- Email: [email protected]
Transition radiation detectors in particle astrophysics
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 522 (2004) 9–15
Transition radiation detectors in particle astrophysics Dietrich Muller . Department of Physic and Enrico Fermi Institute, University of Chicago, Chicago, IL 60637, USA
Abstract Transition radiation detectors (TRDs) have been used for three decades to study highly relativistic cosmic rays. Their relatively low weight but large sensitive area makes them particularly suitable for measurements above the atmosphere, on balloons or in space. A review is given of the different requirements and experimental solutions for threshold detectors and energy-measuring devices, and the astrophysical significance of the measurements is briefly summarized. Finally, the importance of TRD devices for future works is discussed. r 2004 Elsevier B.V. All rights reserved. PACS: 95.55.n Keywords: Cosmic-ray; Space detectors
1. Introduction The seminal paper on transition radiation (TR) has been published more than half a century ago [1], but it took several decades until transition radiation detectors (TRDs) became practical and powerful devices for particle detection and identification. Cosmic-ray physicists have appeared among the first users of this technique because they need detectors of large area that also have low weight in order to permit measurements above the earth’s atmosphere. Exactly these properties are provided by TRDs. The intensity of transition radiation depends on the Lorentz-factor g ¼ E=mc2 of the primary particle. For highly relativistic particles, the classic TRD configuration consists of a radiator of plastic foils, foam or fibers, followed by a detector, typically gaseous and containing xenon for efficient photoelectric conversion of X-rays. TR X-rays are emitted E-mail address: [email protected] (D. Muller). .
in the forward direction, and the X-ray signal in the detector is superimposed upon the ionization signal of the particle. For singly charged primary particles, the number of photons generated in a radiator of B100 interfaces is of order unity, and the statistical fluctuations in the signal are large. Therefore, one usually performs repeated and independent measurements in a sandwich configuration of several radiator–detector pairs. A likelihood analysis of the signal [2] may then be performed in order to ascertain the presence of TR X-rays on an eventby-event basis. Alternatively, the details of the time structures of the signals may be investigated (‘‘cluster counting’’ [3,4]).
2. Threshold detectors Typically, TR X-rays can be observed above a Lorentz factor threshold g0 whose value is around g0 E103 but can be varied somewhat by choice of the geometry of the radiator, and saturation sets in
0168-9002/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2004.01.010
ARTICLE IN PRESS 10
D. Muller / Nuclear Instruments and Methods in Physics Research A 522 (2004) 9–15 .
at g values about an order of magnitude higher than g0 : Hence, a TRD is frequently used as a threshold detector to distinguish between particles of the same energy but with different mass, for instance between electrons, pions, and protons. Exactly this application motivated the design of the first cosmic-ray detector using a TRD in balloon flights above the atmosphere in the 1970s [5–7]. A combination of a TRD sandwich using plastic foam radiators [8] with a short electromagnetic shower counter was employed to identify high-energy cosmic-ray electrons among an overwhelming flux of protons. This work helped to solve a major question in cosmic-ray astrophysics at that time: it showed that the energy spectrum of cosmic-ray electrons was different, i.e., falling much more steeply with increasing energy, than that of protons and nuclei, due to radiative energy losses of electrons in the galaxy. The resulting energy spectrum indicates a lifetime of electrons in the galaxy of about 107 years. This initial work led to a much more difficult task, namely the separate identification of electrons and positrons in the cosmic rays. Cosmic-ray positrons were originally identified below a few GeV in the 1960s [9], and found to be even less abundant (by a factor of B10) than negative electrons. To extend the measurement to higher energies, a discrimination power against protons better than 105 is needed. The HEAT balloon instrument [10], shown in Fig. 1, has accomplished this task by combining a superconducting magnet spectrometer with an electromagnetic calorimeter and a TRD. The TRD signals were processed both by a likelihood analysis and by a cluster-counting technique, leading to a proton rejection, with the TRD alone, of 6 103 ; at an electron acceptance of 90%: A similar instrument was used by the WIZARD collaboration [11,12]. One motivation for the HEAT experiment was the search for possibly exotic contributions to the positron intensity. The results shown in Fig. 2 [13,14] indicate that the positron fraction in the cosmic rays can be explained by assuming that nearly all positrons are generated as secondary particles, from interactions of nuclear cosmic rays in the interstellar gas. However, a small feature remains in the data at high energy that defies easy interpretation [15].
