- Email: [email protected]
Development of the Emulsion Chamber Detector for Space Observations
CHINESE ASTRONOMY AND ASTROPHYSICS
ELSEVIER
Chinese 32(2008) (2008)449–458 449–458 ChineseAstronomy Astronomy and and Astrophysics Astrophysics 32
Development of the Emulsion Chamber Detector for Space Observations † CHANG Jin1 GONG Yi-zhong1 ZHANG Ren-jian1 HU Yi-ming1,2 WANG Nan-sen1 TANG He-sen1 Torii S.3 Nishimura J.4 5 3 Kobayashi T. Shimizu Y. Makino F.3 1
Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008 2 Graduate School of Chinese Academy of Sciences, Beijing 100039 3 Advanced Research Institute for Science and Engineering, Waseda University, Tokyo 169-8555, Japan 4 Institute of Space and Astronautical Science, JAXA, Japan 5 Department of Physics, Aoyamagakuin University, Japan
Abstract The emulsion chamber detector on board the “ShiJian-8” satellite is the first one in China designed especially for observing in space the highenergy electrons and γ-rays. In this paper, the principle of the detector design, the method of data processing and the preliminary results of observations are introduced. The design lifetime of the detector is 15 days on the orbit, and the energy range of detectable particles is 100 GeV∼5 TeV. Key words: instrumentation: detector—instrumentation: miscellaneous—dark matter
1. INTRODUCTION At present, the research of dark matter is a hot point. There has been strong evidence for the existence of the dark matter[1,2] . The updated theoretical model suggests that the dark matter may exist in the form of some special particles, and that the dark-matter particles †
Supported by National Natural Science Foundation and CAS Innovation Foundation Received 2007–04–17; revised version 2007–05–27 A translation of Acta Astron. Sin. Vol. 49, No. 2, pp. 233–242, 2008 [email protected]
0275-1062/08/$-see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.chinastron.2008.10.012
450
HU Yi-ming et al. / Chinese Astronomy and Astrophysics 32 (2008) 449–458
can be searched for by observing the fundamental particles produced by the decay and interaction of dark matters[3,4] . With the aid of the emulsion chamber detector, by observing in space the electrons (as well as positrons) and γ-rays produced by the decay or interaction of dark matters, we hope that the evidence that the dark matter exists in the form of particles can be found, and that some enlightenment on the problem what is really the dark matter can be obtained; at the same time, to observe the high-energy electrons and γ-rays will be very helpful for solving the scientific problems about the origin and acceleration of cosmic rays. In the research area of high-energy cosmic rays, the emulsion chamber detector, as a kind of mature technique, has played a very important role. But at present, the types of emulsion chamber observations are still restricted to the high-mountain observation, underground observation and balloon observation. So far, it is the first time in China to use the emulsion chamber detector on board a scientific experiment satellite for observing in space the high-energy electrons and γ-rays.
2. OVERALL DESIGN OF THE EMULSION CHAMBER DETECTOR 2.1 Detection Principle The basic structure of the “ShiJian-8” emulsion chamber detector is shown in Fig.1. Between two action layers (tungsten plates) a photosensitive layer is placed, and it is composed of the X-ray films and nuclear emulsion films with different photosensitivities[5,6] . When the high-energy γ-rays (or electrons) enter the emulsion chamber, the interaction with the tungsten plate causes the electromagnetic cascade shower, via the Bremsstrahlung radiation the high-energy electrons produce γ-rays, and the high-energy γ-rays will give rise to positron-electron pairs by the effect of electron pairs. The high-energy electrons produce γ-rays via the Bremsstrahlung radiation, and γ-rays will produce new electrons, thus the cascaded electron-γ-ray interaction will continue uninterruptedly. As a result, a great number of γ-rays and electrons are produced, these particles form latent images on the emulsion film inside the photosensitive layer. After some processing, the track of electrons will be left on the emulsion film, simultaneously some black spots will appear on the X-ray emulsion film after exposure, thus these particles are recorded by the emulsion chamber as an event, as shown in Fig.2[7] . Besides, the strong interaction between the high-energy hadrons and the tungsten atomic nuclei[8] will produce many secondary particles, in which the π 0 meson will rapidly decay into the γ-ray, each high-energy nuclear action will produce many secondary particles, and causes in turn the electromagnetic cascade shower in the emulsion chamber, these will be recorded as well by the emulsion chamber. 2.2 The Measurement of Detector Signals At present, most authors judge the energy of incident particles by rebuilding the profile of the particle electromagnetic cascade shower. As mentioned above, passing through the photosensitive layer, the shower will form different black spots on the X-ray film. And on the emulsion film at the position corresponding to each black spot, the track of a bundle of shower electrons will appear. By the 3-dimensional theory of electromagnetic cascade shower, the development of the electromagnetic cascade shower in the emulsion chamber
