RU2441290C1 - Method to produce radioisotope strontium-82 - Google Patents

Abstract

FIELD: power engineering.

SUBSTANCE: method to produce radioisotope strontium-82 includes radiation of α-particles or 3He target from krypton by accelerated beams. The target is one isotope or cascade from several isotopes of crypton, every of which represents crypton enriched by i isotope to concentration that exceeds concentration of i isotope in natural mix of crypton isotopes, and simultaneously exceeding concentration of any other isotope in mixture of crypton isotopes, at the same time crypton isotopes in the cascade are arranged in series in direction of the accelerated particle beam in the decreasing order of atomic masses of isotopes having maximum concentration in the mixture of crypton isotopes, and in process of one or more threshold nuclear reactions ^{80,82,83,84,86}Kr(α,xn)⁸²Sr or accordingly one or more threshold nuclear reactions ^{80,82,83,84,86}Kr(3He,xn)⁸²Sr, the target radioisotope ⁸²Sr is accumulated in the target.

EFFECT: invention makes it possible to increase yield of target radioisotope ⁸²Sr and provides for control and minimisation of the value of accompanying activities of the main interfering admixtures ⁸⁵Sr and ⁸³Sr.

2 cl, 1 dwg

Description

The invention relates to a technology for producing radioisotopes for nuclear medicine on charged particle accelerators.

At present, one of the most promising and dynamically developing areas of nuclear medicine is cardiac diagnostics based on the method of positron emission tomography (PET).

The work of most PET centers is now based on the use of ultrashort β ^- emitting radionuclides ( ¹¹ C, ¹³ N, ¹⁵ O, ¹⁸ F) and suggests the presence of two mandatory components in the PET center: positron emission tomography and cyclotron with E _p <20 MeV for production of the above radioisotopes. The condition of binding to the cyclotron, of course, inhibits the spread of the PET method, since not every clinic can afford to have and operate a cyclotron. A more rational approach to solving this problem is known.

Recently, mainly in the USA, PET technology has been developed based on the use of ⁸² Sr- ⁸² Rb isotope generators.

Below are some of the nuclear physical characteristics of this isotopic pair:

⁸² Sr: T _{1 | 2} = 25.3; EZ (100%); no accompanying gamma quanta.

⁸² Rb: T _{1 | 2} = 1.27 min; β ⁺ (95%), EZ (5%), the main accompanying γ-quanta: E _γ = 511 keV (189%) and E _γ = 776.9 keV (12.5%).

Obtained in the isotopic generator of rubidium, being a physiological analogue of potassium, when introduced into the patient's body, it is mainly localized in the myocardium.

The indicated physical and physiological properties of this radioisotope pair make this generator very convenient for use in the PET method. This generator is compact, can be easily delivered to any clinic, including over long distances, and operated quite a long time. At the same time, there is no need to have and operate a cyclotron in the clinic (bulky and expensive equipment that requires special facilities and maintenance personnel).

The present invention can be used to obtain the ⁸² Sr radioisotope on relatively inexpensive medium-energy accelerators (E _α , _3He ≤60 MeV). The proposed method for producing ⁸² Sr allows one to exclude from the procedure for isolating strontium from a cyclotron target a laborious and expensive operation of radiochemical processing of a target, characteristic of other methods. Obtained by this method ⁸² Sr will have a higher radionuclide purity compared to existing analogues, since the cascade configuration of a gas krypton target in this method will allow you to adjust and optimize the composition and amount of radionuclide impurities in the final product.

State of the art

Currently, ⁸² Sr is produced by proton irradiation (E _p ≈100 ÷ 800 MeV) of solid targets from molybdenum, metallic rubidium or its compounds at high-energy accelerators.

