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Cosmic neutron intensity at sea level near the zero degree geomagnetic latitude
Appl. Radiat. Isot. Vol.49, No. 4, pp. 415--421,1998 © 1998ElsevierScienceLtd. All rights reserved Printed in Great Britain PII: S0969-8043(97)00283-2 0969-8043/98 $19.00+ 0.00
Pergamon
Cosmic Neutron Intensity at Sea Level Near the Zero Degree Geomagnetic Latitude C. C H U N G * , C. Y. C H E N a n d C. H. K U N G Department of Nuclear Science, National Tsing Hua University, Hsinchu, 30043, Taiwan, R.O.C. (Received 31 March 1997; accepted 3 June 1997)
The cosmic neutron intensity at sea level was monitored in the South China Sea where the vertical cut-off of geomagnetic rigidity is highest. The lowest neutron intensity, measured near the geomagnetic latitude at zero degrees, is 1.13 _ 0.12 n Sv h -~. Land survey on the nearby coral reef yields a lower reading. Shipboard data in the open sea are also compared to those measured in harbors and further inland. The progressive decrease of cosmic neutron dose rates from ship to ground indicates that the air-sea-land interfaces play an essential role in attenuating the cosmic neutron intensity. © 1998 Elsevier Science Ltd. All rights reserved
Introduction Cosmic neutrons, as a secondary cosmic radiation, are generated by the interaction of primary cosmic rays from the galaxy with air particles in the atmosphere. After penetrating down through the troposphere, the cosmic neutrons are attenuated, reflected, and absorbed by air particles and eventually strike sea level. The average value of cosmic neutron intensity, measured primarily in the 30°N-65°N region, is reported to be of the order of 10 - 3 n cm - 2 s- f, corresponding to a dose equivalent rate of around 1 n Sv h - ~ at sea level (UNSCEAR, 1993). In the stratosphere, however, the in-flight neutron dose rate was measured to be as high as 7700 n Sv h-~ near the North Pole (Akatov, 1993). The primary cosmic ray comprising charged particles has to penetrate the Earth's magnetic field in order to enter the atmosphere. The strength of its penetrating ability is defined as its geomagnetic rigidity and is given by its momentum divided by the charge of the particles making up the cosmic ray. All primary cosmic rays with the same rigidity, regardless of their charge number and strength of momentum, follow a track of the same curvature in a given magnetic field. For each place within the atmosphere and for every direction from that place, there exists a rigidity below which the primary cosmic rays are not able to penetrate; hence, a lesser secondary radiation such as cosmic neutrons is known to be produced. This specific rigidity is defined as the geomagnetic cut-off. The value of the geomagnetic cut-off, a function of the Earth's magnetic field, is much higher near the *To whom correspondence should be addressed.
equator than in the region of the poles; therefore, cosmic neutron density is distributed in the converse way, having the lowest density near the equator. The vertical cut-offs of geomagnetic rigidity have been calculated in the stratosphere, at 20 km above sea level (Smart and Shea, 1977), where the production of secondary cosmic neutrons at this altitude reaches its highest rate; these vertical cut-off values are contour-mapped in Fig. I(A). The vertical cut-off of geomagnetic rigidity certainly affects the distribution of the secondary cosmic radiation, including elementary particles, electromagnetic radiation, and neutrons. At sea level, the relative cosmic ray intensity is evaluated (Smart and Shea, 1977) and plotted in Fig. I(B). The shaded areas, as shown in both Fig. I(A) and I(B), indicate the region having the highest geomagnetic rigidity for vertical cut-off in the stratosphere and the lowest cosmic neutron intensity at sea level. Although cosmic neutron intensity contributes only a minor part, up to several percent, of the total cosmic radiation at sea level (Reitz, 1993), it varies at different geographical latitudes, as shown in Fig. 1. It has been reported that the relative cosmic neutron intensity increases by a factor of two from a region with the highest vertical cut-off (17.5 GV near the equator) to that near the pole, with the lowest vertical cut-off (1 GV) in the Atlantic Ocean; however, only uncalibrated counting rates for cosmic neutrons have been published (Moraal et al., 1989). In this work, the cosmic neutron intensity at sea level with the highest vertical cut-off of geomagnetic rigidity in the South China Sea was measured for the first time. The neutron counting system was carried by naval vessels to the South China Sea to make an extensive survey of the neutron dose equivalent rate.
415
416
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GEOGRAPH IC LONGITUDE Fig. 1. Global distribution of (A) the calculated vertical cut-offs of geomagnetic rigidity in GV in the stratosphere at an altitude of 20 km, and (B) relative cosmic ray intensity at sea level (Smart and Shea, 1977). Shaded areas have the highest vertical cut-off of geomagnetic rigidity and the lowest cosmic radiation intensity.
Data are compared with those measured at coastal plains, at harbors, and at a coral islet, and the effect of interfaces between air and water and air and land is assessed.
