons is lost, a determination of the energy of the primary electron or the incident radiation is practically impossible.

This limitation is overcome by an improved arrangement of the gas electron multiplier with respect to the at least one elementary anode. The elementary anode is directly arranged on the insulator of the gas electron multiplier. Accordingly, the gas electron multiplier comprises a single metal cladding on the face facing the cathode of the radiation detector. The elementary anode used for detection of the secondary electrons delimits and thus seals the hole arranged in the insulator providing the field condensing area. As the hole is completely sealed by the elementary anode, almost all secondary electrons that are released within the field condensing area reach the elementary anode. Consequently, the energy resolution and sensitivity of the radiation detector is increased.

The increased sensitivity and energy resolution of the radiation detector allows for a usage of the radiation detector in various different applications including medical applications requiring determination of the energy of incident ionizing radiation combined with a spatial resolution. In particular, the radiation detector may be used for medical imaging or as a device providing supplementary information used in the field of medicine.

It is understood that the ionizing radiation may be any radiation capable of ionizing, in particular photons of sufficient energy, even more particularly x-rays.

Additionally, direct arrangement of the at least one elementary anode on the insulator of the gas electron multiplier results in a detector geometry of decreased width. The detector width may be decreased in comparison to the radiation detectors known in the prior art by 20 to 25 % while attaining a comparable registration efficiency, in particular for x-rays. According to an embodiment of the invention, the elementary anode delimiting the hole arranged in the insulator has a planar shape.

However, a planar arrangement of an elementary anode in the area of the hole does not mean that the radiation detector has to have a substantially planar overall design. Typically, the radiation detector comprises a large number of holes constituting a matrix of field condensing areas. The diameter of each hole is tiny compared to the dimensions of the detector. Hence, the detector geometry may be chosen from suitable geometries. In particular, the radiation detector may comprise a cylindrical or spherical detector geometry or may be shaped as a segment of a cylinder or sphere. In another embodiment the radiation detector itself has a planar design.

According to an alternative embodiment, the hole arranged in the insulator is delimited by a bulge of the elementary anode facing away from the cathode. The elementary anode shaped as the bulge generates, when a suitable voltage is applied thereto, an electric field in its proximity that corresponds to an electric field generated by an elementary anode that is located at a distance from gas electron multiplier. Hence, this embodiment is capable of generating an electric field that corresponds - to the electric field generated by an elementary anode spaced away from the insulator while increasing that chance that all secondary electrons reach the anode.

The metal cladding arranged on the face of the insulator facing the cathode is preferably made from a metal with a suitable electrical conductivity, in particular copper.

The geometric shape of the hole providing the field condensing area influences the distribution of the electric field and a break down probability of the radiation detector. In one embodiment of the invention, the hole has a cylindrical shape that may, for example, be produced by a method of etching. Such methods are inexpensive to implement so that the radiation detector may be cost-efficiently manufactured.

In another embodiment of the invention, the hole providing the field condensing area has at least one conical section. Accordingly, the diameter of the hole may be smaller on the side facing the at least one elementary anode. A strong electric field is generated in the area of reduced width, so that the probability of generating secondary electrons from a primary electron is increased.

In another alternative embodiment, the hole comprises two frustoconical sections arranged opposite to each other so that the diameter of the hole is smallest in a plane intermediate to the face of the insulator facing the cathode and the face of the insulator facing the at least one elementary anode.

Preferably, the gas electron multiplier comprises at least one matrix of field condensing areas, wherein each of the field condensing areas is adapted to produce a local electric field amplitude enhancement proper to generate in the gas an electron avalanche from one of the primary electrons, at least one of the field condensing areas comprising the hole arranged in the insulator that is delimited by the at least one elementary anode so that the at least one field condensing area is arranged as a blind hole in the composite of the at least one elementary anode and the insulator having the metal cladding. The plurality of holes providing the matrix of field condensing areas allows for a spatial resolution of the detected radiation. Thus, the radiation detector of the invention provides, within certain limits, a spatial resolution combined with an energy resolution. The radiation detector may be applied in various fields, including the field of medicine, in particular medical imaging.

Preferably, the holes disposed in the insulator are identically shaped and regularly distributed over at least a part of the composite of the at least one elementary anode and the insulator having the metal cladding.

In a preferred embodiment of the invention, each hole of the insulator is delimited by one of the elementary anodes so that each field condensing area is arranged as a blind hole in composite of the elementary anodes and the insulator having the metal cladding. The plurality of anodes directly arranged on the insulator are connected to a suitable read-out electronics so that origin and energy of the ionizing radiation generating primary electrons in the gas may be determined.

In the following, the invention is described in detail with reference to the figures, wherein

Fig.l shows a sectional view of a hole providing a field condensing area according to the prior art,

Fig.2 shows a perspective and schematic view of a radiation detector according to the prior art,

Fig.3 shows a schematic illustration of the typical behaviour of charged particles following electric field lines generated in the radiation detector according to the prior art,

Fig.4 a gas electron multiplier for a radiation detector according to a first embodiment of the invention,

Fig.5 a gas electron multiplier according to a second embodiment of the invention,

Fig.6 a gas electron multiplier according to a third embodiment of the invention.

