Precision half-life measurement of ¹⁸N

Half-life of neutron-rich nuclei is essential for r-process nucleosynthesis in supernova, or neutron star merger, which produces heavy nuclei by neutron capture reaction and β-decay. However, since it is difficult to produce them and to measure short half-life of 〜100 ms, it has not been determined yet for many neutron-rich nuclei.

In order to develop a method to determine the short half-life of nuclei , we performed an experiment for ¹⁸N produced by the ⁹Be+¹⁸O reaction with E(¹⁸O)=9.4 MeV/u at RCNP RI beam line of Osaka university.
The ¹⁸N beam with intensity of 〜1300 pps was implanted in a gold stopper. β-ray was detected by 5 plastic scintillators with efficiency of 〜80 %.
The ¹⁸N beam was pulsed with duration of beam ON time of 1.9 s and beam OFF time of 4.3 s, which was optimized to determine half-life with most high precision.

However, for high precision measurement, dead-time correction is indispensable, because dead-time changes significantly as the count rate changes drastically. Therefore, we developed a unique method of dead-time correction by measuring dead-time event by event with 40 MHz clock signals.

We successfully determined half-life of 622.8(6) ms for ¹⁸N by the careful analysis with correction of dead-time, and subtraction of back ground components.