WO2013084004A1

WO2013084004A1 - Neutron source

Info

Publication number: WO2013084004A1
Application number: PCT/GB2012/053060
Authority: WO
Inventors: Rebecca SEVIOUR; Ian Bailey; Hywel OWEN
Original assignee: Lancaster University; University of Manchester
Current assignee: Lancaster University; University of Manchester
Priority date: 2011-12-09
Filing date: 2012-12-07
Publication date: 2013-06-13
Anticipated expiration: 2014-06-09
Also published as: GB201121182D0

Abstract

A neutron source (200), method of generating neutrons and target stage (206) are described. The neutron source includes a source of deuterons (218), an accelerator (230) and a target stage including an oxygen containing target (254) and a moderator (292). The accelerator is in communication with the source and is operable to accelerate deuterons from the source of deuterons and output a deuteron beam (282) of at least 2 MeV. The target stage is in communication with an output of the accelerator, and the oxygen containing target (286) is positioned to be impacted by the deuteron beam output by the accelerator and generate neutrons which are moderated to generate an output neutron beam (294). The target material can be solid carbon dioxide. This can provide a compact neutron source particularly suited to industrial and commercial applications such as isotope production.

Description

Neutron Source

The present invention relates to neutron sources and in particular to methods and apparatus providing a compact source of neutrons.

There are a wide variety of industrial applications for neutrons, such as neutron radiography, security scanning, production of isotopes, for example for medical use, and structural determination. Other applications using neutrons are also known. There are many industrial and medical uses for isotopes. In particular, neutron irradiation is widely used to induce transmutation, and is often provided by the neutron flux of a nuclear fission reactor. It is a strategic weakness of the global healthcare system that much of the global supply of medical isotopes is currently produced by a small number of such nuclear reactors. Consequently, operational problems result in shortages, and there is a need for less capital-intensive means of production. The principal nuclear reactors currently in use are ageing and becoming increasingly unreliable. The costs and other resources involved in building a new nuclear reactor are very large.

At present there are two different general approaches to generating neutrons: fusion based approaches; and fission based approaches. Fusion based approaches are based on nuclear fusion in which nuclear material is combined and which directly or indirectly results in the release of neutrons. Fission based approaches are based on nuclear fission in which nuclear material is split and which directly or indirectly results in the release of neutrons. There are some advantages and disadvantages to both fusion and fission based approaches.

A benefit of fusion based approaches is that the associated apparatus is generally smaller than that used in fission based approaches. However, fusion based approaches can often result in radioactive material, which gives rise to a number of health, safety and security risks. Also, some fusion based approaches require specific starting nuclear materials which themselves can be scarce, hard to obtain or also give rise to safety issues.

Fission based approaches are less likely to result in nuclear materials giving rise to safety concerns. Hence, fission based approaches have a number of advantages compared to fusion based approaches. However, the types of apparatus used in fission reactions, such as cyclotron rings, are very large, expensive and consume large amounts of energy.

A large number of different neutron sources are currently available and they use a wide variety of different nuclear interactions. Also, there are a very large number of nuclear interactions in general which can give rise to neutron release either directly or indirectly. However, many of these are not practicable for industrial applications and are suitable for research purposes only. Various attempts have been made to find non-reactor neutron sources to diversify the supply chain for the production of medical isotopes. US Patent Application Serial No. 10/985,323 proposes using fast neutrons from a suitable proton conversion target which would be moderated to suitable energies to overlap the resonance peaks in the capture cross-section of the parent isotope.

A spallation target could be used at high energy, giving many neutrons per proton (as advocated by Buono et al. ["An accelerator-driven production of Mo-99 at CERN", internal CERN report, Stefano Buono, July 2009 ]) but this technique would require large (~300 m) and expensive (greater than £10 million) accelerator infrastructure.

Fusor reactions are commercially available at low collision energies. For example the reaction T(d,n)₄He produces 14 MeV neutrons using -120 kV deuterons. However, such systems are currently limited by target ablation and have a typical lifetime of 10⁹-10¹² shots (i.e. several hundred hours at 1 kHz repetition rate). Also, tritium is highly radiotoxic resulting in difficult handling and regulatory requirements even with small sample sizes. Other low-energy reactions are also possible but either share the toxicity problems of the D-T reaction, or require impractical energies.

