Determination of impurity radionuclides in 131Cs solutions

Abstract

The radiochemical purity of a 131 Cs solution used in brachytherapy is studied. After separating 131 Cs from the neutron irradiated targets BaO, Ba(NO 3 ) 2 , and BaCO 3 , the contribution of impurities was evaluated: 0.015% for 124 Sb and 0.012% for 132 Cs. The contribution of the parent 131 Ba to cesium solutions was, on average, 0.0067% for BaO, 0.01% for Ba(NO 3 ) 2 , and 0.011% for BaCO 3 .Brachytherapeutic treatment methods for cancer are widely used in nuclear medicine. The essence of brachytherapy lies in the fact that several tens of microsources of ionizing radiation are introduced into tissue tumors using special needles without any surgical intervention. The microsources are small titanium capsules-grains containing a radionuclide which possesses definite nuclear characteristics, such as the presence of low-energy radiation for effective action on a tumor and absence of high-energy radiation to prevent damage to healthy tissue. Radiation sources based on 131 Cs were first used in 2004 in clinics in the USA and it was determined that 131 Cs suppresses the growth of cancer cells more quickly and effectively than 125 I and 103 Pd, which are used now [1,2]. Radiation microsources based on 131 Cs have proven themselves well for treatment of various types of tumors but especially cancer of the prostate gland [1].Radionuclides used in nuclear medicine and, specifically, brachytherapy must meet stringent requirements with respect to radiochemical purity. The preparations used must possess high purity (99.99%) and the content of impurity radionuclides must not exceed 0.01% [3,4]. For this reason, the radiochemical purity of a preparation after 131 Cs has been separated is the main characteristic of its suitability for medical use. Detecting and qualitatively monitoring impurity radionuclides are special problems in obtaining 131 Cs for brachytherapy.It is best to obtain 131 Cs by irradiating naturally occurring barium with thermal neutrons according to the reaction 130 Ba (n, γ) → 131 Ba → 131 Cs → 131 Xe, as a result of which 131 Ba is formed from low-abundance (0.1%) 130 Ba. The parent radionuclide 131 Ba with T 1/2 = 11.8 days forms as a result of βdecay the daughter radionuclide 131 Cs with T 1/2 = 9.7 days, which has only the x-ray lines K α = 29.65 keV and K β = 33.61 keV. As a result of βdecay, 131 Cs transforms into stable 131 Xe [5,6].Calculations were performed to evaluate the activity of radionuclides which are formed as a result of irradiating barium targets in a nuclear reactor. As Table 1 shows, for prolonged irradiation of barium, aside from the desired 131 Ba, more than 10 radionuclides whose half-life ranges from 2.5 min to 10.5 yr and specific activity from 1.6·10 5 to 5.9·10 10 Bq can form [6,7].Different barium compounds can be irradiated to obtain 131 Cs: BaCO 3 , Ba(NO 3 ) 2 , BaO, and BaCl 2 ·2H 2 O. However, prolonged irradiation of hygroscopic barium compounds damages the quartz ampul because of the elevated internal pressure. Experiments showed that barium chloride dihydrate is simil...