the calet mission for observing high energy cosmic … · 7 department of physics and astronomy,...

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*1 torii.shoji＠waseda.jp*2 Universities Space Research Association, USA1 Advanced Research Institute for Science and Engineering, Waseda University, 3–4–1, Okubo, Shinjuku-ku, Tokyo, Japan2 Institute of Space and Astronautical Science, JAXA, Japan3 Faculty of Engineering, Kanagawa University, Japan4 University of Siena and INFN, Italy5 NASA/Goddard Space Flight Center, USA6 University of Florence, Italy7 Department of Physics and Astronomy, Louisiana State University, USA8 Washington University in St. Louis, USA9 Purple Mountain Observatory, Chinese Academy of Science, China

10 INFN sezione di Pisa and Scuola Normale Superiore, Italy11 Department of Physics, Hirosaki University, Japan12 Department of Physics, Yokohama National University, Japan13 National Institute of Radiological Sciences, Japan14 Department of Physics and Astronomy, University of Denver, USA15 Space Environment Utilization Center, JAXA, Japan16 Department of Electronic & Information Systems, Shibaura Institute of Technology, Japan17 Department of Physics, Rikkyo University, Japan18 Department of Physics, Aoyama Gakuin University, Japan19 Kanagawa University of Human Services, Japan20 Department of Physics, Saitama University, Japan21 Department of Earth and Planetary Physics, University of Tokyo, Japan

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J. Jpn. Soc. Microgravity Appl. Vol. 24 No. 1 2007 (120–126)

Special Issue: Sixth Japan/China Workshop on Microgravity Sciences

The CALET Mission for Observing High Energy Cosmic Rayson Japanese Experiment Module of ISS

Shoji TORII1*1 for the CALET Collaboration

Abstract

The CALorimetric Electron Telescope, CALET, mission is proposed for the Japanese Experiment Module Ex-posed Facility, JEM–EF, of the International Space Station. The mission goal is to reveal the high-energy phenomenain the universe by carrying out a precise measurement of the electrons in 1 GeV–10 TeV and the gamma-rays in 20MeV–several TeV. The instrument will be composed of an imaging calorimeter of scintillating ˆbers and a total ab-sorption calorimeter of BGO. The total thickness of absorber is 36 r.l for electromagnetic particles and 1.6 m.f.p forprotons. Total weight of the payload is nearly 2,500 kg, and the eŠective geometrical factor for the electrons could belarger than 0.5¿1 m2 sr. The CALET has a unique capability to measure the electrons and the gamma-rays over 1TeV since the hadron rejection power might be 106 and the energy resolution of electromagnetic particles better than afew z over 100 GeV. Therefore, it is promising to detect the change of energy spectra and the g-ray line expectedfrom candidates of the dark matter. We are expecting to launch the CALET around 2012 by the Japanese H–IITransfer Vehicle, HTV, and to observe for three years.

Y. SHIMIZU1, N. HASEBE1, N. HAREYAMA1, S. KODAIRA1, J. NISHIMURA2, T. YAMAGAMI2, Y. SAITO2, H. FUKE2,M. TAKAYANAGI2, H. TOMIDA2, S. UENO2, T. TAMURA3, N. TATEYAMA3, K. HIBINO3, T. YUDA3, K. YOSHIDA3,S. OKUNO3, P. S. MARROCCHESI4, P. MAESTRO4, M. G. BAGLIESI4, V. MILLUCCI4, M. MEUCCI4, G. BIGONGIARI4,R. ZEI4, R. E. STREITMATTER5, J. W. MITCHEL5, L. M. BARBIER5, A. A. MOISEEV5*2, J. F. KRIZMANIC5,O. ADRIANI6, P. PAPINI6, P. SPILLANTINI6, L. BONECHI6, E. VANNUCCINI6, G. CASE7, M. L. CHERRY7,T. G. GUZIK7, J. B. ISBERT7, J. P. WEFEL7, W. R. BINNS8, M. H. ISRAEL8, H. S. KRAWZCZYNSKI8, J. CHANG9,W. GAN9, T. LU9, F. LIGABUE10, F. MORSANI10, S. KURAMATA11, M. ICHIMURA11, M. SHIBATA12, Y. KATAYOSE12,Y. UCHIHORI13, H. KITAMURA13, J. F. ORMES14, F. MAKINO15, K. KASAHARA16, H. MURAKAMI17, T. KOBAYASHI18,Y. KOMORI19, K. MIZUTANI20 and T. TERASAWA21

