FIELD OF THE INVENTION This invention relates to a method and system for detecting neutrons. BACKGROUND TO THE INVENTION

Nuclear fission reactors produce heat thro ugh a controlled nuclear chain reaction in a critical mass of fissile material (usually uranium-235, plutonium- 239 or plutonium-241). Upon absorption of a neutron by a relatively large fissile atomic nucleus, the original heavy nucleus splits into two or more lighter nuclei, releasing kinetic energy, gamma radiation and free neutrons. Some of the released neutrons may later be absorbed by other fissile atoms, triggering further fission events and releasing more neutrons in a nuclear- chain reaction. The fraction of neutrons permitted to cause further fission events is controlled by the addition of neutron moderators, which reduce the velocity of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235. Neutron- absorbing materials, also called poisons, are inserted into some types of reactors in order to lower the high reactivity of their initial fresh fuel load. The power output of the reactor is therefore monitored and controlled by detecting and controlling how many neutrons are available and able to produce more fission events.

Furthermore, the energy of various neutrons released during a fission event differs: fast neutrons have an energy greater than 1 eV; thermal neutrons have an energy of about 0.025 eV; and epithermal neutrons have an energy just above thermal. In order to accurately monitor the fission rate and temperature in a nuclear reactor, it is therefore necessary to detect the fast neutrons, the thermal neutrons, and the epithermal neutrons. The major challenges faced by neutron detection include background noise such as gamma radiation, high neutron detection rates, neutron neutrality, and varying neutron energies. Although specialized electrical equipment for neutron detection exists, it cannot be placed too close to the nuclear reactor due to rapid radiation damage of electronic equipment. Other detection methods, such as neutron radiography, make use of radiographic film and are therefore not suited to real-time monitoring of neutron flux. Furthermore, unlike X-rays which sensitize radiographic film directly, neutrons pass through film without substantial interaction with the emulsion. The film therefore requires backing with a conversion screen having a thin film of a material such as gadolinium for absorbing the neutrons. However, high quality screens are a specialized product which in recent years have become extremely difficult to obtain. Furthermore, the use of radiography is not suited to real-time monitoring of neutron flux.

A need, therefore, exists for an improved method and apparatus for neutron detection that is sensitive, stable, efficient, and capable of wide dynamic range and real-time detection of neutron flux in the high radiation environment of a nuclear reactor.

OBJECT OF THE INVENTION

It is an object of this invention to provide a method and system for detecting neutrons which will, at least partially, alleviate some of the abovementioned problems.

SUMMARY OF THE INVENTION

In accordance with the invention there is provided a method of detecting neutron radiation which includes

a. placing a target material which produces alpha particles when exposed to neutron radiation in a container; b. exposing the container and target material to radiation; and c. measuring the alpha particles in the container.

Further features of the invention provide for the target material to be one or more of lithium, carbon, beryllium and boron; for the container to be sealed, alternatively in flow communication with a detector capable of measuring alpha particles; for the target material to be held at a low pressure in the container; alternately for the container to be filled with an inert gas, such as xenon.

A still further feature of the invention provides for discrete sampling to be used to measure alpha particles in the container.

Yet further features of the invention provide for alpha particles to be measured with a mass spectrometer; and for the number of alpha particles measured to be calculated to so determine the energy range of the neutron radiation.

Still further features of the invention provide for the container to be made from stainless steel, preferably nuclear grade stainless steel.

Further features of the invention provide for the method to include the further step of circulating an inert gas through the container and measuring the alpha particles therein; for the inert gas to be xenon; for the alpha particles to be measured by cooling the inert gas sufficiently to liquefy it whilst the alpha particles, in the form of helium, remain in a gaseous form, and separating and measuring the helium gas from the liquefied inert gas.

The invention also provides a neutron detection system which includes

a. a target material capable of producing alpha particles upon exposure to neutron radiation; b. a container in which the target material can be placed and exposed to radiation and alpha particles retained; and

c. a detector capable of measuring alpha particles within the container.

Further features of the invention provide for the target material to be one or more of lithium, carbon, beryllium and boron; for the container to be sealed, alternatively in continuous flow communication with the detector; for the target material to be held at a low pressure in the container; alternately for the target material to be held in an inert gas, such as xenon, within the container; and for there to be a plurality of containers, each having a different target material; and for the containers to be made from nuclear grade stainless steel. Still further features of the invention provide for the detector to be in selective flow communication with the interior of the or each container; and for the detector to be capable of obtaining discrete samples of alpha particles from the or each container. Yet further features of the invention provide for the detector to be a mass spectrometer; and for the mass spectrometer to be located more than 1 m, preferably more than 10 m, from the source of the neutron radiation.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments of the invention will be described, by way of example only, with reference to the drawings in which: Figure 1 is schematic view of containers for a target material adjacent a radiation source; / Figure 2 is a schematic representation of a neutron detection system; shows a neutron flux radial profile in three different energy ranges from the core to the outside border of the citadel wall of a reactor; shows an MCNP simulation for neutron absorption reaction rate on target material ⁶ Li as a function of neutron energy; shows MCNP simulations for target material ¹² C neutron reaction rates as a function of the neutron energy; shows a schematic representation of a second embodiment of a neutron detection system; is a plot of helium partial pressure as a function of proton beam current strength; and shows a schematic representation of a third embodiment of a neutron detection system.

