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Sensitivity of a LR-115 based radon dosemeter
Radiation Measurements
PERGAMON
Radiation Measurements31 (1999) 347-350
SENSITIVITY OF A LR-115 BASED RADON DOSEMETER F. BAGNOLI *, F. BOCHICCHIO ** AND S. BUCCI *** * Dipartimento di Matematica Applicata, Universita di Firenze, via Santa Marta 3, 1-50139 Firenze, Italy ** Laboratorio di Fisica, Istituto Superiore di Sanitd, viale Regina Elena 299, 1-00161 Roma, Italy *** Fisica Ambientale, ARPAT, via San Salvi 12,1-50135 Firenze, Italy
ABSTRACT The first results of a study on the sensitivity of a LR-115 based radon dosemeter as a function of the absorber thickness are presented. The theoretical sensitivity was analytically calculated considering a constant detector response to alpha particles within a given energy range and up to a critical angle of incidence. The results are presented in two extreme situations: i) both radon and its decay products uniformly distributed in the chamber volume; ii) radon decay products uniformly deposited on the chamber walls. The agreement with the experimental curve shape appears better in the former case, suggesting that either the parameter values of the model could be different from the chosen values, or the model was too simplified. KEYWORDS Radon; LR-115; sensitivity; model.
~TRODUCTION The radon dosemeter mostly used in Italy contains two LR-115 detectors covered by a thin foil of absorber, used as an energy degrader in addition to the air enclosed within the measuring cylindrical chamber. This dosemeter was designed by Tommasino (Azimi-Garakani et al., 1988) and used for the Italian National Survey on Natural Radiation Indoors (Bochicchio et al., 1996). In order to better understand the dosemeter response, a model has been developed to calculate the sensitivity and to study its dependence on design parameters. In this paper, preliminary theoretical results of the sensitivity dependence on the absorber thickness are presented and compared with experimental results. This relationship has only been theoretically studied in a few number of papers (Bonetti et al., 1991; Sima, 1995).
METHODS Theoretical evaluation of sensitivity The model assumes two simplified criteria for track detectability: 1) the energy of the incident alpha particle on the detector surface has to be in a given range (Emi, + Em~x); 2) the incidence angle 0 with respect to the normal to the detector surface has to be less than the critical etching angle 0o. These parameters depend on detector characteristics, etching conditions and track counting method. For this preliminary analysis the following values foi" the model parameters have been used: Er~ = 1.7 MeV, Emax = 4.2 MeV and 0c = 50 °, considering those found in the literature for similar, but not identical, experimental conditions (Djeffal et al., 1997; J6nsson, 1981; Nakahara et al., 1980; Nikezic and Baixeras, 1996). 1350-4487/99/$ - see front matter © 1999 ElsevierScience Ltd. All rights reserved. PII: S 1350-4487(99)00165-1
E Bagnoli et al./Radiation Measurements 31 (1999) 347-350
348
Our approach is based on the deterministic computation of the effective volume, which is the part of space in the dosemeter from which a fraction of the emitted alpha arrives on the detector surface with energy E and incidence angle 0 satisfying criteria 1 and 2. For each alpha emission energy Eo of radon and its decay products and for a given absorber thickness s, the effective volume is obtained by computing the portion of space delimited by the surfaces corresponding to E ~ and F-.m~, the cone corresponding to 0c and the dosemeter walls. The surfaces S(E0,Emm,S)and S(Eo,Em~,S) are determined by the range of alpha particles in air and in the absorber, obtained from the Bethe-Bloch formula for the stopping power. Examples of sections of effective volumes are reported in Fig. 1.
30
30
25
25
20
20
15
15
10
10s = 12~tm
5 0 -40
t
i
i
-20
0
20
222Rn
50 -4O
40
30
30 i
25
25 ~
20
20-
15
15-
s=23 i
i
r
-20
0
20
. . . . . . . . . .
