18F half-life measurement using a high-purity germanium detector

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Abstract

The half-life of 18F has been measured using HPGe detectors with a 137Cs reference source. The counting ratio of 511 keV γ-rays from 18F to 622 keV γ-rays from 137Cs was fitted for the half-life with a weighted least-square method. Uncertainties due to the systematic effects arising from the measurement of a high activity 18F source were studied in detail. The half-life of 18F was found to be (109.72±0.19) min. The result is in a good agreement with the recommended value of (109.728±0.019) min evaluated at the Laborotaire National Henri Becquerel (LNHB).

Highlights

► The 18F half-life was measured with a reference source and without it using HPGe detectors. ► We found the systematic effect ‘activity dynamic range effect’ by monitoring the counts of the reference source. ► This activity dynamic range effect was corrected by using the reference source method. ► The 18F half-life using the reference source method was in a good agreement with the recommended value of LNHB.

Introduction

Positron emission tomography has been widely used for cancer diagnosis with short-lived positron emitting nuclei such as 11C, 13N, 15O and 18F. The short half-life makes it difficult to calibrate dose calibrators directly in hospitals, however. More precise values of lifetimes would improve the precision of calibration. The first measurement of 18F lifetime was performed by Snell (1937). Later, a neutron and triton beams were used with a muscovite plate by Blaser et al. (1949) and Jarmie (1955). Schrader (2004) measured the 18F half-life with an ionization chamber. García-Toraño et al. (2010) measured the 18F half-life using ionization chambers and HPGe detectors in coincidence with fast scintillators.

Measurement of half-lives in the range from a few hours to days is more difficult than that for much shorter or much longer half-lives. Measurement of half-lives of a few seconds brings little serious systematic instability because of the short measurement time. On the other hand, half-lives of very long-lived nuclei can be measured using an isotopic composition ratio as in the case of 176Lu measurement performed by Nir-El and Lavi (1998).

An ionization chamber can be used to measure a half-life because of its long term stability. However, since this kind of detector has no capability for energy discrimination, it can only be used for a single radionuclide analysis. In this measurement method, it is necessary to find other means to take into account a source impurity and the potential non-linearity of detector response. On the other hand, an HPGe detector is well suited in this respect for the measurement of these half-lives. An HPGe detector was used with a reference source method in this report.

Systematic effects of using the HPGe detector were corrected by using 137Cs as a reference source which decays with a half-life of 10 976±30 d (Bé et al., 2000).

Section snippets

Experiment

Two HPGe detectors, an n-type detector with a 10% relative efficiency and a p-type detector with a 70% relative efficiency, were used separately for the measurements. The total acquisition time was approximately 8.3 hours, which is 4.5 times longer than the 18F half-life.

In the case of the measurement using the n-type detector FDG (flurodeoxyglucose) in a glass vial was used as a volume source. A 137Cs point source was used as a reference source while measuring the 18F source . This reference

Result and analysis

Fig. 3 shows a γ-ray energy spectrum for the 18F and the reference source measured for 301 s. The MCA has a 1 s latency before the next measurement. The 511 keV peak is originated from 18F and the 662 keV peak is originated from 137Cs. The y-axis is the counts per each energy bin width of 0.985 keV.

Fig. 4. shows the interesting peak and the specific counts which are needed to calculate the net counts.

The 511 keV γ-ray peaks of 18F were analysed by Gamma (ORTEC, ver 5.3). The ROI (Region Of Interest)

Discussion

The two half-life measurements of the present work indicated with square brackets were compared with the half-life determined by the Laboratoire National Henri Becquerel (LNHB) in Fig. 9.

The half-life of 18F measured using the reference source method (109.72±0.19) min is consistent with the LNHB evaluated value within a 1σ uncertainty interval. The half-life of 18F determined without using the reference source was further from the other two values due to the systematic errors, especially the

Conclusion

Measuring the half-life of 18F using an HPGe detector was experimentally feasible and led to a good agreement with the recommended half-life value, especially when a reference source was used. The experiment using a reference source allowed cancellation of systematic errors, which is a powerful advantage especially for a short half-life radionuclide such as 18F. Using the dead time correction evaluated by the MCA caused a systematic effect in the half-life measurement. The systematic effect,

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