Experience in induced radioactivity measurements in the Institute of Nuclear Study

The measurements of the induced activity due to the cyclotron and the synchrotron in the Institute of Nuclear Study are reported, which were carried out a few times so far. The apparent half-lives and gamma-ray spectra of the targets (C, Si, Al, Cu, stainless steel, Ta) irradiated with the particles of various energy and intensity for a few minutes were measured. The result showed that the apparent half-lives were unexpectedly long, some 8 to 20 minutes even for carbon targets. The products were found to be all positron emitters. Accordingly, the caution is necessary even in case of short term irradiation because the exposure to residual activity is expected. In case of the synchrotron, the residual activity is distributed over the whole accelerating tube, especially high dose rate was observed in the vacuum chamber. The dose rate distribution shows several patterns according to the operating conditions of the synchrotron. Therefore it is necessary to start the works in the synchrotron room after the conditions of dose rate distribution has been found by measuring them with a survey meter everytime. The apparent half-lives due to the residual activity in the synchrotron room are divided into three groups of 15 minutes, 60 minutes and long half-life. Since most of nuclides have comparatively short half-lives as described above, the dose rate decreases to a half of initial value 15 minutes after the synchrotron was stopped. The nuclides observed as residual activity were 54Mn, 59Fe, 56Co, 57Co, 58Co, 60Co and 65Zn one month after the synchrotron was stopped. (Wakatsuki, Y.)