TOF
TRD
TWO COIL MAGNET
DTH
EC
1 METER Fig. 1. Cross-section of the HEAT spectrometer for measurements of electrons and positrons. TOF: top time-of-flight scintillator; TRD: transition radiation detector; DTH: drifttube hodoscope; EC: electromagnetic calorimeter.
New measurements currently planned for detectors in space should improve the counting statistics and energy coverage that was possible with the balloon-borne instruments. Furthermore, these next-generation measurements will also be free of systematic uncertainties due to positron production in the residual atmosphere at balloon altitude. Two instruments are under construction, PAMELA [16], and AMS [17], and both will employ combinations of a magnet spectrometer with a TRD and an electromagnetic calorimeter.
3. Energy measurement For threshold TRDs, it is desirable that the transition from gog0 (no TR) to g > g0 (saturated TR) occurs rapidly, over a small range in g: If, on the other hand, the g-dependence of the TR signal around g0 is to be utilized for an actual measurement of g in that region, one may be interested in a more gradual change in the response curve.
ARTICLE IN PRESS D. Muller / Nuclear Instruments and Methods in Physics Research A 522 (2004) 9–15 .
11
muon signals, because signal fluctuations precluded an event-by-event analysis. A qualitatively different situation may be encountered in cosmic ray measurements above the atmosphere. While the majority of cosmic rays are protons and a-particles, the nuclei of all heavier elements are present, and decisive information on the acceleration and interstellar propagation of cosmic rays can be obtained from measurements of their relative abundances and individual energy spectra. The signal in a TRD for nuclei of charge number Z scale with Z 2 ; and the relative statistical fluctuations decrease like 1=Z: This has dramatic consequences, as illustrated in Fig. 3, where we compare signal distributions in a
Fig. 2. HEAT measurement of the positron fraction in cosmic rays. The solid curve indicates the trend of the expected positron fraction if all positrons were purely interstellar secondary particles.
However, because of the large statistical fluctuations in the signal, such a measurement becomes very problematic if the primary particle is singly charged. To overcome the fluctuations, a very large number of independent measurements would be necessary, making the detector unwieldy in size, and no longer thin in units of radiation or interaction lengths. As particle physics experiments on accelerators almost always deal with singly charged particles, a precise measurement of g in such experiments is therefore difficult to perform. The situation is more favorable for measurements of cosmic-ray generated muons at high energies in underground installations, because the background of low-energy particles is relatively low, and because muons are not likely to be lost by interactions even in a fairly massive installation. Such underground detectors have been considered in the 1980s [18], and have been installed as part of the MACRO detector [19]. The TRD of MACRO has analyzed the energies of muon events underground and derived constraints on the cosmic-ray composition around 1015 eV: The study was based on the aggregate signal from a large number of
Fig. 3. Simulated pulse height histograms in a single TRD detector for protons (Z ¼ 1; upper panel) and for iron (Z ¼ 26; lower panel). For each species, two histograms are shown, characterizing the response for g ¼ 100 (left histogram) and for g ¼ 2800 (right histogram).
