DEVICE FOR THE PRODUCTION, MODERATION AND CONFIGURATION OF NEUTRON BEAMS FOR NEUTRON CAPTURE THERAPY

Abstract

The invention provides a device for the production, moderation and configuration of a neutron beam, comprising: an inlet opening (1) through which a proton beam is directed; a target (2) against which the proton beam is accelerated in order to generate neutrons; a moderator (3) to bring the neutrons to energies of the epithermal range; a reflective cover (4) surrounding the moderator (3); a filtration stage (5); an outlet opening (6) for the neutron beam, and a shield (7) to suppress the neutrons and gamma-radiation that do not exit the device via said outlet opening. The filtration stage (5) comprises at least three layers to filter respectively: rapid neutrons, thermal neutrons and gamma-radiation The invention is of use in neutron capture therapies, and more specifically, in boron therapies.

Claims

1. A device for the production, moderation and configuration of a neutron beam, based on the production of neutrons from a proton beam, comprising: an inlet opening (1) through which the proton beam is directed; a target (2) arranged in the path of direction of the proton beam in order to nuclearly interact with said beam and produce a neutron beam which defines a main axis of propagation; a cooling module for cooling the target (2); a moderator (3) contiguous to the target (2), with a thickness (W2) from the target (2) and in the direction of the beam to the filtration stage (5), of between 19.62 and 23.98 cm, where the neutrons produced in the nuclear interaction are moderated to energies of the epithermal range, a reflective cover (4) surrounding the moderator (3) to redirect diverted neutrons to the main axis and increase the epithermal neutron flux; an outlet opening (6) through which the neutron beam exits; and a shield (7) arranged around the outlet opening (6) to suppress the neutrons and gamma-radiation that do not exit the device via said outlet opening (6); and a filtration stage (5) contiguous to the outlet opening (6) and also surrounded by the reflective cover (4), to filter the neutron beam before it exits the device via the said outlet opening (6); where said filtration stage (5) comprises at least three layers: a rapid neutron filtration layer, a thermal neutron filtration layer and a gamma-radiation filtration layer.

2. The device according to the preceding claim, wherein the total thickness of the moderator (L4+L5) is comprised between 24.64 and 36.96 cm, covering the target and part of the tube of the accelerator to the inlet opening.

3. The device according to the preceding claim, wherein the moderator (3) is manufactured with at least one of the following materials: graphite, D.sub.2O, AlF.sub.3 CaF.sub.2, Li.sub.2CO.sub.3, MgF.sub.2, Al.sub.2O.sub.3, or combinations thereof.

4. The device according to the preceding claim, wherein the moderator (3) is manufactured with MgF.sub.2.

5. The device for the production, moderation and configuration of a neutron beam according to any of the preceding claims, wherein the rapid neutron filtration layer comprised in the filtration stage has a thickness (L1) of 1.00 cm ±15%.

6. The device for the production, moderation and configuration of a neutron beam according to any of the preceding claims, wherein the thermal neutron filtration layer comprised in the filtration stage (5) has a thickness (L2) of 0.20 cm ±20%.

7. The device for the production, moderation and configuration of a neutron beam according to any of the preceding claims, wherein the gamma-radiation filtration layer comprised in the filtration stage (5) has a thickness (L3) of 1.00 cm ±15%.

8. The device according to any of claims 5 to 7, characterized in that the rapid neutron filtration layer comprises Al, Fe or Ni.

9. The device according to the preceding claim, characterized in that the rapid neutron filtration layer is manufactured with Al.

10. The device according to any of claims 5 to 9, characterized in that the thermal neutron filtration layer comprises .sup.10B, .sup.6Li, Gd, Cd or LiF.

11. The device according to the preceding claim, characterized in that the thermal neutron filtration layer is manufactured with LiF.

12. The device according to any of claims 5 to 11, characterized in that the gamma-radiation filtration layer comprises Pb or Bi.

13. The device according to the preceding claim, characterized in that the gamma-radiation filtration layer is manufactured with Bi.

