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The High-energy Burst Spectrometer for SMESE Mission
CHINESE ASTRONOMY AND ASTROPHYSICS
ELSEVIER
Chinese 33(2009) (2009)113–120 113–120 ChineseAstronomy Astronomy and and Astrophysics Astrophysics 33
The High-energy Burst Spectrometer for SMESE Mission † GU Qiang1,2
CHANG Jin1,3
1
Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008 2 Graduate School of Chinese Academy of Sciences, Beijing 100039 3 National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012
Abstract The SMall Explorer for Solar Eruptions (SMESE) is a small satellite being developed jointly by China and France. It is planed to launch around the next solar maximum year (∼ 2011) for observing simultaneously the two most violent types of eruptive events on the sun (the coronal mass ejection (CME) and the solar flare) and investigating their relationship. As one of the 3 main payloads of the small satellite, the high energy burst spectrometer (HEBS) adopts the upto-date high-resolution LaBr3 scintillation detector to observe the high-energy solar radiation in the range 10 keV—600 MeV. Its energy resolution is better than 3.0% at 662 keV, 2-fold higher than that of current scintillation detectors, promising a breakthrough in the studies of energy release in solar flares and CMEs, particle acceleration and the relationship between solar flares and CMEs. Key words: instrumentation: spectrographs—space vehicles: instruments
1. A BRIEF INTRODUCTION OF SMESE The SMall Explorer for Solar Eruption (SMESE) is a small satellite which is being developed jointly by China and France. Using the Myraid small satellite platform of French Space Agency, SMESE is planned for launch around 2011 to carry out space observations for 3 years. The orbit of SMESE is a sun-synchronous orbit of 650—750 km, and the platform has a pointing accuracy better than 36 . The 3 main payloads of the satellite are: a †
Supported by National Natural Science Foundation Received 2007–06–25; revised version 2007–07–24 A translation of Acta Astron. Sin. Vol. 49, No. 3, pp. 339–347, 2008 guqiang @hotmail.com
0275-1062/08/$-see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.chinastron.2009.01.009
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Lyman-α solar full-disk high-resolution telescope and coronagraph, LYOT (LYman Orbiting Telescopes), a solar far-infrared telescope DESIR (Detection of Eruptive Solar InfraRed emission), and a solar high-energy burst spectrometer HEBS (High Energy Burst Spectrometer). Fig.1 shows the designed layout of the satellite’s payloads (HEBS is on the lower-left).
Fig. 1
The scientific instruments onboard the SMESE satellite
As mentioned, LYOT consists of two telescopes, a full-disk high-resolution telescope working in Lα and a coronagraph. The former works at the wavelength of 121.6 nm, has a field of view of 1.2 solar radii and an angular resolution of 1 .1. The operating wavelength of the coronagraph is also 121.6 nm, its field of view is 1.15-2.5 solar radii, and the angular resolution is 2 .3. Depending on the solar activity, the temporal resolutions of the two telescopes can reach 0.2 s. DESIR has two operating wavebands, 35-80 μm, and 100-250 μm. It observes solar radio emission at frequencies corresponding to higher than 1012 Hz. The size of the detector is 245×325 pixels, each of size about 44 . It can provide information on the position and size of emission sources during a solar burst, and its time resolution can reach the order of milliseconds. HEBS consists mainly of 3 large-size (7.6 cm×7.6 cm) LaBr3 scintillators of high energy resolution. It observes solar radiation in the high-energy range 10 keV–600 MeV, with an energy resolution better than 3.0% at 662 keV and with an adjustable temporal resolution, which can reach 32 ms. The three payloads are put together to observe solar radiation stretching from farinfrared to γ-ray, it is the first time in the world for a single satellite to make observations over such a wide energy range. Compared with the other similar satellites in the world that make solar observations from space, SMESE, although a small satellite, has concentrated in it all the essentials of the three France-China small satellites, namely: (1) For the first time in the world, a single satellite makes cross-checking observation