ARTICLE IN PRESS 12
D. Muller / Nuclear Instruments and Methods in Physics Research A 522 (2004) 9–15 .
single-radiator TRD for singly charged particles and for iron nuclei ðZ ¼ 26Þ: The overlap of the distributions of particles with and without TR, respectively, rapidly diminishes with increasing Z; and the distributions become well-defined narrow Gaussians. Thus, for heavier nuclei, the TR intensity can be utilized for an accurate measurement of g: That the Z2 -scaling described here is indeed valid has been verified by experimental evidence [20]. It is very important that the energy response of the TRD is precisely known (more exactly: the Lorentz-factor response; we assume that the mass of the nucleus is independently determined, and hence, we use the terms ‘‘energy’’ or ‘‘Lorentz factor’’ indiscriminately). This calibration requires accelerator measurements with beams of electrons or pions/muons, but great care is required to obtain the signals and their distributions with high precision. Further, the parameters of the radiator/ detector combination must be ‘‘tuned’’ in order to ascertain g-sensitive response over a large range of Lorentz factors. For instance, Fig. 4 shows the response curve of the first TRD flown in space [21],
the ‘‘Cosmic Ray Nuclei Detector (CRN)’’. Here, special care was taken to obtain a detector that permits measurements for g values below 103 : Consequently, the radiators are composed of combinations of different plastic fiber blankets, including fibers with very thin diameters ðo5 mmÞ: The resulting response exhibits a g-sensitive region over nearly two decades, from gE500 to 2 104 : A cross-section of CRN is shown in Fig. 5. Highenergy cosmic-ray nuclei with ð5pZp26Þ are detected, and the overwhelming background of low-energy particles is reduced with the aid of gasCherenkov counters. This instrument was flown on the Space Shuttle in 1985 and has determined the energy spectra of cosmic-ray nuclei up to energies above 103 GeV=nucleon (corresponding to total energies of 1013 –1014 eV per particle). These measurements have established a number of important details about the cosmic radiation at high energies [22,23]. They indicate that all cosmic ray species are generated at the acceleration sites with the same source energy spectrum which, however, has a power law shape pE 2:2 ; much harder (flatter) than the observed spectrum. This spectral shape at the source is close to what theories of supernova-shock acceleration would predict. The available data are limited by counting statistics, and there is strong scientific motivation to extend the measurements towards the cosmicray ‘‘knee’’ above 1015 eV per particle. As a first
GAS CERENKOV C1 C1 SCINTILLATOR T1
Photomultipliers
T1
Radiators
TRANSITION RADIATION DETECTORS SCINTILLATOR T2
1.1m Multiwire Proportional Chambers T2 3.7m C2
GAS CERENKOV C2
Fig. 4. Average signal versus Lorentz factor: calibration for the CRN detector. The open squares describe the response if the radiators are replaced by solid plates of polyethylene. Note the zero offset of the vertical scale.
2.0m 2.7m
Fig. 5. Cross-section of the CRN detector.
ARTICLE IN PRESS D. Muller / Nuclear Instruments and Methods in Physics Research A 522 (2004) 9–15 .
step in this direction, one may utilize longduration balloon flights, which now provide the opportunity of flight durations of several weeks. A balloon-instrument, ‘‘Transition radiation array for cosmic energetic radiation’’ (TRACER), has been constructed for such flights, and verified in a short test flight in 1999 [24]. Fig. 6 shows a cross-section of TRACER. The instrument uses layers of single-wire proportional tubes instead of multiwire proportional chambers.
2m
scintillator
13
The tubes easily withstand an overpressure of several atmospheres; hence, the detector does not require a costly and heavy pressurized enclosure. The instrument also employs layers of single wire proportional tubes (without TR radiators) to select relativistic particles below the onset of TR by means of the relativistic rise in specific ionization. TRACER now awaits a circumpolar long-duration balloon flight scheduled for winter 2003/4 in Antarctica. A sample of scientific results for iron nuclei from the test flight is shown in Fig. 7 [24], together with the measurements made earlier with the CRN instrument, and with measurements on HEAO-3.
proportional tube array
1.2 m
radiator
4. Outlook transition radiation detector
scintillator 2m
Cerenkon counter
Fig. 6. Cross-section of the TRACER balloon instrument.
Fig. 7. Differential energy spectra of iron nuclei measured with HEAO-3, CRN and TRACER. Note that E is the energy per amu, and that the intensities are multiplied with E 2:5 :
Future observations should advance knowledge of the individual energy spectra of cosmic ray nuclei to as high an energy as possible. While the overall energy spectrum is measured over many orders of magnitude, details on the particle composition are currently known only below B1013 eV=particle; and consequently, the origin and galactic propagation of cosmic rays remain enigmatic. Detectors using TRD devices appear to be capable of extending the range of direct measurements above the atmosphere up to the knee region around 1015 eV per particle. At still higher energies, air shower observations are currently the only experimental technique available. These measurements are indirect and difficult to interpret, and would be greatly helped by a cross-calibration with data from direct observations. To utilize the full potential of the TRDtechnique, long exposure times above the atmosphere are essential. An instrument called ‘‘Cosmic Ray Energetics and Mass’’ (CREAM) which contains a TRD is currently under construction, possibly for ultralong balloon flight of B100 days duration [25]. However, eventually a space instrument is required not only because it permits exposures that are still larger by at least an order of magnitude, but also because of the lack of atmospheric overburden, and hence, of background from spallation-produced nuclei in the atmosphere.
