WO2019215651A1

WO2019215651A1 - Neutron detector

Info

Publication number: WO2019215651A1
Application number: PCT/IB2019/053806
Authority: WO
Inventors: Andrew Cedric BUFFLER; Angus Craig COMRIE
Original assignee: University of Cape Town
Current assignee: University of Cape Town
Priority date: 2018-05-09
Filing date: 2019-05-09
Publication date: 2019-11-14
Anticipated expiration: 2020-11-09
Also published as: GB201807560D0

Abstract

This invention relates to a neutron detector (10), and more particularly but not exclusively to a neutron detector utilizing organic scintillator materials. The invention also extends to a method of determining the direction from which the neutron radiation emanates. The neutron detector (10) includes a plurality of scintillators (30), at least one photomultiplier (24) associated with each scintillator; and at least one moderator (40). There is provided for the scintillators to have parallel longitudinal axes, with the moderator located between the scintillators. The scintillators are parallel to and spaced about a longitudinal axis of the moderator, and the moderator is configured at least partially to shield the scintillators from one another.

Description

NEUTRON DETECTOR

BACKGROUND TO THE INVENTION

THIS invention relates to a neutron detector, and more particularly but not exclusively to a neutron detector utilizing organic scintillator materials. The invention also extends to a method of determining the direction from which the neutron radiation emanates.

There is growing interest in the development of compact neutron detectors for use at both low and high energy levels. These neutron detectors are for example needed for use in radiation monitoring around nuclear installations and laboratories, for radiation protection in aircraft and spacecraft, where neutrons with energies up to 100 MeV need to be measured, and for a wide range of industrial and security applications, such as monitoring ports of entry for special nuclear materials.

Detectors for neutron radiation are well-known in industry and existing designs vary greatly depending on the particular application. A scintillation counter is one type of neutron detector, and is essentially an instrument for use in detecting and measuring ionizing radiation by using the excitation effect of incident radiation on a scintillator material, and detecting and analysing the resultant light pulses. A scintillation counter typically consists of a scintillating material which generates photons in response to incident radiation, a sensitive photomultiplier device which converts the light so generated into an electrical signal, and electronics and data acquisition components for use in processing the electrical signal.

Organic scintillators are widely used for neutron detection and have been around since the 1960s. Organic scintillators are widely useful and are available in both liquid and solid plastic form. Liquid scintillators are associated with certain toxic and fire hazards, and are therefore not ideal for use in compact, more mobile (or even handheld) types of neutron detectors. Solid plastic scintillators can also be fabricated in any geometrical form, thus further increasing the design freedom when designing and constructing a neutron detector.

Until recently light collection from scintillator detectors was achieved using large glass photomultiplier tubes which required high voltage (e.g. 1000 V). In the last 10 years, extremely compact silicon-based chips have been developed for light collection which operate at less than 30 V. These compact silicon photomultipliers (SiPMs) are already widely used in the field of neutron detection, and their small size allows for the total size of neutron detectors to be significantly reduced.

A critical part of neutron detection relates to the processing of the signals received from the scintillator and photomultiplier. Pulse shape discrimination (PSD) is a technique which analyses the electronic signals from a detector to separate those pulses associated with the detection of neutrons from those associated with gamma-rays. Electromagnetic gamma radiation is nearly always present as a“background” to neutron radiation. Until recently, PSD was only possible in liquid scintillators, which are associated with the limitations already mentioned above. The recent development of solid plastic scintillators therefore opened up new opportunities in terms of reducing the size and robustness of neutron detectors. PSD is a well-known and well documented technique, in which a charged particle recoiling in the scintillator is identified based on the decay characteristics of the electric pulse emanating from the photomultiplier. PSD is essentially based on the idea that information unique to a particle type is carried in the shape of the pulse, generated from the detector, therefore, allowing the type of recoiling particle to be identified. The emergence of digital data acquisition and processing systems allows a number of flexible pulse shape discrimination algorithms to be implemented and dynamically optimised in software form.

Neutron detectors are not only used to detect the presence of neutron radiation, but are often required to be used as spectrometers in which special analyses are used to produce an energy spectrum of the detected radiation.