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between solar flare and corona mass ejection ; (2) For the first time in the world, a single detector makes simultaneous high-resolution observations of solar X-ray and high-energy γ-ray radiations ; (3) For the first time in the world, solar far-infrared imaging observation is made in space; (4) For the first time in the world, high-resolution solar full-disk and near-corona observation at the Lyman-α band is made. 2. SCIENTIFIC OBJECTIVES OF THE HIGH-ENERGY BURST SPECTROMETER The solar high-energy burst spectrometer (HEBS) is one of the three main payloads of SMESE. HERB is mainly used for observing the high-energy solar radiation of 20 keV— 600 MeV with a high energy resolution and temporal resolution. Its main technical specifications are given in Table 1. Table 1
Main specifications of HEBS
Energy range Energy resolution Time resolution Effective area Sensitivity (300keV-10MeV)
10keV to 600MeV 3%@662keV 1s (solar quiet),32ms (solar eruption) > 60cm2 @1MeV better than 3X10−3 photons/cm2 /s
The main scientific objective of HEBS is to study the high-energy physical processes that lie at the heart of solar flares and CMEs. It is concerned with the rapid release of magnetic energy stored in magnetic structures which are not in steady-state, the rapid transformation to the kinetic energy of accelerated particles, as well as to the kinetic and thermal energies of plasmas, the propagation of energetic particles in the atmosphere, the following heating and secondary response of the surrounding atmosphere, the propagation of plasma in space, and so on. As the solar hard X-ray comes from the Bremsstrahlung caused by the accelerated secondary relativistic electrons interacting with the materials of the surrounding atmosphere, the γ-ray continuum reflects the contributions of the Bremsstrahlung of relativistically-accelerated electrons and the decay of the π-mesons caused by the interaction of high-energy ions with the solar atmosphere, and the γ-ray line emission is a direct product of the nuclear reaction between the accelerated ions and solar atmospheric materials, so the X-ray and γ-ray observations can offer the most direct diagnosis on the particle acceleration. The previous hard X-ray observations have shown that the direct results of flare triggers are the pulsating hard X-ray busts and γ-ray bursts, in which electrons are accelerated to several hundred MeV in a few seconds, and ions are accelerated to several tens of GeV, then to release the energy of 1032 —1033 ergs in 102 ∼103 seconds[1−3] , and that the total energy of the non-thermally accelerated electrons responsible for the hard X-ray burst accounts for a dominant proportion in the total energy release of a flare, all this demonstrates that the pulsating energy release of flares is closely related with the particle acceleration. The main purpose of HEBS is to study this key problem of the flare process, and to study the essence of the eruptive energy release in solar magnetized plasmas and the particle acceleration process by observing flares hard X-ray and γ-ray emissions, and to find the relationship between the local burst and the coronal global burst.
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If the SMESE will be successfully launched around 2011, then in its expected 3-year lifetime, we shall be able to observe thousands of solar hard X-ray bursts, hundreds of solar γ-ray burst events and about 100 flares with γ-ray spectral lines.