14. The device according to any of claims 5 to 13, wherein the gamma-radiation filtration layer is arranged last according to the direction of travel of the neutron beam produced.

15. The device according to the preceding claim, wherein the rapid neutron filtration layer, the thermal neutron filtration layer and the gamma-radiation filtration layer are arranged in that order according to the direction of travel of the neutron beam produced.

16. The device for the production, moderation and configuration of a neutron beam according to any of the preceding claims, wherein the reflective cover (4) surrounding the moderator (3) to redirect diverted neutrons to the main axis, has a thickness (L6) of 25.00 cm ±40% from the moderator to the inlet opening, in the direction of the beam.

17. The device according to any of the preceding claims, wherein the reflective cover (4) comprises at least one of the following materials: Ni, Pb, BeO or Bi.

18. The device according to the preceding claim, wherein the reflective cover (4) is manufactured with Pb.

19. The device according to any of the preceding claims with an outlet opening having a diameter A±2δ, having the following radial dimensions: Diameter (Ø2) of the rear part of the moderator: A±2δ+36 cm ±10% Diameter (Ø3) of the cover (4): A±2δ+106 cm ±15% Diameter (Ø4) of the device: A±2δ+116 cm ±20% Inner radius (R1) of the collimator of the front shield (7): (0.5 A±δ+3 cm ±10%) Outer radius (R2) of the collimator of the front shield (7): (0.5 A±δ+22.5 cm ±50%).

20. The device according to any of the preceding claims, wherein the target (2) comprises .sup.7Li and the nuclear interaction with the incident proton beam is .sup.7Li(p,n).sup.7Be.

21. The device according to any of the preceding claims, wherein the outlet opening (6) for the neutron beam comprises at least one section with one of the following geometries: cylindrical, conical, prismatic, truncated pyramid.

22. The device according to any of the preceding claims, wherein the outlet opening (6) for the neutron beam comprises a movable closure to stop irradiation.

23. The device according to any of the preceding claims, wherein the shield (7) arranged around the outlet opening (6) for the neutron beam comprises at least one of the following materials: LiF, .sup.6LiF, B.sub.4C, polyethylene, Pb, Bi.

24. The device according to any of the preceding claims, for use thereof in boron neutron capture therapy.

Description

DESCRIPTION OF THE FIGURES

[0044] A set of figures which are integral part of the description and illustrate a preferred embodiment of the invention is provided to complete the description of the invention. Said figures should be interpreted in a non-limiting illustrative manner and are described in detail below.

[0045] FIG. 1 shows the preferred implementation of the invention by means of a two-dimensional axial section in which the main dimensions of the device, as well as the materials making up same, are indicated.

[0046] FIG. 2 illustrates a three-dimensional view of the invention shown in FIG. 1, with a radial section to enable viewing the distribution of materials and layers in the inside thereof.

[0047] FIG. 3 shows the spectrum of neutrons at different points of the invention, from the Li source to the outlet opening, in order to demonstrate how the suitable selection of materials minimizes the thermal neutron and rapid neutron fluxes without reducing epithermal neutrons and reaching spectral maximum at 2-3 keV. The epithermal range (0.5 eV-10 keV) is indicated in the shaded region.

[0048] FIG. 4 depicts, in logarithmic scale, the final spectrum of the radiation obtained in the outlet opening of the invention, in comparison with two of the reference BNCT treatment facilities: FiR-1 in Helsinki (Finland) and C-BENS in Kyoto (Japan). The epithermal range (0.5 eV-10 keV) is indicated in the shaded region.

[0049] FIG. 5 illustrates, in linear scale, the final spectrum of the radiation obtained in the outlet opening of the invention, in comparison with the same facilities of FIG. 4. The epithermal range (0.5 eV-10 keV) is indicated in the shaded region.

[0050] FIG. 6 shows, in logarithmic scale, a lateral profile of the (epithermal, thermal and rapid) neutron flux and that of gamma-radiation, in the radial direction right at the outlet opening of the invention.

