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Thoron contributions in radon measurements in the environment
Radiation Measurements, Vol. 25, Nos 1--4,pp. 615--616,1995
Pergamon
Copyright© 1995ElsevierScienceLtd Printed in Great Britain. All rights reserved 1350-4487/95$9.50+ .00 1350-4487(95)00199-9
THORON
CONTRIBUTIONS IN RADON MEASUREMENTS IN THE ENVIRONMENT
R. SHWEIKANI*t and S. A. DURRANIt ~'School of Physics & Space Research, University of Birmingham, Birmingham B15 2TT, U.K.
ABSTRACT SSNTDs have found wide use for the determination of indoor radon levels. Most of these types of detector involve filtration, which assumes that any thoron present will decay completely before the alpha particles from the gas itself and its daughter products can be registered on the plastic surface. A surface barrier detector (SBD) was placed at the same position as, and instead of, the plastic detector employed in a routine measurement. An active method of measuring ambient thoron concentrations has been devised. KEYWORDS Surface barrier detector, NRPB dosimeter, Can technique, Thoron and radon diffusion, Filtration. INTRODUCTION The health hazards from radon (Rn-222) and thoron (Rn-220) and their progeny are of considerable concern to the public ~. Few studies on the indoor levels of thoron and its decay products, and their contribution to health hazards, have been made 2'3. Solid state nuclear track detectors are widely used for radon measurements and usually calibrated by affixing them inside dosimeters and then placing the dosimeter inside a chamber with known radon activity concentration for a certain time4. The track density on the detector is a measure of radon exposure. This is only true ff the assumption that thoron can not diffuse into the dosimeter is correct. This work is aimed at studying the contribution of thoron to radon measurements in two different types of radon dosimeters which are used in houses and in the field. EXPERIMENTAL TECHNIQUE The first type of dosimeter, which was used in this work, was for evaluating radon concentration in soil as a measure of the potential radon ha7~rd in the surrounding area, this is called the 'can' technique. Figure (1) shows the cross section of this dosimeter. In this work a SBD was placed air-tight at the normal position of the plastic detector. The second dosimeter was the NRPB-type dosimeter which is often used for indoor radon measurements. Figure (2) shows the cross section of this dosimeter. The SBD was placed air-tight in the lower half in the compartment where the plastic detector is usu,311yaffixed. These dosimeters containing the SBD were placed separately inside a cylindrical air-tight metallic chamber of 20 cm diameter and 50 cm depth containing a mixed environment of thoron and radon gases, and a counting time of 66 hours was set. Another 66 hours was also set for the same SBD to count bare (i.e. without the dosimeter housing). In both cases, the active surface of the detector was at the same height from the sources (about 30 era). An attempt was also made to measure the activity concentration of thoron in the radon/thoron chamber. The idea was to draw a known volume of air through a filter paper, which was immediately afterward monitored by a SBD. From the area under the Po-212 peak (8.79 MeV) of the first spectrum obtained (aRer 1 minute of drawing and for 1 minute counting) the activity concentration of Bi-212 could be calculated. After waiting for about 10 hours radon daughters and also Bi-212 which is a thoron daughter will be dead and the only remaining radioactive material is Pb-212 which has a half life of 10.6 hours. From the areas under the two peaks (Bi-212 of 6.05 Mev and Po-212 of 8.79 Mev), the activity concentration of the Pb212 could be determined RESULTS AND DISCUSSION The two spectra obtained from the multichannel analyser (MCA) for the contribution of thoron inside the two types of dosimeters ('can' technique and NRPB-type) are shown in figs(3) and (4) respectively. Fig (5) shows *Permanent address: Atomic Energy Commission of Syria, P.O. Box 6091, Damascus, Syria
615
616
R. SHWEIKANI and S. A. DURRANI
the spectrum which was obtained by the SBD placed bare inside the chamber at the same height from the source as was mentioned before. By comparing these graphs it is clear that the filter on the top of the can does reduce the entrance ofthoron as well as radon. It seems that thoron finds it more difficult to get into the NRPB type. This means that the tracks on the plastic detector using the can technique for radon measurements is not only from radon and its progeny hut also from thoron and its progeny. Fig(6) shows the spectrum obtained 10 hours aRer the end of the air drawing. The clear two peaks are the two alpha particles produced from the decay of Pb-212 down to Pb-208. The areas under the two peaks were found to be 540 and 907 respectively. These results give a ratio of (37.3 5: 1.6)% for the first peak and (62.75: 2) for the second peak. It is clear from this result that these two peaks are purely a result of the Bi-212 decay which has come direct from the 15 decay of Pb-212. Therefore, the activity of Pb-212 on the filter could be easily calculateds. Aluminum
(
Can
10 cm x.6.5 cm)
Z'Ji-: kill
t
[11.214
i1 *l l
Plastic detector (CR-39 ~ 2.5cm)
Filter paper I~-ll !
