Research Article
Measurement of Cosmic Radiation Dose to Organs of An Anthropomorphous Phantom at Yangbajing International Cosmic Ray Observatory
https://doi.org/10.21203/rs.3.rs-3719310/v1
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The distribution of cosmic ray dose in organs of an anthropomorphous phantom at the Yangbajing International Cosmic Ray Observatory was measured using thermoluminescent detectors. Then the measurements were compared with the calculations of the theoretical equation for the variation of cosmic ray dose rate with altitude and measurements of the active gamma-ray dose meter, which provided basic data for evaluating natural cosmic radiation dose in the Yangbajing area.
Yangbajing
cosmic ray observatory
anthropomorphic phantom
organs
cosmic rays
Cosmic rays, which consist of primary cosmic rays coming from outer space and secondary cosmic rays resulting from the interaction of primary cosmic rays with atomic nuclei in the Earth’s atmosphere, can be hazardous to humans when the radiation is strong. In recent years, as the International Commission on Radiological Protection (ICRP) recommended including aircrews in the population exposed to vocational radiation, great importance has been attached nationally and internationally to the monitoring and evaluation of the cosmic radiation dose aircrews exposed [1, 2, 3].
Particles in cosmic rays that contribute significantly to the human effective dose include neutrons, electrons, positrons, photons, and protons, and the intensity of these particles increases with altitude. At 5000m above sea level, the cosmic radiation effective dose rate received by people is about 8 times higher than at sea level. At this altitude, the compositions and proportions of particles contributing to the human effective dose are the same as that in the flight altitudes. In addition, the percentage of the contribution these particles make to the effective dose does not vary greatly between altitudes(See Fig. 1) [4, 5, 6]. Those cosmic rays dose measurement methods, which are developed at high altitudes, can therefore be applied to dose monitoring for aircrew, and if accompanied by heavy nuclear dose monitoring technology, to manned spaceflight dose monitoring [7, 8]. The development of related monitoring technologies on the ground can be easier and less costly as the safety of the overall work and related equipment (which is extremely strict in aviation and aerospace) need not be considered too much during development.
In this context, several high-altitude observatories or research stations abroad have carried out ground-based research on advanced technology for cosmic radiation dose measurement on high-altitude mountains and have applied the technologies to the aerospace sector. A typical example is a joint work carried out by the Chacaltaya Observatory in Bolivia and the Testa Grigia Research Station in Italy. They have worked on the assessment of cosmic radiation dose to human beings at high altitudes since 2003 [9]. Chinese scientists have also noted that research on radiation protection should also be extended to the Sun-Earth system so as to ensure the safety of human beings who explore space and work and live in space [10].
In this paper, the gamma dose distribution in different organs of the phantom was measured using the thermoluminescent detectors at Yangbajing International Cosmic Ray Observatory, which collected long-term baseline data on the human radiation dose, provide information for the individuals or the public exposed to radiation and provide important data for medical treatment and epidemiological studies and are of great significance for basic research in space science.
Yangbajing, Tibet[11] (Yangbajing town, Naqu county, Tibet, 4300 m a.s.l., 90°53’E, 30°11’N) was chosen as the experimental site. Given the variable climate in the Yangbajing area, a suitable location should be chosen carefully for the measurement without damaging the phantom. Finally, the experiment was carried out in the Yangbajing International Cosmic Ray Observatory, where the phantom was placed on the top layer of the hall and right underneath the plastic wrap to minimise the blocking of the cosmic ray. Thermoluminescent detectors were placed in different parts of the phantom. The placement of the phantom in the hall is shown in Fig. 2.
In this experiment, thermoluminescent detectors were mainly used to measure the gamma-ray dose to different organs of the phantom. A compound measurement method was used when gamma-ray dosimeters were employed to measure the gamma-ray dose rate at the Yangbajing International Cosmic Ray Observatory and thermoluminescent detectors (TLD) placed on the phantom were used to measure absorbed gamma-ray dose to different organs of the phantom.
The thermoluminescent detectors used for this experiment were produced by the Beijing Haiyangbochuang Company, with an overall dimension of Φ4.5mm×0.8mm. The main component of a thermoluminescent detector was natural LiF (Mg, Cu, P) with a uniformity within ± 3%.