[0051] A series of reference numbers, corresponding to the following elements, accompany the mentioned figures: [0052] (1) Inlet opening [0053] (2) Target [0054] (3) Moderator [0055] (4) Reflective cover [0056] (5) Filtration stage [0057] (6) Outlet opening [0058] (7) Shield [0059] (W1) Thickness of air in the direction of the beam, from the last filtration layer (5) to the inlet opening (1). [0060] (W2) Thickness of the moderator (3) in the direction of the beam, from the target (2) to the filtration stage (5). [0061] (L1) Thickness of the rapid neutron filtration layer [0062] (L2) Thickness of the thermal neutron filtration layer [0063] (L3) Thickness of the gamma-radiation filtration layer [0064] (L4) Thickness of the anterior or front part of the moderator in the embodiments in which the diameter thereof is reduced as it approaches the outlet opening. [0065] (L5) Thickness of the rear part of the moderator in which the diameter (∅3) thereof remains constant. [0066] (L6) Thickness of the cover (4) from the moderator (3) to the inlet opening, in the direction of the beam. [0067] (∅2) Diameter of the rear part of the moderator [0068] (∅3) Diameter of the cover (4) [0069] (∅4) Diameter of the device, including the side shield (7) [0070] (R1) Inner radius of the collimator of the front shield (7) [0071] (R2) Outer radius of the collimator of the front shield (7)

DETAILED DESCRIPTION OF THE INVENTION

[0072] FIG. 1 shows a preferred implementation of the invention which has axial symmetry and a cylindrical-conical geometry. First, it is required to accelerate a proton beam by means of an accelerator (not shown in the figure) up to 2.1 MeV (however, other values can also be used), and by directing said proton beam through the inlet opening (1) or a directing tube of the invention, all the IAEA recommendations are met. At this energy of the proton beam, the neutrons generated by the invention have a mean energy of 108.4 keV and a maximum energy of 350.4 keV. Next, a proton beam bombards a target (2) which, in this case, is a lithium sheet (.sup.7Li) located at the end of the directing tube of the accelerator, and neutrons are produced by means of a nuclear reaction .sup.7Li(p,n).sup.7Be. Said proton beam is oriented according to a main axis. However, other preferred embodiments of the invention may involve different nuclear reactions to generate neutrons, for example, .sup.9Be(p,n). The invention also comprises a cooling module for cooling the target (2) which is not shown in the figures but is necessary in order to prevent said target (2) from melting due to the high temperatures that can be reached.

[0073] Surrounding the lithium target (2) and extending in the front direction and to the sides, there is placed the core of the moderator (3) with a thickness (W2) in said front direction, from the target (2) to the filter (5), of 21.80 cm ±10%, preferably 21.8 cm. Preferably, the moderator will be manufactured with MgF.sub.2. Other materials which can be used as the moderator (3) are combinations of Al, Mg or Ca metals with F, as well as carbon in the form of graphite, or water (H.sub.2O or heavy water, D.sub.2O). These materials are chosen based on their neutron interaction properties. Therefore, isotopes having a large elastic cross-section and a small absorption cross-section are required. Neutrons can lose energy gradually by means of elastic collisions. This energy loss by collision is greater for lightweight isotopes, such as hydrogen or deuterium, or carbon. The suitability of the elements Al, Mg, Ca or F is mainly due to their energy resonances of the range of tens to hundreds of keV, which allows reducing the amount of high-energy neutrons which transition to the epithermal range through moderation. MgF.sub.2 is chosen over other materials due to its larger elastic cross-section, while the production of gamma-radiation per capture is lower than in other materials such as water. Furthermore, the nuclei are not as lightweight as to lose a large proportion of energy in each collision, which allows the neutrons to lose their energy much more gradually and controllably. Other metals such as Al or Ca have higher energy resonances, so the use thereof as low-energy sources is not as suitable as Mg. In other preferred implementations of the invention, the moderator can be manufactured with at least one of the following materials: D.sub.2O, Al.sub.2O.sub.3, Li.sub.2CO.sub.3, or others having equivalent or similar properties known by one skilled in the art. In FIG. 1, the moderator (3) includes a cylindrical section and another conical section. However, in other preferred embodiments the moderator (3) comprises at least one section or profile with one of the following geometries: cylindrical, prismatic, truncated pyramid, conical.

