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Ice-based altitude distribution of natural radiation annual exposure rate in the Antarctica zone over the latitude range 69°S–77°S using a pair-filter thermoluminescence method
~
AppL Radiat. Isot. Vol.46, No. 12, pp. 1363-1368, 1995
Pergamon
0969-8043(95)00237-5
Copyright © 1995ElsevierScienceLtd
PrintedinGreatBritain.All rightsreserved 0969-8043/95 $9.50+ 0.00
Ice-Based Altitude Distribution of Natural Radiation Annual Exposure Rate in the Antarctica Zone over the Latitude Range 69°S-77°S using a Pair-filter Thermoluminescence Method TOSHIYUKI NAKAJIMA, l TAKAYOSHI KAMIYAMA) YOSHIYUKI FUJII, 2 H I D E A K I M O T O Y A M A 2 and S H U U I C H I E S U M I 3 ~National Institute of Radiological Sciences, Isozaki-cho. Hitachinaka-shi, Japan 2National Institute of Polar Research, Kaga-chou, Itabashi-ku, Tokyo, Japan 3Shimane Prefectural Institute for Public Health and Environmental Sciences, Nishihamasada, Yonago-shi, Japan (Received 28 April 1995; in revised form 13 June 1995)
Both ice-based altitude distributions of natural ionizing radiation exposure and the quasi-effectiveenergy of natural radiation over Antarctica over the latitude range 69°S-77°S during approx. 500 days were measured using thermoluminescent dosimeters, The results shows that dependence on altitude above sea level of the exposure rate increases by almost three-fold with each increase of 2000m of altitude, thus deviating from the general rule stating that the exposure rate should double with each 2000 m. Although the exposure rate shows a dependence on altitude, altitude dependence of the quasi-effectiveenergy of natural radiation over Antarctica is not observed. In the present study it is observed that natural radiation occurring over the ice base of Antarctica consists mainly of cosmic rays.
1. Introduction Natural radiation measurements provide physical data such as quasi-effective energy and exposure rate. These data vary according to geological, geographical and other physical conditions of each region. It has been reported that the relationship between the exposure rate and the quasi-effective energy of natural radiation can be expressed by a hyperbolic function (Nakajima, 1988). In accordance with the previous result (Nakajima, 1988), the minimum value of the quasi-effective energy of natural radiation emanating from the ground can be estimated from this relationship, as can the minimum value of the exposure rate of cosmic rays according to latitude and altitude. However, the maximum value of the quasi-effective energy of cosmic rays and the maximum value of the exposure rate from naturally radioactive nuclides in the ground can only be guessed at, and there are no solid data to support such estimates. In particular, to measure the quasi-effective energy of cosmic rays, it is necessary to select a location that is not exposed to radiation produced by naturally occurring radioactive nuclides in the ground or in ocean water,
The exposure rate of cosmic rays on the earth's surface is known to fluctuate with altitude and terrestrial magnetism latitudes (hereafter called "latitudes"), The latitude and altitude dependency of the exposure rate of cosmic rays has been calculated by O'Brien (1973) using a Monte-Carlo method. In addition, Wang (1985) has measured natural radiation exposure rates with a scintillation counter and obtained the altitude dependence of the cosmic components from these results. However, measurements designed to distinguish between the various ground and cosmic-ray components of natural radiation have produced unclear results. To measure the exposure rate and a strong altitude dependency for cosmic rays, these rays must be measured under conditions where they are not appreciably affected by natural radiation from the ground or ocean. To satisfy these conditions, it was necessary to select as the measuring location a region covered by a wide sheet of ice with a thickness of at least 10 m. For example, the ice-based Antarctica is almost completely unaffected by radiation from ~K, U, Th and other natural radionuclides found in the ocean and in the ground, and, as such, constitute a unique environmental region.
1363
Toshiyuki Nakajima et al.
1364
Thermoluminescent dosimeters (TLD) were positioned on the Antarctic continent away from the effects of radiation from the ground and ocean. The altitude dependence of the exposure rate and the quasi-effective energy of cosmic rays was obtained after a long period of exposure to these radiations. The quasi-effective energy of cosmic rays from the correlation between the measured exposure rate and quasi-effective energy of natural radiation was also estimated from these results. They indicate that the natural ionizing radiation over Antarctica consists mainly of cosmic-rays.
intensities in the lead and lucite filters was used as the calibration curve to evaluate the quasi-effective energy of natural radiation. The following conditions were considered when selecting monitoring positions in the Antarctic.