The South China Sea Survey Routes As illustrated in Fig. 1, the region of the highest vertical cut-off of the geomagnetic rigidity, or the lowest cosmic neutron intensity, covers the area of the Indian Ocean, the South China Sea, and the Western Pacific to the north of the equator. In this work, the survey route crosses the South China Sea from Taiwan and was selected to monitor the lowest cosmic neutron intensity at sea level. The survey routes, as shown in Fig. 2, begin at the port of Taiwan, cross the Luzon Strait into the South China Sea, and turn round at the roadstead in the Tizard Bank of the Spratly Islets where the geomagnetic latitude is near zero degrees. Trafficking routes in the Formosa Strait are also selected for cosmic neutron monitoring to allow for a comparison
with the inland data on Taiwan. These routes represent the highest vertical cut-off of geomagnetic rigidity on the Earth, ranging from 16 GV to 17.5 GV. The neutron counting system of FHT-751 was carried out by the naval vessels of the Republic of Chinese Navy in Taiwan (ROCN) in the period from December 1993 to November 1995. As also illustrated in Fig. 2, the FHT-751 counter was positioned on the flight deck of a guided-missile destroyer (DDG), on the captain's deck of a tank landing ship (LST), and on the upper deck of a coastguard cutter (WPF), where the neutron counter could monitor the cosmic neutrons coming down from the hemispherical atmosphere. There was a total of seven missions carried out for the cosmic neutron intensity survey during 1993-1995. Two missions were conducted in the South China Sea from Taiwan, using D D G and LST; the others were conducted around Taiwan by WPF. A total survey time was logged at around 1040 h on the high sea, covering a distance of 5100 nautical miles.
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GEOGRAPHIC LONGITUDE,E Fig. 2. The survey routes in the South China Sea and the naval vessels of ROCN carrying the FHT-751 neutron counting system: (A) guided-missile destroyer DDG; (B) tank landing ship LST; (C) coastguard cutter WPF.
Cosmic Neutron Monitoring In order to make an extensive survey of cosmic neutron intensity in the South China Sea, where the vertical cut-off o f geomagnetic rigidity is high, a portable neutron counting system with high sensitivity is used. The system has been calibrated by a reference neutron source and normalized to the energy spectrum of cosmic radiation, using a neutron transport calculation, in our laboratory (Chen and Chung, 1997); we therefore only briefly describe its characteristics here. The cosmic neutron counting system used in this work, the BIOREM FHT-751 made by the F A G Co., consists of four components: a BF3 counter tube, an Andersson-Braun type polyethylene neutron moderator, a DC battery, and a notebook computer. The random and system noise rates are extremely low, down to 0.000 19 -I- 0.000 01 counts per second (cps), and it is free from the interference of intense electromagnetic radiation. To determine the neutron dose equivalent rate, the FHT-751 was first calibrated by a horizontal neutron beam emitted from a 2s2Cf reference source, then normalized to the energy spectra of cosmic neutrons from all directions using the Monte Carlo Neutron-Photon (MCNP) transport code calculation. The conversion factor of the FHT-751 counting system is evaluated as 935 n Sv (h cps)-~ at sea level. Since the shipboard materials underneath the FHT-751 neutron counting system are mostly high-tensile steel plates and plywood surrounded by open sea, the cosmic neutrons coming down from the atmosphere are attenuated, reflected, and absorbed
by both the shipboard materials and the sea water. The response function of the FHT-751 neutron counter on board the coast guard cutter WPF, in terms of cps per n cm-2 s - ' , is calculated by the MCNP transport code with a neutron energy of less than 20 MeV and is shown in Fig. 3. The highest response rate, in the neutron energy range of 0.1-7 MeV, also matches the maximum peak of the cosmic neutron energy spectrum at sea level (Hajnal et al., 1971). On the high sea, the FHT-751 counting system was covered by a waterproof bag and connected to the shipboard electric power supply via an uninterrupted tt)
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Fig. 3. Neutron response function of the BIOREM FHT-751 counting system on the upper deck of ROCN coastguard cutter WPF.
418
C. Chung et al.
power device with reserved DC battery. The counting system was set to store the count rate into the notebook computer every 12 h, equivalent to an average cosmic neutron measurement across 60 nautical miles of open sea. In order to compare the shipboard data with land measurements, the FHT-751 neutron counting system was positioned in all harbors around Taiwan to monitor the cosmic neutron intensity at the sea-land interface. The neutron counter was also brought to the coral islet of Itu Aba (10°23'N, 114°21'E) in the Tizard Bank of the Spratly Islets, South China Sea, for measurements of cosmic neutron intensity on the flat coral reef, representing the neutron interaction at the interface of sea water and coral sand. Finally, the FHT-751 counter was positioned on the coastal plain of Taiwan to acquire the inland data representing the ground effect. A total of 25 land measurements, each with a counting period of one week, was conducted during 1993-1995.