[0074] Preferably, there is arranged around the moderator (3) a reflective cover (4), made with a material with a large elastic collision cross-section and at the same time a high mass number, such that the neutrons do not lose more energy in these collisions, and furthermore the probability of backward collision is higher, which allows neutrons to be recovered towards the core of the moderator. The reflector therefore redirects the diverted neutrons to the main axis of the neutron beam, which increases the epithermal neutron flux. In its rear part, from the moderator to the inlet opening, the cover (4) has a thickness (L6) of 25.00 cm ±40% in the direction of the beam.

[0075] The moderator also covers the target and part of the tube of the accelerator to the rear part (beam inlet). In a preferred embodiment, the total thickness of the moderator (L4+L5) is comprised between 24.64 and 36.96 cm.

[0076] Furthermore, the material of the reflective cover (4) must absorb the gamma-radiation generated in the moderation process. For this reason, materials with a high atomic number are ideal for this task. In this way, the optimal materials for this function are lead and bismuth. Pb has an elastic collision cross-section that is larger than Bi, such that it has been used as a reflector in general, except in the front direction, where the divergence of the beam is a factor to be considered. In the radial as well as rearward directions, increasing the thickness (L6) of the Pb layer allows recovering more neutrons which have escaped the core of the moderator (3). This relationship is maintained until a maximum thickness, at which the increase in epithermal neutrons becomes saturated, is reached.

[0077] A filtration stage (5), also surrounded by the reflective cover (4), is arranged after the core of the moderator (3). Said filtration stage (5) functions to provide an additional filtration of the spectrum of neutrons which do not contribute to BNCT treatment (rapid and thermal neutrons), as well as to reduce the gamma-radiation minimally affecting the epithermal neutron flux. Preferably, the mentioned filtration stage (5) comprises at least these layers: [0078] Rapid neutron filtration layer: this layer complements moderation and fundamentally performs the function of filtering the spectrum of neutrons obtained by selectively removing some rapid neutrons due to their resonances. The thickness of this layer (L1) is 1.00 cm ±15%. In FIG. 1, this layer consists of an aluminum sheet and its thickness is 1 cm. However, other elements such as Fe or Ni may be suitable. [0079] Thermal neutron filtration layer: the purpose of this layer is to suppress the thermal neutrons of the beam. The moderation process subsequently produces a considerable amount of thermal neutrons which must be removed from the beam prior to the use thereof in BNCT treatment. Li is a material with a large thermal capture cross-section. This characteristic makes it an optimal thermal neutron filter.

[0080] The thickness of this layer (L2) is 0.20 cm ±20%. In particular, in the implementation of FIG. 1, this layer is a LiF sheet of only 2 mm thick but allows removing more than half of the thermal neutron flux reaching it. Other materials which can also be used for this thermal neutron filtration layer are B and Cd which, though having larger capture cross-sections, in exchange also generate gamma-radiation and require a larger amount of Pb or Bi in the reflective cover (4) in order to remove same. [0081] Gamma-radiation filtration layer: this layer is designed for mitigate gamma-radiation but by affecting the epithermal neutron flux as little as possible. The thickness of this layer (L3) is 1.00 cm ±10%. In FIG. 1, this layer consists of a Bi sheet of 1 cm thick, which allows obtaining a greater neutron flux with a lower divergence.

[0082] Preferably, the gamma-radiation filtration layer is arranged last according to the direction of travel of the neutron beam produced. Even more preferably, the thermal neutron filtration layer and the gamma-radiation filtration layer are arranged in that order according to the direction of travel of the neutron beam produced.

[0083] It can be seen in FIG. 1 that the filtration stage (5) comprises the aluminum, LiF and Bi layers in said order according to the direction of travel of the neutron beam produced. However, the mentioned layers can be arranged in any technically possible order, and can even be interchanged with part of the block of the moderator. Other preferred embodiments of the invention comprises a plurality of one or more of the layers comprised in the filtration stage (5).