I
O ~ll~Plestic
l
Flange
,
I
?
,
I
)
,
l
i
,
,
5
5
I
, I
,
.. ¢~.----I m.
I
. ;
id,.
Fig(3) Spectrum obtained by the SBD placed inside can-type dosimeter
Fig(2) Schematic diagram of NRPB dosimeter
Fig (1) Schematic diagram of the can-type dosimeter
•
7(.
,
Io. I~-11201,'11I~V)
,_
lao-ll4
~ee
*me
!i 1/
,~
Cklmlt ne
Fig(4) Spectrum obtained by the SBD placed inside NRPB dosimeter
Po-212
• ~" " ~ : ; "
'-"--. . . . ". . . . . ,In. . . . . . . . ~h~.d
m
;~
Bi-212 (MI i~)
" ,
IIl.lilil
,i
ill
I = = .
,
o o
¢,l~aana0~mlMw
Fig(5) Sl~tmm obtained by the Fig(6) Alpha spectrum obtained by the SBD from the sampling tilter paNr SBD Without dosimeter (bare)
ACKNOWLEDGEMENT R. Shweikani would like to thank the AECS for their financial support to study at University of Birmingham. REFERENCES 1- Bigu J. and Raze V. (1984), "Extended capabilities of a personal alpha dosimeter ..." Radiat. Protec. Dosim. 8, 173-176. 2- Khan H.A., Tufail M. and Qureshi I.E., (1991), "Migration and distribution of radon/thoron and their daughters in tube dosimeters of different lengths". Nucl. Tracks Radiat. Meas. 19, 351-352 3- Bigu J., (1986), "A method for measuring thoron and radon gas concentrations using solid-state alphaparticle detectors". Appl. Radiat. 1sot. 37, 567-573. 4- Fleischer 1L and Turner, (1983), "Passive measurement of working levels and effective diflhsion constants of radon daughters by the nuclear track technique". Health Phys. 47, 9-19. 5- NazaroffW.W., Nero A.V. and Revzan K.L. (1982), "Alpha spectroscopic techniques for field measurement of radon daughters". The Natural Radiation Environment, International Symposium on natural radiation environment, New Delhi.
Pergamon
Copyright© 1995ElsevierScienceLtd Printed in Great Britain. All rights reserved 1350-4487/95$9.50+ .00 1350-4487(95)00199-9
THORON
CONTRIBUTIONS IN RADON MEASUREMENTS IN THE ENVIRONMENT
R. SHWEIKANI*t and S. A. DURRANIt ~'School of Physics & Space Research, University of Birmingham, Birmingham B15 2TT, U.K.
ABSTRACT SSNTDs have found wide use for the determination of indoor radon levels. Most of these types of detector involve filtration, which assumes that any thoron present will decay completely before the alpha particles from the gas itself and its daughter products can be registered on the plastic surface. A surface barrier detector (SBD) was placed at the same position as, and instead of, the plastic detector employed in a routine measurement. An active method of measuring ambient thoron concentrations has been devised. KEYWORDS Surface barrier detector, NRPB dosimeter, Can technique, Thoron and radon diffusion, Filtration. INTRODUCTION The health hazards from radon (Rn-222) and thoron (Rn-220) and their progeny are of considerable concern to the public ~. Few studies on the indoor levels of thoron and its decay products, and their contribution to health hazards, have been made 2'3. Solid state nuclear track detectors are widely used for radon measurements and usually calibrated by affixing them inside dosimeters and then placing the dosimeter inside a chamber with known radon activity concentration for a certain time4. The track density on the detector is a measure of radon exposure. This is only true ff the assumption that thoron can not diffuse into the dosimeter is correct. This work is aimed at studying the contribution of thoron to radon measurements in two different types of radon dosimeters which are used in houses and in the field. EXPERIMENTAL TECHNIQUE The first type of dosimeter, which was used in this work, was for evaluating radon concentration in soil as a measure of the potential radon ha7~rd in the surrounding area, this is called the 'can' technique. Figure (1) shows the cross section of this dosimeter. In this work a SBD was placed air-tight at the normal position of the plastic detector. The second dosimeter was the NRPB-type dosimeter which is often used for indoor radon measurements. Figure (2) shows the cross section of this dosimeter. The SBD was placed air-tight in the lower half in the compartment where the plastic detector is usu,311yaffixed. These dosimeters containing the SBD were placed separately inside a cylindrical air-tight metallic chamber of 20 cm diameter and 50 cm depth containing a mixed