Results The cosmic neutron survey in the South China Sea contains a data set of 39 measurements on DDG, 23 on LST, and 22 on WPF. Since the structure of shipboard compartments underneath the FHT-751 counter differs from ship to ship, data measured on various naval vessels have to be normalized. Data points taken on D D G and LST are normalized to those measured at 6 nautical miles off the naval base
GEOMAGNETIC 0 3.0t
'
Tsoying (22°42'N, 120°10'E) on board the WPF, since the W P F provided the least amount of interference for neutron measurement by shipboard materials as she had the shortest freeboard, shallowest draft, and narrowest breadth, as shown in Fig. 2. The dose equivalent rate of cosmic neutrons at sea level, from geomagnetic latitudes 0°N up to 16°N, are logged and illustrated in Fig. 4. The neutron dose equivalent rate at the roadstead in Tizard Bank, where the vertical cut-off of geomagnetic rigidity is the highest in the world, is measured as 1.13 __+0.12 n Sv h -~. Data for neutron dose rates in the open sea fluctuate around an average value of 1.21 + 0.16 n Sv h-J throughout the region from the geomagnetic latitude 0°N to 16°N; minor variation is due primarily to changes in meteorological conditions, which will be discussed later. The FHT-751 neutron counter was also brought to the Itu Aba islet on the Tizard Bank where the geomagnetic latitude is near zero degrees. The three-day monitoring of the cosmic neutron intensity at the center of this tiny islet, with a land area of less than 0.5 km 2, gave a neutron dose equivalent rate of 0.84 + 0.13 n Sv h-~, or 74% of that measured 5000 yards away in the roadstead. In order to assess the effect of the air-sea-land interfaces on cosmic neutron intensity, we measured land, harbor, and open sea cosmic neutron intensity around Taiwan for comparison; the results are shown in Fig. 5. The inland data, taken on the coastal plain
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Fig. 4. Cosmic neutron dose equivalent rates measured on board the naval vessels of ROCN in the South China Sea where the vertical cutoff of geomagnetic rigidity is high; the dashed line represents the average value.
Cosmic neutron intensity at sea level
419
2.0_COSMIC NEUTRON DOSE RATE, nSvlk
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Fig. 5. Cosmic neutron dose equivalent rates measured on land, in harbor, and on the open sea around Taiwan in the region of geographic latitudes 20°N-26°N.
4-50 km away from the shore line and up to 100 m above sea level, give an average of 0.62 + 0.03 n Sv h - ~. Data measured on board naval vessels alongside the pier of all major harbors in Taiwan yield an average reading of 1.09 _+ 0.12 n Sv h - L The more abundant data points in the open sea around Taiwan, within the geomagnetic latitude range II°N-15°N, give an average neutron dose equivalent rate of 1.22 + 0.06 n Sv h - ' . The inland data is only 51% of that measured on board on the high sea off Taiwan, while in the harbors, where the sea and land interface, the cosmic neutron intensity is 89% of that on the high sea. Apparently, soil, rock, and coral sand absorb more cosmic neutrons than sea water.
Discussion The average cosmic neutron dose equivalent rate, measured for the first time in the South China Sea, is 1.21 n S v h -~, which is in fact less than the reported value of the world average of 3.6 n Sv h - 1 (UNSCEAR, 1993). One should be reminded that the UNSCEAR value is weighted mostly from data taken at high latitudes in the industrialized regions of the northern hemisphere. It has been concluded (Bhatt, 1986) that the latitude effect due to the change of rigidity cut-off raises the cosmic neutron intensity by a factor of 2.5 from geomagnetic latitudes 7.8°N to 41°N in Central Asia. Although the South China Sea, located in the highest vertical cut-off of geomagnetic rigidity, has the lowest cosmic neutron intensity, the region is also situated in the tropical ocean where the air moisture changes drastically. It has been calculated (Filippov, 1985) that the variation in moisture content may influence the cosmic neutron intensity above the sea. However, this effect is at its smallest when the
neutron counter is placed 8 m above the water surface. It was for this reason that we selected the upper deck of the naval vessels to position the FHT-751 neutron counter. The neutron data measured in the South China Sea fluctuate up to 28% of the average value, as illustrated in Fig. 4; this is due primarily to the variation of air pressure and overhead cloud amounts during the survey. In seven missions, the barometric readings on board the naval vessels varied between 995 hPa and 1016 hPa with various cloud coverage ranging from cloudless to cloudy with heavy thunderstorms. In our cyclone study (Chung and Chen, 1997), an onset of four times the cosmic neutron intensity was reported during a typhoon strike with a substantial air pressure drop, as much as 48 hPa, coupled with a drastic change of cloud coverage. It is believed that the higher air pressure represents thicker air in the atmosphere, allowing fewer cosmic neutrons originating in the stratosphere to reach sea level. On the other hand, the denser cloud contains enough moisture to effectively moderate high energy cosmic neutrons with E, > 10 MeV into the lower energy range, resulting in greater detector response. The air pressures measured in the South China Sea, 1005 + 10 hPa, correspond to a change of neutron dose equivalent rate around + 0.34 n Sv h - ~ in the case of our typhoon study, which was almost the same magnitude of fluctuation that we observed on the high sea. The primary cosmic ray will interact with all materials to produce secondary cosmic neutrons. The background measurement at sea, on land, or in harbor is made up of neutrons produced, attenuated, reflected, and absorbed by ship, air, seawater, or ground. In an earlier study (O'Brien et al., 1978) a neutron transport code was used to calculate the
420
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cosmic neutron energy spectrum and associated flux for several one-dimensional slab geometries such as air-iron, air-ground, and air-sea; the total cosmic neutron flux was calculated as 0.077, 0.0064, and 0.0031 n cm-2s ~, respectively. Our data display a similar but not identical trend. First, all measurements indicate that the shipboard geometry at open sea yields the largest neutron dose equivalent rate, as shown in Fig. 5. However, the shipboard geometry is certainly far away from the infinitely large slab of an iron plate as assumed in O'Brien's calculation. Once the shipboard neutron counter moves into the harbor where the land mass and sea water interface, the measured neutron dose equivalent rate drops to 89% of that on board in the open sea. By moving the neutron counter further inland, the reading is only 51% of that measured at sea. However, the data obtained on a coral reef is 74% of that measured on board, considerably higher than the inland data. This is due to the lower atomic weight and lighter density of coral sand with respect to those in soil and rock, resulting in less attenuation of the cosmic neutrons, as found in our previous study (Chung et al., 1995). Although the monitoring period in the South China Sea lasted for two years, the variation of cosmic neutron intensity at sea level due to solar activity can be neglected. It has been found (Neher, 1971) that the cosmic ray flux in the stratosphere, 31 km above sea level, varies as much as 100%0 in an l 1-year full solar cycle. However, the variation drops to 33% at the tropopause, 16 km above sea level, and further down to 5% at 7 km above sea level (Anderson, 1973). Hence, we do not expect any impact from solar activity during the course of our lengthy survey period, even when we are approaching the current cycle of solar minimum.