2. Experimental Method
The following arrangements were made when setting up the TLD elements:
Due to Antarctica's severe natural environment and lack of power facilities, it is difficult for researchers to spend long times in the field or to traverse between monitoring positions in order to measure radiation levels. Therefore, it was decided that dosimeters should be used that could be left in the field for extended periods, that were capable of preserving radiation effects for extended periods under severe environmental conditions, and that did not require power facilities. Accordingly, integrated measuring devices were chosen that were small, highly sensitive, and easy to use, that would allow several dosimeters to be used at the same time, and that could derive an average exposure value over an extended period. The thermoluminescent dosimeter (hereafter abbreviated as "TLD") meets these conditions. In the experiment, TLD elements of a CaSO4(Tm) phosphor encapsulated in a small glass tube (about 2 mm in external diameter and 12 mm length) were used, showing high radiation sensitivity. The glass tubes are made of nominally potassium-free glass to eliminate the effect of exposure on the TLD phosphor from 4°K in the holder. A pair-filter TLD system (Nakajima and Chiba, 1986) was used for obtaining both the exposure rate and quasi-effective energy of natural radiation. The pair filter consists of a combination of 10g/cm 2 wall thickness, 50 mm length cylindrical lead and 0.45 g/cm 2 wall thickness, 50 mm length cylindrical lucite vessels. Three TLD elements were placed into each cylinder of the pair-filter TLD system. The TLDs in the lucite vessel of the pair-filter TLD system provide information about exposure from radiation sources. The TL intensity from CaSO4(Tm) in the lucite vessel is calibrated with v-rays from 6°Co sources. On the other hand, the TL intensity ratio observed by the pair-filter TLD system was evaluated in the laboratory as a function of the effective energy of x- and v-radiation and the TL ratio between the TL intensities in the lead and lucite filters was found to increase linearly in the energy range from about 200 keV to about 10 MeV or more effective energy in a semilog presentation (Nakajima and Chiba, 1986). In this work, the curve of relationship between the TL
1. The ice thickness at monitoring points must not fall below 10 m for the duration of the monitoring period, even during the summer. 2. Monitoring points must have an altitude above sea level ranging from several hundred meters to approx. 4000 m or higher.
1. Immediately before the CaSO4(Tm) TLDs were placed at the monitoring points, they were given a thermal annealing treated at 450°C for 15 min at the Showa base laboratory (the Japanese research center in Antarctica) to eliminate any history of radiation exposure occurring during their transportation from Japan to the base. Some of the thermally treated TLD elements were used as background TLD elements. These TLD elements were placed inside lead containers with wall thickness of 5 cm and stored at the Antarctic Showa base, where they were used to measure the background radiation dose at the base. 2. The TLD elements arranged at the monitoring points were placed into the pair-filter TLD system. The pair-filter TLD systems were inserted into 10-cm depth holes made in the ice at the monitoring points. Between 15 and 24 TLD elements were positioned at each monitoring point at specific locations across Antarctica. 3. After the monitoring was completed, the recovered TLD elements were placed inside lead containers with wall thickness of 5cm and stored at the Showa base until the Antarctic winter team returned to Japan. On the day that the research team left the Showa base by ship, half of the stored background TLD elements were annealed again and used to obtain the background exposure of radiation from the day the team set sail until the TLD elements were measured in Japan. 4. A monitoring period of approx. 500 days was used to reduce the extraneous effects that might occur during the TLD set-up and recovery periods. 5. After the TLDs were thermally treated, the extraneous radiation dose received by the TLDs during set up and collecting was estimated from the product of the daily background dose received at the Showa base and the monitoring period.
Ice-based altitude distribution
1s t annealing
EVENT
2nd annealing (leaving date) setting
TLD mea~~Jmment
collecting
I1
PERIODS
1365
]z
r
r
A MEANINGof
(TLD at Monitoring Points)
BG-A (Back Ground TLD at Base Camp)
TLD SIGNALS
I
BG-B
j
F
(Back Ground
TLD) Fig. 1, Sequential schedule of TLD annealing, set up, collection, measurements and meaning of TLD signals.
F i g u r e 1 s h o w s the s e q u e n c e o f T L D m e a s u r e m e n t e v e n t s f o r different p e r i o d s at the times o f their a r r a n g e m e n t at the m o n i t o r i n g p o i n t s as well as u p o n collection a n d T L r e a d o u t . T h e latitudes, l o n g i t u d e s a n d altitudes o f the m o n i t o r i n g p o i n t s are listed in T a b l e 1, a l o n g w i t h r e a d i n g s of annual exposures. T h e e x p o s u r e received at e a c h m o n i t o r i n g p o i n t w a s o b t a i n e d a c c o r d i n g to the following c o n ditions:
the T L D s were set u p at the m o n i t o r i n g p o i n t s until their m e a s u r e m e n t . 2. T h e T L D signal ( B G - B ) f r o m the b a c k g r o u n d r a d i a t i o n d o s e received f r o m the s e c o n d t h e r m a l t r e a t m e n t stage at the S h o w a base until the T L D s were m e a s u r e d in J a p a n . 3. T h e a v e r a g e daily T L D intensity ( B G - r a t e ) f r o m the b a c k g r o u n d r a d i a t i o n d o s e at the S h o w a b a s e w a s o b t a i n e d as follows: B G - r a t e = [ ( B G - A ) - (BG-B)]/(ll + r + l 2). 4. T h e T L signal A o f each T L D set u p at the monitoring points was obtained.