HU Yi-ming et al. / Chinese Astronomy and Astrophysics 32 (2008) 449–458)
Fig. 1
Fig. 2
Basic structure of the emulsion chamber
Illustration of the emulsion chamber detector
451
452
HU Yi-ming et al. / Chinese Astronomy and Astrophysics 32 (2008) 449–458
Fig. 3
N − E − t curves
can be described. Adopting the axial approximation method proposed by J. Nishimura, the number of the electron tracks on the nuclear emulsion film as a function of the radial distance r and the depth t from the shower’s beginning point is[9,10] : Hl N (E, r/k, t; d, g, β) = AF (s) s
−1/2 2 Er 2π μλ (s)t + 1 F (s) exp[aλl (s)t+bs], l s2 F (s) kdgβ
and at the same time, aλl (s)t + ln
1 F (s) Er +b− + = 0. kdgβ s F (s)
Here F (s) is the slow function of s: F (s) =
6 3s2 + 11s + 14 , 7 (s + 1)(s + 2)(s + 3)
in which s is the age parameter of the shower; d and g are respectively the dilution factor and decay factor determined by the structure of the emulsion chamber itself; β is the inclination factor introduced for the shower of inclined incidence; A, a, b are all constants; and Hl (s), λl (s), λl (s), λl (s) have been calculated already by Rossi[10] . We can find that the density distribution of the electron tracks is a function of the shower’s energy and penetration depth. In other words, if we have measured the density of electron tracks of each photosensitive layer, the energy of the shower can be determined, and if we have measured the position of the shower in each layer we can know the direction that the shower comes from. Based on the above formula, we can obtain the profile curve of the electron shower as
HU Yi-ming et al. / Chinese Astronomy and Astrophysics 32 (2008) 449–458)
453
shown in Fig.3, namely the N − E − t curve[5,9] , in which N is the number of incident electrons, E is the initial energy of incident electrons, t is the depth of the shower’s beginning point (in units of radiation length). But in general situations, to measure the number of electron tracks is a tedious job, wasteful in time and energy; besides, because of the quality difference of emulsion films and the interference of cosmic-ray background, it is very difficult to keep count of all electron tracks and make the energy calibration, accurately. In practical measurements, we calibrate the energy of an event by using the method of maximum blackness[7]. The key point of this method is to connect the density of electron tracks on the emulsion film with the blackness of the spots on the X-ray film. At first, we make the characteristic curve of the blackness of the X-ray film and the electron number density ρ. This curve can be expressed by the following analytical formula[9] : D = D0 (1 − e−αρ ) or
D = D0 1 −
1 1 + αρ
,
in which D0 is the saturation blackness, α is the coefficient of the film itself, proportional to the mean cross-section of photosensitive grains. If D1 is the background blackness caused by the density ρ1 of background electrons, D2 is the pure blackness of the event caused by the density ρ2 of shower electrons, D12 is the actual blackness caused by ρ1 and ρ2 , then the actually measured blackness of the shower event can be derived as: D1 D2 . D = D12 − D1 = 1 − D0 Afterwards, we can obtain the energy and direction of the shower only by measuring the blackness and position of the black spot on the X-ray film. In principle, any shower with energy higher than TeV (1012 eV) can be observed by the emulsion chamber. This method is rapid and easy. At present, most people adopt rebuilding the electromagnetic cascaded shower to judge the energy of particles as mentioned above, the profile curves are different for the electron showers with different energies, and the experiments indicate that the energy resolution can reach 10%. 2.3 Background Analysis The emulsion chamber detector has a very high spatial resolution, it can discriminate the particles’ categories by observing the number of the secondary particles produced by the interaction of particles with the materials. As electrons produce γ-rays via the Bremsstrahlung radiation, and high-energy γ-rays form positron-electron pairs by the effect of electron pairs, at the first action point of electrons, there appear generally 3 or 5 tracks (see Fig.4), but the interaction of hadrons with the materials will give rise to a great quantity of secondary particles. So, for the hadron shower, in most cases the number of tracks at the first action point will be much greater than 5 (see Fig.5). The secondary particles of the hadron shower are numerous, the hadron shower has a very large opening angle, and its number of secondary tracks depends on the energy, the greater the energy, the larger the number of secondary tracks of the shower (taking protons as an example, see Table 1).
