BACKG ROU N D OF TH E I NVENTION The present invention relates to the technology of detecting ionizing radiation . It refers to a detector arrangement for the detection of ionizing radiation .

It further relates to a method for operating such a detector arrangement.

PRIOR ART

Document WO 201 2/007734 A2 describes a radiation detector for neutrons and gamma rays with a conversion layer comprising a neutron capturing material and a phosphor, such that neutrons are converted into light signals. These light signals are then allowed to enter into a light guide, the light guide contains a wavelength shifter that absorbs the light and reemits at a longer wavelength . Reemission is isotropic, such that a fraction of the reemitted light can be transported via total internal reflection inside the light guide, until detected via a photoelectric converter, e.g. photomultiplier or similar device. In one embodiment, the light guide may be a scintillator.

Document CA 2 3 1 2 593 A 1 describes a scintillator surrounded by a neutron conversion layer within a gamma radiation shield . The scintillator detects gamma rays of characteristic energy emitted by said conversion layer upon neutron capture. Other prior art such as document US 2005/0258373 A 1 also use conversion layers to detect thermal neutrons in proportional counters The US 2005/0258373 thereby exploits a charge collection approach.

Document WO 2007/ 1 2 1 876 A2 describes a radiation detector based on noble gas scintillation , where pulse shape discrimination is used to distinguish different particle in teractions, such as for example fast neutrons from gamma rays.

However, known radiation detector arrangements are not very flexible in their operation, and mostly have a complicated configuration.

SU M MARY OF TH E I NVENTION

It is an object of the invention , to improve the area of application of such detector arrangements.

It is another object of the invention to provide detector arrangements, which are more compact and /or more versatile.

It is another object of the invention to disclose a method for operating such a detector arrangement.

These and other objects are achieved by a detector arrangement for the detection of ion izing radiation comprising at least one light sensing device and a multifunctional coating arranged in an interacting relation to said at least one light sensing device, whereby said multifunctional coating is configured to perform the functions of a ) reflecting light of a given wavelength; and b) converting at least part of thermal and/or epi-thermal neutrons entering said multifunctional coating into light.

According to an embodiment of the invention said multifunctional coating is capable of emitting particles such as neutron conversion products in accompaniment of the emitted light.

Preferably, a geometry is chosen to maximize the area of the multifunctional coating per volume of the detector arrangement or per sensitive area of the light sensitive device.

According to another embodiment of the invention said multifunctional coating is capable of shifting short wavelength light impinging upon it, reemitting light with a wavelength to which it is reflective.

According to another embodiment of the invention said multifunctional coating comprises a first layer of a neutron conversion material and a second layer of a wavelength shifting material.

Specifically, said first layer contains Li-6 or B- 1 0, and said second layer contains Tetra Phenyl Butadiene (TPB).

More specifically, the thickness of said first layer is between 1 m and 40 m and said second layer has a thickness between 0. 1 m and 1 μιτι.

According to another embodiment of the invention said first and second layers are applied to a flexible substrate, especially of Tyvek® , or a PTFE membrane.

According to another embodiment of the invention , the volume between the multifunctional coating and the light sensing device includes a vacuum. According to a further embodiment of the invention a scintillator volume is provided, which is in optical contact with said multifunctional coating, such that said multifunctional coating reflects light of a given wavelength coming from the scintillator volume, back through said scintillator volume, and said light from neutron conversion in said mul- tifunctional coating is emitted into the scintillator volume, whereby the light is detected by said light sensing device.

Specifically, said light sensing device is one of a photomultiplier or pixelated Geiger mode avalanche photodiode.

According to another embodiment of the invention a scintillator volume is provided, which is in optical contact with said multifunctional coating, such that said multifunctional coating reflects light of a given wavelength coming from the scintillator volume, back through said scintillator volume, shifts light of shorter wavelengths to be re-emitted back through said scintillator volume at a wavelength at which said multifunctional coating is reflective; and emits light from neutron conversion into the said scintillator volume, whereby the light is detected by said light sensing device.

Specifically, said light sensing device is one of a photomultiplier or pixelated Geiger mode avalanche photodiode.