1. Introduction

We propose CALET as an instrument to observevery high energy electrons and g-rays on the Japanese

Experiment Module Exposure Facility, JEM–EF, ofISS. The objective of the CALET Mission is to explorea new frontier at higher energies for the origin of cos-mic-rays (CR), the propagation of CR and to search

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Fig. 1 Expected energy spectrum of electrons from a diŠusion model calculation under diŠerent assumptions, comparing with thepresent data. The assumptions are following: (I) no cut-oŠ energy (Ec＝∞), instant acceleration time (DT＝0 yr) and the diŠu-sion coe‹cient at 1 TeV of D0＝2×1029 cm2/s. (II) D0＝5×1029 cm2/s. (III) existence of cut-oŠ energy (Ec＝20 TeV). (IV) exis-tence of cut-oŠ energy (Ec＝20 TeV), ˆnite acceleration time (DT＝104 yr). The parameters not shown in (II), (III) and (IV) aresame with (I).

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The CALET Mission for Observing High Energy Cosmic Rays on Japanese Experiment Module of ISS

J. Jpn. Soc. Microgravity Appl. Vol. 24 No. 1 2007

for dark matter signatures. We will measure electronsfrom 1 GeV to ¿10 TeV and gamma-rays from 20MeV to several TeV, free from the hadron back-grounds, with an excellent energy resolution beyond100 GeV. We are considering using CALET to meas-ure protons and heavy nuclei from 10 GeV to ¿1,000TeV range. CALET is designed on the basis of ex-perience in balloon observations1–3).

It is a calorimeter, combining an imaging part and atotal absorption part, and it will have an excellentcapability for proton rejection, ¿106, which is neces-sary to select electrons in the TeV region. It is also suit-able for a precise measurement of the energy spec-trum, since the energy resolution is better than a fewz for energies greater than 100 GeV. CALET cansimultaneously detect electrons and gamma-rays by us-ing a multi-triggering system. Since we do not need anextra detector for particle identiˆcation other than alight weight Silicon Pad Detector (for precise chargeidentiˆcation of relativistic nuclei), the experiment canhave a larger geometrical factor for its weight. As aresult, CALET is unique in its detection capabilityabove 1 TeV.

2. Scientiˆc Objectives

2.1 ElectronsThe most important goal for the CALET mission is

to directly detect the nearby electron sources by ob-

serving the energy spectrum in the TeV region. As iswell known, high-energy electrons lose their energy(per unit time) in proportion to square of the energy,by synchrotron radiation and inverse-Compton scat-tering. Therefore, in the TeV region, only the electronsat a distance within 1 kpc from the sources and with anage less than ¿105 years, can reach the Earth. Sincethe number of such possible sources is very limited, theenergy spectrum observed might have a characteristicstructure4), and the arrival directions are expected toshow a detectable anisotropy5). The diŠusion processin the Galaxy also strongly aŠects the electron ‰ux.The energy spectrum could, therefore, give a directevidence of nearby cosmic ray sources and aknowledge of particle diŠusion characteristics in inter-stellar space.