ARTICLE IN PRESS 14
D. Muller / Nuclear Instruments and Methods in Physics Research A 522 (2004) 9–15 .
A space-borne detector, ‘‘advanced cosmic-ray composition experiment for the space station’’ (ACCESS) has been proposed and studied since the late 1990s [26]. Whether attached to the Space Station, or deployed as a free-flying satellite for several years, ACCESS should provide comprehensive cosmic-ray composition measurements up to at least 1015 eV: The concept of ACCESS would combine a large area TRD for measurements of the heavier nuclei with a smaller nuclear calorimeter for the more abundant protons and alphaparticles. To perform measurements over the full range of energies, the TRD for ACCESS should cover a Lorentz factor range from below 1000 to at least 105 : It has been shown in accelerator tests, and is illustrated in Fig. 8, that a response over this large a range can indeed be achieved with a classical TRD configuration. (For further detail, see the contribution of Scott Wakely to this conference [27].) An alternate technique to reach
very high Lorentz factors by observing hard Compton-scattered transition-radiation X-rays has been considered by Cherry and Wefel [28] at the previous Bari meeting.
5. Conclusions For nearly 30 years now, the transition radiation technique has been developed for a variety of measurements in particle astrophysics. It has been applied for muon detection at sea level and underground, and in instruments at balloon altitudes and on the Space Shuttle, and has made possible measurements of cosmic-ray electrons, positrons, and of highly relativistic heavy nuclei. A major advantage of a TRD above the atmosphere is its low mass-to-area ratio. Another important feature is the possibility of calibrating the response of TRDs at accelerators, with singly charged particles of high Lorentz factors, taking into account the fact that TR emission scales precisely with Z 2 : Such accelerator calibrations are, however, tedious, require great care in preparing clean beams, and deserve more attention than often given in a busy accelerator schedule. Equally important is progress in computer simulations of TR-emission. It appears that TRDs will continue to play an important role in particle astrophysics for years to come. Two instruments, TRACER and CREAM, currently use TRDs in long-duration balloon flights; and two spacecraft instruments, PAMELA and AMS, will include TRDs during the next few years. Finally, the ACCESS mission, with a large TRD system for the observation of heavy nuclei up to the cosmic-ray ‘‘knee’’, is now well-defined and should become reality before too long.
References
Fig. 8. Average signal versus Lorentz factor for a composite radiator/detector configuration consisting of plastic foils, foam, and fibers (triangles), and for a radiator of parallel Mylar foils of 76 mm thickness (squares). Note that the signal reaches saturation around gE105 :
[1] V.L. Ginzburg, I.M. Frank, Sov. Phys. JETP 16 (1946) 15. [2] M. Cherry, D. Muller T. Prince, Nucl. Instr. and Meth. 15 (1974) 141. [3] T. Ludlam, et al., Nucl. Instr. and Meth. 180 (1981) 413. [4] C.W. Fabjan, et al., Nucl. Instr. and Meth. 185 (1981) 119. [5] G. Hartmann, D. Muller, T. Prince, Phys. Rev. Lett. 38 (1977) 1368.
ARTICLE IN PRESS D. Muller / Nuclear Instruments and Methods in Physics Research A 522 (2004) 9–15 . [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
[17]
T. Prince, Astrophys. J. 227 (1979) 676. K.K. Tang, Astrophys. J. 278 (1984) 881. T. Prince, et al., Nucl. Instr. and Meth. 123 (1975) 231. J.L. Fanselow, et al., Astrophys. J. 158 (1969) 771. S.W. Barwick, et al., Nucl. Instr. and Meth. A 400 (1997) 34. E. Barbarito, et al., Nucl. Instr. and Meth. A 357 (1995) 588. R. Bellotti, et al., Astropart. Phys. 7 (1997) 219. S.W. Barwick, et al., Astrophys. J. 482 (1997) 191. M.A. Du Vernois, Astrophys. J. 552 (2001) 296. S. Coutu, et al., Astropart. Phys. 11 (1999) 429. M. Ambriola, et al., TRDs for the Third Millennium, Frascati Physics Series, INFN Vol. XXV, Bari 2001, pp. 177. T. Kirn, et al., TRDs for the Third Millennium, Frascati Physics Series, Vol. XXV, 2001, pp. 161.
