The background gamma-ray study before and after the Chernobyl accident

The problem of maintaining a low-level spectrometer free of contamination under conditions of high environmental contamination is of significant concern to monitoring laboratories. A chance to test the procedures for protecting our HP-Ge detector arose during the first stages of the Chernobyl incident. The background gamma-ray spectra of the 8% efficiency shielded low-background HP-Ge detector in the energy range of 60–2700 keV before and after the Chernobyl accident had been investigated. In the measurement performed just the day before a “radioactive cloud” reached the area, the total count rate in that energy range was found to be 0.58 c.p.s. The highest rates between 0.8 and 0.9 c.p.s. were obtained 4–7 days after the accident. In the measurement from about 70 days after the accident, the rate was 0.59 c.p.s. (i.e., very close to the rate value before the accident). The presence of some 14–15 fission products have been confirmed in measurements of gamma spectra of samples taken from outside the laboratory, shortly after a radioactive cloud spread all over the country. In the background spectrum taken 70 days later, only gamma lines of three isotopes remained: 103Ru(39.2d), 134Cs(2.06y), and 137Cs(30.14y).