Conclusion The cosmic neutron intensity at sea level has been extensively surveyed in the South China Sea region for the first time where the vertical cut-off of geomagnetic rigidity is the highest in the world. An FHT-751 neutron counting system is placed on board naval vessels of ROCN to monitor cosmic neutrons in the open sea. The counter is calibrated by a reference neutron source and the response function is normalized to the energy spectrum of cosmic neutrons. The shipboard data measured in the South China Sea yield an average value of 1.21 n Sv h ~. The reading at the roadstead of Tizard Bank, where the geomagnetic latitude is near zero degrees, is 1.13 + 0.12 n Sv h-~; measurement on the nearby coral reef of Tizard Bank, however, is only 0.84_+ 0.13 n Sv h ~. This is due primarily to the large absorbing power of the cosmic neutrons by the coral sand with respect to that of sea water.
Furthermore, data measured in the South China Sea displayed only a slight variation from the average value; this is due primarily to the change of air pressure and amount of cloud coverage. In order to assess the effect of the air-sea-land interfaces on cosmic neutron measurements, we also monitored the neutron background on the ground, in harbors, and in the open sea around Taiwan, resulting in average neutron dose equivalent rates of 0.62+0.03, 1.09+0.12, and 1 . 2 2 + 0 . 0 6 n S v h -~, respectively. It is believed that the interfacing geometry and material therein yield varying degrees of attenuation, reflection, and absorption to the incoming cosmic neutrons, resulting in the highest reading occurring on board with air-iron-sea geometry in the open sea, and the lowest reading in the air-ground geometry inland. Although the cosmic neutron dose rate is slightly higher on board than on land, the total dose caused by natural radiation, including cosmic ray and naturally occurring radioactive nuclides, is much lower on board than on land. The annual background dose of cosmic neutrons for all crew members is around 10 600 n Sv. The annual radiation dose from all sources has been recommended not to exceed 1000000 n Sv (ICRP, 1991); hence, there is no serious health threat to the crew members from cosmic neutrons in the open sea.
work is financially supported by the National Science Council of the Republic of China under contract number NSC 84-2112-M007-040Z. The authors wish to thank all crew members of the naval vessels of ROCN who supported the measurements on the high sea without reservation. Acknowledgements--This
References Akatov, Yu. A. t1993) Some results of dose measurements along civil airways in the USSR. Radiat. Prot. Dosim. 48, 59. Anderson, H. R. (1973) Cosmic ray total ionization, 1970-1972. J. Geophys. Res. 78, 3958. Bhatt, V. L. (1986) Altitude and latitude effects of atmospheric neutrons. J. Geophys. Res. A 91, 8971. Chen, C. Y. and Chung, C. (1997) Low intensity cosmic neutron measurement using a portable counting system. J. Nucl. Instr. Methods Phys. Res. A 397, in press. Chung, C. and Chen, C. Y. (1997) Variation of cosmic neutron intensity in typhoon. Accepted for J. Geophys. Res. D.
Chung, C., Chen, C. Y., Wei, Y. Y. and Hsu, C. N. (1995) Monitoring of environmental radiation on the Spratly Islets in the South China Sea. J. Radioanal. Nucl. Chem. Art. 194, 291. Filippov, E. M. (1985) Influence of the change in moisture content of atmospheric air on the distribution of the cosmic-background neutron flux above a water surface. Atomnaya Energiya 59, 61. Hajnal, F., McLaughlin, J. E. and Weinstein, M. S. (1971) Sea level cosmic ray neutron measurements in 1970. Report HASL-241, Health and Safety Laboratory, New York, USA.
Cosmic neutron intensity at sea level International Commission on Radiological Protection (1991) Recommendation of the ICRP, ICRP Publication 60. Ann. ICRP 21 (1-3), Pergamon Press, Oxford, UK. Moraal, H., Potgieter, M. S. and Stoker, P. H. (1989) Neutron monitor latitude survey of cosmic ray intensity during the 1986/1987 solar minimum. J. Geophys. Res. A 94, 1459. Neher, H. V. (1971) Cosmic rays at high latitudes and altitudes covering four solar maxima. J. Geophys. Res. 76, 1637. O'Brien, K., Sandmeier, H. A., Hansen, G. E. and Campbell, J. E. (1978) Cosmic ray induced neutron back-
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ground sources and flux for geometries of air over water, ground, iron, and aluminum. J. Geophys. Res. 83, 114. Reitz, G. (1993) Radiation environment in the stratosphere. Radiat. Prot. Dosim. 48, 5. Smart, D. F. and Shea, M. A. (1977) The use of offset dipole coordinates for interpolating cosmic ray cutoff rigidities in three dimensions. In 15th Int. Cosmic Ray Conf, Vol. H, p. 256. Bulgarian Academy of Science, Sofia. United Nations Scientific Committee on the Effect of Atomic Radiation (1993) UNSCEAR Report--1993. United Nations, New York.