[0084] As a result of the filtration stage (5), the spectrum of the neutrons obtained is suitable for BNCT treatment (the rapid and thermal neutrons have been filtered, allowing the thermal neutron flux to be kept below 5% of the total flux, and also residual gamma-radiation has been removed).

[0085] Next there is arranged an outlet opening (6) which determines the shape and focalization of the beam. In particular, it has a cylindrical-conical section, as shown in FIG. 1. To that end, said opening comprises a collimation cone, also preferably made with Pb, which makes it possible to shape the neutron beam taking into account the inclination of its generatrix. A too large inclination (i.e., a very elongated collimation cone) reduces the epithermal flux too much. In contrast, a very low inclination (i.e., a very flattened cone) generates excessive divergence of the beam. The neutron beam that can be used for BNCT treatment exits through said outlet opening (6), filled with air. It should be noted that in some preferred implementations of the invention, the outlet opening (6) comprises slits and/or projections. Likewise, in other advantageous implementations the opening (6) comprises a movable closure (also known as beam shutter) that can be remotely actuated to stop irradiation, comprising a doble Li and Pb layer.

[0086] Finally there are arranged various layers of shield (7) which act as protection of the patient against gamma-radiation, as well as absorb the neutrons which would otherwise exit somewhere other than via the outlet opening (6). After the outlet opening (6) there are placed various materials which make it possible to absorb the neutrons which would otherwise exit the invention in an uncontrolled manner. As shown in FIG. 1, this shield (7) comprises at least two layers, one of each of the following materials: [0087] Lithiated polyethylene, comprising .sup.6Li-enriched polyethylene to absorb thermal neutrons, as a coating for the reflective cover (4). As said polyethylene contains hydrogen, it allows the energy of the neutrons to be rapidly reduced to the thermal range, while the .sup.6Li captures them without generating gamma-radiation. As a result of this layer, the neutrons that may exit in these directions are thermalized and absorbed as an additional radiation protection measure. [0088] Lead, to absorb gamma-radiation.

[0089] The shield (7) may contain only the above two layers, the Pb layer and the lithiated polyethylene layer, in case a mixture with a sufficiently high proportion of .sup.6Li is achieved. Alternatively, the following two layers can be used additionally to increase the concentration of the .sup.6Li isotope: a natural LiF layer and a .sup.6Li-enriched LiF layer, which together with the natural LiF layer prevent contamination of the neutron beam obtained with contributions off the main axis.

[0090] In different preferred implementations of the invention, there can be a plurality of one or more layers of the shield (7) of the aforementioned materials: lithiated polyethylene, lead, LiF, .sup.6Li-enriched LiF. In other implementations, the lithiated polyethylene can be replaced with borated polyethylene.

[0091] The layers of the shield (7) have been described in the sense in which they would be traversed by the neutron beam. However, they can be arranged in any technically possible order. The outlet opening (6) is thus covered by lead and LiF layers in both its conical and cylindrical cross-section. However, it should be noted that the arrangement of layers of the shield (7) shown FIG. 1 is only an illustrative example and may vary depending on the design requirements.

[0092] In summary, the DPMC must be capable of producing an epithermal neutron beam with high flux, while at the same time thermal and rapid neutron fluxes must be reduced to a minimum. The optimal energy of the neutrons which are applied in BNCT is a few keV, since it allows the treatment of deep tumors, so the maximum of the spectrum must be in this energy range. Likewise, the contamination of the beam by gamma-radiation must be avoided as much as possible. Finally, the neutron beam must have good collimation and a not very high divergence. In other words, the generated neutrons must exit towards the front and the beam must not open up excessively after exiting thorough the outlet opening (6). It should be noted that in the invention there are two different gamma-radiation filters, but their function is different: a layer included in the filtration stage (5), which filters said radiation for the beam that is to exit through the outlet opening (6) and that is to be used in the treatment; while the gamma-radiation filters included in the shield (7) only affect the diverted neutrons with respect to the main axis.