DETAILED DESCRIPTION WITH REFERENCE TO THE DRAWINGS The method of detecting neutron radiation of the invention is based on measuring alpha particles which are produced as break-up products of target materials that have been subjected to neutron radiation. Knowing the quantity of the target material used and the amount of alpha particles produced enables the neutron radiation to be detected and its flux to be calculated. More specifically, the method includes enclosing a target material which produces alpha particles when exposed to neutron radiation in a container and exposing the container and the target material to radiation. This causes alpha particles to be produced in thermal atomic helium form and these are trapped in the container where they can conveniently be measured by a detector, for example, a helium mass spectrometer. The energy range of the neutron radiation can subsequently be calculated from the alpha particle measurement as indicated above.

The container can either be sealed and the alpha particles be selectively removed and measured, or the container can be in continuous flow communication with the detector. Suitable target materials include lithium ( ⁶ Li), carbon ( ¹² C), beryllium ( ⁹ Be) and boron ( ¹⁰ B). ⁶ Li material has a high neutron capture cross section for thermal neutrons, ^{σ = 942} b at neutron energy of 0.0253 eV. When ⁶ Li is bombarded with low energy neutrons, nearly all of the absorption is proposed to take place by means of the (n, He) reaction. Thus, ⁶ Li or ¹⁰ B, is sensitive to thermal and epithermal neutrons but is insensitive to fast neutrons. The break-up products from lithium therefore give information about the flux of low energy neutrons.

¹² C(n,a) ⁹ Be and ¹² C(n,n')3a reactions occur when the kinetic energy of the incident neutrons is above the threshold energies of 6.18 MeV and 7.96 MeV, respectively. Measuring the counts of the alpha particles from these break-up reactions provide the information about the neutron flux in the region 6.18 MeV and higher. Separate containers, each containing one of the target materials lithium, carbon, beryllium and boron, can thus be located proximal the site of radioactive decay. Lithium is used for the detection of slow or thermal neutrons, carbon and beryllium for the detection of fast neutrons, and boron for the detection of neutrons with epithermal energy and above. The nature of the target material chosen for use will be dependent on the magnitude of the energy of incident neutrons to be detected. The use of separate containers, each having one of lithium, carbon, beryllium and boron permits comprehensive measurement of most of the neutrons emitted by the radioactive decay but in many instances it will be necessary to use only two of the target materials . Further features of the method will become apparent from the following description of a system according to the invention for detecting neutron radiation. Referring to Figure 1 , two sealable, leak-tight stainless steel containers (1) are filled with 10 g of ⁶ Li liquid (3) and 1000 g of 5 μιη graphite powder (4) respectively and the pressure in each lowered to approximately 10 ^"5 mbar. In this specification, "low pressure" shall mean pressures lower than about 10 ^"4 mbar. The containers (1) are then located adjacent a source of neutron radiation (7) to cause the containers (1) and the target materials (3, 4) to become irradiated and the target materials to produce alpha particles in the form of helium gas.