A known method of producing ⁸² Sr by the reaction of Mo (p, spallation) (Thomas KE Strontium-82 Production at Los Alamos National Laboratory. - Applied Radiation and Isotopes, 1987, v. 38, No. 3, p. 175-180). Targets made of metal molybdenum with a diameter of 1.9-6.4 cm and a thickness of 1.25-1.9 cm were irradiated with proton beams of 800 MeV energy. As a result of the cleavage reaction, ⁸² Sr. was formed. The duration of irradiation of various targets ranged from 2 to 30 days. The nominal current of the proton beam is 500 μA. Then the targets were dissolved in a mixture of nitric and phosphoric acids in the presence of hydrogen peroxide. After that, ⁸² Sr. was isolated by a multi-stage chemical redistribution

This method has significant disadvantages, which are as follows:

- To obtain ⁸² Sr, a unique and expensive installation is used, mainly intended for basic research: the meson factory of the Los Adamos National Laboratory of the United States;

- the technology is based on the use of a disposable target;

- along with ⁸² Sr, a large amount of radioactive impurities is formed in the target;

- the release of ⁸² Sr is associated with the need for multi-stage radiochemical redistribution of the target and the disposal of a large amount of radioactive waste;

- high content in the target product of the main interfering impurity ⁸⁵ Sr, the activity of which is comparable to the activity of the target product.

It is known that one of the significant factors determining the quality of ⁸² Sr is its radionuclide purity. The main radionuclide impurities are ⁸³ Sr and ⁸⁵ Sr. The activity of ⁸³ Sr can be significantly reduced by the exposure of the irradiated target (T _{1 | 2} = 32.4 hours for ⁸³ Sr). As for the long-lived ⁸⁵ Sr (T _{1 | 2} = 64.73 days), its presence in ⁸² Sr significantly increases the dose load on the patient and the medical staff and complicates the PET studies (with partial “breakthrough” of strontium in the ⁸² Sr- ⁸² separation column Rb of the isotope generator), since ⁸⁵ Sr has an intense line Е _γ = 514 keV, close to the annihilation line ⁸² Rb (Е _γ = 511 keV). In addition, the presence of ⁸⁵ Sr substantially increases the radiation protection requirements of the ⁸² Sr- ⁸² Rb isotope generator and reduces its service life before recharging.

A known method of producing ⁸² Sr by the reaction of Rb (p, xn) on targets from rubidium chloride (Mausner LF, Prach T., Srivastava SC Production of ⁸² Sr by Proton Irradiation of RbCl. - Applied Radiation and Isotopes, 1987, v. 38, No. 3, p. 181-184). In order to dehydrate, rubidium chloride was kept in vacuum for 48 hours, then pressed with a force of 75 tons into a 35 g tablet 0.81 cm thick and 4.44 cm in diameter. A rubidium chloride tablet was placed in a stainless steel capsule and sealed under vacuum with an electron beam. Then, the capsule with rubidium chloride was irradiated with protons at the accelerator of the Brookhaven National Laboratory, which allows accelerating protons to an energy of 200 MeV. The proton beam current was 45 μA. After irradiation, the capsule was transported in a protective container to a hot laboratory and after 6 days the extracts were opened. Then rubidium chloride was dissolved in 100 ml of 0.1 M NH ₄ OH: 0.1 M NH ₄ Cl and ⁸² Sr. was isolated after multistage radiochemical redistribution.

The disadvantages of this method include:

- the use to obtain ⁸² Sr of an expensive high-energy accelerator;

- rather complicated procedure for manufacturing the target;

- the technology is based on the use of a disposable target;

- due to the poor thermal conductivity of rubidium chloride at currents above several μA, overheating in the center of the target and sublimation of rubidium chloride may occur, which leads to a decrease in the effective thickness of the target and, accordingly, the yield of the target product;

- the allocation of ⁸² Sr is associated with the need for multi-stage radiochemical redistribution of the target;

- as in the previous example, the high content in the target product of the main interfering impurity ⁸⁵ Sr, the activity of which is comparable to the activity of the target product.