There are a wide variety of known small neutron sources. A first type is based on radioactive isotopes which undergo spontaneous fission and neutron emission. The most commonly used radioactive isotope is Californium-252. Spontaneous fission neutron sources are usually produced by irradiating uranium, or another transuranic element, in a nuclear reactor, where neutrons are absorbed in the starting material and its subsequent reaction products, transmuting the starting material into the spontaneous fission isotope. Such neutron sources are small and initially emit between 10⁷ to 10⁹ neutrons per second, but this neutron output rate drops to half in 2.6 years. Also, a major deficiency with these sources is that they cannot be promptly turned off. A second type is based on radioisotopes which decay when alpha particles impinge on a low atomic weight isotope, such as lithium, beryllium, carbon and oxygen. This nuclear reaction can be used to construct a neutron source by inter-mixing a radioisotope that emits alpha particles such as radium or polonium with a low atomic weight isotope, usually in the form of a mixture of powders of the two materials. Typical emission rates for alpha reaction neutron sources range from 10 to 10 neutrons per second. The useful lifetime for these types of sources is highly variable, depending upon the half-life of the radioisotope that emits the alpha particles. The size and cost of these neutron sources are also comparable to spontaneous fission sources. Usual combinations of materials are plutonium-beryllium (PuBe), americium-beryllium (AmBe), or americium-lithium (AmLi).

A third type is based on radioisotopes which decay with high energy photon interaction and which are co-located with beryllium or deuterium. Gamma radiation with an energy exceeding the neutron binding energy of a nucleus can eject a neutron. Two examples and their decay products are: ⁹Be + >1.7 MeV photon→ 1 neutron + 2 ⁴He; and ²H (deuterium) + >2.26 MeV photon→ 1 neutron + 1H.

A fourth type uses sealed tube neutron generators. These particle accelerator based neutron generators work by inducing nuclear fusion between beams of deuterium and/or tritium ions and metal hydride targets which also contain these isotopes.

There are also a wide variety of medium sized neutron sources. A first type is plasma focus and plasma pinch devices. A plasma focus neutron source produces controlled nuclear fusion by creating a dense plasma within which ionized deuterium and/or tritium gas is heated to temperatures sufficient for creating fusion. A second type of device is a light ion accelerator. Traditional particle accelerators with hydrogen (H), deuterium (D), or tritium (T) ion sources may be used to produce neutrons using targets of deuterium, tritium, lithium or beryllium. Typically these accelerators operate with voltages in the > 1 MeV range. A third type are known as high energy photoneutron/photofission systems. So-called photoneutrons are produced when photons above the nuclear binding energy of a substance are incident on that substance, causing it to undergo giant dipole resonance after which it either emits a neutron (photodisintegration) or undergoes fission

(photofission). The number of neutrons released by each fission event is dependent on the substance. Typically photons begin to produce neutrons on interaction with normal matter at energies of about 7 to 40 MeV, which means that megavoltage photon radiotherapy facilities may produce neutron radiation as well, and require special shielding. In addition, electrons of energy over about 50 MeV may induce giant dipole resonance in nuclides by a mechanism which is the inverse of internal conversion, and thus produce neutrons by a mechanism similar to that of photoneutrons.

A wide variety of large sized neutron sources are also known. Nuclear fission reactors rely on nuclear fission within a nuclear reactor and which produces very large quantities of neutrons and can be used for a variety of purposes including power generation and experiments. Subcritical reactors can be also used. Nuclear fusion systems combine the heavy isotopes of hydrogen, and also have the potential to produce large quantities of neutrons. Small scale fusion systems exist for research purposes at universities and laboratories around the world. A small number of large scale nuclear fusion systems also exist including the National Ignition Facility in the USA, JET in the UK, and soon the recently started ITER experiment in France. High energy particle accelerators provide a spallation source which is a high- flux source in which protons that have been accelerated to high energies hit a target material, prompting the emission of neutrons. Well known examples are the ISIS neutron source and the Spallation Neutron Source. US 5,854,531 describes a storage ring system and method for high yield nuclear production which uses a large 1 lm diameter storage ring into which projectile particles are injected so as to repeatedly travel around the ring and pass through a target until a stable particle beam intensity is reached. An O¹⁶ gas jet can serve as the target and can first act to convert deuterium ions into positively charge deuterons which circle the ring and then re-encounter the O¹⁶ gas jet to produce neutrons. However, the storage ring is large and complex and relies on recirculation of particles around the ring and multiple passes through a target which is itself part of the ring accelerator structure. WO 92/12415 describes a contraband detection system which includes a neutron source for producing a pulsed beam of fast white neutrons having a sufficient energy range whereby removal of neutrons from the beam can be used to determine the presence of contraband indicating elements in a sample object between the source and a detector. The neutron source includes a tandem accelerator for generating a pulsed deuteron beam on the order of 3.0 MeV to 8.0 MeV directed to a target having a composition such that it generates a white neutron beam having energies in the range of about 0.5 MeV to 3.0 MeV. The target can be made of a single material such as carbon or beryllium or have layers of different materials, such as a first layer of gaseous oxygen 16 (or silicon dioxide) and a second layer of carbon 13 which has a high yield of neutrons for deuteron energies from 2.5 MeV down to lower energies.