The containers (1) are then sequentially connected to a detector, in this embodiment a helium mass spectrometer (10), through a vacuum line as shown in Figure 2. The mass spectrometer (10) is mounted on the intake flange of a vacuum pump (12), in this embodiment a diffusion pump, and a backing pump (14) evacuates the diffusion pump (12) via valve (16). A container (1) is evacuated via the test gas connection with valve (18) open. When the container (1) is evacuated, it is connected to a backing vacuum (19), or to a tap on the diffusion pump via valve (20). When the pressure reaches 10 ^"5 mbar, the container (1) is opened allowing the gas particles to flow out. The helium present in the residual gas flows counter to the pumping direction through valves (18) and (16) and through the diffusion pump (12) to the mass spectrometer (10) where it is detected. The differing compression ratios of the diffusion pump (12) for helium and air, which differ by multiple powers of ten, are utilized in this regard. While the high compression ratio of the diffusion pump (12) keeps air away from the mass spectrometer (10), the helium arrives there at a relatively high partial pressure. The diffusion pump (12) thus acts as a selective amplifier for helium, hence it is a discrimination method against the gamma ray background because only the break-up particles introduced in the helium mass spectrometer are measured. The neutron radiation can subsequently be calculated from measurement. The high neutron flux detection system above is intended for use with a quasi-monoenergetic neutron beam produced by the ⁷ Li(p,n) ⁷ Be reaction. A 10 μΑ proton beam of incident energy 66 MeV impinging on a 30 mm thick ⁷ Li target produces about 1.84 x 10 ¹¹ n.cm ^"2 . The estimates of the reaction rate for the ⁶ Li and ¹² C materials are determined using the MCNP code. A 10g of 96 % enriched ⁶ Li in a stainless steel container placed 4.0 cm from the neutron source gives a reaction rate of 3.2705x10 ^"5 per 1 starting particle per second, see Figure 4. A natural graphite powder gives a reaction rate of 3.8685x10 ^"6 per 1 starting particle per second for ¹² C(n,n')3a reaction and 2.271 1 x10 ^"5 per 1 starting particle per second for ¹² C(n,a) ⁹ Be reaction as shown in Figure 5, respectively.

The mass spectrometer (10) is located a suitable distance away from the site of radioactive decay (7) so that the radiation emitted therefrom does not substantially damage the electronic components of the mass spectrometer (10). Monte Carlo N-Particle Transport Code (MCNP) simulations predict that a large number of neutrons escape from the reactor to the area between the Reactor Pressure Vessel (RPV) and Reactor Cavity Cooling System (RCCS) as shown in Figure 3. A suitable location for the containers (1 ) is proposed to be on the heat shield of the RCCS.

The neutron detection system of the invention is sensitive, stable, efficient, and capable of wide dynamic range and real-time detection of neutron flux in the high radiation environment of a nuclear reactor. Importantly, the helium mass spectrometer included in the neutron detection system of the invention is able to discriminate between alpha particle emissions and gamma radiation. It will be appreciated that many other systems exist which fall within the scope of the invention, particularly as regards the containers and the method of obtaining and measuring the alpha particles contained therein. For example, referring to Figure 6 in which like features to those of the system above have like numbering, a container (1 ) can be continuously connected under a vacuum to the mass spectrometer (10) to permit continuous flow of the alpha particles to the mass spectrometer instead of in discrete batches. A further feature of the system in this embodiment is the use of a cryogenic trap (25) to further reduce the pressure in the system.

A cryogenic trap is a very low temperature absorber or molecular sieve. In this embodiment it was built from thin copper plates and wires to make a 15 x 5 x 5 cm rectangular box lined with a fine stainless steel mesh. The copper material was used because it has good heat conduction properties and the thin mesh allows gas molecules to enter inside the trap. The trap is placed inside the evacuated pipe and fixed to a 16 mm diameter copper rod from a liquid nitrogen tank.

The molecular sieve has an effective pore opening or kinetic diameter of 5 angstroms (0.5 nm). When the sieve is chilled to the temperature of liquid nitrogen, it absorbs all kinds of molecules with kinetic diameters of less than 0.5 nm. Kinetic diameter is the diameter of a pore needed to let those specific molecules to pass. For example, water has a kinetic diameter of 0.265 nm, smaller than that of hydrogen which is 0.289 nm, 0 ₂ 0.273 nm, N ₂ 0.29 nm, CO 0.376 nm, and saturated hydrocarbon 0.43 nm. These molecules are removed from the system by the trap. The efficiency of the trap was monitored by measuring the total pressure of the system with the mass spectrometer (10), in this embodiment a residual gas analyser (RGA) which covers a mass range from 1 to 200 amu (atomic mass units). The total pressure was reduced by a factor of 100 when the cryogenic trap was chilled. The helium gas produced from the neutron radiation is identified and quantitatively measured by the RGA which is connected directly to a vacuum pump (12). The RGA includes a probe and an electronics control unit (ECU) which contains all the electronics necessary to operate the instrument. The probe equipment consists of three parts: the ionizer (electron impact), the quadrupole mass filter and a dual detector system comprising a Faraday cup and an electron multiplier (EM or SEM) and mounts directly onto the vacuum chamber. The pressure is measured using the Penning or ionization gauges and the Piranni gauges from the RGA. The Penning gauges are particularly attractive because of their wide measurement range (from 10 ^"2 to 10 ^"9 torr).

Put simply, the RGA analyzes residual gases by ionizing a fraction of the gas molecules to positive ions with the ionizer, separating the resulting ions according to their respective masses with the quadrupole mass filter and measuring the ion currents at each mass with the faraday cup detector.

