Istutuzioni di Fisica 10 Elettricit e circuiti

1
Preliminary measurements of the absolute fluxes
of heavy nuclei from Carbon to Silicon with the
second CREAM flight
Paolo Maestro University of Siena INFN on
behalf of the CREAM-II collaboration
2
CREAM-II collaboration
University of Maryland, USA H.S. Ahn, O. Ganel,
J.H. Han, K.C. Kim, M.H. Lee, A. Malinin, E.S.
Seo, R. Sina, P. Walpole, J. Wu, Y.S.Yoon, S.Y.
Zinn Ohio State University, USA P.S. Allison, J.
J. Beatty, T. J. Brandt University of Siena and
INFN, Italy M.G. Bagliesi, G. Bigongiari, P.
Maestro, P.S. Marrocchesi, R. Zei NASA Goddard
Space Flight Center, USA L. Barbier University
of Minnesota, USA J.T. Childers, M.A.
Duvernois Penn State University, USA N.B.
Conklin, S. Coutu, S.I. Mognet Ewha Womans
University, Republic of Korea J.A. Jeon, S. Nam,
I.H. Park, N.H. Park, J. Yang Kent State
University, USA S. Minnick Northern Kentucky
University, USA S. Nutter
Thanks to
3
Cosmic Ray Energetics And Mass
CREAM can measure individual energy spectra and
elemental composition
(1 ? Z ? 26) of cosmic rays up to 1000 TeV

• Main science goals
• search for a cutoff in the proton spectrum at E
  100 TeV
• search for a change in the elemental composition
  approaching the knee
• measurement of secondary over
• primary ratio (test of propagation
• models).
• CREAM-I measured
• B/C and N/O up to 1.5 TeV/n
• arXiv0808.1718v1 astro-ph

• NASA Long Duration Balloons (LDB).
• Three flights over Antarctica from McMurdo
• CREAM-I 42 days (Dec. 16th 2004 - Jan. 27th
  2005)
• CREAM-II 28 days (Dec. 16th 2005 - Jan. 13th
  2006)
• CREAM-III 28 days (Dec. 19th 2007 - Jan.
  16th 2008)
• Altitude 38-40 km.
• In flight 85 kbps telemetry (TRDSS) of the high
• energy data. Low energy data recorded on
  board.

4
CREAM-II instrument

• Cerenkov counter
• 1 cm thick plastic radiator with blue wavelength
  shifter
• low energy particles veto

• Timing Charge Detector (TCD)
• 5 mm thick fast (lt 3 ns) plastic scintillator
  paddles
• charge measurement from H to Fe (s 0.2-0.35 e)
  
• backscatter rejection by fast pulse shaping

• Tungsten-SciFi calorimeter
• Preceded by a graphite target
• ( 0.5 ?int)
• Active area 50 50 cm2
• 20 layers, each 3.5 mm W
• 0.5 mm SciFi ? 20 X0, 0.7 ?int
• 1 cm transverse granularity
• 2560 channels (40 HPDs)

• Silicon Charge Detector (SCD)
• 2 planes, 2496 Si pixels each
• Active area 0.52 m2 . No dead area
• charge measurement from Z1 to Z33

5
High-Z nuclei analysis

• Selection cuts for high-Z nuclei
• trajectory reconstruction
• charge-ID with SCD
• energy measurement
• Energy deconvolution
• Definition of geometrical factor (GF)
• Live Time estimate
• Corrections for interactions in the atmosphere
  (TOA) and in the instrument (TOI)
• Preliminary absolute fluxes of C, O, Ne, Mg, Si

6
Display of a detected event
Charge-ID with SCD
Charge-ID 26 (Fe) Energy deposit 105
GeV Primary particle rec. energy 70 TeV
Shower imaging with CAL
7
Trajectory reconstruction

• Two steps algorithm
• 1 CAL tracking. Shower axis is projected
• back to SCD planes. Search for hit pixels
  
• in the circle of confusion (R 3 cm)
• 2 new fit including the matched SCD
• pixels ? This improves the accuracy of
• pathlength correction

Flight data
MC

• Impact point resolution on SCD is estimated
  comparing the reconstructed impact point with the
  position of the pixel with the highest count. lt
  7mm
• Accuracy of zenith angle measure
• 0.7 (estimated from MC)

8
Charge measurement
Si
SCD
Z- top plane
Mg
Counts
O
C
Ne
O
N
NOT representative of elemental abundances.
C
B
Z- bottom plane
Si
Mg
N
B
Ne
Charge Z combined top bottom planes
Excellent charge resolution 0.2 e from C to Si
9
Energy measurement
All-particles energy deposit in the calorimeter

• Very good CAL performance in flight
• HPD high voltage system was very stable,
  operating _at_7 kV unpressurized in weak vacuum
  environment (few mbar).
• Low electronics noise. Very stable channels
  gains.
• Accurate pedestals subtraction to correct for
  pedestals drift with temperature.

