RU2610301C1 - Neutron-generating target - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: in order to generate optimum neutron flux using the ⁷Li(p,n)⁷Be reaction in the disclosed invention, instead of using 3-layer targets comprising: a neutron-generating layer, a proton beam absorbing layer and a heat-removing layer, which also provides mechanical strength of the entire structure, the proton beam absorber is merged with the heat-removing layer and is made of tantalum. To provide mechanical strength and small temperature drop during heat removal, the neutron-generating target is made from 20 tantalum tubes with diameter of 5 mm with wall thickness of 0.2 mm, length off 113 mm, arranged in two rows and soldered into a copper housing (shell).

EFFECT: longer service life, low level of undesirable accompanying radiation and high efficiency of heat removal in order to maintain the lithium layer in a solid state.

3 cl, 2 dwg

Description

The invention relates to nuclear physics and medicine and can be used in neutron sources based on a charged particle accelerator. Such sources are intended for use mainly in medical equipment used in neutron therapy, mainly in boron neutron capture therapy (BNCT).

The concept of boron neutron capture therapy in oncology was proposed in 1936, four years after the discovery of the neutron. Its physical principle is as follows. A solution containing the stable isotope boron-10 is introduced into the blood of a person, and after a while boron is sorbed mainly in the tumor cells. Then the tumor is irradiated with a stream of epithermal (with energies from 0.5 eV to 10 keV) neutrons. As a result of neutron absorption by the stable ¹⁰ V isotope, a nuclear reaction occurs, and the resulting energetic α-particle and ⁷ Li ion are rapidly inhibited along the length of the cell size and release energy of ~ 2.3 MeV within the very cell that contained the boron nucleus, which leads to her defeat. Thus, boron-neutron capture therapy allows for selective damage to malignant tumor cells.

The feasibility of developing neutron capture therapy technology is due to its focus on the treatment of such types of malignant tumors that are practically not amenable to any other methods - brain glioblastomas and melanoma metastases.

In the early 1950s, Dr. Sweet demonstrated for the first time that certain boron compounds provide a higher concentration of boron in cancer cells compared to healthy [W. Sweet The uses of nuclear disintegration in the diagnosis and treatment of brain tumor. N. Engl. J. Med. 245 (1951) 875-878; W. Sweet, M. Javid. The possible use of slow neutrons plus boron-10 in the therapy of intracranial tumors. Trans. Am. Neurol. Assoc. 76 (1951) 60-63]. Since the mid-1950s, the methodology has been tested with varying success at nuclear reactors in a number of countries. The main achievements were associated with the name of the Japanese neurosurgeon Hiroshi Hatanaka. Hatanaka began to use the intra-arterial administration of ¹⁰ V isotope-enriched sodium borocaptate (BSH, Na ₂ ¹⁰ B ₁₂ H ₁₁ SH), synthesized by Dr. Soloway [A. Soloway, N. Hatanaka, M. Davis. Penetration of brain and brain tumor. VII. Tumor binding sulfhydryl boron compounds. J. Med. Chem. 10 (1967) 714-717], performed open irradiation of the tumor after surgical treatment and achieved impressive results - the 5-year survival rate was 58% for a group of patients with malignant gliomas of grade 3 and 4 [Hatanaka N. Clinical results of boron neutron capture therapy . Basic Life Sci. 54 (1990) 15-21]. In 1987, Mishima treated superficial malignant melanoma using ¹⁰ V isotope enriched borphenylalanine in the optically isomeric form L (BPA, (HO) ₂ ¹⁰ B-C ₆ H ₄ -CH ₂ CH (NH ₂ ) -CO ₂ H) [ Y. Mishima, et al. Selective thermal neutron capture therapy and diagnosis of malignant melanoma: from basic studies to first clinical treatment. Basic Life Sci. 50 (1989) 251-260]. The use of these pharmaceuticals allows you to create a concentration of ¹⁰ V isotope in tumor tissue up to 40 μg / g, which is 3.5 times more than in healthy tissue. Such concentration and contrast make it possible to make the contribution of background radiation reasonably small, and indeed provide the possibility of selective damage to tumor cells.

In the 1990s, along with work on reactors, intensive discussions began on the development and creation of a neutron source based on a compact and inexpensive accelerator, which could be equipped with almost every cancer clinic. Various principles have been proposed for constructing NCT systems based on an accelerator with a beam of charged particles, in particular with a beam of protons interacting with a target for generating neutrons. Basically, four reactions are considered: ⁷ Li (p, n), ⁹ Be (p, n), ⁹ Be (d, n) and ¹³ C (d, n) [T. Blue and J. Yanch. Accelerator-based epithermal neutron sources for boron neutron capture therapy of brain tumors. Journal of Neuro-oncology 62 (2003) 19-31; C. Lee, X. Zhou. Thick target neutron yields for the ⁷ Li (p, n) ⁷ Be reaction near threshold. NIM B 152 (1999) 1-11].

The best reaction for generating epithermal neutrons is the proton bombardment of lithium: the neutron yield is large and the energy spectrum is relatively soft. Thus, a proton beam of 10 mA 2.5 MeV leads to a neutron flux of 8.9 × 10 ¹² s ^-1 at an average neutron energy of 0.55 MeV.