Neutron detectors used to be bulky and fragile due to the use of liquid scintillators, large and complex photomultiplier tubes and bulky electronics, but the advent of solid state, digital electronics, compact photomultipliers and plastic scintillators which allow PSD opened the door for the reduction in size, and improvement in robustness. The shift to SiPMs also resulted in a much lower voltage requirement, thus further reducing the complexity of the operational infrastructure.

Neutron detectors can also be characterised by their energy range. Many existing neutron detectors operate at a relatively low energy range. However, neutron radiation produced by cosmic rays is both significant at high altitude and in space, has a significant peak at high energies (100 MeV). Any detector which purports to be useful in these conditions needs to be able to reach 100 MeV, which is not that common in industry. Examples of neutron detectors incorporating the above features are described in the following articles:

- A.C. Comrie, A. Buffler, F.D. Smit and H. Woertche,“Tests of pulse shape discrimination with EJ299-33 plastic scintillator for use in portable spectroscopy," Proceedings of Science (TIPP2014) 251 ;

- A.C. Comrie, A. Buffler, F.D. Smit and FI. Woertche, “Digital neutron-gamma discrimination with an organic scintillator at energies between 1 MeV and 100 MeV,” Nuclear Instruments and Methods A 772 (2015) 43-49.

- A. Buffler, A.C. Comrie, F.D. Smit and FI. Woertche, “Neutron spectrometry with EJ299-33 plastic scintillator for En= 10-100 MeV,” IEEE Transactions on Nuclear Science 62 3 (2015) 1422-1428.

The content of these articles are incorporated herein by reference.

It is clear that major strides forward have been made in the field of neutron detector design, and in particular organic scintillator neutron detector design. Flowever, some challenges still exist. These include the fact that conventional neutron detectors, including the improved detectors disclosed in the articles above, are not directionally sensitive. They can accordingly determine the presence of radiation, but not the direction from where the neutrons emanate. In addition, neutron detectors are not always well suited to be used as spectrometers, and in theory it will be even more challenging to design a neutron detector that is both directionally sensitive, and capable of producing calibrated energy distributions of the neutron radiation detected.

It is accordingly an object of the invention to provide a neutron detector that will, at least partially, alleviate the above shortcomings.

It is also an object of the invention to provide a neutron detector which will be a useful alternative to existing neutron detectors. It is a particular object of the invention to provide a neutron detector which is directionally sensitive.

It is also an object of the invention to provide a neutron detector which is compact in size, capable of performing neutron-gamma discrimination, functional over a wide energy range (e.g. 1 to 120 MeV) and capable of being used as a spectrometer.

SUMMARY OF THE INVENTION

According to the invention there is provided a neutron detector including:

a plurality of scintillators;

at least one photomultiplier associated with each scintillator; and

at least one moderator;

the scintillators having parallel longitudinal axes, with the moderator located between the scintillators; characterized in that the scintillators are parallel to and spaced about a longitudinal axis of the moderator, and in that the moderator is configured at least partially to shield the scintillators from one another.

There is provided for the moderator to include a central core, with radially outwardly extending extensions extending from the core, wherein each extension is located between two or more adjacently located scintillators.

There is provided for the moderator to include four orthogonally extending extensions extending from the core, wherein each extension is located between two or more adjacently located scintillators. There is provided for the moderator to be cross-shaped or plus-shaped in cross-section.

There is provided for the neutron detector to include at least two scintillators having parallel longitudinal axes, with the moderator located between the scintillators.

Preferably, the moderator will extend substantially the full length of the two scintillators.

There is provided for the neutron detector to include at least three scintillators.

In one embodiment the neutron detector includes four scintillators, with the longitudinal axes of the scintillators defining the four corners of a square in cross-section. In this embodiment, there is provided for the moderator to be cross-shaped or plus-shaped in cross-section in order to comprise a central core, and four orthogonally extending extensions extending from the core. There is provided for each extension to be located between two adjacently located scintillators.

There is provided for the scintillators to be of an elongate configuration, and for at least one side of each scintillator not to be covered by the moderator, in order for the neutron detector to be capable of being used as a direction sensitive detector when a side of the detector, and hence the sides of the scintillators, face a radioactive source.