As well as there being a wide variety of different approaches, for each approach there is a very wide range of potential nuclear materials which can be used. It is hard to predict the behaviour of nuclear materials and which may or may not be suitable owing to their rate of reaction, the rate of neutron production, the energies required for interactions to occur and any reaction products.

However, there are a wide variety of draw backs with many of these approaches which mean that they are not practicable for everyday industrial application, such as their size, cost, energy consumption, experimental nature, use of radioactive, scarce or carefully controlled materials and the attendant security or safety issues arising from use, handling and transportation of such materials. The wide variety of approaches, wide variety of nuclear materials available, large number of drawbacks and unpredictability mean that to date a compact source of neutrons suitable for everyday industrial use is not yet available.

It would therefore be advantageous to be able to provide a compact source of neutrons which does not suffer from one or more of the disadvantages associated with existing approaches to generating neutrons.

A first aspect of the invention provides a neutron source, comprising: a source of deuterons, an accelerator and a target stage including an oxygen containing target. The accelerator can be in communication with the source and can be operable to accelerate deuterons from the source of deuterons and output a deuteron beam of at least 2 MeV. The target stage can be in communication with an output of the accelerator. The oxygen containing target can be positioned to be impacted by the deuteron beam output by the accelerator and generate a neutron beam.

The neutron source may further include a moderator arranged to moderate the neutrons. The moderated neutrons can then form an output neutron beam. The moderator can act to shift the neutron energy spectrum. The moderator can be selected depending on the intended application of the neutron source so that the output neutron beam is suitable for the intended application.

The moderator can be a different and/or separate entity to the oxygen containing target. The oxygen containing target can itself act as the moderator.

The neutron source can further include a transmutation target. The target stage can further comprise the transmutation target and which can be located within the target stage. The transmutation target can be part of a transmutation cell. The transmutation cell can include a moderator. The moderator can act to shift the energy of the neutron. In particular the moderator can act to shift the energy of the neutrons into the resonant part of a transmutation parent isotope's neutron capture cross section. The accelerator can be operable to accelerate deuterons from the source of deuterons and output a deuteron beam of at least 2.5 MeV, preferably at least 2.8 MeV and more preferably at least 3 MeV. The energy of the deuteron beam can be in the range of approximately 2 MeV to approximately 10 MeV, or preferably approximately 2.5 MeV to approximately 8 MeV.

Various types of accelerator can be used. The accelerator can be a travelling wave accelerator, such as, for example, an RFQ accelerator. This can help to provide a source with a small linear size. The oxygen containing target can comprise a fluid. The oxygen containing target can be a gas or a liquid.

The oxygen containing target can comprise a solid. This improves the ease with which the target can be handled.

The oxygen containing target can comprise oxygen or carbon dioxide, and preferably solid carbon dioxide. The neutron source can further comprise a deuteron beam impact position adjuster operable to vary the position that the deuteron beam impacts the oxygen containing target. This can be used to help reduce local heating effects.

The deuteron beam impact position adjuster can move the deuteron beam relative to the oxygen containing target, or vice versa.

The oxygen containing target material can have an oxygen number density sufficient to provide a commercially useful neutron beam strength or power. The oxygen number density can be greater than that of air at STP (standard temperature and pressure) and preferably at least one, two, three, four, or greater, orders of magnitude greater. The oxygen containing target material can have an oxygen number density of at least 1 x 10²³ , preferably at least 1 x 10 m^" , more preferably at least 1 10 m and most preferably at least 1 x 10 m^" . The oxygen containing target material can have an oxygen number density in the range of approximately 1 x 10²³ m^"3 to approximately 1 x 10 m^" , preferably in the range of approximately 1 10 m^" , to approximately 1 x 10 m^"3 and more preferably in the range of approximately 1 x 10²⁶ m^"3, to approximately 1 x 10²⁹ m-³.

The neutron source can be a compact neutron source. For example, the neutron source can have a maximum linear dimension of not more than 5 m.

