GB2154047A

GB2154047A - Process for producing thallium-201

Info

Publication number: GB2154047A
Application number: GB08400751A
Authority: GB
Inventors: Viktor Andreevich Ageev; Alexandr Alexandro Kljuchnikov; Alexandr Fedorovich Linev; Vladimir Alexeevich Khalkin; Natalya Gavrilovna Zaitseva
Original assignee: SP KT BJURO S EX PROIZV I YADE
Current assignee: SP KT BJURO S EX PROIZV I YADE
Priority date: 1984-01-12
Filing date: 1984-01-12
Publication date: 1985-08-29
Also published as: DE3402348C2; GB8400751D0; CH659340A5; GB2154047B; DE3402348A1

Abstract

The herein-proposed process for producing thallium-201 resides in that a target from pure lead- 206 enriched for at least 95%, is subjected to irradiation with a bunch of protons having an energy of 50 to 70 MeV, this resulting in a reaction <206>Pb(p,6n)<201>Bi, the irradiated target is kept for a period of time sufficient for a transition <201>Bi<->201>Pb<->201>TI. Said target is treated with an acid to complete dissolution, the thallium-201 contained in the resulting solution is oxidized to its trivalent state, and then lead is precipitated from the solution. After that the traces of lead and thallium-201 remaining in said solution are separated by way of cation exchange, and then thallium-201 is eluted with hydrochloric acid.

Description

SPECIFICATION Process for producing thallium-201 The present invention relates to the production of radioactive isotopes and, more particularly, to a process for producing thallium201.

The process of the invention makes it possible to produce high-purity thallium-201 which will find extensive application in nuclear medicine, as a scanning element for diagnostics of the state of the myocardium and coronary arteries.

Known in the art is a process for producing high-purity thallium-201 (US Patent No.

3,993,538) which has a high specific activity, residing in that a thallium target containing at least 99.9% of thallium-203 is subjected to the action of a bundle of protons with an energy of 20 to 30 MeV. The target thickness is selected to be not over 0.2 g/cm2 for the reaction 203Tl(p, 3n)201Pb to proceed at a minimum energy of the protons. The irradiated target is dissolved in an acid, and the resulting solution is treated to obtain a lead-containing eluate. This eluate is kept for not over 24 hours, during which period it undergoes a decay with the formation of thallium-201 and a small quantity of thallium-203. Then the eluate containing the undecayed lead, the formed thallium-201, and thallium-203 is passed through an ionexchange column to isolate said thallium isotopes which in further treatment are converted to chlorides.

The yield of the thallium-201 isotope is less than 0.7 mCi/uA.h. for the target of natural thallium.

Said process is characterised by a low yield of the desired product.

With a view to increase the yield of the desired product per microampere-hour, a process for producing thallium-201 is described (M.C.Lagunas-Solar, J.A.Jungerman, D.W.Paulson, J.Appl.Radiat. a Isotop. 31, p.11 7-1 21, 1980), consisting in that a thallium target comprising 99.46% of thallium205 isotope is irradiated with protons having an energy of 34 to 60 MeV; the formation of thallium-201 proceeds according to the reaction 205Tl(p,5n)201Pb201Tl and is characterised by a higher yield of thallium-201; 2 mCi/pA.h. The existing processes for producing thallium-201 with the use of natural thallium as the target, as well as with the use of enriched isotopes of thallium-203 and thallium-205, are characterised by a low yield of the final product per unit current of the proton bunch and by a complicated two-stage method of the radiochemical isolation of the final product from the irradiated target: separation of lead from the large quantity of the material of the target-thallium, and subsequent isolation of thallium-201 from the previously isolated lead-201 after accumulation thereof in the lead.

The first stage of the process must be accomplished during the shortest possible period of time, directly after the irradiation of the target, since thallium-201 which forms in the decay of parent lead-201 cannot be isolated from the thallium target.

The two-stage nature of the process and the necessity of working with a high-activity "hot" target add to the production costs and exposure dose to the personnel. The thallium201 formed from the parent lead-201 during the irradiation proves to be lost, since it cannot be isolated from the thallium target; therefore, it is unreasonable to irradiate the target for a long period of time, though it might considerably increase the ultimate quantity of thallium-201.

The above-described processes for producing thallium-201 from thallium targets are characterised by a low yield of the desired product and cannot meet the increasing needs of nuclear medicine in this isotope.

At present a number of scientific papers have appeared in which the possibility is considered of producing thallium-201 upon irradiation of targets from natural lead with a bunch of protons having an energy of 40 to 66 MeV (M.S. Lagunas-Solar F.E.Little, J.A. Jungerman, Intern. J. Appl. Radiat. and Isotop. 31, p. 817-822, 1981; M.S.Lagunas Solar, F.E.Little, S.L.Weters, J.A. Jungerman, J. Lab. Comp. a Radiopharm., 18, p.