3. THE HEBS DETECTOR 3.1 Detector Selection Worldwide solar high-energy observations have a long history, and the NaI scintillation detector (represented by SMM) and the BGO scintillation detector (represented by YOHKOH) are the commonly used devices for space solar high-energy observations. Because of their technical maturity, their rather high detection efficiency and large area, they are widely used for the space solar observations, their main drawback lies in its energy resolving power: being only typically 7—8% at 662 keV, or 25 times worse than the high-purity Ge detector. High-purity Ge semiconductor detector represents a developing trend in modern solar high-energy observations. Its advantage is the very high energy resolution (25 times better than that of the NaI scintillation detector, as just mentioned), so it is especially suitable for observing the solar γ-ray spectrum. The disadvantage is its low operating temperature (below 80 K), which must be realized by large-area passive radiation-cooling or by active cooling. Because of its very stringent requirements on the power consumption and weight, its usage is restricted for common small-satellite observations. An additional problem is radiation damage: when the high-purity Ge-crystal is exposed to high-energy space particles ove a long time, its energy resolution and noise performance will be degraded. And this point is of key importance for such a long-lifetime exploration project as SMESE. Following technical advances, the year 2004 saw the appearance of a kind of scintillation crystal similar to LaBr3 [4] . Owing to its high light yield (1.3 times the common NaI scintillation crystal) and uniform lighting, its energy resolution can be 3 times better than that of a common NaI scintillation detector. Because it has a high energy resolution and there is no need of cooling, and due to the high atomic numbers of the La and Br atoms, it has a higher detection efficiency in comparison with the high-purity Ge, the LaBr3 detector has become the first choice of high-energy space explorations as soon as it appeared. Many planned satellites in the world will be adopting the LaBr3 scintillation detector[5,6] . Table 2 compares the performance of LaBr3 scintillator and the other detectors in common use. HEBS will also adopt the LaBr3 crystal. If SMESE is successfully launched at the scheduled time, it will be the first time in the world to observe high-energy radiations of celestial objects with this new technology. Table 2
Comparison of performance of the LaBr3 scintillator and the other detectors in common use
Parameter NaI(Tl) CsI(Tl) BGO LaBr3 Ge Scintillation efficiency relative to NaI(Tl) 100% 45% 10% 130% Light decay constant (μs) 0.23 1.0 0.3 0.016 3.67 4.51 7.13 5.29 5.36 Density(g/cm−3 ) Energy resolution at 662keV(FWHM) 7.5% 9.0% 13.5% 3.0% 0.3% Highest Z (high-Z element) 53(I) 55(Cs) 83(Bi) 57(La) 32(Ge)
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3.2
The HEBS Detector The whole HEBS detector is shown in Fig.2. The main detecting elements are the 3 LaBr3 scintillators, 7.6 cm in diameter, 7.6 cm in height, which is the maximum size availabe at present. In order to reduce the background level, the LaBr3 scintillator is encased in plastic scintillators of thickness 1.5 cm, and 3 cm longer than the LaBr3 scintillator. It should reduce the background caused by charged particles coming obliquely from the back of the photoelectric tubes. Above the anti-coincident crystals are two photoelectric tubes of 3 cm diameter. When charged particles penetrate through the detector, the two small photoelectric tubes will yield a signal, and with this signal we can judge whether the output signals of the main detectors belong to the background or the sun. Our simulation result indicates that this device can reduce over 97% of the background resulted directly from the high-energy charged particles. Besides, by analyzing the relations between the output signals of the three main detectors, the backgrounds of cosmic γ-ray higher than 1 MeV and of the satellite’s secondary γ-ray can be reduced by about 30%. In the rear of the main crystals are the three 7.6 cm photoelectric tubes. To cover the wide energy range from 10 keV to 600 MeV, one anode output is certainly not enough, so the anode and 3 dynodes of each photoelectric tube will output signals, and owing to the different gains of the anode and dynodes, the different outputs correspond to different energy bands, so by this device the energy range from 10 keV to 600 MeV is covered. In the laboratory we have applied this technique to an American balloon observation project (in this project, the 3 dynodes of a photoelectric tube are adopted to observe the energy deposition from 12 MeV to 20 TeV)[8] .
Fig. 2
Schematic diagram of the detector
3.3 Detector Performance The effective area as a function of energy is shown in Fig.3. The solid line refers to HEBS and the dotted line, to the γ-ray spectrometer on board the SMM satellite[9] . This figure shows that at 1 MeV the effective area of the HEBS detector is about 70 m2 . This is 65% the area of the γ-ray spectrometer on SMM, but the latter has 8 times the weight of HEBS. Fig.4 shows the energy resolution as a function of energy for HEBS (the solid line) and for the γ-ray spectrometer on SMM[9] (dashed line). This figure shows that the energy
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resolution of HEBS is more than two times better than the SMM spectrometer.