According to another embodiment of the invention a scintillator volume is provided, which is in optical contact with said multifunctional coating, such that said multifunc- tional coating reflects light of a given wavelength coming from the scintillator volume, back through said scintillator volume, shifts light of shorter wavelengths to be re-emitted back through said scintillator volume at a wavelength at which said multifunctional coating is reflective; and emits light as well as one or more particles with mass such as conver- sion products into the said scintillator volume, the particle causing scintillation inside the scintillator volume, when falling back upon the multifunctional coating said scintillation light can be shifted and re-emitted back through said scintillator volume, and the light from the multifunctional coating and the scintillator volume is detected by said light sensing device, directly or after having been shifted in wavelength .

Specifically, said light sensing device is one of a photomultiplier or pixelated Geiger mode avalanche photodiode.

According to another embodiment of the invention said scintillator volume consists primarily of noble gas such as helium, argon or xenon or a mixture of noble gas, such as helium doped with xenon. Due to the fact that these gases scintillate in the vacuum ultraviolet (VUV) region at wavelengths that are difficult to collect and detect, the multifunctional coating's wavelength shifting property is useful: It can shift scintillation light from VUV to visible, which can be reflected including by the multifunctional coating , and detected by light sensing devices such as photomultiplier tubes.

Specifically, said scintillator volume is predominantly helium, thereby allowing the simultaneous measurement and distinction of fast neutrons, thermal neutrons, and/or photons and electrons produced by the interaction of photons with a detector wall.

Alternatively, said scintillator volume is predominantly xenon , thereby allowing gamma spectrometry to be performed while also measuring neutrons.

According to another embodiment of the invention said scintillator volume is predominantly PVT or a liquid scintillator, thereby allowing the simultaneous measurement of gammas and neutrons. According to another embodiment of the invention solid state light sensors such as pixe- lated Geiger mode avalanche photodiodes are immersed in the scintillating gas.

According to another embodiment of the invention an in-situ gas purification device such as a getter is immersed in the gas of said scintillator volume, thereby assuring a stable gas composition .

According to another embodiment of the invention a plurality of light sensing devices is interspersed in the area of said multifunctional coating.

In the inventive method for operating a detector arrangement according to the invention the signals from said neutron conversion are discerned from signals from said scintillator volume by pulse shape discrimination , whereby the signals involving light emitted by said multifunctional coating typically have a different time structure than the signals from said scintillator volume.

According to an embodiment of the inventive method , the light signals from said neutron conversion combined with the light signals from particle emission from the multifunctional coating into said scintillator volume are discerned from signals from said scintillator volume alone by pulse shape discrimination , whereby the signals from neutron conversion typically have a different time structure than the signals from said scintillator volume.

BRI EF DESCRI PTION OF TH E DRAWI NGS

The present invention is now to be explained more closely by means of different embodiments and with reference to the attached drawings. Fig. 1 shows in a longitudinal cross section a detector arrangement comprising a multifunctional coating with diffuse reflective properties according to an embodiment of the invention ;

Fig. 2 shows, similar to Fig. 1 , part of a detector arrangement comprising a multi- functional coating with wavelength shifting properties according to another embodiment of the invention;

Fig. 3a shows, similar to Fig. 2, part of a detector arrangement comprising a multifunctional coating converting thermal neutrons to light signals according to another embodiment of the invention ; Fig. 3 b shows, similar to Fig. 3a, part of a detector arrangement comprising a multifunctional coating converting thermal neutrons to light signals and emitting one or more particles according to another embodiment of the invention.

Fig. 4 shows the notional difference in signal shape for gamma rays interacting in the scintillator ( Fig . 4a ), a fast neutron or heavy particle interacting in the scintillator ( Fig. 4b), a charged particle interacting with the multifunctional coating ( Fig. 4c) and a massive particle such as a neutron conversion product producing light in the multifunctional coating , then producing light in the scintillator ( Fig. 4d ) detected by the detector arrangement according to the invention ; Fig. 4bis shows the difference in time structure of actual measured signals detected by the detector arrangement according to the invention ; shows in a diagram how signal analysis enables the distinction between different radiation types; shows in a schematic diagram the multifunctional coating capable of converting neutrons to light, and reflecting visible light; shows in a schematic diagram the multifunctional coating capable of converting neutrons to light as well as particles, and reflecting visible light; shows in a schematic diagram similar to Fig . 6a the multifunctional coating capable of converting neutrons to light, reflecting visible light, and shifting short wavelength light to visible light; shows in a schematic diagram similar to Fig. 6b the multifunctional coating capable of converting neutrons to light as well as particles, reflecting visible light, and shifting short wavelength light to visible light; shows a multifunctional coating with integrated distributed light sensors;

Fig. 9 shows a single detector for thermal neutrons, fast neutrons, and gamma rays;

and

Fig. 1 0 shows an alternative geometry for the detector according to Fig. 9.

