Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01T—MEASUREMENT OF NUCLEAR OR X-RADIATION
  • G01T1/00—Measuring X-radiation, gamma radiation, corpuscular radiation, or cosmic radiation
  • G01T1/16—Measuring radiation intensity
  • G01T1/20—Measuring radiation intensity with scintillation detectors
  • G01T1/201—Measuring radiation intensity with scintillation detectors using scintillating fibres
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01T—MEASUREMENT OF NUCLEAR OR X-RADIATION
  • G01T1/00—Measuring X-radiation, gamma radiation, corpuscular radiation, or cosmic radiation
  • G01T1/16—Measuring radiation intensity
  • G01T1/20—Measuring radiation intensity with scintillation detectors
  • G01T1/204—Measuring radiation intensity with scintillation detectors the detector being a liquid

Definitions

• the invention relates to a two-dimensional detector of ionizing particles.
• the invention applies, for example, to the field of imaging of particles with high penetrating power.
• Such detectors are used, for example, in the fusion of deuterium (DD) or of a mixture of deuterium (D) and tritium (T) by inertial confinement using a power laser.
• the fusion of these isotopes of hydrogen occurs in a volume of characteristic dimension 50 ⁇ .
• the nuclear fusion reaction is accompanied by the release of a fast neutron of 14.1 MeV for a DT mixture or
• the neutron image allows to locate the area where burning isotopes • hydrogen.
• the neutron image or the gamma image is formed either by a pinhole camera or by a coded aperture such as a penumbra diaphragm or a ring. Detectors with high detection efficiency and capable of locating the interaction point of the particle are necessary for the recording of this image.
• the two-dimensional detectors of Ionizing particles are produced by assembling thousands of fibers with a plastic scintillator, each fiber typically having a length of between 1 and 10 cm and constituting a detector pixel.
• a detector is shown in Figures 1A and IB.
• a set of fibers 2 with plastic scintillator are held in a cylinder 1.
• Each fiber with plastic scintillator 2 has a diameter D substantially equal, for example, to 1mm.
• a plastic scintillator fiber is shown in FIG. 2. It consists of a plastic scintillator bar 3 with a high refractive index (typically of the order of 1.6) surrounded by a sheath 4 of lower optical index ( typically around 1.5).
• the incident particles to detect P have a trajectory parallel to the axis of the fiber and deposit their energy in the plastic scintillator.
• Tertiary photons Ph3 constitute a visible scintillation light which is guided to one end of the fiber where an image is recorded using a CCD detector (CCD (Charge Coupled Device)).
• CCD detector Charge Coupled Device
• this technology limits the minimum fiber diameter to around 0.5 mm.
• sampling an image limits the ultimate resolution in the source to twice the size of a pixel divided by the magnification of the imaging system.
• the magnification of an imaging system must therefore be of the order of 200 to obtain spatial resolutions lower than the size of the source, for example resolutions of the order of 5 ⁇ m.
• the measuring instrument then extends over significant distances which can be greater than ten meters.
• the realization of a detector is obtained by the tedious assembly of several thousand pixels one by one. This results in imperfections in the regular arrangement of the pixels.
• the lack of rigidity of the fibers with plastic scintillator and their significant expansion does not guarantee a precise collinearity between each fiber.
• the interaction of fast neutrons in a plastic scintillator is dominated by elastic scattering on hydrogen.
• the recoil ions I deposit their energy on a cylinder with a typical diameter of 1 mm when the incident particles (neutrons, gamma radiation) have an energy of 14.1 MeV.
• Another limitation of the spatial resolution in the source is therefore the width of the energy deposit (diameter of the cylinder) divided by the magnification.
• the technology for manufacturing two-dimensional detectors limits it. it the performance of the instruments in which these detectors are installed.
• the spatial resolution of the neutron detector is limited to 1.4 mm for neutrons of 14.1 MeV and to 1 mm for neutrons of 2, 45 MeV.
• the invention does not have the drawbacks mentioned above.
• the invention relates to a two-dimensional detector of ionizing particles comprising a matrix of detector fibers, each detector fiber constituting a pixel of the detector and comprising a scintillator for emitting scintillation light, characterized in that each detector fiber consists of a glass capillary filled with liquid scintillator, the chemical composition of which is chosen so that the mean free path of primary scintillation photons is negligible compared to the diameter of the capillary.
• FIG. 1A shows a two-dimensional detector of ionizing particles according to the prior art
• Figure IB shows a detailed view of Figure 1A