Pergamon
Cosmic Neutron Intensity at Sea Level Near the Zero Degree Geomagnetic Latitude C. C H U N G * , C. Y. C H E N a n d C. H. K U N G Department of Nuclear Science, National Tsing Hua University, Hsinchu, 30043, Taiwan, R.O.C. (Received 31 March 1997; accepted 3 June 1997)
The cosmic neutron intensity at sea level was monitored in the South China Sea where the vertical cut-off of geomagnetic rigidity is highest. The lowest neutron intensity, measured near the geomagnetic latitude at zero degrees, is 1.13 _ 0.12 n Sv h -~. Land survey on the nearby coral reef yields a lower reading. Shipboard data in the open sea are also compared to those measured in harbors and further inland. The progressive decrease of cosmic neutron dose rates from ship to ground indicates that the air-sea-land interfaces play an essential role in attenuating the cosmic neutron intensity. © 1998 Elsevier Science Ltd. All rights reserved
Introduction Cosmic neutrons, as a secondary cosmic radiation, are generated by the interaction of primary cosmic rays from the galaxy with air particles in the atmosphere. After penetrating down through the troposphere, the cosmic neutrons are attenuated, reflected, and absorbed by air particles and eventually strike sea level. The average value of cosmic neutron intensity, measured primarily in the 30°N-65°N region, is reported to be of the order of 10 - 3 n cm - 2 s- f, corresponding to a dose equivalent rate of around 1 n Sv h - ~ at sea level (UNSCEAR, 1993). In the stratosphere, however, the in-flight neutron dose rate was measured to be as high as 7700 n Sv h-~ near the North Pole (Akatov, 1993). The primary cosmic ray comprising charged particles has to penetrate the Earth's magnetic field in order to enter the atmosphere. The strength of its penetrating ability is defined as its geomagnetic rigidity and is given by its momentum divided by the charge of the particles making up the cosmic ray. All primary cosmic rays with the same rigidity, regardless of their charge number and strength of momentum, follow a track of the same curvature in a given magnetic field. For each place within the atmosphere and for every direction from that place, there exists a rigidity below which the primary cosmic rays are not able to penetrate; hence, a lesser secondary radiation such as cosmic neutrons is known to be produced. This specific rigidity is defined as the geomagnetic cut-off. The value of the geomagnetic cut-off, a function of the Earth's magnetic field, is much higher near the *To whom correspondence should be addressed.
equator than in the region of the poles; therefore, cosmic neutron density is distributed in the converse way, having the lowest density near the equator. The vertical cut-offs of geomagnetic rigidity have been calculated in the stratosphere, at 20 km above sea level (Smart and Shea, 1977), where the production of secondary cosmic neutrons at this altitude reaches its highest rate; these vertical cut-off values are contour-mapped in Fig. I(A). The vertical cut-off of geomagnetic rigidity certainly affects the distribution of the secondary cosmic radiation, including elementary particles, electromagnetic radiation, and neutrons. At sea level, the relative cosmic ray intensity is evaluated (Smart and Shea, 1977) and plotted in Fig. I(B). The shaded areas, as shown in both Fig. I(A) and I(B), indicate the region having the highest geomagnetic rigidity for vertical cut-off in the stratosphere and the lowest cosmic neutron intensity at sea level. Although cosmic neutron intensity contributes only a minor part, up to several percent, of the total cosmic radiation at sea level (Reitz, 1993), it varies at different geographical latitudes, as shown in Fig. 1. It has been reported that the relative cosmic neutron intensity increases by a factor of two from a region with the highest vertical cut-off (17.5 GV near the equator) to that near the pole, with the lowest vertical cut-off (1 GV) in the Atlantic Ocean; however, only uncalibrated counting rates for cosmic neutrons have been published (Moraal et al., 1989). In this work, the cosmic neutron intensity at sea level with the highest vertical cut-off of geomagnetic rigidity in the South China Sea was measured for the first time. The neutron counting system was carried by naval vessels to the South China Sea to make an extensive survey of the neutron dose equivalent rate.
415
416
C. Chung et al.
60"
J l
"
2
30*
.
-30* 2
~
4
,--I 6O*
<
30-
--30 °
-60* O"
30°
60*
90*
120"
150"
180"
210"
240'
270*
300*
330*
360'
GEOGRAPH IC LONGITUDE Fig. 1. Global distribution of (A) the calculated vertical cut-offs of geomagnetic rigidity in GV in the stratosphere at an altitude of 20 km, and (B) relative cosmic ray intensity at sea level (Smart and Shea, 1977). Shaded areas have the highest vertical cut-off of geomagnetic rigidity and the lowest cosmic radiation intensity.
Data are compared with those measured at coastal plains, at harbors, and at a coral islet, and the effect of interfaces between air and water and air and land is assessed.