A second aspect of the invention provides a method for generating neutrons. The method can comprise accelerating deuterons to form a deuteron beam having an energy of at least 2 MeV. The deuteron beam can be impacted with an oxygen containing target. This can cause a nuclear fusion reaction which converts oxygen nuclei into fluorine nuclei and releases free neutrons. The method can further comprise moderating the free neutrons. This can generate an output neutron beam. The energy of the deuteron beam can be at least 2.5 MeV or more preferably at least 3 MeV. The energy of the deuteron beam can be in the range of approximately 2 MeV to approximately 10 MeV, or preferably approximately 2.5 MeV to approximately 8 MeV.

The neutron beam can be a continuous wave beam or a pulsed beam.

Preferred features of the neutron source aspect of the invention can also be preferred counterpart features of the second aspect of the invention.

A third aspect of the invention provides a target stage for a neutron source. The target stage can have a housing. The housing can be of a material capable of reflecting incident neutrons. The housing can have an input window for receiving a beam of deuterons. An oxygen containing target material can be provided within the housing. A moderator can be provided within the housing. The housing can have an output window which is at least partially transparent to a beam of neutrons created by a nuclear fusion reaction between oxygen nuclei of the oxygen containing target material and the beam of deuterons.

The moderator can act to shift the energy of the neutrons. The oxygen containing target can itself act as the moderator.

The target stage can further comprise a transmutation target located within the target stage. The transmutation target can be part of a transmutation cell. The transmutation cell can include a moderator. The moderator can act to shift the energy of the neutrons, and preferably into the resonant part of a transmutation parent isotope's neutron capture cross section. The oxygen containing target material can have an oxygen number density sufficient to provide a commercially useful neutron beam strength or power. The oxygen number density can be greater than that of air at STP (standard temperature and pressure) and preferably at least one, two, three, four, or greater, orders of magnitude greater. The oxygen containing target material can have an oxygen number density of at least 1 10 m^" , preferably at least 1 x 10 m^" , more preferably at least 1 x 10 m^" and most preferably at least 1 x 10²⁸ m^"3

The target can further include a vacuum system operable to reduce the pressure within the target below atmospheric pressure. The vacuum system can be operable to reduce the pressure within the target to below 10^"1 atmospheres and preferably to below or about 10^" ² atmospheres.

The target can further include a cooling system operable to reduce the temperature within the target to below ambient temperature.

The oxygen containing target material can be a gas, a liquid or a solid. The oxygen containing material can be gaseous oxygen. The oxygen containing material is preferably carbon dioxide and most preferably solid carbon dioxide.

A fourth aspect of the invention provides a method for manufacturing a target isotope of an atom, comprising: generating neutrons using the method of the second aspect;

directing the neutron beam at a material including an initial atom; and the neutron beam, or a neutron thereof, causing the initial atom to transmute directly, or indirectly, into the target isotope.

The method aspect of the invention has particular utility in the manufacture of isotopes, for example medical isotopes. An embodiment of the invention will now be described in detail, and by way of example only, with reference to the accompanying drawings in which:

Figure 1 is a graphical representation of the underlying fusion reaction used in the invention; Figure 2 shows a schematic drawing of a neutron source according to the invention; and Figure 3 shows a schematic cross sectional drawing of a detector part of the neutron source illustrated in Figure 2 also according to an aspect of the invention. Similar items in different Figures share common reference signs unless indicated otherwise.

With reference to Figure 1, there is shown a graphical representation of the nuclear fusion process 100 underlying the invention. The invention is based on colliding sufficiently energetic deuterons (the nucleus of a deuterium atom) with oxygen nuclei which transmute into fluorine nuclei with release of neutrons. As shown in Figure 1 , a deuteron 102, being a bound proton and neutron (and hence positively charged), is accelerated up to an energy of at least approximately 2.5 MeV. The energetic deuteron 102 is then collided with an ¹⁶0 nucleus 104 (comprising eight protons and eight neutrons) in an oxygen rich target material. It is understood that that the oxygen nucleus absorbs the proton of the deuteron and transmutes 106 into a F nucleus 108 (compnsing nine protons and eight neutrons) and the released free neutron 109. The ¹⁷F nucleus 108 fusion product is unstable, having a half-life of approximately 90 s, and decays 110 by gamma emission 112 back into a stable ^I60 nucleus 114.