15
[18] M.L. Cherry, Proceedings, Cosmic-Ray Workshop, Utah, 1983, pp. 149. [19] E. Barbarito, et al., Nucl. Instr. and Meth. A 365 (1995) 214. [20] S.P. Swordy, et al., Phys. Rev. D 42 (1990) 3197. [21] J. L’Heureux, et al., Nucl. Instr. and Meth. A 295 (1990) 246. [22] D. Muller, et al., Astrophys. J. 374 (1991) 356. [23] S.P. Swordy, et al., Astrophys. J. 403 (1993) 658. [24] F. Gahbauer, et al., Astrophys. J. (2004), in press. [25] E.S. Seo, et al., Adv. Space Res. 30 (2002) 1263. [26] M. Israel, et al., ACCESS: a cosmic journey, Report of NASA Working Group, 2000. [27] S. Wakely, TRDs for the Third Millennium, Nucl. Instr. and Meth. A, (2004) these proceedings. [28] M.L. Cherry, J.P. Wefel, TRDs for the Third Millennium, Frascati Physics Series, INFN Vol. XXV, Bari 2001, pp. 35.
Nuclear Instruments and Methods in Physics Research A 522 (2004) 9–15
Transition radiation detectors in particle astrophysics Dietrich Muller . Department of Physic and Enrico Fermi Institute, University of Chicago, Chicago, IL 60637, USA
Abstract Transition radiation detectors (TRDs) have been used for three decades to study highly relativistic cosmic rays. Their relatively low weight but large sensitive area makes them particularly suitable for measurements above the atmosphere, on balloons or in space. A review is given of the different requirements and experimental solutions for threshold detectors and energy-measuring devices, and the astrophysical significance of the measurements is briefly summarized. Finally, the importance of TRD devices for future works is discussed. r 2004 Elsevier B.V. All rights reserved. PACS: 95.55.n Keywords: Cosmic-ray; Space detectors
1. Introduction The seminal paper on transition radiation (TR) has been published more than half a century ago [1], but it took several decades until transition radiation detectors (TRDs) became practical and powerful devices for particle detection and identification. Cosmic-ray physicists have appeared among the first users of this technique because they need detectors of large area that also have low weight in order to permit measurements above the earth’s atmosphere. Exactly these properties are provided by TRDs. The intensity of transition radiation depends on the Lorentz-factor g ¼ E=mc2 of the primary particle. For highly relativistic particles, the classic TRD configuration consists of a radiator of plastic foils, foam or fibers, followed by a detector, typically gaseous and containing xenon for efficient photoelectric conversion of X-rays. TR X-rays are emitted E-mail address: [email protected] (D. Muller). .
in the forward direction, and the X-ray signal in the detector is superimposed upon the ionization signal of the particle. For singly charged primary particles, the number of photons generated in a radiator of B100 interfaces is of order unity, and the statistical fluctuations in the signal are large. Therefore, one usually performs repeated and independent measurements in a sandwich configuration of several radiator–detector pairs. A likelihood analysis of the signal [2] may then be performed in order to ascertain the presence of TR X-rays on an eventby-event basis. Alternatively, the details of the time structures of the signals may be investigated (‘‘cluster counting’’ [3,4]).
2. Threshold detectors Typically, TR X-rays can be observed above a Lorentz factor threshold g0 whose value is around g0 E103 but can be varied somewhat by choice of the geometry of the radiator, and saturation sets in
