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Calibration of a solid state nuclear track detector for the measurements of radon concentration in air
Pergamon
Appl. Radiat. lsot. Vol. 46, No. 6/7, pp. 483-484, 1995 Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0969-8043/95 $9.50 + 0.00
0969-8043(95)00061-5
CALIBRATION OF A SOLID STATE NUCLEAR TRACK D E T E C T O R F O R THE MEASUREMENTS OF RADON CONCENTRATION IN AIR K. HAKAM OUM, M. LFERDE and M. BERRADA Laboratoire de Physique Nucleaire, Faculte des Sciences, B.P. 1014, Rabat, Morocco
Time-integrated measurements of environmental radiation activity are commonly carried out using solid state nuclear track detectors (SSNTD). This paper reports the results of experiments conducted to calibrate cellulose nitrate films LR-115 type II used for measurements of radon concentration in indoor air in Morocco. Calibration measurements were made in our laboratory using a calibration chamber and a radon source. The calibration chamber is a cylindrical box (2613,6 cm 3) made of aluminium. The radon source is a natural sample (14,514 g) rich in uranium (5053 ppm). Its emanation rate (34%) and its activity (17,29 +__ 0,12 Bq/g) were determined by a gamma spectrometry technique (Berrada et al., 1989). The first experiment was carried out, in which a gold silicon surface barrier semiconductor detector (SCD) was placed on a stand in the middle of the chamber (Fig. 1) and connected to a multichannel analyser through an outlet. We have seen that there is a clear growth to a steady-state plateau in the semiconductor count-rate; so that there is no leakage of radon from the calibration chamber. LR-115 films cut in pieces of 3,4 x 2,5 cm2 were placed in a detector holder and enclosed in a heat-sealed polyethylene bag (40 tim). The LR-115 is sensitive to alpha particles with an energy less than 4 MeV (Hakam, 1993). The energy of the alpha particles from radon and its short-lived daughters are 5, 6 and 7,7 MeV and the air in the sensitive volume of the detector holder is not sufficient to reduce their energy to less than 4 MeV. Therefore a thin foil (20/zm) of aluminized polycarbonate has been introduced between the detectors and the sensitive volume. The films are exposed inside the calibration chamber for different times. Following the exposure, the films were chemically etched in NaOH (2,5 N) at 60°C for 120 minutes. The number of registered alpha particles tracks were counted with an optical microscope and the background was then subtracted. The number of tracks counted depends on the removed thickness of the detector during the chemical treatment (Hakam, 1993) and it is expressed at least in the range 5 to 8/zm by: D(6) = D / (1 + 0,27(t-6) )
(1)
Where D is the observed track density, D(6) is the track density normalized to 6/zm and t is the removed thickness. 483
484
K. Hakam Oum et al.
Equation (1) makes it possible to correct the results on the basis of the removed thickness by normalizing the data to a nominal thickness of 6 t~m. The calibration factor of the plastic-bag radon sample based on LR-115 alpha track registration by optical microscope is found to be about 0,58 tracks.era 2 / kBq.h.m 3 (Table 1) at the reference removed thickness of 6/~m. TABLE 1: Calibration factor for LR-115 type II films Tracks density (tracks.cm -2)
Integrated activity concentration for radon (kBq.h.m "3)
Calibration factor (tracks.cm-2)/(kBq.h.m -3)
................................................................................................................
380+49 1036 + 60 1335 +90 2217 +___136 3028_____139 4129_____126
685 + 9 2466 + 22 2466+30 3831 +47 4896 +60 6853 -t-84
0,55 0,59 0,54 0,58 0,62 0,60 mean = 0,58 To alpha spectrometer
Stopper ~ 1 1 ~
i111I
1 1 1 1 1 1 1 1 1 1 1
,,
Seal SCD Polyethylene bag Detector holder
L
11 f • '
Radon source
~
Fig. 1 : Calibration chamber for the determination of LR-115 calibration factor for radon measurement REFERENCES Berrada, M., Choukri, A., and E1 Khoukhi, T,, (1989). Uranium, Radium and Radon emanating rates of Moroccan Phosphate samples. In workshop on radon monitoring in radioprotection environmental radioactivity and earth sciences. Trieste, Italy. Hakam, O., (1993). Mesure des activites volumiques du radon dans l'air des habitations et des enceintes de travail par les detecteurs solides de traces nucleaires. These de 3 emecycle, Rabat, Morocco.