There is provided for the moderator to be made from a high density polyethylene, or any other suitable material. There is provided for the scintillators to be in the form of organic scintillators, and more preferably solid plastic organic scintillators which has the capability of pulse shape discrimination.

The scintillators may be of elongate configuration, and in one embodiment is square in cross-section.

There is provided for a photomultiplier to be provided at each end of a scintillator.

There is provided for each photomultiplier to be in the form of a silicon photomultiplier, preferably a compact silicon-based chip.

In a preferred embodiment, four photomultipliers are mounted on a single printed circuit board, with one such printed circuit board being located at each end of the neutron detector.

According to a further aspect of the invention there is provided a method of determining the direction from which radiation emanates, the method including the steps of:

- providing a neutron detectors including a plurality of scintillators that are at least partially shielded from one another;

- determining the radiation detected by each scintillator; and

- calculating the direction from which radiation emanates by analyzing the radiation detected by each scintillator.

According to a still further aspect of the invention there is provided a method of determining both the direction and the characteristics of radiation using the same neutron detector, the method including the steps of:

- providing a neutron detector including a plurality of elongate scintillators that are at least partially shielded from one another;

- determining the radiation detected by each scintillator; - calculating the direction from which radiation emanates by analyzing the radiation detected by each scintillator;

- orientating the neutron detector in order for longitudinal axes of the scintillators to be aligned with the direction from which radiation emanates; and

- determining the calibrated energy distributions of the neutron radiation detected.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the invention is described by way of a non limiting example, and with reference to the accompanying drawings in which:

Figure 1 is a perspective view of a first embodiment of a neutron detector in accordance with the invention;

Figure 2 is a perspective view of the neutron detector of

Figure 1 with end casing and internal cover of the neutron detector removed, in order for circuit boards carrying the photomultipliers to be visible;

Figure 3 is an exploded perspective view of the neutron detector of Figure 1 ;

Figure 4 is a plan view of an end casing with the internal cover omitted, with four printed circuit boards (each carrying a photomultiplier) located in the end casing; Figure 5 is a perspective view of a second embodiment of a neutron detector in accordance with the invention;

Figure 6 is a schematic side view of the scintillators and moderator of the neutron detector of Figure 1 or Figure 5;

Figure 7(a) and (b) is a graph showing radiation counts as a function of light output parameter L and pulse shape parameter S for events induced by neutrons and gamma-rays from an Am-Be radioisotopic source, with (a) using a reference scintillator, and (b) a single scintillator in similar to the scintillators used in Figure 1 ;

Figure 8 is a histogram showing the counts versus shape parameter S for events in the energy range depicted by the cut between the broken lines in Figures 7(a) and (b);

Figure 9 is a graph showing the unfolded energy spectra of the reference scintillator and the new scintillator of Figures 7(a) and (b);

Figure 10(a) shows counts as a function of light output parameter

L and pulse shape parameter S for events in a plastic scintillator which is capable of pulse shape discrimination, when exposed to neutrons and gamma-rays produced by the irradiation of a Li target by a proton beam of energy 140 MeV;

Figure 10(b) shows the light output (L) spectra of the neutron events of Figure 10(a); Figure 1 1 (a) is a graph showing test energy spectra compared to unfolded energy spectra, illustrating the capability of the scintillator to produce energy spectra;

Figure 1 1 (b) shows the input pulse height spectrum together with the re-folded spectrum obtained from the fit;

Figure 12 is a histogram of the number of counts as a function of light output (L) using the neutron detector of Figure 1 , with a radiation source being located at different angles relative to the neutron detector;

Figure 13 is a repeat of Figure 12, but shown in colour;

Figure 14 shows reconstructed angle (determined from the detector) versus incident radiation angle (actual) obtained using the neutron detector of Figure 1 ;

Figure 15(a) shows test spectra measured with 14 MeV neutrons after the standard unfolding process;

Figure 15(b) shows the shows the input pulse height spectrum together with the re-folded spectrum obtained from the fit; and

Figure 16 show the difference in response of the neutron detector of Figure 5 at the different orientations.