The South China Sea Survey Routes As illustrated in Fig. 1, the region of the highest vertical cut-off of the geomagnetic rigidity, or the lowest cosmic neutron intensity, covers the area of the Indian Ocean, the South China Sea, and the Western Pacific to the north of the equator. In this work, the survey route crosses the South China Sea from Taiwan and was selected to monitor the lowest cosmic neutron intensity at sea level. The survey routes, as shown in Fig. 2, begin at the port of Taiwan, cross the Luzon Strait into the South China Sea, and turn round at the roadstead in the Tizard Bank of the Spratly Islets where the geomagnetic latitude is near zero degrees. Trafficking routes in the Formosa Strait are also selected for cosmic neutron monitoring to allow for a comparison
with the inland data on Taiwan. These routes represent the highest vertical cut-off of geomagnetic rigidity on the Earth, ranging from 16 GV to 17.5 GV. The neutron counting system of FHT-751 was carried out by the naval vessels of the Republic of Chinese Navy in Taiwan (ROCN) in the period from December 1993 to November 1995. As also illustrated in Fig. 2, the FHT-751 counter was positioned on the flight deck of a guided-missile destroyer (DDG), on the captain's deck of a tank landing ship (LST), and on the upper deck of a coastguard cutter (WPF), where the neutron counter could monitor the cosmic neutrons coming down from the hemispherical atmosphere. There was a total of seven missions carried out for the cosmic neutron intensity survey during 1993-1995. Two missions were conducted in the South China Sea from Taiwan, using D D G and LST; the others were conducted around Taiwan by WPF. A total survey time was logged at around 1040 h on the high sea, covering a distance of 5100 nautical miles.
Cosmic neutron intensity at sea level II
1(
•
soc..., uT.
=
LUZON STRAIT
-~ /--FHT751
20-1 metlr
~
-,,z,2,,-//, 115
lot
II
/jo,q 120
I
-I
(B)
1/,
Ii Ss. V ! o
417
I
': o
,oo ,oo
1125
GEOGRAPHIC LONGITUDE,E Fig. 2. The survey routes in the South China Sea and the naval vessels of ROCN carrying the FHT-751 neutron counting system: (A) guided-missile destroyer DDG; (B) tank landing ship LST; (C) coastguard cutter WPF.
Cosmic Neutron Monitoring In order to make an extensive survey of cosmic neutron intensity in the South China Sea, where the vertical cut-off o f geomagnetic rigidity is high, a portable neutron counting system with high sensitivity is used. The system has been calibrated by a reference neutron source and normalized to the energy spectrum of cosmic radiation, using a neutron transport calculation, in our laboratory (Chen and Chung, 1997); we therefore only briefly describe its characteristics here. The cosmic neutron counting system used in this work, the BIOREM FHT-751 made by the F A G Co., consists of four components: a BF3 counter tube, an Andersson-Braun type polyethylene neutron moderator, a DC battery, and a notebook computer. The random and system noise rates are extremely low, down to 0.000 19 -I- 0.000 01 counts per second (cps), and it is free from the interference of intense electromagnetic radiation. To determine the neutron dose equivalent rate, the FHT-751 was first calibrated by a horizontal neutron beam emitted from a 2s2Cf reference source, then normalized to the energy spectra of cosmic neutrons from all directions using the Monte Carlo Neutron-Photon (MCNP) transport code calculation. The conversion factor of the FHT-751 counting system is evaluated as 935 n Sv (h cps)-~ at sea level. Since the shipboard materials underneath the FHT-751 neutron counting system are mostly high-tensile steel plates and plywood surrounded by open sea, the cosmic neutrons coming down from the atmosphere are attenuated, reflected, and absorbed
by both the shipboard materials and the sea water. The response function of the FHT-751 neutron counter on board the coast guard cutter WPF, in terms of cps per n cm-2 s - ' , is calculated by the MCNP transport code with a neutron energy of less than 20 MeV and is shown in Fig. 3. The highest response rate, in the neutron energy range of 0.1-7 MeV, also matches the maximum peak of the cosmic neutron energy spectrum at sea level (Hajnal et al., 1971). On the high sea, the FHT-751 counting system was covered by a waterproof bag and connected to the shipboard electric power supply via an uninterrupted tt)
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Fig. 3. Neutron response function of the BIOREM FHT-751 counting system on the upper deck of ROCN coastguard cutter WPF.
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power device with reserved DC battery. The counting system was set to store the count rate into the notebook computer every 12 h, equivalent to an average cosmic neutron measurement across 60 nautical miles of open sea. In order to compare the shipboard data with land measurements, the FHT-751 neutron counting system was positioned in all harbors around Taiwan to monitor the cosmic neutron intensity at the sea-land interface. The neutron counter was also brought to the coral islet of Itu Aba (10°23'N, 114°21'E) in the Tizard Bank of the Spratly Islets, South China Sea, for measurements of cosmic neutron intensity on the flat coral reef, representing the neutron interaction at the interface of sea water and coral sand. Finally, the FHT-751 counter was positioned on the coastal plain of Taiwan to acquire the inland data representing the ground effect. A total of 25 land measurements, each with a counting period of one week, was conducted during 1993-1995.