[0093] In the preferred embodiment of the invention laid out in FIG. 1, the design is generated from two parameters which must previously be imposed: A (14 cm) and ∅1 (8 cm). The rest of the parameters represent the optimal dimensions of the design. However, the DPMC is operative within a certain tolerance range of said parameters, such that the optimal value of each parameter is indicated below, in centimeters, together with the percentage variation allowed without significantly degrading its operation: ∅2 (50, ±10%), ∅3 (120, ±15%), ∅4 (130, ±20%), R1 (10, ±10%), R2 (45, ±50%), W1 (19.80, ±10%), W2 (21.80, ±10%), L1 (1.00, ±15%), L2 (0.20, ±20%), L3 (1.00, ±10%), L4 (6.20, ±10%), L5 (24.60, ±10%), L6 (25.00, ±40%), L7 (57.00, ±20%), L8 (70.80, ±20%), L9 (1.00, ±50%), L10 (1.00, ±50%), L11 (5.00, ±10%), L12 (4.00, ±25%), L13 (3.00, ±25%), T1 (2.00, ±50%), T2 (2.00, ±50%). The size of the outlet opening has been designed with the value A=14 cm. However, a modification A±2δ would entail changes in other related radial dimensions, as follows: ∅2, ∅3 and ∅4 would vary ±2δ (the same as A), while R1 and R2 would vary half as much as A (±δ). In this way, the inlet opening (1) for the beam could be modified according to the directional requirements of the target (2). The design of FIG. 1 is also shown in FIG. 2, in a three-dimensional view. It should be noted that the device has axial symmetry.

[0094] The validation of the preferred embodiment shown in FIGS. 1 and 2 has been carried out by comparing the quality parameters imposed by the IAEA with those obtained with said design. First, the DPMC has been verified by means of Monte Carlo simulation for a proton beam at 2.0 and 2.1 MeV, respectively, and for two types of phantoms: a cylindrical phantom model filled with ICRU-4 standard tissue and another Snyder phantom model.

[0095] The results of the simulations on the phantoms are summarized in Table 1.

TABLE-US-00001 Parameter ICRU-4 (2.0 MeV) ICRU-4 (2.1 MeV) Snyder (2.0 MeV) Snyder (2.1 MeV) AD (cm) 9.54 9.53 9.29 9.27 TD (cm) 7.38 7.37 7.24 7.23 ADDR (Gy-Eq/min) 0.16 0.30 0.13 0.25 TR 4.72 4.71 4.67 4.65

[0096] The following equation has been used to estimate the total radiation equivalent dose (D.sub.T), in units of equivalent grays (Gy-Eq.):

[00001]$D_T=\left(w\left|\left|th+\text{}\chi \text{}r\cdot w_{\text{}\text{}\text{}B}\right)\text{}\text{}\right)\right)D_{th}+w_{\text{}\text{}\text{}fast}\cdot D_{fast}+D,$

where D.sub.th is the thermal neutron dose, r = 0.422 and χ denotes the concentration of boron applied with the treatment (its value is about 35 in tumor tissue and 10 in healthy tissue); while w.sub.th,w.sub.fast and w.sub.B are a series of weighting parameters depending on the relative biological effectiveness (RBE) of the contributions of thermal neutrons, rapid neutrons, and boron, respectively. In particular, w.sub.th as w.sub.fast are equal to 3.2 both for tumor tissue and for healthy tissue. In contrast, the weighting of boron is higher in tumor tissue (where w.sub.B = 3.8) in comparison with healthy tissue (w.sub.B = 1.3).

[0097] Secondly, in Table 2 various quality parameters of the neutron beam obtained in the DPMC are compared based on the IAEA recommendations. In particular, the results for a proton beam at 2.0 and 2.1 MeV, respectively, are shown. The epithermal limit of energy (LE) is set at 10 and 20 keV. It should be noted that about half of the dose produced by rapid neutrons corresponds to neutrons with an energy in the range of 10-20 keV. Furthermore, all the IAEA recommendations are met with the proposed invention when the incident proton beam has an energy of 2.1 MeV, while in the case of 2.0 MeV, it only falls below in terms of epithermal flux. Another aspect to be noted is that as a result of the proposed preferred embodiment, the IAEA requirements can be met considering exclusively epithermal neutrons in the range of 0.5 eV-10 keV, which are the most suitable for BNCT treatment. This is achieved as a result of the filtration stage (5) and the shield (7), as well as the materials forming same.