The Faraday cup is connected to an electronic amplifier which typically takes the form of an electrometer amplifier or a charge integration system. The detected charge from ions at the detector cup is amplified and scaled, typically to be displayed as a partial pressure signal by the instrument software. Since the sensitivity is typically around 5 ^"4 A/mbar, it follows that the signal at the Faraday cup will be relatively small, especially at low pressures. This means that at pressures below the 10 ^"10 mbar range, the signal to noise ratio for Faraday cup detection exceeds acceptable levels and prevents reasonable acquisition due to the excessive filtering required to differentiate it. To solve this problem and increase the detection limit of the RGA, an electron multiplier is turned on. The electron multiplier has a coating which readily emits the secondary electrons when a large negative potential is applied to the top of the device. This greatly increases electron signal which is then detected by a secondary Faraday cup at the base of the multiplier and fed to the signal amplifier. The gain is set to 10 ⁴ times and allows the amplifier to detect a greater signal from the secondary Faraday plate for a very low ion current at the top of the device. Signal to noise ratio is improved. Hence, the detection limit of the analyzer is increased.

In this embodiment, all measurements are done with the SEM on and the pressures are amplified by a factor of 10 000 because the partial pressure of helium is below 10 ^"10 mbar.

For experimental purposes, the container (1 ) with 1 kg of 5pm grain ¹² C graphite powder was bombarded with a 66 MeV proton beam. The proton beam with an intensity ranging from 10 ¹¹ to 10 ¹² particles per second was used as neutron flux is much lower than proton flux over a distance of 120cm, the distance of the container from the radiation source (7), and as the proton inelastic scattering cross-section with ¹² C is larger than the neutron inelastic cross-section with ¹² C and thus more helium is produced. The beam current was increased in four uneven steps from 50 to 143 nA. At each step the beam current was stabilized and the response of the detector was measured in a form of a helium pressure in the vacuum system. The neutron detection system showed an immediate response when the beam was switched on and off. For example, when a 50 nA proton beam bombarded the graphite powder, helium atoms were produced and the partial pressure increased from about 1 ^"14 mbar to 4.21 ^"14 mbar. When current was increased to 94 nA the helium partial pressure also increased to 6.3 ^"14 mbar. A plot of helium partial pressure vs beam current is shown in Figure 7 and from this it can be seen that there is an almost linear relationship. In a system such as that shown in Figure 6, real time measurements of neutron flux can thus be made from the measured partial pressure of helium in the system. The system should, however, take into account the slow diffusion of helium through the graphite powder. This was discovered after it was noted that a small amount of helium continued to flow to the RGA after the beam was switched off and a period of time was required to reduce to the background pressure level of around 1 ^"14 mbar.

In a further embodiment the container can be filled with inert gas, such as xenon, instead of a vacuum being formed therein. The alpha particles, as helium, obtained from the target material will be mixed with the xenon which can then be pumped out of the container and cooled to at least -110°C in a first stage separation chamber and down to -200° C for increased purification under vacuum. Upon cooling to these temperatures, the xenon gas will condense while the helium will not. The liquid/solid xenon can then be separated from the gaseous helium and the later directed to the mass spectrometer for detection and quantification.

Also, detection efficiency in the energy range can be increased by the use of multiple detectors or mass spectrometers with different energy detection efficiencies. It is envisaged that a further system offering substantially continuous measuring may be provided by providing a plurality of secondary containers (30) arranged to move in a continuous loop as shown in Figure 8. Like features to those described above also have like numbering in this illustration. These will each be moved between the container (1) with the target sample and the mass spectrometer (10) to transfer the alpha particles in the container (1) to the mass spectrometer (10) for measurement. A vacuum will be provided in each of the secondary containers (30) after transfer of the alpha particles to the mass spectrometer (10) whereafter each moves sequentially to the container (1 ) and is coupled to it to draw the alpha particles therefrom. In this way, continuous discrete readings can be taken in real time with the mass spectrometer removed some distance, preferably 10 to 20 meters, away from the radiation source (7). Clearly, many different configurations of secondary container can be used. It may even be possible to drive the helium along a passage extending between the container and detector using a screw conveyor of suitable construction and configuration. The system of the invention offers much better longevity than existing systems as the active part, the container, is made of very basic material and is only required to maintain a vacuum. The container can have any suitable shape and construction and may be fitted with an access opening and valve. Also, the system can be used to measure neutron flux much stronger than 10 ¹⁴ and is sensitive to a broader range of incident neutron energies than existing systems.

Furthermore, by using appropriate materials, the system of the invention can operate in temperatures up to a few thousand degrees. With stainless steel material it is possible to operate at temperatures between about 1300° and 1400°C, which is suitable for high temperature reactors.