I. T h e T L D signal ( B G - A ) f r o m the overall b a c k g r o u n d r a d i a t i o n d o s e received f r o m the time
Table 1. Location, altitude, monitoring period, covered snow depth of the pair-filter TLD systems at monitoring points covering Antarctica, altitudinal distribution of cosmic-ray annual exposure rate and quasi-effective energy with the standard deviation of the mean value in the Antarctica zone
Point
Location coordinates
Altitude (m)
Monitoring period (day)
Snow depth (cm)
Annual exposure ( × 2.58 x 10- 7c/kg.yr)
$20 H192 7__42 MD44 MDI00
$69"01.11' $69°37.79 ' $70°20.07" $71°08.91 ' $71 39.0'
E40°l 2.32' E42°06.04 ' E43°41.08' E44°07.20 ' E43°57.06 '
682 1582 2101 2381 2492
491 487 481 475 --
93 51 1 30 --
38.0 _+ 1.7 57.6_+ 1.8 69.4 _+7.1 74.0 _+7.9 --
MDI44 MD254 MD364 MD364
$72°02.57 ' $73~'01.55' $74°00.48 ' S74°00.48 '
E43°48.88' E43°25.27' E42°59.80 ' E42"59.80'
2683 3078 3353 3341
470 465 457 --
19 49 9 --
82.9 _+7.5 89.5 _+3.6 118.3 +_6.7 --
MD472 MD568 MD664 DF80
$74°58.97" S75°50.12' $67°41.73 ' S77°22.39'
E42°14.05' E41°21.63' E40"/23.75 ' E39°36.99 '
3586 3682 3751 3807
451 446 442 430
13 6 24 21
152.3 + 5.8 169,4 __.5.1 161.8 + 14.3 180.7 + 9.6
--Data without annual exposure.
Quasi-effective energy (MeV) 10.3 _+0.4 10.5 _+0.8 9.8 + 2.2 -1.7
11.5_+0.6 10.7 + 1.7 - - 1.4 9.6 ::t:0.3 9.2 :t: 0.4
1366
Toshiyuki Nakajima et al.
14
*t
12108e-
6J 2-
.&
&
o
soo
10'oo
l s'oo
zooo
2s'oo
30'00
3s'oo
4000
Altitude(m) Fig. 2. Quasi-effective energy of natural radiation as a function of ice-based altitude above sea level at TLD monitoring points on Antarctica. 5. The exposure rate (per annum) at the monitoring points was obtained as follows: .~. = 1.261365{A - (BG-B) - (BG-rate) (/~ + 12)}]
quasi-effective energy Eq is indicated by the subscript "q". The quasi-effective energy of natural radiation over Antarctica was previously obtained using the pair-filter TLD method (Nakajima, 1988).
r
(1) In equation (1), 1.2 is the TLD density-effect coefficient for cosmic-rays, examples of which for LiF-TLD have been proposed by Lowder and de Planque (1977), O'Brien (1978) and Maiello et al. (1990); 6 is the conversion coefficient used to convert the TL signal into a given radiation exposure; 11is the period from the thermal treatment before set up to the time of arrangement of the TLDs; r is the time period during which the TLDs were set up at the monitoring points;/2 is the period from TLD collection to the date of disembarkation from the Showa base. Cosmic-rays are comprised of both neutrons (and other indirectly ionizing radiation such as photons) and directly ionizing radiation (such as charged particles). Since most TLD phosphors such as CaSO4(Tm) are relatively insensitive to neutrons, this experiment obtained the exposure rate and quasieffective energy of other indirectly ionizing radiation (i.e. photons). In this experiment, the TLD intensity was calibrated with ~-rays from a 6°Co source, which are widely used and fairly closely approximate the quasi-effective energy of cosmic rays. Therefore, the exposure corresponds to that for laboratory ),-rays. After irradiation for calibration, the irradiated TLDs were thermally treated for about 20 s in boiling water, because the fading effect of CaSO4(Tm)-TLD collected from the monitoring points in Antarctica can thereby be effectively cancelled. Subsequently, those TL intensities were measured for calibration of the TL sensitivity. Quasi-effective energy is defined as follows. When a mixture of radiation consisting of radiation beams and other forms exhibits a half-value layer similar to that for monochromatic ?-rays, this radiation also possesses quasi-effective energy approximately that of a particular energy of monochromatic ~,-rays. This
3. Results and Discussion Figure 2 shows the dependence of the quasi-effective energy of natural radiation over Antarctica on the ice-based altitude above sea level. Natural radiation emanating from the ground is mainly from 4°K, U and Th, and is comprised of low-energy radiation compared with cosmic-rays. The exposure rate of this radiation varies according to the ratio of naturally radioactive nuclides to the constituent elements of the ground. Moreover, the quasi-effective energy of natural radiation varies greatly according to the ratio of the exposure rate of radiation from the cosmos and the ground. As a result, the quasi-effective energy of natural radiation never rises above the value of the quasi-effective energy of cosmic rays. In addition, this quasi-effective energy has been shown to vary according to the geological and geographical conditions at the monitoring points. Conversely, it can be assumed that due to the relatively isotropic source of cosmic rays, the quasieffective energy of cosmic rays does not change with altitude. The results displayed in Fig. 2 show that the quasi-effective energy of natural radiation over Antarctica, which is covered with wide sheets of thick ice, is not dependent on altitude. In addition, the average value of this energy is Eq-- 10.1 +_ 1.02 MeV (lg). As a result, it can be assumed that the natural radiation over regions covered with wide sheets of thick ice is comprised almost entirely of cosmic rays. In addition, since the quasi-effective energy of natural radiation over Antarctica does not vary between monitoring points, the dose of y-rays from fall-out nuclides and other man-made radioactive nuclides in the ice is negligible compared to that for cosmic rays (Pourchet et al., 1983) and therefore can be ignored.