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HU Yi-ming et al. / Chinese Astronomy and Astrophysics 32 (2008) 449–458
Besides, as the first action point of electrons is generally located within a radiation length, so the first action point is commonly recorded on the film in the upper shallow layer of the emulsion chamber, but because of the long nuclear-action length of the hadron shower, the hadron shower will appear in the deep layer of the emulsion chamber[8] . According to these rules, the categories of particles can be easily distinguished, and the major part of the background can be subtracted.
Fig. 4 Electron tracks at the first action point
Table 1
Fig. 5 Hadron tracks at the first action point
Number of the secondary particles produced by the protons interacting with Pb Energy(GeV) 10 100 103 104 Average number of 43.8 67.0 101 142.7 secondary particles
2.4 Detector Design Fig.1 shows the internal structure of the emulsion chamber. Between two tungsten plates (action layers) the photosensitive layer is inserted. The tungsten plates of five different thicknesses are selected, from the top layer to the bottom layer of the emulsion chamber, the thickness of the tungsten plate increases gradually (to be 0.3 mm, 0.5 mm, 1.0 mm, 1.5 mm and 3.0 mm, respectively), in order to increase the effective action depth and reduce the height of the detector. Considering that the materials of the high-sensitivity emulsion chamber for detecting electrons are still short in China, the materials of the photosensitive layers in the emulsion chamber are provided by the Institute of Cosmic Ray Research (ICRR), Tokyo University, Japan. The detailed information is given in Table 2, in which, the tungsten plate is the action layer, and the emulsion film, Fuji-IX-150 X-ray film and HR16+HR-HA30 film, constitute the photosensitive layer. As the satellite with the on-board detector is a recoverable satellite, the requirement on the mechanical environment of the detector is critical. Although the total weight of the detector is only 5 kg, but in order to meet the proposed mechanical requirements, the whole
HU Yi-ming et al. / Chinese Astronomy and Astrophysics 32 (2008) 449–458)
455
detector is mainly composed of two parts, namely the aluminum outer casing and the inner emulsion chamber box, as shown in Fig.6. The metallic outer casing is fabricated by the method of integral shaping, in addition to setting flanges on the screw holes and adopting the strengthening ribs. These measures make the emulsion chamber detector capable to withstand the shock of 600 g, and have the mechanical strength and anti-vibration capability of the structure increased. Fig.7 is the sectional drawing of the emulsion chamber after the Table 2
Materials of the emulsion chamber detector
Material Layer Weight (g/cm2 ) Thickness (mm) Rad.length Tungsten 25 49.21 25.5 7.29 Emulsion 23 4.14 22.5 0.19 Fuji IX 150 x-ray film 21 0.65 4.41 0.039 HR16 HR-HA30 film 9 3.83 11.5 0.387 White paper 42 0.34 3.36 0.01 Glassine 31 0.09 0.93 0.003 Black paper 3 0.02 0.24 0.001
chamber box is enclosed by the outer case, the inner is the Ti-alloy box. In the dark room, the materials of every layer are packed sequentially in the Ti-alloy protective box of 2 mm thickness, and pressed tightly by adjusting the screws. Finally, the Ti-alloy protective box is put into the outer case and sealed. Such a design is favorite to the positioning and assembling of the emulsion chamber, and makes it fit the adverse environments during launching, onorbit operating and recovering. The sealing of the emulsion chamber has a very stringent requirement, the operation should be conducted by hands in the dark room of 20◦ constant temperature. In cooperation with the technicians of the Waseda University, ICRR, etc., we have completed the sealing of the preliminary and formal samples of the detector in the underground dark room of the ICRR, and completed the mechanical environment and reliability tests of the emulsion chamber detector at the headquarter of the Institute of Space and Astronautical Science in Japan. The result indicates that the emulsion chamber detector satisfies completely the design requirements, and that it can be used for space observations. 2.5 Design Performance of the Emulsion Chamber Detector Benefited by the high-photosensitivity materials, the “ShiJian-8” emulsion chamber detector can detect also the low-energy electrons, compared with other emulsion chambers it possesses a marked advantage. For space observations, the problem of atmospheric secondary electrons can be solved (for the balloon observation of high-energy electrons, the major problem is unable to discriminate the cosmic electrons from the atmospheric secondary electrons); even though the satellite cabin of finite thickness will also give rise to secondary electrons, but this effect can be totally eliminated by the image analysis of the ground emulsion chamber, in order to improve the observational efficiency. Table 3 indicates the predicted performance of the emulsion chamber detector in the 15-day space observations.