2~Spo
10
10-
s= 12pro
5 0 -40
i
T
i
-20
0
20
30
30
25
25
20
20
15
15
10
10 __
5
~
_ ~i
s=12pm
i
-40
40
-20
0
20
40
m
s=23
r
i
0
20
40
s = 2 3 ~m
-40
i
i
i
-20
0
20
Fig. 1. Section of effective volumes, relative to the detector central point, for the alpha emitters Rn, 2 1 8 Po, 2 1 4 Po, and for mylar absorber thickness of 12 p m (left) and 23 p m (right). All X and Y values are expressed in millimeters, but the Y axis scale is expanded. The dotted lines show the dosemeter cylindrical chamber (diameter = 24 ram, height = 11.5 mm). Parameters used m the model: Em~ = 1.7 MeV, Em~ = 4.2 MeV, 0c = 50 °. 222
pm
214po
0 -40
-20
.0
5
0
40
21gpo
50
pm
40
349
E Bagnoli et al./Radiation Measurements 31 (1999) 347-350
Since the track density is not constant over the detector surface, an average sensitivity has been obtained from the values calculated for each point of the counting surface. For a given dosemeter size, the importance of inhomogeneity depends on the absorber thickness and the energy of the nuclide considered. The calculations have been performed in two extreme situations: i) a uniform distribution in the chamber volume of both radon and its decay products; ii) a uniform distribution of radon in the chamber volume and a uniform distribution on the chamber surfaces of radon decay products. Experimental measurements
In order to determine experimentally the radon sensitivity as a function of the absorber thickness, 9 groups of dosemeters with absorber thickness from 6 to 46 lam were exposed all together in a room. Two groups included 45 dosemeters, while all the other 7 groups included 30 dosemeters. Aluminised polyethylene terephtalate (mylar) was used as absorber for all the groups. The radon concentration in the exposure room was measured with a diffusion type calibrated scintillation cell. All LR-115 detectors were etched in 10% NaOH at 60 *(2. Three etching times (100, 110 and 120 minutes) were used in order to determine the response as a function of the detector thickness after etching; however, in this paper the results have been normalised to 6.5 lam of detector thickness, corresponding to about 110 minutes etching time. Track density has been measured with a sparkcounter.
RESULTS The computed sensitivity as a function of mylar absorber thickness is shown in Fig. 2 for both cases i) and ii), together with the experimental data results.
1.4 x ~.
1.2 i
x 1.0 .......
0.8
e
......... i ........ ,
Case i)
-
...............
O
× 0.6 0.4 0.2
...................... ~...............................................................~i. . . . . . . . .
~:"~4 .................
0.0 0
10
20 30 absorber thickness (lam)
40
50
Fig. 2. Computed and experimental sensitivity vs. mylar absorber thickness. Model results are reported for two different distributions of radon decay products inside the chamber: i) uniform in the volume; ii) uniform on the chamber surfaces. The parameters used in the model w e r e : Emi n = 1.7 MeV, Emax= 4.2 MeV, 0c = 50°.
350
E Bagnoli et al. / Radiation Measurements31 (1999)347-350
The experimental sensitivity curve has a main maximum at around 12 lam and a secondary maximum at around 36 lam. The position of the secondary maximum is quite uncertain due to the limited experimental points in this region, but its presence is supported by the small experimental error. Regarding the position of the main maximum, the agreement between the model and the experimental data is excellent for case i) and good for case ii), while the calculated sensitivity value is overestimated by a factor of about 1.7. A secondary maximum is present in the model results only in case i), although for a lower absorber thickness. These latter results are somewhat in contrast with the room model, which foresees radon decay products mostly plated-out on surfaces, in case of small chambers (e.g. McLaughlin and Fitzgerald, 1994). CONCLUSIONS The calculated sensitivity can be considered in sufficient agreement with the experimental data, taking into account the very simplified model used for track detectability. With the chosen model parameter values, the agreement seems better in the case of uniform distribution of radon decay products in the chamber volume, than in the case of uniform distribution on the chamber surfaces. These results suggest us to carry out a systematic study with many different values of the parameters En~n, Em~ and 0c in order to check whether a better agreement with the experimental data, and with the room model results, can be obtained.