Ice-based altitude distribution In Table 1, the annual exposure rate and quasieffective energy of cosmic rays at each monitoring point compensated for the TLD density effect was shown. It is also shown that the annual exposure rate of cosmic rays increases with altitude. This increasing tendency is also indicated in Fig. 3. The pair-filter TLD systems were positioned 10 cm below the surface of the ice floe to prevent them from being blown around by strong winds. However, the pair filter TLD systems had become covered with snow to the depths listed in Table 1 when they were recovered after approx. 500 days. Since the period during which they were covered with snow was unclear, the shielding effect of the snow from cosmic-rays was ignored when evaluating the exposure rate. Figure 3 displays a function of ice-based altitude above sea level the dose values of the ionized components of cosmic-rays (exposure rates) at a latitude of 25°N as calculated using O'Brien's Monte-Carlo method (Fujitaka, 1994) and the exposure rates due to the ionized components of cosmic rays measured between the latitudes of 69°S to 77°S. As shown in Fig. 3, both the results calculated using the Monte-Carlo method and the present measured results show a tendency to increase with altitude. However, the results for high latitudes differed from those for low latitudes in that the rate of increase in the exposure rate increased more sharply with increase in altitude. For example, Fujitaka's (1994) results show that at low latitudes the exposure rate approximately doubles with each 2000 m increase in altitude. At high latitudes of approx. 69°S-79°S, however, the results showed that the increase in the exposure rate did not follow this empirical doubling rule for each 2000 m increase in altitude, but instead almost tripled. This is thought to be because the effects of the earth's magnetic field magnify the exposure rate of cosmic25 Present result
20 -~o
~
15
-
5
• Latitude:69°S-77°S O Latitude:25°N
~• la
/
/
/(/"
/
./
rays at the higher altitudes. This relationship can also be used to estimate the exposure rate of cosmic rays inside airplanes flying at high altitudes in different latitudes. However, the latitude dependency of the coefficients must first be derived. In summary, the present work shows that, although the quasi-effective energy is independent of altitude, the absorbed dose of natural radiation is strongly dependent on altitude. As a result, it may be concluded that natural radiation occurring over the ice floes of Antarctica consists almost entirely of cosmic-rays. In addition, the maximum value of the quasi-effective energy in the hyperbolic function expressing the relationship between the exposure rate and quasi-effective energy of natural radiation (Nakajima, 1988) was estimated and assessed at approx. 10 MeV. Furthermore it is suggested that even when an aircraft flies at constant altitude, the absorbed dose of persons in the craft from cosmic radiation is dependent on the latitude.
4. Conclusion
and Summary
The exposure rate and quasi-effective energy of natural radiation over Antarctica was measured with integrating TLD dosimeters, and the following results were obtained: 1. In high-latitude regions, the empirical doubling with-altitude rule cannot be applied to the correlation between altitude and the exposure rate of cosmic-rays, and the exposure rate is instead found to almost triple with altitude. This relationship can be affected by magnetic field variations latitude at the monitoring points. 2. The quasi-effective energy of natural radiation over Antarctica is approx. 10 MeV. This value is not dependent on the altitude above sea level of the measuring point, and is considered to be the quasi-effective energy of the cosmic-rays. 3. Based on these results, the natural over the ice-based Antarctica consists almost entirely of cosmic rays, and, in general, natural radiation in regions covered with wide sheets of ice at least 10 m thick is presumed to consist almost exclusively of cosmic-rays.
/
O'Brien'sequation
t I I I I I I I I 500 1000 1500 2000 2500 3000 3500 4000 4500 Altitude (m) Fig. 3. Experimental annual exposure rate of natural radiation as a function of ice-based altitude above sea level at TLD monitoring points on Antarctica in the latitude range 69°S-77°S (solid curve) and calculated cosmic ray exposure rate at 27°N latitude obtained by Monte-Carlo method (dotted curve) and experimental exposure rate (circle points) (Fujitaka, 1994). ARI 46/12--F
1367
Acknowledgements--The authors are indebted to Dr K. Fujitaka, Division Head, National Institute of Radiological Sciences, for his useful and helpful discussions on the work. Grateful acknowledgements are also due to the Japanese Antarctic winter teams from 1991 to 1993 for their unfailing support and encouragement accorded to this work. The work was financially supported by both the Science and Technology Agency and Ministry of Education of the Japanese Government. References Fujitaka K. (1994) Private Communication. Lowder W. M. and de Planque G. (1977) The response of LiF thermoluminescence dosimeters to natural environment radiation. HASL-313 (Health & Safety Lab.)
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Toshiyuki Nakajima et al.
Energy Research and Development Administration, U.S. Department of Energy. Maiello M. L., Gulbin J. F., de Planque G. and Gesell T. F. (1990) 8th international intercomparison of environmental dosimeters. Radiat. Protect. Dosim. 32, 91-98. Nakajima T. (1988) Measurement of exposure rate and quasi-effective energy of natural radiation in Japan by pair-filter thermoluminescencedosimeter method. Radiat. Protect. Dosim. 25, 191-200. Nakajima T. and Chiba M. (1986) Evaluating method of effective energy of radiation due to thermoluminescence dosimeter. J. Nucl. Sci. Technol. 23, 258-266. O'Brien K. (1973) Luin--a code for the calculation of
cosmic ray propagation in the atmosphere. Report HASL-275 (Health & Safety Lab.) U.S. Atomic Energy Commission. O'Brien K. (1978) The response of LiF thermoluminesccnce dosimeters to the ground-level cosmic-ray background. Int. J. Appl. Radiat. Isot. 29, 735-739. Pourchet M. F., Pinglot and Lorius C. (1983) Some meteorological applications of radiative fallout measurements in antarctive snows. J. Geophys. Res. 88, 6013-6020. Wang Q. (1985) Correction of instrument readings and assessment of population doses for surveying external dose rate of natural background radiation in China. Chin. J. Radiol. Med. Protec. 5, 81-82.