Among the candidates, Vela is the most promisingas an observable nearby source since both the distance,¿0.25 kpc, and the age, ¿104 years, satisfy the con-straints listed above. Fig. 1 shows the expected energyspectra calculated by a diŠusion model under diŠerentassumptions. Several parameters are chosen toreproduce a spectrum consistent with the present databelow 100 GeV. These include an injection spectrumof E－2.4, total energy of 1048 erg, the size of the Galac-tic disc, the diŠusion coe‹cient and the energy lossrate6). Possible candidates which contribute in the TeVregion are Vela, Cygnus loop and Monogem in order

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Table 1 Performance for gamma rays compared with other instruments

EGRET GLAST (SRD) CALET

Energy Range (GeV) 0.02–30 0.02–300 0.02–1×104

EŠective Area 1500 À8000 7.9×103 (＠10 GeV)

(cm2) 4.6×103 (À100 GeV)

R.O.V. (sr) 0.5 À2 0.5–1.8

Angular Res. 5.8 (＠100 MeV) º3.5 (＠100 MeV) 5.0 (＠100 MeV)

(deg) º0.15 (À10 GeV) º0.24 (À10 GeV)

Energy Res. (z) 10 º10 7/ E/10 GeV

Point Source Sensitivity(cm－2s－1) (À100 MeV) 5×10－8 6×10－9 1×10－8

Fig. 2 Simulated energy spectrum of e＋＋e－ power law spec-trum with Kaluza-Klein dark matter annihilation for300 GeV mass. The observation time is assumed to be 3years in simulation.

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Shoji TORII, et al.


of the strength. DiŠerences in the parameters assumedfor these sources causes the changes of ‰ux seen in theˆgures. In the CALET observation, the number ofelectrons over 1 TeV is expected to be about one thou-sand, assuming that the ``distant'' electron spectrumhas a simple diŠerential power index of －3.3. In thiscase, it should be very easy to detect the signature ofVela above the background and to resolve the diŠer-ences in the spectra as suggested in Fig. 1.

The electron energy spectrum from 10 GeV to 1 TeVis the result of contributions from unresolved sources,and its measurement will give us accurate knowledgeof the average features of the source spectrum, thediŠusion time, and the density of sources. The ‰ux be-low 10 GeV is strongly modulated by solar activity andlong-term observation in this energy regime can supplyus with information to understand the modulationmechanism. There are some expectations that posi-trons have a line signature around several 100 GeVfrom the dark matter candidates; neutralinos in theSUSY theory7) and Kaluza-Klein (extra dimensional)particles8). Although CALET has no capability to dis-tinguish positive and negative charges, an excess of

positron and electrons might be detected due to the ex-cellent energy resolution of the instrument as shown inFig. 2.2.2 Gamma-rays

Since CALET has a large ˆeld of view (¿2 sr) and awide eŠective area (¿1 m2＠10 GeV), it can observethe whole sky without any attitude control. Thecoverage per one day is ¿70z and the entire sky canbe observed in 20 days. The observation period forpoint sources is 48 days on average per one year. Mostof the GeV sources detected by EGRET have not beenobserved in the TeV region by Air Cherenkov observa-tions although the detection e‹ciency should beenough for the case that there is no break in the spec-trum.

CALET will have the ability to detect gamma-raysfrom point sources to ˆll the energy gap between theEGRET and Air Cherenkov observations. In Table 1,the performance is compared with EGRET andGLAST. The most important targets of observationinclude: Galactic and extra-Galactic diŠuse compo-nents, supernova remnants, pulsars, AGNs, and gam-ma-ray bursts. In particular, the diŠuse Galactic com-ponent of gamma-rays above 10 GeV is strongly relat-ed to the electron energy spectrum since the gamma-rays are mainly produced by inverse-Compton scatter-ing with electrons near the source region9).