DETAILED DESCRIPTION OF INVENTION Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of "including," "comprising," or "having" and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. It is noted that, as used in this specification and the appended claims, the singular forms "a," "an," and "the," and any singular use of any word, include plural referents unless expressly and unequivocally limited to one referent. As used herein, the term“include” and its grammatical variants are intended to be non-limiting, such that recitation of items in a list is not to the exclusion of other like items that can be substituted or added to the listed items.

Referring to the drawings, in which like numerals indicate like features, a non-limiting example of a neutron detector in accordance with the invention is generally indicated by reference numeral 10.

The neutron detector 10 comprises shown in Figures 1 and 5 both include four elongate scintillators 30, with a moderator 40 located between the scintillators. The only significant difference between the neutron detectors of Figures 1 and 5 reside in the lengths of the scintillators used (50 mm in Figure 1 vs 150 mm in Figure 5). As to the rest, the two detectors are conceptually the same, and the description of the first embodiment with reference to Figures 1 to 4 equally applies to the embodiment of Figure 5. It will of course be appreciated that more (e.g. 3) or less (e.g. 5) scintillators may be used, provided that they are separated by a suitably configured moderator. The first embodiment of the neutron detector in accordance with the invention is now described with reference to Figures 1 to 4. The neutron detector 10 comprises four scintillators 30, each scintillator being in the form of an elongate cylinder being square in cross-section. In this embodiment, each scintillator is 50 mm long, and 6 mm in width and height. Each scintillator therefore has a clearly defined longitudinal axis, and comprises an elongate body 31 that terminates in two opposing end sections 32. The scintillators are organic scintillators, and more particularly solid plastic organic scintillators. Although the specific type of scintillator selected for the initial design is an EJ299-33 pulse-shape discriminating plastic scintillator, it is appreciated that other scintillators will also suffice, for example the EJ276 family of scintillators.

The four scintillators 30 are spaced apart, with parallel longitudinal axes of the scintillators defining a cuboid. A moderator 40 is located between the scintillators 30. In this example the moderator 40 is made from a high density polyethylene (HDPE). The moderator 40 is configured and dimensioned to shield the four scintillators from one another, and in this example is accordingly cross- or plus-shaped in cross-section. More particularly, the moderator 40 has a core section 41 , with four orthogonally extending shielding sections 42 extending outwardly from the core section 41. Each of the shielding sections 42 is located between two adjacent scintillators 30. The core section of the 41 of the moderator 40, and hence the moderator in totality, has a longitudinal axis that is parallel to the longitudinal axes of the scintillators spaced around the moderator 40. It will be appreciated that the neutron detector will be able to function as intended even if the moderator only partially shields the scintillators, as one would be able to pick up a differential reading between the scintillators. However, in a preferred embodiment the moderator will fully extend between the scintillators so as to fully shield the scintillators from one another. A photomultiplier 24 is provided at each end 32 of a scintillator 30, meaning that eight photomultipliers 24 are used in the neutron detector 10 in accordance with the invention. Each photomultipliers produces an electrical pulse dependent on the amount and time dependence of the photons of light received. Each photomultiplier is in the form of a silicon photomultiplier, and more particularly a solid-state single-photon-sensitive device built from an avalanche photodiode array on a common silicon substrate. Each photomultiplier is located on a printed circuit board 20. It will be appreciated that each scintillator is services by two photomultipliers - one at each end 32.

The neutron detector 10 furthermore includes two opposing end casings 1 1 that are securable relative to one another by way of threaded connecting rods 13, which keeps all the components of the neutron detector together. The printed circuit boards 20 are housed inside the end casings 1 1 , and are sandwiched between the end casings 1 1 and internal covers 12. Apertures are provided in the internal covers 12, and align with the position of the photomultipliers 24 located in the printed circuit boards, and the ends 32 of the scintillators.

A compact power support system (not shown) is also provided, and features eight channels for providing power to the eight PCBs. The electronics furthermore includes an amplifier, stabilization unit, programmable field array (FPGA) processor, and communications (wired or wi-fi). The detector and electronics are housed together and data may be stored on the device for later processing. Alternatively, the device may be coupled to a display such as a mobile smartphone.