environment of thoron and radon gases, and a counting time of 66 hours was set. Another 66 hours was also set for the same SBD to count bare (i.e. without the dosimeter housing). In both cases, the active surface of the detector was at the same height from the sources (about 30 era). An attempt was also made to measure the activity concentration of thoron in the radon/thoron chamber. The idea was to draw a known volume of air through a filter paper, which was immediately afterward monitored by a SBD. From the area under the Po-212 peak (8.79 MeV) of the first spectrum obtained (aRer 1 minute of drawing and for 1 minute counting) the activity concentration of Bi-212 could be calculated. After waiting for about 10 hours radon daughters and also Bi-212 which is a thoron daughter will be dead and the only remaining radioactive material is Pb-212 which has a half life of 10.6 hours. From the areas under the two peaks (Bi-212 of 6.05 Mev and Po-212 of 8.79 Mev), the activity concentration of the Pb212 could be determined RESULTS AND DISCUSSION The two spectra obtained from the multichannel analyser (MCA) for the contribution of thoron inside the two types of dosimeters ('can' technique and NRPB-type) are shown in figs(3) and (4) respectively. Fig (5) shows *Permanent address: Atomic Energy Commission of Syria, P.O. Box 6091, Damascus, Syria
615
616
R. SHWEIKANI and S. A. DURRANI
the spectrum which was obtained by the SBD placed bare inside the chamber at the same height from the source as was mentioned before. By comparing these graphs it is clear that the filter on the top of the can does reduce the entrance ofthoron as well as radon. It seems that thoron finds it more difficult to get into the NRPB type. This means that the tracks on the plastic detector using the can technique for radon measurements is not only from radon and its progeny hut also from thoron and its progeny. Fig(6) shows the spectrum obtained 10 hours aRer the end of the air drawing. The clear two peaks are the two alpha particles produced from the decay of Pb-212 down to Pb-208. The areas under the two peaks were found to be 540 and 907 respectively. These results give a ratio of (37.3 5: 1.6)% for the first peak and (62.75: 2) for the second peak. It is clear from this result that these two peaks are purely a result of the Bi-212 decay which has come direct from the 15 decay of Pb-212. Therefore, the activity of Pb-212 on the filter could be easily calculateds. Aluminum
(
Can
10 cm x.6.5 cm)
Z'Ji-: kill
t
[11.214
i1 *l l
Plastic detector (CR-39 ~ 2.5cm)
Filter paper I~-ll !
I
O ~ll~Plestic
l
Flange
,
I
?
,
I
)
,
l
i
,
,
5
5
I
, I
,
.. ¢~.----I m.
I
. ;
id,.
Fig(3) Spectrum obtained by the SBD placed inside can-type dosimeter
Fig(2) Schematic diagram of NRPB dosimeter
Fig (1) Schematic diagram of the can-type dosimeter
•
7(.
,
Io. I~-11201,'11I~V)
,_
lao-ll4
~ee
*me
!i 1/
,~
Cklmlt ne
Fig(4) Spectrum obtained by the SBD placed inside NRPB dosimeter
Po-212
• ~" " ~ : ; "
'-"--. . . . ". . . . . ,In. . . . . . . . ~h~.d
m
;~
Bi-212 (MI i~)
" ,
IIl.lilil
,i
ill
I = = .
,
o o
¢,l~aana0~mlMw
Fig(5) Sl~tmm obtained by the Fig(6) Alpha spectrum obtained by the SBD from the sampling tilter paNr SBD Without dosimeter (bare)
ACKNOWLEDGEMENT R. Shweikani would like to thank the AECS for their financial support to study at University of Birmingham. REFERENCES 1- Bigu J. and Raze V. (1984), "Extended capabilities of a personal alpha dosimeter ..." Radiat. Protec. Dosim. 8, 173-176. 2- Khan H.A., Tufail M. and Qureshi I.E., (1991), "Migration and distribution of radon/thoron and their daughters in tube dosimeters of different lengths". Nucl. Tracks Radiat. Meas. 19, 351-352 3- Bigu J., (1986), "A method for measuring thoron and radon gas concentrations using solid-state alphaparticle detectors". Appl. Radiat. 1sot. 37, 567-573. 4- Fleischer 1L and Turner, (1983), "Passive measurement of working levels and effective diflhsion constants of radon daughters by the nuclear track technique". Health Phys. 47, 9-19. 5- NazaroffW.W., Nero A.V. and Revzan K.L. (1982), "Alpha spectroscopic techniques for field measurement of radon daughters". The Natural Radiation Environment, International Symposium on natural radiation environment, New Delhi.