DETAI LED DESCRI PTION OF DI FFERENT EM BODI M ENTS OF TH E I NVENTION

Fig. 1 shows in a longitudinal cross section a detector arrangement comprising a multifunctional coating with diffuse reflective properties according to an embodiment of the invention . In Fig 1 the multifunctional coating 1 2 of the detector arrangement 1 0 is in contact with a scintillator volume 1 3 , such as noble gas. Said multifunctional coating 1 2 has reflective properties, such that light or light signals 1 5 can be transported by means of diffuse reflection until impinging on the sensitive area of a light sensing device 1 4, such as a photomultiplier tube or a solid state light detector. The multifunctional coating 1 2 can be coated directly onto the wall 1 1 of the detector arrangement 1 0 or onto a substrate such as PTFE or Tyvek® , which is then inserted into the detector arrangement 1 0.

In an embodiment of the invention according to Fig. 2 the multifunctional coating 1 2 contains a first layer 1 2a containing a neutron conversion material such as Li-6. This conversion layer 1 2 is overcoated with a second layer 1 2b of fast wavelength shifting material such as Tetraphenyl Butadiene (TPB). This wavelength shifting coating 1 2b can convert short wavelength noble gas scintillation light 1 7 into longer wavelength visible light 1 5 that is better suited for detection with a photomultiplier tube or solid state light detection devices.

As shown in Fig 3a, the conversion layer 1 2a may contain ⁵ LiF. In one embodiment of the invention , the conversion layer is made of ⁵ LiF in an epoxy matrix. ⁵ Li has a high probability of capturing thermal neutrons, leading to the decay into ⁴ He + ³ T. These two conversion products 1 9 will share a kinetic energy of almost 5 MeV. The thickness of the conversion layer 1 2a is chosen to be thin enough, such as to allow a high probability for conversion products 1 9 to exit the conversion layer 1 2a and enter into the overlying wavelength shifter layer 1 2b, causing this to emit visible light 1 5 into the scintillation volume. The light produced in the fast wavelength shifter 1 2b has a time structure that is shorter and faster than scintillation signals from the scintillator volume 1 3 , allowing the distinction between a neutron conversion event in the multifunctional coating 1 2 and radiation interactions in scintillator volume 1 3. As shown in Fig 3 b, the conversion layer 1 2a may contain ⁵ LiF. In one embodiment of the invention , the conversion layer is made of ⁵ LiF in an epoxy matrix. ⁵ Li has a high probability of capturing thermal neutrons, leading to the decay into ⁴ He + ³ T. These two conversion products 1 9 will share a kinetic energy of almost 5 MeV. The thickness of the conversion layer 1 2a is chosen to be thin enough, such as to allow a high probability for conversion products 1 9 to exit the conversion layer 1 2a and enter into the overlying wavelength shifter layer 1 2b, causing this to emit visible light 1 5 into the scintillation volume. Furthermore according to Fig 3 b the shifter layer 1 2b is chosen to be thin enough, such as to allow a high probability for a conversion product 1 9 to exit into the scintillator volume 1 3. The scintillation light produced by the conversion product in the scintillator, upon falling upon the multifunctional coating, can be shifted by the coating to the visible range. The light produced in the fast wavelength shifter 1 2b has a time structure that is shorter and faster than scintillation signals from the scintillator volume 1 3 , allowing the distinction between a neutron conversion event in the multifunctional coating 1 2 combined with the scintillator 1 3 and radiation interactions in scintillator volume 1 3 alone.

In embodiments of the invention where the scintillator volume 1 3 is liquid or gaseous, the multifunctional coating 1 2 can be coated directly onto the inside of the wall 1 1 .

In embodiments of the invention where the volume between the multifunctional coating 1 2 and the light sensing device 1 4 is a vacuum, the multifunctional coating 1 2 can be coated directly onto the inside of the wall 1 1 .

In another embodiment of the invention , the multifunctional coating is coated onto a substrate such as Tyvek® or a membrane, foil, or structured material of PTFE. Said substrate can be an efficient diffuse reflector. If the scintillator volume 1 3 is a fluid, said substrate can be inserted along the inner wall of a detector. If the scintillator volume 1 3 is a solid , said substrate can be used to wrap the scintillator. Alternatively, the multifunctional coating 1 2 may be coated directly onto the solid scintillator. If the volume between the multifunctional coating 1 2 and the light sensing device 1 4 contains a vacuum, the multifunctional coating 1 2 can be coated on a substrate that can be inserted along the inner wall of a detector.