As mentioned above, the neutron detector of Figure 5 is conceptually the same as the neutron detector of Figure 1 , with the only significant difference being the dimensions of the scintillators. In this case each scintillator is 150 mm long, and is still 6 mm in width and height. The additional length of the scintillators makes the device suitable for use for neutrons up to 100 MeV.

The use of a plurality of, in this case four, shielded scintillators is a critical aspect of this invention, as this result in the neutron detector being capable to be used in a direction-sensitive manner. It will be appreciated that a scintillator on a specific side of the detector that happens to face the radiation source will generate more counts than a scintillator on an opposite side of the neutron detector. By using suitable data processing techniques, a person skilled in the art will then be able to determine the direction from which the neutron radiation emanates. It will also be appreciated that more or less scintillators, for example 3 or 5, can be utilized, with the important aspect being the scintillators being shielded from one another, and located about an axis defined by the longitudinal axis of the moderator. The scintillators are therefore not arranged to be axially aligned, but are spread apart. It is important for the scintillators, and hence the neutron detector, to be of elongate configuration, so as to enable the neutron detector to detect both the direction and the nature of the radioactive source. This is discussed in more detail below.

In addition to, and in combination with, the direction sensitivity, the neutron detector can also be used as a spectrometer. More specifically, the elongated configuration of the scintillators allows for the neutron detector to be used as a spectrometer once the direction of the radioactive source has been identified. Once it has been established from which direction the neutrons emanate (as explained in more detail in the examples hereinbelow) the neutron detector can be orientated so that the elongate axes of the scintillators are aligned with the direction of radiation. This feature can be linked back to the essential operational characteristics of a scintillator material. Neutrons interact with the hydrogen and carbon nuclei in the scintillator material. The distance that the recoiling charged particles (such as neutrons) travel in the material depends on the energy of the incoming neutron, as well as the recoil angle of the charged particle. Recoil protons that carry the full energy of the neutron are scattered forward (i.e. in the direction of the incoming neutron). Therefore, in order to accurately function as a spectrometer, one needs to capture the full energy of these protons. When the detector is placed side-on (i.e. direction-sensitive mode), the majority of forward -scattered protons will not deposit their full energy in the scintillator before escaping. When the detector is placed lengthways, forward-scattered protons can travel a much longer distance before escaping the scintillator. Thus, they are far more likely to deposit their full energy in the scintillator, allowing one to reconstruct the energy spectrum of the incoming neutrons more accurately. The new and inventive neutron detector design will therefore provide the capability to determine both the direction and the type of radiation that is being detected.

The inventors believe that the new and novel neutron detector is a significant improvement over the state of the art. The present invention provides a compact neutron detector based on solid plastic scintillator material coupled to silicon photomultipliers for light collection, and digital electronics for pulse processing and display. The scintillator material produces pulses which may be analysed to accept neutron signals and reject gamma-ray signals through a digital application of pulse shape discrimination. The data analysis system automatically produces an energy spectrum of the detected neutrons and thus serves as a neutron spectrometer. Multiple segments of scintillator are deployed, separated by moderating, which allows the direction of the neutron radiation to be discerned by the detector. The detector can be used over a wide energy range (1 MeV to 100 MeV) and thus may find to measure secondary neutrons produced by cosmic rays. The compact size of the detector system, including the analysis and display, allows it to be used both in hand-held and fixed location mode. The detector may thus find use for dosimetry at high altitudes and in space, for security monitoring and contraband detection, and in general radiation protection applications. It will be appreciated that the above is only two embodiments of the invention and that there may be many variations without departing from the spirit and/or the scope of the invention. It is easily understood from the present application that the particular features of the present invention, as generally described and illustrated in the figures, can be arranged and designed according to a wide variety of different configurations. In this way, the description of the present invention and the related figures are not provided to limit the scope of the invention but simply represent selected embodiments.

The skilled person will understand that the technical characteristics of a given embodiment can in fact be combined with characteristics of another embodiment, unless otherwise expressed or it is evident that these characteristics are incompatible. Also, the technical characteristics described in a given embodiment can be isolated from the other characteristics of this embodiment unless otherwise expressed.