Before performing experiments in the Yangbajing area, the thermoluminescent detectors calibrated TLD chips encapsulated in plastic bags using 137 Cs rays from the Radiation Measurement Station of the China Institute for Radiation Protection (CIRP). The air kerma of the radiation field was 1 mGy. During calibration, the normal direction of the round surface of the TLD chips was parallel to the incidence direction of radiation. In order to reduce errors, seven dose chips were calibrated as a group and the results were averaged in the end. After calibration, the dose chips were read using a TLD-3500 reader.
To facilitate the installation of the TLD, a method of encapsulation was designed. First, the detectors were installed in the corresponding positions on the encapsulations, and then a packaging film specifically designed for the experiment was used to warp the encapsulations, which then were inserted into the corresponding holes of the phantom as a whole.
Passive detector: As shown in Fig. 3, its overall size is 13 x 13 x 8 (mm). The circular hole (Ф4.8mm) in the middle can hold 7–8 TLD chips.
A batch of thermoluminescent detectors was placed outside and inside the phantom at Yangbajing International Cosmic Ray Observatory for 96 days. 6 TLD chips were placed on the shoulders of the phantom to measure the gamma-ray dose outside the phantom, and the measurements were averaged; 7–8 TLD chips were inserted into the holes of the phantom to measure the gamma-ray dose to different organs. Meanwhile, in order to compare the gamma-ray radiation dose from cosmic rays at different altitudes, a batch of TLD chips was placed on indoor windowsills in Taiyuan for 104 days. The altitude of Taiyuan is about 798 m a.s.l.
The gamma dose rate meter, consisting of a principal machine and a highly sensitive NaI detection system, gives a response within 1s in the background case and its measurement range is between 0.05 µSv/h and 300 µSv/h. It can be used to measure gamma-ray in the environment. The detection system of the gamma dose rate meter is shown in Fig. 4.
Data read from calibrated TLD chips is shown in Table 1
| Radiation quality /KV | Corresponding Ray energy */keV | TLD chip reading /nC | Average value /nC | ||||||
|---|---|---|---|---|---|---|---|---|---|
| 1# | 2# | 3# | 4# | 5# | 6# | 7# | |||
| 137Cs | 662 | 530.9 | 530.5 | 538.2 | 534.6 | 525.5 | 539.4 | 536.6 | 533.7 |
The data read from the thermoluminescent detectors are shown in Table 2, where A represents the average deposited dose of six TLD chips placed on the shoulders of the phantom for 96 days and it is approximately 1.14mGy and the annual cumulative gamma-ray dose is 4.33 mGy/a; B represents the average deposited dose of 6 six TLD chips placed on the indoor windowsills in Taiyuan for over 104 days and it is approximately 0.275 mGy. The annual cumulative gamma-ray dose is 0.965 mGy/a.
| Region | Thermoluminescent Detector Readings / nC in the ARGO Experimental Hall | Average value / nC | Deposited dose / mGy | |||||
| 1# | 2# | 3# | 4# | 5# | 6# | |||
| Yangbajing | 577.9 | 628.1 | 585.3 | 605.3 | 630.3 | 634.2 | 610.2 | A |
| 1.14 | ||||||||
| Taiyuan | 148.0 | 146.3 | 147.8 | 144.7 | 147.9 | 144.5 | 146.5 | B |
| 0.275 | ||||||||
Thermoluminescent detectors were used to measure the gamma-ray dose distribution in the organs of the phantom at Yangbajing International Cosmic Ray Observatory. The organs measured are the brain, thyroid, esophagus, lung, heart, spine, muscles (muscle 1, muscle 2...muscle 5 indicates different muscle parts), liver, pancreas, kidney, stomach, spleen, intestine, bladder, pelvis and testicles. The phantom was measured for three months and the distribution of gamma dose absorbed by different organs is shown in Fig. 5:
From Fig. 5, it can be seen that the doses absorbed by different organs are different, with the left testicle absorbing the most doses. The dose it absorbed for three months is 0.611mGy and its annual cumulative dose is 2.32mGy/a; The spleen absorbed the least doses, with a three-month cycle dose of 0.512mGy and an annual cumulative dose of 1.95mGy/a. According to the National Standard of the People’s Republic of China GB-18871, the annual average effective dose to the public should not exceed 1mSv/a. However, the dose to the public in the Yangbajing area exceeds this limit. According to the standard, natural radiation is not included in the limit, which means it is unnecessary to take measures to reduce radiation dose to the local. However, in terms of numbers alone, the measurements of this experiment indicate that the gamma dose in the Yangbajing area is on the high side. These measurements are an important reference for the environmental radiation survey. It can also provide reference data to ensure the radiation safety of people in Yangbajing town.