Acknowledgments - Grateful acknowledgment is due to the colleagues F. Felici and G. Moroni (Istituto Superiore di Sanit~t)for radon detector etching and track counting.
REFERENCES Azimi-Garakani D., Flores B., Piermattei S., Susanna A.F., Siedel J.L., Tommasino L. and Torri G. (1988) Radon gas sampler for indoor and soil measurements and its applications. Radiat. Prot. Dosim. 24(1-4), 269-272. Bochicchio F., Campos Venuti G., Nuccetelli C., Piermattei S., Risica S., Tommasino L. and Torri G. (1996) Results of the representative Italian national survey on radon indoors. Health Phys. 71(5), 741-748. Bonetti R., Capra L., Chiesa A., Guglielmetti A. and Migliorino C. (1991) Energy response of LR-115 cellulose nitrate to alpha particle beams. Nucl. Tracks Radiat. Meas. 18(3), 321-324. Djeffal S., Lounis Z. and Allab M. (1997) Design of a radon measuring device based on the diffusion principle using LR-115 detector. Radiat. Meas. 28(1-6), 629-632. J6nsson G. (1981) The angular sensitivity of Kodak LR-film to alpha particles. Nucl. Instr. Meth. 190, 407-414. McLaughlin J.P. and Fitzgerald B. (1994) Models for determining the response of passive alpha detectors to radon and it's progeny in cylindrical detecting volumes. Radiat. Prot. Dosim. 56(1-4), 241-246. Nakahara H., Kudo H., Akiba F. and Murakami Y. (1980) Some basic studies on the absolute determination of radon concentration in the air by a cellulose nitrate track detector. NucL Instr. Meth. 171, 171-179. Nikezic D. and Balxeras C. (1996) Radon, radon progeny and equilibrium factor determination using an LR-115 detector. Radiat. Meas. 26(2), 203-213. Sima O. (1995) Computation of the calibration factor for the cup type SSNTD radon monitor. Radiat. Meas. 25(14), 603-606.
PERGAMON
Radiation Measurements31 (1999) 347-350
SENSITIVITY OF A LR-115 BASED RADON DOSEMETER F. BAGNOLI *, F. BOCHICCHIO ** AND S. BUCCI *** * Dipartimento di Matematica Applicata, Universita di Firenze, via Santa Marta 3, 1-50139 Firenze, Italy ** Laboratorio di Fisica, Istituto Superiore di Sanitd, viale Regina Elena 299, 1-00161 Roma, Italy *** Fisica Ambientale, ARPAT, via San Salvi 12,1-50135 Firenze, Italy
ABSTRACT The first results of a study on the sensitivity of a LR-115 based radon dosemeter as a function of the absorber thickness are presented. The theoretical sensitivity was analytically calculated considering a constant detector response to alpha particles within a given energy range and up to a critical angle of incidence. The results are presented in two extreme situations: i) both radon and its decay products uniformly distributed in the chamber volume; ii) radon decay products uniformly deposited on the chamber walls. The agreement with the experimental curve shape appears better in the former case, suggesting that either the parameter values of the model could be different from the chosen values, or the model was too simplified. KEYWORDS Radon; LR-115; sensitivity; model.
~TRODUCTION The radon dosemeter mostly used in Italy contains two LR-115 detectors covered by a thin foil of absorber, used as an energy degrader in addition to the air enclosed within the measuring cylindrical chamber. This dosemeter was designed by Tommasino (Azimi-Garakani et al., 1988) and used for the Italian National Survey on Natural Radiation Indoors (Bochicchio et al., 1996). In order to better understand the dosemeter response, a model has been developed to calculate the sensitivity and to study its dependence on design parameters. In this paper, preliminary theoretical results of the sensitivity dependence on the absorber thickness are presented and compared with experimental results. This relationship has only been theoretically studied in a few number of papers (Bonetti et al., 1991; Sima, 1995).