An extensive search of the scientific literature has revealed a small number of papers relating to oxygen fusion reactions. S.B. Welles, Phys. Rev. 59, 679, 1941, investigated deuteron bombardment of oxygen, but reported only a (deuteron, gamma) reaction with no production of neutrons. Studies of the oxygen neutron capture cross section have been published (Newson, Physical Review 11, 1937; Gruhle et al. , Nuclear Physics A 186, 1972; Haddou, Berrada, Pai, Journal of Radio Analytical and Nuclear Chemistry, Vo. 102, 159-175, 1986) which appear to show that the neutron capture cross section grows from above 2 MeV and starts to become significant around approximately 2.5 MeV before peaking about 3 MeV. However, there has never been any suggestion or teaching that ¹⁶0 might be a suitable target material for a compact, commercially useful neutron source. The applicant has discovered that unexpectedly, out of all the potential nuclear materials available, the nuclear properties of ¹⁶0 are suitable for a deuteron fusion approach to neutron generation. A significant benefit of the invention is that the fusion reaction does not produce any dangerous, harmful or weaponisable fusion products, merely the desired neutrons 109 and the stable ^I60 nucleus.

While not wishing to be bound by theory, the above is believed to reflect the underlying physics of the apparatus and method of the invention now to be described.

With reference to Figure 2, there is shown a schematic diagram of a compact neutron source 200 according to the invention. The illustrated embodiment would have a length of approximately four metres and hence is sufficiently small to be of real world industrial utility as it can be located in a room rather than requiring a whole building. The neutron source 200 comprises three general stages: a source of deuterons 202; an accelerator 204 stage; and a target stage 206.

While the source 202 and accelerator 204 are required, they can in some embodiments be largely generic components which simply provide deuterons at a suitable energy for the target stage 206 and it is the target stage which is particularly important to the invention. The source 202 and accelerator can take many forms. For example, the accelerator could be: a mini-cyclotron; an electrostatic accelerator; a linear accelerator; or an RFQ. Hence, the invention is not limited to the specific source and accelerator of the embodiment described below.

The deuteron source stage 202 is based on a Taylor design which is generally known to persons of ordinary skill in the art. Such sources can efficiently produce a high current density deuteron beam, with a high reliability, long life and low maintenance

requirements. For example, Taylor design ion sources are described generally in

T.Taylor and J.F.Mouris, Nucl. Instr. Meth. Phys. Research, 1993, A336: 1-5. The deuteron source stage 202 includes a source of hydrogen gas 210, such as a cylinder, which will naturally include some proportion of the deuterium isotope. The hydrogen source 210 is connected via a conduit, such as a hose or similar, to an ion source 218 which includes a plasma chamber and permanent magnets located about the plasma chamber which generate a magnetic field of approximately 0.873 T within the plasma chamber and which define an electron cyclotron resonant frequency of approximately 2.45 GHz. A microwave source 212 is also provided and is coupled to the plasma chamber by a curved waveguide 216 or similar. The microwave source is operated to apply an RF field to the hydrogen gas within the plasma chamber of the ion source at the resonant frequency of the plasma chamber which acts to strip the outer electrons to produce, hydrogen, deuteron and triton nuclei. The stripped electrons are directed to the walls of the ion source 218 by the magnetic field component of the RF field and grounded away by a ground connection 214.

By way of example, a 2.45 GHz RF source 212 providing 200 W can generate a 10 mA deuteron beam. Increasing the power of the RF source to 1 kW would increase the deuteron beam current to approximately 44 mA

The deuteron source stage also includes an electrostatic accelerating section (not shown) and an ion selector component 220 at an output of the ion source to filter out foreign ions.

The ion selector component 220 is provided at an output of the ion source and acts to select only those ions having the specific mass/charge ratio of a deuteron ion in order to remove any hydrogen, triton or other undesired nuclei from the ion beam. The selector works similarly to selectors used in mass spectrometers and applies mutually

perpendicular magnetic and electric fields to the direction of travel of the ion beam. The magnetic and electric field strengths are set so that only ions having the desired mass/charge ratio of a deuteron pass out through the output of the beam selector.

The overall length of the deuteron source stage can be about 0.5m.

