[Daily fluctuation of background count rate of a liquid scintillation counter]

3H channel count rate of 3 background samples were measured for 8 days. Sample A and C were different scintillators in 100 ml teflon vials and sample B in a 20 ml glass vial. Count rate of sample A fluctuated from 3 to 7 cpm, and that of C from 6 to 12 cpm, respectively. However, count rate of sample B was about 6 cpm and rather constant. Trend of fluctuation in count rate of sample A and C were observed to be similar. On weekdays, count rate increased at night and decreased in day time. At weekend, count rate increased from Saturday evening and was constant from Sunday noon till Monday morning. Tritium Research Center where LSC is placed is ventilated from 9 a.m. to 6 p.m. on weekdays, and the ventilation is suspended on Sunday. The observed fluctuation of background was apparently related to the operation and suspension of ventilation. When ventilation was operated continuously, count rates of sample A (4 cpm) and sample C (6 cpm) became constant and corresponded to their minimum value. The fluctuation of background may be caused by radon, concentration of which varies in association with the operation and suspension of ventilation.