FR2552887A1 - SYSTEM FOR DETECTING NEUTRONS AND / OR GAMMA RAYS - Google Patents

Abstract

A. SYSTEM FOR DETECTION OF NEUTRONS AND OR GAMMA RAYS WITH A LARGE SURFACE RECEIVING RADIATION TO BE DETECTED WHICH PRODUCES PHOTONS IN THE MATERIAL PRODUCING SCINTILLATION, PHOTONS WHICH ARE CAPTURED IN THE OPTICAL FIBERS FORMING THE CONDUCTOR. B. SYSTEM CHARACTERIZED IN THAT THE OPTICAL FIBERS 6 ARE EXTENDED OVER THE ENTIRE RECEPTION SURFACE 2 IN THE MATERIAL PRODUCING THE SCINTILLATIONS 5, THAT THE LIGHT COLLECTION 4 IS OPERATED OVER THE ENTIRE ENVELOPING SURFACE OF THE OPTICAL FIBERS 6 AND THAT THE CONDUCTIVE OPTICAL FIBERS 6 ARE ESTABLISHED TO BE ANISOTROPIC. C. THIS SYSTEM ALLOWS SIGNIFICANTLY REDUCED LOSSES BY LIGHT ABSORPTION IN EXTENDED DETECTOR SYSTEMS.

Description

"Neutron detection system and / or

gamma rays ".

  The invention relates to a neutron and / or gamma-ray detection system having a large receiving surface for the irradiation to be detected which produces in the scintillation material photons which will be captured in an optical fiber light conductor. In particular, the invention relates to a system

for a rotary chamber lock.

  The commissioning of neutron detection is planned in personnel locks, as a supplement to nuclear material monitoring. It increases the probability of discovery of an unauthorized transfer of nuclear material out of the 15 areas where material balance is practiced because the protection measures on the subtraction side are much more difficult than in the detection of Zn radiation. S (Ag) is one of the substances whose scintillation is the most important among those available for 20 For the purpose of the construction of an efficient detector, however, it is also the optimal collection and transmission of the light that will be produced to a photomultiplier.

  used light collectors whose construction was as follows.

  Directly on the PTM (PTM can mean Photomultiplier) a PTM input window detector disc according to KH Sun, Malmberg PR, FA Pecjak, Scintillation Counters, Nucleonis (July 1956), which has the advantage that the The path of the transmitted light is shorter and, as a result, the losses are lower. The defect lies in the limitation of the diameter of the input window of the photomultiplier used. Another construction, WS Emmerich, A Fast Neutron Szintillator, Rev. Scien Inst, Vol 25, No 1, Jan 1954, provides a five-fold increase in the useable area by means of

  However, here too it is only possible to construct small units which, in a rotating chamber lock for example, embrace a too small angle of the space.

  The drawbacks of the current isotropic light conductors are: the dispersion losses that occur during direct optical contact with the optically denser scintillator material, losses by weakening (light absorption over the required transmission path), unfavorable surfaces between the light conductor section (the exit surface)

and the collecting surface.

  The output yield of the large scintillator crystals is, depending on the refractive index 25 in an order of magnitude of 5 to 11%.

  These values given for the input and transmission efficiency of the isotropic light conductors (air-limited surface, optically less dense material), show that for large surface scintillators, optical systems must be suitable to satisfy the following requirements: decoupling of the light source and the light conductor by interposition of an intermediate optical layer, having a refractive index which is smaller than that of the scintillator (n 1.5 Y) to avoid

  dispersion losses on the boundary surface.

  capturing sufficient light intensity to overcome the distance of the transmission to the photomultiplier. The mode of light capture by the front face of an optical fiber is already used for the construction of residual light amplifiers, WB Allan, Fiber Optics, Theory and Practice, Plenum Press, L Ondon, NY 1973, and large format projection screens, when deviating the path of light necessary for the use of the detector The disadvantage is the high consumption of fibers, and the weakness of the reduction factor obtained for the input surface ratio / output surface These drawbacks lead to consider a mode of capture of light that is not

used until now.