Because the energy resolution improves at higherenergies, CALET can precisely measure the change inthe gamma-ray energy spectrum index around from 10to ¿100 GeV. Such changes might be brought by thedecrease of acceleration power and/or the absorptionby starlight photons in the extra-Galactic space. Fi-nally, observations of gamma-ray lines from the anni-hilation of SUSY particles7) are also feasible if suchparticles exist in su‹cient numbers.2.3 Protons and nuclei

Although CALET is an electromagnetic calorime-ter, it can detect protons up to 1,000 TeV as the ab-sorber thickness corresponds to 1.8 m.f.p. (mean freepath) for protons as described in next section. A pix-elated silicon detector module will have su‹cientcharge separation capability and dynamic range to

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Fig. 3 Simulated energy spectrum of a gamma-ray line at 690GeV from neutralino annihilation, including the back-ground of the Galactic diŠuse emission. The observa-tion time is assumed to be 3 years in simulation.

Fig. 4 The energy dependence of the ratio of boron relative tocarbon. The expected data for CALET mission areplotted with ˆlled circles up to the TeV/n region.Previous measure-ments are covering the energy regionless than 100 GeV/n. The observation time is assumedto be 3 years in simulation.

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identify relativistic nuclei in the range from proton toIron and above. Determining the energy spectrum ofprotons in proximity of the Knee region is very im-portant for resolving the acceleration limits of protonsand heavy nuclei. Further, validity of the leaky boxmodel will be tested up to the energy region of 10 TeVby measuring the cosmic ray secondary to primary ra-tio energy dependence as presented in Fig. 4.2.4 Solar physics

The CALET measurement of long-term variationsof the energy spectrum of electrons in 1–100 GeV willproduce a wealth of data to investigate electron propa-gation in the heliosphere. CALET is not able to distin-guish positive charge from negative. However, we canevaluate the charge sign dependence of solar modula-tion using correlation with the neutron monitors, since

most particles are negative electrons in this energyrange. We can also estimate transport parameters,mainly the energy dependence of diŠusion coe‹cientin the heliosphere. CALET will also have a capabilityof measuring the short-term variation of around tenForbush decreases (Fds) for three years. Precise mea-surements of the energy spectral variation of Fds willgive a conclusion of the energy dependence of Fds.

3. Detector Development

3.1 Detector ConceptCALET is a combination of an imaging calorimeter,

IMC, with a total absorption calorimeter, TASC. TheIMC is used for identiˆcation of the incident particleand energy measurement below 10 GeV, while theTASC is for proton rejection in the TeV region and forenergy measurements above 10 GeV. The detectorweight is nearly 1,760 kg and the eŠective geometricalfactor (for electrons) is ¿1 m2 sr. A schematic struc-ture of the CALET detector is shown in Fig. 5.

The IMC consists of 17 layers of lead plates eachseparated by 2 layers of 1 mm square cross sectionscintillating ˆber (SciFi) belts arranged in the x and ydirection and is capped by an additional x, y SciFi lay-er pair. The dimension of the IMC is about 100 cm by100 cm. While the total thickness of the IMC is 4 radi-ation length (X0) , and about 0.13 proton interactionlengths (l), the ˆrst 10 lead-SciFi layers sample theparticle at 0.1 X0, followed by 5 layers that are 0.2 X0

thick and ˆnally 2 layers that are 1 X0 deep. Thisprovides the precision necessary to 1) separate theincident particle from backscattered particles, 2) pre-cisely determine the starting point for the electro-magnetic shower, and 3) identify the incident particle.The readout for the SciFi layers is currently envisionedto consist of multi-anode photomultiplier tubes(MA–PMT), such as the Hamamatsu R5900. EachR5900 MA–PMT has 64 anodes, and, consequently,about 16 MA–PMTs will be needed to read each belt.The front-end electronics for the IMC will be basedupon a high density ASIC such as the 32 channel Vik-ing chip (VA32HDR14). Current work involves de-veloping readout electronics with the dynamic rangeand low noise characteristics.

The TASC measures the development of the elec-tromagnetic shower to 1) determine the total energy ofthe incident particle and 2) separate electrons and gam-ma-rays from hadrons. The TASC is composed of 14layers of Bismuth Germanate (BGO) ``logs'' whereeach log has dimensions of 2.5 cm×2.5 cm×35 cm.There are 56 such logs in each layer. Alternate layersare orientated 909to each other to provide an x, ycoordinate for tracking the shower core. The total areaof the TASC is about 0.5 m2 and the vertical thicknessis 32 X0 and 1.6 l. It is anticipated that each BGO logwill be read by a photodiode and laboratory tests haveconˆrmed that such a system is capable of detecting

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Fig. 5 Schematic side view of CALET detector structure.