AppL Radiat. Isot. Vol.46, No. 12, pp. 1363-1368, 1995
Pergamon
0969-8043(95)00237-5
Copyright © 1995ElsevierScienceLtd
PrintedinGreatBritain.All rightsreserved 0969-8043/95 $9.50+ 0.00
Ice-Based Altitude Distribution of Natural Radiation Annual Exposure Rate in the Antarctica Zone over the Latitude Range 69°S-77°S using a Pair-filter Thermoluminescence Method TOSHIYUKI NAKAJIMA, l TAKAYOSHI KAMIYAMA) YOSHIYUKI FUJII, 2 H I D E A K I M O T O Y A M A 2 and S H U U I C H I E S U M I 3 ~National Institute of Radiological Sciences, Isozaki-cho. Hitachinaka-shi, Japan 2National Institute of Polar Research, Kaga-chou, Itabashi-ku, Tokyo, Japan 3Shimane Prefectural Institute for Public Health and Environmental Sciences, Nishihamasada, Yonago-shi, Japan (Received 28 April 1995; in revised form 13 June 1995)
Both ice-based altitude distributions of natural ionizing radiation exposure and the quasi-effectiveenergy of natural radiation over Antarctica over the latitude range 69°S-77°S during approx. 500 days were measured using thermoluminescent dosimeters, The results shows that dependence on altitude above sea level of the exposure rate increases by almost three-fold with each increase of 2000m of altitude, thus deviating from the general rule stating that the exposure rate should double with each 2000 m. Although the exposure rate shows a dependence on altitude, altitude dependence of the quasi-effectiveenergy of natural radiation over Antarctica is not observed. In the present study it is observed that natural radiation occurring over the ice base of Antarctica consists mainly of cosmic rays.
1. Introduction Natural radiation measurements provide physical data such as quasi-effective energy and exposure rate. These data vary according to geological, geographical and other physical conditions of each region. It has been reported that the relationship between the exposure rate and the quasi-effective energy of natural radiation can be expressed by a hyperbolic function (Nakajima, 1988). In accordance with the previous result (Nakajima, 1988), the minimum value of the quasi-effective energy of natural radiation emanating from the ground can be estimated from this relationship, as can the minimum value of the exposure rate of cosmic rays according to latitude and altitude. However, the maximum value of the quasi-effective energy of cosmic rays and the maximum value of the exposure rate from naturally radioactive nuclides in the ground can only be guessed at, and there are no solid data to support such estimates. In particular, to measure the quasi-effective energy of cosmic rays, it is necessary to select a location that is not exposed to radiation produced by naturally occurring radioactive nuclides in the ground or in ocean water,
The exposure rate of cosmic rays on the earth's surface is known to fluctuate with altitude and terrestrial magnetism latitudes (hereafter called "latitudes"), The latitude and altitude dependency of the exposure rate of cosmic rays has been calculated by O'Brien (1973) using a Monte-Carlo method. In addition, Wang (1985) has measured natural radiation exposure rates with a scintillation counter and obtained the altitude dependence of the cosmic components from these results. However, measurements designed to distinguish between the various ground and cosmic-ray components of natural radiation have produced unclear results. To measure the exposure rate and a strong altitude dependency for cosmic rays, these rays must be measured under conditions where they are not appreciably affected by natural radiation from the ground or ocean. To satisfy these conditions, it was necessary to select as the measuring location a region covered by a wide sheet of ice with a thickness of at least 10 m. For example, the ice-based Antarctica is almost completely unaffected by radiation from ~K, U, Th and other natural radionuclides found in the ocean and in the ground, and, as such, constitute a unique environmental region.
1363
Toshiyuki Nakajima et al.
1364
Thermoluminescent dosimeters (TLD) were positioned on the Antarctic continent away from the effects of radiation from the ground and ocean. The altitude dependence of the exposure rate and the quasi-effective energy of cosmic rays was obtained after a long period of exposure to these radiations. The quasi-effective energy of cosmic rays from the correlation between the measured exposure rate and quasi-effective energy of natural radiation was also estimated from these results. They indicate that the natural ionizing radiation over Antarctica consists mainly of cosmic-rays.
intensities in the lead and lucite filters was used as the calibration curve to evaluate the quasi-effective energy of natural radiation. The following conditions were considered when selecting monitoring positions in the Antarctic.