  The problem which lies at the basis of the invention then consists in producing a large-area detection system of the type mentioned, beginning in such a way that a scintillator substance which is effective from the point of view of nuclear physics can be used. and also take advantage of the benefits of fiber for the transmission of

  light to detection systems.

  For this purpose, the invention proposes a system characterized in that the optical fibers are extended over the entire receiving surface in the scintillating material, the light is captured over the entire enveloping surface of the optical fibers and that

  Conductive optical fibers are set up to be anisotropic.

  Some of the following are described below.

  advantageous improvements of the invention.

  The special advantages of this new solution lie in the use of anisotropic light-conducting systems, which, unlike optically isotropic systems, cause a separation of the light-conducting light collector functions. The light intensity picked up on the surface of the conductive optical fiber then reaches the optoelectronic amplifier, coupled to the end of the optical fiber, to a large extent.

without notable impairment.

  The mechanism for capturing light through the enveloping surface of the fiber is based on the synchronization of: phase velocities, polarization directions, surface wave fractions of the natural oscillations

  which can be excited in the core of the fibers.

  It has been confirmed experimentally that using cylindrosymmetric fibers, due to the geometry of the electromagnetic fields, appear properties that allow a light supply through the enveloping surface of the fiber. The volume mode (number of auto-oscillations) possible) increasing with the increase of the radius of the central core of the fiber or the diameter of this fiber increases the probability of the synchronization of the two fields in the enveloping surface of the fiber This point is particularly important, especially for the introduction 25 non-coherent light with phase changes

  random in time and space.

  These previously known properties of the cylindromesymmetric fibers are limited until now: to the use of anisotropic thin-film optical fibers 30, with flat geometry, which lead to other marginal conditions of light introduction, when using tunnel optical effect, the introduction of coherent light sources

  (Laser) which lead to permanent mating conditions.

  However, it has not been used technically so far, in particular, optically anisotropic optical fiber conductors have not been used for

  the construction of neutron detectors.

  The invention is described below in more detail with reference to theoretical and experimental results, as well as to an exemplary embodiment, and with the help of the figures

1 to 4.

  The principle of a fiber detector 1 with light capture by the receiving surface 2 of the base body (matrix Zn S (Ag) boron) is shown schematically in FIG. 1. The neutron flux O (3) produced lightning flashes 4 in the scintillating material of the base body 5, flashes which are collected by the fibers 6 placed parallel to each other, possibly in superimposed layers (here 3 layers) The fibers 6 are then assembled on both surfaces front of the basic body in bundles 7,

and go to the photomultiplier 8.

  The following discussion accounts for the relationship between light intensity and surface ratios of the light collecting surfaces incorporated in the scintillator (sum of enveloping surfaces of the different fibers 6) for different rays of these insulated fibers. light that can be obtained at the location 2 of the light capture (boundary surface nl / n 2) correspond to the ratio of the collector surfaces of the light A that fix the rays R of the different fibers If we continue In decreasing the radius R of the optical fibers, the enveloping surface decreases proportionally to R, the front surface of these fibers being reduced in proportion to R 2. To maintain a constant light collecting surface in the scintillator, it is necessary, if reduces the diameter, proportionally increase the number of fibers As the front surface of the fiber bundle decreases as the square of At the radius R of the fibers, the following equation is valid for the increase of the surface by introduction of other fibers 6: R 1

R 1 R 2

  Enveloping asurface = for R 1> R 2

R 2 R 1

  Enveloping Asurface The size of the embedded enveloping surface 10 causes a decrease in the distance separating the different point light sources from the scintillator with respect to the surface of the optical fibers and consequently an increase in the intensity of the light introduced (captured).

in the fiber optic cable 6.

  The following table shows, for common fiber optic diameters, light source /

  point of introduction under the marginal conditions indicated.

  Fibers O Number for the Enveloped Area Distance LQ / (*) (t A) fiber L-fiber detector 2 Lob (min)

1500 1100 8 3 400.

1050 2270 12 0 315.