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HU Yi-ming et al. / Chinese Astronomy and Astrophysics 32 (2008) 449–458
Fig. 6
Fig. 7
Table 3
3-D model of the detector
Sectional drawing of the detector
Design performance of the emulsion chamber detector
Energy > 100 GeV > 200 GeV > 400 GeV Number of electrons 24∼30 6∼8 1.5∼2 Energy Number of hadrons
1 TeV 94
2 TeV 28
3 TeV 14
4 TeV 5 TeV 10 TeV 8 5.6 1.7
3. THE OBSERVATIONAL DATA OF THE DETECTOR On 9th Sep. 2006, the emulsion chamber detector, on board the “ShiJian-8” scientific breeding satellite, started its 15-day space observations, and it was successfully recovered on 24th Sep. 2006. Right now, it is at the positioning stage of shower tracks. As the apogee height of the “ShiJian-8” satellite is 461.49∼447.54 km, much greater than the initial design value, and the magnitude of the background of space particles is squarely proportional to the orbit height, so that the background is the 4 times of the detector’s design value. This causes the low-energy electrons to be completely buried in the background, unable to be
HU Yi-ming et al. / Chinese Astronomy and Astrophysics 32 (2008) 449–458)
457
analyzed. Fig.8a and Fig.8b are respectively the plane and 3-dimensional figures of the shower tracks in a high-energy particle event, totally 14 tracks with the energy higher than 1 TeV are observed, and the detailed parameters are listed in Table 4. Among the 14 shower tracks, most tracks are concentrated in the 17th∼18th layers, and one track (No.14) appears in the 13th∼17th layers. As the first action point of electrons is generally located within a radiation length, so the first action point is commonly recorded on the film in the upper shallow layer, and the hadron shower appears often in the deep emulsion chamber; besides, Fig.9 shows 3 tracks. Therefore, it is very possible that the No.11 shower track belongs to the high-energy electron shower, and this will be further verified in later analysis.
Fig. 8
14 shower tracks found in the emulsion chamber detector
Fig. 9
Events in the emulsion chamber
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HU Yi-ming et al. / Chinese Astronomy and Astrophysics 32 (2008) 449–458
Table 4 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Parameters of shower tracks
Layer Length(mm) Height(mm) Angle 18∼21 36 17.805 64.6 18∼21 8.5 17.805 26.5 18∼21 12 17.805 59.4 17∼21 1.5 22.78 3.8 17∼21 11 22.78 25.8 18∼21 5 17.805 16.3 18∼20 6 11.39 27.8 18∼21 24 17.805 54.6 19∼21 54 11.39 78.1 17∼19 11.5 11.39 45.3 13∼17 22 16.78 52.7 19∼21 6 11.39 27.8 17∼20 9 17.805 27.8 17∼18 10 5.695 60.3
4. CONCLUSIONS The space emulsion chamber can be used to observe the high-energy electrons and γ-rays, to upgrade the emulsion chamber observation from the ground-level and low-level observations to the space observation is very meaningful in science. The developed emulsion chamber detector has a simple structure and a wide range of detectable energies. At present, it has finished the step of space observations, and has obtained some preliminary scientific results. As the next step, we will make a detailed analysis on its recorded data. References 1
Zwicky F., ApJ, 1937, 86, 217
2
Begeman K. G., et al., MNRAS, 1991, 249, 523
3
Kammionkowski M., Turner M. S., Phys. Rev., 1991, D43, 1774
4
Feng J. L., et al., Phys. Rev., 2003, 91, L011302
5