A known method of producing ⁸² Sr by the reaction of Rb (p, xn) on targets from metal rubidium (Zhuikov B. L., Kokhanyuk V. M., Glushchenko V. N. et al. Obtaining strontium-82 from a target metal rubidium in a proton beam with an energy of 100 MeV. - Radiochemistry, 1994, Volume 36, pp. 494-498). Rubidium metal targets were a disc with a diameter of 30 mm and a thickness of 11 mm, enclosed in sealed stainless steel shells. The thickness of the entrance window of the shell was 0.13-0.2 mm. The shells were charged with metallic rubidium in a box in an inert atmosphere. To do this, rubidium in an ampoule was heated by an electric furnace to 80-90 ° C, taking liquid rubidium with a medical syringe, liquid metal was introduced through a nozzle into the shell. Target irradiation was carried out on a linear accelerator with a beam with a proton energy of 100 MeV at beam currents of 6–10 μA. The exposure time reached 10 days. The technology for processing the target included mechanical opening of the cartridge and dissolution of the target in isobutanol, destruction of the rubidium isobutonolate formed during dissolution of the target and separation of the organic phase by distillation, separation of strontium isotopes from rubidium on an ion-exchange column.

The disadvantages of this method, as in the previous example, include:

- the use to obtain ⁸² Sr of an expensive high-energy accelerator;

- rather complicated procedure for manufacturing the target;

- the technology is based on the use of a disposable target;

- the allocation of ⁸² Sr is associated with the need for multi-stage radiochemical redistribution of the target;

- high content in the target product of the main interfering impurity ⁸⁵ Sr, the activity of which is comparable to the activity of the target product.

In addition, a significant disadvantage of this method should be considered a high explosion hazard due to the use of metallic rubidium.

As a prototype, a method was chosen for producing ⁸² Sr in the reactions Kr (α, xn) and Kr (3Не, xn) when irradiated with accelerated beams of α particles or 3Не targets from natural krypton (Tarkanyi F., Qaim SM, Stocklin G. Excitation Functions of ³ He- and α-Particle Induced Nuclear Reactions on Natural Krypton: Production of ⁸² Sr at a Compact Cyclotron. - Applied Radiation and Isotopes, 1988, v. 39, No. 2, p. 135-143). When using natural krypton and accelerated α particles or 3He with an initial energy of 60–80 MeV as a target, ⁸² Sr production is possible on all Kr isotopes except ⁷⁸ Kr. However, the ⁸² Sr production time on each of the isotopes using natural krypton is not optimal, since the cross sections for nuclear reactions leading to the formation of ⁸² Sr on each of the krypton isotopes vary over a wide range (from 0 to σ _max ) in the energy range of braking in target charged particles.

As a result, this method has the following significant disadvantages:

- a relatively low yield of ⁸² Sr in the target from natural krypton;

- high content in the target product of the main interfering impurity ⁸⁵ Sr, the activity of which is comparable to the activity of the target product.

Disclosure of invention

The technical result is to increase the yield of the target radioisotope ⁸² Sr and the ability to regulate and minimize the associated activities of the main interfering impurities: ⁸³ Sr and ⁸⁵ Sr.

To this end, a method for producing a strontium-82 radioisotope is proposed, which involves irradiating beams of accelerated α particles or 3He targets from krypton, moreover, one isotope or cascade of several krypton isotopes is taken as a target, each of which is krypton enriched in the ith isotope to a concentration exceeding the concentration of the ith isotope in a natural mixture of krypton isotopes, and at the same time exceeding the concentration of any other isotope in a mixture of krypton isotopes, while the krypton isotopes in the cascade are arranged flax in the direction of the beam of accelerated particles in order of decreasing atomic mass isotope having a mixture of krypton isotope maximum concentration, and in the process the one or more thresholds nuclear reactions ^{80,82,83,84,86} Kr (α, xn) ⁸² Sr or respectively , one or more threshold nuclear reactions ^{80.82.83.84.86} Kr (3He, xn) ⁸² Sr accumulate in the target the target radioisotope ⁸² Sr.

In addition, a cyclotron or linear accelerator is used as a source of accelerated particles.

The figure shows the dependences of the cross sections of the threshold nuclear reactions of ^80.82.83 Kr (α, xn) ⁸² Sr of the energy of accelerated particles in the range from the energy thresholds of reactions to 60 MeV.