500 10,000 25 1,150.

250,000 126 0 30.

70 510 200 180 0 21.

2,777,780,420 0 9.

25,000,000 1257 0 3.

(t LQ: light source

  L Lob: optical conductor surface.

  It emerges that between 1500 t and 70 fibers there is a light intensity ratio of 1/363 at the location of the light introduction, the surface

limit nl / n 2.

  Current achievements provide guidance for the geometric construction of the detection using

optical fibers.

  Important points are also: the mechanism of capture of light by the enveloping surface, the transmission properties of light energy

captured by the fibers.

  The anisotropic optical fibers consist of the two core components, with the refractive index N 1 and the enveloping surface N 2: the electromagnetic energy 10 is localized, during the transmission of the energy, mainly in the material whose refractive index

  n 1 is the highest, N S Kapany, J J Burke, Fiber Optics IX.

  Waveguide Effects, J Am Opt Soc Vol 51, No 10, Oct 1961.

  The propagation conditions emerge from the laws of geometrical optics (Snellius-Fresnellsche Gesetze der Brechung und Reflection, W Pohl, Optik und Atomphysik, Springer 1928), for the total reflection which occurs on the boundary layer towards the optically larger medium. easy to go through Without going back in detail on the theory, we will indiau quer important consequences: The transmission of light is based on the principle of total reflection in the optically easiest layer to cross N 2 With the final diameter of the optical fibers , it establishes, according to the wavelength of the light>, the numerical aperture (NA) and the radius a of the core of the fibers, discrete conditions of

  propagation (eigen modes) in the fiber.

  Contrary to the conclusion of the optical geometry, in the case of total reflection, the energy arrives in the "forbidden" medium N 2 optically easier to cross next, M Born, Optik, Springer, Berlin 1933 The wave vector Here k extends parallel to the boundary layer n 1 / n 2 in N 2 The losses of the core of the fibers, due to radiation losses on the surrounding material, are determined by the thickness S of the enveloping surface (region In general, the thickness of the layer amounts to about 7 to 10 lengths of the light wave for multi-modal optical fibers, for

  magnitude of wavelength X 0.5 / t (visible light).

  A coupling of the external light of the scintillator with the synchronous surface wave field in the tunnel region of the material N 2 makes it possible, without additional optical coupler (prism for example), that the propagation of the light is formed in a manner helical The light rays 10 emitted in the scintillator L Qn propagate in the form of c 6 ne, and tangentially reach the boundary surface nl / n 2 ("rasante" indication of the light). In case of concordance of the synchronization conditions for the speeds phase, direction of polarization corresponding to the choice of the angles of incidence vet 8 v in the zone of the boundary layer nl / n 2, the excitation of the electromagnetic auto-oscillations of the order v operates, in the

  the core of the fiber on the superficial wave field appearing to self-oscillation, by resonance.

  The introduction of light by taking advantage of the geometric effect of the curved boundary surfaces in practice offers the following advantages: Any additional coupling system, as used for the capture of light on optical films thin (prism coupler) becomes superfluous. The coupling mechanism is anisotropic with respect to the preferential path of the introduced light. As a result, the captured light energy remains in the

  fiber, even on long transmission paths.

  The entire length of the fiber is available as a sensing zone. From the light energy captured, it is only lost at about 10% / m, depending on the quality of the optical fiber, in the decoupling losses. and absorption. In order to verify the design of the detector, a surface element of the detector for locking is provided. This element is shown in section in FIG. 2. The box 9, of parallelepipedal shape, consists of a part forming a lid and a part 10, 11, which are screwed together and which, by means of a seal which makes it around 12, seal against the environment of a cavity 13, intended to receive the body of 10 base 5 (FIG. 1) The bundles 7 of optical fibers leave, being fixed by them, by the two terminals 14, 15,

  mostly offset from each other diagonally.