Nishimura J., et al., ApJ, 1980, 238, 394
6
Ohta I., et al., Nuclear Instrum. and Methods, 1979, 161, 35
7
Qiu Jin-fa, High-energy Physics and Nuclear Physics, 1989, 13, 967
8
Ren Jing-ru, Lu Hui-ling, Jie Wei, et al., Science in China (Ser.A), 1997, 27, 636
9 10
Bai Guang-zhi, Journal of Chongqing Architecture University, 1996, 18, 31 Ren Jing-ru, Journal of Shandong University, 1991, 15, 103
ELSEVIER
Chinese 32(2008) (2008)449–458 449–458 ChineseAstronomy Astronomy and and Astrophysics Astrophysics 32
Development of the Emulsion Chamber Detector for Space Observations † CHANG Jin1 GONG Yi-zhong1 ZHANG Ren-jian1 HU Yi-ming1,2 WANG Nan-sen1 TANG He-sen1 Torii S.3 Nishimura J.4 5 3 Kobayashi T. Shimizu Y. Makino F.3 1
Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008 2 Graduate School of Chinese Academy of Sciences, Beijing 100039 3 Advanced Research Institute for Science and Engineering, Waseda University, Tokyo 169-8555, Japan 4 Institute of Space and Astronautical Science, JAXA, Japan 5 Department of Physics, Aoyamagakuin University, Japan
Abstract The emulsion chamber detector on board the “ShiJian-8” satellite is the first one in China designed especially for observing in space the highenergy electrons and γ-rays. In this paper, the principle of the detector design, the method of data processing and the preliminary results of observations are introduced. The design lifetime of the detector is 15 days on the orbit, and the energy range of detectable particles is 100 GeV∼5 TeV. Key words: instrumentation: detector—instrumentation: miscellaneous—dark matter
1. INTRODUCTION At present, the research of dark matter is a hot point. There has been strong evidence for the existence of the dark matter[1,2] . The updated theoretical model suggests that the dark matter may exist in the form of some special particles, and that the dark-matter particles †
Supported by National Natural Science Foundation and CAS Innovation Foundation Received 2007–04–17; revised version 2007–05–27 A translation of Acta Astron. Sin. Vol. 49, No. 2, pp. 233–242, 2008 [email protected]
0275-1062/08/$-see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.chinastron.2008.10.012
450
HU Yi-ming et al. / Chinese Astronomy and Astrophysics 32 (2008) 449–458
can be searched for by observing the fundamental particles produced by the decay and interaction of dark matters[3,4] . With the aid of the emulsion chamber detector, by observing in space the electrons (as well as positrons) and γ-rays produced by the decay or interaction of dark matters, we hope that the evidence that the dark matter exists in the form of particles can be found, and that some enlightenment on the problem what is really the dark matter can be obtained; at the same time, to observe the high-energy electrons and γ-rays will be very helpful for solving the scientific problems about the origin and acceleration of cosmic rays. In the research area of high-energy cosmic rays, the emulsion chamber detector, as a kind of mature technique, has played a very important role. But at present, the types of emulsion chamber observations are still restricted to the high-mountain observation, underground observation and balloon observation. So far, it is the first time in China to use the emulsion chamber detector on board a scientific experiment satellite for observing in space the high-energy electrons and γ-rays.