METHODS Theoretical evaluation of sensitivity The model assumes two simplified criteria for track detectability: 1) the energy of the incident alpha particle on the detector surface has to be in a given range (Emi, + Em~x); 2) the incidence angle 0 with respect to the normal to the detector surface has to be less than the critical etching angle 0o. These parameters depend on detector characteristics, etching conditions and track counting method. For this preliminary analysis the following values foi" the model parameters have been used: Er~ = 1.7 MeV, Emax = 4.2 MeV and 0c = 50 °, considering those found in the literature for similar, but not identical, experimental conditions (Djeffal et al., 1997; J6nsson, 1981; Nakahara et al., 1980; Nikezic and Baixeras, 1996). 1350-4487/99/$ - see front matter © 1999 ElsevierScience Ltd. All rights reserved. PII: S 1350-4487(99)00165-1
E Bagnoli et al./Radiation Measurements 31 (1999) 347-350
348
Our approach is based on the deterministic computation of the effective volume, which is the part of space in the dosemeter from which a fraction of the emitted alpha arrives on the detector surface with energy E and incidence angle 0 satisfying criteria 1 and 2. For each alpha emission energy Eo of radon and its decay products and for a given absorber thickness s, the effective volume is obtained by computing the portion of space delimited by the surfaces corresponding to E ~ and F-.m~, the cone corresponding to 0c and the dosemeter walls. The surfaces S(E0,Emm,S)and S(Eo,Em~,S) are determined by the range of alpha particles in air and in the absorber, obtained from the Bethe-Bloch formula for the stopping power. Examples of sections of effective volumes are reported in Fig. 1.
30
30
25
25
20
20
15
15
10
10s = 12~tm
5 0 -40
t
i
i
-20
0
20
222Rn
50 -4O
40
30
30 i
25
25 ~
20
20-
15
15-
s=23 i
i
r
-20
0
20
. . . . . . . . . .
2~Spo
10
10-
s= 12pro
5 0 -40
i
T
i
-20
0
20
30
30
25
25
20
20
15
15
10
10 __
5
~
_ ~i
s=12pm
i
-40
40
-20
0
20
40
m
s=23
r
i
0
20
40
s = 2 3 ~m
-40
i
i
i
-20
0
20
Fig. 1. Section of effective volumes, relative to the detector central point, for the alpha emitters Rn, 2 1 8 Po, 2 1 4 Po, and for mylar absorber thickness of 12 p m (left) and 23 p m (right). All X and Y values are expressed in millimeters, but the Y axis scale is expanded. The dotted lines show the dosemeter cylindrical chamber (diameter = 24 ram, height = 11.5 mm). Parameters used m the model: Em~ = 1.7 MeV, Em~ = 4.2 MeV, 0c = 50 °. 222
pm
214po
0 -40
-20
.0
5
0
40
21gpo
50
pm
40
349
E Bagnoli et al./Radiation Measurements 31 (1999) 347-350
Since the track density is not constant over the detector surface, an average sensitivity has been obtained from the values calculated for each point of the counting surface. For a given dosemeter size, the importance of inhomogeneity depends on the absorber thickness and the energy of the nuclide considered. The calculations have been performed in two extreme situations: i) a uniform distribution in the chamber volume of both radon and its decay products; ii) a uniform distribution of radon in the chamber volume and a uniform distribution on the chamber surfaces of radon decay products. Experimental measurements
In order to determine experimentally the radon sensitivity as a function of the absorber thickness, 9 groups of dosemeters with absorber thickness from 6 to 46 lam were exposed all together in a room. Two groups included 45 dosemeters, while all the other 7 groups included 30 dosemeters. Aluminised polyethylene terephtalate (mylar) was used as absorber for all the groups. The radon concentration in the exposure room was measured with a diffusion type calibrated scintillation cell. All LR-115 detectors were etched in 10% NaOH at 60 *(2. Three etching times (100, 110 and 120 minutes) were used in order to determine the response as a function of the detector thickness after etching; however, in this paper the results have been normalised to 6.5 lam of detector thickness, corresponding to about 110 minutes etching time. Track density has been measured with a sparkcounter.