The output of the beam selector is connected to a low energy beam transport (LEBT) system in the form of a stainless steel conduit connected to the main accelerator stage 204. The LEBT 224 also electrostatically accelerates the deuteron beam to

approximately 60 keV and uses electrostatic focussing components to centre the beam before it is passed to the input of the main accelerator stage 204. Using the LEBT to accelerate the deuteron beam to a few tens of keV reduces the load placed on the subsequent main accelerator stage 204. Switching and/or modulation of the deuteron ion beam may be applied at this stage if required for the specific application of the neutron beam. For example, in certain security applications, pulses of neutrons are used rather than a continuous stream. As illustrated by dashed line 222 the deuteron source components can be located within a vacuum vessel so as to provide a low pressure environment of approximately 10^"6 to 10^"9 Torr. The main accelerator stage 204 includes a Radio Frequency Quadrupole (RFQ) linear accelerator 230 which provides a travelling wave linear accelerator for accelerating the deuteron beam up to an energy above approximately 2 MeV, and preferably about 3 to 4 MeV. The use and construction of RFQ linear accelerators is generally known to a person of ordinary skill in the art. The RFQ linear accelerator 230 comprises four metallic vanes running down its longitudinal axis and parallel to the beam direction. The vanes are arranged at approximately 90° intervals about the central beam path. The vanes each have a sinusoidal or cosinusoidal form and operate in pairs of opposing electrical polarity. The vanes are typically made of copper. By applying time varying voltages to the vanes of appropriate magnitude and frequency, a time varying electric field is generated in the vicinity of the central beam path which creates a travelling wave of electric field which accelerates the deuterons.

Compared to other technologies, accelerators based on RFQs offer many advantages (such as simultaneous acceleration and focusing). RFQs can handle intense beams up to 100 mA, and, because of bunching ability, when a DC beam is directly injected, high beam transmission can be obtained. This gives RFQs a flexibility to operate over a very wide parameter space. RFQs have demonstrated the capability of accelerating deuterons up to 8 MeV.

The preferred operating parameters for the RFQ linear accelerator are: a deuteron input energy of 60 keV, a deuteron output energy of 4 MeV, a deuteron beam current of 10 mA, a duty factor 6-100%, an RF frequency of 352.2 MHz, a maximum accelerating gradient, Es, of 33 MV/m, a deuteron beam power of 170 kW, an RFQ power of 420 kW, a total power consumption of 590 kW, and a total length of the entire neutron source of approximately 4.75 m. As illustrated in Figure 2, the main RFQ linear accelerator 230 is connected to an RF power supply 234 which supplies radio frequency (RF) voltage to the vanes of the RFQ via co-axial cables 236, 238 terminated in RF couplers. The RFQ is located within a vacuum vessel 240, which may be made from a suitable material such as stainless steel. A pump is connected to the vacuum vessel 240 and is operable to pump on the interior of the vacuum vessel to reduce the pressure to about 10^"8 Torr. The length of the LEBT and RFQ linear accelerator can be approximately 3 m.

The output of the main accelerator stage 204 is connected by a curved stainless steel beam pipe 250 to a target stage containing an oxygen rich target material. An output end of the beam pipe 250 is flared as illustrated in Figure 2. The curved beam pipe 250 is located in a bending magnetic field provided by a bending magnet (not shown) in the form of a plurality of suitably arranged solenoids. The target stage can be housed by shielding 260 which may be concrete or graphite or any other suitable material. The target part 254 of the target stage 206 is shown in cross section in Figure 3.

The accelerator 230 directs energetic deuterons into an oxygen rich target material containing oxygen atoms, so that neutrons are generated via the fusion reaction

1⁶0(d,n)¹⁷F. This reaction has a cross-section with a peak around 3 MeV at

approximately 0.7 barn, and its neutron spectrum is quasi-monochromatic.

As a general indication of the neutron yield that might be expected from an oxygen- bearing target, the neutron yield of deuterons with energies of a few MeV incident on a thin water target is estimated to be of order 10¹⁵ neutrons per second per centimetre of water target per mA of deuteron beam.

Suitable oxygen-bearing target materials include water, oxygen gas as well as solid carbon dioxide (colloquially referred to as "dry ice"). In general there exists the possibility of by-production of other isotopes if elements other than oxygen are present in the oxygen rich target material. In the case of a water target, there may be by-production of heavy water and the possibility of trace amounts of tritium over long periods of operation. It is therefore desirable to minimise the dwell time of hydrogen within the target in this particular case. The fusor target geometry is designed to maximise neutron production efficiency while minimising unwanted activation of adjacent accelerator components.

The rate of production of neutrons depends on the number density of oxygen atoms in the oxygen rich target material and is preferably in the range exemplified by water (liquid or solid) or solid carbon dioxide. For example, the oxygen atom number density in liquid water is approximately 3 x 10 28 m^" 3 and in dry ice approximately 4 x 1028 m^" 3. Hence, various oxygen rich target materials can be used having oxygen atom number densities generally in the range of approximately 1 x 10 26 m^" 3 to approximately 1 x 1029 m^" 3 , wi *th number densities toward the higher end of that range being more preferred owing to the increased neutron beam power available.