The method is as follows.

Since each point of the target on the axis of the beam corresponds to a very specific energy of accelerated particles, the modular layout of the target with krypton isotopes allows for each isotope to select the energy range (and its corresponding place in the cascade) so that ⁸² Sr on each krypton isotope is produced in the energy region effectiveness. In other words, the ⁸² Sr operating time is carried out where the cross section of the ⁸² Sr formation reaction on this particular krypton isotope is higher than on any other isotope. The selection of the corresponding energy ranges is carried out using the energy dependences of the reaction cross sections ^{80.82.83.84.86} Kr (α, xn) ⁸² Sr and ^{80.82.83.84.86} Kr (3He, xn) ⁸² Sr, leading to the formation ⁸² Sr. The choice of specific krypton isotopes and their number for the production of ⁸² Sr depends on the initial energy of charged particles. The higher the initial energy of charged particles, the higher the atomic number of the krypton isotope, on which the threshold reaction will take place with the formation of ⁸² Sr. The energy thresholds of nuclear reactions leading to the formation of ⁸² Sr increase with increasing atomic number of the krypton isotope. This method allows to obtain a maximum yield of ⁸² Sr for each fixed set of krypton isotopes and a fixed initial energy of charged particles. The absolute maximum yield of ⁸² Sr in the krypton target (for a fixed initial energy of charged particles) is obtained by using the maximum enrichment of krypton isotopes in this method.

Along with optimizing the yield of ⁸² Sr, the modular layout of the cascade target allows minimizing the main interfering impurities: ⁸³ Sr and ⁸⁵ Sr. The minimization procedure is the same as in the case of optimizing the yield of ⁸² Sr. The only difference is that the operating time of ⁸³ Sr and ⁸⁵ Sr on each krypton isotope is carried out in the energy regions of its minimum efficiency. In other words, the operating time of ⁸³ Sr and ⁸⁵ Sr will be carried out where the cross sections for the formation of ⁸³ Sr and ⁸⁵ Sr on this particular krypton isotope are lower than on any other isotope. In this case, the selection of the corresponding energy ranges is carried out using the energy dependences of the reaction cross sections ^{80.82.83.84.86} Kr (α, xn) ⁸³ Sr, ^82.83.84.86 Kr (α, xn) ⁸⁵ Sr, ^{82.83 , 84.86} Kr (3He, xn) ⁸³ Sr and ^83.84.86 Kr (3He, xn) ⁸⁵ Sr, resulting in the formation of ⁸³ Sr and ⁸⁵ Sr.

In practice, the conditions for optimizing the yield of ⁸² Sr and minimizing the content of impurity activities of ⁸³ Sr and ⁸⁵ Sr can contradict each other. In this case, taking into account consumer requirements for the target product, a compromise layout of the cascade krypton target is implemented.

The proposed method for producing a radioisotope ⁸² Sr has significant advantages compared to analogues described in the literature:

- The method can be implemented on relatively cheap to operate medium-energy accelerators (E _{α, 3He} ≤60-70 MeV).

- The target device can be used repeatedly.

- Isolation of ⁸² Sr from the target is not associated with its destruction and multistage radiochemical redistribution.

- In this method, the activity of the main interfering impurity ⁸⁵ Sr can be reduced by an order of magnitude compared to analogues.

The proposed method for producing a radioisotope ⁸² Sr has significant advantages compared to the prototype:

- The ⁸² Sr yield in a cascade target with krypton isotopes can be increased several times in comparison with the target exit from natural krypton.

- The activity of the main interfering impurity ⁸⁵ Sr in this method can be reduced by an order of magnitude compared with the prototype.