• FIG. 2 shows the interaction of ionizing particles to be detected in a plastic scintillator fiber according to the prior art
• FIG. 3 shows a two-dimensional detector of ionizing particles according to a preferred embodiment of the invention.
• FIG. 3 represents a two-dimensional detector of ionizing particles according to the invention.
• the two-dimensional detector according to the invention comprises a matrix of capillaries 6 filled with liquid scintillator.
• the matrix of capillaries. 6 is placed in a tank 5.
• the capillaries have, for example, a lower average diameter of than or equal to 500 .mu.m up to, for example, 20 microns.
• the refractive index of the glass of the capillaries is, for example, 1.49.
• the parallelism of the capillaries is less than 100 micro-radians.
• the trajectory of the incident particles is parallel to the axis of the capillaries.
• the liquid scintillator for example, has a refractive index of 1.57.
• the chemical composition of the liquid scintillator is chosen so that the primary scintillation photons have a negligible mean free path in front of the capillary diameter.
• the primary scintillation photons induced in the solvent for example, have a wavelength of 300 nm.
• the liquid scintillator is either a binary liquid scintillator or a ternary liquid scintillator. In the first case, the liquid scintillator comprises a first scintillator component which absorbs the UV photons of primary scintillation to emit a secondary emission of greater wavelength, for example 370 nm.
• the liquid scintillator comprises, in addition to the first component, a second scintillator component which absorbs the secondary emission emitted by the first component to in turn emit at a wavelength between 400 nm and 500 nm, by example 420nm.
• the refractive index of the scintillating liquid and the refractive index of the glass which constitutes the capillary are chosen to guide the scintillation light towards an exit end of the capillary.
• the solvent that makes up the capillary is, for example, PXE (PXE for phenyl-o-xylylethane).
• PXE PXE for phenyl-o-xylylethane
• the binary liquid scintillator has a spatial resolution of 6 ⁇ m and emits at 370 nm and the ternary liquid scintillator has a spatial resolution of 7 ⁇ m and emits at 420 nm.
• the binary and ternary scintillators can thus be, for example, the components marketed respectively under the references EJ-399-05C2 and EJ-399-05C1.
• the liquid scintillator contains deuterium.
• deuterium advantageously makes it possible to reduce by a factor of 2 the width of the neutron energy deposition zone around from its point of interaction.
• the liquid may also contain a solution of lithium or of an element with an atomic mass greater than lithium.
• the scintillation emission sees its intensity divided by the factor e (e ⁇ 2.71828) in a few nanoseconds. This property makes it possible to select the energy band of the neutrons by time of flight. This property also makes it possible to differentiate neutrons from photons which generally accompany the production of neutrons.
• the binary scintillator has a rise time of a few tens of pico-seconds.
• the tank 5 comprises a first wall 7 provided with a glass porthole transparent to the scintillation wavelength and a second wall 8, located opposite the second wall, and made of a mirror reflecting this length of wave.
• the capillaries are placed between the window and the mirror and their axis is perpendicular to the mirror and the window.
• the particles to be detected enter the detector through the mirror.
• the scintillation light is collected by the window 7. This light being emitted isotropically, the fraction of light emitted which leaves towards the mirror is reflected by the latter and returned to the outlet window.
• respective elastic membranes 9 and 10 absorb the thermal expansions of the scintillator.
• the detector array has, for example, a section of the order of 100 ⁇ 100 mm 2 and a thickness E which can range from 10 to 50 mm. It is produced in a single block by multiple assembly of macro beams containing elementary beams. This technique makes it possible to produce large section monolithic detectors.
• the capillary matrix is preferably produced on a thickness much greater than the desired thickness so as to ensure good collinearity between capillaries (for example less than 100 ⁇ radians).
• a digital example of a detector used to acquire the neutron image of a 1 mm diameter capsule, filled with deuterium and imploded by a 30 kJ laser is given below.
• the capillary matrix is a paver with a side of 100 mm and a thickness of 50 mm. Each capillary has a diameter of 250 ⁇ m.
• the stainless steel tank is closed by a mirror and a glass porthole. Four elastic membranes allow thermal expansion of the scintillator.

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• General Physics & Mathematics (AREA)
• High Energy & Nuclear Physics (AREA)
• Molecular Biology (AREA)
• Spectroscopy & Molecular Physics (AREA)
• Measurement Of Radiation (AREA)

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