Like parts are indicated in all figures by a like reference symbols.

Fig.l shows in a sectional view of an area of a gas electron multiplier 1 known in the prior art. The gas electron multiplier 1 is part of a radiation detector 2 illustrated in more detail in figure 2. The radiation detector 2 is filled with a noble gas, in particular argon.

The gas electron multiplier 1 consists of an insulator 3 having two faces with metal claddings 4 made from copper. The insulator 3 is made from a polyimide. A through hole 5 providing a so-called field condensing area is arranged in the sandwich structure of the insulator 3 having the two metal claddings 4.

A voltage is applied across the metal claddings 4 arranged on the opposite faces of the insulator 3 so as to generate an electric dipole field in the hole 5. When the gas electron multiplier 1 is brought into an external electric field generated by a cathode 6 and an anode 7, a local electric amplitude enhancement is generated in the field condensing area that is capable of generating an avalanche of secondary electrons from a primary electron that was previously released by incident ionizing radiation.

The field condensing area, respectively the gas electron multiplier 1, acts as a preamplifier of given gain for the primary electrons. Primary electrons are accelerated in the electric field so that the probability of releasing secondary electrons from the primary electron is increased. The charge contained in the avalanche of secondary electrons is read-out when the secondary electrons reach the anode 7 of the radiation detector.

The hole 5 shown in fig. 1 comprises two sections of conical shape, so that the diameter of the hole 5 is smallest in a plane intermediate to the two faces having the metal claddings 4.

Fig.2 shows a schematic and perspective view of the radiation detector 2 comprised in the state of art. One contribution of the electric field is generated by suitable potential difference applied between the cathode 6 and the anode 7. The anode 7 is made from a plurality of elementary anodes 8 that are connected to a suitable readout circuit schematically indicated by diodes 9. The read-out electronics allows for a spatial resolution of the detected radiation. In particular, the position of detection along the plane defined by the anode 7 comprising the plurality of elementary anodes 8 may be determined.

The gas electron multiplier 1 is arranged in the space intermediate to the cathode 6 and the anode 7. Accordingly, the gas electron multiplier 1 is spaced away from both the cathode 6 and the anode 7. The gas electron multiplier 1 comprises a plurality of holes 5 each providing one local field condensing area. Thus a matrix of field condensing areas is provided by the sandwich structure of the insulator 3 having the metal claddings 4.

At least the space between the cathode 6 and the anode 7 is filled with a noble gas like argon. Radiation may enter this space via a window or the like that is a least transparent for incident radiation of a suitable wave length. In particular, a section may be provided that is transparent for x-rays.

Fig.3 shows schematically the main disadvantage of the conventional radiation detector 2. The electric field is indicated by field lines 9 schematically shown in fig. 3. Charged particles like ions 10 or electrons 1 1, 12 are accelerated along the field lines 9. The positively charged ions 10 are accelerated towards the cathode 6, whereas primary electrons 1 1 drift towards the anode 7 through the hole 5 providing the field condensing area. When passing the field condensing area, the primary electron 11 is accelerated so that eventually an avalanche of secondary electrons 12 is generated from the primary electron 1 1. However, a substantial part of secondary electrons 12 are immediately redirected towards the positively charged, lower metal cladding 4 of the gas electron multiplier 1. Thus, a part of the avalanche of secondary electrons 12 never reach the anode 7 for detection.

Accordingly, the detector sensitivity and, more particularly, energy resolution of the radiation detector 2 according to the state of the art is limited.

Fig.4 shows a schematic and detailed view of a section of a radiation detector 12 according to a first embodiment of the invention. In principle, the lower metal cladding 4 of the sandwich structure constituting the gas electron multiplier 1 of the radiation detector 2 according to the prior art is replaced by an elementary anode 108 that is directly arranged on an insulator 103 having at least one hole 105 providing a local field condensing area. The face of the insulator 105 opposite to the elementary anode 108 is cladded with a metal cladding 104.

The insulator 103 is made from a polymer and, more particularly from a polyimide. The metal cladding 104 is made, according to alternative embodiments, from copper, gold or another suitable conducting metal or alloy. The area between the elementary anode 108 and a cathode 106 is filled with an inert gas, in particular a noble gas.

Fig.5 shows another schematic arrangement of a gas electron multiplier 101 for a radiation detector 1 12 according to a second embodiment of the invention. The gas electron multiplier 101 is directly arranged on the elementary anode 108 that has a substantially planar shape. The hole 105 disposed in the insulating material 104 that is delimited by the anode 108 has a frusto-conical shape. Accordingly, the diameter of the hole 105 is largest on the face facing the cathode 106.

Fig.6 shows a third embodiment of the invention, wherein the elementary anode 108 is shaped as a bulge in the area of the hole 105. The bulge-shaped elementary anode 108 creates in its proximity an electric field that is similar to a planar elementary anode spaced away from the insulator 103.

However, since the elementary anode 108 is directly attached to the insulator

103, a loss of secondary electrons 12 is minimized. The hole 105 shown in fig. 6 has two sections of frustoconical shape arranged opposite to each other. The diameter of the hole 105 is smallest in a plane intermediate to the two faces of the insulator 103.