0168-9002/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2004.01.010
ARTICLE IN PRESS 10
D. Muller / Nuclear Instruments and Methods in Physics Research A 522 (2004) 9–15 .
at g values about an order of magnitude higher than g0 : Hence, a TRD is frequently used as a threshold detector to distinguish between particles of the same energy but with different mass, for instance between electrons, pions, and protons. Exactly this application motivated the design of the first cosmic-ray detector using a TRD in balloon flights above the atmosphere in the 1970s [5–7]. A combination of a TRD sandwich using plastic foam radiators [8] with a short electromagnetic shower counter was employed to identify high-energy cosmic-ray electrons among an overwhelming flux of protons. This work helped to solve a major question in cosmic-ray astrophysics at that time: it showed that the energy spectrum of cosmic-ray electrons was different, i.e., falling much more steeply with increasing energy, than that of protons and nuclei, due to radiative energy losses of electrons in the galaxy. The resulting energy spectrum indicates a lifetime of electrons in the galaxy of about 107 years. This initial work led to a much more difficult task, namely the separate identification of electrons and positrons in the cosmic rays. Cosmic-ray positrons were originally identified below a few GeV in the 1960s [9], and found to be even less abundant (by a factor of B10) than negative electrons. To extend the measurement to higher energies, a discrimination power against protons better than 105 is needed. The HEAT balloon instrument [10], shown in Fig. 1, has accomplished this task by combining a superconducting magnet spectrometer with an electromagnetic calorimeter and a TRD. The TRD signals were processed both by a likelihood analysis and by a cluster-counting technique, leading to a proton rejection, with the TRD alone, of 6 103 ; at an electron acceptance of 90%: A similar instrument was used by the WIZARD collaboration [11,12]. One motivation for the HEAT experiment was the search for possibly exotic contributions to the positron intensity. The results shown in Fig. 2 [13,14] indicate that the positron fraction in the cosmic rays can be explained by assuming that nearly all positrons are generated as secondary particles, from interactions of nuclear cosmic rays in the interstellar gas. However, a small feature remains in the data at high energy that defies easy interpretation [15].
TOF
TRD
TWO COIL MAGNET
DTH
EC
1 METER Fig. 1. Cross-section of the HEAT spectrometer for measurements of electrons and positrons. TOF: top time-of-flight scintillator; TRD: transition radiation detector; DTH: drifttube hodoscope; EC: electromagnetic calorimeter.
New measurements currently planned for detectors in space should improve the counting statistics and energy coverage that was possible with the balloon-borne instruments. Furthermore, these next-generation measurements will also be free of systematic uncertainties due to positron production in the residual atmosphere at balloon altitude. Two instruments are under construction, PAMELA [16], and AMS [17], and both will employ combinations of a magnet spectrometer with a TRD and an electromagnetic calorimeter.
3. Energy measurement For threshold TRDs, it is desirable that the transition from gog0 (no TR) to g > g0 (saturated TR) occurs rapidly, over a small range in g: If, on the other hand, the g-dependence of the TR signal around g0 is to be utilized for an actual measurement of g in that region, one may be interested in a more gradual change in the response curve.
ARTICLE IN PRESS D. Muller / Nuclear Instruments and Methods in Physics Research A 522 (2004) 9–15 .
11
muon signals, because signal fluctuations precluded an event-by-event analysis. A qualitatively different situation may be encountered in cosmic ray measurements above the atmosphere. While the majority of cosmic rays are protons and a-particles, the nuclei of all heavier elements are present, and decisive information on the acceleration and interstellar propagation of cosmic rays can be obtained from measurements of their relative abundances and individual energy spectra. The signal in a TRD for nuclei of charge number Z scale with Z 2 ; and the relative statistical fluctuations decrease like 1=Z: This has dramatic consequences, as illustrated in Fig. 3, where we compare signal distributions in a
Fig. 2. HEAT measurement of the positron fraction in cosmic rays. The solid curve indicates the trend of the expected positron fraction if all positrons were purely interstellar secondary particles.
However, because of the large statistical fluctuations in the signal, such a measurement becomes very problematic if the primary particle is singly charged. To overcome the fluctuations, a very large number of independent measurements would be necessary, making the detector unwieldy in size, and no longer thin in units of radiation or interaction lengths. As particle physics experiments on accelerators almost always deal with singly charged particles, a precise measurement of g in such experiments is therefore difficult to perform. The situation is more favorable for measurements of cosmic-ray generated muons at high energies in underground installations, because the background of low-energy particles is relatively low, and because muons are not likely to be lost by interactions even in a fairly massive installation. Such underground detectors have been considered in the 1980s [18], and have been installed as part of the MACRO detector [19]. The TRD of MACRO has analyzed the energies of muon events underground and derived constraints on the cosmic-ray composition around 1015 eV: The study was based on the aggregate signal from a large number of
Fig. 3. Simulated pulse height histograms in a single TRD detector for protons (Z ¼ 1; upper panel) and for iron (Z ¼ 26; lower panel). For each species, two histograms are shown, characterizing the response for g ¼ 100 (left histogram) and for g ¼ 2800 (right histogram).
