1. Balloon observation of electrons and gamma rays with CALET prototype
Y. Shimizu, S. Torii, K. Kasahara, Y. Akaike, Y. Fukuta, K. Hidaka, K. Taira (Waseda Univsersity)
T. Tamura (Kanagawa University), K. Yoshida (Shibaura Institute of Technology)
Y. Katayose (Yokohama National University), H. Murakami (Rikkyo University)
for the CALET collaboration
We carried out a balloon observation of cosmic rays using a prototype of the CALET at the Sanriku Balloon Center of the Japan Aerospace Exploration Agency. The prototype detector consists of 1024 scintillating fibers for track imaging and 24 BGO scintillator “logs” for total absorption of showers. The observation was carried at an altitude between 35 and 37 km for about 3.5 hours. We measured electrons and gamma rays in the energy region between 1 to 100 GeV, and 20 MeV to 1 GeV, respectively. In addition to verification of the prototype system, we have obtained the electron flux which is useful to investigate solar modulation. Now we are planning a series of balloon experiments with larger-scale detectors and longer-duration flights, which includes one-month observation by a super-pressure balloon.
CALET prototype
Balloon experiment
CALET measures high energy cosmic rays by imaging technique of the electromagnetic shower.
We have developed its elemental components such as scintillating fibers (SciFi) and BGO scintillators.
A prototype was constructed for the verification of observation capability of the CALET.
The Balloon experiment was carried out at the Sanriku Balloon Center of the Japan Aerospace Exploration Agency on 31 May.
The duration of the flight was about 6 hours.
Cosmic rays were observed at the altitude between 35 and 37 km for 3.5 hours.
The payload was recovered on the Pacific Ocean.
The prototype consists of imaging calorimeter (IMC), total absorption calorimeter (TASC) and trigger scintillators.
IMC – tungsten plates
1024 SciFi’s (1×1mm2 cross section)
64ch-multi-anode PMTs
TASC BGO’s (25×25 mm2 cross section)
photodiodes
Trigger - 2 plastic scintillators (S1, S2)
the first BGO layer (B)
PMTs
Balloon
Volume ,000m3
Total weight kg
(including 84kg of ballast)
Altitude km
6: launching
8: level flight above 35km
11:44 cutting off the payload
12:15 landing　on ocean
e- 5GeV (simulation)
γ 1GeV (simulation)
Balloon trajectory
Altitude
Trigger setting
e S1 > 0.7 S2 > 0.7 B > 5.0 (MIPs)
γ S1 < S2 > 0.7 B > 0.5 (MIPs)
Electronics
IMC
launching
landing
recovery
FEC+PMT
Power
TASC
Data Analysis
Results
To derive energy spectrum of electrons, we analyzed 3×103 electron trigger events obtained at an altitude between 35km and 37km.
We picked up partially (up to the second BGO layer) and fully (up to the bottom BGO layer) contained events. Among these events, selection of electron candidates was carried out by criteria imposed by an analysis of the simulation events. Using an estimated acceptance by simulation, the electron flux has been derived. The result is consistent with expected values by calculation using the COSMOS code.
Our current data analysis is focused on electron events during the level flight observation.
Using the shower image measured by IMC and TASC, we have obtained the incident energy and the incident direction, and the eliminated proton background.
Detection capabilities were estimated by the Monte-Carlo simulation using a code, EPICS.
Acceptance
Energy resolution
Estimated capability of 10GeV electron
Acceptance cm2sr (full)
12cm2sr (partial)
Energy resolution - 3%(full)
4.5%(partial)
Proton rejection (full)
110 (partial)
Flux of selected events
Electron flux
The incident energy was determined by the sum of the deposit energy in IMC and TASC. Energy loss in tungsten plates was estimated from the observed energy in SciFi layer.
The incident direction was obtained by the best-fit line of shower cores in each layer. We estimated the core position by the SciFi position with the highest signal in each layer for IMC and by the energy weighted center of four BGOs in one layer for TASC.
Proton background was rejected by imaging the shower development. The vertical development was estimated by an equation written below. We used parameters (a, b) and χ2 of the best fit. The transverse development was distinguished by energy concentration around the estimated trajectory and the standard deviation from the energy weighted centers.
Event viewer (e- 3GeV)
partially contained
fully contained
Future plan
For further development of CALET and much more precise observation, a longer balloon experiment is prospected. At first, we are preparing for one-day flight with a larger detector( 1/16 scale of CALET) in new balloon-experimental cite, Taiki-cho in Hokkaido. One-month flight of the CALET prototype (1/4 scale) is planned around 2009 with a super-pressure balloon from Brazil. There is a possibility to observe mono-energetic electrons produced by the WIMP dark matter annihilation in the energy range of several hundred GeV by the prototype.
Expected Electron flux
by CALET Prototype
Structure of CALET Prototype
t - thickness
E0 - incident energy
a, b - parameters