  The sensing surface of the detector is 900 cm 2. An isolated fiber diameter of 1500 / A, incorporated into a single layer bed, is used in a Zn S (Ag) -boric acid matrix. This important section of the fibers is admittedly optically unfavorable, but offers technical advantages for the fabrication. To obtain a sufficient capture efficiency with the rather large fibers used, the enveloping surface (surface area) was roughened. outer limit N 0 / n 2) The dispersion centers appearing on the surface act in the manner of a grid optical coupler, as used in conjunction with flat light conductors. the scintillator light falling perpendicular to the boundary surface N 1 / n 2 in synchronous incidence directions for coupling with the surface waves results in an increase in the intensity of the light captured. which is about 20 times that one

would get on a smooth surface.

  The p-ray sensitivity measurements were made under the following marginal conditions: setting the optimum amplification parameters with reference to the specifications given in W. SC Chang, Periodic Structures and their Application in Integrated Optics, IEEE Trans on Microwave Theory and

  Techniques, Dec 1973, p 775 ff, and by tests.

  The amplifying electronics used (Philips XP 2230 photomultiplier) is identical to that of the IRT liquid scintillation system for the detection of radii. Determination of the discrimination threshold and 2 μ the window width U w from the ratio 52 / B, signal measured noise for different energies The energy field under consideration ranges from Am-241 (E '60 ke V to U U 0.1 V),

  beyond Ba-133 (E 360 ke V) to Co-60 (E% 1170 ke V).

  The discriminator setting was set as the result of the measurement for Uu c 0.1 V and Uw = 3 V. The signal-to-noise ratio for different distances from the sources with respect to the detector plate, calculated for the center of the plate, is illustrated in Figure 3, and it is also indicated 52 / B = 5 6502 which corresponds to a probability of detection of 95% with a rate of false alarms of 0, 01% The alarm threshold adjusted , 4 a, takes into account variations in the stability of the electronics The probability of detection of 95% corresponds to a net signal

  S = G-B of 1.65 au above the alarm threshold.

  For the required signal-to-noise ratio is valid the equation

52 / B = (5.65 VB) / B = 32

  regardless of the absolute count rate B 52 / B is a quality criterion of a detector, to a large extent independent of the setting of the amplifier parameters.

  The measurement results (FIG. 3) were determined for different distances z above the center of the plate

(x, y) = (5.5).

  FIG. 4 shows, for z = a constant, the dependence of the ratio 52 / B signal / noise for different distances with respect to the photomultiplier, the position 1 l y = 1 denoting the smallest distance, which is 30 cm,

  y = 9, a distance of about 60 cm.

  The measurement results confirm the ability of anisotropic optical fibers to function as a collection and transmission agent, even if

  optical fibers whose section is large.

  Differences in the efficiency of gamma-ray and neutron count rates due to differences in signal strength suggest that the Zn S (Ag) boric acid detector can be commissioned in Canada. same time for the detection of gamma rays, the separation of signals being

  obtained by setting different energy thresholds.

Claims (5)

  1) A system for detecting neutrons and / or gamma rays having a large surface for receiving the radiation to be detected which produces photons in the photon scintillation producing material which are captured in the optical fibers forming the conductor, characterized in that the optical fibers (6) are extended over the entire receiving surface (2) in the scintillation-producing material (5), the light-catching (4) is effected over the entire enveloping surface of the optical fibers (6) and that the conductive optical fibers (6) are established so as to be anisotropic.   2) neutron and / or gamma detection system according to claim 1, characterized in that the optical fibers (6) are incorporated parallel to each other in the scintillating material (5), in one or more layers which are directed parallel to the surface receiver (2).   3) System according to one of claims 1 and 2, characterized in that the output (7) of the light is effected   by the end faces of the optical fibers (6).   4) System according to one of claims 1 to 3,   characterized in that the optical fibers (6) are cylindrosymmetrically set, the refractive index (n 1) of the core 25 of these optical fibers being larger than that (n 2) of the enveloping surface.   System according to one of Claims 1 to 4, characterized in that the enveloping surface   optical fibers (6) are roughened.   6) System according to any one of claims 1 to 5, characterized in that the receiving surface   (2) is square or rectangular.