In an embodiment of the invention according to Fig. 4, where the scintillator volume 1 3 is noble gas, gamma ray and fast neutron interaction can be distinguished by their pulse shape according to WO 2007/ 1 2 1 876 A2. The signal of thermal conversion in the multifunctional coating 1 2 gives a signal with yet another pulse shape, such that the interactions can be distinguished . Fig . 4 notionally depicts the difference in pulse shape between these interactions, whereby a signal shape according to Fig . 4a relates to interactions of gamma radiation in the scintillator volume 1 3 , Fig. 4b relates to fast neutrons or heavy particles interacting in the scintillator volume 1 3 , Fig . 4c to charged particles interacting in the multifunctional coating, and Fig. 4d to the combination of the processes described in connection with Fig . 4b and 4c.

Fig 4b/s shows actual signals measured by an embodiment of the invention . The upper two signals are caused by fast neutron interactions, the lower left by a gamma interaction , and the lower rig ht by a thermal neutron interaction . The time structure difference between event types can be seen . A variety of possible methods to discern between interaction types becomes evident, for example by comparing integrals over short times, over the full signal, pulse height few nanoseconds after signal start, and maximum pulse height.

Fig. 5 shows actual measurement data taken with a detector according to the invention , whereby the scintillator volume 1 3 is predominantly helium. Fast neutrons, thermal neu- trons and gamma rays can clearly be distinguished on the basis of their pulse shape (see Fig. 4): The signals are integrated from signal start over a short time period (x axis) and a longer time period (y-axis). For neutron detection , gamma rejection is often a key attribute. Measurements with this detector demonstrated the capability to detect neutrons while rejecting gamma radiation fields up to 400 Sv/hr.

According to Fig . 6a the multifunctional coating 1 2 converts thermal neutrons 1 8 to light signals 1 5 and transmits these to the scintillator volume 1 3. Furthermore, the multifunctional coating 1 2 is reflective, allowing light signals 1 5 to be reflected until they can be detected by a light sensing device 1 4 such as a photo multiplier.

According to Fig. 6b the multifunctional coating 1 2 converts thermal neutrons 1 8 to light signals 1 5 and transmits these to the scintillator volume 1 3. Also, conversion products 1 9 are emitted into the scintillator volume, causing scintillation . Furthermore, the multifunctional coating 1 2 is reflective, allowing light signals 1 5 to be reflected until they can be detected by a light sensing device 1 4 such as a photo multiplier.

According to Fig . 7a the multifunctional coating 1 2 converts thermal neutrons 1 8 to light signals 1 5 and transmits these to the scintillator volume 1 3. Furthermore, the multifunctional coating 1 2 is reflective, allowing light signals 1 5 to be reflected until they can be detected by a light sensing device 1 4 such as a photo multiplier. In addition , the multifunctional coating 1 2 is capable of shifting shorter wavelength scintillation light 1 7 coming from the scintillator volume 1 3 , re-emitting visible light 1 5 back into the volume of scintillator.

According to Fig. 7b the multifunctional coating 1 2 converts thermal neutrons 1 8 to light signals 1 5 and transmits these to the scintillator volume 1 3. Also, conversion products 1 9 are emitted into the scintillator volume, causing scintillation . Furthermore, the multifunctional coating 1 2 is reflective, allowing light signals 1 5 to be reflected until they can be detected by a light sensing device 1 4 such as a photo multiplier. In addition , the multifunctional coating 1 2 is capable of shifting shorter wavelength scintillation light 1 7 coming from the scintillator volume 1 3 , re-emitting visible light 1 5 back into the volume of scintillator.

Fig. 8 shows an embodiment of the invention whereby the multifunctional coating 1 2 is interspersed with a plurality of solid state light detectors 1 4 such as pixelated Geiger mode avalanche photomultipliers, thereby reducing the number of reflections between light creation and detection . The solid state light detectors 1 4 are connected to a common signal processing unit 22. Not only does this maximize light collection but also offers means to localize the point of origin of the light by a centre of gravity method . This interspersed multifunctional coating can form the inner lining of a detector filled with a noble gas scintillator, making a very robust detector consisting only of noble gas, wall, and circuitry without containing any fragile components. Such a detector can be capable of detecting and discerning gamma radiation , fast neutrons, and thermal neutrons.