ARTICLE IN PRESS 12
D. Muller / Nuclear Instruments and Methods in Physics Research A 522 (2004) 9–15 .
single-radiator TRD for singly charged particles and for iron nuclei ðZ ¼ 26Þ: The overlap of the distributions of particles with and without TR, respectively, rapidly diminishes with increasing Z; and the distributions become well-defined narrow Gaussians. Thus, for heavier nuclei, the TR intensity can be utilized for an accurate measurement of g: That the Z2 -scaling described here is indeed valid has been verified by experimental evidence [20]. It is very important that the energy response of the TRD is precisely known (more exactly: the Lorentz-factor response; we assume that the mass of the nucleus is independently determined, and hence, we use the terms ‘‘energy’’ or ‘‘Lorentz factor’’ indiscriminately). This calibration requires accelerator measurements with beams of electrons or pions/muons, but great care is required to obtain the signals and their distributions with high precision. Further, the parameters of the radiator/ detector combination must be ‘‘tuned’’ in order to ascertain g-sensitive response over a large range of Lorentz factors. For instance, Fig. 4 shows the response curve of the first TRD flown in space [21],
the ‘‘Cosmic Ray Nuclei Detector (CRN)’’. Here, special care was taken to obtain a detector that permits measurements for g values below 103 : Consequently, the radiators are composed of combinations of different plastic fiber blankets, including fibers with very thin diameters ðo5 mmÞ: The resulting response exhibits a g-sensitive region over nearly two decades, from gE500 to 2 104 : A cross-section of CRN is shown in Fig. 5. Highenergy cosmic-ray nuclei with ð5pZp26Þ are detected, and the overwhelming background of low-energy particles is reduced with the aid of gasCherenkov counters. This instrument was flown on the Space Shuttle in 1985 and has determined the energy spectra of cosmic-ray nuclei up to energies above 103 GeV=nucleon (corresponding to total energies of 1013 –1014 eV per particle). These measurements have established a number of important details about the cosmic radiation at high energies [22,23]. They indicate that all cosmic ray species are generated at the acceleration sites with the same source energy spectrum which, however, has a power law shape pE 2:2 ; much harder (flatter) than the observed spectrum. This spectral shape at the source is close to what theories of supernova-shock acceleration would predict. The available data are limited by counting statistics, and there is strong scientific motivation to extend the measurements towards the cosmicray ‘‘knee’’ above 1015 eV per particle. As a first
GAS CERENKOV C1 C1 SCINTILLATOR T1
Photomultipliers
T1
Radiators
TRANSITION RADIATION DETECTORS SCINTILLATOR T2
1.1m Multiwire Proportional Chambers T2 3.7m C2
GAS CERENKOV C2
Fig. 4. Average signal versus Lorentz factor: calibration for the CRN detector. The open squares describe the response if the radiators are replaced by solid plates of polyethylene. Note the zero offset of the vertical scale.
2.0m 2.7m
Fig. 5. Cross-section of the CRN detector.
ARTICLE IN PRESS D. Muller / Nuclear Instruments and Methods in Physics Research A 522 (2004) 9–15 .
step in this direction, one may utilize longduration balloon flights, which now provide the opportunity of flight durations of several weeks. A balloon-instrument, ‘‘Transition radiation array for cosmic energetic radiation’’ (TRACER), has been constructed for such flights, and verified in a short test flight in 1999 [24]. Fig. 6 shows a cross-section of TRACER. The instrument uses layers of single-wire proportional tubes instead of multiwire proportional chambers.