2. Experimental Method
The following arrangements were made when setting up the TLD elements:
Due to Antarctica's severe natural environment and lack of power facilities, it is difficult for researchers to spend long times in the field or to traverse between monitoring positions in order to measure radiation levels. Therefore, it was decided that dosimeters should be used that could be left in the field for extended periods, that were capable of preserving radiation effects for extended periods under severe environmental conditions, and that did not require power facilities. Accordingly, integrated measuring devices were chosen that were small, highly sensitive, and easy to use, that would allow several dosimeters to be used at the same time, and that could derive an average exposure value over an extended period. The thermoluminescent dosimeter (hereafter abbreviated as "TLD") meets these conditions. In the experiment, TLD elements of a CaSO4(Tm) phosphor encapsulated in a small glass tube (about 2 mm in external diameter and 12 mm length) were used, showing high radiation sensitivity. The glass tubes are made of nominally potassium-free glass to eliminate the effect of exposure on the TLD phosphor from 4°K in the holder. A pair-filter TLD system (Nakajima and Chiba, 1986) was used for obtaining both the exposure rate and quasi-effective energy of natural radiation. The pair filter consists of a combination of 10g/cm 2 wall thickness, 50 mm length cylindrical lead and 0.45 g/cm 2 wall thickness, 50 mm length cylindrical lucite vessels. Three TLD elements were placed into each cylinder of the pair-filter TLD system. The TLDs in the lucite vessel of the pair-filter TLD system provide information about exposure from radiation sources. The TL intensity from CaSO4(Tm) in the lucite vessel is calibrated with v-rays from 6°Co sources. On the other hand, the TL intensity ratio observed by the pair-filter TLD system was evaluated in the laboratory as a function of the effective energy of x- and v-radiation and the TL ratio between the TL intensities in the lead and lucite filters was found to increase linearly in the energy range from about 200 keV to about 10 MeV or more effective energy in a semilog presentation (Nakajima and Chiba, 1986). In this work, the curve of relationship between the TL
1. The ice thickness at monitoring points must not fall below 10 m for the duration of the monitoring period, even during the summer. 2. Monitoring points must have an altitude above sea level ranging from several hundred meters to approx. 4000 m or higher.
1. Immediately before the CaSO4(Tm) TLDs were placed at the monitoring points, they were given a thermal annealing treated at 450°C for 15 min at the Showa base laboratory (the Japanese research center in Antarctica) to eliminate any history of radiation exposure occurring during their transportation from Japan to the base. Some of the thermally treated TLD elements were used as background TLD elements. These TLD elements were placed inside lead containers with wall thickness of 5 cm and stored at the Antarctic Showa base, where they were used to measure the background radiation dose at the base. 2. The TLD elements arranged at the monitoring points were placed into the pair-filter TLD system. The pair-filter TLD systems were inserted into 10-cm depth holes made in the ice at the monitoring points. Between 15 and 24 TLD elements were positioned at each monitoring point at specific locations across Antarctica. 3. After the monitoring was completed, the recovered TLD elements were placed inside lead containers with wall thickness of 5cm and stored at the Showa base until the Antarctic winter team returned to Japan. On the day that the research team left the Showa base by ship, half of the stored background TLD elements were annealed again and used to obtain the background exposure of radiation from the day the team set sail until the TLD elements were measured in Japan. 4. A monitoring period of approx. 500 days was used to reduce the extraneous effects that might occur during the TLD set-up and recovery periods. 5. After the TLDs were thermally treated, the extraneous radiation dose received by the TLDs during set up and collecting was estimated from the product of the daily background dose received at the Showa base and the monitoring period.
Ice-based altitude distribution
1s t annealing
EVENT
2nd annealing (leaving date) setting
TLD mea~~Jmment
collecting
I1
PERIODS
1365
]z
r
r
A MEANINGof
(TLD at Monitoring Points)
BG-A (Back Ground TLD at Base Camp)
TLD SIGNALS
I
BG-B
j
F
(Back Ground
TLD) Fig. 1, Sequential schedule of TLD annealing, set up, collection, measurements and meaning of TLD signals.
F i g u r e 1 s h o w s the s e q u e n c e o f T L D m e a s u r e m e n t e v e n t s f o r different p e r i o d s at the times o f their a r r a n g e m e n t at the m o n i t o r i n g p o i n t s as well as u p o n collection a n d T L r e a d o u t . T h e latitudes, l o n g i t u d e s a n d altitudes o f the m o n i t o r i n g p o i n t s are listed in T a b l e 1, a l o n g w i t h r e a d i n g s of annual exposures. T h e e x p o s u r e received at e a c h m o n i t o r i n g p o i n t w a s o b t a i n e d a c c o r d i n g to the following c o n ditions:
the T L D s were set u p at the m o n i t o r i n g p o i n t s until their m e a s u r e m e n t . 2. T h e T L D signal ( B G - B ) f r o m the b a c k g r o u n d r a d i a t i o n d o s e received f r o m the s e c o n d t h e r m a l t r e a t m e n t stage at the S h o w a base until the T L D s were m e a s u r e d in J a p a n . 3. T h e a v e r a g e daily T L D intensity ( B G - r a t e ) f r o m the b a c k g r o u n d r a d i a t i o n d o s e at the S h o w a b a s e w a s o b t a i n e d as follows: B G - r a t e = [ ( B G - A ) - (BG-B)]/(ll + r + l 2). 4. T h e T L signal A o f each T L D set u p at the monitoring points was obtained.
I. T h e T L D signal ( B G - A ) f r o m the overall b a c k g r o u n d r a d i a t i o n d o s e received f r o m the time
Table 1. Location, altitude, monitoring period, covered snow depth of the pair-filter TLD systems at monitoring points covering Antarctica, altitudinal distribution of cosmic-ray annual exposure rate and quasi-effective energy with the standard deviation of the mean value in the Antarctica zone
Point
Location coordinates
Altitude (m)
Monitoring period (day)
Snow depth (cm)
Annual exposure ( × 2.58 x 10- 7c/kg.yr)
$20 H192 7__42 MD44 MDI00
$69"01.11' $69°37.79 ' $70°20.07" $71°08.91 ' $71 39.0'
E40°l 2.32' E42°06.04 ' E43°41.08' E44°07.20 ' E43°57.06 '
682 1582 2101 2381 2492
491 487 481 475 --
93 51 1 30 --
38.0 _+ 1.7 57.6_+ 1.8 69.4 _+7.1 74.0 _+7.9 --
MDI44 MD254 MD364 MD364
$72°02.57 ' $73~'01.55' $74°00.48 ' S74°00.48 '
E43°48.88' E43°25.27' E42°59.80 ' E42"59.80'
2683 3078 3353 3341
470 465 457 --
19 49 9 --
82.9 _+7.5 89.5 _+3.6 118.3 +_6.7 --
MD472 MD568 MD664 DF80
$74°58.97" S75°50.12' $67°41.73 ' S77°22.39'
E42°14.05' E41°21.63' E40"/23.75 ' E39°36.99 '
3586 3682 3751 3807
451 446 442 430
13 6 24 21
152.3 + 5.8 169,4 __.5.1 161.8 + 14.3 180.7 + 9.6
--Data without annual exposure.