Since lithium is characterized by a low melting point (182 ° С), low thermal conductivity (71 W / (m⋅K) in the solid state and 43 W / (m⋅K) in the liquid state at a temperature near the melting point) and high chemical activity, the manufacture lithium neutron generating target is a complex technical task. When implementing it, the following requirements must be taken into account:

1. The lithium neutron-generating layer must be thin so that the protons are inhibited in it to the threshold of neutron generation. This will significantly reduce the concomitant flux of 0.478 MeV gamma rays and reduce the temperature on the lithium surface.

2. The lithium neutron-generating layer must be of pure lithium for maximum neutron yield. The neutron yield from lithium hydride, oxide, and fluoride is 1.43, 2, and 3.3 times less than from pure lithium, respectively.

3. The lithium neutron-generating layer must be in a solid state to prevent the spread of lithium vapor and the resulting radioactive isotope of beryllium-7 according to the installation.

4. The substrate on which the lithium neutron-generating layer is sprayed should be thin. This will allow you to place the optimal moderator as close as possible to the neutron generation site and form the best-quality therapeutic neutron beam.

5. The substrate must be intensively cooled to maintain the lithium layer in the solid state (below 182 ° C) when it is heated by a stationary proton beam with a power of 25 kW.

6. The substrate must be resistant to radiation damage.

7. The substrate should be easy to manufacture.

8. The substrate must be easily removable for disposal after activation.

Due to the complexity of the task, some groups of researchers switched to the development and use of a target from beryllium, despite the fact that the neutron yield decreased by 4 times.

In order to fulfill the set requirements, various ideas were considered and the necessary experimental and theoretical studies were carried out. Their results are reflected in the following patents and scientific articles:

- US Patent No. 4666651 dated 05/19/1987 (Barjon Robert).

- US Patent No. 5920601 dated July 6, 1999 (Dawid Nigg et al.).

- US patent No. 4582667 from 04/15/1986 (Gunter Bauer).

- US patent US 2010/0067640 dated 03/18/2010 (Carl Willis, Donald Swenson).

- T. Mitsumoto, S. Yajiima, H. Tsutsui, et al. Cyclotron-based neutron source for BNCT. Proc. XIV International Congress on Neutron Capture Therapy, October 25-29, 2010, Buenos Aires, Argentina, p. 510-522.

- C. Willis, J. Lenz, D. Swenson. High-power lithim target for acelerator-based BNCT. Proc. XIV Linear Accelerator Conference, 29 September - 3 October 2008, Victoria, Canada, p. 223-225.

- E. Forton, F. Stichelbaut, A. Cambriani, et al. Overview of the IBA accelerator-based BNCT system. Applied Radiation and Isotopes 67 (2009) S262-S265.

- S. Park, H. Joo, B. Jan, et al. Thermally optimized lithium neutron producing target design for accelerator-based BNCT. 12th Intern. Congress on Neutron Capture Therapy, Takamatsu, Japan, October 9-13, 2006, p. 319-322.

The closest prototype analogue of the present invention is a lithium target, described in patent US 2010/0067640 dated 03/18/2010, authors: Carl Willis and Donald Swenson, name: High-power-density lithium target for neutron production (high density neutron-generating lithium target heating power).

The target diagram is shown in FIG. 1. The basis of the target site is a housing 14 made of a material with a high coefficient of thermal conductivity. The use of a material that conducts heat well is due to the need to minimize, as far as possible, the temperature drop across the metal thickness from the lithium layer to the surface to be cooled. Such material may be copper, characterized by a thermal conductivity of 400 W / (m⋅grad). Channels (24 and 26) were made in the case for cooling with water (water inlet is marked with the number 20, output - 22). The housing is designed to provide effective cooling of the target by a turbulent flow of water, proposed and studied by us [V. Bayanov, V. Belov, V. Kindyuk, E. Oparin, S. Taskaev. Lithium neutron producing target for BINP accelerator-based neutron source. Applied Radiation and Isotopes 61 (2004) 817-821]. It should be noted that the authors of the patent US 2010/0067640 of 03/18/2010 in the description refer to our subsequent work [S. Taskaev, V. Bayanov, V. Belov and E. Zhoorov. Development of lithium target for accelerator based neutron capture therapy. Advances in Neutron Capture Therapy 2006, p. 292-295].

The copper case is covered with a thin layer of palladium 12, on which a thin layer of lithium 10 is sprayed. The lithium layer is designed to generate neutrons and has a thickness on which the protons are decelerated to an energy of 1.882 MeV - the threshold of neutron generation of the ⁷ Li (p, n) ⁷ Be reaction. Further inhibition of protons and their absorption is carried out in palladium. Palladium is characterized by a high diffusion coefficient of hydrogen in it and, according to the authors of the patent, it will create a target that is resistant to radiation blistering and, as a result, with a long operating time. The palladium thickness should be greater than the proton path (about 20 μm), should be sufficient to dissolve and retain hydrogen (protons), but should not be too large, since the thermal conductivity coefficient of palladium (76 W / (m⋅grad)) is much less than copper.