The radiation detector 1 12 comprises a plurality of arrangements shown in figs. 4 to 6 disposed at regular intervals relative to each other similar to the arrangement shown in fig. 2. Thus, the radiation detector 112 comprises a plurality of blind holes 105 disposed in the composite of the insulator 103 having the metal cladding 104 and the elementary anodes 108. The plurality of holes 105 are evenly distributed over the surface of said composite of metal cladding 104, insulator 103 and elementary anodes 108. As the elementary anode 108 used for detection of the secondary electrons 12 is directly arranged on the insulator 103, almost all secondary electrons 12 are detected. Hence, the sensitivity and energy resolution of the radiation detector 112 is improved. A voltage is applied across the cathode 6, the metal cladding 104 and elementary anode 108 so that an electric field having a strong field component in the local field condensing area is generated.

The working principle of the detector 1 12 is similar to the one comprised in the prior art. In particular, primary electrons 1 1 are released into the gas by ionizing radiation. The primary electrons 11 drift towards one of the local field condensing areas provided by the blind holes 105 in the composite of the elementary anode 108, the insulator 103 and the metal cladding 104. An avalanche of secondary electrons 12 is generated from each primary electron 11 in the local field condensing area. The total charge of the avalanche of secondary electrons 1 1 is collected via the elementary anode 108.

Although the present invention has been described in detail with reference to the preferred embodiment, the present invention is not limited by the disclosed examples from which the skilled person is able to derive other variations without departing from the scope of the invention.

Reference numeral list

1 gas electron multiplier

2 radiation detector

3 insulator

4 metal cladding

5 hole

6 cathode

7 anode

8 elementary anode

9 field lines

10 ion

11 primary electron

12 secondary electron

101 gas electron multiplier

102 insulator

104 metal cladding

105 hole

106 cathode

108 elementary anode

1 12 radiation detector

Claims

1. A radiation detector (112) in which primary electrons (1 1) are released into a gas by ionizing radiation and drifted to a collection electrode by means of an electric field, the radiation detector (1 12) including a gas electron multiplier (101) comprising at least one field condensing area adapted to produce a local electric field amplitude enhancement proper to generate in the gas an electron avalanche from one of the primary electrons (1 1), the gas electron multiplier (101) operating thus as an amplifier of given gain for the primary electrons, the at least one field condensing area comprising a hole (105) arranged in an insulator (103) having a face with a metal cladding (104) facing a cathode (106) of the radiation detector (1 12), characterized in that the insulator (103) is directly arranged on at least one elementary anode (108) delimiting the hole (105) of the at least one field condensing area so that the at least one field condensing area is arranged as a blind hole (105) in the composite of the at least one elementary anode (108) and the insulator (103) having the metal cladding (104).

2. The radiation detector (1 12) according to claim 1, characterized in that the elementary anode (108) delimiting the hole (105) arranged in the insulator (103) has a planar shape.

3. The radiation detector (1 12) according to claim 1, characterized in that the hole (105) arranged in the insulator (103) is delimited by a bulge of the elementary anode (108) facing away from the cathode (106).

4. The radiation detector (112) according to one of the previous claims, characterized in that the metal cladding (104) is made from copper.

5. The radiation detector (1 12) according to one of the previous claims, characterized in that the insulator (103) is made from a polymer, in particular a polyimide.

6. The radiation detector (1 12) according to one of the previous claims, characterized in that the hole (105) has a cylindrical shape.

7. The radiation detector (112) according to one of the claims 1 to 5, characterized in that the hole (105) has at least one conical section.

8. The radiation detector (112) according to claim 7, characterized in that the diameter of the hole (105) is smaller on the side facing the at least one elementary anode (108).

9. The radiation detector (1 12) according to claim 7, characterized in that the hole (105) comprises two frusto-conical sections arranged opposite to each other so that the diameter of the hole (105) is smallest in a plane intermediate to the face of the insulator (103) facing the cathode (106) and the face of the insulator (103) facing the at least one elementary anode (108).

10. The radiation detector (1 12) according to one of the previous claims, characterized in that the gas electron multiplier (101) comprises at least one matrix of field condensing areas, wherein each of the field condensing areas is adapted to produce a local electric field amplitude enhancement proper to generate in the gas an electron avalanche from one of the primary electrons (1 1), at least one of the field condensing areas comprising the hole (105) arranged in the insulator that is delimited by the at least one elementary anode (108) so that the at least one field condensing area is arranged as a blind hole (105) in the composite of the at least one elementary anode (108) and the insulator (103) having the metal cladding (104).

11. The radiation detector (112) according to claim 10, characterized in that the holes (105) are identical in shape and regularly distributed over at least a part of the composite of the at least one elementary anode (108) and the insulator (103) having the metal cladding (104).

12. The radiation detector (112) according to claim 10 or 11, characterized in that each hole (105) of the insulator (103) is delimited by one of the elementary anodes (108) so that each field condensing area is arranged as a blind hole (105) in the composite of the elementary anodes (108) and the insulator (103) having the metal cladding (104).