With reference to Figure 3 there is shown a cross section 300 through an example target stage 206 in which solid carbon dioxide is used as the oxygen atom rich target material. Although as discussed above, other materials can be used for the oxygen rich target material, such as water and oxygen gas. It will be appreciated that the target stage design will be modified to incorporate a liquid target material (such as water) or a gaseous target material (such as oxygen gas, which may also be pressurised).

The target stage 254 has a main body 270 which acts as a reflector to prevent neutrons escaping from the target stage and which can be made of graphite. The flared end 252 of the beam tube 250 is attached to an entrance aperture 272 of the reflector body. A thin sheet of Mylar (a registered Trade Mark in some countries) material, or similar, provides a deuteron transparent window 274 into the target stage. The window can have a thickness of less than 1 mm. A coil 278 is arranged adjacent to the flared end of the beam tube connected to a controllable current source 280. Coil 278 is arranged to produce a magnetic field perpendicular to the deuteron beam 282 so as to change the position at which the deuteron beam impacts the window 274. By using current controller 280 to supply a suitable time varying current to the coil 278 a time varying magnetic field is applied to the deuteron beam and causes the beam to move so as to trace out a circular path over the plane of the window (which extends in a direction perpendicular to the sheet of drawings). This helps to reduce heating effects which would otherwise be caused were the position of the deuteron beam to remain static.

Within a chamber 284 defined by the housing 270 and window 274 is located a sheet of solid carbon dioxide 286 of thickness approximately 2 mm which provides the oxygen rich target material. An output window 288 in the form of a sheet of aluminium is provided covering an output aperture 290 of the housing 270. As illustrated in Figure 3, a moderator and/or transmutation cell 292 may optionally be provided between the oxygen rich target material 286 and output window 288. The shape of the output neutron beam 294 is shown in Figure 3 and has the general form of a lobe. A channel 296 is provided in a side wall of the housing 270 to provide communication with the chamber 284 and is connected to a pump 298, such as a roughing pump. The pump 298 is operable to pump on the chamber to reduce the pressure within the chamber to approximately 10^"2 atmospheres. This helps to reduce the pressure differential across window 274 as the pressure within beam tube 252 caused by vacuum pump 242 will typically be of order 10^"6 to 10^"7 atmospheres. A plurality of cooling channels 300 extend throughout the main body of the housing as part of a cooling system including a source of coolant 302 which pumps coolant along the channels in order to maintain the temperature inside the chamber 284 below the sublimation temperature of solid carbon dioxide. This also helps to reduce heat loading of the stage caused by the incoming energetic deuteron beam.

As discussed above, depending on the application of the neutron source, a moderator 292 may be provided. A medium-Z material may be used for the moderator in order efficiently to shift the neutron spectrum. A suitable material for the moderator can be copper. Different moderation assemblies may be used depending on the required application of the neutrons. A further advantage of using carbon dioxide as the oxygen rich target material is that the carbon atoms present provide a natural moderator for the neutrons. Therefore, the oxygen rich target material can itself be engineered to act as a moderator and provide moderation of neutrons to desired application energies.

In the application of the neutron source to neutron-induced transmutation, the neutrons may be used in an adjacent apparatus, or a suitable transmutation target 292 may be co- located with the target stage. The transmutation cell includes a compact neutron moderator assembly which is used to shift the fast neutron energies into the resonant part of the transmutation parent isotope's capture cross-section. The neutrons interact with this parent isotope to generate product isotopes. For example the neutrons may be conveniently used for the production of technetium for medical purposes, via the molybdenum/technetium route, as is well known to those skilled in the art.

Different deuteron energies may be generated by the accelerator depending on the application of the neutron source. Deuterons can be accelerated to energies around 3 MeV which corresponds to the neutron absorption peak cross-section of oxygen. In another approach, higher energy deuterons are used to produce neutrons with higher energies and in greater numbers, which are then moderated to produce more neutrons in the desired energy range. Such moderation is conveniently achieved using light water, as is generally well known in the art. Hence, the accelerator is preferably capable of accelerating deuterons to energies above 3 MeV and up to around 8 MeV.