EXPERIMENTAL VERIFICATION a. Characterization of Scintillator

As a first step of the design and verification process, the specific scintillator used in this invention was tested and characterised. In particular, the pulse shape discrimination capabilities of the scintillator were verified. This does not in itself form part of the gist of the invention, but did provide more information regarding the characteristics and the suitability of the scintillator to be used in the invention. The test setup comprised a single scintillator having dimensions of 6 x 6 x 50 mm³, two silicon photomultipliers (SiPMs) and two channels of data. Figures 7(a) and 7(b) show counts as a function of light output parameter L and pulse shape parameter S for events induced by neutrons and gamma- rays from an Am-Be radioisotopic source in EJ299-33 scintillators of dimension (a) 50 mm diameter c 50 mm (reference scintillator) and (b) 6 c 6 x 50 mm scintillator (scintillator tested). Loci associated with recoiling protons (associated with neutron events) and electrons (associated with gamma-ray events) are identified. The broken line is a cut used to separate neutrons and gamma rays. The vertical dotted lines indicate the cut used to select a particular energy to illustrate in the histogram of Figure 8, which shows the counts versus shape parameter S for events in the energy range depicted by the particular cut. The separation of event type in the test scintillator (red spectrum - denoted SIPM) is superior to that achieved by the reference detector (blue spectrum - denoted PMT), as seen in Figure 8.

Idealised response functions were subsequently determined for the test scintillator using the specialised radiation transport code GEANT4. In order to evaluate the capability of the test scintillator as a neutron spectrometer, spectrum unfolding was performed. An AmBe neutron source was utilised, and the measured light output spectrum was unfolded using the simulated response matrix. The unfolded energy spectrum (red) was compared to the known reference spectrum for AmBe, as shown in Figure 9. It can be seen that the unfolded spectrum matches the reference spectrum very well, with deviations below 3 MeV most likely due to the interaction of the detector with neutrons scattered off surrounding material.

Measurements were also undertaken with a 140 MeV beam at the fast neutron facility at iThemba LABS national facility near Cape Town. The Separated-Sector Cyclotron of the iThemba LABS accelerates protons in the energy range from 20 MeV to 200 MeV. A beam pulse selector can suppress a chosen fraction of proton bunches to enlarge the time interval between pulses, which allows time of flight measurements to be carried out with less interference from earlier or later pulses. The 7Li(p, n)7Be reaction was employed to produce quasimonoenergetic neutron beams. A proton energy of 140 MeV was directed onto an 8 mm thick natural Li target, and the test scintillator was mounted coaxially at 6.00 m from the target.

Figure 10(a) shows counts as a function of light output parameter L and pulse shape parameter S for events in the test scintillator when exposed to neutrons and gamma-rays produced by the irradiation of a Li target by a proton beam of energy 140 MeV, selecting events with 47.5 < E < 52.5 MeV by time-of-flight. Loci associated with recoiling electrons (e), protons (p), escaping protons (ep), deuterons (d), tritons (t) and alpha-particles (a) are indicated. The dotted line indicates the cut used to separate gamma-ray events from neutron events, which are projected into the L-axis in (b). This is the first time that EJ299-33 was tested at these higher energies.

Tests of unfolding were also undertaken at these higher energies to demonstrate the test scintillator as a neutron spectrometer for high energy neutrons. A response matrix for neutron energies between 10 and 100 MeV was constructed for each detector from pulse height spectra for energy ranges of width 1 MeV, centred from 10 MeV to 100 MeV, in steps of 2 MeV, and selected using time-of-flight measurements. Pulse height spectra for the unfolding tests were created by applying time-of-flight cuts to an independent set of data. In the example shown in Figure 1 1 (a), the test energy spectra consisted of four“boxcar” functions of width 4 MeV, centred at 35, 50, 65 and 80 MeV. The pulse height spectra derived from these energy spectra were unfolded using a “Monte-Carlo” based program. Figure 1 1 (a) shows the test energy spectra together with the unfolded energy spectra. The unfolding process reproduces the position of the centre of each boxcar function and the peak is well resolved in each case. Figure 1 1 (b) shows the input pulse height spectrum together with the re folded spectrum obtained from the fit. b. Testing of the Neutron Detector of Figure 1 Measurements with the neutron detector as shown in Figure 1 , and as described above, were undertaken at a 14 MeV neutron facility. Figure 12 is a histogram of the number of counts as a function of light output (L) using the neutron detector of Figure 1 , with a radiation source being located at different angles relative to the neutron detector, with 0° (top), 90° (middle) and 45° (bottom).