An example embodiment of the invention

The target, which is a cascade of two consecutively located aluminum modules with highly enriched krypton isotopes ⁸⁰ Kr and ⁸² Kr (enrichment> 99.9%, pressure 300 kPa), was mounted on a beam of α-particles with an energy of 60 MeV and ^80.82 Kr in the process of threshold nuclear reactions (α, xn) ⁸² Sr accumulated in it the target radioisotope ⁸² Sr. Based on the experimental values of the reaction cross sections previously obtained by the authors, ^80.82.83 Kr (α, xn) ⁸² Sr (see the figure) and to ensure a maximum yield of ⁸² Sr in the region E _α ≤60 MeV, the cascade krypton target was made of two modules with highly enriched krypton isotopes: with ⁸⁰ Kr and ⁸² Kr, and the geometric boundary between the modules with ⁸⁰ Kr and ⁸² Kr, in the form of Al foil with a thickness of 100 μm, was set at a distance of 12 cm (in the region where E _α ≈50 MeV) from the beam entrance α -particles in the first module with ⁸² Kr. The length of the second module with ⁸⁰ Kr, equal to 29 cm, ensured the complete deceleration of α particles with E _α ≈50 MeV in ⁸⁰ Kr. The cascade target arrangement used allowed us to produce the maximum possible amount of ⁸² Sr in the range of α-particle energies in the range of 60-50 MeV (on the ⁸² Kg isotope) and the maximum possible amount of ⁸² Sr in the range of α-particle energies in the range from the threshold of ⁸⁰ Kr (α, xn) ⁸² Sr reactions up to 50 MeV (on the ⁸⁰ Kr isotope). In addition, in the module with ⁸⁰ Kr, ⁸² Sr was produced, lacking the impurity activity of ⁸⁵ Sr, since there are no reactions initiated by α particles with the formation of ⁸⁵ Sr on ⁸⁰ Kr. After completion of irradiation and prior to the start of the ⁸² Sr extraction procedure, the cascade target was kept for 3 days to allow short-lived activities to decay and to reduce the corresponding dose burden on the personnel. Then the cascade target was transported to a special protective box, where it was connected to the communications of the gas-vacuum radiochemical stand. Krypton was removed from the modules of the cascade target, for example, by cryogenic freezing in special gas traps (each krypton isotope into its own trap) located at the temperature of liquid nitrogen. After krypton was removed, nozzles were introduced into the volumes of the modules of the cascade target through special channels, through which NH ₄ Cl solution was injected under pressure for several seconds, washing off the activity of rubidium and strontium isotopes from the walls of the cascade target. The solution of NH ₄ Cl by gravity was collected in a special receiving container. Then, using the pump, the active solution was fed through the filter to an ion-exchange column, where the activities of rubidium and strontium isotopes were separated.

Thus, this invention allows to increase several times the yield of ⁸² Sr in comparison with the yield of ⁸² Sr in the target from natural krypton. The activity of the main interfering impurity ⁸⁵ Sr in ⁸² Sr can be reduced in this method by an order of magnitude compared with the prototype and the considered analogues. The method can be implemented on relatively inexpensive medium-energy accelerators (E _α3He ≤60-70 MeV).

Isolation of ⁸² Sr from the target is not associated with its destruction and multistage radiochemical redistribution.

The totality of these circumstances allows us to hope that this method will have a commercial perspective and will provide a wider implementation of the diagnostic PET method in our country.

Claims (2)

1. A method of obtaining a radioisotope of strontium-82, comprising irradiating accelerated beams of α particles or 3He targets from krypton, characterized in that one isotope or cascade of several krypton isotopes is taken as a target, each of which is krypton enriched in i- the isotope to a concentration exceeding the concentration of the ith isotope in a natural mixture of krypton isotopes, and at the same time exceeding the concentration of any other isotope in a mixture of krypton isotopes, while the krypton isotopes in the cascade are arranged sequentially about in the direction of the beam of accelerated particles in decreasing order of atomic masses of isotopes having a maximum concentration in the mixture of krypton isotopes and during one or more threshold nuclear reactions ^{80.82.83.84.86} Kr (α, xn) ⁸² Sr or, respectively, one or several threshold nuclear reactions ^{80.82.83.84.86} Kr (3He, xn) ⁸² Sr accumulate in the target the target radioisotope ⁸² Sr. 2. The method according to claim 1, characterized in that a cyclotron or a linear accelerator is used as a source of accelerated particles.