Results The cosmic neutron survey in the South China Sea contains a data set of 39 measurements on DDG, 23 on LST, and 22 on WPF. Since the structure of shipboard compartments underneath the FHT-751 counter differs from ship to ship, data measured on various naval vessels have to be normalized. Data points taken on D D G and LST are normalized to those measured at 6 nautical miles off the naval base
GEOMAGNETIC 0 3.0t
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Tsoying (22°42'N, 120°10'E) on board the WPF, since the W P F provided the least amount of interference for neutron measurement by shipboard materials as she had the shortest freeboard, shallowest draft, and narrowest breadth, as shown in Fig. 2. The dose equivalent rate of cosmic neutrons at sea level, from geomagnetic latitudes 0°N up to 16°N, are logged and illustrated in Fig. 4. The neutron dose equivalent rate at the roadstead in Tizard Bank, where the vertical cut-off of geomagnetic rigidity is the highest in the world, is measured as 1.13 __+0.12 n Sv h -~. Data for neutron dose rates in the open sea fluctuate around an average value of 1.21 + 0.16 n Sv h-J throughout the region from the geomagnetic latitude 0°N to 16°N; minor variation is due primarily to changes in meteorological conditions, which will be discussed later. The FHT-751 neutron counter was also brought to the Itu Aba islet on the Tizard Bank where the geomagnetic latitude is near zero degrees. The three-day monitoring of the cosmic neutron intensity at the center of this tiny islet, with a land area of less than 0.5 km 2, gave a neutron dose equivalent rate of 0.84 + 0.13 n Sv h-~, or 74% of that measured 5000 yards away in the roadstead. In order to assess the effect of the air-sea-land interfaces on cosmic neutron intensity, we measured land, harbor, and open sea cosmic neutron intensity around Taiwan for comparison; the results are shown in Fig. 5. The inland data, taken on the coastal plain
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Fig. 4. Cosmic neutron dose equivalent rates measured on board the naval vessels of ROCN in the South China Sea where the vertical cutoff of geomagnetic rigidity is high; the dashed line represents the average value.
Cosmic neutron intensity at sea level
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2.0_COSMIC NEUTRON DOSE RATE, nSvlk
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Fig. 5. Cosmic neutron dose equivalent rates measured on land, in harbor, and on the open sea around Taiwan in the region of geographic latitudes 20°N-26°N.
4-50 km away from the shore line and up to 100 m above sea level, give an average of 0.62 + 0.03 n Sv h - ~. Data measured on board naval vessels alongside the pier of all major harbors in Taiwan yield an average reading of 1.09 _+ 0.12 n Sv h - L The more abundant data points in the open sea around Taiwan, within the geomagnetic latitude range II°N-15°N, give an average neutron dose equivalent rate of 1.22 + 0.06 n Sv h - ' . The inland data is only 51% of that measured on board on the high sea off Taiwan, while in the harbors, where the sea and land interface, the cosmic neutron intensity is 89% of that on the high sea. Apparently, soil, rock, and coral sand absorb more cosmic neutrons than sea water.
Discussion The average cosmic neutron dose equivalent rate, measured for the first time in the South China Sea, is 1.21 n S v h -~, which is in fact less than the reported value of the world average of 3.6 n Sv h - 1 (UNSCEAR, 1993). One should be reminded that the UNSCEAR value is weighted mostly from data taken at high latitudes in the industrialized regions of the northern hemisphere. It has been concluded (Bhatt, 1986) that the latitude effect due to the change of rigidity cut-off raises the cosmic neutron intensity by a factor of 2.5 from geomagnetic latitudes 7.8°N to 41°N in Central Asia. Although the South China Sea, located in the highest vertical cut-off of geomagnetic rigidity, has the lowest cosmic neutron intensity, the region is also situated in the tropical ocean where the air moisture changes drastically. It has been calculated (Filippov, 1985) that the variation in moisture content may influence the cosmic neutron intensity above the sea. However, this effect is at its smallest when the
neutron counter is placed 8 m above the water surface. It was for this reason that we selected the upper deck of the naval vessels to position the FHT-751 neutron counter. The neutron data measured in the South China Sea fluctuate up to 28% of the average value, as illustrated in Fig. 4; this is due primarily to the variation of air pressure and overhead cloud amounts during the survey. In seven missions, the barometric readings on board the naval vessels varied between 995 hPa and 1016 hPa with various cloud coverage ranging from cloudless to cloudy with heavy thunderstorms. In our cyclone study (Chung and Chen, 1997), an onset of four times the cosmic neutron intensity was reported during a typhoon strike with a substantial air pressure drop, as much as 48 hPa, coupled with a drastic change of cloud coverage. It is believed that the higher air pressure represents thicker air in the atmosphere, allowing fewer cosmic neutrons originating in the stratosphere to reach sea level. On the other hand, the denser cloud contains enough moisture to effectively moderate high energy cosmic neutrons with E, > 10 MeV into the lower energy range, resulting in greater detector response. The air pressures measured in the South China Sea, 1005 + 10 hPa, correspond to a change of neutron dose equivalent rate around + 0.34 n Sv h - ~ in the case of our typhoon study, which was almost the same magnitude of fluctuation that we observed on the high sea. The primary cosmic ray will interact with all materials to produce secondary cosmic neutrons. The background measurement at sea, on land, or in harbor is made up of neutrons produced, attenuated, reflected, and absorbed by ship, air, seawater, or ground. In an earlier study (O'Brien et al., 1978) a neutron transport code was used to calculate the
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cosmic neutron energy spectrum and associated flux for several one-dimensional slab geometries such as air-iron, air-ground, and air-sea; the total cosmic neutron flux was calculated as 0.077, 0.0064, and 0.0031 n cm-2s ~, respectively. Our data display a similar but not identical trend. First, all measurements indicate that the shipboard geometry at open sea yields the largest neutron dose equivalent rate, as shown in Fig. 5. However, the shipboard geometry is certainly far away from the infinitely large slab of an iron plate as assumed in O'Brien's calculation. Once the shipboard neutron counter moves into the harbor where the land mass and sea water interface, the measured neutron dose equivalent rate drops to 89% of that on board in the open sea. By moving the neutron counter further inland, the reading is only 51% of that measured at sea. However, the data obtained on a coral reef is 74% of that measured on board, considerably higher than the inland data. This is due to the lower atomic weight and lighter density of coral sand with respect to those in soil and rock, resulting in less attenuation of the cosmic neutrons, as found in our previous study (Chung et al., 1995). Although the monitoring period in the South China Sea lasted for two years, the variation of cosmic neutron intensity at sea level due to solar activity can be neglected. It has been found (Neher, 1971) that the cosmic ray flux in the stratosphere, 31 km above sea level, varies as much as 100%0 in an l 1-year full solar cycle. However, the variation drops to 33% at the tropopause, 16 km above sea level, and further down to 5% at 7 km above sea level (Anderson, 1973). Hence, we do not expect any impact from solar activity during the course of our lengthy survey period, even when we are approaching the current cycle of solar minimum.