Energy deposit by Si nuclei in CAL Black dots
events in SCD/CAL acceptance Blue dots events in
TCD/SCD/CAL acceptance
10
Absolute flux determination

• Ni are the unfolded counts in energy bin i.
  They are obtained by correcting the
• measured counts for overlap with neighbouring
  bins (energy deconvolution)
• DEi energy bin size. The energy scale is
  divided into equidistant logarithmic
• bins starting from 800 GeV
• TOI Top of instrument correction i.e. survival
  fraction of primary nuclei of
• each species reaching SCD without interacting
  in the above apparatus
• TOA Top of atmosphere correction
• Tlive Live time
• ?i selection cuts efficiency for energy bin i

11
Geometrical Factor and Live Time

• Geometrical acceptance for events crossing
• - TCD (120120 cm2), SCD (7878 cm2)
• and CAL top plane (5050 cm2) ? GFsmall
  0.194 m2 sr
• - SCD and CAL top plane ? GFlarge 0.462 m2
  sr
• Two independent estimates based respectively on
  MC simulation
• and analytical calculation turned out to be in
  good agreement.

TCD CER SCD TARGET CAL
Accepted events
Rejected event
CREAM-2 trajectory

• Live Time is computed for the period
• Dec. 19th 5am - Jan. 12th 730 pm
• Effective Live Time 24246.7 min (?16 days 19h)

12
MC simulation

• A detailed MC simulation of CREAM-2 instrument
  has been done to estimate
• - the trajectory reconstruction and charge
  assignment efficiencies
• - the energy deconvolution or overlap matrix
• - TOI correction for each nucleus
• MC based on FLUKA 2006.3b with hadronic package
  DPMJET-III
• Isotropic generation of nuclei extracted from
  power-laws energy spectra 0.1200 TeV

Each element Aij of the overlap matrix represents
the probability that events in the deposited
energy bin i come from the primary particle
energy bin j
Average longitudinal profile of carbon nuclei
Data MC

• Reconstruction efficiency
• nearly flat _at_ E gt 3 TeV
• not dependent on Z for Zgt6
• 80 (65) in TCD (SCD) acceptance

13
TOI and TOA corrections

• TOI (Top of Instrument) correction ?5 g/cm2 of
  materials above SCD
• TOA (Top Of Atmosphere) correction estimated by
  means of a Fluka based
• MC of the residual atmosphere overburden (?3.9
  g/cm2).
• Zenith angle distribution of nuclei within
  CREAM acceptance is taken into account
• At TeV scale the survival probabilities are
  nearly independent on energy

14
Carbon Oxygen energy spectra
C
O
total no. events 686 events in the last two
bins 2, 1
total no. events 936 events in the last two
bins 6, 2
15
Neon Magnesium energy spectra
Ne
Mg
total no. events 210 events in the last two
bins 3, 3
total no. events 325 events in the last two
bins 3, 3
16
Silicon energy spectrum
Si

• Remarks
• Median energy of each bin is defined according to
  Lafferty Wyatt definition NIMA355 (1995) 541
• High energy intervals (?) are integral bin
• Measured fluxes are well fitted to power-laws
  ?(E) ?0 ? E-g
• Fitted spectral indexes in range 2.6-2.75
• Only statistical errors are reported.
• Systematics under study.
• Data at energy below 1 TeV were prescaled
  during the flight ? larger statistical error in
  the first bin

total no. events 348 events in the last two
bins 4, 1
17
Fluxes ? E2.5
C
O
18
Ne
Mg
Si
19

• Conclusions
• CREAM-2 instrument was capable of an excellent
  charge separation
• and a reliable energy determination of the CR
  nuclei
• Absolute fluxes of C, O, Ne, Mg, Si in the
  range 1-100 TeV have been
• measured. These preliminary results are in
  agreement with both
• low energy measurements from old experiments
  and TeV data
• from the last generation of balloon-borne
  instruments.
• Energy spectra are well fitted to power-laws
  with very similar spectral
• indexes. No significant changes in CR
  composition are yet observed.
• On-going analysis to refine and check the
  current results, understand
• systematics and attempt to measure other
  nuclei (B, N, Fe) spectra.