According to Fig . 9 a detector 2 1 consisting of a scintillator volume 1 3 , in this case noble gas, surrounded by the multifunctional coating 1 2, coated onto the walls 1 1 of the detector. In another embodiment, a flexible substrate is coated and inserted along the walls 1 1 . Radiation interaction within the scintillator in a scintillation event 1 6 produces VUV scintillation light. When said VUV light first reaches the multifunctional coating 1 2, it is shifted to visible light. This visible light 1 5 is then reflected by the multifunctional coating until it reaches the sensitive area of a light sensing device 1 4 such as a PMT. Fig 1 0 shows a different arrangement of such a detector. The multifunctional coating has a geometry that increases its area and therefore the detector's sensitivity to thermal and epithermal neutrons relative to the detector's volume. In the example shown , the detector contains of a plurality of cylindrical sub-volumes 1 3', each lined by the multifunctional coating 1 2 coated directly onto a structure of PTFE. The cylindrical sub-volumes 1 3' contain helium gas acting as a scintillator. This structure may also act as a moderator for neutrons. Thermal neutrons 1 8 are converted by the multifunctional coating , emitting visible light 1 7. Conversion products also escape into the scintillating gas causing scintillation 1 6, as would also be caused by interaction of a fast neutron in the scintillating gas. The diameter of the individual cylinders is chosen to be big enough (a few millimetres in the case of 1 80 bar helium ) such as to allow conversion products emitted from the multifunctional coating to lose all of their energy in the gas causing scintillation prior to hitting the opposite wall. The scintillation light 1 7 , is shifted to visible light 1 5. Visible light 1 5 is reflected on the multifunctional coating until said light is detected by a light sensing device 1 4 such as an immersed solid state light sensor.

The present invention thus has a certain similarity to WO 20 1 2 /007734 A2 , in that the coating produces light upon neutron capture, and that this signal is characteristic enough to be discriminated from other signals detected by the photoelectric converter ( in WO 201 2/007734 A2 from gamma events; in the present invention from gamma and fast neutron events) by pulse shape. The key differences of the present invention to WO 201 2/007734 A2 are: In WO 201 2/007734 A2 , the converter layer serves a single purpose, which is to convert thermal neutrons to light. In the present invention , the layer serves a multiple purpose of: a ) Converting thermal neutrons to light as in WO 201 2/007734 A2, b) being reflective to visible light impinging on the layer, such that this light can be transported to a photomultiplier or similar device by simple reflection on the multifunctional coating and does not require a light guide's total internal reflection facilitated by an internal wavelength shifter. Further, in a preferred embodiment, c) the multifunctional coating contains a wavelength shifter to shift VUV scintillation light, impinging on the multifunctional coating, to wavelengths in the visible spectrum, the multifunctional coating then being reflective to the visible light. In another preferred embodiment d ), the multifunctional coating converts thermal or epi- thermal neutrons to light but also emits one or more particles in addition to the light produced.

Key difference between the aforementioned CA 2 3 1 2 593 A 1 and the present invention is that the conversion layer in CA 2 3 1 2 593 A 1 needs not be in optical contact with the scintillator, and performs no optical function . In the present invention , the conversion layer is in optical contact with the scintillator, and performs the function of a ) reflecting light coming from the scintillator back into the scintillator, and b) upon neutron capture, emitting light ( rather than a gamma ray) into the scintillator.

To those skilled in the art it seems that in CA 2 3 1 2 593 A 1 the distinction of gamma events in the scintillator from neutron capture events occurs via the characteristic energy of the gamma emitted by the converter. This motivates the use of the described external gamma shield , to reduce unwanted gamma events in this energy range. In the present invention , neutron capture events are distinguished from other events via the characteristic signal shape. Unlike the present invention, where the conversion layer emits an optical signal, US 2005/0258373 A1 exploits a charge collection approach. Finally, the present invention allows the detector of WO 2007121876 A2 to be used as a triple detector being able to measure and distinguish gamma rays, thermal neutrons, and fast neutrons (the thermal neutron capability being novel).

List of reference numerals

10,20 detector arrangement

11 wall

12 multifunctional coating

12a neutron conversion layer

12b wavelength shifting layer

13 scintillator volume

14 light sensing device (e.g. photomultiplier or solid state light detector

15 (visible) light

16 scintillating event

17 (scintillation) light

18 thermal neutron

19 conversion products

21 detector

22 signal processing unit