2. OVERALL DESIGN OF THE EMULSION CHAMBER DETECTOR 2.1 Detection Principle The basic structure of the “ShiJian-8” emulsion chamber detector is shown in Fig.1. Between two action layers (tungsten plates) a photosensitive layer is placed, and it is composed of the X-ray films and nuclear emulsion films with different photosensitivities[5,6] . When the high-energy γ-rays (or electrons) enter the emulsion chamber, the interaction with the tungsten plate causes the electromagnetic cascade shower, via the Bremsstrahlung radiation the high-energy electrons produce γ-rays, and the high-energy γ-rays will give rise to positron-electron pairs by the effect of electron pairs. The high-energy electrons produce γ-rays via the Bremsstrahlung radiation, and γ-rays will produce new electrons, thus the cascaded electron-γ-ray interaction will continue uninterruptedly. As a result, a great number of γ-rays and electrons are produced, these particles form latent images on the emulsion film inside the photosensitive layer. After some processing, the track of electrons will be left on the emulsion film, simultaneously some black spots will appear on the X-ray emulsion film after exposure, thus these particles are recorded by the emulsion chamber as an event, as shown in Fig.2[7] . Besides, the strong interaction between the high-energy hadrons and the tungsten atomic nuclei[8] will produce many secondary particles, in which the π 0 meson will rapidly decay into the γ-ray, each high-energy nuclear action will produce many secondary particles, and causes in turn the electromagnetic cascade shower in the emulsion chamber, these will be recorded as well by the emulsion chamber. 2.2 The Measurement of Detector Signals At present, most authors judge the energy of incident particles by rebuilding the profile of the particle electromagnetic cascade shower. As mentioned above, passing through the photosensitive layer, the shower will form different black spots on the X-ray film. And on the emulsion film at the position corresponding to each black spot, the track of a bundle of shower electrons will appear. By the 3-dimensional theory of electromagnetic cascade shower, the development of the electromagnetic cascade shower in the emulsion chamber
HU Yi-ming et al. / Chinese Astronomy and Astrophysics 32 (2008) 449–458)
Fig. 1
Fig. 2
Basic structure of the emulsion chamber
Illustration of the emulsion chamber detector
451
452
HU Yi-ming et al. / Chinese Astronomy and Astrophysics 32 (2008) 449–458
Fig. 3
N − E − t curves
can be described. Adopting the axial approximation method proposed by J. Nishimura, the number of the electron tracks on the nuclear emulsion film as a function of the radial distance r and the depth t from the shower’s beginning point is[9,10] : Hl N (E, r/k, t; d, g, β) = AF (s) s
−1/2 2 Er 2π μλ (s)t + 1 F (s) exp[aλl (s)t+bs], l s2 F (s) kdgβ
and at the same time, aλl (s)t + ln
1 F (s) Er +b− + = 0. kdgβ s F (s)
Here F (s) is the slow function of s: F (s) =
6 3s2 + 11s + 14 , 7 (s + 1)(s + 2)(s + 3)
in which s is the age parameter of the shower; d and g are respectively the dilution factor and decay factor determined by the structure of the emulsion chamber itself; β is the inclination factor introduced for the shower of inclined incidence; A, a, b are all constants; and Hl (s), λl (s), λl (s), λl (s) have been calculated already by Rossi[10] . We can find that the density distribution of the electron tracks is a function of the shower’s energy and penetration depth. In other words, if we have measured the density of electron tracks of each photosensitive layer, the energy of the shower can be determined, and if we have measured the position of the shower in each layer we can know the direction that the shower comes from. Based on the above formula, we can obtain the profile curve of the electron shower as
HU Yi-ming et al. / Chinese Astronomy and Astrophysics 32 (2008) 449–458)
453
shown in Fig.3, namely the N − E − t curve[5,9] , in which N is the number of incident electrons, E is the initial energy of incident electrons, t is the depth of the shower’s beginning point (in units of radiation length). But in general situations, to measure the number of electron tracks is a tedious job, wasteful in time and energy; besides, because of the quality difference of emulsion films and the interference of cosmic-ray background, it is very difficult to keep count of all electron tracks and make the energy calibration, accurately. In practical measurements, we calibrate the energy of an event by using the method of maximum blackness[7]. The key point of this method is to connect the density of electron tracks on the emulsion film with the blackness of the spots on the X-ray film. At first, we make the characteristic curve of the blackness of the X-ray film and the electron number density ρ. This curve can be expressed by the following analytical formula[9] : D = D0 (1 − e−αρ ) or
D = D0 1 −
1 1 + αρ
,
in which D0 is the saturation blackness, α is the coefficient of the film itself, proportional to the mean cross-section of photosensitive grains. If D1 is the background blackness caused by the density ρ1 of background electrons, D2 is the pure blackness of the event caused by the density ρ2 of shower electrons, D12 is the actual blackness caused by ρ1 and ρ2 , then the actually measured blackness of the shower event can be derived as: D1 D2 . D = D12 − D1 = 1 − D0 Afterwards, we can obtain the energy and direction of the shower only by measuring the blackness and position of the black spot on the X-ray film. In principle, any shower with energy higher than TeV (1012 eV) can be observed by the emulsion chamber. This method is rapid and easy. At present, most people adopt rebuilding the electromagnetic cascaded shower to judge the energy of particles as mentioned above, the profile curves are different for the electron showers with different energies, and the experiments indicate that the energy resolution can reach 10%. 2.3 Background Analysis The emulsion chamber detector has a very high spatial resolution, it can discriminate the particles’ categories by observing the number of the secondary particles produced by the interaction of particles with the materials. As electrons produce γ-rays via the Bremsstrahlung radiation, and high-energy γ-rays form positron-electron pairs by the effect of electron pairs, at the first action point of electrons, there appear generally 3 or 5 tracks (see Fig.4), but the interaction of hadrons with the materials will give rise to a great quantity of secondary particles. So, for the hadron shower, in most cases the number of tracks at the first action point will be much greater than 5 (see Fig.5). The secondary particles of the hadron shower are numerous, the hadron shower has a very large opening angle, and its number of secondary tracks depends on the energy, the greater the energy, the larger the number of secondary tracks of the shower (taking protons as an example, see Table 1).
