Effect of the Radon Background Fluctuation on the Low Activity Measurement of the 214 Pb , 214 Bi and 226

In this study, radon background fluctuation effect on the measurement of 214 Pb, 214 Bi and 226 Ra low level activity concentrations were investigated in ventilated laboratory. For this purpose, detectors which had 150% and 20% efficiencies were used. In spite of the air ventilation, radon background fluctuation effect on the low level activity measurement was observed. 20% efficiency detector measured activity discrepancies were 5.7% for 214 Pb, 8.7% for 214 Bi and 5.8% for 226 Ra. 150% efficiency detector measured activity discrepancies were 13.8% for 214 Pb, 11.5% for 214 Bi and 12.7% for 226 Ra.


Introduction
The background spectrum is using for Quality Assurance/Quality Control purposes and for background correction in the spectrum evaluation algorithm.Therefore background contribution to the gamma spectrometric measurement is important.For this reasons background spectrums are obtaining regularly.Background spectrum of an HPGe detector arises from the cosmic radiation, building materials surrounding it and radioactive impurities in the detector and shielding materials.The naturally occurring radionuclides are 238 U and 232 Th decay series and 40 K. Main contributions to the natural background spectrum are these primordial radionuclides [1-4].Generally the activity levels of primordial radionuclides are low but there are encountered high activity concentrations.
Another contribution to the background is radon gas and its daughter nuclides.Radon is a noble gas and concentration in the laboratory changes.Radon permeates in the detector environment and affects the peak count rates in the spectrum through its daughter nuclides.Additionally radon background fluctuation affects the Compton continuum.Radon fluctuation in the laboratory can affect considerably the activity determination and reduces its reliability.The most serious effect of the radon fluctuation in laboratory is on the measured activity concentration of the 214 Pb, 214 Bi daughters of the 226 Ra.
Generally environmental sample contains low content of artificial and natural radionuclides.The determination of low level activities of natural radionuclides in these samples has problem of natural background subtraction.Low detection limits provide more accurate and precise measurements of radionuclides in environmental samples.So to improve low detection limit gamma-ray spectrometry laboratories pay special attention to minimize the radiation background of detectors [5-7].
Measurement sensitivity in energy regions not containing the peaks of interested radionuclides will be affected by radon fluctuations in the background.
Also the minimum detectable activity (MDA) of the photopeaks of the radionuclides in the spectrum will be affected by radon fluctuation in the background.Commercially available gamma spectrometry analysis software packages include peaked background corrections.But this software packages do not consider fluctuation of the radon background level in the laboratory due to the seasonal and climate changes.
In the present work radon background fluctuation effect on the activity measurements in the ventilated gamma spectrometry laboratory is investigated.Background measurements are taken in different days.Each background spectrums is used to evaluate 214 Pb, 214 Bi and 226 Ra activity concentration in IAEA-SOIL-375 soil sample.Also MDA of the each radionuclide is calculated.

Materials and Methods
Saraykoy Nuclear Research and Training Center (SNTRC) Gamma Spectrometry Laboratory (GML) operates 10 coaxial HPGe detectors with relative efficiencies 10-150% and -DSPEC multichannel analyzer.GML accredited by Turkish Accreditation Agency (TURKAK), a member of International Laboratory Accreditation Cooperation, in 2009.GML has taken part in comparison tests and has been inspected by TURKAK experts annually since then.GML has two entrances and two ventilation system mounted on the windows.Both ventilation systems are on 24 hours and 365 days.
Two software programs GAMMAVISION-32 and GENIE-2000 have been used for data analyses.Detectors which were chosen for statistical comparison of background fluctuation are presented in Table 1.Before the measurements, energy calibration and peak shape calibration was done by using peaks of 241 Am, 137 Cs and 60 Co radionuclides which were obtained from a certified radioactive standard point sources.IAEA-RGU was used as standard reference material and IAEA-SOIL-375 was used as sample for activity measurement.Activity concentrations are directly proportional to the net count rate of the samples.Therefore, to determine activity concentrations of samples, the net count rate of reference material and the net count rate of samples were compared with each other and calculated at 2s statistical uncertainty.According to the IAEA-SOIL-375 certificate 226 Ra activity was 20 Bq/kg.Considering that 226 Ra in equilibrium with the daughter 214 Pb and 214 Bi, activity of the 214 Pb and 214 Bi was 20.0 Bq/kg.This value appropriate for to determine the effect of the radon fluctuation on the low level activity.
The activity concentrations of the samples were determined from radionuclide's own energies or gamma ray photopic of their decay products.The activity concentration of 214 Pb was calculated from 295.2, 351.9 keV gamma-ray energies.The activity concentration of 214 Bi was calculated from 609.3 keV.The activity concentration of 226 Ra was calculated from the 295.2, 351.9 keV gamma-ray energies of 214 Pb and 609.3 keV of 214 Bi.
The activity concentrations of the samples are calculated by Equation 1: where N corresponds the net peak area of gamma-ray energy, εγ denotes the absolute efficiency, Pγ is the gamma-ray yield per decay, t and M denotes the counting time and sample mass, respectively.Minimum Detectable Activity (MDA) calculations were performed by Equation 2 [8]: where   stands for standard deviation of the background in the region of interest and equal square root of the number of count for the background spectrum;  is absolute efficiency; P is emission probability of gamma decay; t and w denote measurement time and weight of the dried sample respectively.