Claims

CLAIMS:

1. A neutron source, comprising:

a source of deuterons;

an accelerator in communication with the source and operable to accelerate deuterons from the source of deuterons and output a deuteron beam of at least 2 MeV; and

a target stage in communication with an output of the accelerator, wherein the target stage includes an oxygen containing target positioned to be impacted by the deuteron beam output by the accelerator to generate neutrons and a moderator arranged to moderate the neutrons and generate an output neutron beam.

2. A neutron source as claimed in claim 1 , wherein the oxygen containing target itself acts as the moderator.

3. A neutron source as claimed in claim 1 or 2, wherein the target stage further comprises a transmutation target located within the target stage.

4. A neutron source as claimed in claim 3, wherein the transmutation target is part of a transmutation cell and the transmutation cell includes the moderator, which acts to shift the energy of the neutrons.

5. The neutron source of any preceding claim, wherein the moderator acts to shift the energy of the neutrons into the resonant part of a transmutation parent isotope's neutron capture cross section.

6. A neutron source as claimed in any preceding claim, wherein the accelerator is a travelling wave accelerator.

7. A neutron source as claimed in any preceding claim, wherein the oxygen containing target comprises a fluid.

8. A neutron source as claimed in any of claims 1 to 6, wherein the oxygen containing target comprises a solid.

9. A neutron source as claimed in claim 8, wherein the oxygen containing target comprises carbon dioxide.

10. A neutron source as claimed in any preceding claim, wherein the neutron source further comprises a deuteron beam impact position adjuster operable to vary the position that the deuteron beam impacts the oxygen containing target.

11. A neutron source as claimed in claim 10, wherein the deuteron beam impact position adjuster moves the deuteron beam relative to the oxygen containing target.

12. A neutron source as claimed in any preceding claim, wherein the oxygen containing target has an oxygen number density of at least 1 x 10 23 m^" 3 , preferably at least 1 x 10 m , and more preferably at least 1 x 10 m^" .

13. A neutron source as claimed in any preceding claim, wherein the neutron source is a compact neutron source and has a maximum linear dimension of not more than 5 m.

14. A method for generating neutrons comprising:

accelerating deuterons to form a deuteron beam having an energy of at least 2 MeV;

impacting the deuteron beam with an oxygen containing target, wherein a nuclear fusion reaction converts oxygen nuclei into fluorine nuclei and free neutrons are released; and

moderating the free neutrons to generate an output neutron beam.

15. The method of claim 14, wherein the oxygen containing target itself moderates the free neutrons.

16. The method of claim 14 or 15, further comprising transmutating a parent isotope to generate a product isotope.

17. The method of claim 16, wherein moderating the free neutrons shifts the energy of the neutrons.

18. The method of any of claims 14 to 17, wherein moderating the free neutrons shifts the energy of the neutrons into the resonant part of a transmutation parent isotope's neutron capture cross section.

19. A target stage for a neutron source, comprising:

a housing of a material capable of reflecting incident neutrons;

an input window of the housing for receiving a beam of deuterons;

an oxygen containing target material within the housing;

a moderator within the housing; and

an output window of the housing transparent to a beam of neutrons created by a nuclear fusion reaction between oxygen nuclei of the oxygen containing target material and the beam of deuterons.

20. A target stage as claimed in claim 19, wherein the oxygen containing target itself acts as the moderator.

21. A target stage as claimed in claim 19 or 20, wherein the target stage further comprises a transmutation target located within the target stage.

22. A target stage as claimed in claim 21, wherein the transmutation target is part of a transmutation cell and the transmutation cell includes the moderator, which acts to shift the energy of the neutrons.

23. A target stage as claimed in any of claims 19 to 22, wherein the moderator acts to shift the energy of the neutrons into the resonant part of a transmutation parent isotope's neutron capture cross section.

24. A target stage as claimed in any of claims 19 to 23, wherein the oxygen containing target material has an oxygen number density of at least 1 x 10²³ m^~3, preferably at least 1 x 10²⁶ m^"3, and more preferably at least 1 x 10²⁸ m^"3.

A target as claimed in any of claims 19 to 24, and further including a vacuum operable to reduce the pressure within the target below atmospheric pressure.

26. A target as claimed in any of claims 19 to 25, and further including a cooling system operable to reduce the temperature within the target to below ambient temperature.

27. A target as claimed in any of claims 19 to 26, wherein the oxygen containing target material comprises carbon dioxide.

28. A method for manufacturing a target isotope of an atom, comprising:

generating neutrons using the method of claim 14;

directing the neutron beam at a material including an initial atom; and the neutron beam causing the initial atom to transmute directly, or indirectly, into the target isotope.