Fig. 3
Effective area of HEBS as a function of energy
Fig. 4
Energy resolution of HEBS as
a function of energy
Fig.5 is the HEBS simulated result of the observed solar burst of 2003 Nov 2. Fig.6 shows the photon spectrum of the solar high-energy radiation, obtained from the observed result of RHESSI (Fig.7). From the figures we see that compared with the observed result of RHESSI, although RHESSI uses a high-purity Ge semiconductor, and hence its energy resolution is much higher than that of HEBS, but the C-line and O-line of HEBS are clearer than those of RHESSI. Without a background suppression system, RHESSI has a higher background level than HEBS, in addition the effective area of its detector is also smaller than that of HEBS, therefore it has a lower sensitivity than HEBS.
Fig. 5
Simulated result of the high-energy radiation
of the 2003 Nov.2 solar burst observed by HEBS
Fig. 6 Photon spectrum of the high-energy radiation of the 2003 Nov.2 solar burst
3.4 Laboratory Test Results of the Large-sized LaBr3 Crystal It is not until the end of 2005 that large-size (7.6 cm×7.6 cm) LaBr3 crystal became available. Whether its performance satisfies the scientific requirements on HEBS is a problem of great concern to the SMESE science group. The following figures display the test results
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of our laboratory on the large-size LaBr3 crystal produced by the Saint Gobain Company in 2006. In the tests, the photomultiplier used is the spaceflight-class photoelectric tube R184805 of HAMAMATSU Company, and common silicone is used for the coupling between the photoelectric tube and the crystal. Fig.8 shows the energy spectra of the Am-241 and Co-57 radioactive sources measured by the detector. We see that the detector has energy resolutions of 14% at 59.5 keV and 7.5% at 122 keV, and the detectable lower limit can reach 5 keV, altogether satisfying the 10 keV lower-limit requirement of HEBS. Fig.9 is the result of observation of the Cs-137 radioactive source. As is shown, the energy resolution at 622 keV is 3.5%. Although it is a little worse than the 3.0% specification of HEBS, but considering that the R1848-05 spaceflight-class photoelectric tube is an old product of 10 years ago, lagging behind the the up-to-date products in quantum efficiency and spectral response [10] , so the 3.0% energy resolution required for HEBS is entirely feasible. Fig.10 gives the energy deposition spectrum measured by the detector generated after the Muon particles produced by the interaction between ground cosmic ray and the atmosphere pass through the detector. According to the Monte Carlo simulation, this spectrum should exhibit a peak at 72.1 MeV. The signals were output from the dynodes of the photoelectric tube. We have calibrated the relative gains of the anode and dynodes of the photoelectric tube. We found that the energy deposition peak of Muon particles together with the lowenergy band satisfies the linearity requirement, and there was not any saturation. Fig.11 displays the relationship between the amplitude of the output signal and the incident energy. We see that in the range from 5 keV to 72 MeV, the detector output and incident energy are linearly related.
Fig. 7
Energy spectrum of the 2003 Nov.2
solar burst observed by RHESSI
Fig. 8 Energy spectra of Am-241 and Co-57 measured by the LaBr3 detector
4. CONCLUDING SUMMARY From the above analysis of the present design of HEBS, we may say that HEBS’s energy resolution is better than that of the space scintillation detector onboard SMM, and its effective area at 1 MeV is equivalent to 65% of that of SMM. By adopting a background
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Fig. 9 Energy spectrum of Cs-137 measured by the LaBr3 detector