Thus, the prototype neutron generating target is three-layer. The first layer, lithium, is necessary for the generation of neutrons. The second layer, palladium, serves to absorb protons and must be resistant to radiation blistering, providing a long target life. The third layer, copper, provides heat removal from the previous two layers heated by the proton beam, with a minimum temperature difference and provides mechanical strength of the structure.

The disadvantages of this system include the following:

1. The complexity of the manufacture of the target, in which it is necessary to ensure good adhesion of palladium to copper.

2. At a characteristic target temperature in the region of 150 ° C (below the melting temperature of lithium 182 ° C), palladium, as it was found out later, is not a material that is highly resistant to radiation blistering.

The invention is directed to the creation of a neutron-generating lithium target, which is characterized by a long service life, a minimum level of undesirable associated radiation and effective heat removal to maintain the lithium layer in the solid state when it is heated by a powerful proton beam.

To solve this problem, additional studies were carried out, which gave new experimental data on the resistance of materials to radiation blistering and on the level of power of the accompanying radiation during the absorption of protons in them.

The study of radiation blistering under the proton beam irradiation of various materials heated to a temperature below the melting temperature of lithium is given in work V. Astrelin, A. Burdakov, R. Bykov, et al., Blistering of the selected materials irradiated by intense 200 keV proton beam. J. Nucl. Mater. 396 (2010) 43-48. " It was found that materials such as alpha iron, vanadium and tantalum are most resistant to radiation blistering. So, at a proton beam energy of 2 MeV, a current of 10 mA, a target diameter of 10 cm, and its temperature of 100 ° C, the appearance time of blisters should exceed 200 hours of continuous irradiation. In the case of palladium, this time is 20 hours. Although palladium is considered to be a hydrogen-soluble material with a high diffusion coefficient and resistant to radiation blistering, studies have shown that these palladium properties may be realized at a higher temperature than is required in the case of a lithium target.

The study of the radiation power level during the absorption of protons with an energy of 2 MeV in various structural materials is given in the work “D.A. Kasatov et al. Radiation from the absorption of protons with an energy of 2 MeV in various materials. Nuclear Physics, 2015, Volume 78, No. 11, pp. 963–969. At the tandem accelerator, samples made of lithium, graphite, magnesium fluoride, barium fluoride, aluminum, silicon, titanium, vanadium, stainless steel, copper, molybdenum, and tantalum were irradiated with a proton beam. The dose rate and the spectrum of x-ray and gamma radiation, the dose rate of neutron radiation during the absorption of protons with an energy of 2 MeV in materials, and the radiation spectrum of residual activity were measured. The generation of neutrons from lithium, vanadium, stainless steel, and titanium and the activation of lithium, graphite, and titanium were recorded. It was determined that the absorption of protons with an energy of 2 MeV in molybdenum and tantalum is accompanied by a minimum dose rate of x-ray and gamma radiation and does not lead to the generation of fast neutrons and to residual activity.

Thus, according to a combination of two factors, tantalum is the best material for fabricating a substrate of a neutron-generating lithium target - when a proton beam is absorbed in it, it provides maximum resistance to radiation blistering and, as a result, the maximum operating time of the target, and ensures a minimum level of unwanted radiation power.

The thermal conductivity coefficient of tantalum is 63 W / (m⋅grad). This value is comparable with the coefficient of thermal conductivity of palladium and is much smaller than that of copper. Thus, it is possible to produce a target similar to the prototype, in which the new is that palladium is replaced by tantalum.

In the proposed target, another new idea was proposed - to combine two materials into one material, one of which protons stop, and it should be characterized by high radiation resistance (in the case of the prototype it is palladium), and the other provides mechanical strength and a small temperature difference during heat removal (in the case of the prototype it is copper). We suggest using tantalum tubes. Tantalum as a material has maximum resistance to radiation blistering, and tubes with thin walls can provide the necessary mechanical strength and a small temperature difference due to the small path along which heat is removed to the coolant.

The proposed target is made; her photograph is shown in FIG. 2. The target is made of 20 tantalum tubes with a diameter of 5 mm with a wall thickness of 0.2 mm, a length of 113 mm, placed in two rows and soldered into a copper casing (shell). Soldering was carried out using copper-tin solder. The lithium layer is sprayed onto the tantalum tubes by the thermal method described in the work “B.F. Bayanov, E.V. Zhurov, S.Yu. Taskaev. Measurement of the thickness of the lithium layer. Instruments and experimental equipment, 1 (2008) 160-162 ".

Claims (3)

1. A neutron-generating target with a long life for generating epithermal neutrons in boron-neutron capture therapy, containing a layer of neutron-generating material, a proton beam absorber and a heat sink layer, characterized in that the proton beam absorber and the heat sink layer are one product made of one material. 2. The neutron generating target according to claim 1, characterized in that the material of the proton beam absorber and the heat sink layer can be tantalum. 3. The neutron-generating target according to claim 1, characterized in that the proton beam absorber and the heat-removing layer can be made in the form of a tube with a thin wall.

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