272-275, 1981). Reasoning from insignificant yields of Pb-201 and Pb-200 in the range of proton energies of 42 to 52 MeV, the authors concluded that the nucleus of lead-206 had an enhanced stability and that it was reasonable to use enriched lead-207 as a target. The authors presented theoretical calculations of thallium-201 and thallium-200 yields from a lead-207 target and gave recommendations concerning the use of lead-207 for producing thallium-201. In the abovecited publications the authors gave no description of the radiochemical procedures for isolating thallium-201.

It is an object of the present invention to provide such a process for producing thallium201, which would allow an increase in the yield of the desired product per unit current of the proton bunch.

Said object is accomplished by the provision of a process for producing thallium201, wherein, according to the invention, a target from pure lead-206, enriched for at least 95%, is subjected to irradiation with a bunch of protons having an energy of 50 to 70 MeV, this resulting in a reaction 206Pb(p, 6n)201 Bi, the irradiated target being then kept for a period of time sufficient for the transition 20'Bi > 20'Pbe20'TI, treated with an acid to complete dissolution, thallium-201 contained in the solution thus prepared is oxidized to the trivalent state, whereafter lead is precipitated from the solution, and the traces of lead remaining in said solution and thallium-201 are separated by way of cation exchange, after which the thallium-201 is eluted with hydrochloric acid.

With the process conducted as described above it becomes possible to increase the yield of thallium-201 having a high radiochemical, radionuclide purity to 7 mCi/pA h and to rule out the necessity of working with a high-activity target, so that radiation hazards to the personnel will be diminished.

For a better separation of thallium-201 from macroquantities of lead, it is expedient to use bromine as an oxidant and potassium bromide as a precipitation agent.

For creating optimal conditions ensuring a high yield of thallium-201 with minimum yields of Tl-200 and Tl-202, it is preferable to use said target having a thickness of 0.3 to 2 g/cm2.

The herein-proposed process for producing thallium-201 is effected as follows.

A target from pure lead-206 enriched for at least 95% is subjected to irradiation with a bunch of protons having an energy of 50 to 70 MeV, this resulting in a reaction 206Pb(p,6n)201Bi. Then the irradiated target is kept for a period of time sufficient for the accumulation of a maximum quantity of thallium-201 from the parent products via the decay chain 201Bi201P201Tl.

We have stablished that the conditions of irradiating lead-206 with protons having an energy of 50 to 70 MeV are optimal for the formation of lead nuclei having an atomic mass of 201 with relatively high reaction cross-sections, whereas the yield of radionuclides 200Pb and 202Pb remains minimal. Therefore the yield of thallium-201 from a target of lead-206 increases to 7 mCi/uA.h. At the proton bunch energies lower than 50 and higher than 70 MeV there occurs a considerable diminishing of the yield of the main radionuclide thallium-201, and the cross-sections of the reactions of formation of thallium202 and thallium-200, whose presence in the final product is undesirable, increase accordingly.

For producing thallium-201, we subjected to irradiation with protons having an energy of 50 to 70 MeV targets from enriched lead207, as recommended by M.C.Lagunas-Solar, F.E.Little, S.L.Weters, J.A.Jungerman, J. Lab.

Comp. a Radiopharm., 18, p.272-275, 1981.

However, our experimentai verification disproved the theoretical supposition of the authors of the publication concerning the applicability of enriched lead-207 for producing thallium-201.

As has been shown above, an increase in the yield of thallium-201 depends on the irradiation conditions of the lead-206 target.

But there exists a dependence between the cross-section of the thallium-201 formation reaction and the thickness of the lead-206 target. For the irradiation conditions proposed by us it is preferable to use a target having a thickness of 0.3 to 2 g/cm2. The use of a target having a thickness smaller than or exceeding the above-cited range leads to a considerable reduction of the yield of thallium201, whereas the use of a target having a thickness exceeding the range leads to an increase in the yield of thallium-202, which reduces the quality of the final product.

The irradiated target containing thallium201 is treated with an acid to complete dissolution of the target, e.g. with nitric acid, and the thallium-201 contained in the resulting solution is oxidized to its trivalent state with a corresponding oxidant, preferably Br2.

Macroquantities of the target material, i.e. of lead, are separated from the solution by precipitation, e.g. with potassium bromide. After the separation of the precipitate-lead bromide, the solution containing traces of lead and thallium-201 is passed through an ionexchange column filled with a cation-exchange resin. The lead passes through the column unabsorbed, while the thallium-201 is retained. The thallium-201 absorbed by the cation-exchange resin is eluted with a hydrochloric acid solution in a volume equivalent to two or three free volumes of the column. The radiochemical yield of thallium-201 is 80%.

Given hereinbelow are specific examples of carrying the present process into effect.