Fig. 10 Energy deposition spectrum of Muon particles measured by the LaBr3 detector
Fig. 11 The relationship between the signal amplitude of the LaBr 3 detector and the incident energy
suppression system, its sensitivity is higher than that of RHESSI, satisfying completely the scientific requirements on HEBS set by the SMESE mission. References 1
Akimov V. V., Leikov N. G., Kurt V. G., et al., In: Ryan J. M. & Vestrand W. T. (Eds.), High Energy
2
Akimov V. V., Ambroz P., Belov A. V., et al., Solar Phys., 1996, 166, 107
3
Kanbach G., Bertsch D. L., Fichtel C. E., et al., A&AS, 1993, 97, 349
Solar Phenomena–A New Era of Spacecraft Measurements, AIP Conf. Proc., 1994, 294, 130
4
Van Loef E. V. D., Dorenbos P., Van Ejik C. W. E., et al., Applied Physics Letters, 2001, 79, 1573
5
Cherry M. L., Bloser P. F., Case G. L., et al., Proc. 29th International Cosmic Ray Conference, 2005,
6
Kozyrev A. S., Proc. Lunar and Planetary Science, 2006, 37, in press
7
Gehrels N., Crannell C. J., Forrest D. J., et al., Solar Phys., 1988, 118, 233
8
Guzik T. G., Proc. 26th International Cosmic Ray Conference, 1999, 5, 9
5, 407
9 10
Forrest D. J., Chupp E. L., Ryan J. M., et al., Solar Phys., 1980, 65, 15 http://jp.hamamathu.com/resources/products/etd/pdf /R6091 TPMH1114E02.pdf
ELSEVIER
Chinese 33(2009) (2009)113–120 113–120 ChineseAstronomy Astronomy and and Astrophysics Astrophysics 33
The High-energy Burst Spectrometer for SMESE Mission † GU Qiang1,2
CHANG Jin1,3
1
Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008 2 Graduate School of Chinese Academy of Sciences, Beijing 100039 3 National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012
Abstract The SMall Explorer for Solar Eruptions (SMESE) is a small satellite being developed jointly by China and France. It is planed to launch around the next solar maximum year (∼ 2011) for observing simultaneously the two most violent types of eruptive events on the sun (the coronal mass ejection (CME) and the solar flare) and investigating their relationship. As one of the 3 main payloads of the small satellite, the high energy burst spectrometer (HEBS) adopts the upto-date high-resolution LaBr3 scintillation detector to observe the high-energy solar radiation in the range 10 keV—600 MeV. Its energy resolution is better than 3.0% at 662 keV, 2-fold higher than that of current scintillation detectors, promising a breakthrough in the studies of energy release in solar flares and CMEs, particle acceleration and the relationship between solar flares and CMEs. Key words: instrumentation: spectrographs—space vehicles: instruments
1. A BRIEF INTRODUCTION OF SMESE The SMall Explorer for Solar Eruption (SMESE) is a small satellite which is being developed jointly by China and France. Using the Myraid small satellite platform of French Space Agency, SMESE is planned for launch around 2011 to carry out space observations for 3 years. The orbit of SMESE is a sun-synchronous orbit of 650—750 km, and the platform has a pointing accuracy better than 36 . The 3 main payloads of the satellite are: a †
Supported by National Natural Science Foundation Received 2007–06–25; revised version 2007–07–24 A translation of Acta Astron. Sin. Vol. 49, No. 3, pp. 339–347, 2008 guqiang @hotmail.com
0275-1062/08/$-see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.chinastron.2009.01.009
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Lyman-α solar full-disk high-resolution telescope and coronagraph, LYOT (LYman Orbiting Telescopes), a solar far-infrared telescope DESIR (Detection of Eruptive Solar InfraRed emission), and a solar high-energy burst spectrometer HEBS (High Energy Burst Spectrometer). Fig.1 shows the designed layout of the satellite’s payloads (HEBS is on the lower-left).