Fig. 6 Concept instrument design of CALET.

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0.5 MIP (minimum ionizing particle) and work is cur-rently underway to develop electronics with a high dy-namic range.

For low energy gamma-rays (º10 GeV), the IMC iscovered by plastic scintillators for anti-coincidencewith hadrons. A pixelated silicon detector module willbe placed at the top of the IMC for having su‹cientcharge separation capability and dynamic range toidentify relativistic nuclei in the range from proton toIron and above. It can also aŠord to identify preciseposition of an incident particle among the copiousbackscattered particles at higher energies.3.2 Read-out Electronics

A concept instrument design for CALET is shown inFig. 6. The IMC in the top part of the ˆgure shows theSciFi belt assembly interfacing with an array ofMA–PMTs and front end electronics (FEC). Two suchbelts, oriented 909to each other, are shown beingplaced on a lead plate. The remainder of the IMC lay-ers is assembled in a similar fashion. Below the IMC isthe TASC with its stack of BGO logs. The logs arewrapped on ˆve sides for optical isolation and, foreach layer, are assembled next to each other in tworows with the unwrapped face outward. The pho-todetectors (PD) are then attached to the unwrappedlog faces. Layers are then assembled on top of eachother with each layer rotated by 909relative to theprevious layer. Finally, the PD readout electronics(Pre. AMP＋AMP) boxes are assembled on each faceof the TASC. Details of this assembly, including, forexample, structural support for the individual BGOcrystals, position calibrations for the SciFi belts andanalysis of launch loads, still needs further work.3.3 Simulation Study on Performance

By simulation we ˆnd that primary electrons deposit

about 95z of their energy in the BGO calorimeter, de-pending weakly on energy, while protons on averagedeposit about 40z. After the shower trigger, only theproton events, which have the ˆrst interaction at thetop of CALET, can survive. Such a trigger system hasbeen proven by several ‰ight instruments. Proton in-duced shower should have a wider spread than electrondue to the spread of secondary particles in the nuclearinteractions. This diŠerence is clearly observed in the

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Fig. 7 Scatter plots of F value in diŠerent BGO layer forisotropic incident electrons (dot signs) and protons ofcomparable total energy deposit in the TASC (plussigns).

Table 2 Expected performance for electrons

Energy Range 1 GeV¿10 TeV

Geometrical Factor ¿1 m2sr

Proton Rejection Power ¿106

Energy Resolution 9.2z/ E(10 GeV)

Angular Resolution (À10 GeV) 0.03¿0.1 deg.

Fig. 8 A schematic view of CALET on the JEM platform.

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images detected by SciFi.For studying the performance of CALET, we have

simulated 2×106 isotropic incident proton events withpower law index －2.75 of spectrum which energyabove 1 TeV. We deˆne a function F＝(En/Sum)*square of the r.m.s. of lateral spread, to calculate theshower development in the TASC. Here En is theenergy deposit in the BGO n–th layer, and Sum is thetotal energy deposit in all BGO. Fig. 7 shows a scatterplot of the F values in 13-th BGO layer vs. 14-th BGOlayer, most of proton events are outside the plotregion. It can be seen that if we set a cut as the dashedline, the electron events and proton events are separat-ed very well with each other. Combining the total per-formance of the IMC and the TASC, the total rejec-tion power is nearly 6×105 in the current structure ofCALET under the condition that the electron detec-tion e‹ciency is above 95z. The performance which isexpected by simulations for electron observations issummarized in Table 2.