2m
scintillator
13
The tubes easily withstand an overpressure of several atmospheres; hence, the detector does not require a costly and heavy pressurized enclosure. The instrument also employs layers of single wire proportional tubes (without TR radiators) to select relativistic particles below the onset of TR by means of the relativistic rise in specific ionization. TRACER now awaits a circumpolar long-duration balloon flight scheduled for winter 2003/4 in Antarctica. A sample of scientific results for iron nuclei from the test flight is shown in Fig. 7 [24], together with the measurements made earlier with the CRN instrument, and with measurements on HEAO-3.
proportional tube array
1.2 m
radiator
4. Outlook transition radiation detector
scintillator 2m
Cerenkon counter
Fig. 6. Cross-section of the TRACER balloon instrument.
Fig. 7. Differential energy spectra of iron nuclei measured with HEAO-3, CRN and TRACER. Note that E is the energy per amu, and that the intensities are multiplied with E 2:5 :
Future observations should advance knowledge of the individual energy spectra of cosmic ray nuclei to as high an energy as possible. While the overall energy spectrum is measured over many orders of magnitude, details on the particle composition are currently known only below B1013 eV=particle; and consequently, the origin and galactic propagation of cosmic rays remain enigmatic. Detectors using TRD devices appear to be capable of extending the range of direct measurements above the atmosphere up to the knee region around 1015 eV per particle. At still higher energies, air shower observations are currently the only experimental technique available. These measurements are indirect and difficult to interpret, and would be greatly helped by a cross-calibration with data from direct observations. To utilize the full potential of the TRDtechnique, long exposure times above the atmosphere are essential. An instrument called ‘‘Cosmic Ray Energetics and Mass’’ (CREAM) which contains a TRD is currently under construction, possibly for ultralong balloon flight of B100 days duration [25]. However, eventually a space instrument is required not only because it permits exposures that are still larger by at least an order of magnitude, but also because of the lack of atmospheric overburden, and hence, of background from spallation-produced nuclei in the atmosphere.
ARTICLE IN PRESS 14
D. Muller / Nuclear Instruments and Methods in Physics Research A 522 (2004) 9–15 .
A space-borne detector, ‘‘advanced cosmic-ray composition experiment for the space station’’ (ACCESS) has been proposed and studied since the late 1990s [26]. Whether attached to the Space Station, or deployed as a free-flying satellite for several years, ACCESS should provide comprehensive cosmic-ray composition measurements up to at least 1015 eV: The concept of ACCESS would combine a large area TRD for measurements of the heavier nuclei with a smaller nuclear calorimeter for the more abundant protons and alphaparticles. To perform measurements over the full range of energies, the TRD for ACCESS should cover a Lorentz factor range from below 1000 to at least 105 : It has been shown in accelerator tests, and is illustrated in Fig. 8, that a response over this large a range can indeed be achieved with a classical TRD configuration. (For further detail, see the contribution of Scott Wakely to this conference [27].) An alternate technique to reach
very high Lorentz factors by observing hard Compton-scattered transition-radiation X-rays has been considered by Cherry and Wefel [28] at the previous Bari meeting.
5. Conclusions For nearly 30 years now, the transition radiation technique has been developed for a variety of measurements in particle astrophysics. It has been applied for muon detection at sea level and underground, and in instruments at balloon altitudes and on the Space Shuttle, and has made possible measurements of cosmic-ray electrons, positrons, and of highly relativistic heavy nuclei. A major advantage of a TRD above the atmosphere is its low mass-to-area ratio. Another important feature is the possibility of calibrating the response of TRDs at accelerators, with singly charged particles of high Lorentz factors, taking into account the fact that TR emission scales precisely with Z 2 : Such accelerator calibrations are, however, tedious, require great care in preparing clean beams, and deserve more attention than often given in a busy accelerator schedule. Equally important is progress in computer simulations of TR-emission. It appears that TRDs will continue to play an important role in particle astrophysics for years to come. Two instruments, TRACER and CREAM, currently use TRDs in long-duration balloon flights; and two spacecraft instruments, PAMELA and AMS, will include TRDs during the next few years. Finally, the ACCESS mission, with a large TRD system for the observation of heavy nuclei up to the cosmic-ray ‘‘knee’’, is now well-defined and should become reality before too long.
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Fig. 8. Average signal versus Lorentz factor for a composite radiator/detector configuration consisting of plastic foils, foam, and fibers (triangles), and for a radiator of parallel Mylar foils of 76 mm thickness (squares). Note that the signal reaches saturation around gE105 :
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