Example 1 A target from 95%-enriched lead-206 of 99.999% purity, having a thickness of 0.3 g/cm2 is irradiated with protons having an energy of 50 MeV, this resulting in a reaction 206Pb(p,6n) 201Bi. After the irradiation the target is kept for 35 hours for the accumulation of thallium-201 from the parent products via the decay chain: 20'Bio20'Pbe20'TI. The irradiated target is treated with 4 M nitric acid to complete dissolution thereof, then 1 to 2 ml of bromine are added to oxidize thallium-201 to its trivalent state. To a warm solution potassium bromide solution is added to precipitate lead bromide, in such an amount that the concentration of potassium bromide in the solution should reach 3 M.

After filtering the lead bromide precipitate the solution is passed through a column filled with Dowex 50 x 8 cationexchange resin. The lead passes through the column unabsorbed.

The column is thoroughly washed with a 3 M solution of potassium bromide, and then thal lium-201 is eluted with a 1 M solution of hydrochloric acid in a volume equal to 2 free volumes of the column. The yield of thallium 201 is 2 mCi/A.h.

Example 2 A target from 95%-enriched lead-206 of 99.999% purity, having a thickness of 2 g/cm2 is irradiated with protons having an energy of 65 MeV, this resulting in a reaction 206Pb(p,6n)201Bi. After the irradiation the target is kept for 35 hours for the accumulation of thallium-201 from the parent products via the decay chain: 20'Bi~20'Pb~201TI The irradiated target is treated with 4M nitric acid to complete dissolution thereof, and then 1 to 2 ml of bromine are added to oxidize thallium-201 to its trivalent state. To a warm solution potassium bromide solution is added to precipitate lead bromide, in such an amount that the concentration of potassium bromide in the solution should reach 3 M.

After filtering the lead bromide precipitate, the solution is passed through an ion-exchange column filled with a cation-exchange resin Dowex 50 X 8. The lead passes through the column unabsorbed. The column is thoroughly washed with a 3 M solution of potassium bromide, and thallium-201 is eluted with a 1 M solution of hydrochloric acid in a volume equal to 3 free volumes of the column. The yield of thallium-201 is 7 mCi/ A.h.

Example 3 A target from 96%-enriched lead-206 of 99.999% purity, having a thickness of 1.8 g/cm2 is irradiated with protons having an energy of 65 MeV, this resulting in a reaction 206Pb(p,6n)20'Bi. After the irradiation the target is kept for 32 hours for the accumulation of thallium-201 from the parent products via the decay chain: 20'Bi 20'Pbo20'TI. The irradiated target is treated with 4 M nitric acid to complete dissolution thereof, and then 1 to 2 ml of bromine are added to oxidize thallium201 to its trivalent state. To a warm solution potassium bromide solution is added to precipitate lead bromide, in such an amount that the concentration of potassium bromide in the solution should reach 3 M.After filtering the lead bromide precipitate, the solution is passed through an iorn-exchange column filled with Dowex 50 X 8 cation-exchange resin. The lead passes through the column unabsorbed. The column is thoroughly washed with a 3 M solution of potassium bromide, and thallium-201 is eluted with a 1 M solution of hydrochloric acid in a volume equal to 3 free volumes of the column. The yield of thallium-201 is 7 mCi/p A.h.

Example 4 A target from 95%-enriched lead-206 of 99.999% purity, having a thickness of 2 g/cm2 is irradiated with protons having an energy of 70 MeV, this resulting in a reaction 206Pb(p,6n)20'Bi. After the irradiation the target is kept for 32 hours for the accumulation of thallium-201 from the parent products via the decay chain: 201Bi201P201Tl. The irradiated target is treated with 4 M nitric acid to complete dissolution thereof, then 1 to 2 ml of bromine are added to oxidize thallium-201 to its trivalent state. To a warm solution potassium bromide solution is added to precipitate lead bromide, in such an amount that the concentration of potassium bromide in the solution should reach 3 M.

After filtering the lead bromide precipitate, the solution is passed through an ion-exchange column filled with Dowex 50 x 8 cation-exchange resin. The lead passes through the column unabsorbed. The column is thoroughly washed with a 3 M solution of potassium bromide, and thallium-201 is eluted with a 1 M solution of hydrochloric acid in a volume equal to 3 free volumes of the column. The yield of thallium-201 is 6.8 mCi/ yA.h.

Claims

1. A process for producing thallium-201, residing in that a target from pure lead-206, enriched for at least 95%, is subjected to irradiation with a bundle of protons having an energy of 50 to 70 MeV, this resulting in a reaction 206Pb(p,6n)20'Bi, the irradiated target is kept for a period of time sufficient for the transition 201Bi201Pb201Tl, then the target is treated with an acid to complete dissolution thereof, the thallium-201 contained in the resulting solution is oxidized to its trivalent state with a subsequent precipitation of lead from the solution, and the remaining traces of lead and the thallium-201 are separated by way of a cation exchange, whereafter the thallium-201 is eluted with hydrochloric acid.

2. A process according to Claim 1, wherein use is made of said target having a thickness of 0.3 to 2 g/cm2.

3. A process according to Claims 1, 2, wherein bromine is used as an oxidant.

4. A process according to Claims 1 to 3, wherein precipitation of lead from said solution is effected with potassium bromide.