For gamma-rays below 10 GeV, we set another trig-ger condition: 1): Events which give hits at least 4 con-secutive ˆbre planes (X and Y view). 2): Energydeposit in the anticoincidence which surrounds the im-aging calorimeter is smaller than 0.3 MIP. We simulat-ed 1 million events of earth albedo gamma-rays (above

10 MeV) and cosmic gamma-rays (above 10 MeV).The trigger e‹ciency is 3.3z for earth albedo gamma-rays; the trigger e‹ciency is 16z for cosmic gamma-rays. The eŠective area, the angular resolution and theenergy resolution are obtained as a function of energyin Table 1, comparing with EGRET and GLAST.3.4 Beam Test of Proto-type Detector

In 2003, we carried out a beam test of the prototypeof CALET (in smaller scale) at CERN–SPS. In thisprototype, the detector has similar structure in lon-gitudinal direction with the baseline structure ofCALET for measuring the energy resolution, the an-gular resolution and the hadron rejection power. Weconˆrmed the performance is very consistent with ex-pectations from the simulations for electrons at ener-gies of 50 GeV and 100 GeV and for protons of 150GeV. The energy resolution is 4.0z and 2.3z for 50GeV and 100 GeV electrons, respectively. The proto-type could reject 97.3z of 150 GeV protons and couldidentify 96.8z of electrons. Since the simulationresults are fairly consistent with the beam tests, the ex-trapolation to higher energies might be reliable to exa-mine the performance.3.5 Accommodation Study for JEM

The CALET will be launched by a Japanese carrier,HII Transfer Vehicle (HTV), and attached to the EFU#9, which is capable to maintain a heavy payload up to2,500 kg in mass and has a wider ˆeld of view, 45degrees. Fig. 8 shows a schematic view of the CALETpayload on ISS/JEM. The main structure of CALETis designed by adopting an interface structure of ausual exposed facility. The structure, therefore, in-cludes a pallet to sustain the detector, which is used

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both for launching by HTV and for attaching to JEM.The structure was optimized to meet the requirementsfrom the ISS for the vibration condition.

Also, the heat condition was analyzed in severalphases of the experiment. Preliminary thermal analysisin orbital visiting phase, in both hot and cold caseswith various b-angles gives the temperature of ＋38¿＋449C in hot case and ＋6¿＋129C in cold case atthe detector surface. Although obtained ranges arevery narrow, it was assumed idealistic thermal connec-tion among detectors circuits were achieved. Tempera-ture may become critical in some extreme cases. Aftermore detailed analysis, active thermal control by the‰uid interface may be considered. By a structural anal-ysis, it is proven that the 1st mode eigen value of thestructure is ¿3 Hz on JEM–EF, which satisfy the stiŠ-ness requirement: À2 Hz.

4. Summary and Conclusion

The CALET mission is proposed to perform a cru-cial observations of electrons and g-rays at the highenergy frontier. Potential nearby sources of electronswill be directly identiˆed by observing the energy spec-trum and the distribution of particle arrival directionsin the TeV region. Also, dark matter signatures couldbe discovered by measuring g-ray line features and ex-cesses in the electron spectrum around several 100GeV. Observation of protons and heavy nuclei spectrawill be used to study the acceleration and propagationof cosmic rays at energies close to the Knee. Moreover,solar physics is also investigated.

We have already completed a phase A study forCALET within the last 3 years, and have successfullydeveloped the electronics necessary to read–out theSciFi and the BGO. To conˆrm the detector perfor-mance for the experiment on space station, the ‰ight

test by a balloon is scheduled in 2006. We expect to be-gin operations on the ISS /JEM around 2012 and themission life is supposed to be three-years.

Acknowledgments

This work is carried out as a part of Ground-basedResearch Announcement for Space Utilizationpromoted by JSF and Grants-in-Aid for ScientiˆcResearch. We would like to express our thanks toProf. M. Haguenauer and other staŠs for their help tocarry out successful beam tests in CERN.

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(Received October 8, 2005Accepted for publication, September 27, 2006)

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