[0098] Thirdly, the neutron spectrum in the DPMC has been analyzed. In particular, FIG. 3 shows the neutrons spectrum at different positions within the DPMC and at the outlet opening (6) thereof, reflecting the process of moderation and subsequent filtration of unwanted neutrons. The selection of materials of the design allows thermal and rapid electron fluxes to be minimized without reducing epithermal fluxes. Particularly relevant is the moderation of the neutrons with the highest energy in the MgF.sub.2 layer with which the spectral maximum is achieved at 2-3 keV, which is optimal for BNCT treatments, especially deep tumors.

TABLE-US-00002 Parameter IAEA Standard Invention (at 2.0 MeV) Invention (at 2.1 MeV) LE 10 keV LE 20 keV LE 10 keV LE 20 keV ϕ.sub.epi (n/cm2s) > 5.Math.10.sup.8 5.450.Math.10.sup.8 5.785.Math.10.sup.8 1.012.Math.10.sup.9 1.078.Math.10.sup.9 ϕ.sub.th/ϕ.sub.epi < 0.05 0.0328 0.0309 0.0326 0.0306 J.sub.n/ϕ.sub.n > 0.7 0.7116 0.7116 0.7108 0.7108 D.sub.fast (Gy.Math.cm.sup.2/n) < 2.Math.10.sup.-13 1.81.Math.10.sup.-13 0.93.Math.10.sup.-13 1.99.Math.10.sup.-13 1.05.Math.10.sup.-13 D.sub.γ (Gy.Math.cm.sup.2/n) < 2.Math.10.sup.-13 1.12.Math.10.sup.-13 1.05-10.sup.-13 1.12.Math.10.sup.-13 1.05.Math.10.sup.-13

[0099] In addition, it has been compared with the spectrum provided by other DPMC facilities, in particular Fir-1 and C-BENS. The results are shown in FIG. 4 (in logarithmic scale) and FIG. 5 (in linear scale).

[0100] FIG. 6 shows, in logarithmic scale, the lateral profile of the (epithermal, thermal and rapid) neutron flux and that of gamma-radiation, in the radial direction right at the outlet opening (6). It is remarkable the good definition of the beam at the outlet opening (6), delimited by the dotted vertical dashed lines in the central area of the graph, as well as the efficient suppression of the neutrons outside said outlet opening (6). In particular, the epithermal neutron flux is reduced by two orders of magnitude within the first 15 cm, while for thermal neutrons this attenuation is much more pronounced and occurs in the first 5 cm. In addition, gamma-radiation is well attenuated over the entire irradiation area. This avoids contamination of the neutron beam obtained, both in terms of gamma-radiation and the contribution off the main axis.

[0101] In some preferred embodiments of the invention, the reflective cover (4) comprises at least one of the following materials: Ni, Pb, BeO, Bi. In the case of beryllium oxide, the usable isotope is .sup.9Be. This has a high elastic cross-section, which constitutes almost the entirety of its cross-section up to the MeV range, so that neutron absorption losses would be minimal in this case. However, it has two drawbacks, both due to the low mass of .sup.9Be. On the one hand, neutron collisions do not occur predominantly at large angles (backward bouncing), so many neutrons will follow their path to the outside of the DPMC, slightly diverted and not reflected, thus requiring a larger thickness of material. On the other hand, the kinematics of the elastic collision cause the neutrons to lose much more energy compared to a heavier nucleus, so the neutrons would be excessively moderated. Thus, BeO might be more appropriate in the case of using higher initial energies for the neutrons, beyond the 2-2.1 MeV used in this embodiment of the invention. In such a case it would contribute to moderation together with MgF.sub.2, as a result of its intermediate behavior between a reflector and a moderator.