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Authors (affiliation): 1. Ken-ich Mori (Kindai Univ., Japan), 2. Takahiro Yamada (Kindai Univ., Japan), 3. Yasushi Sato (NMIJ/AIST, Japan), 4. Kotaro Nagatsu (QST, Japan), 5. Hidetoshi Kikunaga (Tohoku Univ., Japan).
Recently, 225Ac has been proposed for Targeted Alpha Therapy. In such medical applications and studies, quantitative activity measurements for evaluating the accumulation of radioactivity in tissues and dosimetry are indispensable. However, because α emitters including 225Ac have complex decay chains, conventional coincidence counting techniques are difficult to apply in a straightforward manner. Liquid scintillation counting (LSC) techniques have been successfully applied to the standardization by several authors. In the measurement of 225Ac using LSC techniques, however, the detection efficiency of α-rays emitted from the decay of a short-lived progeny, 213Po is significantly reduced due to 213Po decay during the dead time generated by its parent nuclide 213Bi β-ray detection. We found that this problem can be solved by sandwiching a source between two ultra-thin plastic scintillator (PS) sheets as only α signals can be selected under the presence of both α and β nuclides. Thus, α-rays emitted from 213Po can be detected with high detection efficiency. In this study, carrier-free 225Ac solutions were produced via the 226Ra(p, 2n)225Ac reaction in the NIRS-AVF-930 cyclotron or chemically separated from 229Th. 225Ac solution was directly dropped onto a 20 µm ultra-thin PS sheet. The source was then covered with an identical PS sheet after drying. To determine individual α-counting efficiency, 4πα-γ anti-coincidence spectrometry techniques were adapted with a 4πα-γ detector configuration composed of a sandwich-type PS sheets detector and a Ge detector. A list-mode MCA (16bit ADC, 40 ns/bit resolution timestamp) which treats shaping amplifier output signals from both α- and γ-channels. A high-speed digitizer (1GHz ADC for a PS, 62.5 MHz for a Ge-detector), which could directly accept pre-amplifier output signals from both channels, was used. The system records pulse height information along with a time stamp for every recorded event and software was developed to generate the coincidence/anti-coincidence spectra or time spectra. In the study, the supplied voltage to the photomultiplier tube (PMT) was adjust appropriately to avoid β contribution on the PS sheets. To validate the detection of α-particles from 213Po, a series of measurements of γ-α/β time difference spectra was conducted with varying voltages supplied to the PMT. The time-difference spectra were obtained as histograms of time-differences from the detection of 440 keV γ-rays due to 213Po γ-rays being emitted promptly after 213Bi β decay. Consequently, most of the time differences were distributed within 1 μs with a higher voltage supply to the PMT. The results demonstrated that few α-particles from 213Po were detected due to the detection of β-rays of 213Bi. However, with a lower voltage supply, the time-difference distribution could be fitted with an exponential function, and a half-life of 3.67±0.12 μs was obtained. This showed that 213Po induced α-rays were detected with minimal β-rays interference. The α efficiencies determined as 1- nanti/nγ were 0.971 and 0.987 for 221Fr and 213Po, respectively. Finally, the activity concentration of 225Ac calculated using the determined α-efficiency was consistent with the results obtained using the Ciemat/Nist-method by NMIJ within their uncertainties.