Fig. 1
The scientific instruments onboard the SMESE satellite
As mentioned, LYOT consists of two telescopes, a full-disk high-resolution telescope working in Lα and a coronagraph. The former works at the wavelength of 121.6 nm, has a field of view of 1.2 solar radii and an angular resolution of 1 .1. The operating wavelength of the coronagraph is also 121.6 nm, its field of view is 1.15-2.5 solar radii, and the angular resolution is 2 .3. Depending on the solar activity, the temporal resolutions of the two telescopes can reach 0.2 s. DESIR has two operating wavebands, 35-80 μm, and 100-250 μm. It observes solar radio emission at frequencies corresponding to higher than 1012 Hz. The size of the detector is 245×325 pixels, each of size about 44 . It can provide information on the position and size of emission sources during a solar burst, and its time resolution can reach the order of milliseconds. HEBS consists mainly of 3 large-size (7.6 cm×7.6 cm) LaBr3 scintillators of high energy resolution. It observes solar radiation in the high-energy range 10 keV–600 MeV, with an energy resolution better than 3.0% at 662 keV and with an adjustable temporal resolution, which can reach 32 ms. The three payloads are put together to observe solar radiation stretching from farinfrared to γ-ray, it is the first time in the world for a single satellite to make observations over such a wide energy range. Compared with the other similar satellites in the world that make solar observations from space, SMESE, although a small satellite, has concentrated in it all the essentials of the three France-China small satellites, namely: (1) For the first time in the world, a single satellite makes cross-checking observation
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between solar flare and corona mass ejection ; (2) For the first time in the world, a single detector makes simultaneous high-resolution observations of solar X-ray and high-energy γ-ray radiations ; (3) For the first time in the world, solar far-infrared imaging observation is made in space; (4) For the first time in the world, high-resolution solar full-disk and near-corona observation at the Lyman-α band is made. 2. SCIENTIFIC OBJECTIVES OF THE HIGH-ENERGY BURST SPECTROMETER The solar high-energy burst spectrometer (HEBS) is one of the three main payloads of SMESE. HERB is mainly used for observing the high-energy solar radiation of 20 keV— 600 MeV with a high energy resolution and temporal resolution. Its main technical specifications are given in Table 1. Table 1
Main specifications of HEBS
Energy range Energy resolution Time resolution Effective area Sensitivity (300keV-10MeV)
10keV to 600MeV 3%@662keV 1s (solar quiet),32ms (solar eruption) > 60cm2 @1MeV better than 3X10−3 photons/cm2 /s
The main scientific objective of HEBS is to study the high-energy physical processes that lie at the heart of solar flares and CMEs. It is concerned with the rapid release of magnetic energy stored in magnetic structures which are not in steady-state, the rapid transformation to the kinetic energy of accelerated particles, as well as to the kinetic and thermal energies of plasmas, the propagation of energetic particles in the atmosphere, the following heating and secondary response of the surrounding atmosphere, the propagation of plasma in space, and so on. As the solar hard X-ray comes from the Bremsstrahlung caused by the accelerated secondary relativistic electrons interacting with the materials of the surrounding atmosphere, the γ-ray continuum reflects the contributions of the Bremsstrahlung of relativistically-accelerated electrons and the decay of the π-mesons caused by the interaction of high-energy ions with the solar atmosphere, and the γ-ray line emission is a direct product of the nuclear reaction between the accelerated ions and solar atmospheric materials, so the X-ray and γ-ray observations can offer the most direct diagnosis on the particle acceleration. The previous hard X-ray observations have shown that the direct results of flare triggers are the pulsating hard X-ray busts and γ-ray bursts, in which electrons are accelerated to several hundred MeV in a few seconds, and ions are accelerated to several tens of GeV, then to release the energy of 1032 —1033 ergs in 102 ∼103 seconds[1−3] , and that the total energy of the non-thermally accelerated electrons responsible for the hard X-ray burst accounts for a dominant proportion in the total energy release of a flare, all this demonstrates that the pulsating energy release of flares is closely related with the particle acceleration. The main purpose of HEBS is to study this key problem of the flare process, and to study the essence of the eruptive energy release in solar magnetized plasmas and the particle acceleration process by observing flares hard X-ray and γ-ray emissions, and to find the relationship between the local burst and the coronal global burst.
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If the SMESE will be successfully launched around 2011, then in its expected 3-year lifetime, we shall be able to observe thousands of solar hard X-ray bursts, hundreds of solar γ-ray burst events and about 100 flares with γ-ray spectral lines.