Gamma dose rate meters were used to measure the gamma dose rate in Taiyuan, Lhasa and Yangbajing International Cosmic Ray Observatory and a gamma dose rate meter has the NaI as its probe.
| Location | 1# | 2# | 3# | 4# | 5# | Mean value/ nGy-h− 1 |
|---|---|---|---|---|---|---|
| Taiyuan | 190 | 180 | 190 | 190 | 170 | 184 |
| Lhasa | 290 | 280 | 290 | 270 | 290 | 284 |
| Yangbajing | 390 | 400 | 380 | 390 | 400 | 392 |
The stability of the gamma dose rate meter is reliable. The gamma dose rate at the Yangbajing International Cosmic Ray Observatory is approximately 392 nGy/h and the annual gamma-ray dose is approximately 3.43 mGy/a, indicating the gamma dose rate in the Yangbajing area is on the high side. Zheng Guowen et al. of the China Institute of Atomic Energy measured cosmic ray dose rates at different altitudes and studied the changing rule of cosmic ray dose rate varies with altitude[12]. The gamma-ray dose in the Yangbajing International Cosmic Ray Observatory can be estimated using the measurements of the high-pressure ionization chamber at different altitudes. The estimated dose rate at the Yangbajing International Cosmic Ray Observatory is about 361.3 nGy/h, and the estimated annual calculative gamma-ray dose in the environment is about 3.16 mGy/a. This theoretical data deviates from the measured data of the gamma dose rate meter by about 8%, which is of great importance since it characterizes the accuracy of the measurements of gamma dose rate meters and the accuracy of the gammy dose measurements of organs of the phantom.
The relevant analytical data are listed in Table 4. In Table 4: A represents the deviation of the measured values of the TLD chips from the theoretically calculated values; B represents the deviation of the measured values of the gamma dose rate meter from the theoretically calculated values.
| Measurement / Calculation | Annual cumulative gamma dose at Yangbajing International Cosmic Ray Observatory, mGy/a | Deviation % |
|---|---|---|
| TLD chips | 4.33 | A, 37.02 |
| Gamma dose rate meter | 3.43 | B, 8.54 |
| Theoretically calculated values | 3.16 | / |
The deviation between the measured values of the active gamma dose rate meter and the theoretically calculated values at the Yangbajing International Cosmic Ray Observatory is around 8%, which indicates that the measurements of the dose rate meters are accurate to a certain extent; The deviation between the measured results of the passive TLD chips and the theoretically calculated results is 37%. The measured results of TLD chips are relatively high. Neutron, which is believed to play a part in radiation at high altitude, reacted with Li-6 of natural Lif in the experiment and contributed to the high absorbed gammy ray dose.
The measured results of the TLD, the gamma dose rate meters and the theoretically calculated results indicate that gamma-ray radiation dose in the Yangbajing area is relatively high; The monitoring results of the gamma dose rate meter and theoretically calculated results indicate that the measured results of TLD are reliable to a certain extent.
The measured results of the distribution of gamma dose in organs of the phantom provide important reference data for figuring out the radiation dose absorbed by people in the Ynagbajing area and are of great importance to the evaluation of the natural radiation dose absorbed by people in the Yangbajing area. These measurements also provide important data to facilitate epidemiological studies in medicine and guarantee the radiation safety of aircrews in aviation and aerospace.
A.C.E. completed the field experiment work and wrote the main manuscript text.B.D. reviewed the manuscript and provided modification suggestions.All authors reviewed the manuscript.
No competing interests reported.
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