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HU Yi-ming et al. / Chinese Astronomy and Astrophysics 32 (2008) 449–458
Besides, as the first action point of electrons is generally located within a radiation length, so the first action point is commonly recorded on the film in the upper shallow layer of the emulsion chamber, but because of the long nuclear-action length of the hadron shower, the hadron shower will appear in the deep layer of the emulsion chamber[8] . According to these rules, the categories of particles can be easily distinguished, and the major part of the background can be subtracted.
Fig. 4 Electron tracks at the first action point
Table 1
Fig. 5 Hadron tracks at the first action point
Number of the secondary particles produced by the protons interacting with Pb Energy(GeV) 10 100 103 104 Average number of 43.8 67.0 101 142.7 secondary particles
2.4 Detector Design Fig.1 shows the internal structure of the emulsion chamber. Between two tungsten plates (action layers) the photosensitive layer is inserted. The tungsten plates of five different thicknesses are selected, from the top layer to the bottom layer of the emulsion chamber, the thickness of the tungsten plate increases gradually (to be 0.3 mm, 0.5 mm, 1.0 mm, 1.5 mm and 3.0 mm, respectively), in order to increase the effective action depth and reduce the height of the detector. Considering that the materials of the high-sensitivity emulsion chamber for detecting electrons are still short in China, the materials of the photosensitive layers in the emulsion chamber are provided by the Institute of Cosmic Ray Research (ICRR), Tokyo University, Japan. The detailed information is given in Table 2, in which, the tungsten plate is the action layer, and the emulsion film, Fuji-IX-150 X-ray film and HR16+HR-HA30 film, constitute the photosensitive layer. As the satellite with the on-board detector is a recoverable satellite, the requirement on the mechanical environment of the detector is critical. Although the total weight of the detector is only 5 kg, but in order to meet the proposed mechanical requirements, the whole
HU Yi-ming et al. / Chinese Astronomy and Astrophysics 32 (2008) 449–458)
455
detector is mainly composed of two parts, namely the aluminum outer casing and the inner emulsion chamber box, as shown in Fig.6. The metallic outer casing is fabricated by the method of integral shaping, in addition to setting flanges on the screw holes and adopting the strengthening ribs. These measures make the emulsion chamber detector capable to withstand the shock of 600 g, and have the mechanical strength and anti-vibration capability of the structure increased. Fig.7 is the sectional drawing of the emulsion chamber after the Table 2
Materials of the emulsion chamber detector
Material Layer Weight (g/cm2 ) Thickness (mm) Rad.length Tungsten 25 49.21 25.5 7.29 Emulsion 23 4.14 22.5 0.19 Fuji IX 150 x-ray film 21 0.65 4.41 0.039 HR16 HR-HA30 film 9 3.83 11.5 0.387 White paper 42 0.34 3.36 0.01 Glassine 31 0.09 0.93 0.003 Black paper 3 0.02 0.24 0.001
chamber box is enclosed by the outer case, the inner is the Ti-alloy box. In the dark room, the materials of every layer are packed sequentially in the Ti-alloy protective box of 2 mm thickness, and pressed tightly by adjusting the screws. Finally, the Ti-alloy protective box is put into the outer case and sealed. Such a design is favorite to the positioning and assembling of the emulsion chamber, and makes it fit the adverse environments during launching, onorbit operating and recovering. The sealing of the emulsion chamber has a very stringent requirement, the operation should be conducted by hands in the dark room of 20◦ constant temperature. In cooperation with the technicians of the Waseda University, ICRR, etc., we have completed the sealing of the preliminary and formal samples of the detector in the underground dark room of the ICRR, and completed the mechanical environment and reliability tests of the emulsion chamber detector at the headquarter of the Institute of Space and Astronautical Science in Japan. The result indicates that the emulsion chamber detector satisfies completely the design requirements, and that it can be used for space observations. 2.5 Design Performance of the Emulsion Chamber Detector Benefited by the high-photosensitivity materials, the “ShiJian-8” emulsion chamber detector can detect also the low-energy electrons, compared with other emulsion chambers it possesses a marked advantage. For space observations, the problem of atmospheric secondary electrons can be solved (for the balloon observation of high-energy electrons, the major problem is unable to discriminate the cosmic electrons from the atmospheric secondary electrons); even though the satellite cabin of finite thickness will also give rise to secondary electrons, but this effect can be totally eliminated by the image analysis of the ground emulsion chamber, in order to improve the observational efficiency. Table 3 indicates the predicted performance of the emulsion chamber detector in the 15-day space observations.