3. THE HEBS DETECTOR 3.1 Detector Selection Worldwide solar high-energy observations have a long history, and the NaI scintillation detector (represented by SMM) and the BGO scintillation detector (represented by YOHKOH) are the commonly used devices for space solar high-energy observations. Because of their technical maturity, their rather high detection efficiency and large area, they are widely used for the space solar observations, their main drawback lies in its energy resolving power: being only typically 7—8% at 662 keV, or 25 times worse than the high-purity Ge detector. High-purity Ge semiconductor detector represents a developing trend in modern solar high-energy observations. Its advantage is the very high energy resolution (25 times better than that of the NaI scintillation detector, as just mentioned), so it is especially suitable for observing the solar γ-ray spectrum. The disadvantage is its low operating temperature (below 80 K), which must be realized by large-area passive radiation-cooling or by active cooling. Because of its very stringent requirements on the power consumption and weight, its usage is restricted for common small-satellite observations. An additional problem is radiation damage: when the high-purity Ge-crystal is exposed to high-energy space particles ove a long time, its energy resolution and noise performance will be degraded. And this point is of key importance for such a long-lifetime exploration project as SMESE. Following technical advances, the year 2004 saw the appearance of a kind of scintillation crystal similar to LaBr3 [4] . Owing to its high light yield (1.3 times the common NaI scintillation crystal) and uniform lighting, its energy resolution can be 3 times better than that of a common NaI scintillation detector. Because it has a high energy resolution and there is no need of cooling, and due to the high atomic numbers of the La and Br atoms, it has a higher detection efficiency in comparison with the high-purity Ge, the LaBr3 detector has become the first choice of high-energy space explorations as soon as it appeared. Many planned satellites in the world will be adopting the LaBr3 scintillation detector[5,6] . Table 2 compares the performance of LaBr3 scintillator and the other detectors in common use. HEBS will also adopt the LaBr3 crystal. If SMESE is successfully launched at the scheduled time, it will be the first time in the world to observe high-energy radiations of celestial objects with this new technology. Table 2
Comparison of performance of the LaBr3 scintillator and the other detectors in common use
Parameter NaI(Tl) CsI(Tl) BGO LaBr3 Ge Scintillation efficiency relative to NaI(Tl) 100% 45% 10% 130% Light decay constant (μs) 0.23 1.0 0.3 0.016 3.67 4.51 7.13 5.29 5.36 Density(g/cm−3 ) Energy resolution at 662keV(FWHM) 7.5% 9.0% 13.5% 3.0% 0.3% Highest Z (high-Z element) 53(I) 55(Cs) 83(Bi) 57(La) 32(Ge)
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3.2
The HEBS Detector The whole HEBS detector is shown in Fig.2. The main detecting elements are the 3 LaBr3 scintillators, 7.6 cm in diameter, 7.6 cm in height, which is the maximum size availabe at present. In order to reduce the background level, the LaBr3 scintillator is encased in plastic scintillators of thickness 1.5 cm, and 3 cm longer than the LaBr3 scintillator. It should reduce the background caused by charged particles coming obliquely from the back of the photoelectric tubes. Above the anti-coincident crystals are two photoelectric tubes of 3 cm diameter. When charged particles penetrate through the detector, the two small photoelectric tubes will yield a signal, and with this signal we can judge whether the output signals of the main detectors belong to the background or the sun. Our simulation result indicates that this device can reduce over 97% of the background resulted directly from the high-energy charged particles. Besides, by analyzing the relations between the output signals of the three main detectors, the backgrounds of cosmic γ-ray higher than 1 MeV and of the satellite’s secondary γ-ray can be reduced by about 30%. In the rear of the main crystals are the three 7.6 cm photoelectric tubes. To cover the wide energy range from 10 keV to 600 MeV, one anode output is certainly not enough, so the anode and 3 dynodes of each photoelectric tube will output signals, and owing to the different gains of the anode and dynodes, the different outputs correspond to different energy bands, so by this device the energy range from 10 keV to 600 MeV is covered. In the laboratory we have applied this technique to an American balloon observation project (in this project, the 3 dynodes of a photoelectric tube are adopted to observe the energy deposition from 12 MeV to 20 TeV)[8] .