Results and Discussion
Background measurements are planned daytime, nighttime, all day time and longtime for the Det#01 and longtime for the Det#02.Each background spectrums is used to evaluate 214

Table 1 .
Specifications Pb,214Bi and226Ra activity concentrations in IAEA-SOIL-375 soil sample.Daytime, nighttime, all day time and longtime measurements were carried out for Det#01.Counting times were 28000, 57000, 83000 and 260000 sec respectively.Measured activity, MDA and deviation from reference values are shown inTable 2, 3, 4, 5, 6.As can be seen at tables, biggest variation of the activity measurement was daytime measurements for Det#01.Table 2 shows the results of the measurements.Minimum and maximum value of the activity measurements for 214 Pb, 214 Bi and 226 Ra are 21.4-23.2Bq/kg, 20.3-22.6 Bq/kg and 20.9-22.9Bq/kg respectively.Maximum deviations from reference value are 13.8% for 214 Pb, 11.5 % for 214 Bi and 12.7 % for 226 Ra.Measurement average value of the 226 Ra was 21.9 Bq/kg.Upper level of 226 Ra activity value in the IAEA-Soil-375 was 22.0 Bq/kg in 95% confidence interval.In table 2, there are 6 values for 214 Pb, 3 values for 214 Bi and 6 values for 226 Ra outside the acceptable range.Deviation from reference value for average measurement value of the 226 Ra was 8.6%.Due to the low detector efficiency only long time measurements were carried out for Det#02.Counting times were 260000 sec.Table 6 shows the results of the measurements.Minimum and maximum value of the activity measurement for 214 Pb, 214 Bi and 226 Ra are 19.5-21.2Bq/kg, 18.4-20.5Bq/kg and 18.9-20.7Bq/kg respectively.Maximum deviation from reference value 5.7% for 214 Pb, -8.7% for 214 Bi and 5.8% for 226 Ra.When we compared these values with the reference values, these values were in the acceptable range.Average measurement value of the 226 Ra was 19.9 Bq/kg.Deviation from reference value was 0.5%.

Table 2 .
Measured activities, deviation from the reference values and MDA values for Det #01(Daytime).

Table 3 .
Measured activities, deviation from the reference values and MDA values for Det #01(Nighttime).

Table 4 .
Measured activities, deviation from the reference values and MDA values for Det #01(Allday time).

Table 5 .
Measured activities, deviation from the reference values and MDA values for Det #01(Longtime).

Table 6 .
Measured activities, deviation from the reference values and MDA values for Det #02Variations of the count rates caused by the changes of the radon levels in laboratory have significant effects on the measured activity concentrations of 214 Pb,214Bi and 226 Ra. 150% and 20% detectors were used to evaluate this effect.GML is ventilated laboratory but effect of the radon background fluctuations on the low activity measurement of the 214 Pb, 214 Bi and 226 Ra is observed.Measured radioactivity concentrations discrepancies for the 150% efficiency detector are bigger than 20% efficiency detector.For 20% efficiency detector measured activity concentration discrepancies were 5.7% for 214 Pb, 8.7% for 214 Bi and 5.8% for 226 Ra.For 150% efficiency detector maximum measured activity concentration discrepancies were 13.8% for 214 Pb, 11.5% for 214 Bi and 12.7% for 226 Ra.Minimum radon background fluctuation for Det#01 and Det#02 is longtime counting.Therefore longtime background counting is appropriate for activity concentrations determination or mean value of the long term monitored background fluctuation can be taken.