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HU Yi-ming et al. / Chinese Astronomy and Astrophysics 32 (2008) 449–458
Fig. 6
Fig. 7
Table 3
3-D model of the detector
Sectional drawing of the detector
Design performance of the emulsion chamber detector
Energy > 100 GeV > 200 GeV > 400 GeV Number of electrons 24∼30 6∼8 1.5∼2 Energy Number of hadrons
1 TeV 94
2 TeV 28
3 TeV 14
4 TeV 5 TeV 10 TeV 8 5.6 1.7
3. THE OBSERVATIONAL DATA OF THE DETECTOR On 9th Sep. 2006, the emulsion chamber detector, on board the “ShiJian-8” scientific breeding satellite, started its 15-day space observations, and it was successfully recovered on 24th Sep. 2006. Right now, it is at the positioning stage of shower tracks. As the apogee height of the “ShiJian-8” satellite is 461.49∼447.54 km, much greater than the initial design value, and the magnitude of the background of space particles is squarely proportional to the orbit height, so that the background is the 4 times of the detector’s design value. This causes the low-energy electrons to be completely buried in the background, unable to be
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analyzed. Fig.8a and Fig.8b are respectively the plane and 3-dimensional figures of the shower tracks in a high-energy particle event, totally 14 tracks with the energy higher than 1 TeV are observed, and the detailed parameters are listed in Table 4. Among the 14 shower tracks, most tracks are concentrated in the 17th∼18th layers, and one track (No.14) appears in the 13th∼17th layers. As the first action point of electrons is generally located within a radiation length, so the first action point is commonly recorded on the film in the upper shallow layer, and the hadron shower appears often in the deep emulsion chamber; besides, Fig.9 shows 3 tracks. Therefore, it is very possible that the No.11 shower track belongs to the high-energy electron shower, and this will be further verified in later analysis.
Fig. 8
14 shower tracks found in the emulsion chamber detector
Fig. 9
Events in the emulsion chamber
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Table 4 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Parameters of shower tracks
Layer Length(mm) Height(mm) Angle 18∼21 36 17.805 64.6 18∼21 8.5 17.805 26.5 18∼21 12 17.805 59.4 17∼21 1.5 22.78 3.8 17∼21 11 22.78 25.8 18∼21 5 17.805 16.3 18∼20 6 11.39 27.8 18∼21 24 17.805 54.6 19∼21 54 11.39 78.1 17∼19 11.5 11.39 45.3 13∼17 22 16.78 52.7 19∼21 6 11.39 27.8 17∼20 9 17.805 27.8 17∼18 10 5.695 60.3
4. CONCLUSIONS The space emulsion chamber can be used to observe the high-energy electrons and γ-rays, to upgrade the emulsion chamber observation from the ground-level and low-level observations to the space observation is very meaningful in science. The developed emulsion chamber detector has a simple structure and a wide range of detectable energies. At present, it has finished the step of space observations, and has obtained some preliminary scientific results. As the next step, we will make a detailed analysis on its recorded data. References 1
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