Conclusion The cosmic neutron intensity at sea level has been extensively surveyed in the South China Sea region for the first time where the vertical cut-off of geomagnetic rigidity is the highest in the world. An FHT-751 neutron counting system is placed on board naval vessels of ROCN to monitor cosmic neutrons in the open sea. The counter is calibrated by a reference neutron source and the response function is normalized to the energy spectrum of cosmic neutrons. The shipboard data measured in the South China Sea yield an average value of 1.21 n Sv h ~. The reading at the roadstead of Tizard Bank, where the geomagnetic latitude is near zero degrees, is 1.13 + 0.12 n Sv h-~; measurement on the nearby coral reef of Tizard Bank, however, is only 0.84_+ 0.13 n Sv h ~. This is due primarily to the large absorbing power of the cosmic neutrons by the coral sand with respect to that of sea water.
Furthermore, data measured in the South China Sea displayed only a slight variation from the average value; this is due primarily to the change of air pressure and amount of cloud coverage. In order to assess the effect of the air-sea-land interfaces on cosmic neutron measurements, we also monitored the neutron background on the ground, in harbors, and in the open sea around Taiwan, resulting in average neutron dose equivalent rates of 0.62+0.03, 1.09+0.12, and 1 . 2 2 + 0 . 0 6 n S v h -~, respectively. It is believed that the interfacing geometry and material therein yield varying degrees of attenuation, reflection, and absorption to the incoming cosmic neutrons, resulting in the highest reading occurring on board with air-iron-sea geometry in the open sea, and the lowest reading in the air-ground geometry inland. Although the cosmic neutron dose rate is slightly higher on board than on land, the total dose caused by natural radiation, including cosmic ray and naturally occurring radioactive nuclides, is much lower on board than on land. The annual background dose of cosmic neutrons for all crew members is around 10 600 n Sv. The annual radiation dose from all sources has been recommended not to exceed 1000000 n Sv (ICRP, 1991); hence, there is no serious health threat to the crew members from cosmic neutrons in the open sea.
work is financially supported by the National Science Council of the Republic of China under contract number NSC 84-2112-M007-040Z. The authors wish to thank all crew members of the naval vessels of ROCN who supported the measurements on the high sea without reservation. Acknowledgements--This
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Chung, C., Chen, C. Y., Wei, Y. Y. and Hsu, C. N. (1995) Monitoring of environmental radiation on the Spratly Islets in the South China Sea. J. Radioanal. Nucl. Chem. Art. 194, 291. Filippov, E. M. (1985) Influence of the change in moisture content of atmospheric air on the distribution of the cosmic-background neutron flux above a water surface. Atomnaya Energiya 59, 61. Hajnal, F., McLaughlin, J. E. and Weinstein, M. S. (1971) Sea level cosmic ray neutron measurements in 1970. Report HASL-241, Health and Safety Laboratory, New York, USA.
Cosmic neutron intensity at sea level International Commission on Radiological Protection (1991) Recommendation of the ICRP, ICRP Publication 60. Ann. ICRP 21 (1-3), Pergamon Press, Oxford, UK. Moraal, H., Potgieter, M. S. and Stoker, P. H. (1989) Neutron monitor latitude survey of cosmic ray intensity during the 1986/1987 solar minimum. J. Geophys. Res. A 94, 1459. Neher, H. V. (1971) Cosmic rays at high latitudes and altitudes covering four solar maxima. J. Geophys. Res. 76, 1637. O'Brien, K., Sandmeier, H. A., Hansen, G. E. and Campbell, J. E. (1978) Cosmic ray induced neutron back-
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ground sources and flux for geometries of air over water, ground, iron, and aluminum. J. Geophys. Res. 83, 114. Reitz, G. (1993) Radiation environment in the stratosphere. Radiat. Prot. Dosim. 48, 5. Smart, D. F. and Shea, M. A. (1977) The use of offset dipole coordinates for interpolating cosmic ray cutoff rigidities in three dimensions. In 15th Int. Cosmic Ray Conf, Vol. H, p. 256. Bulgarian Academy of Science, Sofia. United Nations Scientific Committee on the Effect of Atomic Radiation (1993) UNSCEAR Report--1993. United Nations, New York.