Quasi-effective energy (MeV) 10.3 _+0.4 10.5 _+0.8 9.8 + 2.2 -1.7
11.5_+0.6 10.7 + 1.7 - - 1.4 9.6 ::t:0.3 9.2 :t: 0.4
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Toshiyuki Nakajima et al.
14
*t
12108e-
6J 2-
.&
&
o
soo
10'oo
l s'oo
zooo
2s'oo
30'00
3s'oo
4000
Altitude(m) Fig. 2. Quasi-effective energy of natural radiation as a function of ice-based altitude above sea level at TLD monitoring points on Antarctica. 5. The exposure rate (per annum) at the monitoring points was obtained as follows: .~. = 1.261365{A - (BG-B) - (BG-rate) (/~ + 12)}]
quasi-effective energy Eq is indicated by the subscript "q". The quasi-effective energy of natural radiation over Antarctica was previously obtained using the pair-filter TLD method (Nakajima, 1988).
r
(1) In equation (1), 1.2 is the TLD density-effect coefficient for cosmic-rays, examples of which for LiF-TLD have been proposed by Lowder and de Planque (1977), O'Brien (1978) and Maiello et al. (1990); 6 is the conversion coefficient used to convert the TL signal into a given radiation exposure; 11is the period from the thermal treatment before set up to the time of arrangement of the TLDs; r is the time period during which the TLDs were set up at the monitoring points;/2 is the period from TLD collection to the date of disembarkation from the Showa base. Cosmic-rays are comprised of both neutrons (and other indirectly ionizing radiation such as photons) and directly ionizing radiation (such as charged particles). Since most TLD phosphors such as CaSO4(Tm) are relatively insensitive to neutrons, this experiment obtained the exposure rate and quasieffective energy of other indirectly ionizing radiation (i.e. photons). In this experiment, the TLD intensity was calibrated with ~-rays from a 6°Co source, which are widely used and fairly closely approximate the quasi-effective energy of cosmic rays. Therefore, the exposure corresponds to that for laboratory ),-rays. After irradiation for calibration, the irradiated TLDs were thermally treated for about 20 s in boiling water, because the fading effect of CaSO4(Tm)-TLD collected from the monitoring points in Antarctica can thereby be effectively cancelled. Subsequently, those TL intensities were measured for calibration of the TL sensitivity. Quasi-effective energy is defined as follows. When a mixture of radiation consisting of radiation beams and other forms exhibits a half-value layer similar to that for monochromatic ?-rays, this radiation also possesses quasi-effective energy approximately that of a particular energy of monochromatic ~,-rays. This
3. Results and Discussion Figure 2 shows the dependence of the quasi-effective energy of natural radiation over Antarctica on the ice-based altitude above sea level. Natural radiation emanating from the ground is mainly from 4°K, U and Th, and is comprised of low-energy radiation compared with cosmic-rays. The exposure rate of this radiation varies according to the ratio of naturally radioactive nuclides to the constituent elements of the ground. Moreover, the quasi-effective energy of natural radiation varies greatly according to the ratio of the exposure rate of radiation from the cosmos and the ground. As a result, the quasi-effective energy of natural radiation never rises above the value of the quasi-effective energy of cosmic rays. In addition, this quasi-effective energy has been shown to vary according to the geological and geographical conditions at the monitoring points. Conversely, it can be assumed that due to the relatively isotropic source of cosmic rays, the quasieffective energy of cosmic rays does not change with altitude. The results displayed in Fig. 2 show that the quasi-effective energy of natural radiation over Antarctica, which is covered with wide sheets of thick ice, is not dependent on altitude. In addition, the average value of this energy is Eq-- 10.1 +_ 1.02 MeV (lg). As a result, it can be assumed that the natural radiation over regions covered with wide sheets of thick ice is comprised almost entirely of cosmic rays. In addition, since the quasi-effective energy of natural radiation over Antarctica does not vary between monitoring points, the dose of y-rays from fall-out nuclides and other man-made radioactive nuclides in the ice is negligible compared to that for cosmic rays (Pourchet et al., 1983) and therefore can be ignored.