Appl. Radiat. lsot. Vol. 46, No. 6/7, pp. 483-484, 1995 Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0969-8043/95 $9.50 + 0.00
0969-8043(95)00061-5
CALIBRATION OF A SOLID STATE NUCLEAR TRACK D E T E C T O R F O R THE MEASUREMENTS OF RADON CONCENTRATION IN AIR K. HAKAM OUM, M. LFERDE and M. BERRADA Laboratoire de Physique Nucleaire, Faculte des Sciences, B.P. 1014, Rabat, Morocco
Time-integrated measurements of environmental radiation activity are commonly carried out using solid state nuclear track detectors (SSNTD). This paper reports the results of experiments conducted to calibrate cellulose nitrate films LR-115 type II used for measurements of radon concentration in indoor air in Morocco. Calibration measurements were made in our laboratory using a calibration chamber and a radon source. The calibration chamber is a cylindrical box (2613,6 cm 3) made of aluminium. The radon source is a natural sample (14,514 g) rich in uranium (5053 ppm). Its emanation rate (34%) and its activity (17,29 +__ 0,12 Bq/g) were determined by a gamma spectrometry technique (Berrada et al., 1989). The first experiment was carried out, in which a gold silicon surface barrier semiconductor detector (SCD) was placed on a stand in the middle of the chamber (Fig. 1) and connected to a multichannel analyser through an outlet. We have seen that there is a clear growth to a steady-state plateau in the semiconductor count-rate; so that there is no leakage of radon from the calibration chamber. LR-115 films cut in pieces of 3,4 x 2,5 cm2 were placed in a detector holder and enclosed in a heat-sealed polyethylene bag (40 tim). The LR-115 is sensitive to alpha particles with an energy less than 4 MeV (Hakam, 1993). The energy of the alpha particles from radon and its short-lived daughters are 5, 6 and 7,7 MeV and the air in the sensitive volume of the detector holder is not sufficient to reduce their energy to less than 4 MeV. Therefore a thin foil (20/zm) of aluminized polycarbonate has been introduced between the detectors and the sensitive volume. The films are exposed inside the calibration chamber for different times. Following the exposure, the films were chemically etched in NaOH (2,5 N) at 60°C for 120 minutes. The number of registered alpha particles tracks were counted with an optical microscope and the background was then subtracted. The number of tracks counted depends on the removed thickness of the detector during the chemical treatment (Hakam, 1993) and it is expressed at least in the range 5 to 8/zm by: D(6) = D / (1 + 0,27(t-6) )
(1)
Where D is the observed track density, D(6) is the track density normalized to 6/zm and t is the removed thickness. 483
484
K. Hakam Oum et al.
Equation (1) makes it possible to correct the results on the basis of the removed thickness by normalizing the data to a nominal thickness of 6 t~m. The calibration factor of the plastic-bag radon sample based on LR-115 alpha track registration by optical microscope is found to be about 0,58 tracks.era 2 / kBq.h.m 3 (Table 1) at the reference removed thickness of 6/~m. TABLE 1: Calibration factor for LR-115 type II films Tracks density (tracks.cm -2)
Integrated activity concentration for radon (kBq.h.m "3)
Calibration factor (tracks.cm-2)/(kBq.h.m -3)
................................................................................................................
380+49 1036 + 60 1335 +90 2217 +___136 3028_____139 4129_____126
685 + 9 2466 + 22 2466+30 3831 +47 4896 +60 6853 -t-84
0,55 0,59 0,54 0,58 0,62 0,60 mean = 0,58 To alpha spectrometer
Stopper ~ 1 1 ~
i111I
1 1 1 1 1 1 1 1 1 1 1
,,
Seal SCD Polyethylene bag Detector holder
L
11 f • '
Radon source
~
Fig. 1 : Calibration chamber for the determination of LR-115 calibration factor for radon measurement REFERENCES Berrada, M., Choukri, A., and E1 Khoukhi, T,, (1989). Uranium, Radium and Radon emanating rates of Moroccan Phosphate samples. In workshop on radon monitoring in radioprotection environmental radioactivity and earth sciences. Trieste, Italy. Hakam, O., (1993). Mesure des activites volumiques du radon dans l'air des habitations et des enceintes de travail par les detecteurs solides de traces nucleaires. These de 3 emecycle, Rabat, Morocco.