RESULTS The computed sensitivity as a function of mylar absorber thickness is shown in Fig. 2 for both cases i) and ii), together with the experimental data results.
1.4 x ~.
1.2 i
x 1.0 .......
0.8
e
......... i ........ ,
Case i)
-
...............
O
× 0.6 0.4 0.2
...................... ~...............................................................~i. . . . . . . . .
~:"~4 .................
0.0 0
10
20 30 absorber thickness (lam)
40
50
Fig. 2. Computed and experimental sensitivity vs. mylar absorber thickness. Model results are reported for two different distributions of radon decay products inside the chamber: i) uniform in the volume; ii) uniform on the chamber surfaces. The parameters used in the model w e r e : Emi n = 1.7 MeV, Emax= 4.2 MeV, 0c = 50°.
350
E Bagnoli et al. / Radiation Measurements31 (1999)347-350
The experimental sensitivity curve has a main maximum at around 12 lam and a secondary maximum at around 36 lam. The position of the secondary maximum is quite uncertain due to the limited experimental points in this region, but its presence is supported by the small experimental error. Regarding the position of the main maximum, the agreement between the model and the experimental data is excellent for case i) and good for case ii), while the calculated sensitivity value is overestimated by a factor of about 1.7. A secondary maximum is present in the model results only in case i), although for a lower absorber thickness. These latter results are somewhat in contrast with the room model, which foresees radon decay products mostly plated-out on surfaces, in case of small chambers (e.g. McLaughlin and Fitzgerald, 1994). CONCLUSIONS The calculated sensitivity can be considered in sufficient agreement with the experimental data, taking into account the very simplified model used for track detectability. With the chosen model parameter values, the agreement seems better in the case of uniform distribution of radon decay products in the chamber volume, than in the case of uniform distribution on the chamber surfaces. These results suggest us to carry out a systematic study with many different values of the parameters En~n, Em~ and 0c in order to check whether a better agreement with the experimental data, and with the room model results, can be obtained.
Acknowledgments - Grateful acknowledgment is due to the colleagues F. Felici and G. Moroni (Istituto Superiore di Sanit~t)for radon detector etching and track counting.
REFERENCES Azimi-Garakani D., Flores B., Piermattei S., Susanna A.F., Siedel J.L., Tommasino L. and Torri G. (1988) Radon gas sampler for indoor and soil measurements and its applications. Radiat. Prot. Dosim. 24(1-4), 269-272. Bochicchio F., Campos Venuti G., Nuccetelli C., Piermattei S., Risica S., Tommasino L. and Torri G. (1996) Results of the representative Italian national survey on radon indoors. Health Phys. 71(5), 741-748. Bonetti R., Capra L., Chiesa A., Guglielmetti A. and Migliorino C. (1991) Energy response of LR-115 cellulose nitrate to alpha particle beams. Nucl. Tracks Radiat. Meas. 18(3), 321-324. Djeffal S., Lounis Z. and Allab M. (1997) Design of a radon measuring device based on the diffusion principle using LR-115 detector. Radiat. Meas. 28(1-6), 629-632. J6nsson G. (1981) The angular sensitivity of Kodak LR-film to alpha particles. Nucl. Instr. Meth. 190, 407-414. McLaughlin J.P. and Fitzgerald B. (1994) Models for determining the response of passive alpha detectors to radon and it's progeny in cylindrical detecting volumes. Radiat. Prot. Dosim. 56(1-4), 241-246. Nakahara H., Kudo H., Akiba F. and Murakami Y. (1980) Some basic studies on the absolute determination of radon concentration in the air by a cellulose nitrate track detector. NucL Instr. Meth. 171, 171-179. Nikezic D. and Balxeras C. (1996) Radon, radon progeny and equilibrium factor determination using an LR-115 detector. Radiat. Meas. 26(2), 203-213. Sima O. (1995) Computation of the calibration factor for the cup type SSNTD radon monitor. Radiat. Meas. 25(14), 603-606.