These measurements allowed the direction sensing capability of the new neutron detector to be demonstrated. The important aspect of the graph shown in Figure 12 is not the specific counts of the different scintillators per se, but that the fact that there is a difference between the counts of the four scintillators when the source is located at different angles, which successfully confirms the directional sensitivity of the new neutron detector. Figure 13 is a repeat of Figure 12, but represented in colour.

Figure 14 shows reconstructed angle (determined from the detector) versus incident radiation angle (actual), showing that the new neutron detector is effective for detecting not only the energy of the incident neutrons, but also the direction from which they emanate. Response functions for new neutron detector were determined using the radiation transport simulation code GEANT4, using the geometry as shown in Figure 1. These response functions allowed unfolded tests using the new neutron detector to be undertaken using measurements made at a neutron facility. Figure 15(a) shows test spectra measured with 14 MeV neutrons, after the standard unfolding process, and Figure 15(b) shows the input pulse height spectrum together with the re-folded spectrum obtained from the fit. c. Testing of the Neutron Detector of Figure 5

Measurements with the neutron detector shown in Figure 5 were made using a 14 MeV neutron facility. Figure 16 show different orientations for which measurements were taken using the neutron detector of Figure 5; and also show the difference in response of the neutron detector of Figure 5 at the different orientations. The direction sensitivity of the device is clearly evident, even with these preliminary measurements. It is envisaged for further test measurements to be made, and also at high energy neutron levels.

Claims

CLAIMS:

1 . A neutron detector including:

a plurality of scintillators;

at least one photomultiplier associated with each scintillator; and

at least one moderator;

2. The neutron detector of claim 1 characterized in that the moderator includes a central core, with radially outwardly extending extensions extending from the core, wherein each extension is located between two or more adjacently located scintillators.

3. The neutron detector of claim 2 characterized in that the moderator includes four orthogonally extending extensions extending from the core, wherein each extension is located between two or more adjacently located scintillators.

4. The neutron detector of claim 3 wherein the moderator is cross shaped or plus-shaped in cross-section.

5. The neutron detector of any one of the preceding claims in which the moderator extends substantially the full length of the two scintillators.

6. The neutron detector of claim 1 including at least three scintillators.

7. The neutron detector of any one of the preceding claims in which the scintillators are of an elongate configuration, and wherein at least one side of each scintillator is not covered by the moderator, in order for the neutron detector to be capable of being used as a direction sensitive detector when a side of the detector, and hence the sides of the scintillators, face a radioactive source.

8. The neutron detector of any one of the preceding claims in which the moderator is made from a high density polyethylene.

9. The neutron detector of any one of the preceding claims in which the scintillators are in the form of organic scintillators.

10. The neutron detector of any one of the preceding claims in which the scintillators are in the form of solid plastic organic scintillators.

1 1. The neutron detector of any one of the preceding claims in which a photomultiplier is provided at each end of a scintillator.

12. The neutron detector of claim 1 1 in which each photomultiplier is in the form of a silicon photomultiplier.

13. The neutron detector of claim 1 1 in which four photomultipliers are mounted on a single printed circuit board, with one such printed circuit board being located at each end of the neutron detector.

14. The neutron detector of claim 1 substantially as herein described with reference to the accompanying figures.

15. A method of determining the direction from which radiation emanates, the method including the steps of:

- providing a neutron detector including a plurality of scintillators that are at least partially shielded from one another;

- determining the radiation detected by each scintillator; and calculating the direction from which radiation emanates by analyzing the radiation detected by each scintillator.

16. A method of determining both the direction and the characteristics of radiation using the same neutron detector, the method including the steps of:

- determining the radiation detected by each scintillator;

- calculating the direction from which radiation emanates by analyzing the radiation detected by each scintillator;