Fig. 2
Schematic diagram of the detector
3.3 Detector Performance The effective area as a function of energy is shown in Fig.3. The solid line refers to HEBS and the dotted line, to the γ-ray spectrometer on board the SMM satellite[9] . This figure shows that at 1 MeV the effective area of the HEBS detector is about 70 m2 . This is 65% the area of the γ-ray spectrometer on SMM, but the latter has 8 times the weight of HEBS. Fig.4 shows the energy resolution as a function of energy for HEBS (the solid line) and for the γ-ray spectrometer on SMM[9] (dashed line). This figure shows that the energy
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resolution of HEBS is more than two times better than the SMM spectrometer.
Fig. 3
Effective area of HEBS as a function of energy
Fig. 4
Energy resolution of HEBS as
a function of energy
Fig.5 is the HEBS simulated result of the observed solar burst of 2003 Nov 2. Fig.6 shows the photon spectrum of the solar high-energy radiation, obtained from the observed result of RHESSI (Fig.7). From the figures we see that compared with the observed result of RHESSI, although RHESSI uses a high-purity Ge semiconductor, and hence its energy resolution is much higher than that of HEBS, but the C-line and O-line of HEBS are clearer than those of RHESSI. Without a background suppression system, RHESSI has a higher background level than HEBS, in addition the effective area of its detector is also smaller than that of HEBS, therefore it has a lower sensitivity than HEBS.
Fig. 5
Simulated result of the high-energy radiation
of the 2003 Nov.2 solar burst observed by HEBS
Fig. 6 Photon spectrum of the high-energy radiation of the 2003 Nov.2 solar burst
3.4 Laboratory Test Results of the Large-sized LaBr3 Crystal It is not until the end of 2005 that large-size (7.6 cm×7.6 cm) LaBr3 crystal became available. Whether its performance satisfies the scientific requirements on HEBS is a problem of great concern to the SMESE science group. The following figures display the test results
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of our laboratory on the large-size LaBr3 crystal produced by the Saint Gobain Company in 2006. In the tests, the photomultiplier used is the spaceflight-class photoelectric tube R184805 of HAMAMATSU Company, and common silicone is used for the coupling between the photoelectric tube and the crystal. Fig.8 shows the energy spectra of the Am-241 and Co-57 radioactive sources measured by the detector. We see that the detector has energy resolutions of 14% at 59.5 keV and 7.5% at 122 keV, and the detectable lower limit can reach 5 keV, altogether satisfying the 10 keV lower-limit requirement of HEBS. Fig.9 is the result of observation of the Cs-137 radioactive source. As is shown, the energy resolution at 622 keV is 3.5%. Although it is a little worse than the 3.0% specification of HEBS, but considering that the R1848-05 spaceflight-class photoelectric tube is an old product of 10 years ago, lagging behind the the up-to-date products in quantum efficiency and spectral response [10] , so the 3.0% energy resolution required for HEBS is entirely feasible. Fig.10 gives the energy deposition spectrum measured by the detector generated after the Muon particles produced by the interaction between ground cosmic ray and the atmosphere pass through the detector. According to the Monte Carlo simulation, this spectrum should exhibit a peak at 72.1 MeV. The signals were output from the dynodes of the photoelectric tube. We have calibrated the relative gains of the anode and dynodes of the photoelectric tube. We found that the energy deposition peak of Muon particles together with the lowenergy band satisfies the linearity requirement, and there was not any saturation. Fig.11 displays the relationship between the amplitude of the output signal and the incident energy. We see that in the range from 5 keV to 72 MeV, the detector output and incident energy are linearly related.
Fig. 7
Energy spectrum of the 2003 Nov.2
solar burst observed by RHESSI
Fig. 8 Energy spectra of Am-241 and Co-57 measured by the LaBr3 detector
4. CONCLUDING SUMMARY From the above analysis of the present design of HEBS, we may say that HEBS’s energy resolution is better than that of the space scintillation detector onboard SMM, and its effective area at 1 MeV is equivalent to 65% of that of SMM. By adopting a background
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Fig. 9 Energy spectrum of Cs-137 measured by the LaBr3 detector
Fig. 10 Energy deposition spectrum of Muon particles measured by the LaBr3 detector
Fig. 11 The relationship between the signal amplitude of the LaBr 3 detector and the incident energy
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