Ice-based altitude distribution In Table 1, the annual exposure rate and quasieffective energy of cosmic rays at each monitoring point compensated for the TLD density effect was shown. It is also shown that the annual exposure rate of cosmic rays increases with altitude. This increasing tendency is also indicated in Fig. 3. The pair-filter TLD systems were positioned 10 cm below the surface of the ice floe to prevent them from being blown around by strong winds. However, the pair filter TLD systems had become covered with snow to the depths listed in Table 1 when they were recovered after approx. 500 days. Since the period during which they were covered with snow was unclear, the shielding effect of the snow from cosmic-rays was ignored when evaluating the exposure rate. Figure 3 displays a function of ice-based altitude above sea level the dose values of the ionized components of cosmic-rays (exposure rates) at a latitude of 25°N as calculated using O'Brien's Monte-Carlo method (Fujitaka, 1994) and the exposure rates due to the ionized components of cosmic rays measured between the latitudes of 69°S to 77°S. As shown in Fig. 3, both the results calculated using the Monte-Carlo method and the present measured results show a tendency to increase with altitude. However, the results for high latitudes differed from those for low latitudes in that the rate of increase in the exposure rate increased more sharply with increase in altitude. For example, Fujitaka's (1994) results show that at low latitudes the exposure rate approximately doubles with each 2000 m increase in altitude. At high latitudes of approx. 69°S-79°S, however, the results showed that the increase in the exposure rate did not follow this empirical doubling rule for each 2000 m increase in altitude, but instead almost tripled. This is thought to be because the effects of the earth's magnetic field magnify the exposure rate of cosmic25 Present result
20 -~o
~
15
-
5
• Latitude:69°S-77°S O Latitude:25°N
~• la
/
/
/(/"
/
./
rays at the higher altitudes. This relationship can also be used to estimate the exposure rate of cosmic rays inside airplanes flying at high altitudes in different latitudes. However, the latitude dependency of the coefficients must first be derived. In summary, the present work shows that, although the quasi-effective energy is independent of altitude, the absorbed dose of natural radiation is strongly dependent on altitude. As a result, it may be concluded that natural radiation occurring over the ice floes of Antarctica consists almost entirely of cosmic-rays. In addition, the maximum value of the quasi-effective energy in the hyperbolic function expressing the relationship between the exposure rate and quasi-effective energy of natural radiation (Nakajima, 1988) was estimated and assessed at approx. 10 MeV. Furthermore it is suggested that even when an aircraft flies at constant altitude, the absorbed dose of persons in the craft from cosmic radiation is dependent on the latitude.
4. Conclusion
and Summary
The exposure rate and quasi-effective energy of natural radiation over Antarctica was measured with integrating TLD dosimeters, and the following results were obtained: 1. In high-latitude regions, the empirical doubling with-altitude rule cannot be applied to the correlation between altitude and the exposure rate of cosmic-rays, and the exposure rate is instead found to almost triple with altitude. This relationship can be affected by magnetic field variations latitude at the monitoring points. 2. The quasi-effective energy of natural radiation over Antarctica is approx. 10 MeV. This value is not dependent on the altitude above sea level of the measuring point, and is considered to be the quasi-effective energy of the cosmic-rays. 3. Based on these results, the natural over the ice-based Antarctica consists almost entirely of cosmic rays, and, in general, natural radiation in regions covered with wide sheets of ice at least 10 m thick is presumed to consist almost exclusively of cosmic-rays.
/
O'Brien'sequation
t I I I I I I I I 500 1000 1500 2000 2500 3000 3500 4000 4500 Altitude (m) Fig. 3. Experimental annual exposure rate of natural radiation as a function of ice-based altitude above sea level at TLD monitoring points on Antarctica in the latitude range 69°S-77°S (solid curve) and calculated cosmic ray exposure rate at 27°N latitude obtained by Monte-Carlo method (dotted curve) and experimental exposure rate (circle points) (Fujitaka, 1994). ARI 46/12--F
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Acknowledgements--The authors are indebted to Dr K. Fujitaka, Division Head, National Institute of Radiological Sciences, for his useful and helpful discussions on the work. Grateful acknowledgements are also due to the Japanese Antarctic winter teams from 1991 to 1993 for their unfailing support and encouragement accorded to this work. The work was financially supported by both the Science and Technology Agency and Ministry of Education of the Japanese Government. References Fujitaka K. (1994) Private Communication. Lowder W. M. and de Planque G. (1977) The response of LiF thermoluminescence dosimeters to natural environment radiation. HASL-313 (Health & Safety Lab.)
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Energy Research and Development Administration, U.S. Department of Energy. Maiello M. L., Gulbin J. F., de Planque G. and Gesell T. F. (1990) 8th international intercomparison of environmental dosimeters. Radiat. Protect. Dosim. 32, 91-98. Nakajima T. (1988) Measurement of exposure rate and quasi-effective energy of natural radiation in Japan by pair-filter thermoluminescencedosimeter method. Radiat. Protect. Dosim. 25, 191-200. Nakajima T. and Chiba M. (1986) Evaluating method of effective energy of radiation due to thermoluminescence dosimeter. J. Nucl. Sci. Technol. 23, 258-266. O'Brien K. (1973) Luin--a code for the calculation of
cosmic ray propagation in the atmosphere. Report HASL-275 (Health & Safety Lab.) U.S. Atomic Energy Commission. O'Brien K. (1978) The response of LiF thermoluminesccnce dosimeters to the ground-level cosmic-ray background. Int. J. Appl. Radiat. Isot. 29, 735-739. Pourchet M. F., Pinglot and Lorius C. (1983) Some meteorological applications of radiative fallout measurements in antarctive snows. J. Geophys. Res. 88, 6013-6020. Wang Q. (1985) Correction of instrument readings and assessment of population doses for surveying external dose rate of natural background radiation in China. Chin. J. Radiol. Med. Protec. 5, 81-82.